ADSORPTION ON A SURFACE OF LIQUID. SURFACE-ACTIVE COMPOUNDS. TO DETERMINE THE SURFACE TENSION BY STALAGMOMETRIC METHOD. ADSORPTION ON A SURFACE OF SOLID ADSORBENTS. SURFACE PHENOMENON. AN ION-EXCHANGE ADSORPTION.

COHESION AND ADHESION

Molecules liquid state experience strong intermolecular attractive forces. When those forces are between like molecules, they are referred to as cohesive forces. For example, the molecules of a water droplet are held together by cohesive forces, and the especially strong cohesive forces at the surface constitute surface tension.

When the attractive forces are between unlike molecules, they are said to be adhesive forces. The adhesive forces between water molecules and the walls of a glass tube are stronger than the cohesive forces lead to an upward turning meniscus at the walls of the vessel and contribute to capillary action.

The attractive forces between molecules in a liquid can be viewed as residual electrostatic forces and are sometimes called van der Waals forces or van der Waals bonds.

COHESION AND SURFACE TENSION

The cohesive forces between molecules down into a liquid are shared with all neighboring atoms. Those on the surface have no neighboring atoms above, and exhibit stronger attractive forces upon their nearest neighbors on the surface. This enhancement of the intermolecular attractive forces at the surface is called surface tension.

Introduction: Water has many unusual properties as a result of its ability to hydrogen bond. For example, the density of ice is less than that of the liquid and the predicted boiling point is almost 200 degrees C higher than it would be without hydrogen bonding.

Surface Tension

Liquids sometimes form drops, and sometimes spread over a surface and wet it. Why does this happen, and why are raindrops never a meter wide? A clue to the answer to the second question may be found in pictures of astronauts playing with large blobs of water in their space-craft.

It all comes down to the forces between atoms or molecules, and the forces between them. These particles are unimaginably small. In one gram of water the number of molecules is about 3.3 X 10²², or 33000000000000000000000. If the gram of water were in the form of a 1 cm cube, there would be about 23000000 molecules on a side.

The force between two atoms or molecules is generally repulsive if they are pushed too close together. The force increases so strongly as the distance is reduced that they behave almost as if they were hard objects. Try compressing some water or steel. But at larger distances the force are attractive. Try pulling pulling the bung from a tube which contains only water and no air. Or try pulling a piece of piano wire in two.

The vertical direction represents energy. The horizontal direction represents the distance between the centres of two molecules. Zero is way off the left of the graph because two molecules can't overlap.

The green line represents the zero of energy. At the right, when two molecules are far apart, the mutual energy is nearly zero. As we move them together, the energy goes down. This means that they are attracting each other, just as a falling object is moving towards positions of lower energy. But closer than a certain distance, the energy starts to rise more and more sharply, until it is very steep indeed. It becomes unbelievably hard to push two molecules closer than a certain distance. This is why substances such as water and steel are almost incompressible. Gases are compressible because there are gaps between the molecules. The distance between air molecules is very roughly ten times their diameter.

The white dots represent molecules, running around in the stable area between the red lines. The vertical spread represents their distribution of energies. The upper diagram corresponds to a higher temperature. We can see that another increment of energy would see a few molecules going above the green line, and able to go infinitely far to the right. In other words, they can escape. This is called evaporation. Further small increments in temperature will greatly increase the number above the green line, and the evaporation rate will increase dramatically.

This is characteristic of phenomena with a threshold. When you wash something in warm water, the absolute temperature is not raised much, but the effect is great. Another example is the conductance of a semiconductor, which rises with temperature. In fact, in the early days of semiconductors, great care had to be taken to stop them burning themselves out.

The diagram comes with a bonus - as the temperature is raised, the average separation of the molecules moves to the right, that is, the molecules move apart. In other words, the material expands.

You may argue that a few substance contract on heating, like water from 0 C to 4 C. This can be explained by invoking changes of structure. Similarly, a well ordered substance might shrink if the aligned molecules wriggle about more vigorously.

Also, as the liquid starts to lose its grip, the surface tension goes down, and by the time the boiling point is reached, it is considerably less than at room temperature. But it doesn't disappear completely until the critical temperature is reached. At that point, the interface between liquid and solid vanishes; in fact the distinction between liquid and solid vanishes, leaving no reason to have an interface.

So, between the two regimes of repulsion and attraction, two molecules prefer very strongly to sit at a distance where there is no force between them, where their total energy is a minimum. Of course, they are not stationary - they are jiggling about because of thermal motion. Below a certain temperature, molecules cannot easily get away from their position - in this condition, the substance is a solid. At a higher temperature they can wander around, and we have a liquid. Given a high enough temperature, molecules may may escape - this is evaporation. At higher temperature the liquid turns into a gas. As you might expect, adding pressure changes things.

Thus in a solid or a liquid, the atoms or molecules are in a state of average equilibrium. On average, each one feels no net force from the ones all around. There will be fluctuations in the force on each one as they jiggle around, and in a liquid the molecules will slowly wander about. Two layers of differently coloured water will gradually diffuse into each other. Even in a solid, there can be very slow diffusion.

The answer is already implied in the statements given. Every molecule is on average in a sate of lowest available energy, with no net force on it. If a molecule experienced a net force it would move until it didn't. Molecules in the body of substance are surrounded by neighbours . You might imagine that there could be around twelve packed around it - six in a ring, with three above and three below - that would be hexagonal close packing. A simple cubic lattice would give six closest neighbours, while in diamond there are four. Any molecule is, on average, in equilibrium with all of these neighbours, and will be at the position of lowest energy.

The next diagram shows again a curve representing the variation of energy for one molecules in the field of another. The curve is has mirror symmetry because the second molecule can be on either side of the first. The true situation has, of course, spherical symmetry, which is hard to draw. The blue line represents the energy at infinite separation.

In three dimensions, every molecule sits in the potential well created by all the others, though the steepness of the curve means that only the nearest and next nearest neighbours have much effect. To simplify things, let us consider whole planes of molecules or atoms as objects, whose energy will be described by a different, though similar curve. The next curve shows some of the many curves.

We can obtain the total energy by adding these curves together, as below, on a reduced scale. The vertical blue lines show the positions of the atoms.

We already found that molecules in the surface have fewer neighbours than those in the interior. For hexagonal close packing, the neighbours can be thought of as comprising six equatorial, and three polar on each side. At the surface, one set of three is missing, so the number if neighbours is reduced to three quarters of the normal value. For a simple cubic packing, we find by a similar argument a factor of five sixths. As we are only going to make a qualitative argument, we will take a rough value of four fifths, 0.8. Then a surface molecule will have a binding energy that is only 0.8 of the normal, and will therefore be at a higher energy level. Objects move to positions of lower energy if possible, and this is what happens in the liquid.

To answer this we need to think about the surface molecules. Each time one leaves, the average spacing of the remainder increases, though be very little. That increase means that they are no longer at the minima of their local potential wells. The movement of molecules to in interior continues until the movement of one more molecule loses as much energy from it as the surface gains. Then we have equilibrium. The diagram below shows the well for one molecule.

We now have a surface in which the molecules have an average spacing which is greater than the normal. In a solid object, we would call that stretching, and the object would be in tension. And so it is with this surface layer of molecules - it behaves as if it were in tension.

That the molecules are moving about, that the forces may be considered as classical or quantum mechanical, are irrelevant considerations: the force is based on averages, like any other macroscopical quantity.

Because this region has higher energy for a given area, it will tend to behave so as to minimize its area in any situation. Surface tension is not caused by a skin on the surface of a liquid. For a free blob of liquid, the smallest area is obtained with a sphere. In more complicated cases the shape of the surface reflects the complexity of the situation.

For the smaller drops of water, gravity has little effect on the shape. All the drops have shapes which minimise the total energy.

In terms of forces, there must be a balance between all the forces at any of the boundaries between the substances, together with the weights if they are not negligible.

The weight of an object is proportional to the cube of its length, is the shape remains the same. But the surface tension forces are only proportional to length. So expanding an object by a factor of ten increases the ratio of weight to surface tension by a factor of a hundred. Nobody will ever make a boat float by surface tension. On the other hand, the barbules of birds' feathers are close enough, when oiled by the bird, to repel water and keep the bird dry. But buoyancy, not surface tension, keeps birds afloat.

The drops in the middle of the picture show the spherical shape, while those on the wire are influenced by gravity, adhesion to the wire, and surface tension.

Larger drops of water that sit on a non-wetted surface are not spherical - the shape is the result of the combination of the surface effect with the force of gravity. Because some surfaces have an affinity for water, drops can hang from them.

The diagram above gives a very rough idea of what happens at the surface of a liquid. The molecules are shown as hard edged balls for simplicity. On the left we see a sharp demarcation between the liquid and its vapour. On the right we see an exaggerated picture of the surface layer, in which the density falls smoothly from the liquid to the vapour. It is this layer of more thinly spread molecules which produces the "skin-effect" which is responsible for surface tension.

As the temperature rises, the distinction between liquid and gas becomes less clear, and the surface tension decreases. At the critical temperature, the distinction between liquid and gas disappears altogether, and there is no surface at all. Above that temperature we cannot speak of "gas" and "liquid": the substance is just a fluid.

To see a moving version of the diagram click here and click on "run in current location". To quit the demo press "q".

When people say that liquids behave as if there is a skin, they convey the wrong picture: the "skin" has less density than the body of the liquid, not more. But it's not like a balloon: as you inflate a balloon, the skin stretches, and the tension increases. But if you add water to a drop, the surface tension remains the same.

Some liquid and solid combinations have little or no affinity. In these cases a liquid drop sits on the surface. But if there is a degree of affinity, the shape of the drop is modified. With strong affinity, the liquid wets the surface and spreads out as a thin layer. What matters is the relative energy of the three interfaces - air-liquid, liquid-solid, and air-solid. In practice, because of surface variations, impurities and foreign bodies, the situation is complex, and real drops may take shapes that cannot be computed, as you can see in some of the photographs above.

What are the shapes of water-drops in ideal circumstances? To some extent they reflect the symmetry of the situation. Very small drops floating in a space craft are close to being spherical, because there is no preferred direction, but as larger and larger drops are more and more affected by effects that can dominate the short-range inter-molecular forces. The kinetic energy and momentum of different parts of large drops makes them wobble around in a rather unstable manner. If the waves that travel around a drop should add in a suitable manner (Interference) the drop may split.

A similar effect is a part of the explanation of the finite list of chemical elements found on earth, which terminates at number 92, uranium. The particles in an atomic nucleus experience short range forces analogous to those between molecules in a liquid. So larger and larger nuclei can behave rather like liquid drops. Because the number of nucleons is so small, a large proportion of them lie in or near the surface. In uranium, about 40 or so lie in the surface, which is about one sixth of the total. In a drop of water, the number of molecules is unimaginably large, about 1.4 X 10¹⁴, or 140 million million. This is the number in a cube with 52000 molecules on a side. The number in a surface layer about one molecule deep is about 2.7 X 10⁹, which is a very small fraction, 1/52000, of the total.

The diagram below shows how the density of a uranium nucleus varies with the distance from the centre. The horizontal axis is in fermis, which are 10^-15 m.

When the big nucleus begins to divide into two smaller ones, enormous tidal effects are set up. Just as the gravitational attraction of earth and moon sets up tides in both, the electrostatic repulsion of the putative nuclei produces huge deformations. This happens because the force varies so strongly with the distance. If you are stretched out by the gravitational field of a black hole, it isn't the strength of the field that gets you: it is the variation with position. The water of the oceans forms a prolate ellipsoid, bulging along the line joining earth and moon. Jupiter produces much bigger forces in its moons. The fleeing nuclei will be momentarily oblate, squashed along the line of flight. As the stable states are spherical, the oval nuclei have a lot of energy to get rid of. They are neutron rich, and some energy may be shed in the form of surplus neutrons. Although nuclear forces can act on time scales as short as 10^-23 seconds, some neutrons are delayed long enough to allow a stable feedback loop to be created, enabling energy producing reactors to be built.

When we get to the dimensions of boats, the effects of surface tension are obviously negligible. (Who went to sea in a sieve?) Birds keep the water out because the feathers are composed of very tiny parts, with tiny gaps, and so the effective size is small. By keeping the feathers oiled, they prevent them being wetted by the water, which cannot generate enough pressure to penetrate the gaps, because the required pressure is inversely proportional to the width of the gaps.

Cormorants and kingfishers have wettable feathers, and indeed young kingfishers drown if they don't learn how to dive and emerge properly. Cormorants spend a lot of time with outspread wings, drying off. Perhaps after millions of years, some of their descendents may have evolved natural oils.

Other considerations, such as quantum mechanics, modify the behaviour of nuclei, but a large enough nucleus can be quite unstable. Furthermore, some of the particles (protons) in a nucleus carry electric charge. These repel each other, like similar magnetic poles. Because the electric force has long range, each proton feels the effect of all the others, whereas it feels only the nearest neighbours for the nuclear force.

So adding particles to a nucleus doesn't make it bind more strongly, while the electric repulsion gets bigger. So very large nuclei are very unstable. Large enough nuclei have such short lives that they have not survived the age of the earth, even if they were originally present. Nevertheless, during the last sixty years, scientists have been able to create those bigger nuclei and some that probably never existed on earth. An exciting recent development is the discovery of very heavy ones which have anomalously long lives. On the chart of nuclear types this would appear as an island of stability, which was predicted to exist many years ago. This has no counterpart in drops of water - it is a quantum mechanical effect. See also Blobs

Since it is the repulsion between protons which makes big nuclei unstable, it might be thought possible to make nuclei without them, using only the neutral particles - neutrons. One of the reasons why not is that neutrons are unstable, decaying into protons. They only survive in existing nuclei because their energy is effectively reduced in that environment.

But there is another island of stability, which does indeed comprise only neutrons. In a neutron star the force of gravity generated by a huge number of particles crushes everything together into an enormously dense mass. The electric charges are sent packing in the form of electrons - in effect the protons have decayed into neutrons. Surface tension plays no part here - the surface is a negligible part of the whole. Neutron stars are like no other object. Nature seldom repeats herself exactly.

If the symmetry is broken, drops are no longer spherical. The sticky blobs on a spider web have circular, but not spherical, symmetry because the threads are cylinders (Spider Webs). They also have mirror symmetry because the threads have no preferred direction. For water drops resting on a horizontal surface, gravity breaks the up-down symmetry, although the drops are circular in plan. The pressure in the drop increases slightly with the depth, and so the total radius of curvature decreases from top to bottom.

The next picture show approximate simulations for a range of drop sizes, for the case where liquid has no affinity at all for the solid surface. The tiniest drops are a

If the density of the liquid increases, or the surface tension decreases, or we look at larger drops, the drops will be flatter for a given radius or volume. The second, third and fourth diagrams above show examples. The scales decrease from top to bottom in order to accommodate the greater widths. Eventually the drops do not become higher - they just become wider and flatter.

Here is a plastic barrel of grit for use in icy road conditions. The open end of the barrel has been distorted by the weight of the gravel into a shape which is reminiscent of a water drop. The shape differs for several reasons, for example, the tension is not the same all round the barrel, the system is based on a cylinder and not a sphere, and the shape is distorted by the closed end of the barrel, which retains its circular shape. But the relationship with a water drop is clear.

To see other examples based on cylinders, look at pictures of penguins, sea-lions and seals lying on flat surfaces. You will see the same kind of flattened shape, more round in the penguins, less round in the large mammals. And animals of the size of killer whales and other whales cannot survive on land: if they beach themselves, they are doomed, unless they can be refloated. Their weight squashes their bodies far out of shape, making a large flat area underneath. Their lungs cannot expand against the weight above them, and the animals slowly suffocate. At the other extreme, an earthworm is barely troubled by gravity - it is almost perfectly round.

The photograph above shows very small drops that are nearly spherical, with a few larger ones which are noticeably flattened.

The photograph above shows very large drops tending to constant depth, except that in the photograph the angle of contact is different from the value in the diagram, because the water had some affinity for the surface. This is no problem, because we merely need to remove a slice from the bottom of each of the earlier simulations to see these shapes.

The next set of curves show drops of water hanging from a sphere or a tube. The diagram is to scale, and shows drops ranging from radius 0.1 mm to 2.1 mm at their position of greatest curvature. They cannot come to a point, because the length of the circle at the top must be enough for the surface tension to hold the weight of the drop.

Surface Tension and Droplets

Surface tension is responsible for the shape of liquid droplets. Although easily deformed, droplets of water tend to be pulled into a spherical shape by the cohesive forces of the surface layer. The spherical shape minimizes then necessary "wall tension" of the surface layer according to LaPlace's law. At left is a single early morning dewdrop in an emerging dogwood blossom.

Surface tension and adhesion determine the shape of this drop on a twig. It dropped a short time later, and took a more nearly spherical shape as it fell. Falling drops take a variety of shapes due to oscillation and the effects of air friction.

A water droplet can act as lens and form an image as a simple magnifier.

The relatively high surface tension of water accounts for the ease with which it can be nebulized, or placed into aerosol form. Low surface tension liquids tend to evaporate quickly and are difficult to keep in an aerosol form. All liquids display surface tension to some degree. The surface tension of liquid lead is utilized to advantage in the manufacture of various sizes of lead shot. Molten lead is poured through a screen of the desired mesh size at the top of a tower. The surface tension pulls the lead into spherical balls, and it solidifies in that form before it reaches the bottom of the tower.

All these curves are examples of the way that large and small things are not simply comparable. Small insects can ride on the surface tension of a pond, but large ones cannot. Nobody will ever make a boat that floats by surface tension. That doesn't mean that going to sea in a sieve is impossible - you just need fine enough holes in an unwettable material.

Some insects, such as spring-tails and pond-skaters and whirligig beetles, can live on top of the surface of water. The surface is dimpled by their weight, rather like a trampoline, until it provides an upward force equal to the weight. Some insect larvae which live below the surface can push breathing apparatus through the surface, and cling there while they take in air. The breathing tubes often have feathery tips to increase the length of the perimeter without much increasing the weight. Surface tension, by definition, always produces forces in proportion to the length of any edges, and energies proportional to area.

These diagrams show what happens when long needles of different densities float on the surface of water. The last one is on the point of falling through the surface, and it is doubtful whether this condition could ever be obtained, because of the effects of imperfections or vibrations.

This picture of a pond skater is poor, because it does not show the dimples in the surface. Ideally, the photograph should be taken from a position that shows the dimples by the way they distort the reflection of skylight. Note how four legs are used for support, with the ends almost horizontal, indeed curved upward at the ends, to get a good length of contact. The two forelegs are used to sense the vibrations made by insects that fall into the water. The skater can then approach them and suck out their fluids. Click on the picture to see a bigger version.

This piece of aluminium foil is about 12 cm wide. It is floating in a tray of water. A piece like this will float even if a part of the edge is pushed under the water.

The reason that surface tension affects only small objects is that it is confined to a layer only a few molecules thick. Its energy is proportional to the surface area. But other effects are often related to the volume. Multiplying the length of a shape by ten increases the area a hundred-fold, but the volume a thousand-fold.

The spider Argyroneta aquatica makes a net under the water, under which it traps air to make a home in which it can live and breed. The net needs only to be fine enough for surface tension to stop the air from getting through any holes. The curvature of a bubble or a drop is proportional to the pressure difference between the inside or the outside. If the hole is small enough, there won't be enough pressure to make the small radius needed for a bubble to get through the hole. The pressure difference across a water-air surface is proportional to the curvature, that is, inversely proportional to the radius of curvature. So a small drop has a bigger pressure difference than a big one. The same is true of bubbles.

This is also how an umbrella or a tent works - the holes are too small to let the water through. Of course, the material must not be wetted by the water. If you have ever been lucky enough to see an Argyroneta making its bubble house then you will probably never forget it.

This is another example of nature's versatility in the use of a simple material, in this case, spider web. Not only are there innumerable types in the air and on the ground, here is one in the water.

If we had a deep container with a bottom made of umbrella material, and we started to fill it, the pressure would eventually force the water through. Why not try it with an inverted umbrella, pouring the water in slowly and carefully.

Let's see if we can get reasonable numbers for the effect of surface tension on diving birds. The diagram below shows the water pushing through between the barbules of the feathers, or between the threads of an umbrella or tent.

This idealised picture represents the point of no return, because the excess pressure across a curved surface decreases as the radius increases, which is what will happen if the water goes any further. So the calculation below is for the maximum depth of a bird in water.

The excess pressure at depth D in a liquid of density d is dDg, where g is the acceleration due to gravity. If we assume rectangular gaps of width w between the tiny parts of the feathers, then the excess pressure is 2T/w, where T is the surface tension of water. T is 0.070 N/m for pure water: we will use this in the absence of knowledge about water found in rivers, lakes and seas.

So we can set dDg = 2T/w for the maximum depth, giving the formula

Dw = 2T / dg = 2 x 0.07 / (1000 x 9.8),

Dw = 0.0000142.

If we express w in mm instead of metres we get

Dw = 0.0142.

So for a depth D of 1 metre, w = 0.0142 mm. If you look at a feather, do you think this is reasonable? You really need to look at the feather of a diving bird. Perhaps there is another possibility: the oil in the feathers might fill the gaps, and it might have a higher surface tension than that of water. Furthermore, as the bird dives down, the trapped air is compressed to a higher pressure.

If a liquid wets a surface, drops can hang, as shown below.

These drops are hanging from a spiders web. Each one probably hangs from one sticky blob on a thread. The largest drop shows an interesting divergence from sphericity. It looks almost like the shape of an inverted hot air balloon. This is not coincidence - the drop has the same surface tension everywhere, and that is the ideal condition for the balloon. The shape results from this constancy, combined with the vertical variation in pressure inside the object, caused by weight.

Here are some snail eggs. Between some of them you can see a viscous fluid that holds them together. The shapes are surfaces of minimum energy.

These pictures show lens shaped drops of organic material on the surface of water.

Glass is a sort of super-cooled liquid, with a viscosity so high at normal temperatures, that its flow is measured in terms of hundreds of years. In fact, during such a period, many specimens of glass will begin to crystallise. During the shorter times with which we are familiar, glass behaves much like a solid substance, though when it is heated, it does not melt at a specific temperature. The picture shows a piece of glass that has been hit by a small projectile. Like the liquid drop, it displays non-random behaviour - the spacing of the cracks is not far from uniform.

The next two diagrams show the distribution of distances between the little radial splashes, for the actual drop, and for a simulated one using random positions. The 37 distances were measured with a precision of one degree of arc, and then put in descending order of size. If the distances were all the same, the graph would be flat. The distribution in the second is an exponential, which is what you get with a random uniform distribution of positions, with statistical fluctuations. The distribution for an actual drop is nearly straight, and lies between a flat distribution and an exponential one.

The third graph is a linear combination of a flat graph and an exponential graph, which does not fit the actual shape very well, and is certainly not the correct way to combine them. But it does show that the actuality is far from random, as the shape is composed of 0.6 of the flat graph and 0.4 of the random one. What allows the behaviour of different parts of the drop to be correlated is the surface tension waves which travel around the surface. This picture was made using a drop of liquid from some pickled beetroot, falling on to paper. Perhaps a smooth surface like glass would have made an even more uniform splash. The picture below shows the result of smearing some cleaning fluid on the side of a bath. A similar semi-random spacing is seen.

You can see waves on the surface of the blobs of liquid that astronauts made in their space-craft for demonstration purposes. Extremely tiny versions of the same type of waves run around the surface of very heavy atomic nuclei. In the heaviest nuclei, the repulsion of the electric charges is almost enough to overcome the surface tension. A slight input of energy makes waves which can be big enough to allow the nucleus to break in two, forming two smaller ones. Even the largest nucleus has less than 300 nucleons in it, so a significant proportion of them are "in the surface" - much more so than in a water drop. Like the forces between water molecules, the nuclear force has a short range, affecting mainly nearest neighbours.

The ocean is like a large drop of water, though it is almost filled by the earth inside it. It experiences waves, caused by the wind, and sometimes by earthquakes. There is a big rotating wave caused by the moon and the sun. None of these waves are connected with surface tension.

A black hole is a sort of large drop, with a surface which is so strong that nothing can escape. But it is unrelated to surface tension.

The largest known "drop" is the universe. This universe is very peculiar, because if information really cannot travel faster than light, we have problems in explaining how regions that are in space-like connection appear to have the same physical properties in every measurable way. If we make an electron-positron pair here, it will have, as far as we know, exactly the same properties as one made a million light-years away. The same is true of the spectrum lines of hydrogen and helium, which are detectable in the light from distant stars. Indeed, the only differences are the red-shifts which first gave a clue to the expanding universe.

.Earlier, we met the idea that a small change in a variable, such as temperature, can make a big difference in some effect. Let's look into that a bit more. Here is yet another graph -

The vertical axis represents the number of molecules, electrons, or what you like: the horizontal axis represents energy. The curves are exponentials, reflecting the common property that in a statistical system, low energies are more likely than high ones. The yellow curve represents a change in some controlling variable, such as temperature, by 10 %, or a factor of 1.1. The two curves at the left look fairly similar, which is what we might expect. But if we look at the two right hand curves, which represent the original vertical values multiplied by fifty, we something very interesting. Above the red line, which stands for the threshold of some process, the yellow curve is about twice as high as the blue. This illustrates the sensitivity of the exponential distribution to changes in the parameter. Similarly, if everyone's income increases by 20%, the number of millionaires goes up very much more than that, without any change in the nature of the income distribution. Many chemical reactions speed up dramatically as temperature increases for the same reason.

In a similar way, a small increase in the mean level of a river or sea can result in a significant increase in the frequency of floods which surpass a given level. The same is true for increases in local rainfall, sometimes with distressing results. A flood every twenty years one thing: a flood every year is another.

Here is a nice example of the way that systems tend to minimise their energy. There are not enough bubbles to cover the surface, and we see that the distribution in size of the bubbles varies with the distance from the hole. There are no big bubbles near it. The effect of this is to smooth the top and bottom surface of the sheet of bubbles, minimising the areas of those interfaces which have the largest pressure differential. These are between bubble and free air, and bubble and water.

Balloons

An earlier diagram represented some drops of water hanging from a circular support. If we turn these curves upside down, and choose the right ones - what do we see? We see possible shapes for hot air balloons. Ideally, the tension in the envelope of the balloon would be the same everywhere. You will object that these shapes do not look right. That is because they were the ones for water turned upside down. But the envelope of the balloon has weight, while the surface of water does not. The curves must be done again with the weight put in.

A real hot air balloon is flatter at the top than the shape shown above, because the weight of the envelope is pressing down. As we look further down the balloon, the effect of the weight on the shape decreases, because it acts downwards, while the pressure of the gas is more horizontal. In the next diagram, an approximate correction has been included, but no payload has been added. The weight of the basket, propane, burners and people would probably change the shape further.

Hot air balloons can be made in many shapes, but all are less efficient than the one shown above. These other shapes have more surface area per unit volume; therefore the envelope is too heavy. The envelope is not funicular, and in the extreme case shown here, there are probably internal tethers to prevent the huge flat surfaces from bulging. Fun is usually less efficient than science, and so is advertising.

The behaviour of a child's rubber balloon is completely different. Whereas the envelope of a hot air balloon barely changes in size, that of the rubber balloon stretches quite easily. As it is inflated, the tension in the balloon grows in importance, relative to the pressure of the air, and the minimum energy shape tends towards a sphere. For hot air balloons of different sizes we see the opposite effect. Although, on average, the gas and the air have the same pressure, (or there would be an inflow or outflow) this is not true in detail. At the top, the domed surface shows that the gas has slightly higher pressure than the air.

High altitude balloons are a different matter entirely. The envelope is not intended to take much tension, and is not open to the atmosphere. To attain the correct tension at the design altitude, the balloon must be limp at low altitudes, because the volume of the gas is almost inversely proportional to the pressure. Because the gas is lighter than air, the minimum energy condition at the ground is with the gas pushing the envelope into a round shape at the top, with the sides more or less straight, but pleated. Although, on average, the gas and the air have the same pressure, this is not true in detail. At the top, the domed surface shows that the gas has slightly higher pressure than the air.

Alveoli of the Lungs

The oxygen exchange in the lungs takes place across the membranes of small balloon-like structures called alveoli attached to the branches of the bronchial passages. These alveoli inflate and deflate with inhalation and exhalation. The behavior of the alveoli is largely dictated by LaPlace's law and surface tension. It takes some effort to breathe in because these tiny balloons must be inflated, but the elastic recoil of the tiny balloons assists us in the process of exhalation. If the elastic recoil of the alveoli is compromised, as in the case of emphysema, then it is difficult to exhale forcibly.

Inflating the Alveoli

Inflating the alveoli in the process of respiration requires an excess pressure inside the alveoli relative to their surroundings. This is actually accomplished by making the pressure in the thoracic cavity negative with respect to atmospheric pressure.

The amount of net pressure required for inflation is dictated by the surface tension and radii of the tiny balloon-like alveoli. During inhalation the radii of the alveoli increase from about 0.05 mm to 0.1 mm . The normal mucous tissue fluid surrounding the alveoli has a nominal surface tension of about 50 dynes/cm so the required net outward pressure is:

The remarkable property of the surfactant which coats the alveoli is that it reduces the surface tension by a factor of about 15 so that the 1 mmHg pressure differential is sufficient to inflate the alveoli. Other factors affecting the remarkable efficiency of oxygen transport across the lung membranes is characterized in Fick's Law.

Surfactant Role in Respiration

One of the remarkable phenomena in the process of respiration is the role of the fluid coating the walls of the alveoli of the lungs. This fluid, called a surfactant, lowers the surface tension of the balloon-like alveoli by about a factor of 15 compared to the normal mucous tissue fluid in which they are immersed. There appears to be a nearly constant amount of this surfactant per alveolus, so that when the alveoli are deflated it is more concentrated on the surface. Since the surface-tension-lowering effect of the surfactant depends on this concentration, it diminishes the required pressure for inflation of the alveoli at their most critical phase. For a given surface tension, the pressure to inflate a smaller bubble is greater. It is the surfactant which makes possible the inflation of the alveoli with only about 1 mmHg of pressure excess over their surroundings. The baby's first breath depends upon this surfactant and is made more difficult in premature infants by the incomplete formation of the surfactant.

Alveoli and Exhalation

The alveoli of the lungs act much like balloons in that there is some effort involved to inflate them, but when the inflating pressure is released, the recoil of the elastic walls provides the pressure necessary to deflate them. The lungs are suspended in the thoracic cavity which is normally at a slight negative pressure. When the diaphragm is lowered, that pressure becomes more negative and the lungs expand into the cavity. Air from the atmosphere moves into the resulting partial vacuum and inflates the alveoli. One is aware of the effort, but it is not extreme as in the case of the baby's first breath . Once the alveoli are fully inflated, exhalation can be accomplished by merely relaxing the diaphragm, since the wall tension in all the tiny alveoli will act to force the air out of them. By forcing the diaphragm upward, we can exhale forcefully by adding the diaphragm effort to the recoil of the elastic alveoli. In diseases like emphysema, the elasticity of the alveoli is lost and exhalation becomes a laborious process. Surface Energy

Surfaces have energy associated with them because work is needed to form them.
Surface energy is the work per unit area done by the force that creates the new surface.

From the table, the surface energy is very large for Cast Iron, which is a brittle material that shatters without much warning. Since brittle fracture creates new surfaces, the surface energy varies inversely with the tendency to brittle failure.

Surface Energy and Temperature

In the bulk, atoms are evenly surrounded and the cohesive forces between the atoms tend to balance. On the surface there are atoms on one side only, so there is a net inward cohesive force. This creates a force on the surface that tries to minimise its area. When considered as a force rather than an energy, the force is called "surface tension".

As temperature increases, the atoms in a solid vibrate more, and reduce the cohesive force binding the atoms. The surface energy depends on the net inward cohesive force and so surface energy decreases with increasing temperature. The surface energy for many metals (e.g. Ag, Au, and Cu) goes down by about 0.5 mJ.m².K^-1 with increasing temperature. Water goes down by about 160 mJ.m².K^-1.

Surface Energy and Contamination

Contaminant molecules adhere to the surface ("like" cohere and "unlike" adhere).
The contaminant molecules thus change the balance of forces and reduce the net inward force. Since the net inward force is related to the surface energy, the surface energy is reduced by contaminants.

Measuring the surface energy of solids

• Fracture method:

A crack is opened up by forces pulling the edges apart.
A "double cantilever" forms. The work done by the applied force is equal to the potential energy of the "leaf springs" and the surface energy. Solving for the surface energy (eventually) gives:

Measuring Young's modulus, E, and the lengths x, y and d, will give T.

Indentation method:

With small specimens an indentation method is used. A diamond point is forced into the surface and microcracks appear at the sharp edges. It can be shown (but not in this course) that the surface energy is given by:

Measuring the lengths, a, and c, and the indenting force, F, will give the surface energy.

Surface Tension

In dealing with liquids, it is more usual to use the idea of Surface Tension rather than Surface energy, even though they refer to the same dimensional quantity. This is shown in the following dimensional analysis.

The net inward force on the surface of a liquid makes the surface act as if it was an elastic skin that constantly tries to decrease its area.

, acts in the surface and normal to an imaginary line in the surface.

Measuring Surface tension

To measure surface tension, the "wire frame" method is often used. A rectangular wire frame is suspended into a liquid and pulled upwards with force, F, to balance the downward force of surface tension, T. Make the applied upward force, F up, balance the surface tension force, from the two surfaces clinging to the top of the frame.

Surface tensions for some liquids in contact with air.

Liquid	Surface Tension	Temperature °C
Neon	5.2 mN.m^-1	-247
Oxygen	15.7 mN.m^-1	-193
Ethyl alcohol	22.3 mN.m^-1	20
Olive Oil	32.0 mN.m^-1	20
Water	58.9 mN.m^-1	100
	66.2 mN.m^-1	60
	72.8 mN.m^-1	20
	75.6 mN.m^-1	0
Mercury	465. mN.m^-1	20
Silver	800. mN.m^-1	970
Gold	1.0 N.m^-1	1070
Copper	1.1 N.m^-1	1130

Angle of contact

For a solid/liquid/gas interface, the adhesion between the liquid and the solid will curve the liquid surface to form a meniscus (Greek word for "crescent").
The angle of contact is always measured through the liquid.

The forces act along the interfaces, as shown.
F_SG is the upward force between the solid and the gas. F_SL is the downward force between the solid and the liquid. F_LG is the inclined force between the liquid and the gas.

Resolving the vertical forces, with the proviso that the force between solid and gas, F_SG is much smaller than the other two forces:

When F_SL and F_LG are in the same direction: cosα is positive i.e. α is less than 90° the meniscus is positive, and the liquid "wets" the surface.

When F_SL and F_LG are in the opposite direction:• cosα is negative i.e. α is greater than 90° the meniscus is negative, and the liquid does not "wet" the surface.

Capillary Action

As a result of surface tension acting around the inner circumference of a small-bore tube (or capillary), that is partially immersed in a liquid, there will be a raised or depressed column of liquid inside it. The case of a raised column is shown on the right.

The upward component of the surface tension force will balance the weight of the liquid column.
From this, the height of the column can be calculated.

The same maths applies if α is greater than 90° but there is a depressed column.
Pressure difference for a gas bubble in a liquid

A gas bubble in a liquid has two balancing forces that determine its size.
These are the outward force from internal gas pressure, and the inward force from surface tension trying to reduce the surface area.

Changing to energy, and using (force)×(distance) = (pressure)×(volume)

The surface energy of the gas bubble is due to the difference between the bubble filled with gas and the bubble filled with liquid.

Divide top and bottom by the radius.

How the volume and surface area change with radius is now calculated.

The final result is that the pressure difference between the inner gas and the outer liquid is directly proportional to the surface tension and inversely proportional to the radius of the bubble.

What happens as a bubble rises and the outer liquid pressure decreases?

Laplace's law (Pressure difference across a tube of liquid)

For a cylinder of radius R and length l such as a blood vessel, the wall supplies an inward force and the liquid supplies an outward pressure.

The volume and surface area of the cylinder are given by:

This gives:

There is a greater pressure difference for a smaller radius than a larger one. This inverse relationship is called Laplace's law. Note that if the outside pressure decreases, the inside pressure also decreases so the radius increases as expected.

The water molecules at the surface of water are surrounded partially by air and partially by water. These surface molecules would be much more stable if they could be in the interior of the liquid where all their hydrogen bonds could be fulfilled (cohesion). Therefore, water normally tends to have the smallest surface possible, i.e. it has a high surface tension, in order to achieve the lowest possible energetic state.

If a solid material more dense than water is placed on the surface of water, what happens next depends on the nature of the material. If the material is hydrophilic ("water loving") it has a surface to which water is attracted. The adhesion of water to the surface of this material coats the surface of the object with water, reduces the surface tension, and causes the object to sink.

If the solid object is hydrophobic ("water fearing"), the unfavorable interactions between the water surface and the object make it difficult to wet the surface. Two forces now come into play -- the energy it would take to overcome this repulsion and the force of gravity. If the force of gravity is strong enough, it will prevail and the object will sink (assuming that the object has a density greater than water). If the gravitational force is less than the surface tension then the object will float on the surface of the water.

Surface tension is what permits water striders and other insects to walk across the surface of water and what enables a needle to float. Of course, the critical feature here is the amount of force per unit area -- put a needle into water end-on instead sideways and the needle will immediately sink.

In the demo shown below, sulfur is sprinkled on the surface of water in a large beaker. The sulfur floats because the particles are very small and sulfur is a hydrophobic molecular solid.

When one drop of liquid detergent is added to the beaker without stirring, the sulfur suddenly sinks to the bottom of the beaker.

If you have Apple's (free) Quicktime 3.0 installed, you can watch a color movie of the demonstration. This movie is 1.05 Mb in size, so it may take a while to download if you have a slow Internet connection.

To view the movie, simply click on the picture below:

SURFACTANTS

Surfactants are a large group of surface active substances with a great number of (cleaning) applications. Most surfactants have degreasing or wash active abilities. They reduce the surface tension of the water so it can wet the fibres and surfaces, they loosen and encapsulate the dirt and in that way ensure that the soiling will not re-deposit on the surfaces.

Surfactants have a hydrophobic (water repellent) part and a hydrophilic (‘water loving’) part. The hydrophobic part consists of an uncharged carbohydrate group that can be straight, branched, cyclic or aromatic.

Dependent on the nature of the hydrophilic part the surfactants are classified as an-ionic, non-ionic, cat-ionic or amphoteric.

Anionic surfactants

When the hydrophilic part of the surfactant consists of a negatively charged group like a sulphonate, sulphate or carboxylate the surfactant is called anionic. Basic soaps are anionic surfactants. Over the last 50 years many soaps have been replaced with more efficient substances like alkyl sulphates, alkyl sulphonates and alkyl benzene sulphonates.

Anionic surfactants are sensitive to water hardness.

Nonionic surfactants

A surfactant with a non-charged hydrophilic part, e.g. ethoxylate, is non-ionic. These substances are well suited for cleaning purposes and are not sensitive to water hardness.

They have a wide application within cleaning detergents and include groups like fatty alcohol polyglycosides, alcohol ethoxylates etc.

Cationic surfactants

For this category the hydrophilic part is positively charged – e.g. with a quaternary ammonium ion. This group has no wash activity effect, but fastens to the surfaces where they might provide softening, antistatic, soil repellent, anti bacterial or corrosion inhibitory effects.

The most typical applications are for softeners and antistatics.

The cationic surfactant called DADMAC was formerly used, but now almost substituted.

Please consult section on ‘fabric softeners’ for further information.

Amphoteric surfactants

For the amphoteric surfactants the charge of the hydrophilic part is controlled by the pH of the solution. This means that they can act as anionic surfactant in an alkalic solution or as cationic surfactant in an acidic solution.

Environmental properties

Most surfactants are more or less toxic to aquatic organisms due to their surface activity which will react with the biological membranes of the organisms.

The biological degradability varies according to the nature of the carbohydrate chain. Generally the linear chains are more readily degradable than branched chains.

Also the toxic effects vary with the chain structure. Generally an increase of the chain length in the range of 10 to 16, leads to an increase in toxicity to aquatic organisms.

The properties of surfactants most often used in laundry detergents are given below.

Specific chemical groups

Alkane sulfonates (anionic),

linear alcohol ethoxylates (non-ionic) and

branched alcohol ethoxylates (non-ionic)

Most of these surfactants are readily degradable with varying eco-toxicity towards aquatic organisms.

Linear alkyl benzene sulphonates - LAS (anionic)

Probably the most frequently used group of surfactants for cleaning and laundering.

Linear alkyl benzene sulphonates (LAS) have been under some debate over the recent years due to the fact that they do not biodegrade under anaerobic conditions. Under aerobic conditions LAS are readily biodegradable.

Eco-toxicity towards aquatic organisms is fairly low.

Alkyl phenol ethoxylates, APEO (non-ionic)

Formerly this group was widely used for cleaning and laundering. Now it has been replaced to a great extent due to the negative environmental effects.

During the biological degradation, alkyl phenol ethoxylates bare transformed to alkyl phenols, e.g. nonyl phenol ethoxylate (NPEO) degrades to nonyl phenol (NP), which is known to be toxic and have hormone like effects.

Pulmonary surfactant is a surface-active lipoprotein complex (phospholipoprotein) formed by type II alveolar cells. The proteins and lipids that comprise the surfactant have both a hydrophilic region and a hydrophobic region. By adsorbing to the air-water interface of alveoli with the hydrophilic head groups in the water and the hydrophobic tails facing towards the air, the main lipid component of surfactant, dipalmitoylphosphatidylcholine (DPPC), reduces surface tension.

Detergents are a class of chemicals that contain hydrophobic (non-polar) hydrocarbon "tails" and a hydrophilic (polar) "head" group. This general class of molecules are called surfactants. Surfactants can interact with water in a variety of ways, each of which disrupts or modifies the hydrogen bonding network of water. Since this reduces the cohesive forces in water, this leads to reduction in the surface tension and our sulfur sinks.

A typical example of a detergent molecule is sodium lauryl sulfate (read that shampoo bottle of yours!). The structure can be represented in several different ways. Notice that in the models the Na ion has been left off because the anion and cation completely dissociate in water:

If you have the MDL Chime plug-in installed, you can play with this interactive 3-D model of a sodium lauryl sulfate molecule. You can rotate it, change the display features, enlarge/shrink, display solvent accessible surfaces and more...click and play:

When a detergent is placed in water, the long non-polar hydrocarbon tails tend to aggregate because of favorable intermolecular interactions ("like dissolves like" in the interior and ion-dipole interactions at the exterior). The surfactant molecules thereby organize themselves into 3-dimensional spheres called micelles which have a hydrocarbon core and sulfate groups around the outer surface. Here's a 2-D representation:

Without detergent, we can not remove a greasy oily stain from clothing because grease and oil are large, non-polar, hydrophobic molecules. However, the interior core of a micelle is quite greasy just like an oily stain. When we add detergent to our wash water, the oil or grease on our clothes can dissolve in the interior of the micelles and thereby go into solution.

Surfactants can also form other structures. Rather than form a sphere, some surfactants can coat the surface of the water to form a layer one molecule thick, a molecular monolayer. This is shown diagrammatically below:

A good example of a monolayer is oil on water. A small amount of oil can be spread over a large surface of water when the oil is only one monolayer thick! A variety of related structures are also known, particularly in cell walls (lipid bilayers etc.).

There are many, many other Real World examples and applications of surfactants! Here's just one: your body uses surfactants to reduce surface tension in the lungs. The human body does not start to produce lung surfactants until late in fetal development. Therefore, premature babies are often unable to breathe properly, a condition called Respiratory Distress Syndrome. Untreated, this is a serious illness and is often fatal, but administration of artificial surfactants virtually eliminates this health problem.

Surfactants are compounds that lower the surface tension of a liquid, the interfacial tension between two liquids, or that between a liquid and a solid. Surfactants may act as detergents, wetting agents, emulsifiers, foaming agents, and dispersants.

Etymology and definition

The term surfactant/surfactants is a blend of surface active agents.

In Index Medicus and the United States National Library of Medicine, surfactant/surfactants is reserved for the meaning pulmonary surfactant. For the more general meaning, surface active agent/s is the heading.

A micelle-the lipophilic tails of the surfactant ions remain on the inside of the micelle due to unfavourable interactions. The polar "heads" of the micelle, due to favourable interactions with water, form a hydrophilic outer layer that in effect protects the hydrophobic core of the micelle. The compounds that make up a micelle are typically amphiphilic in nature, meaning that micelles are soluble not only in protic solvents such as water but also in aprotic solvents as a reverse micelle.

Composition and structure

Surfactants are usually organic compounds that are amphiphilic, meaning they contain both hydrophobic groups (their tails) and hydrophilic groups (their heads). Therefore, a surfactant contains both a water insoluble (or oil soluble) component and a water soluble component. Surfactants will diffuse in water and adsorb at interfaces between air and water or at the interface between oil and water, in the case where water is mixed with oil. The insoluble hydrophobic group may extend out of the bulk water phase, into the air or into the oil phase, while the water soluble head group remains in the water phase. This alignment of surfactants at the surface modifies the surface properties of water at the water/air or water/oil interface.

World production of surfactants is estimated at 15 Mton/y, of which about half are soaps. Other surfactants produced on a particularly large scale are linear alkylbenzenesulfonates (1700 kton/y), lignin sulfonates (600 kton/y), fatty alcohol ethoxylates (700 ktons/y), alkylphenol ethoxylates (500 kton/y).

Sodium stearate, the most common component of most soap, which comprise about 50% of commercial surfactants.

4-(5-Dodecyl) benzenesulfonate, a linear dodecylbenzenesulfonate, one of the most common surfactants.

Structure of surfactant phases in water

In the bulk aqueous phase, surfactants form aggregates, such as micelles, where the hydrophobic tails form the core of the aggregate and the hydrophilic heads are in contact with the surrounding liquid. Other types of aggregates such as spherical or cylindrical micelles or bilayers can be formed. The shape of the aggregates depends on the chemical structure of the surfactants, depending on the balance of the sizes of the hydrophobic tail and hydrophilic head. This is known as the HLB, Hydrophilic-lipophilic balance. Surfactants reduce the surface tension of water by adsorbing at the liquid-gas interface. The relation that links the surface tension and the surface excess is known as the Gibbs isotherm.

Dynamics of surfactants at interfaces

The dynamics of adsorption of surfactants is of great importance for practical applications such as foaming, emulsifying or coating processes, where bubbles or drops are rapidly generated and need to be stabilized. The dynamics of adsorption depends on the diffusion coefficient of the surfactants. Indeed, as the interface is created, the adsorption is limited by the diffusion of the surfactants to the interface. In some cases, there exists a barrier of energy for the adsorption or the desorption of the surfactants, then the adsorption dynamics is known as ‘kinetically limited'. Such energy barrier can be due to steric or electrostatic repulsions. The surface rheology of surfactant layers, including the elasticity and viscosity of the surfactant layers plays a very important role in foam or emulsion stability.

Characterization of interfaces and surfactant layers

Interfacial and surface tension can be characterized by classical methods such as the -pendant or spinning drop method Dynamic surface tensions, i.e. surface tension as a function of time, can be obtained by the Maximum Bubble Pressure apparatus

The structure of surfactant layers can be studied by ellipsometry or X-Ray reflectivity.

Surface rheology can be characterized by the oscillating drop method or shear surface rheometers such as double-cone, double-ring or magnetic rod shear surface rheometer.

Detergents in biochemistry and biotechnology

In solution, detergents help solubilize a variety of chemical species by dissociating aggregates and unfolding proteins. Popular surfactants in the biochemistry laboratory are SDS and CTAB. Detergents are key reagents to extract protein by lysis of the cells and tissues: They disorganize the membrane's lipidic bilayer (SDS, Triton X-100, X-114, CHAPS, DOC, and NP-40), and solubilize proteins. Milder detergents such as (OctylThioGlucosides) are used to solubilize sensible proteins (enzymes, receptors). Non-solubilized material is harvested by centrifugation or other means. For electrophoresis, for example, proteins are classically treated with SDS to denature the native tertiary and quaternary structures, allowing the separation of proteins according to their molecular weight.

Detergents have also been used to decellularise organs. This process maintains a matrix of proteins that preserves the structure of the organ and often the microvascular network. The process has been successfully used to prepare organs such as the liver and heart for transplant in rats.^[4] Pulmonary surfactants are also naturally secreted by type II cells of the lung alveoli in mammals.

Classification of surfactants

The "tail" of most surfactants are fairly similar, consisting of a hydrocarbon chain, which can be branch, linear, or aromatic. Fluorosurfactants have fluorocarbon chains. Siloxane surfactants have siloxane chains.

Many important surfactants include a polyether chain terminating in a highly polar anionic group. The polyether groups often comprise ethoxylated (polyethylene oxide-like) sequences inserted to increase the hydrophilic character of a surfactant. Polypropylene oxides conversely, may be inserted to increase the lipophilic character of a surfactant.

Surfactant molecules have either one tail or two; those with two tails are said to be double-chained.

Surfactant classification according to the composition of their head: nonionic, anionic, cationic, amphoteric.

Most commonly, surfactants are classified according to polar head group. A non-ionic surfactant has no charge groups in its head. The head of an ionic surfactant carries a net charge. If the charge is negative, the surfactant is more specifically called anionic; if the charge is positive, it is called cationic. If a surfactant contains a head with two oppositely charged groups, it is termed zwitterionic. Commonly encountered surfactants of each type include:

Anionic

Sulfate, sulfonate, and phosphate esters

Anionic surfactants contain anionic functional groups at their head, such as sulfate, sulfonate, phosphate, and carboxylates. Prominent alkyl sulfates include ammonium lauryl sulfate, sodium lauryl sulfate (SDS, sodium dodecyl sulfate, another name for the compound) and the related alkyl-ether sulfates sodium laureth sulfate, also known as sodium lauryl ether sulfate (SLES), and sodium myreth sulfate.

Docusates: dioctyl sodium sulfosuccinate, perfluorooctanesulfonate (PFOS), perfluorobutanesulfonate, linear alkylbenzene sulfonates (LABs).

These include alkyl-aryl ether phosphates and the alkyl ether phosphate

Carboxylates

These are the most common surfactants and comprise the alkyl carboxylates (soaps), such as sodium stearate. More specialized species include sodium lauroyl sarcosinate and carboxylate-based fluorosurfactants such as perfluorononanoate, perfluorooctanoate (PFOA or PFO).

Cationic head groups

pH-dependent primary, secondary, or tertiary amines: Primary and secondary amines become positively charged at pH < 10: Octenidine dihydrochloride;

Permanently charged quaternary ammonium cation:

Alkyltrimethylammonium salts: cetyl trimethylammonium bromide (CTAB) a.k.a. hexadecyl trimethyl ammonium bromide, cetyl trimethylammonium chloride (CTAC

Cetylpyridinium chloride (CPC)

Benzalkonium chloride (BAC)

Benzethonium chloride (BZT)

5-Bromo-5-nitro-1,3-dioxane

Dimethyldioctadecylammonium chloride

Cetrimonium bromide

Dioctadecyldimethylammonium bromide (DODAB)

Zwitterionic surfactants

Zwitterionic (amphoteric) surfactants have both cationic and anionic centers attached to the same molecule. The cationic part is based on primary, secondary, or tertiary amines or quaternary ammonium cations. The anionic part can be more variable and include sulfonates, as in CHAPS (3-[(3-Cholamidopropyl)dimethylammonio]-1-propanesulfonate). Other anionic groups are sultaines illustrated by cocamidopropyl hydroxysultaine. Betaines, e.g., cocamidopropyl betaine. Phosphates: lecithin

Nonionic surfactant

Many long chain alcohols exhibit some surfactant properties. Prominent among these are the fatty alcohols cetyl alcohol, stearyl alcohol, and cetostearyl alcohol (consisting predominantly of cetyl and stearyl alcohols), and oleyl alcohol.

Polyoxyethylene glycol alkyl ethers (Brij): CH₃–(CH₂)_10–16–(O-C₂H₄)_1–25–OH:

Octaethylene glycol monododecyl ether

Pentaethylene glycol monododecyl ether

Polyoxypropylene glycol alkyl ethers: CH₃–(CH₂)_10–16–(O-C₃H₆)_1–25–O

Glucoside alkyl ethers: CH₃–(CH₂)_10–16–(O-Glucoside)_1–3–OH:

Decyl glucoside,

Lauryl glucoside

Octyl glucoside

Polyoxyethylene glycol octylphenol ethers: C₈H₁₇–(C₆H₄)–(O-C₂H₄)_1–25–OH:

Triton X-100

Polyoxyethylene glycol alkylphenol ethers: C₉H₁₉–(C₆H₄)–(O-C₂H₄)_1–25–OH:

Nonoxynol-9

Glycerol alkyl esters:

Glyceryl laurate

Polyoxyethylene glycol sorbitan alkyl esters: Polysorbate

Sorbitan alkyl esters: Spans

Cocamide MEA, cocamide DEA

Dodecyldimethylamine oxide

Block copolymers of polyethylene glycol and polypropylene glycol: Poloxamers

Polyethoxylated tallow amine (POEA).

Surface Tension

Surface Tension of Water: The surface tension of water is 72 dynes/cm at 25°C . It would take a force of 72 dynes to break a surface film of water 1 cm long. The surface tension of water decreases significantly with temperature as shown in the graph. The surface tension arises from the polar nature of the water molecule.

Hot water is a better cleaning agent because the lower surface tension makes it a better "wetting agent" to get into pores and fissures rather than bridging them with surface tension. Soaps and detergents further lower the surface tension.

The cohesive forces between liquid molecules are responsible for the phenomenon known as surface tension. The molecules at the surface do not have other like molecules on all sides of them and consequently they cohere more strongly to those directly associated with them on the surface. This forms a surface "film" which makes it more difficult to move an object through the surface than to move it when it is completely submersed.

Surface tension is typically measured in dynes/cm, the force in dynes required to break a film of length 1 cm. Equivalently, it can be stated as surface energy in ergs per square centimeter. Water at 20°C (Decrease in water surface tension with heating) has a surface tension of 72.8 dynes/cm compared to 22.3 for ethyl alcohol and 465 for mercury.

Cohesion and Surface Tension:The cohesive forces between molecules down into a liquid are shared with all neighboring atoms. Those on the surface have no neighboring atoms above, and exhibit stronger attractive forces upon their nearest neighbors on the surface. This enhancement of the intermolecular attractive forces at the surface is called surface tension.

Cohesion and Adhesion: Molecules liquid state experience strong intermolecular attractive forces. When those forces are between like molecules, they are referred to as cohesive forces. For example, the molecules of a water droplet are held together by cohesive forces, and the especially strong cohesive forces at the surface constitute surface tension.

When the attractive forces are between unlike molecules, they are said to be adhesive forces. The adhesive forces between water molecules and the walls of a glass tube are stronger than the cohesive forces lead to an upward turning meniscus at the walls of the vessel and contribute to capillary action.

The attractive forces between molecules in a liquid can be viewed as residual electrostatic forces and are sometimes called van der Waals forces or van der Waals bonds.

Surface Tension Examples

Walking on water: Small insects such as the water strider can walk on water because their weight is not enough to penetrate the surface.

Floating a needle: If carefully placed on the surface, a small needle can be made to float on the surface of water even though it is several times as dense as water. If the surface is agitated to break up the surface tension, then needle will quickly sink.

Common tent materials are somewhat rainproof in that the surface tension of water will bridge the pores in the finely woven material. But if you touch the tent material with your finger, you break the surface tension and the rain will drip through.

Soaps and detergents: help the cleaning of clothes by lowering the surface tension of the water so that it more readily soaks into pores and soiled areas.

Clinical test for jaundice:Normal urine has a surface tension of about 66 dynes/cm but if bile is present (a test for jaundice), it drops to about 55. In the Hay test, powdered sulfur is sprinkled on the urine surface. It will float on normal urine, but sink if the S.T. is lowered by the bile.

Washing with cold water: The major reason for using hot water for washing is that its surface tension is lower and it is a better wetting agent. But if the detergent lowers the surface tension, the heating may be unneccessary.

Surface tension disinfectants: Disinfectants are usually solutions of low surface tension. This allow them to spread out on the cell walls of bacteria and disrupt them. One such disinfectant, S.T.37, has a name which points to its low surface tension compared to the 72 dynes/cm for water.

Surface Tension and Bubbles: The surface tension of water provides the necessary wall tension for the formation of bubbles with water. The tendency to minimize that wall tension pulls the bubbles into spherical shapes (LaPlace's law).

The pressure difference between the inside and outside of a bubble depends upon the surface tension and the radius of the bubble. The relationship can be obtained by visualizing the bubble as two hemispheres and noting that the internal pressure which tends to push the hemispheres apart is counteracted by the surface tension acting around the cirumference of the circle.

For a bubble with two surfaces providing tension tension, the pressure relationship is:

Sessile Drop Method

Optical contact angle measurement to determine the wetting behaviour of solids

Task:Determination of the static and dynamic contact angle and of the surface free energy of solids

Test results: interface-specific parameters and measuring ranges of typical instrument systems

· Measurement of the static contact angle of sessile drops of liquid on a surface as a function of time or temperature

· Measurement of the dynamic contact angle as a function of the dosing rate , as advancing angle or as receding angle

· Measurement of the difference between advancing angle and receding angle (contact angle hysteresis) by metered addition or removal of liquid

· Measurement of the contact angle or until the rolling off of the drop on a plate inclined with the angle (Tilting Plate method)

· Calculation of the critical surface tension and of the surface free energy : determination of the dispersion as well as the non-dispersion parts (e.g. polar parts , acid/base parts , hydrogen bonding parts ) from contact angle measurements with various test liquids

· Typical measuring ranges : 0 ... 180°/0,1 mN/m ... 1000 mN/m

Pendant Drop Method

Task: Determination of the interface and surface tension of liquids

Test results: interface-specific parameters and measuring ranges of typical instrument systems

· Measurement of the static interfacial or surface tension as a function of time or of temperature

· Measurement of the adsorption/diffusion coefficients of surfactant molecules in vibrating/relaxing drops

· Typical measuring range : : 0,05 ... 1000 mN/m

ADSORPTION

General. The situation existing at the surface of а liquid or а solid is different from that in the interior. For example, а molecule in the interior of а liquid is completely surrounded by other molecules on all sides and hence the intermolecular forces of attraction are exerted equally in all directions. However, а molecule at the surface of а liquid is surrounded by larger number of molecules in the liquid phase and fewer molecules in the vapour phase i.е. in the space above the liquid surface. As а result, these molecules lying at the surface, experience some net inward force of attraction which causes surface tension. Similar inward forces of attraction exist at the surface of а solid. Alternatively, in case of certain solids such as transition metals (like Ni) there are unutilized free valencies at the surface.

Because of the unbalanced inward forces of attraction or free valencies at the surface, liquids and solids have the property to attract and retain the molecules of а gas or а dissolved substance onto their surfaces with which they come in contact.

The phenomenon of attracting and retaining the molecules of а substance on the surface of а liquid or а solid resulting into a higher concentration of the molecules on the surface is called adsorption. The substance thus adsorbed on the surface is called the adsorbate and the substance on which it is adsorbed is called adsorbent. The reverse process e. removal of the adsorbed substance from the surface is called desorption. The adsorption of gases on the surface of metals is called occlusion.

Difference between adsorption and absorption. The term adsorption differs from the term absorption in the fact that whereas the former refers to the attraction and retention of the molecules of а substance on the surface only, the latter involves passing of the substance through the surface into the bulk of the liquid or the solid. Where there is а doubt whether the process is true adsorption or absorption (i.е. both adsorption and absorption take place) the term sorption is simply used.

Thus in adsorption whereas the concentration is different at the surface than in the bulk, in absorption, the concentration is same throughout. Moreover whereas adsorption is fast in the beginning and then the rate decreases till equilibrium is attained, absorption takes place at uniform speed. Thus the main points of difference between adsorption and absorption may be summed up as follows:

Adsorption:

1. It is а surface phenomenon i.е. it occurs only at the surface of the adsorbent.

2. In this phenomenon, the concentration on the surface of adsorbent is different from that in the bulk.

3. Its rate is high in the beginning and then decreases till equilibrium is attained.

Absorption:

1. It is а bulk phenomenon i.e. occurs throughout the body of the material.

2. In this phenomenon, the concentration is same throughout the material.

3. Its rate remains same throughout the process.

Examples of adsorption, absorption and sorption.

(i) If silica gel is placed in а vessel containing water vapours, the latter are adsorbed on the former. On the other hand, if anhydrous CaCl₂ is kept in place of silica gel, absorption takes place as the water vapours are uniformly distributed in CaCl₂ to form hydrated calcium chloride (CaCO₃ ^. 2H₂O).

(ii) Ammonia gas placed in contact with charcoal gets adsorbed on the charcoal whereas ammonia gas placed in contact with water gets absorbed into water, giving NH₄OH solution of uniform concentration.

(iii) Dyes get adsorbed as well as absorbed in the cotton fibres i.е. sorption takes place.

Positive and Negative Adsorption. In case of adsorption by solids from the solutions, mostly the solute is adsorbed on the surface of the solid adsorbent so that the concentration of solute on the surface of the adsorbent is greater than in the bulk. This is called positive adsorption. However in some cases, the solvent from the solution may be adsorbed by the adsorbent so that the concentration of the solution increases than the initial concentration. This is called negative adsorption. For example, when а concentrated solution of KCI is shaken with blood charcoal, it shows positive adsorption but with а dilute solution of КС1, it shows negative adsorption. To sum up:

When the concentration of the adsorbate is more on the surface of the adsorbent than in the bulk. it is called positive adsorption. If the concentration of the adsorbate is less relative to its concentration in the bulk, it is called negative adsorption.

Factors affecting adsorption of gases by solids. Almost all solids adsorb gases to some extent. However, the exact amount of а gas adsorbed depends upon а number of factors, as briefly explained below:

(i) Nature and Surface area of the adsorbent. If is observed that the same gas is adsorbed to different extents by different solids at the same temperature. Further, as may be expected, the greater the surface area of the adsorbent, greater is the volume of the gas adsorbed. It is for this reason that substances like charcoal and silica gel are excellent adsorbents because they have highly porous structures and hence large surface areas.

For the same reason, finely divided substances have larger adsorption power than when they are present in the compact form.

Since the surface area of adsorbents cannot always be determined readily, the common practice is to express the gas adsorbed per gram of the adsorbent (The surface area per gram of the adsorbent is called specific area).

(ii) Nature of the gas being adsorbed. Different gases are adsorbed to different extents by the same adsorbent at the same temperature.

(iii) Temperature. Studying the adsorption of any particular gas by some particular adsorbent. It is observed that the adsorption decreases with increase of temperature and vice versa. For example, one gram of charcoal adsorbs about 10 cm³ of N₂ at 273 K, 20 cm³ at 244 K and 45 cm³ at 195 K. The decrease of adsorption with increase of temperature may be explained as follows:

Like any other equilibrium, adsorption is а process involving а true equilibrium. The two opposing processes involved are condensation (i.е. adsorption) of the gas molecules on the surface of the solid and evaporation (i.е. desorption) of the gas molecules from the surface of the solid into the gaseous phase. Moreover, the process of condensation (or adsorption) is exothermic so that the equilibrium may be represented as:

Applying be Chatelier’s principle, it can be seen that increase of temperature decreases the adsorption and vice versa.

The amount of heat evolved when one mole of the gas is adsorbed on the adsorbent is called the heat of adsorption.

(iv) Pressure. At constant temperature, the adsorption of а gas increases with increase of pressure. It is observed that at low temperature, the adsorption of а gas increases very rapidly as the pressure is increased from small values.

(v) Activation of the solid Adsorbent. It constant temperature, the adsorbing power of an adsorbent. This is usually done by increasing the surface area (or the specific area) of the adsorbent which can be achieved in any of the following ways:

(а) By making the surface of the adsorbent rough e.g. by mechanical rubbing or by chemical action or by depositing finely dispersed metals on the surface of the adsorbent by electroplating.

(b) By subdividing the adsorbent into smaller pieces or drains. No doubt this method increases the surface area but it has а practical limitation, that is, if the adsorbent is broken into too fine particles that it becomes almost powder, then the penetration of the gas becomes difficult and this will obstruct adsorption.

(с) By removing the gases already adsorbed e.g. charcoal is activated by heating in superheated steam or in vacuum at а temperature between 623 to 1273 К.

Types of adsorption. An experimental study of the adsorption of various types on solids shows that there are two main types of adsorption. These are briefly explained below:

(i) Physical adsorption or van der Waal's adsorption or physicosorption. When а gas is held (adsorbed) on the surface of а solid by van-der-Waal’s forces (which are weak intermolecular forces of attraction) without resulting into the formation of any chemical bond between the adsorbate and the adsorbent, it is called “physical adsorption” or “van-der-Waal’s adsorption” or “physicosorption”. This type of adsorption is characterized by low heats of adsorption i.e. about 40 kJ per mole. Further, physical adsorption of а gas by а solid is generally reversible. Increase of pressure causes more gas to be adsorbed and the release of pressure frees the adsorbed gas. Similarly, decrease of temperature increases adsorption but the gas adsorbed at low temperature can be freed again by heating.

(ii) Chemical adsorption or Chemisorption or Langmuir adsorption. When а gas is held on to the surface of а solid by forces similar to those of а chemical bond, the type of adsorption is called chemical adsorption or chemisorption. This type of adsorption results into the formation of what is called а “surface compound”. That the forces involved are similar to those of chemical bond is confirmed by the fact that the heats evolved during chemisorption are high (i.е. about 400 kJ/mole) which are of the same magnitude as those involved in chemical reactions. Further, as chemisorption is something similar to а chemical change, it is usually irreversible. The efforts to free the adsorbed gas often gives some definite compound instead of the free gas. For example, oxygen adsorbed on tungsten or carbon is liberated as tungsten oxide or as carbon monoxide and carbon dioxide.

Another aspect in which chemisorption differs from physical adsorption is the fact that whereas physical adsorption takes place between every gas and а solid i.е. is not specific in nature (because it involves van der Waal's forces which exist among the molecules of every two substances), the chemisorption is specific in nature and occurs only where there is а tendency towards compound formation between the gas and the adsorbent. Further unlike physical adsorption, the chemisorption like the most of chemical changes, increases with increase of temperature. For this reason, а gas may be physically adsorbed at low temperature but chemisorbed at higher temperature. For example, it happens in case of adsorption of hydrogen on nickel. When chemisorption takes place by raising the temperature i.е. by supplying activation energy, the process is called “activated adsorption”.

Physical adsorption:

1. The forces operating in these cases are weak van-der-Waal’s forces.

2. The heats of adsorption are low viz. about 20 – 40 kJ/mol

3. No compound formation takes place in these cases.

4. The process is reversible i.е. desorption of the gas occurs by increasing the temperature or decreasing the pressure.

5. It does not require any а activation energy.

б. This type of adsorption decreases with increase of temperature.

7. It is not specific in nature i.е. all gases are adsorbed on all solids to some extent.

8. The amount of the gas adsorbed is related to the ease of liquefaction of the gas.

9. It forms multimolecular layer.

Chemisorption:

1. The forces operating in these cases are similar to those of а chemical bond.

2. The heats of adsorption are high viz. about 400-400 kJ/mol

3. Surface compounds are formed.

4. The process is irreversible. Efforts to free the adsorbed gas give some definite compound.

5. It requires activation energy.

6. This type of adsorption first increases with increase of temperature. The effect is called activated adsorption.

7. It is specific in nature and occurs only when there is some possibility of compound formation between the gas being adsorbed and the solid adsorbent.

8. There is no such correlation.

9. It forms unimolecular layer.

Adsorption of gases-Freundlich’s. Adsorption isotherm. The extent of adsorption on а given surface generally increases with increase in pressure (for gases) and concentration (for solution) at constant temperature. At low temperatures, the adsorption of а gas increases very rapidly as the pressure rises. When the temperature is high, the increase in adsorption is relatively less.

To understand the effect of pressure on adsorption, we should consider adsorption as an equilibrium process. When the adsorbent and the adsorbate are enclosed in а closed vessel, the amount of gas adsorbed equals the amount desorbed when the equilibrium stage is attained. Therefore, after an initial decrease in the pressure of the gas, gas pressure as well as the amount of gas adsorbed reach constant or equilibrium values.

The amount of gas adsorbed depends on the surface area of the adsorbent or on its mass if the adsorbent is taken in the form of powder.

The extent of adsorption is usually expressed as x/m, where m is the mass of the adsorbent and x is the mass of the adsorbate when adsorption equilibrium is reached.

The specific surface area of а solid (in the form of а powder or porous mass) is the surface area in square meters per gram of the adsorbent. Highly active solids with large surface area (several hundred square meters per gram) are used as adsorbents.

А graph between the amount (х/m) adsorbed by an adsorbent and the equilibrium pressure (or concentration for solutions) of the adsorbate at а constant temperature is called the adsorption isotherm.

The simplest type of adsorption isotherm is shown in Fig. At а value of р_s of equilibrium pressure, x/m reaches its maximum value and then it remains constant even though the pressure p is increased. This is the saturation state and р_s is the saturation pressure. This type of adsorption isotherm is observed when the adsorbate forms а uniform molecular layer of it on the surface of the adsorbent.

Fig. Variation of x/m with increase in pressure at constant temperature (General adsorption isotherm)

А relationship between the amount adsorbed (х/m) and the equilibrium pressure (р) can be obtained as follows:

At low values of р, the graph is nearly straight and sloping. This is represented by the following equation:

x/m µ p¹ or x/m = constant x p¹

At high pressure х/m becomes independent of the values of p. In this range of pressure

x/m µ p⁰ or x/m = constant x p⁰

In the intermediate range of pressure, х/m will depend on p raised to powers between 1 and 0 i.е. fractions. For а small range of pressure values, we can write:

x/m µ p^1/n or x/m = Kp^1/n

where n is а positive integer and n and К are constants depending upon the nature of the adsorbate and adsorbent.

This relationship was originally put forward by Freundlich and is known as Freundlich adsorption isotherm.

To test the validity of this equation, taking logarithms of both sides, we get

log x/m = log K + 1/n log p

А graph between log x/m against log p should, therefore, give а straight 1inе with slope equal to 1/n and ordinate intercept equal to log К. The experimental values, when plotted, however, show some deviation from linearity, specially at high pressures. The relation is hence considered as an approximate one and is suitable at low pressures.

Fig. Freundlich adsorption isotherm. Linear graph between log x/m and log p.

Adsorption from solutions. Solid surfaces can also adsorb solutes from the solutions. An application of adsorption from solution is the use of activated charcoal for decolorising sugar solutions. Activated charcoal can adsorb colouring impurities from the solutions of organic compounds. Adsorption from solution can also involve colourless solutions. Adsorption of ammonia from ammonium hydroxide solution and acetic acid from its solution in water by activated charcoal are such examples.

This type of adsorption is also affected by temperature and concentration. The extent of adsorption decreases with increase in temperature and increases with increase in concentration. The isotherm for the adsorption of solutes from solutions (by the solid adsorbents) is found to be similar to that shown in Fig. 2. Hence the relationship between x/m (mass of the solute adsorbed per gram of the adsorbent) and the equilibrium concentration, С of the solute in the solution is also similar i.e:

X/m =KC^1/n

Taking logarithms of both sides of the equation, we get:

log x/m = log К + 1/n log C

This equation implies that а plot of log x/m against log С should be а straight line with slope1/n and intercept log Х. This is found to be so over small ranges of concentration.

The equation for adsorption from solutions is found to give better results than for adsorption of gases by solids.

Adsorption isobars. As already discussed, adsorption is а case of dynamic equilibrium in which forward process (adsorption) is exothermic while backward process (desorption) is endothermic. Thus applying be Chatelier's principle, increase of temperature will favour the backward process i.е., adsorption decreases.

А graph drawn between the amount adsorbed (x/m) and temperature 't' at а constant equilibrium pressure of adsorbate gas is known as adsorption isobar.

Adsorption isobars of physical adsorption and chemical adsorption show important difference [Fig.3 (а) and (b)] and this difference is helpful in distinguishing these two types of adsorption. The physical adsorption isobar shows а с1есгеаье in х/m throughout with rise in temperature, the chemisorption isobar shows an initial increase with temperature and then the expected decrease. The initial increase is because of the fact that the heat supplied acts as activation energy required in chemisorption (like chemical reactions).

b

Fig.3. (а) Physical adsorption isobar. (b) Chemisorption isobar.

Application of adsorption. Adsorption finds extensive applications both in research laboratory and in industry. А few applications are briefly described below:

In preserving vacuum. In Dewar flasks activated charcoal is placed between the walls of the flask so that any gas which enters into the annular space either due to glass imperfection or diffusion through glass is adsorbed.

In gas masks. All gas masks are devices containing suitable adsorbent so that the poisonous gases present in the atmosphere are preferentially adsorbed and the air for breathing is purified.

In clarification of sugar. Sugar is decolorised by treating sugar solution with charcoal powder. The latter adsorbs the undesirable colours present.

In chromatographic analysis. The selective adsorption of certain substances from а solution by а particular solid adsorbent has helped to develop technique for the separation of the components of the mixture. This technique is called chromatographic analysis. For example, in column chromatography, а long and wide vertical tube is filled with а suitable adsorbent and the solution of the mixture poured from the top and then collected one by one from the bottom.

In catalysis. The action of certain solids as catalysts is best explained in terms of adsorption. The theory is called adsorption theory. According to this theory, the gaseous reactants are adsorbed on the surface of the solid catalyst. As а result, the concentration of the reactants increases on the surface and hence the rate of reaction increases. The theory is also able to explain the greater efficiency of а catalyst in the finely divided state, the action of catalytic promoters and poisons.

In paint industry. The paint should not contain dissolved gases as otherwise the paint does not adhere well to the surface to be painted and thus will have а poor covering power. The dissolved gases are, therefore, removed by suitable adsorbents during manufacture. Further, all surfaces are covered with layers of gaseous, liquid or solid films. These have to be removed before the paint is applied. This is done by suitable liquids which adsorb these films. Such liquids are called wetting agents. The use of spirit as wetting agent in furniture painting is well known.

In adsorption indicators. Various dyes, which owe their use to adsorption, have been introduced as indicators particularly in precipitation titrations. For example, KBr is easily titrated with AgNO₃ using eosin as an indicator.

In softening of hard water. The use of ion exchangers for softening of hard water is based upon the principle of competing adsorption just as in chromatography.

In removing moisture from air in the storage of delicate instruments. Such instruments, which may be harmed by contact with the moist air, are kept out of contact with moisture using silica gel.

Adsorption is the adhesion of atoms, ions, or molecules from a gas, liquid, or dissolved solid to a surface. This process creates a film of the adsorbate on the surface of the adsorbent. This process differs from absorption, in which a fluid (the absorbate) permeates or is dissolved by a liquid or solid (the absorbent). Note that adsorption is a surface-based process while absorption involves the whole volume of the material. The term sorption encompasses both processes, while desorption is the reverse of adsorption. It is a surface phenomenon.

Similar to surface tension, adsorption is a consequence of surface energy. In a bulk material, all the bonding requirements (be they ionic, covalent, or metallic) of the constituent atoms of the material are filled by other atoms in the material. However, atoms on the surface of the adsorbent are not wholly surrounded by other adsorbent atoms and therefore can attract adsorbates. The exact nature of the bonding depends on the details of the species involved, but the adsorption process is generally classified as physisorption (characteristic of weak van der Waals forces) or chemisorption (characteristic of covalent bonding). It may also occur due to electrostatic attraction.

Adsorption is present in many natural physical, biological, and chemical systems, and is widely used in industrial applications such as activated charcoal, capturing and using waste heat to provide cold water for air conditioning and other process requirements (adsorption chillers), synthetic resins, increase storage capacity of carbide-derived carbons, and water purification. Adsorption, ion exchange, and chromatography are sorption processes in which certain adsorbates are selectively transferred from the fluid phase to the surface of insoluble, rigid particles suspended in a vessel or packed in a column. Lesser known, are the pharmaceutical industry applications as a means to prolong neurological exposure to specific drugs or parts thereof.

Isotherms

Adsorption is usually described through isotherms, that is, the amount of adsorbate on the adsorbent as a function of its pressure (if gas) or concentration (if liquid) at constant temperature. The quantity adsorbed is nearly always normalized by the mass of the adsorbent to allow comparison of different materials.

Freundlich

The first mathematical fit to an isotherm was published by Freundlich and Küster (1894) and is a purely empirical formula for gaseous adsorbates,

where is the quantity adsorbed, is the mass of the adsorbent, is the pressure of adsorbate and and are empirical constants for each adsorbent-adsorbate pair at a given temperature. The function is not adequate at very high pressure because in reality has an asymptotic maximum as pressure increases without bound. As the temperature increases, the constants and change to reflect the empirical observation that the quantity adsorbed rises more slowly and higher pressures are required to saturate the surface.

Langmuir

In 1916, Irving Langmuir published a new model isotherm for gases adsorbed to solids, which retained his name. It is a semi-empirical isotherm derived from a proposed kinetic mechanism. It is based on four assumptions:

1. The surface of the adsorbent is uniform, that is, all the adsorption sites are equivalent.

2. Adsorbed molecules do not interact.

3. All adsorption occurs through the same mechanism.

4. At the maximum adsorption, only a monolayer is formed: molecules of adsorbate do not deposit on other, already adsorbed, molecules of adsorbate, only on the free surface of the adsorbent.

These four assumptions are seldom all true: there are always imperfections on the surface, adsorbed molecules are not necessarily inert, and the mechanism is clearly not the same for the very first molecules to adsorb to a surface as for the last. The fourth condition is the most troublesome, as frequently more molecules will adsorb to the monolayer; this problem is addressed by the BET isotherm for relatively flat (non-microporous) surfaces. The Langmuir isotherm is nonetheless the first choice for most models of adsorption, and has many applications in surface kinetics (usually called Langmuir-Hinshelwood kinetics) and thermodynamics.

Langmuir suggested that adsorption takes place through this mechanism: , where A is a gas molecule and S is an adsorption site. The direct and inverse rate constants are k and k₋₁. If we define surface coverage, , as the fraction of the adsorption sites occupied, in the equilibrium we have:

or

where is the partial pressure of the gas or the molar concentration of the solution. For very low pressures and for high pressures

is difficult to measure experimentally; usually, the adsorbate is a gas and the quantity adsorbed is given in moles, grams, or gas volumes at standard temperature and pressure (STP) per gram of adsorbent. If we call v_mon the STP volume of adsorbate required to form a monolayer on the adsorbent (per gram of adsorbent), and we obtain an expression for a straight line:

Through its slope and y-intercept we can obtain v_mon and K, which are constants for each adsorbent/adsorbate pair at a given temperature. v_mon is related to the number of adsorption sites through the ideal gas law. If we assume that the number of sites is just the whole area of the solid divided into the cross section of the adsorbate molecules, we can easily calculate the surface area of the adsorbent. The surface area of an adsorbent depends on its structure; the more pores it has, the greater the area, which has a big influence on reactions on surfaces.

If more than one gas adsorbs on the surface, we define as the fraction of empty sites and we have:

Also, we can define as the fraction of the sites occupied by the j-th gas:

where i is each one of the gases that adsorb.

BET

Main article: BET theory

Often molecules do form multilayers, that is, some are adsorbed on already adsorbed molecules and the Langmuir isotherm is not valid. In 1938 Stephen Brunauer, Paul Emmett, and Edward Teller developed a model isotherm that takes that possibility into account. Their theory is called BET theory, after the initials in their last names. They modified Langmuir's mechanism as follows:

A_(g) + S ⇌ AS

A_(g) + AS ⇌ A₂S

A_(g) + A₂S ⇌ A₃S and so on

Langmuir isotherm (red) and BET isotherm (green)

The derivation of the formula is more complicated than Langmuir's (see links for complete derivation). We obtain:

x is the pressure divided by the vapor pressure for the adsorbate at that temperature (usually denoted ), v is the STP volume of adsorbed adsorbate, v_mon is the STP volume of the amount of adsorbate required to form a monolayer and c is the equilibrium constant K we used in Langmuir isotherm multiplied by the vapor pressure of the adsorbate. The key assumption used in deriving the BET equation that the successive heats of adsorption for all layers except the first are equal to the heat of condensation of the adsorbate.

The Langmuir isotherm is usually better for chemisorption and the BET isotherm works better for physisorption for non-microporous surfaces.

Kisliuk

Two adsorbate nitrogen molecules adsorbing onto a tungsten adsorbent from the precursor state around an island of previously adsorbed adsorbate (left) and via random adsorption (right)

In other instances, molecular interactions between gas molecules previously adsorbed on a solid surface form significant interactions with gas molecules in the gaseous phases. Hence, adsorption of gas molecules to the surface is more likely to occur around gas molecules that are already present on the solid surface, rendering the Langmuir adsorption isotherm ineffective for the purposes of modelling. This effect was studied in a system where nitrogen was the adsorbate and tungsten was the adsorbent by Paul Kisliuk (1922–2008) in 1957. To compensate for the increased probability of adsorption occurring around molecules present on the substrate surface, Kisliuk developed the precursor state theory, whereby molecules would enter a precursor state at the interface between the solid adsorbent and adsorbate in the gaseous phase. From here, adsorbate molecules would either adsorb to the adsorbent or desorb into the gaseous phase. The probability of adsorption occurring from the precursor state is dependent on the adsorbate’s proximity to other adsorbate molecules that have already been adsorbed. If the adsorbate molecule in the precursor state is in close proximity to an adsorbate molecule that has already formed on the surface, it has a sticking probability reflected by the size of the S_E constant and will either be adsorbed from the precursor state at a rate of k_EC or will desorb into the gaseous phase at a rate of k_ES. If an adsorbate molecule enters the precursor state at a location that is remote from any other previously adsorbed adsorbate molecules, the sticking probability is reflected by the size of the S_D constant.

These factors were included as part of a single constant termed a "sticking coefficient," k_E, described below:

As S_D is dictated by factors that are taken into account by the Langmuir model, S_D can be assumed to be the adsorption rate constant. However, the rate constant for the Kisliuk model (R’) is different to that of the Langmuir model, as R’ is used to represent the impact of diffusion on monolayer formation and is proportional to the square root of the system’s diffusion coefficient. The Kisliuk adsorption isotherm is written as follows, where Θ_(t) is fractional coverage of the adsorbent with adsorbate, and t is immersion time:

Solving for Θ_(t) yields:

Adsorption enthalpy

Adsorption constants are equilibrium constants, therefore they obey van 't Hoff's equation:

As can be seen in the formula, the variation of K must be isosteric, that is, at constant coverage. If we start from the BET isotherm and assume that the entropy change is the same for liquefaction and adsorption we obtain

,

that is to say, adsorption is more exothermic than liquefaction.

Adsorbents: Characteristics and general requirements

Activated carbon is used as an adsorbent

Adsorbents are used usually in the form of spherical pellets, rods, moldings, or monoliths with hydrodynamic diameters between 0.5 and 10 mm. They must have high abrasion resistance, high thermal stability and small pore diameters, which results in higher exposed surface area and hence high surface capacity for adsorption. The adsorbents must also have a distinct pore structure that enables fast transport of the gaseous vapors.

Most industrial adsorbents fall into one of three classes:

· Oxygen-containing compounds – Are typically hydrophilic and polar, including materials such as silica gel and zeolites.

· Carbon-based compounds – Are typically hydrophobic and non-polar, including materials such as activated carbon and graphite.

· Polymer-based compounds – Are polar or non-polar functional groups in a porous polymer matrix.

Silica gel

Silica gel is a chemically inert, nontoxic, polar and dimensionally stable (< 400 °C or 750 °F) amorphous form of SiO₂. It is prepared by the reaction between sodium silicate and acetic acid, which is followed by a series of after-treatment processes such as aging, pickling, etc. These after treatment methods results in various pore size distributions.

Silica is used for drying of process air (e.g. oxygen, natural gas) and adsorption of heavy (polar) hydrocarbons from natural gas.

Zeolites

Zeolites are natural or synthetic crystalline aluminosilicates, which have a repeating pore network and release water at high temperature. Zeolites are polar in nature.

They are manufactured by hydrothermal synthesis of sodium aluminosilicate or another silica source in an autoclave followed by ion exchange with certain cations (Na⁺, Li⁺, Ca²⁺, K⁺, NH₄⁺). The channel diameter of zeolite cages usually ranges from 2 to 9 Å (200 to 900 pm). The ion exchange process is followed by drying of the crystals, which can be pelletized with a binder to form macroporous pellets.

Zeolites are applied in drying of process air, CO₂ removal from natural gas, CO removal from reforming gas, air separation, catalytic cracking, and catalytic synthesis and reforming.

Non-polar (siliceous) zeolites are synthesized from aluminum-free silica sources or by dealumination of aluminum-containing zeolites. The dealumination process is done by treating the zeolite with steam at elevated temperatures, typically greater than 500 °C (930 °F). This high temperature heat treatment breaks the aluminum-oxygen bonds and the aluminum atom is expelled from the zeolite framework.

Activated carbon

Activated carbon is a highly porous, amorphous solid consisting of microcrystallites with a graphite lattice, usually prepared in small pellets or a powder. It is non-polar and cheap. One of its main drawbacks is that it is reacts with oxygen at moderate temperatures (over 300 °C).

Activated carbon nitrogen isotherm showing a marked microporous type I behavior

Activated carbon can be manufactured from carbonaceous material, including coal (bituminous, subbituminous, and lignite), peat, wood, or nutshells (e.g., coconut). The manufacturing process consists of two phases, carbonization and activation. The carbonization process includes drying and then heating to separate by-products, including tars and other hydrocarbons from the raw material, as well as to drive off any gases generated. The process is completed by heating the material over 400 °C (750 °F) in an oxygen-free atmosphere that cannot support combustion. The carbonized particles are then "activated" by exposing them to an oxidizing agent, usually steam or carbon dioxide at high temperature. This agent burns off the pore blocking structures created during the carbonization phase and so, they develop a porous, three-dimensional graphite lattice structure. The size of the pores developed during activation is a function of the time that they spend in this stage. Longer exposure times result in larger pore sizes. The most popular aqueous phase carbons are bituminous based because of their hardness, abrasion resistance, pore size distribution, and low cost, but their effectiveness needs to be tested in each application to determine the optimal product.

Activated carbon is used for adsorption of organic substances and non-polar adsorbates and it is also usually used for waste gas (and waste water) treatment. It is the most widely used adsorbent since most of its chemical (e.g. surface groups) and physical properties (e.g. pore size distribution and surface area) can be tuned according to what is needed. Its usefulness also derives from its large micropore (and sometimes mesopore) volume and the resulting high surface area.

Protein adsorption of biomaterials

Protein adsorption is a process that has a fundamental role in the field of biomaterials. Indeed, biomaterial surfaces in contact with biological media, such as blood or serum, are immediately coated by proteins. Therefore, living cells do not interact directly with the biomaterial surface, but with the adsorbed proteins layer. This protein layer mediates the interaction between biomaterials and cells, translating biomaterial physical and chemical properties into a "biological language". In fact, cell membrane receptors bind to protein layer bioactive sites and these receptor-protein binding events are transduced, through the cell membrane, in a manner that stimulates specific intracellular processes that then determine cell adhesion, shape, growth and differentiation. Protein adsorption is influenced by many surface properties such as surface wettability, surface chemical composition and surface nanometre-scale morphology.

Adsorption chillers

Combining an adsorbent with a refrigerant, adsorption chillers use heat to provide a cooling effect. This heat, in the form of hot water, may come from any number of industrial sources including waste heat from industrial processes, prime heat from solar thermal installations or from the exhaust or water jacket heat of a piston engine or turbine.

Although there are similarities between absorption and adsorption refrigeration, the latter is based on the interaction between gases and solids. The adsorption chamber of the chiller is filled with a solid material (for example zeolite, silica gel, alumina, active carbon and certain types of metal salts), which in its neutral state has adsorbed the refrigerant. When heated, the solid desorbs (releases) refrigerant vapour, which subsequently is cooled and liquefied. This liquid refrigerant then provides its cooling effect at the evaporator, by absorbing external heat and turning back into a vapour. In the final stage the refrigerant vapour is (re)adsorbed into the solid. As an adsorption chiller requires no moving parts, it is relatively quiet.

Portal site mediated adsorption

Portal site mediated adsorption is a model for site-selective activated gas adsorption in metallic catalytic systems that contain a variety of different adsorption sites. In such systems, low-coordination "edge and corner" defect-like sites can exhibit significantly lower adsorption enthalpies than high-coordination (basal plane) sites. As a result, these sites can serve as "portals" for very rapid adsorption to the rest of the surface. The phenomenon relies on the common "spillover" effect (described below), where certain adsorbed species exhibit high mobility on some surfaces. The model explains seemingly inconsistent observations of gas adsorption thermodynamics and kinetics in catalytic systems where surfaces can exist in a range of coordination structures, and it has been successfully applied to bimetallic catalytic systems where synergistic activity is observed.

In contrast to pure spillover, portal site adsorption refers to surface diffusion to adjacent adsorption sites, not to non-adsorptive support surfaces.

The model appears to have been first proposed for carbon monoxide on silica-supported platinum by Brandt et al. (1993). A similar, but independent model was developed by King and co-workers to describe hydrogen adsorption on silica-supported alkali promoted ruthenium, silver-ruthenium and copper-ruthenium bimetallic catalysts. The same group applied the model to CO hydrogenation (Fischer-Tropsch synthesis). Zupanc et al. (2002) subsequently confirmed the same model for hydrogen adsorption on magnesia-supported caesium-ruthenium bimetallic catalysts. Trens et al. (2009) have similarly described CO surface diffusion on carbon-supported Pt particles of varying morphology.

Adsorption spillover

In the case catalytic or adsorbent systems where a metal species is dispersed upon a support (or carrier) material (often quasi-inert oxides, such as alumina or silica), it is possible for an adsorptive species to indirectly adsorb to the support surface under conditions where such adsorption is thermodynamically unfavorable. The presence of the metal serves as a lower-energy pathway for gaseous species to first adsorb to the metal and then diffuse on the support surface. This is possible because the adsorbed species attains a lower energy state once it has adsorbed to the metal, thus lowering the activation barrier between the gas phase species and the support-adsorbed species.

Hydrogen spillover is the most common example of an adsorptive spillover. In the case of hydrogen, adsorption is most often accompanied with dissociation of molecular hydrogen (H₂) to atomic hydrogen (H), followed by spillover of the hydrogen atoms present.

The spillover effect has been used to explain many observations in heterogeneous catalysis and adsorption.^]

Polymer adsorption

Main article: polymer adsorption

Adsorption of molecules onto polymer surfaces is central to a number of applications, including development of non-stick coatings and in various biomedical devices. Polymers may also be adsorbed to surfaces through polyelectrolyte adsorption.

Adsorption in viruses

Adsorption is the first step in the viral infection cycle. The next steps are penetration, uncoating, synthesis (transcription if needed, and translation), and release. The virus replication cycle, in this respect, is similar for all types of viruses. Factors such as transcription may or may not be needed if the virus is able to integrate its genomic information in the cell's nucleus, or if the virus can replicate itself directly within the cell's cytoplasm.

In popular culture

The game of Tetris is a puzzle game in which blocks of 4 are adsorbed onto a surface during game play. Scientists have used Tetris blocks "as a proxy for molecules with a complex shape" and their "adsorption on a flat surface" for studying the thermodynamics of nanoparticles.

Ion exchange

Ion exchange is an exchange of ions between two electrolytes or between an electrolyte solution and a complex. In most cases the term is used to denote the processes of purification, separation, and decontamination of aqueous and other ion-containing solutions with solid polymeric or mineralic 'ion exchangers'.

Typical ion exchangers are ion exchange resins (functionalized porous or gel polymer), zeolites, montmorillonite, clay, and soil humus. Ion exchangers are either cation exchangers that exchange positively charged ions (cations) or anion exchangers that exchange negatively charged ions (anions). There are also amphoteric exchangers that are able to exchange both cations and anions simultaneously. However, the simultaneous exchange of cations and anions can be more efficiently performed in mixed beds that contain a mixture of anion and cation exchange resins, or passing the treated solution through several different ion exchange materials.

Ion exchangers can be unselective or have binding preferences for certain ions or classes of ions, depending on their chemical structure. This can be dependent on the size of the ions, their charge, or their structure. Typical examples of ions that can bind to ion exchangers are:

H⁺ (proton) and OH⁻ (hydroxide)

Single-charged monatomic ions like Na⁺, K⁺, and Cl⁻

Double-charged monatomic ions like Ca²⁺ and Mg²⁺

Polyatomic inorganic ions like SO₄²⁻ and PO₄³⁻

Organic bases, usually molecules containing the amino functional group -NR₂H⁺

Organic acids, often molecules containing -COO⁻ (carboxylic acid) functional groups

Biomolecules that can be ionized: amino acids, peptides, proteins, etc.

Along with absorption and adsorption, ion exchange is a form of sorption.

Ion exchange is a reversible process and the ion exchanger can be regenerated or loaded with desirable ions by washing with an excess of these ions.

Ion exchanger

Ion exchange resin beads

Ion exchange column, used for protein purification