1. Colloidal solutions. Methods of preparation and purification of colloidal solution.

2. Essence of the electrophoresis and the electroosmosis. Coagulation of sols by electrolytes. The coagulation threshold.

3. Solutions of high-molecular compounds and they protective effect. Swelling and dissolution of polymers.

True Solution, Suspension and Colloidal Solution

Based on distinct properties, solutions can be classified into True Solution, Suspension and Colloid. This classification is necessary to understand concepts of colloidal solutions and distinguish it from rest of the types.

True Solution

True Solution is a homogeneous mixture of two or more substances in which substance dissolved (solute) in solvent has the particle size of less than 10-9 m or 1 nm. Simple solution of sugar in water is an example of true solution. Particles of true solution cannot be filtered through filter paper and are not visible to naked eye.

Suspensions

Suspension is a heterogeneous mixture in which particle size of one or more components is greater than 1000nm.

When mud is dissolved in water and stirred vigorously, particles of mud are distributed evenly in water. After some time, the particles of this solution settle under water due to influence of gravity. This solution is an example of Suspension (see picture below). Contrary to True Solution, particles of suspension are big enough to be seen with naked eye.

Suspension Settled Muddy Water

Colloidal Solution

Colloidal Solution is a heterogeneous mixture in which particle size of substance is intermediate of true solution and suspension i.e. between 1-1000 nm. Smoke from a fire is example of colloidal system in which tiny particles of solid float in air. Just like true solutions, Colloidal particles are small enough and cannot be seen through naked eye.They easily pass through filter paper. But colloidal particles are big enough to be blocked by parchment paper or animal membrane.

Black Cloud of Smoke with Fire in Forest

Solutions

 A solution is a homogeneous mixture of two or more components. The dissolving agent is the solvent. The substance which is dissolved is the solute. The components of a solution are atoms, ions, or molecules, which makes them 10-9 m or smaller in diameter.

 Example: Sugar and Water

Suspensions

 The particles in suspensions are larger than those found in solutions. Components of a suspension can be evenly distributed by a mechanical means, like by shaking the contents, but the components will settle out.

 Example: Oil and Water

Colloids

 

 Particles intermediate in size between those found in solutions and suspensions can be mixed such that they remain evenly distributed without settling out. These particles range in size from 10-8 to 10-6 m in size and are termed colloidal particles or colloids. The mixture they form is called a colloidal dispersion. A colloidal dispersion consists of colloids in a dispersing medium.

 Example: Milk

More Dispersions

 Liquids, solids, and gases all may be mixed to form colloidal dispersions.

Aerosols: solid or liquid particles in a gas.

Examples: Smoke is a solid in a gas. Fog is a liquid in a gas.

 

Sols: solid particles in a liquid.

Example: Milk of Magnesia is a sol with solid magnesium hydroxide in water.

Emulsions: liquid particles in liquid.

 

Example: Mayonnaise is oil in water.

Gels: liquids in solid.

         

Examples: gelatin is protein in water. Quicksand is sand in water.

Telling Them Apart

You can tell suspensions from colloids and solutions because the components of suspensions will eventually separate. Colloids can be distinguished from solutions using the Tyndall effect. A beam of light passing through a true solution, such as air, is not visible. Light passing through a colloidal dispersion, such as smoky or foggy air, will be reflected by the larger particles and the light beam will be visible.

Solution

1. a homogeneous mixture of one or more substances (solutes) dispersed molecularly in a sufficient quantity of dissolving medium (solvent).

2. in pharmacology, a liquid preparation of one or more soluble chemical substances, which are usually dissolved in water. For names of specific solutions, see under the name.

3. the process of dissolving or disrupting.

4. a loosening or separation.

Preparation of Solutions.

Formula for preparing solutions from a pure drug:

For example, to prepare 2000 mL of a 2 per cent solution from boric acid crystals, the proportion would be

Formula for preparing solutions from stock solutions:

For example, to prepare 1000 mL of a 2 per cent solution from a 4 per cent stock solution, the proportion would be

aqueous solution one in which water is used as the solvent.

             

BCG solution an aqueous suspension of bacille Calmette-Guérin for instillation into the bladder to activate the immune system in treatment of superficial bladder cancers. It reduces the risk of a subsequent bladder cancer developing, although the exact mechanism of action is unknown.

buffer solution one that resists appreciable change in its hydrogen ion concentration (pH) when acid or alkali is added to it.

     

colloid solution (colloidal solution) imprecise term for colloid.

hyperbaric solution one having a greater specific gravity than a standard of reference.

hypertonic solution one having an osmotic pressure greater than that of a standard of reference.

 

hypobaric solution one having a specific gravity less than that of a standard of reference.

hypotonic solution one having an osmotic pressure less than that of a standard of reference.

isobaric solution a solution having the same specific gravity as a standard of reference.

isotonic solution one having an osmotic pressure the same as that of a standard of reference.

molar solution a solution in which each liter contains 1 mole of the dissolved substance; designated 1 M. The concentration of other solutions may be expressed in relation to that of molar solutions as tenth-molar (0.1 M), etc.

 

 

normal solution a solution in which each liter contains 1 equivalent weight of the dissolved substance; designated 1 N.

ophthalmic solution a sterile solution, free from foreign particles, for instillation into the eye.

saturated solution one in which the solvent has taken up all of the dissolved substance that it can hold in solution.

standard solution one that contains in each liter a definitely stated amount of reagent; usually expressed in terms of normality (equivalent weights of solute per liter of solution) or molarity (moles of solute per liter of solution).

supersaturated solution an unstable solution containing more of the solute than it can permanently hold.

volumetric solution one that contains a specific quantity of solvent per stated unit of volume.

Electro-osmosis

Electroosmotic flow (or electro-osmotic flow, often abbreviated EOF; synonymous with electroosmosis or electroendosmosis) is the motion of liquid induced by an applied potential across a porous material, capillary tube, membrane, microchannel, or any other fluid conduit. Because electroosmotic velocities are independent of conduit size, as long as the double layer is much smaller than the characteristic length scale of the channel, electroosmotic flow is most significant when in small channels. Electroosmotic flow is an essential component in chemical separation techniques, notably capillary electrophoresis. Electroosmotic flow can occur in natural unfiltered water, as well as buffered solutions.

Difference Between Electrophoresis and Electroosmosis

Physical separation methods like filtering, distillation, column chromatography are not easy methods when comes to separation of some molecules. Electrophoresis and electro-osmosis are two other separation techniques which can be used to separate charged particles.

What is Electrophoresis?

Electrophoresis is a technique of separating molecules based on their sizes. Fundamental for this separation is the charge of the molecule and their ability to move in an electric field. This is the most common and main technique in molecular biology to separate molecules, especially DNA and proteins. This is mostly in use because it is relatively easy and inexpensive. The apparatus for the electrophoresis can be bit complicated, and preparation of it takes some time. But we can easily make an electrophoresis apparatus from the things we have in the laboratory. Electrophoresis techniques can vary depending on our purposes. We can use one dimensional electrophoresis for the separation of DNA or protein. Two dimensional electrophoresis is used when more resolved samples are required (as in the case of finger printing). A gel is used as the support medium to separate the molecules. This gel can be prepared as flat sheets or in tubes. Basis of this procedure is to separate molecules depending on their rate of movement through a gel when an electric field is supplied. Negatively charged molecules like DNA tend to travel towards the positive pole in this electric field while positively charged molecules tend to travel to the negative pole. Two types of gels are used in electrophoresis as agarose and polyacrylamide. These two have different resolving powers. The gel acts as a sieve to filter the different sizes of molecules. The electrostatic charges set up in the gel act as the force.

What is Electro-osmosis?

This is the process of moving a liquid through a material using an applied electric field. The movement can be through a porous material, along a capillary, membrane etc. This can be used as a separation technique (especially capillary electro-osmosis). The velocity of the liquid is linearly proportional to the applied electric field. It is also dependent on the material used to build the channel and the solution used. In the interface, solution and material have obtained opposite charges and, this is known as an electrical double layer. When an electrical field is applied to the solution, the electrical double layer moves by the resulting Coulomb force. This is known as the electro-osmotic flow.

 

 

What is Electro Osmosis?

 

In 1809, F.F. Reuss originally demonstrated electro-osmosis in an experiment that showed that water could be forced to flow through a clay-water system when an external electric field was applied to the soil. Research has since shown that flow is initiated by the movement of cations, (positively charged ions), present in the pore fluid of clay, or similar porous medium such as concrete; and the water surrounding the cations moves with them.

 

Electro-osmosis can be used to arrest or cause flow of water as well as the ions in it. Electro-osmosis has been used for many years in civil engineering to dewater dredging and other high-water content waste solids, consolidate clays, strengthen soft sensitive clays, and increase the capacity of pile foundations. It has also received significant attention in the past 5 years as a method to remove hazardous contaminants from groundwater or to arrest water flow. The basic physics and chemistry of electro osmosis can be found in several textbooks and treatises (e.g. Glasstone, 1946 and Tikhomolova, 1993).

Electro osmotic systems for waterproofing masonry walls were introduced in the 1960’s by the Europeans, (Adams 1978, Smith, 1984). The first applications were to prevent rising damp, (the wicking up of soil moisture by masonry due to capillary action). To prevent the upward movement of water, these systems established an electric field at the point at which a damp course is installed. “Active” systems that supplied a direct current, and “passive” systems that used the natural electrical potential between the saline saturated wall and the earth were installed. The natural electrical potential difference was claimed to halt the migration of moisture above a copper strand installed in the mortar joint between two courses of brick. In practice, however, there was little proof that the passive system worked and ionisation of the copper anode resulted in physical depletion. Early active systems did work, but were also subject to rapid copper anode corrosion. The introduction of newer conductive materials such as platinised titanium overcame these problems and, provided that a calculated controlled D.C. current was used, longevity was assured.

Electrokinesis

Electrokinesis is the particle or fluid transport produced by an electric field acting on a fluid having a net mobile charge.

Electrokinesis was first observed by Reuss in 1809 and has been studied extensively since the 19th century. Such study is known as electro-hydrodynamics or electro-kinetics, and was documented by Thomas Townsend Brown in 1921. It was later refined in scientific terms during the 1930s in conjunction with Dr. Paul Alfred Biefeld. The flow rate in such a mechanism is linear in the electric field. Electrokinesis is of considerable practical importance in micro-fluidics, since it offers a way to manipulate and convey fluids in Microsystems using only electric fields, with no moving parts.

 If the electrodes are free to move within the fluid, while keeping their distance fixed from each other, then such a force will actually propel the electrodes with respect to the fluid.

Electrophoresis

The migration of electrically charged particles in solution or suspension in the presence of an applied electric field. Each particle moves toward the electrode of opposite electrical polarity. For a given set of solution conditions, the velocity with which a particle moves divided by the magnitude of the electric field is a characteristic number called the electrophoretic mobility. The electrophoretic mobility is directly proportional to the magnitude of the charge on the particle, and is inversely proportional to the size of the particle. An electrophoresis experiment may be either analytical, in which case the objective is to measure the magnitude of the electrophoretic mobility, or preparative, in which case the objective is to separate various species which differ in their electrophoretic motilities under the experimental solution conditions.

What do you mean by Coagulation?

 Coagulation: Coagulation is a process which involves coming together of colloidal particles so as to change into large sized particles which ultimately settle as a precipitate or float on the surface.

Coagulation is generally brought about by the addition of electrolytes. When an electrolyte is added to a colloidal solution, the particles of the sol take up the ions which are oppositely charged and thus get neutralized. The neutral particles then start accumulating to form particles of a larger size which settle down.

Hardy Schulze law:

 The quantity of the electrolyte which is required to coagulate a definite amount of a colloidal solution depends upon the valency of the ion having a charge opposite to that of the colloidal particles. This observation oh Hardy and Schulze are known as Hardy Schulze law.

It can be defined as:

 Greater is the valency of the oppositely charged ion of the electrolyte being added, the faster is the coagulation.

Hence, for the coagulation of negatively charged arsenious sulphide sol., trivalent cations are far more effective than divalent cations which in turn are more effective than monovalent cations. Similarly for coagulation of positively charged ferric hydroxide sol, tetravalent anions are more effective than trivalent anions which are more effective than divalent anions which in turn are more effective than monovalent anions.

The minimum amount of an electrolyte that must be added to one litre of a colloidal solution so as to bring about complete coagulation or flocculation is called the coagulation or flocculation value of the electrolyte. Thus smaller is the flocculation value of an electrolyte; greater is its coagulating or precipitating power.

Preparation of Colloidal Solutions

As mentioned earlier, lyophilic colloids have a strong affinity for the dispersion medium and readily form a sol by bringing them into contact or by warming them with the medium. For example, sols of starch, gelatin, gum Arabic etc. can be prepared just by warming them with water. Similarly, a colloidal solution of cellulose nitrate can be prepared by dissolving it in an organic solvent such as ethyl alcohol. The product obtained is commercially called collodion.

The hydrocarbon residue R of RCOO– ion dissolves in the greasy dirt on the cloth (b) Soap micelle containing greasy dirt (Grease micelle)

Since lyophobic colloids practically have no affinity for the dispersion medium, they do not readily pass into the medium to form a colloidal solution. Hence special methods are required for the preparation of lyophobic sols. The methods used for the preparation of lyophobic sols can broadly be divided into the following two categories.

(i) Dispersion methods

(ii) Condensation or aggregation methods

Dispersion Methods

In these methods, the bigger particles of a substances are broken down to form smaller particles of colloidal dimensions thus obtained are stabilized by the addition of certain stabilizing agents. Some important dispersion methods are as follows:

(i)                                             Mechanical dispersion method: In this method, the dispersion of the coarse material (whose colloidal solution is to be prepared) is carried out in a machine called colloid mill. It consists of two heavy steel discs separated by a little gap. The gap may be adjusted according to the particle size desired. The two discs rotate at high speed (about 8000 revolutions per minute) in the opposite direction. A suspension of the substance in water is introduced into the mill. The coarse particles present in the suspension are grinded to the particles of colloidal dimensions and get dispersed in water to form a sol. Finer dispersion can be obtained by adding an inert diluents which prevents the colloidal particles to grow in size. For example, glucose is used as diluents in the preparation of sulphur sol.

Preparation of Colloidal Solutions

(ii) Electrical dispersion method (Bredig’s are method): This method is used for the preparation of sols metals such as gold, silver, platinum etc. In this method, an electric are is struck between the two electrodes of the metal (whose colloidal solution is to be prepared) immersed in the dispersion medium (say water). The dispersion medium is cooled by surrounding it with a freezing mixture. High temperature of the arc vaporizes some of the metal. The vapour condenses to the particles of colloidal size on cooling. The colloidal particles thus formed get dispersed in the medium to form a sol. of the metal.

Peptization: In this method, a freshly prepared precipitate of the substance is made to pass into the colloidal state by the addition of a suitable electrolyte. The process of dispersing a freshly prepared precipitate into colloidal form by using a suitable electrolyte is called peptization. The electrolyte added is called peptizing agent.

Some examples of peptization are given below:

(a) When a small amount of ferric chloride solution is added to the freshly precipitated ferric hydroxide, a reddish brown coloured colloidal solution of ferric hydroxide is obtained. This occurs due to the adsorption of Fe3+ ions over ferric hydroxide particles which causes them to disperse into the solution due to the electrostatic repulsions between the similarly charged particles.

(b) When a freshly prepared precipitate of silver iodide is shaken with a dilute solution of silver nitrate, a colloidal solution of silver iodide is obtained.

[B] Condensation Methods (Aggregation Method)

In condensation methods, the smaller particles of the dispersed phase are aggregated to form larger particles of colloidal dimensions. Some important condensation methods are described below.

1. Chemical Methods

Some chemical reactions may be used to aggregate smaller particles of atomic or ionic sizes to form large particles of colloidal dimensions. These reactions actually involve the formation of the dispersed phase as insoluble reaction products. Some important reactions leading to the formation of hydrophobic sols are as follows.

(a) Oxidation: Colloidal solution of sulphur can be prepared by oxidizing an aqueous solution of H2S with a suitable oxidizing agent such as bromine water, nitric acid or SO2.

(b) Reduction: Sols of gold, silver, platinum etc. can be obtained by the reduction of dilute solutions of their salts with a suitable reducing agent. For example, gold sol can be obtained by reducing a dilute aqueous solution of gold with stannous chloride.

The gold sol thus obtained is called purple of Cassius.

(c) Hydrolysis:

 

Sols of ferric hydroxide and aluminium hydroxide can be prepared boiling the aqueous solution of the corresponding chlorides. For example,

FeCI3    +   3H2S            -->      Fe(OH)3    +   3HCI

(d) Double decomposition:

The sols of inorganic insoluble salts such as arsenous sulphide, silver halides etc. may be prepared by using double decomposition reaction. For example, arsenous sulphide sol can be prepared by passing H2S gas through a dilute aqueous solution of arsenous oxide.

       As2O3      +       3H2S       -->         As2S3(OH)3     +     3H2O

2. Physical Methods

(i) Exchange of solvent:

This method involves the pouring of the true solution to another solvent in which the solute is insoluble but the solvent is completely miscible. An exchange of solvent gives the colloidal solution of the solute. The method may be used for the preparation of the sols of sulphur and phosphorus. For example, sulphur is soluble in alcohol but less soluble in water. When an alcoholic solution of sulphur is poured into water, a colloidal solution of sulphur is obtained. 

Purification of Colloidal Solutions (Sols)

 

Colloidal solutions prepared by the above mentioned methods usually contain the impurities of electrolytes. The presence of electrolytes in smaller concentrations stabilizes a sol but their presence in large concentration tends to destabilize the colloidal solution. Therefore, it is necessary to purify colloidal solutions by removing the impurities of electrolytes present in them. Following methods are generally used for the purification of colloidal solutions (sols).

1. Dialysis: We have already seen that an animal membrane allows the passage of crystalloids but retains the larger colloidal particles. This property of animal membranes is utilized for the purification of sols. The process involved is called dialysis. It may be defined as follows.

The process of separating the impurity particles of true solution dimensions (crystalloids) from an impure sol by means of diffusion through a suitable membrane such as parchment paper or cellophane membrane is called dialysis.

The apparatus used in this method is called dialyser. It consists of a bag made of parchment or cellophane. The bag is filled with the impure sol to be purified and is suspended in a tank through which pure water is circulated. The impurities of electrolytes present in the sol diffuse out of the bag leaving behind pure sol in the bag.

Electrodialysis: Dialysis is a slow process. However, it can be expedited by applying an electric field. Under the influence of electric field, the impurity ions move faster to the oppositely charged electrodes and the process gets quickened. This process is referred to as electrodialysis.    

2. Ultrafiltration: The pores of an ordinary filter paper are large enough to allow the passage of both impurity particles as well as colloidal particles. Therefore an ordinary filter paper cannot be used for removing the impurities of electrolytes from an impure sol. However, if the pore size of ordinary filter paper is reduced, it can be used for separating the impurities from impure sols. This is achieved by treating an ordinary filter paper with collodion or gelatin followed by its hardening by dipping it in formaldehyde solution. This treatment reduces the pore size and enables it to check the passage of colloidal particles through it. Filter papers thus obtained are called ultrafilters. Filtration through ultrafilters is called ultrafiltration.

2. Ultrafiltration: The pores of an ordinary filter paper are large enough to allow the passage of both impurity particles as well as colloidal particles. Therefore an ordinary filter paper cannot be used for removing the impurities of electrolytes from an impure sol.

However, if the pore size of ordinary filter paper is reduced, it can be used for separating the impurities from impure sols. This is achieved by treating an ordinary filter paper with collodion or gelatin followed by its hardening by dipping it in formaldehyde solution. This treatment reduces the pore size and enables it to check the passage of colloidal particles through it. Filter papers thus obtained are called ultrafilters. Filtration through ultrafilters is called ultrafiltration.

In ultrafiltration, the ultrafilter is supported over a wire mesh and the impure sol is poured over it. The impurity particles (electrolytes) pass through the ultrafilter while the larger colloidal particles are retained.

The process is very slow. However, it can be expedited by applying pressure on sol side or by using a suction pump on the filtrate side. By using a series of graded ultrafilters, impurities of different size can easily be removed and it is even possible to separate colloidal particles of different size from one another

3. Ultra-centrifugation: Ultracentrifugation involves the separation of colloidal particles from the impurities by centrifugal force. The impure sol is taken in a tube and the tube is placed in an ultra-centrifuge. The tube is rotated at high speeds. On account of this, the colloidal particles settle down at the bottom of the tube and the impurities remain in the solution. This solution is termed as centrifugate. The settled colloidal particles are removed from the tube and are mixed with an appropriate dispersing medium. Thus, the pure sol is obtained.   

                                             Colloid

A colloid is a substance microscopically dispersed evenly throughout another substance.

Colloidal solutions have dispersed phase particle, which size from 10-9 to 10-7m or 1 nm to 100 nm

A colloidal system consists of two separate phases:

 a dispersed phase (or internal phase) and a continuous phase (or dispersion medium). A colloidal system may be solid, liquid, or gaseous.

Many familiar substances are colloids, as shown in the chart below. In addition to these naturally occurring colloids, modern chemical process industries utilize high shear mixing technology to create novel colloids.

The dispersed-phase particles have a diameter of between approximately 5 and 200 nanometers. Such particles are normally invisible in an optical microscope, though their presence can be confirmed with the use of an ultramicroscope or an electron microscope. Homogeneous mixtures with a dispersed phase in this size range may be called colloidal aerosols, colloidal emulsions, colloidal foams, colloidal dispersions, or hydrosols. The dispersed-phase particles or droplets are affected largely by the surface chemistry present in the colloid.

Some colloids are translucent because of the Tyndall effect, which is the scattering of light by particles in the colloid. Other colloids may be opaque or have a slight color.

Colloidal solutions (also called colloidal suspensions) are the subject of interface and colloid science. This field of study was introduced in 1861 by Scottish scientist Thomas Graham.

 

 

 

Classification

Because the size of the dispersed phase may be difficult to measure, and because colloids have the appearance of solutions, colloids are sometimes identified and characterized by their physico-chemical and transport properties. For example, if a colloid consists of a solid phase dispersed in a liquid, the solid particles will not diffuse through a membrane, whereas with a true solution the dissolved ions or molecules will diffuse through a membrane. Because of the size exclusion, the colloidal particles are unable to pass through the pores of an ultrafiltration membrane with a size smaller than their own dimension. The smaller the size of the pore of the ultrafiltration membrane, the lower the concentration of the dispersed colloidal particules remaining in the ultrafiltred liquid. The exact value of the concentration of a truly dissolved species will thus depend on the experimental conditions applied to separate it from the colloidal particles also dispersed in the liquid. This is, a.o., particularly important for solubility studies of readily hydrolysed species such as Al, Eu, Am, Cm, ... or organic matter complexing these species. Colloids can be classified as follows:

In some cases, a colloid can be considered as a homogeneous mixture. This is because the distinction between "dissolved" and "particulate" matter can be sometimes a matter of approach, which affects whether or not it is homogeneous or heterogeneous.

Milk is an emulsified colloid of liquid butterfat globules dispersed within a water-based solution.

Colloidal solutions have dispersed phase particle, which size between 10-9 to 10-7m or 1 nm to 100 nm.

 Structure of colloidal parts

 

 

 

 

 

                     

 

Soap foam bubbles

A foam is a substance that is formed by trapping gas in a liquid or solid in a divided form, i.e. by forming gas regions inside liquid regions, leading to different kinds of dispersed media. In general, gas is present in large amount so it will be divided in polydisperse gas bubbles separated by liquid regions which may form films, thinner and thinner when the liquid phase is drained out of the system films.When the principal scale is small, id est for fine foam, this dispersed medium can be considered as a type of colloid.

The term foam may also refer to anything that is analogous to such a phenomenon, such as quantum foam, polyurethane foam (foam rubber), XPS foam, Polystyrene, phenolic, or many other manufactured foams. This is not the purpose of this page.

Structure of foams

A foam is in many cases a multiscale system.

One scale is the bubble one: real-life foams are typically disordered and have a variety of bubble sizes. At larger sizes, the study of idealized foams is closely linked to the mathematical problems of minimal surfaces and three-dimensional tessellations, also called honeycombs. The Weaire-Phelan structure is believed to be the best possible (optimal) unit cell of a perfectly ordered foam[citation needed], while Plateau's laws describe how soap-films form structures in foams.

At lower scale than the bubble one, is the thickness of the film for dry enough foams, which can be considered as a network of interconnected films called lamellae. Ideally, the lamellae are connected by three and radiate 120° outward from the connection points, known as Plateau borders.

An even lower scale is the one of the liquid-air interface at the surface of the film. Most of the time this interface is stabilized by a layer of amphiphilic structure, often made of surfactants, particles (Pyckering), or more complex associations.

Cappuccinos are topped with a layer of steamed-milk foam.

 

Liquid foams

Liquid foams can be used in fire retardant foam, such as those that are used in extinguishing fires, especially oil fires.

In some ways, leavened bread is a foam, as the yeast causes the bread to rise by producing tiny bubbles of gas in the dough.

The unique property of gas-liquid foams having very high specific surface area are exploited in the chemical processes of froth flotation and foam fractionation.

Solid foams

Solid foams form an important class of lightweight cellular engineering materials. These foams can be classified into two types based on their pore structure: open-cell-structured foams (also known as reticulated foams) and closed-cell foams.

Open-cell-structured foams contain pores that are connected to each other and form an interconnected network that is relatively soft. Open-cell foams will fill with whatever they are surrounded with. If filled with air, a relatively good insulator is the result, but, if the open cells fill with water, insulation properties would be reduced. Foam rubber is a type of open-cell foam.

Closed-cell foams do not have interconnected pores. The closed-cell foams normally have higher compressive strength due to their structures. However, closed-cell foams are also in general denser, require more material, and as a consequence are more expensive to produce. The closed cells can be filled with a specialized gas to provide improved insulation. The closed-cell structure foams have higher dimensional stability, low moisture absorption coefficients, and higher strength compared to open-cell-structured foams. All types of foam are widely used as core material in sandwich-structured composite materials.

From the early 20th century, various types of specially manufactured solid foams came into use. The low density of these foams made them excellent as thermal insulators and flotation devices, and their lightness and compressibility made them ideal as packing materials and stuffings. A modern application of foam technology is aerogel, which is a closed-cell foam with very good insulatory properties, that is also very light. It is usually based on alumina, chromia, and tin oxide, with carbon aerogels first developed in the late 1980s.

Syntactic foam

A special class of closed-cell foams is known as syntactic foam, which contains hollow particles embedded in a matrix material. The spheres can be made from several materials, including glass, ceramic, and polymers. The advantage of syntactic foams is that they have a very high strength-to-weight ratio, making them ideal materials for many applications, including deep-sea and space applications.[6] One particular syntactic foam employs shape memory polymer as its matrix, enabling the foam to take on the characteristics of shape memory resins and composite materials; i.e., it has the ability to be reshaped repeatedly when heated above a certain temperature and cooled. Shape memory foams have many possible applications, such as dynamic structural support, flexible foam core, and expandable foam fill.

Integral skin foam

Integral skin foam, also known as self-skin foam, is a type of foam with a high-density skin and a low-density core. They can be formed in an open-mold process or a closed-mold process. In the open-mold process, two reactive components are mixed and poured into an open mold. The mold is then closed and the mixture is allowed to expand and cure. Examples of items produced using this process include arm rests, baby seats, shoe soles, and mattresses. The closed-mold process, more commonly known as reaction injection molding (RIM), injects the mixed components into a closed mold under high pressures.

An emulsion is a mixture of two or more liquids that are normally immiscible (un-blendable). Emulsions are part of a more general class of two-phase systems of matter called colloids.

Although the terms colloid and emulsion are sometimes used interchangeably, emulsion is used when both the dispersed and the continuous phase are liquid. In an emulsion, one liquid (the dispersed phase) is dispersed in the other (the continuous phase). Examples of emulsions include vinaigrettes, milk, and some cutting fluids for metal working. The photo-sensitive side of photographic film is an example of a colloid.

A. Two immiscible liquids, not yet emulsified

 B. An emulsion of Phase II dispersed in Phase I

 C. The unstable emulsion progressively separates

 D. The surfactant (purple outline around particles) positions itself on the interfaces between Phase II and Phase I, stabilizing the emulsion

Emulsions, being liquid, do not exhibit a static internal structure; the droplets dispersed in the liquid matrix (the “dispersion medium”) are assumed to be statistically distributed.

To understand the formation and properties of emulsions, consider that the dispersed phase exhibits a "surface" that is covered ("wetted") by a different "surface". These "surfaces" form an interface. Both surfaces have to be created, which requires an energy input. Oil and water do not mix and usually separate from each other, forming two layers.

Oil-in-water emulsions are common in food:

Crema in espresso – coffee oil in water (brewed coffee), unstable

Hollandaise sauce – similar to mayonnaise

Mayonnaise – vegetable oil in lemon juice or vinegar, with egg yolk lecithin as emulsifier

Vinaigrette – vegetable oil in vinegar; if prepared with only oil and vinegar (without an emulsifier), yields an unstable emulsion

Homogenized milk – milk fat in water and milk proteins

In pharmaceutics, hairstyling, personal hygiene, and cosmetics, emulsions are frequently used. These are usually oil and water emulsions, but which is dispersed and which is continuous depends on the pharmaceutical formulation.

These emulsions may be called creams, ointments, liniments (balms), pastes, films, or liquids, depending mostly on their oil and water ratios and their route of administration. The first 5 are topical dosage forms, and may be used on the surface of the skin, transdermally, ophthalmically, rectally or vaginally. A very liquidy emulsion may also be used orally, or it may be injected. Popular medicated emulsions include calamine lotion, cod liver oil, Polysporin, cortisol cream, Canesten, and Fleet.

Microemulsions are used to deliver vaccines and kill microbes. Typical emulsions used in these techniques are nanoemulsions of soybean oil, with particles that are 400-600 nm in diameter.

 The process is not chemical, as with other types of antimicrobial treatments, but mechanical. The smaller the droplet, the greater the surface tension and thus the greater the force to merge with other lipids. The oil is emulsified using a high shear mixer with detergents to stabilize the emulsion, so, when they encounter the lipids in the membrane or envelope of bacteria or viruses, they force the lipids to merge with themselves. On a mass scale, this effectively disintegrates the membrane and kills the pathogen. This soybean oil emulsion does not harm normal human cells or the cells of most other higher organisms.

The exceptions are sperm cells and blood cells, which are vulnerable to nanoemulsions due to their membrane structures. For this reason, these nanoemulsions are not currently used intravenously. The most effective application of this type of nanoemulsion is for the disinfection of surfaces. Some types of nanoemulsions have been shown to effectively destroy HIV-1 and various tuberculosis pathogens, for example, on non-porous surfaces.

A gel is a solid, jelly-like material that can have properties ranging from soft and weak to hard and tough. Gels are defined as a substantially dilute cross-linked system, which exhibits no flow when in the steady-state.

By weight, gels are mostly liquid, yet they behave like solids due to a three-dimensional cross-linked network within the liquid. It is the crosslinks within the fluid that give a gel its structure (hardness) and contribute to stickiness (tack).

In this way gels are a dispersion of molecules of a liquid within a solid in which the solid is the continuous phase and the liquid is the discontinuous phase.

                 

An upturned vial of hair gel

Gels consist of a solid three-dimensional network that spans the volume of a liquid medium and ensnares it through surface tension effects.

This internal network structure may result from physical bonds (physical gels) or chemical bonds (chemical gels), as well as crystallites or other junctions that remain intact within the extending fluid. Virtually any fluid can be used as an extender including water (hydrogels), oil, and air (aerogel).

Both by weight and volume, gels are mostly fluid in composition and thus exhibit densities similar to those of their constituent liquids. Edible jelly is a common example of a hydrogel and has approximately the density of water.

As said earlier, the dissolution of a polymer is generally a slow process, which can take even several weeks, depending on the structure and the molecular weight of a given polymer.

When a low molecular weight solute such as sucrose is added to water, the dissolution process takes place immediately. The sugar molecules leave the crystal lattice progressively, disperse in water, and form a solution.

But polymer molecules are rather different. They constitute long chains with a large number of segments, forming tightly folded coils which are even entangled to each other. Numerous cohesive and attractive both intra and intermolecular forces hold these coils together, such a dispersion, dipole-dipole interaction, induction, and hydrogen bonding (Figure 1a).

Based on these features, one may expect noticeable differences in the dissolution behavior shown by polymers. Due to their size, coiled shape, and the attraction forces between them, polymer molecules become dissolved quite slowly than low molecular weight molecules. Billmeyer Jr. (1975) points out that there are two stages involved in this process: in the first place, the polymer swelling, and next the dissolution step itself.

When a polymer is added to a given solvent, attraction as well as dispersion forces begin acting between its segments, according to their polarity, chemical characteristics, and solubility parameter. If the polymer-solvent interactions are higher than the polymer-polymer attraction forces, the chain segment start to absorb solvent molecules, increasing the volume of the polymer matrix, and loosening out from their coiled shape (Figure 1b). We say the segments are now "solvated" instead of "aggregated", as they were in the solid state.

The whole "solvation-unfolding-swelling" process takes a long time, and given it is influenced only by the polymer-solvent interactions, stirring plays no role in this case. However, it is desirable to start with fine powdered material, in order to expose more of their area for polymer-solvent interactions.

When crystalline, hydrogen bonded or highly crosslinked substances are involved, where polymer-polymer interactions are strong enough, the process does stop at this first stage, giving a swollen gel as a result.

If on the contrary, the polymer-solvent interactions are still strongly enough, the "solvation-unfolding-swelling" process will continue until all segments are solvated. Thus, the whole loosen coil will diffuse out of the swollen polymer, dispersing into a solution. At this stage, the disintegration of the swollen mass can be favored by stirring, which increases the rate of dissolution.

However, once all the chain segments have been dispersed in the solvent phase, they still retain their coiled conformation, yet they are now unfolded, fully solvated, and with solvent molecules filling the empty space between the loosen segments. Hence, the polymer coil, along with solvent molecules held within, adopts a spheric or ellipsoid form, occupying a volume known as hydrodynamic volume of the polymer coil.

The particular behavior shown by polymer molecules, explains the high viscosity of polymer solutions. Solvent and low molecular weight solutes have comparable molecular size, and the solute does not swell when dissolving. Since molecular mobility is not restricted, and therefore intermolecular friction does not increase drastically, the viscosity of the solvent and the solution are similar. But the molecular size of polymer solutes is much bigger than that of the solvent. In the dissolution process such molecules swell appreciably, restricting their mobility, and consequently the intermolecular friction increases. The solution in these cases, becomes highly viscous.