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-
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-
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-
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
3. the process of dissolving or
disrupting.
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
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
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.