Colloid

In general, a colloid or colloidal dispersion is a two-phase system of matter; a type of mixture intermediate between homogeneous mixtures and heterogeneous mixtures.

Many familiar substances, including butter, milk, cream, aerosols (fog, smog, smoke), asphalt, inks, paints, glues and sea foam, are colloids. This field of study was introduced in 1861 by Scottish scientist Thomas Graham.

The size of dispersed phase particles in a colloid range from 0.001 to 1 micrometers. Dispersions where the particle size is in this range are referred to as colloidal aerosols, colloidal emulsions, colloidal foams, or colloidal suspensions or dispersions. Colloids may be colored or translucent because of the Tyndall effect. The Tyndall effect is the scattering of light by particles in the colloid.

Classification of colloids

Colloids can be classified as follows:

Dispersed phase
Gas
Liquid
Solid
Continuous Phase Gas None: all gases are soluble Liquid aerosol,
Examples: fog, mist
Solid aerosol,
Examples: Smoke, dust
Liquid Foam,
Examples: Whipped cream
Emulsion,
Examples: Milk, mayonnaise, hand cream, blood
Sol,
Examples: Paint, pigmented ink
Solid Solid foam,
Examples: Aerogel, Styrofoam, Pumice
Gel,
Examples: Gelatin, jelly, cheese, Opal
Solid sol,
Examples: Cranberry glass, Ruby glass

Interaction between colloid particles

Colloids usually are too large to be affected by quantum effects. However, they are light enough to be affected by the thermic motion happening in the suspension.

The following forces play an important role in the interaction of colloid particles:

• Excluded Volume Repulsion: This refers to the repulsive force genetated between the particles when they come close to each other(due to overlap of electron gas of the two particles).
• Electrostatic interaction: Colloids can be manufactured so that they carry a charge. The Coulomb Potential is proportional to $\displaystyle \frac{1}{r}$ . However, if there are solvent particles with a charge opposite to that of the colloids, they assemble around the colloids and screen the repulsion. The potential is proportional to $\displaystyle e^{-\kappa r}/r$ .
• van der Waals forces: This is due to dipole-dipole interaction(permanent/induced). Even if the particles don't have a permanent dipole there could be fluctuations of the electron gas giving rise to a temporary dipole. So these type of forces are always present.
• Entropic forces: According to the second law of thermodynamics, a system evolves to a state in which entropy is maximized. This can result in effective forces even between hard spheres.

Stabilization of colloid suspensions

Stabilization is the means to keep the colloids from all settling on the ground of the container or glueing together. Steric stabilization and electrostatic stabilization are the two main mechanisms for colloid stabilization. Electrostatic stabilization is based on the mutual repulsion of like electrical charges. Different phases generally have different charge affinities, so that a charge double-layer forms at any interface. Small particle sizes lead to enormous surface areas, and this effect is greatly amplified in colloids. In a stable colloid, mass of a dispersed phase is so low that its buoyancy or kinetic energy is too little to overcome the electrostatic repulsion between charged layers of the dispersing phase. The charge on the dispersed particles can be observed by applying an electric field: all particles migrate to the same electrode and therefore must all have the same charge.

The destruction of a colloid, called coagulation, can be accomplished by heating or by adding an electrolyte. Heating increases the velocities of the particles, causing them to collide with enough energy that the barriers are penetrated and the particles can aggregate. Since this is repeated many times, the particle grows to be large enough to form a precipitate. Adding an electrolyte neutralizes the adsorbed ion layers on the surface of the colloidal particles.

Colloids as a model system for atoms

In physics, colloids are an interesting model system for atoms. For instance, crystallization and phase transitions can be observed.

• It is possible to manufacture the shape of interaction between colloid particles. Thereby atomic potentials can be imitated.
• Colloids are much bigger than atoms, and can therefore be observed with an optical microscope.
• Due to their size, the velocity of diffusion of colloids is slower. Processes like crystallization that happen within picoseconds in atomic systems, are slow enough to be observed in detail.
• Colloids are too big to be significantly affected by quantum effects. Therefore their dynamics are much easier to understand than that of atoms.

Colloids in biology

In the early 20th century, before enzymology was well understood, colloids were thought to be the key to the operation of enzymes; i.e., the addition of small quantities of an enzyme to a quantity of water would, in some fashion yet to be specified, subtly alter the properties of the water so that it would break down the enzyme's specific substrate, such as a solution of ATPase breaking down ATP. Furthermore, life itself was explainable in terms of the aggregate properties of all the colloidal substances that make up an organism. As more detailed knowledge of biology and biochemistry developed, of course, the colloidal theory was replaced by the macromolecular theory, which explains an enzyme as a collection of identical huge molecules which act as very tiny machines, freely moving about between the water molecules of the solution and individually operating on the substrate, no more mysterious than a factory full of machinery. The properties of the water in the solution are not altered, other than the simple osmotic changes that would be caused by the presence of any solute.