Colloid

A colloid is a mixture of substances in which the particles of one substance are of greater than molecular size but are stabilized, at least for a time, with respect to forming larger particles or settling under gravity. The particles are referred to as the disperse phase, while the other phase is termed the dispersion medium or continuous phase. Smoke is a colloidal suspension of solid particles in air, while fog is a colloidal suspension of water droplets in air, and milk is a colloidal suspension of oil droplets in water. Chemists are most often concerned with colloids in which solid or liquid particles are suspended in a liquid. The special properties of colloids result from the large contact area between particle and solvent, which may reach thousands of square feet per ounce (or hundreds of square meters per gram).

An ingestible colloidal suspension of gold particles in oil was used as potable gold by medical alchemists during ancient times. The first systematic (scientific) studies of inorganic colloids were reported between 1845 and 1850 by Italian chemist Francesco Selmi (1817–1881). The term colloid itself was introduced in 1861 by the British physical chemist Thomas Graham (1805–1869) to distinguish glue-like materials that would not pass through a parchment filter from the majority of substances that in solution would pass through filters with ease. The latter he termed crystalloids, as they could be crystallized from solution. Graham also coined the term dialysis to describe the use of membranes to remove dissolved substances from a colloidal suspension, and the terms sol and gel to indicate the fluid and more rigid phase of a colloidal suspension, respectively.

Colloids are generally classified as either lyophyllic (solvent-loving), lyophobic (solvent-hating), or association colloids. When the continuous phase is an aqueous solution, the older terms hydrophilic and hydrophobic are also used. Lyophillic colloids are those in which the interaction of the particle surfaces and the solvent is energetically favorable. Aqueous solutions of proteins and other macromolecules are colloids of this type. They will form spontaneously when the solvent is added to the dry particles. When water is the solvent, the particles of a lyophillic colloid typically carry a significant surface charge, compensated by ions of the opposite sign in true solution.

In lyophobic colloids, the particle-solvent interaction is energetically unfavorable and the suspension will sooner or later separate. Lyophobic colloids are often prepared by vigorous agitation. The homogenization of milk is a process of this type. Association colloids are formed in solutions of molecules that include both lyophilic and lyophobic regions. The most important examples of association colloids are the micelles formed by surfactant molecules in water in which the non-polar regions of the molecules aggregate together in the center so that only the polar groups are exposed to the surface. The ability of surfactant micelles to accommodate additional oily material is the basis of their detergent action.

Colloids are also a type of particle that is intermediate in size between a molecule and the type of particles normally visible to the naked eye. Colloidal particles are usually from one to 1, 000 nanometers in diameter (where one nanometer is one-billionth the length of one meter). When a colloid is placed in water, it forms a mixture which is similar in some ways to a solution, and similar in some ways to a suspension. Like a solution, the particles never settle to the bottom of the container. Like a suspension, the dispersion is cloudy.

The size of colloidal particles accounts for the cloudiness of a colloidal dispersion. A true solution, such as obtained by dissolving table salt in water, is transparent, and light will go through it with no trouble, even if the solution is colored. A colloidal dispersion, on the other hand, is cloudy. If it is held up to the light, at least some of the light scatters as it goes through the dispersion. This is because the light rays bounce off the larger particles in the colloid, and bounce away from the human eye.

A colloidal dispersion does not ever settle to the bottom of the container. In this way, it is like a solution. The particles of the dispersion, though relatively large, are not large and heavy enough to sink. The solvent molecules support them for an indefinite time.

Everyone has seen what looks like dust particles moving about in a beam of sunlight. What is seen is light reflected from colloid-sized particles, in motion because of tiny changes in air currents surrounding the suspended particles. This type of motion, called Brownian motion, is typical of colloids, even those in suspension in solution, where the motion is actually caused by bombardment of the colloidal particles by the molecules of the liquid. This constant motion helps to stabilize the suspension, so the particles do not settle.

Another commonly visible property of a colloidal dispersion is the Tyndall effect. If a strong light is shined through a translucent colloidal dispersion that will let at least some of the light through, the light beam becomes visible, like a column of light. This is because the large particles of the dispersed colloid scatter the light, and only the most direct beams make it through the medium.

Milk is the best known colloidal dispersion, and it shows all these properties. They can be seen by adding several drops of milk to a glass of water. Most of its cloudiness is due to fat particles that are colloidal in size, but there is also a significant amount of protein in it, and some of these are also in the colloidal size range.

Understanding the interaction between colloidal particles is a matter of great theoretical and practical importance. The particles are constantly being brought into contact with each other through Brownian motion. Should they adhere to each other, the particles would rapidly coagulate into a single mass. An atmosphere of ionic compensating material will surround each charged colloidal particle. Beyond a distance known as the Debye length, the particle and its atmosphere will appear to be electrically neutral. Particles separated by more than a Debye length will attract each other weakly due to the van der Waals force. As the particles move closer, the atmospheres will overlap and the electrostatic repulsion will begin to result in a repulsive motion. However, if the Debye length is decreased, through the addition of additional ions (particularly ions of a higher charge) the colloidal particles may approach closely enough for a bond to form. The concentration at which this action occurs is known as the critical coagulation concentration (c.c.c.), named for the particular salt being added. The process, as a whole, is sometimes called the salting-out of the colloid.

The flow characteristics of colloids are also of great interest. A very desirable property in many applications is thixotropy, in which the material behaves as a gel or very viscous liquid at rest when subjected to a mild shear, but flows freely when subjected to a larger shear. This property is highly desirable in paints, which must be transported on a brush but then flow freely as the brush is moved against a stationary surface.

There are numerous practical applications for the field of colloid science. The wood pulp and clays used in papermaking are both colloidal, as are the inks used in writing and printing. In fact, recent research into the formation and growth of colloidal particles at the nano-particle level has been conducted for many different industrial applications. For instance, inks within the printing industry require very precise controls concerning the scattering and absorbance properties of colloids at the nanoparticle level. This control is especially important to the quality of the final product. In addition, colloidal phenomena are important in the separation of minerals from their ores by particle flotation, and are also the basis for numerous adhesives, cosmetics, and other products in widespread use today.

See also Brownian motion.