Colloid | Research & Encyclopedia Articles

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. Fog is a colloidal suspension of water droplets. 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 hundreds of square meters per gram.

An ingestible colloidal suspension of gold particles in oil was used as "potable gold" by medical alchemists. The first systematic studies of inorganic colloids were reported between 1845 and 1850 by Francesco Selmi (1817-1881), an Italian chemist. The term colloid itself was introduced in 1861 by the British physical chemist, Thomas Graham (1805-1869) to distinguish "gluelike" materials which would not pass through a parchment filter from the majority of substances which in solution 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

Colloids are generally classified as either lyophyllic(solvent-loving), either lyophobic(solvent hating), or as either association colloids. When the continuous phase is an aqueous solution, the older terms "hydrophillic" 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 are the micelles formed by surfactant molecules in water in which the nonpolar 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.

Physically, colloids are characterized by light scattering and microscopy, and by their electrial properties. Colloidal particles have typical dimensions comparable to or less than the wavelengths of visible light, roughly 500 nm and below. As a result colloids scatter light strongly, a phenomenon known as the Tyndall effect, but can not be clearly imaged under the light microscope. The invention of the ultramicroscope by H. F. W. Siedentopf and Richard Zsigmondy in 1903 made it possible to image the scattered light well enough to allow the particles to be counted and their Brownian motion to st udied. The subsequent invention of electron microscopy has allowed the imaging of colloidal particles with the solvent evaporated. Charged colloidal particles will migrate in an applied electric field, a process called electrophoresis. The ratio of their velocity to the strength of the driving field is called the electrical mobility of the particles, from which one can calculate the zeta potential, the electrostatic potential of the particles at their radius of contact with the continuous phase.

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 and should they adhere to each other would rapidly coagulate into a single mass. An "atmosphere" of compensating charges ionic will surround each charged colloidal particle. Beyond a distance known 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 repulsion. If the Debye length is decreased, however, 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 occurs is known as the critical coagulation concentration (c.c.c.) for the particular salt being added and 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 or subject to 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 of colloid science. The wood pulp and clays used in papermaking are both colloidal, as are the inks used in writing and printing. Colloidal phenomena are key in the separation of minerals from their ores by particle flotation. The colloidal natures of detergents, paint, and milk products were mentioned above. Colloidal phenomena are also the basis for numerous adhesive, cosmetic and other products in widespread use today.