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Colloids, Solutions, and Mixtures
Classification of Colloids
One way of classifying colloids is to group them according to the phase (solid, liquid, or gas) of the dispersed substance and of the medium of dispersion. A gas may be dispersed in a liquid to form a foam (e.g., shaving lather or beaten egg white) or in a solid to form a solid foam (e.g., styrofoam or marshmallow). A liquid may be dispersed in a gas to form an aerosol (e.g., fog or aerosol spray), in another liquid to form an emulsion (e.g., homogenized milk or mayonnaise), or in a solid to form a gel (e.g., jellies or cheese). A solid may be dispersed in a gas to form a solid aerosol (e.g., dust or smoke in air), in a liquid to form a sol (e.g., ink or muddy water), or in a solid to form a solid sol (e.g., certain alloys).
A further distinction is often made in the case of a dispersed solid. In some cases (e.g., a dispersion of sulfur in water) the colloidal particles have the same internal structure as a bulk of the solid. In other cases (e.g., a dispersion of soap in water) the particles are an aggregate of small molecules and do not correspond to any particular solid structure. In still other cases (e.g., a dispersion of a protein in water) the particles are actually very large single molecules. A different distinction, usually made when the dispersing medium is a liquid, is between lyophilic and lyophobic systems. The particles in a lyophilic system have a great affinity for the solvent, and are readily solvated (combined, chemically or physically, with the solvent) and dispersed, even at high concentrations. In a lyophobic system the particles resist solvation and dispersion in the solvent, and the concentration of particles is usually relatively low.
Formation of Colloids
Properties of Colloids
One property of colloid systems that distinguishes them from true solutions is that colloidal particles scatter light. If a beam of light, such as that from a flashlight, passes through a colloid, the light is reflected (scattered) by the colloidal particles and the path of the light can therefore be observed. When a beam of light passes through a true solution (e.g., salt in water) there is so little scattering of the light that the path of the light cannot be seen and the small amount of scattered light cannot be detected except by very sensitive instruments. The scattering of light by colloids, known as the Tyndall effect, was first explained by the British physicist John Tyndall. When an ultramicroscope (see microscope) is used to examine a colloid, the colloidal particles appear as tiny points of light in constant motion; this motion, called Brownian movement, helps keep the particles in suspension. Absorption is another characteristic of colloids, since the finely divided colloidal particles have a large surface area exposed. The presence of colloidal particles has little effect on the colligative properties (boiling point, freezing point, etc.) of a solution.
The particles of a colloid selectively absorb ions and acquire an electric charge. All of the particles of a given colloid take on the same charge (either positive or negative) and thus are repelled by one another. If an electric potential is applied to a colloid, the charged colloidal particles move toward the oppositely charged electrode; this migration is called electrophoresis. If the charge on the particles is neutralized, they may precipitate out of the suspension. A colloid may be precipitated by adding another colloid with oppositely charged particles; the particles are attracted to one another, coagulate, and precipitate out. Addition of soluble ions may precipitate a colloid; the ions in seawater precipitate the colloidal silt dispersed in river water, forming a delta. A method developed by F. G. Cottrell reduces air pollution by removing colloidal particles (e.g., smoke, dust, and fly ash) from exhaust gases with electric precipitators. Particles in a lyophobic system are readily coagulated and precipitated, and the system cannot easily be restored to its colloidal state. A lyophilic colloid does not readily precipitate and can usually be restored by the addition of solvent.
Thixotropy is a property exhibited by certain gels (semisolid, jellylike colloids). A thixotropic gel appears to be solid and maintains a shape of its own until it is subjected to a shearing (lateral) force or some other disturbance, such as shaking. It then acts as a sol (a semifluid colloid) and flows freely. Thixotropic behavior is reversible, and when allowed to stand undisturbed the sol slowly reverts to a gel. Common thixotropic gels include oil well drilling mud, certain paints and printing inks, and certain clays. Quick clay, which is thixotropic, has caused landslides in parts of Scandinavia and Canada.
a disperse system consisting of droplets of a liquid (the dispersed phase) distributed evenly throughout another liquid (the dispersion medium).
A distinction is made between emulsions of the oil-in-water type (with droplets of a nonpolar liquid, such as a mineral oil, dispersed in a polar medium, usually water) and reverse emulsions of the water-in-oil type (with droplets of a polar liquid in a non-polar medium). Multiple emulsions are also encountered, in which the droplets of the dispersed phase serve as the dispersion medium for even finer droplets of another dispersed phase.
Emulsions are also divided into lyophilic and lyophobic types (seeLYOPHILIC AND LYOPHOBIC COLLOIDS). Lyophilic emulsions are thermodynamically stable, reversible systems that are formed spontaneously at temperatures close to the critical displacement temperature for two interacting liquids. Lyophobic emulsions are thermodynamically unstable systems formed by the mechanical, acoustic, or electrical dispersion of one liquid in another or by the separation of droplets from a supersaturated solution or melt; such emulsions may exist for prolonged periods only if mixed with an emulsifier. Lyophilic emulsions are highly dispersed (colloidal) systems, the droplets of which measure no more than 10–5 cm. Lyophobic emulsions are coarsely (poorly) dispersed systems with droplet size usually ranging from 10–5 to 10–2 cm. If the dispersed phase and dispersion medium differ greatly in density, the emulsion will be kinetically unstable—that is, the particles of the dispersed phase will tend either to sink to the bottom or rise to the top. The sedimentation of emulsion droplets that are well protected against coalescence may lead to the concentration of the droplets and the formation of creams or sediments of continuous two-liquid phases not separated into discrete layers.
The type and properties of an emulsion depend on such factors as its composition, the relative proportions of the liquid phases, the quantity and chemical nature of the emulsifier, the method of emulsification, and the temperature at which the emulsification is carried out. A change in the composition of an emulsion or in the action of the emulsifier may produce a phase inversion, in which an oil-in-water emulsion becomes a water-in-oil emulsion or vice versa.
Dilute emulsions are typical liquids, with droplets that move freely and independently of one another in a highly mobile medium. In emulsions with droplets of uniform size, as the concentration of the dispersed phase exceeds 74 percent by volume, the viscosity of the system increases abruptly, and the emulsion becomes a gel. In the process, droplets that initially had a spherical shape are highly deformed in such a way that they come to resemble polyhedrons. The content of the dispersed phase in highly concentrated emulsions may be as high as 99 percent by volume; in such cases, the dispersion medium is retained between the droplets in the form of fine layers that resemble the liquid films between bubbles in foams.
Emulsions with various compositions and properties are commonly used in industry, agriculture, and medicine; they also have household uses. Many foods, such as milk and egg yolks, are multicomponent emulsions, as are unrefined petroleum and the milky juices of plants.
Among the products that take the form of emulsions are cooling lubricants and various pesticides, cosmetics, drugs, and binders for latex paints. Asphalt emulsions are used in construction.
REFERENCESVoiutskii, S. S. Kurs kolloidnoi khimii, 2nd ed. Moscow, 1975. Pages 367–81.
Emul’sii. Leningrad, 1972. (Translated from English.)
Becher, P. Emulsions: Theory and Practice, 2nd ed. New York, 1965.
Emulsions and Emulsion Technology, parts 1–2. Edited by K. J. Lissant. New York, 1974.
L. A. SHITS