emulsion

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colloid

colloid (kŏlˈoid) [Gr.,=gluelike], a mixture in which one substance is divided into minute particles (called colloidal particles) and dispersed throughout a second substance. The mixture is also called a colloidal system, colloidal solution, or colloidal dispersion. Familiar colloids include fog, smoke, homogenized milk, and ruby-colored glass.

Colloids, Solutions, and Mixtures

The Scottish chemist Thomas Graham discovered (1860) that certain substances (e.g., glue, gelatin, or starch) could be separated from certain other substances (e.g., sugar or salt) by dialysis. He gave the name colloid to substances that do not diffuse through a semipermeable membrane (e.g., parchment or cellophane) and the name crystalloid to those which do diffuse and which are therefore in true solution. Colloidal particles are larger than molecules but too small to be observed directly with a microscope; however, their shape and size can be determined by electron microscopy. In a true solution the particles of dissolved substance are of molecular size and are thus smaller than colloidal particles; in a coarse mixture (e.g., a suspension) the particles are much larger than colloidal particles. Although there are no precise boundaries of size between the particles in mixtures, colloids, or solutions, colloidal particles are usually on the order of 10−7 to 10−5 cm in size.

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

There are two basic methods of forming a colloid: reduction of larger particles to colloidal size, and condensation of smaller particles (e.g., molecules) into colloidal particles. Some substances (e.g., gelatin or glue) are easily dispersed (in the proper solvent) to form a colloid; this spontaneous dispersion is called peptization. A metal can be dispersed by evaporating it in an electric arc; if the electrodes are immersed in water, colloidal particles of the metal form as the metal vapor cools. A solid (e.g., paint pigment) can be reduced to colloidal particles in a colloid mill, a mechanical device that uses a shearing force to break apart the larger particles. An emulsion is often prepared by homogenization, usually with the addition of an emulsifying agent. The above methods involve breaking down a larger substance into colloidal particles. Condensation of smaller particles to form a colloid usually involves chemical reactions—typically displacement, hydrolysis, or oxidation and reduction.

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.

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The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

Emulsion

 

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.

REFERENCES

Voiutskii, 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

The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.

emulsion

[ə′məl·shən]
(chemistry)
A stable dispersion of one liquid in a second immiscible liquid, such as milk (oil dispersed in water).
(graphic arts)
In photography, the photosensitized material on film, plates, and various photographic papers.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.

emulsion

1. A mixture of liquids insoluble in one another, in which one is suspended in the other in the form of minute globules.
2. A mixture in which solid particles are suspended in a liquid in which they are insoluble, as a mixture of bitumen and water, with uniform dispersion of the bitumen globules. The cementing action needed in roofing and waterproofing takes place as the water evaporates.
McGraw-Hill Dictionary of Architecture and Construction. Copyright © 2003 by McGraw-Hill Companies, Inc.

emulsion

emulsionclick for a larger image
A suspension of a light-sensitive silver salt, especially silver chloride or silver bromide in a colloidal medium—usually gelatin—which is used for coating photographic films, plates, and paper.
An Illustrated Dictionary of Aviation Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved

emulsion

1. Photog a light-sensitive coating on a base, such as paper or film, consisting of fine grains of silver bromide suspended in gelatine
2. Chem a colloid in which both phases are liquids
3. a type of paint in which the pigment is suspended in a vehicle, usually a synthetic resin, that is dispersed in water as an emulsion. It usually gives a mat finish
4. Pharmacol a mixture in which an oily medicine is dispersed in another liquid
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005
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