colloid [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.

Colloid

Colloids are mixtures whose particles are larger than the size of a molecule but smaller than particles that can be seen with the naked eye. Colloids are one of three major types of mixtures, the other two being solutions and suspensions. The three kinds of mixtures are distinguished by the size of the particles that make them up. The particles in a solution are about the size of molecules, approximately 1 nanometer (1 billionth of a meter) in diameter. Those that make up suspensions are larger than 1,000 nanometers. Finally, colloidal particles range in size between 1 and 1,000 nanometers. Colloids are also called colloidal dispersions because the particles of which they are made are dispersed, or spread out, through the mixture.

Types of colloids

Colloids are common in everyday life. Some examples include whipped cream, mayonnaise, milk, butter, gelatin, jelly, muddy water, plaster, colored glass, and paper.

Every colloid consists of two parts: colloidal particles and the dispersing medium. The dispersing medium is the substance in which the colloidal particles are distributed. In muddy water, for example, the colloidal particles are tiny grains of sand, silt, and clay. The dispersing medium is the water in which these particles are suspended.

Colloids can be made from almost any combination of gas, liquid, and solid. The particles of which the colloid is made are called the dispersed material. Any colloid consisting of a solid dispersed in a gas is called a smoke. A liquid dispersed in a gas is referred to as a fog.

Types of Colloids

Properties of colloids

Each type of mixture has special properties by which it can be identified. For example, a suspension always settles out after a certain period of time. That is, the particles that make up the suspension separate from the medium in which they are suspended and fall to the bottom of a container. In contrast, colloidal particles typically do not settle out. Like the particles in a solution, they remain in suspension within the medium that contains them.

Colloids also exhibit Brownian movement. Brownian movement is the random zigzag motion of particles that can be seen under a microscope. The motion is caused by the collision of molecules with colloid particles in the dispersing medium. In addition, colloids display the Tyndall effect. When a strong light is shone through a colloidal dispersion, the light beam becomes visible, like a column of light. A common example of this effect can be seen when a spotlight is turned on during a foggy night. You can see the spotlight beam because of the fuzzy trace it makes in the fog (a colloid).

col·loid / ˈkälˌoid/ • n. a homogeneous, noncrystalline substance consisting of large molecules or ultramicroscopic particles of one substance dispersed through a second substance. Colloids include gels, sols, and emulsions. ∎ Anat. & Med. a substance of gelatinous consistency. • adj. of the nature of, relating to, or characterized by a colloid or colloids. DERIVATIVES: col·loi·dal / kəˈloidl/ adj.

colloid
1. A substance that is composed of two homogeneous phases, one of which is dispersed in the other.

2. (pedol.) Soil colloids are substances of very small particle size, either mineral (e.g. clay) or organic (e.g. humus), which therefore have a large surface area per unit volume. Colloids usually provide surfaces with high cation exchange capacity, and also exhibit an instability controlled by soil chemistry.

colloid
1. A substance that is composed of two homogenous phases, one of which is dispersed in the other.

2. (pedol.) Soil colloids are substances of very small particle size, either mineral (e.g. clay) or organic (e.g. humus), which therefore have a large surface area per unit volume. Colloids usually provide surfaces with high cation exchange capacity, and also exhibit an instability controlled by soil chemistry.

colloid
1. Substance composed of two homogeneous phases, one of which is dispersed in the other.

2. Soil colloids are substances of very small particle size, either mineral (such as clay) or organic (such as humus), which therefore have a large surface area per unit volume. Colloids usually provide surfaces with high cation exchange capacity, and also exhibit an instability controlled by soil chemistry.

colloid Particles (the disperse phase) suspended in a second medium (the dispersion medium); can be solid, liquid, or gas suspended in a solid, liquid, or gas. Examples of gas‐in‐liquid colloids are beaten egg‐white and whipped cream; of liquid‐in‐liquid colloids, emulsions such as milk and salad cream. See also emulsifying agents; stabilizers.

colloid Substance composed of fine particles which can be readily dispersed throughout a second substance. A sol is a solid dispersed in a liquid, an aerosol is a solid or liquid in a gas, an emulsion is a liquid in a liquid, and a foam is a gas in either a liquid or solid.

colloid (kol-oid) n. a mixture in which particles of one component (diameter 10⁻⁶−10⁻⁴ mm) are dispersed in a continuous phase of another component.
—colloidal adj.

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