colloid

(kŏl`oid) [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 dialysisdialysis
, in chemistry, transfer of solute (dissolved solids) across a semipermeable membrane. Strictly speaking, dialysis refers only to the transfer of the solute; transfer of the solvent is called osmosis.
..... Click the link for more information. . 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 microscopemicroscope,
optical instrument used to increase the apparent size of an object. Simple Microscopes

A magnifying glass, an ordinary double convex lens having a short focal length, is a simple microscope. The reading lens and hand lens are instruments of this type.
..... Click the link for more information. ) is used to examine a colloid, the colloidal particles appear as tiny points of light in constant motion; this motion, called Brownian movementBrownian movement or motion,
zigzag, irregular motion exhibited by minute particles of matter when suspended in a fluid.
..... Click the link for more information. , helps keep the particles in suspension. Absorptionabsorption
[Lat.,=sucking from], taking of molecules of one substance directly into another substance. It is contrasted with adsorption, in which the molecules adhere only to the surface of the second substance.
..... Click the link for more information. 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 propertiescolligative properties,
properties of a solution that depend on the number of solute particles present but not on the chemical properties of the solute. Colligative properties of a solution include freezing point (see freezing), boiling point, osmotic pressure (see osmosis), and
..... Click the link for more information. (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

[′käl‚ȯid] (chemistry) The phase of a colloidal system made up of particles having dimensions of 10-10,000 angstroms (1-1000 nanometers) and which is dispersed in a different phase.

colloid

A gelatinous substance so finely divided that it remains in suspension when dispersed in a liquid.

colloid

1. a mixture having particles of one component, with diameters between 10^--7 and 10^--9 metres, suspended in a continuous phase of another component. The mixture has properties between those of a solution and a fine suspension FORMULA 2. Physiol a gelatinous substance of the thyroid follicles that holds the hormonal secretions of the thyroid gland 3. Pathol of or relating to the gluelike translucent material found in certain degenerating tissues

colloid

[kol´oid] 1. gluelike.2. the translucent, yellowish, gelatinous substance resulting from colloid degeneration.3. a chemical system composed of a continuous medium (the continuous phase) throughout which are distributed small particles, 1 to 1000 nm in size (the disperse phase), which do not settle out under the influence of gravity. Colloidal particles are not capable of passing through a semipermeable membrane, as in dialysis. Solutes that can pass through a semipermeable membrane are sometimes called crystalloids. adj., adj colloid´al.dispersion colloid colloid" >colloid (def. 3), particularly an unstable colloid system.emulsion colloid lyophilic colloidrarely, emulsion.lyophilic colloid a stable colloid system in which the disperse phase is relatively liquid, usually comprising highly complex organic substances, such as glue or starch, which readily absorb solvent, swell, and distribute uniformly through the continuous phase.lyophobic colloid an unstable colloid system in which the disperse phase particles tend to repel liquids, are easily precipitated, and cannot be redispersed with additional solvent.stannous sulfur colloid a sulfur colloid containing stannous ions; complexed with technetium 99m it is used as a diagnostic aid (bone, liver, and spleen imaging).suspension colloid lyophobic colloid.

col·loid

(kol'oyd), 1. Aggregates of atoms or molecules in a finely divided state (submicroscopic), dispersed in a gaseous, liquid, or solid medium, and resisting sedimentation, diffusion, and filtration, thus differing from precipitates.
See also: hydrocolloid. 2. Gluelike. 3. A translucent, yellowish, homogeneous material of the consistency of glue, less fluid than mucoid or mucinoid, found in the cells and tissues in a state of colloid degeneration. Synonym(s): colloidin4. The stored secretion within follicles of the thyroid gland. For individual colloids not listed below, see the specific name. [G. kolla, glue, + eidos, appearance]

colloid

(kŏl′oid′)n.1. Chemistry a. A system in which finely divided particles, which are approximately 1 to 1,000 millimicrons in size, are dispersed within a continuous medium in a manner that prevents them from being filtered easily or settled rapidly.b. The particulate matter so dispersed.2. The gelatinous stored secretion of the thyroid gland, consisting mainly of thyroglobulin.3. Gelatinous material resulting from degeneration in diseased tissue.adj. Of, relating to, containing, or having the nature of a colloid.

col·loi′dal (kə-loid′l, kŏ-) adj.col·loi′dal·ly adv.

colloid

Chemistry
A liquid containing 1.0 mm to 1.0 nm microscopic and submicroscopic particles.
Physiology
The thyroglobulin-rich, homogeneous pale pink (by H&E stain) liquid which is secreted into the follicles by the thyroid cells

col·loid

(kol'oyd) 1. Aggregates of atoms or molecules in a finely divided state (submicroscopic), dispersed in a gaseous, liquid, or solid medium, and resisting sedimentation, diffusion, and filtration, thus differing from precipitates.
See also: hydrocolloid2. Gluelike. 3. A translucent, yellowish, homogeneous material of the consistency of glue, less fluid than mucoid or mucinoid, found in the cells and tissues in a state of colloid degeneration.
Synonym(s): colloidin. 4. The stored secretion within follicles of the thyroid gland. [G. kolla, glue, + eidos, appearance]

colloid

A substance in which particles are in suspension in a fluid medium. The particles are too small to settle by gravity or to be readily filtered. The colloid state lies between that of a solution and that of an emulsion.

colloid

a mixture of two substances which are immiscible (see MISCIBLE), but where the particles of one are too small to settle out, and so remain suspended indefinitely. Glue is a colloid of animal gelatin in water; the water is defined as the matrix and the gelatin as the inclusion. Colloid particles measure 1 x 10^-4 to 1 x 10^-6 mm in diameter, forming either a SOL or GEL structure which does not diffuse through cell membranes. Colloids are common in cells, where their large surfaces are important for chemical changes constantly in progress there.

col·loid

(kol'oyd) Aggregates of atoms or molecules in a finely divided state, dispersed in a gaseous, liquid, or solid medium, and resisting sedimentation, diffusion, and filtration, thus differing from precipitates. [G. kolla, glue, + eidos, appearance]

colloid

col·loid

colloid

col•loid

col·loid

colloid

colloid

colloid

Colloids, Solutions, and Mixtures

Classification of Colloids

Formation of Colloids

Properties of Colloids

colloid

colloid

colloid

colloid

colloid

col·loid

colloid

colloid

col·loid

colloid

colloid

col·loid

colloid

Words related to colloid

noun a mixture with properties between those of a solution and fine suspension

Related Words