Colloids

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Colloids

Colloids. Colloid is a word that comes from the Greek word kolle, meaning glue. It was originally applied only to glue-like substances such as starch or gelatin. These substances when mixed with water pass through animal and vegetable membranes very slowly or not at all. This fact was discovered in 1861 by a Scottish chemist, Thomas Graham (1805-1896), who performed a series of experiments with such substances as starch, glue, salt, and sugar. In the experiments he enclosed water solutions of the materials in parchment bags and then suspended them in water. He noted that solutions of substances that crystalize well, such as sugar and salt, readily passed through the walls of the parchment bag and solutions of substances that didn’t crystalize didn’t. Since Graham’s time the word colloid has been broadened to include any dispersion of particles of very fine size that are larger than molecules.

Def. Colloid. A mixture in which very small particles of one substance are distributed evenly throughout another substance. The particles are generally larger than those in a solution, and smaller than those in a suspension, ranging in size from about one millionth of a millimeter to one hundred millionth of a millimeter.

Examples: butter, milk, smoke, fog, ink, paint

The particles may be any one of the three states of matter: solid, liquid, or gaseous. The dispersing agent in which they are suspended may also be solid, liquid, or gaseous.

Colloidal dispersions Examples

Solid in a solid Some alloys of metals, colored glass

Solid in a liquid Liquid glue, white of an egg, India ink

Solid in a gas A cloud of smoke in the air

Liquid in a solid Jelly, gelatin desserts

Liquid in a liquid Milk, mayonnaise, other emulsions

Liquid in a gas Fog, clouds in the sky

Gas in a solid Bread dough, marshmallow

Gas in a liquid Gas in ginger ale or soda water, whipped cream

Properties of colloids. Colloids have the following properties:

1. Non-settling. They are non-settling. Some larger colloidal particles may settle very slowly but finer particles remain suspended indefinitely.

2. Exhibit Brownian movement. They exhibit Brownian movement (i.e. the particles are in constant motion, following random, zigzag paths).

3. Exhibit Tyndall effect. They exhibit the Tyndall effect (i.e. the fine particles scatter light) causing a diffusion of light passing through them.

4. Enormous adsorptive power. They exhibit enormous adsorptive power due to the enormous surface area of the myriad of particles.

5. Electrical charge. The particles usually acquire an electrical charge, either positive or negative.

Large surface area of a colloidal suspension. Because of their small size, a given mass of colloidal particles has an astonishing total surface area. Suppose we divide a one-inch cube as shown in Fig. 1, producing 8 half-inch cubes. The surface area of the one-inch cube is 6 sq. in. The total surface area of the eight half-inch cubes is 12 sq. in. [ 8•6•(½)² = 12 ]. Thus this operation has doubled the surface area. If we now apply the same operation to each of the eight half-inch cubes, we will double their total surface area to 24 sq. in. If we now repeatedly do this until we arrive at particles the size of colloidal particles we come up with an astonishing total surface area of over 200 acres! This gives colloids tremendous adsorptive power. An example is activated carbon which, in a gas mask, will adsorb many times its own weight in poisonous gases.

Electrical charge acquired by colloids. Examples of colloidal particles which acquire a positive charge include ferric hydroxide, aluminum hydroxide, albumen, and hemoglobin. Examples of colloidal particles which acquire a negative charge include clay, silver chloride, starch, oil emulsions, arsenic sulfide, activated charcoal, silicic acid, and metals.

Methods of precipitating colloids.

1. Heat. Heat causes the particles of some colloidal dispersions to coagulate. An example is dropping an egg into hot water to poach it. The heat coagulates the colloidal suspension of albumen, forming a white mass.

2. Adding an acid or base to neutralize the charge of the colloidal suspension. Latex, the natural fluid from the rubber tree, which is slightly alkaline, can be made to release the rubber by the addition of formic acid or acetic acid. This causes the rubber particles to coagulate. Another example is found in the manufacture of soap, where common salt is added to cause the colloidal soap particles to coagulate and collect on top of the suspension in which the soap in formed.

Methods of producing of colloidal suspensions. There are two general methods of producing colloids, called condensation and dispersion:

1. Condensation. Increase the size of extremely tiny particles up to colloidal size.

2. Dispersion. Break larger particles down to colloidal size.

Preparing colloidal suspensions by condensation - examples.

Example 1. If one adds hydrogen peroxide to a solution of hydrogen sulfide in water, colloidal particles of sulfur are dispersed in the water. That the particles are colloidal becomes apparent if we try to filter the mixture. The particles are so small that they pass right through the filter paper.

Example 2. We can produce colloidal ferric hydroxide by adding ferric chloride solution slowly to boiling water.

Methods of preparing colloidal suspensions by dispersion.

1. The electric arc

2. Grinding

3. Homogenizing

4. Shaking

The electric arc. One can produce an electric arc under water by momentarily touching two wires and then separating them just far enough so the arc is maintained. If the wires are of gold, colloidal particles of gold are torn loose from the wires, creating a colloidal suspension of gold. Such a colloidal suspension remains suspended indefinitely. Colloidal suspensions of other metals may be made in a similar way.

Grinding. Cement manufactures grind their product to colloidal size in large grinding mills.

Homogenizing. Homogenizers are colloid mills that usually have a grooved cone that rotates at very high speeds. The material to be homogenized is forced through a narrow slit that separates the rotating cone from a stationary housing. A shearing action breaks up the material into particles of colloidal size. Milk is homogenized to break up the fat globules into colloidal size.

Shaking. If we shake a few drops of oil in a jar of water, the oil will be broken up into colloidal particles dispersed in the water. In this case, the dispersion is not permanent, as the water will soon separate and come to the top.

Methods of stabilizing colloids.

Example 1. In manufacturing a graphite lubricant called aquadag, tannin is used to stabilize the suspension of graphite.

Example 2. When a pharmacist makes an emulsion he triturates the oil and the dispersing medium with gum arabic or gum tragacanth which makes the emulsion much more stable.

Example 3. Manufacturers of mechanical emulsions, toilet creams, and tooth pastes stabilize the colloidal dispersions in their products by grinding them in colloid mills.

References

1. Dull, Metcalfe, Brooks. Modern Chemistry.