A colloidal dispersion is a mixture of two substances, one of which is distributed evenly throughout the other. The substance occurring in greater quantity is called the dispersing (or solvent-like) material, while the other substance is called the dispersed (or solute-like) material. The size of the dispersed particles is the property that defines a mixture as a colloid. In order to be identified as a colloid, the dispersed particles must have diameters between about 10 to 2,000 angstroms.
Difference between a Colloid and a Solution
The solute particles in a solution are much smaller than the dispersed particles in a colloid. They typically consist of single atoms, such as sodium and chlorine in a saltwater solution, or may consist of rather small molecules, like sugars and alcohols. In contrast, colloidal particles are often larger aggregations of many smaller molecules, but they may even be a single giant molecule, such as hemoglobin. At the other end of the size spectrum, if the dispersed particles are too large, they will settle to the bottom and separate under the influence of gravity. In this case, the combination is no longer classified as a colloidal suspension but is simply referred to as a mixture.
Types of Colloidal Dispersions
A colloid can exist as any phase dispersed in any other phase, except for gas/gas, which is always a solution. An example of a colloid that is a liquid dispersed in a gas is fog, while an example of a solid dispersed in a gas is smoke. Both types of colloids are known as aerosols. A colloidal dispersion of a gas in a liquid is known as foam, while a gas dispersed in a solid is called solid foam. Whipped cream is an example of a foam while a marshmallow is an example of a solid foam. A colloidal dispersion of a liquid in a liquid is known as an emulsion, while a liquid dispersed in a solid is called a solid emulsion. Milk is an example of an emulsion, while butter is an example of a solid emulsion. A colloidal dispersion of a solid in a liquid is known as a sol, while a solid dispersed in a solid is called a solid sol. An example of a sol is paint, while an example of a solid sol is a pearl.
The Tyndall Effect
One way to identify a mixture as a colloidal dispersion is if it demonstrates the Tyndall effect. The dispersed particles in a liquid or gaseous colloid are large enough to scatter any light that passes through the material. As a result, the beam path can be clearly observed in a colloidal dispersion. The Tyndall effect can be observed in a smoky movie theater. The beam of light can clearly be seen extending from the projector as it scatters off millions of microscopic smoke particles and into your eyes.
Removal of Colloidal Particles
It is sometimes desirable to remove the dispersed particles from a colloid. Due to their small size, this can be difficult to do by simple filtration. A process of enlarging the particles, known as coagulation, is often used to make filtration more efficient. If the coagulated particles are made large enough, they may even settle out of the dispersing medium as a result of gravity. Two methods are commonly used to produce coagulation: heating and adding an electrolyte. Heating the colloid causes the particles to move faster and, thus, undergo more collisions, sticking together as they collide. An electrolyte is an extra chemical that can be added to the colloid, which will neutralize any surface charges on the dispersed particles. This eliminates electrostatic repulsions between the particles and, therefore, also increases the rate collisions.