Thomas Graham, in 1861, during his work on diffusion, found that while certain substances, such as sugars, salts, acids and bases diffused readily through a parchment membrane, others, such as gelatin, albumen and glue, diffused at a very slow rate. The substances belonging to the former category which generally exist in crystalline state, were called crystalloids while the substances belonging to the second category were given the name colloids. These observations led to the development of a new branch known as colloidal science.
In a true solution as sugar or salt in water, the solute particles are dispersed in the solvent as single molecules or ions. Thus the diameter of the dispersed particles ranges from 1A o to 10A o . On the other hand, in a suspension as sand stirred into water, the dispersed particles are aggregates of millions of molecules. The diameter of these particles is of the order of 2000A o or more. The colloidal solutions are intermediate between true solutions and suspensions. When the diameter of the particles of a substance dispersed in a solvent ranges from about 10A o to 2000A o , the system is termed a colloidal solution.
Types of Colloids
A colloidal system is made up of two phases. The substance distributed as the colloidal particles is called the dispersed phase. The second continuous phase in which the colloidal particles are dispersed is called the dispersion medium. Depending upon the physical state of dispersed phase and dispersion medium, the following types of colloidal solutions are possible.
Dispersed phase + Dispersion medium = Name with Examples
Solid + Solid = Solid-sol Eg. Ruby glass, alloys
Solid + Liquid = Sol Eg. paint, ink
Solid + Gas = Aerosol Eg. Smoke, haze
Liquid + Solid = Gel Eg. Curd, cheese
Liquid + Liquid = Emulsion Eg. Milk, Cream
Liquid + Gas = Liquid Aerosol Eg. Cloud, mist
Gas + Solid = Solid form Eg. Cork, pumice stone
Gas + Liquid = Foam Eg. Froths of air
A colloidal solution of gas in gas is not possible as gases are completely miscible and always form true solutions.
Lyophobic and Lyophilic Colloids
Colloidal solutions in which the dispersed phase has very little affinity for the dispersion medium are termed as lyophobic (solvent hating) colloids. Colloidal solutions of metals which have negligible affinity for solvents and sulphur in water are examples of this type.
Colloidal solutions in which the dispersed phase has considerable affinity for the dispersion medium are called lyophilic (solvent loving) colloids. Gelatin, protein and starch are examples of this type.