Depending upon the affinity of the dispersed phase for the dispersion medium, colloidal systems can be classified into following two categories.
(i) Lyophilic sols (ii) Lyophobic sols
A brief description of these two types of sols is given below.
1. Lyophilic sols: These tem lyophilic means liquid-loving (i.e. solvent loving). Certain substances have an affinity for certain liquids and readily form colloidal dispersions with them. The substances which when mixed with a suitable liquid (dispersion medium) readily form colloidal solutions are called lyophilic colloids orintrinsic colloids and the sols thus formed are called lyophilic sols. When water is used as the dispersion medium, such colloids are termed as hydrophilic colloids and their colloidal dispersion in water are known as hydrophilic sols.
Arabic gum, gelatin, albumin, starch etc. are some common examples of lyophilic colloids. Gum sol, starch sol, sols of proteins in water, sols of polymers in organic solvents etc. is some examples of lyophilic sols.
Lyophilic sols are stable and do not get precipitated easily. In fact, they are self stabilized, i.e. they do not require any stabilizing agent to preserve them. An important characteristic of these sols is that if dispersed phase is separated from dispersion medium (say by evaporation), the dispersed phase can again be brought in the sol state simply by mixing it with the dispersion medium. This is why hydrophilic sols are also known as reversible sols.
2. Lyophobic sols: The term lyophilic means liquid-hating (i.e. solvent-hating). The substances which do not pass much affinity for the dispersion medium and do not readily pass into the sol state when mixed with the medium are called hydrophobic colloids or extrinsic colloids. Their sols are prepared by using special techniques and they are referred to as hydrophobic sols. Sols of metals e.g. gold sol, platinum sol etc. and the sols of insoluble substances such as metal sulphides and oxides are some examples of lyophilic sols.
Lyophilic sols are relatively less stable as compared to lyophilic sols. They are easily precipitated (or coagulated) on addition of small amounts of electrolytes, by heating or agitation. Moreover, the precipitated dispersed phase cannot be brought back into the sol state by simply mixing it with the dispersion medium. This is why hydrophobic sols are also known as irreversible sols. The lyophobic sols need stabilizing agents to keep them in the sol form for a longer time.