Classification Based on the Nature of Interaction Between Dispersed Phase and Dispersion Medium

Colloidal systems, depending on the nature of attraction between the dispersed phase and the dispersion medium are classified into lyophobic (solvent hating) and lyophilic (solvent loving). If water is the dispersion phase is water, then the colloids are either hydrophilic or hydrophobic.

1) Lyophilic colloids

In this type of colloids sols, the dispersed phase has great attraction for the dispersion medium. In such colloids, the dispersed phase does not precipitate easily and the sols are quite stable. If the dispersion medium is separated from the dispersed phase, the sol can be reconstituted by simply remixing with the dispersion medium. Hence, these sols are called reversible sols. Examples of lyophilic sols include sols of gum, gelatine, starch, proteins and certain polymers in organic solvents.

2) Lyophobic colloids

In this type of colloidal sols, the dispersed phase has little affinity for the dispersion medium. These colloids are easily precipitated on the addition of small amounts of electrolytes, by heating or by shaking and therefore are not stable. Once precipitated, it is not easy to reconstitute the sol by simple mixing with the dispersion medium. Hence, these sols are called irreversible sols. Examples of lyophobic sols include sols of metals and their insoluble compounds like sulphides and oxides. Lyophobic sols need stabilizing agents to keep the dispersed phase from precipitating out.
Hydrophobic sols are often formed when rapid crystallization takes place. With rapid crystallization, many centres of crystallization called nuclei are formed at once. Ions are attracted to these nuclei and very small crystals are formed. These small crystals are prevented from settling out by the random thermal motion of the water molecules.

interaction of lyophobic particles(oil) with the solvent(water) through addition of an emulsifier(soap)

Classification of Colloids Based on Type of Particles of the Dispersed Phase

1) Multimolecular colloids
2) Macromolecular colloids
3) Associated colloids.

Multimolecular colloids

In this type of colloids the colloidal particles are aggregates of atoms or small molecules with molecular size less than one nanometer (1 nm). For e.g., gold sol consists of particles of various sizes which are clusters of several gold atoms. Similarly, sulphur sol consists of colloidal particles which are aggregates of S₈ molecules. The molecules in the aggregates are held together by Van der Waal forces.

Macromolecular colloids

Macromolecular colloidal particles are formed when on dissolution in a suitable solvent, the macromolecules have sizes which are in the colloidal range. Naturally occurring macromolecules are starch, proteins and cellulose. Man made macromolecules are polymers such as polyethylene, nylon and polystyrene. These colloids are quite stable and resemble true solutions in many respects.

Associated colloids (Micelles)

Certain substances behave as strong electrolytes at low concentration but at higher concentrations these substances exhibit colloidal characteristics due to the formation of aggregated particles. These aggregated particles are called micelles. Micelles are called associated colloids. The formation of micelles takes place only above a particular temperature called Kraft Temperature (T_k) and above particular concentration called the Critical micelle concentration (CMC). On dilution, these colloids revert back to individual ions. Surface active molecules such as soaps and synthetic detergents form associated colloids in water. For soaps, the CMC is about 10^-4 to 10^-3 mol L^-1. Micelles have both a lyophilic and lyophobic parts. Micelles may consists of more than 100 molecules. 

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.

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