Colloids

A colloid is a heterogeneous system in which one substance is dispersed (dispersed phase) as very fine particles in another substance called dispersion medium.

The essential difference between a solution and a colloid is that of particle size. While in a solution, the constituent particles are ions or small molecules, in a colloid, the dispersed phase may consist of particles of a single macromolecule (such as protein or synthetic polymer) or an aggregate of many atoms, ions or molecules.

Colloidal particles are larger than simple molecules but small enough to remain suspended. Their range of diameters is between 1 and 1000 nm (10^–9 to 10^–6 m).

Colloidal particles have an enormous surface area per unit mass as a result of their small size.

This enormous surface area leads to some special properties of colloids.

Classification of colloids

Colloids are classified on the basis of the following criteria:

i. Physical state of dispersed phase and dispersion medium

ii. Nature of interaction between dispersed phase and dispersion medium

iii. Type of particles of the dispersed phase.

Classification of colloids based on physical state of dispersed phase and dispersion medium

Sols are solids in liquids, gels are liquids in solids and emulsions are liquids in liquids.

Types of colloidal systems

Dispersed phase	Dispersion medium	Type of colloid	Examples
Solid	Solid	Solid sol	Some coloured glasses and gem stones
Solid	Liquid	Sol	Paints, cell fluids
Solid	Gas	Aerosol	Smoke, dust
Liquid	Solid	Gel	Cheese, butter, jellies
Liquid	Liquid	Emulsion	Milk, hair cream
Liquid	Gas	Aerosol	Fog, mist, cloud, insecticide sprays
Gas	Solid	Solid sol	Pumice stone, foam rubber
Gas	Liquid	Foam	Froth, whipped cream, soap lather

Classification of colloids based on nature of interaction between dispersed phase and dispersion medium

Lyophilic colloids

The word ‘lyophilic’ means solvent attracting or liquid-loving. If water is the dispersion medium, the term used is hydrophilic. Colloidal sols directly formed by mixing substances like gum, gelatine, starch, rubber, etc., with a suitable liquid (the dispersion medium) are called lyophilic sols. These sols can be reconstituted by simply remixingwith the dispersion medium. That is why these sols are also called reversible sols. These sols are quite stable and cannot be easily coagulated.

Lyophobic colloids

The word ‘lyophobic’ means solvent repelling or liquid-hating. If water is the dispersion medium, the term used is hydrophobic. Substances like metals, their sulphides, etc., when simply mixed with the dispersion medium do not form the colloidal sol. Their colloidal sols can be prepared only by special methods. These sols are readily precipitated (or coagulated) on the addition of small amounts of electrolytes, by heating or by shaking and hence, are not stable. Further, once precipitated, they do not give back the colloidal sol by simple addition of the dispersion medium. Hence, these sols are also called irreversible sols. Lyophobic sols need stabilizing agents for their preservation.

Lyophilic Colloids (liquid loving)	Lyophobic colloids(liquid hating)
Some substances which can from colloids directly on mixing them with a suitable liquid (dispersion medium).	Some substances cannot form colloid just by directly mixing them with a liquid. Their colloidal sols are prepared by special methods and are called lyophobic colloids.
Examples of these substances are gum, gelatin, starch, rubber.	Examples of these substances are metals, metal sulphides.
They are also called Reversible sols as in these sols (colloids) when the dispersion phase is separated from the dispersion medium (by say evaporation), the sol can be formed again by just mixing the dispersion phase and medium again.	They are also called Irreversible colloids as on precipitation, they don’t give back the colloid on simply mixing the dispersed phase and the dispersed medium.
They are also very stable and cannot be coagulated	They are unstable and coagulate easily by heating shaking or adding electrolytes. Stabilizing agents are used to preserve them

Classification of colloids based on type of particles of the dispersed phase

Multi-molecular colloids

On dissolution, a large number of atoms or smaller molecules of a substance aggregate together to form species having size in the colloidal range (diameter < 1 nm). For example, a gold sol may contain particles of various sizes having many atoms. Sulphur sol consists of particles containing a thousand or more of S₈ sulphur molecules.

Macromolecular colloids

Macromolecules in suitable solvents form solutions in which the size of the macromolecules may be in the colloidal range. Such systems are called macromolecular colloids. These colloids are quite stable and resemble true solutions in many respects. Examples are starch, cellulose, proteins, enzymes, polythene, nylon, polystyrene, synthetic rubber, etc.

Associated colloids (Micelles)

There are some substances which at low concentrations behave as normal strong electrolytes, but at higher concentrations exhibit colloidal behaviour due to the formation of aggregates. The aggregated particles thus formed are called micelles. These are also known as associated colloids.

The formation of micelles takes place only above a particular temperature called Kraft temperature (T_k) and above a particular concentration called critical micelle concentration (CMC). On dilution, these colloids revert back to individual ions.

Surface active agents such as soaps and synthetic detergents belong to this class. For soaps, the CMC is 10^–4 to 10^–3 mol L^–1. These colloids have both lyophobic and lyophilic parts. Micelles may contain as many as 100 molecules or more.

Mechanism of micelle formation

Soap is sodium or potassium salt of a higher fatty acid and may be represented as RCOO^–Na⁺ (e.g., sodium stearate CH₃(CH₂)₁₆COO^–Na⁺, which is a major component of many bar soaps). When dissolved in water, it dissociates into RCOO^– and Na⁺ ions. The RCOO^– ions, however, consist of two parts — a long hydrocarbon chain R (also called non-polar ‘tail’) which is hydrophobic (water repelling), and a polar group COO^– (also called polar-ionic ‘head’), which is hydrophilic (water loving).

Cleansing action of soaps

The cleansing action of soap is due to the fact that soap molecules form micelle around the oil droplet in such a way that hydrophobic part of the stearate ions is in the oil droplet and hydrophilic part projects out of the grease droplet like the bristles. Since the polar groups can interact with water, the oil droplet surrounded by stearate ions is now pulled in water and removed from the dirty surface. Thus soap helps in emulsification and washing away of oils and fats. The negatively charged sheath around the globules prevents them from coming together and forming aggregates.