Hydrophilic and Hydrophobic Colloids

Surface Chemistry of Class 12

Hydrophilic and Hydrophobic Colloids

Colloids in which the continuous phase is water are also divided into two major classes: hydrophilic and hydrophobic colloids.

A Hydrophilic Colloid is a solid in which there is a strong attraction between the dispersed phase and the continuous phase (water). Many such colloids consist of macromolecules (very large molecules) dispersed in water. Except for the large size of the dispersed molecules, these are like normal solutions. Protein solution such as gelatin in water, are hydrophilic colloids. Gelatin molecules are attracted to water molecules by London force and hydrogen bonding very firmly.

A Hydrophobic Colloid is a colloid in which there is a lack of attraction between the dispersed phase and the continuous phase (water). Hydrophobic colloids are basically unstable on given sufficient time, the dispersed phase comes out of solution by aggregating into larger particles. In this behavior, they are quit unlike true solutions and hydrophilic colloid. The time taken to separate may be extremely long,

However, Hydrophobic sols are often formed when solid crystallizes rapidly from a chemical reaction or a supersaturated solution and very small crystals are formed. These small crystals are prevented from settling out by the random thermal motion of the solvent molecules, which continue to buffet them.

We might expect these very small crystals to aggregate into large crystals because the aggregation would bring ions of opposite charge into contact. However, sol formation appears to happen when, for some reason, each of the small crystals gets a preponderance of one kind of charge on its surface. For example, iron (III) hydroxide forms a colloid because an excess of iron (III) ion is present on the surface, given each crystal an excess of positive charge. These positively charged crystals repel one another, so aggregation to large particles is prevented.

Multimolecular, Macromolecular And Associated Colloids

(I) Multimolecular Colloids:

In these colloidal systems, the dispersed Phase (or colloidal particle) consists of large aggregates of simple atoms and molecules, having diameter less than 10-7cm. The atoms or molecules in these aggregates are held together by weak van der Waal’s forces. For example, gold sol., hydrated ferric oxide sol, silic acid sol, colloidal sulphur sol, metal hydroxide sols, metal sulphide sols etc., represents multimolcular colloidal solutions.

(II) Macromolecular colloids:

In these colloidal systems the colloidal particles (or dispersed phase) consist of large molecules, called macromolecules. These molecules are highly polymeric with very large molecular−weights, so that their molecular sizes are big enough to be of colloidal dimensions. Their molecular weight ranges from many thousands to even millions. Example of macromolecular collides are starch, cellulose, proteins polymers (e.g., polystyrene, nylon, synthetic rubber, enzymes etc).

(III) Associated colloids or colloidal electrolytes:

These are the colloidal solutions, in which the colloidal particles behave as normal strong electrolytes at low concentrations, but at higher concentration, they from aggregated particles of colloidal dimensions, called micelles, which exhibit colloidal properties. The minimum concentration at which micelle formation starts, is designed as the “critical micellization concentration (cmc)”. The parent substance, generally contains a large lyophobic (or organic) part as well as a lyophilic part. A solution of soap (the salt of higher fatty acid) is an example of an electrolytic solution that has colloidal properties.

We have some solid evidence to say so. For example, the apparent molecular−weight of a soap in solution, as measured by osmotic pressure or freezing point depression is found to be much grater than expected, thus showing that the molecules are associated into aggregates of considerable size.

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