Engineering Chemistry S Datta

COLLOIDS 287
Emulsion and Gel
Emulsions are colloidal systems in which both dispersed phase and dispersion medium
are liquids (immiscible). The familiar examples are–milk, which consists of liquid fat dispersed
in water (oil in water type); butter, which consists of water dispersed in liquid fat (water in oil
type).
Emulsions are prepared merely by shaking one liquid with the other, and hence they
are not stable and easily separate into two immiscible layers. Emulsions are stabilized by
adding a third substance (emulsifier) as stabilizer. This substance forms a thin layer around
the dispersed phase thereby preventing it from coalescing and coagulating. The emulsifier
also decreases the surface tension of the two liquids.
Albumin, casein, gum, agar-agar, soaps and detergents are the common emulsifiers.
Emulsifiers are selected according to the type of emulsion, e.g., soaps of alkali metals, gelatin
etc. are more soluble in aqueous phase, hence they are used as stabilizers for oil-in-water-type
of emulsion whereas heavy metal soaps of palmitic and oleic acid are soluble in fat and hence
used in water-in-oil-type emulsion.
Emulsions have properties similar to those of lyophobic colloids like Tyndall effect,
Brownian movement etc.
Gel: Many lyophilic sol and few lyophobic sol when coagulated under certain conditions
form semi-solid mass. Such a product is called gel and the process is called gelation, e.g., jam,
jelly, cheese, curd etc. This is formed by entrapment of the dispersion medium inside the loose
network of the dispersed particle. Gels have high viscosity and swell if placed in the dispersion
medium and some shrink in volume.
Gel formation is utilized for preparation of shoe polish, cheap soaps, curds, jellies, solid
fuel of alcohol with calcium oxalate.
Reversible colloid. When a colloid can be reversibly converted from a sol to gel and
vice versa, it is called a reversible colloid, e.g., solution of gelatin can be reconverted into sol by
heating. Other colloids which cannot be reversibly transformed are called irreversible colloid,
e.g., normal Au colloid.
(v) Zeta potential. In lyophobic colloids, particles are of similar charge, which do not
allow the particles to come closer and coalesce and hence bring stability to the system.
The charge is embedded in the colloidal particle and an equivalent amount of opposite
charge remains outside and these two layers form a double layer. A part of this opposite
charge is fixed on the particle and another part is mobile and distributed throughout the
medium.
Surface of solid
–
+
+
+
+
+
+
+
+
+
Liquid
Surface of solid
–
+
+
+
+
+
+
zeta pot.
+
+ – +
+ –
+ –
Liquid
+
+ –
+
+ –
+
–
+
–
Fixed
Movable
Fixed
Movable
Helmholtz double layer
Diffuse double layer
Fig. 12.8 Zeta potential.