Introduction to Surfaces and Colloids - Surface and Colloid Chemistry: Principles and Applications - page 7

table 1.1
typical colloidal Systems

Phases

dispersed continuous System name

Liquid Gas Aerosol fog, spray

Gas Liquid Foam, thin films, froth

Fire extinguisher foam

Liquid Liquid Emulsion (milk) mayonnaise, butter

Solid Liquid Sols, AgI, photography films

Suspension wastewater
Cement, coal slurry

Biocolloids Corpuscles Serum Blood, blood-coagulants

Hydroxyapatite Collagen Bone

Liquid Solid Solid emulsion (toothpaste)

Solid Gas Solid aerosol (dust)

Gas Solid Solid foam—expanded (polystyrene)

Insulating foam

Solid Solid Solid suspension/solids in plastics

biological and technological significance. There are three types of colloidal systems
(Adamson and Gast, 1997; Birdi, 2002, 2008):

1. In simple colloids, a clear distinction can be made between the dis-
perse phase and the disperse medium, for example, simple emulsions of
oil-in-water (o/w) or water-in-oil (w/o).

2. Multiple colloids involve the coexistence of three phases, of which two
are finely divided, such as multiple emulsions (mayonnaise, milk) of
water-in-oil-in-water (w/o/w) or oil-in-water-in-oil (o/w/o).

3. Network colloids have two phases forming an interpenetrating network, for
example, a polymer matrix.

Colloidal (as solids or liquid drops) stability is determined by the free energy (sur-
face free energy or the interfacial free energy) of the system. The main parameter
of interest is the large surface area exposed between the dispersed phase and the
continuous phase. Since the colloid particles move about constantly, their dispersion
energy is determined by Brownian motion . The energy imparted by collisions with
the surrounding molecules at temperature T = 300 K is 3/2 k B T = 3/2 1.38 10 −23 300
= 10 −20 J (where k B is the Boltzmann constant). This energy and the intermolecular
forces would thus determine colloidal stability.
In the case of colloid systems (particles or droplets), the kinetic energy trans-
ferred on collision will be thus k B T = 10 −20 J. However, at a given moment, there is
a high probability that a particle may have a larger or smaller energy. Further, the
probability of total energy several times k B T (over 10 times k B T ) thus becomes very