Page 12 - Introduction to Colloid and Surface Chemistry

The colloidal state 3

The natural laws of physics and chemistry which describe the
behaviour of matter in the massive and molecular states also, of
course, apply to the colloidal state. The characteristic feature of
colloid science lies in the relative importance which is attached to the
various physicochemical properties of the systems being studied. As
we shall see, the factors which contribute most to the overall nature
of a colloidal system are:

Particle size
Particle shape and flexibility
Surface (including electrical) properties
Particle-particle interactions
Particle-solvent interactions

Classification of colloidal systems

Colloidal systems may be grouped into three general classifications:
1. Colloidal dispersions are thermodynamically unstable owing to
their high surface free energy and are irreversible systems in the
sense that they are not easily reconstituted after phase separation.
2. True solutions of macromolecular material (natural or synthetic)
are thermodynamically stable and reversible in the sense that they
are easily reconstituted after separation of solute from solvent.
3. Association colloids which are thermodynamically stable (see
Chapter 4).

Dispersions

The particles in a colloidal dispersion are sufficiently large for definite
surfaces of separation to exist between the particles and the medium
in which they are dispersed. Simple colloidal dispersions are,
therefore, two-phase systems. The phases are distinguished by the
terms dispersed phase (for the phase forming the particles) and
dispersion medium (for the medium in which the particles are
distributed) - see Table 1.1. The physical nature of a dispersion
depends, of course, on the respective roles of the constituent phases;
for example, an oil-in-water (O/W) emulsion and a water-in-oil (W/O)
emulsion could have almost the same overall composition, but their
physical properties would be notably different (see Chapter 10).