Page 221 - Introduction to Colloid and Surface Chemistry

8 Colloid stability

A most important physical property of colloidal dispersions is the
tendency of the particles to aggregate. Encounters between particles
dispersed in liquid media occur frequently and the stability of a
dispersion is determined by the interaction between the particles
during these encounters.
The principal cause of aggregation is the van der Waals attractive
forces between the particles, which are long-range forces. To
counteract these and promote stability, equally long-range repulsive
forces are required. Solvation tends to be too short-range; however,
the molecular ordering associated with solvation can propagate
several molecular diameters into the liquid phase and may exert some
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influence on stability . The principal stabilising options are electro-
static (i.e. the overlap of similarly charged electric double layers) and
polymeric. Polymeric and/or surfactant additives can influence
stability by a variety of mechanisms and the overall situation is often
very complicated.

Lyophobic sols

Ideally, lyophobic sols are stabilised entirely by electric double-layer
interactions and, as such, present colloid stability at its simplest.
Critical coagulation concentrations*-Schulze-Hardy rule

A most notable property of lyophobic sols is their sensitivity to
coagulation by small amounts of added electrolyte. The added

*ln this book, coagulation refers to a primary minimum effect (see page 219) and
ftocculation refers to a secondary minimum or polymer bridging effect (see pages 220,
238 and 241). In much of the colloid science literature, these terms (together with the
generic term aggregation) are used interchangeably.