- The Hamaker constant does not vary greatly in most of the
systems considered (see Table 12.2). Nevertheless, the variation may be
important, especially because it affects the depth of the secondary minimum
if the ionic strength is fairly high. - Thesurface potentialhas a very large effect, since electrostatic
repulsion is proportional toc^20. This means that the pH will have a large
effect on aggregation in many food dispersions. - Also theionic strengthhas a large effect. IfIis very small, the
system is nearly always stable to aggregation (unlessc 0 !0), whereas it is
unstable at highIvalues. As discussed before, multivalent ions affect the
value ofIfar more than monovalent ions do, and undissociated salts have
no effect. Ionic strength primarily determines therangeover which repulsion
acts, whereas pH (via c 0 ) determines the maximum repulsive energy.
However, salt may also affectc 0 , because counterions may associate with
ionic groups on the particle surface. This greatly depends on the chemical
constitution of the afore-mentioned ionic groups and of the counterions, as
well as on the counterion activity. In practice, it is often difficult to
differentiate between the two mechanisms. - Equations (12.1) and (12.7) show that interaction energy between
spheres is proportional to theradiusat any value ofh. As shown in Figure
12.4d, this may imply, for instance, that the larger particles in a dispersion
would be stable against aggregation in the primary minimum, whereas the
smaller particles are not. In practice, this is often not observed. In other
words, the dependence of stability on particle size is smaller than predicted.
Application. To apply the DLVO theory in practice, several pieces
of information have to be collected. Particle size (distribution) and shape
can generally be experimentally determined. Hamaker constants often are to
be found in the literature or can be calculated from Lifshits theory. The
surface potential can be approximated by the zeta potential obtained in
electrophoretic experiments. The ionic strength is generally known (or can
be calculated) from the composition of the salt solution. All the other
variables needed are generally tabulated in handbooks. This then allows
calculation ofV(h). To arrive at an aggregation rate, more information is
needed; this is discussed in Section 13.2.
The DLVO theory has been very successful in predicting (in) stability
against aggregation for many, especially inorganic, systems. Although
developed for lyophobic colloids, the theory can often be usefully applied to
lyophilic colloids; these are often found in biogenic systems, including most
foods. However, some complications and other interaction forces may come
into play.