Emulsions and foams 267(bj I H" Io7( ~—"—Figure 10.1 Stabilisation of emulsions by finely divided solids: (a) preferential
wetting by water leading to an O/W emulsion; (b) preferential wetting by oil leading to
a W/O emulsionwetting phase is the dispersion medium). For example, bentonite
clays (which are preferentially wetted by water) tend to give O/W
emulsions, whereas carbon black (which is preferentially oil-wetted)
tends to give W/O emulsions. This preferential wetting theory can be
extended to cover other types of emulsifying agent. The type of
emulsion which tends to form depends on the balance between the
hydrophilic and the lipophilic properties of the emulsifier - alkali-
metal soaps favour the formation of O/W emulsions because they are
more hydrophilic than lipophilic, whereas the reverse holds for
heavy-metal soaps.
The generalisation that 'the phase in which the emulsifying agent is
the more soluble tends to be the dispersion medium' is known as
Bancroft's rule.
The amphiphilic nature of many emulsifying agents (particularly
non-ionic surfactants) can be expressed in terms of an empirical scale
of so-called HLB (hydrophile-lipophile balance) numbers^222 (see
Table 10.1). The least hydrophilic surfactants are assigned the lowest
HLB values. Several formulae have been established for calculating
HLB numbers from composition data and they can also be determined
experimentally - e.g. from cloud-point measurements^123 '^125. For
mixed emulsifiers, approximate algebraic additivity holds.
The optimum HLB number for forming an emulsion depends to
some extent on the nature of the particular system. Suppose that 20
per cent sorbitan tristearate (HLB 2.1) plus 80 per cent polyoxyethylene
sorbitan monostearate (HLB 14.9) is the optimum composition of a
mixture of these emulsifiers for preparing a particular O/W emulsion.
The HLB of the mixture is, therefore, (0.2 x 2.1) + (0.8 x 14.9) =
12.3. The theory is that an HLB of 12.3 should be optimum for the