conditions are equal, except for the phase in which the (small-molecule)
surfactant is present. It is seen that the retardation of the approach is
stronger if the surfactant is in the continuous phase and that it then even
tends to zero velocity at very small distance. This further underpins
Bancroft’s rule.
Note that the rule does not speak of the phase in which the surfactant
is initially dissolved, but that it concerns their solubility ratio, for small-
molecule surfactants given by their HLB value.
Note It concerns solubility in terms of thermodynamic activity,
hence surfactant present in micelles should not be included in the
solubility ratio.If the HLB value is not greatly different from 7, the surfactant can probably
be dissolved in either phase. After agitation has started, most of the
surfactant will readily move to the phase in which it is best soluble. For
HLB&7, it will reach about equal concentrations in both phases; this is
presumably the reason why such surfactants tend to be poor emulsifiers.
Bancroft’s rule also explains why proteins cannot be used as
surfactants to make a W–O emulsion: they are insoluble in oil.
11.4.3 Surface Layers of Proteins
Changes occurring in proteins due to adsorption are discussed in Section
7.2.2, topic ‘‘Adsorption.’’ In Section 10.3.2 some particulars of proteins as
surfactants are given. During emulsion or foam formation with protein(s) as
surfactant, however,no equilibrium adsorptionis attained. In Figure 11.15a
an adsorption isotherm forb-casein at the O–W interface (obtained by slow
adsorption onto macroscopic interfaces at quiescent conditions) is
compared with an apparent adsorption isotherm calculated from emulsions
made (d 32 ¼ 1 mm) at various protein concentrations. It is seen that the
apparent surface activity of the casein in the emulsions is far smaller, the
bulk concentration needed to obtain a plateau value ofG; being about 3
orders of magnitude larger than for quiescent adsorption. In the emulsion
the plateau value ofG; is higher by about 30%.
For small-molecule surfactants, emulsification leads to equilibrium
adsorption. This means that knowledge of the adsorption isotherm,
surfactant concentration, and total droplet surface area permits calculation
ofG;. This is not possible for proteins.
Figure 11.15b gives the same result forb-casein, but now plotted asG;
versus concentration over specific surface area. If emulsions are made at