Theoretical Study of Phase Transition Behaviour in Mixed Biopolymer + Surfactant Interfacial Layers Using the Self-Consistent-Field Approach - Food Colloids: Self-Assembly and Material Science - page 246

closely associated with disordered proteins like b-casein, involves the ability of
such biopolymers to act as effective steric stabilizers, preventing close approach
of droplets through the provision of long-range repulsive forces. 1
The different roles played by protein stabilizers and low-molecular-weight
emulsifiers necessitates that both type of molecules be present in many food
colloid formulations. This gives rise to some more complex types of behaviour,
the study of which has attracted much interest in recent years. 5-10 The asso-
ciation and formation of complexes between surfactant molecules and bio-
polymers can have important implications for the surface activity, the net
charge, and the subsequent functionality of both kinds of species. For example,
it has been reported 9 that sodium caseinate forms complexes with certain
surfactants at moderately low pH values ( 5.5), and that this increases the

foam stabilizing properties of the protein. While some surfactants interact
favourably with protein, there are others, mostly non-ionic water-soluble
surfactants, which do not show any tendency for association 11 and may even
exhibit a degree of incompatibility in bulk solution. Such surfactants compete
with the amphiphilic biopolymers for coverage of the interface. At typical
concentrations of practical interest in foods, the competitive adsorption leads
to displacement of protein from the surface, which is sometimes followed by
undesirable destabilization and break up of the emulsion, 6,12 particularly if the
system is sheared.
Due to the relatively large size of protein molecules and their high adsorption
energies, their displacement from the interface by surfactant species is a rather
slow process. It is therefore reasonable, at any stage during the displacement
process, to view the interfacial layer as one with a fixed degree of coverage by
the biopolymer. In contrast, the surfactant molecules can quickly exchange
between the bulk solution and the interface. Therefore the amount of adsorbed
surfactant is determined by the equilibrium conditions. In turn, these condi-
tions are dictated by the bulk concentration of surfactant and the amount of
polymer still remaining on the film at any given time.
We have recently considered 13 the phase separation behaviour in binary
interfacial layers in which either one or both components maintains equilibrium
with the bulk solution. It was concluded that, for the latter case, no matter how
unfavourable are the interactions between the species, no phase separation can
occur and the composition of the surface will remain homogeneous. However,
in the former situation, where the surface coverage of one of the components
remains fixed, a certain degree of incompatibility between the two species is
sufficient to induce surface phase separation. 13 These predictions were based on
a simple lattice model which took no account of the internal configurational
degrees of freedom of the molecules. For biopolymers, however, the configu-
rational entropy is expected to make an important contribution to the phase
properties of the mixed layer. A number of theoretical studies have attempted
to overcome this problem, by assuming a small number of finite states, each
with a different degree of surface area coverage, that the chains may adopt. 14,15
In these studies the interactions between surfactant and biopolymers have
generally been assumed to be net attractive.

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