Food colloids also contain surfactants (emulsifiers),
which compete with proteins for space at fluid interfaces.
The review by Miller et al. (pp. 350–356) describes
recent progress in modelling the adsorption of proteins
and protein + surfactant mixtures from a classical statistical thermodynamics point of view. Their approach
assumes that proteins exist in a number of different states
with different molar areas, varying from a maximum
value at very low surface coverage to a minimum value at
high surface coverage. The derived surface equation of
state can then describe the equilibrium surface tensions of
protein + surfactant mixtures in terms of the properties of
the pure components. In particular, the competitive
character of the adsorption from the mixture is nicely
demonstrated, with the protein surface coverage falling
systematically with increasing surfactant bulk concentration. By incorporating the surface equation of state and
the adsorption isotherm into the standard theory of
diffusion-controlled adsorption, a kinetic description of
the adsorption process emerges. Considering the inevitable
limitations of this approach, ignoring as it does any
structural inhomogeneities in the adsorbed layers, the
predicted dynamic surface tensions of the mixed solutions
turn out to be in surprisingly reasonable agreement with
experiment.