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 252

Figure 4 As Figure 1, but now the bulk volume fraction of surfactant molecules is kept
fixed at 0.0265, with the number of biopolymer chains at the interfacial region
being made to vary
identical. As the surfactant concentration is further increased above 0.0265,
however, it is now the high-coverage surfactant phase that has the lower
free energy and is therefore the stable phase. These results are precisely those
one would expect arising from the existence of a first-order phase transition in
the system, with the volume fraction of surfactant in the first layer in contact
with the solid surface acting as a measure of the order parameter for this
transition.
During the gradual displacement of protein by surfactant in food systems, it
is the amount of biopolymer at the interface that slowly decreases. It is
therefore useful to consider a slightly different situation in which the bulk
concentration of the surfactant is kept constant and the number of chains at
the interface is altered. The results of such an exercise, for the same biopolymer
and surfactant system as the one above, are displayed in Figure 4. The
bulk concentration of surfactant is now fixed at 0.0265. Starting from a high
surface coverage of protein molecules, as the amount of protein on the
surface decreases, the number of surfactant segments in contact with the wall
increases smoothly at first. However, as the number of chains per unit mon-
omer area (a 0 ) reaches the transition value of 0.005, once again there is a
rapid and abrupt jump in the value of G c from 0.13 to 0.61. The above
result indicates that the displacement of protein molecules from the interface
may be accompanied by configurational and structural phase transitions in the
mixed monolayer, involving the sudden large uptake of surfactant mole-
cules. Phase transition behaviour of this type also has further consequences
for the emergence of phase-separated surface domains, resulting in a hetero-
geneous film if the protein molecules possess a reasonable degree of lateral
mobility.