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 249

biopolymer chains at the interface is constant, the segments of the polymers in
contact with the solid surface can vary greatly as chains adopt different
configurations. The w parameter for the unfavourable interactions between
hydrophobic monomers of the polymer and surfactant species is set to 4 kT
here. It should be mentioned that the presence of such a strong interaction,
between hydrophobic groups of two different species, is unlikely to arise
directly in food systems. However, in our recent Brownian dynamics simulation
study, we showed 23 that the effect of the formation of strong (yet reversible)
bonds between some segments on one set of molecules, but not the other, is
effectively similar to the presence of a strong repulsion between the two species.
We have attributed 23 the observed surface phase separation in mixed biopol-
ymer layers, as reported 24 for some combinations of adsorbed protein mole-
cules, to the presence of such an effective repulsion between the different
species.
16.3 Results and Discussion
16.3.1 Adsorption Isotherms
We begin the discussion of the results by presenting our calculated adsorption
curves for the surfactant molecules, as they exchange between the bulk and an
interface containing a fixed amount of biopolymer. As an example, we take a
case where the amount of biopolymer in the entire interfacial layer is set to
0.005 chains per unit monomer area (a 0 ). Figure 1 shows the variation in the
volume fraction of segments belonging to the surfactant molecules, G c ,inthe
first layer in direct contact with the solid surface. This is plotted against the
bulk concentration of surfactant c s . At low values of the surfactant bulk
concentration (c s o 0.0265), the number density of surfactant segments in
contact with the surface is seen to increase smoothly (the solid line), following a
typical adsorption isotherm. However, as the bulk concentration reaches the
value of 0.0265, we find that the amount of the adsorbed surfactant shows a
sudden jump, with the value of G c changing abruptly from a relatively low value
of 0.125 to a much higher value of 0.614. This discontinuous change is indicated
by the arrow in Figure 1. For c s > 0.0265, the curve is once again smooth,
increasing monotonically with surfactant concentration. Associated with this
change in G c , we also observe a similar corresponding abrupt drop in the
amount of biopolymer in contact with the solid surface. This drop occurs at
precisely the same surfactant bulk concentration of 0.0265. This is shown in
Figure 2, where the volume fraction of the polymer chains, G c , in the layer in
direct contact with the wall, is plotted against c s . The value of G c at c s ¼ 0.0265
falls from 0.452 down to a much lower value of 0.0096. This is interpreted as
signalling a sudden change in the configuration of the chains, from one with
chains originally laying flat on the surface to one with chains extending into the
bulk. These results are not unique to the particular case considered here. We
have found that they can also be reproduced for a variety of other similar
systems, with somewhat different interaction strengths, adsorption energies,