activity, likeas1-casein andb-casein, can displace each other within about
15 min when the one is present at the surface of emulsion droplets and the
other is added to the emulsion afterwards. It appears likely that a flexible
polymer is not desorbed at once, but that one segment (or a short train of
segments) at the interface is displaced by one of another molecule at the
time, etc. This would not be possible for globular proteins adsorbed as
depicted in Figure 10.12c; indeed, the evidence for fairly rapid exchange or
mutual displacement of these proteins is less convincing. Desorption rate is
further discussed in Section 10.4.
The surface equation of state of proteins is highly nonideal, as
mentioned before: see, e.g., Figure 10.8, lysozyme. Equation (10.5) indicates
that the expansion of a molecule in the interface (larger radius) would lead
to a larger value ofyand hence to a higher value ofP. It also makes a
difference onto what surface the protein is adsorbed, as illustrated in Figure
10.14. Presumably, the diversity is due to differences in the amount of
surface area occupied by each protein molecule in the interface, which will,
in turn, depend on the state of unfolding of the peptide chain. For instance,
FIGURE10.14 Relation between surface pressure (P) and surface excess (G) for
two proteins at the A–W and O–W interfaces. The broken line is for predenatured
lysozyme at the A–W interface. (From results by D. Graham, M. Phillips. J. Colloid
Interf. Sci. 70 (1979) 403, 415.)