and for the amphiphile of the order of 1kBT. Consequently, the affinity of
the protein for the surface is far greater than that of the amphiphile.
On the other hand, the value ofPreached is clearly larger for the
amphiphile, provided that the surfactant concentration is high enough. The
explanation must be that for a polymer, and hence a protein, a very dense
packing of surfactant material at the interface cannot be reached. This is in
accordance with the observation that for most polymers and for most
amphiphiles the surface excesses expressed in unit mass are roughly the
same: a few mg?m^2 , despite the ‘‘thickness’’ of the polymer layer being
clearly higher for a polymer (say, 10 nm) than for an amphiphile (about
2.5 nm).
For some proteins at some conditions,multilayer adsorptioncan occur,
as indicated by a dotted curve forG/G?; a second layer is very weakly
FIGURE10.13 Surface pressure (P) and surface excess (G; the plateau valueG?is
indicated near the curves) at the triglyceride oil–water interface as a function of the
concentration in solution (c) for a protein (b-casein) and an amphiphile (SDS). The
value ofgwithout surfactant&30 mN?m^1.