Physical Chemistry of Foods

1. If thesurface fractioncoveredyis appreciable, large deviations
   from Eq. (10.4) occur. The discrepancy is especially large for large
   molecules, because the value ofG(to be expressed in moles per m^2 ) remains
   very small. In first approximation, the correction term would be (1
   y)
   ^2 ,
   leading to

P¼

GRT

ð 1 
yÞ^2

y¼pr^2 NAVG

ð 10 : 5 Þ

whereris the radius of a surfactant molecule in the plane of the interface.
This equation is especially useful for globular proteins as surfactants. As an
example the relation for lysozyme, a hard globular protein of 14.6 kDa, is
given in Figure 10.8, and it appears that the correction for finite surface
fraction is overriding. (The value calculated forrequals about 1.6 nm, in
good agreement with the radius of the molecule in solution, about 1.7 nm.)

2. There often is anet interaction energybetween the surfactant
   molecules in the interface, and if the interaction is attractive,Pwill be
   smaller (at the sameG); if it is repulsive,Pwill be higher. The former
   appears to be the case for Na-stearate (Figure 10.8). For most small-
   molecule surfactants, the attractive interaction appears to be smaller at the

FIGURE10.8 Surface equations of state, i.e., the relation between surface pressure
(P) and surface excess (G) for Na-dodecyl sulfate (SDS), Na-stearate (C18), and
lysozyme at the A–W interface. Also the relations according to Eq. (10.4) are given
(broken lines).