Physical Chemistry of Foods

lowest value ofc, because transport of surfactant by diffusion is slower, as it
has to occur over a greater distance. The dependence ofESDon surfactant
concentration is, for a constant value oft, like that in Figure 10.35.
The second case concerns athin film. Here the change inPis governed
by the limited amount of surfactant in the bulk liquid in the film, since by far
most of the surfactant will be in the adsorbed layers. The time for
diffusional transport (normal to the surfaces) is taken to be negligible in the
thin film; it would nearly always be<0.1 s. The modulus then is purely
elastic. Rather than the modulus, theGibbs elasticityof the film is given, by

EG¼ 2

dP=dlnG

1 þðd= 2 Þdc=dG

ð 10 : 23 Þ

FIGURE10.35 Gibbs elasticityEGof films made of solutions of SDS of various

concentrationsc, for two film thicknesses (indicated). The broken line gives an

approximate example of results to be expected for a mixture of surfactants. (Adapted

from J. Lucassen, in E. H. Lucassen-Reynders (Ed.). Anionic Surfactants. Surfactant

Series Vol. 11. Marcel Dekker, New York, 1981, p. 232.)