10.3.3 Mixtures
In this section mixtures of amphiphiles and proteins are considered. As seen
in Figure 10.13, an amphiphile tends to give a lower interfacial tension than
a protein does, if the concentration of the amphiphile is high enough.
Suppose that an aqueous solution contains both protein and amphiphile in
about equal mass concentrations. If the surfactant concentrations are on the
order of the CMC of the amphiphile, one would expect the amphiphile to
predominate in the interface (A–W or O–W), possibly even displacing all of
the protein, because that would give the lowest free energy in the system. At
concentrations far below the CMC of the amphiphile, the protein would
dominate in the interface. Such phenomena do indeed occur.
Figure 10.15 illustrates what happens if an emulsion made with a
protein (b-casein) is subjected to increasing concentrations of anionic
FIGURE10.15 Surface excess (G) in an O–W emulsion and interfacial tension (g)
at the O–W interface forb-casein as a function of the concentration of Na-dodecyl
sulfate (SDS) present.gis also given for SDS only. (From results by J. A. de Feijter
et al. Colloids Surfaces 27 (1987) 243.)