Analysis of the Asymmetric Synergy in the Adsorption of Zwitterionic-Ionic Surfactant Mixtures at the Air-Water Interface below and above the Critical Micelle Concentration

Surface tension (ST) and neutron reflection (NR) measurements have been made on a series of mixtures of two ionic surfactants, one anionic (SDS) and one cationic (C(12)TAB), with the two zwitterionic surfactants dodecyldimethylammonium propanesulfonate (C12SB) and dodecyldimethylammonium acetate C12CB. The anionic surfactant SDS interacts equally strongly with both zwitterionics and the C(12)TAB less strongly. For the SDS-C12SB mixtures simultaneous fitting of ST and NR data made it possible to use the pseudophase approximation with an expansion of the excess free energy, G(E), up to and including the quartic term. G(E) is asymmetric for the adsorbed layer and the minimum occurs at a surface mole fraction, x(SDS), of 0.38 with a depth of -2.8RT. NR was also used to follow the adsorption above the CMC, and the changes showed that the intramicellar interaction is more asymmetric, but weaker than the surface interaction with a depth of G(E) of -2.2RT at the minimum of x(SDS) = 0.23. A strong synergy in the total surface excess was observed above the mixed CMC with an enhancement over the adsorption of the pure components of about 45%, which appears to result from a sharp variation of the packing with surface composition coupled with the effects of the strongly asymmetric micellization. NR data on SDS-C12CB showed that G(E) for both surface and micelles was similarly asymmetric to those for SDS-C12SB, but there is no strong synergy in adsorption. This is attributed to the more rigid headgroup. C(12)TAB-C12SB has an asymmetric G(E) for both surface and micelle similar to those for SDS-C12SB but the depths are smaller at -0.6RT and -0.5RT, respectively, and there is no synergy in the total adsorption.