A disjoining pressure study of foam films stabilized by mixtures of nonionic and ionic surfactants

Studying the disjoining pressure Pi as a function of the film thickness h (Pi-h curves) of foam films stabilized by ionic and nonionic surfactants, one finds that the surface charge density q(0) of films stabilized by ionic surfactants increases with increasing surfactant concentration, while the opposite holds true for nonionic surfactants. Thus, it should be possible to tune the surface charge density with mixtures of nonionic and ionic surfactants. To address this question, we studied foam films stabilized by aqueous solutions of surfactant mixtures. The mixtures consisted of the nonionic beta-dodecylmaltoside (beta-C(12)G(2)) and the cationic dodecyl trimethylammonium bromide (C(12)TAB) with mixing ratios of beta-C(12)G(2)/C(12)TAB = 1:0, 50:1, 1:1, 1:50, 0:1. The addition of small amounts of C(12)TAB to beta-C(12)G(2) first neutralizes the negative surface charge of the beta-C(12)G(2) films and finally leads to a charge reversal from negatively to positively charged surfaces. On the other hand, by adding small amounts of beta-C(12)G(2) to C(12)TAB, one observes the formation of stable CBFs which was also observed for the pure C(12)TAB. However, in contrast to the pure C(12)TAB, the resulting Pi-h curves for the mixtures cannot be described with the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory; the slope of the curves is too steep, and it barely changes with changing electrolyte concentration. A possible explanation for this observation will be given and discussed.