A Disjoining Pressure Study of Foam Films Stabilized by Mixtures of Nonionic and Ionic Surfactants
journal contributionposted on 08.05.2007, 00:00 by Natalie Buchavzov, Cosima Stubenrauch
Studying the disjoining pressure Π as a function of the film thickness h (Π−h curves) of foam films stabilized by ionic and nonionic surfactants, one finds that the surface charge density q0 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 β-dodecylmaltoside (β-C12G2) and the cationic dodecyl trimethylammonium bromide (C12TAB) with mixing ratios of β-C12G2/C12TAB = 1:0, 50:1, 1:1, 1:50, 0:1. The addition of small amounts of C12TAB to β-C12G2 first neutralizes the negative surface charge of the β-C12G2 films and finally leads to a charge reversal from negatively to positively charged surfaces. On the other hand, by adding small amounts of β-C12G2 to C12TAB, one observes the formation of stable CBFs which was also observed for the pure C12TAB. However, in contrast to the pure C12TAB, the resulting Π−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.