Drainage of foam films stabilized by n-dodecyl-β-d-maltoside or dodecyl trimethylammonium bromide and their mixtures

The thinning of foam films from aqueous solutions of single non-ionic- [n-dodecyl-β-d-maltoside (β-C12G2)] and ionic-[dodecyl trimethylammonium bromide (C12TAB)] surfactants, as well as of their mixtures at Ct = CMC, was studied at different molar ratios (50:1; 1:1; 1:50), with or without additional electrolyte (0.1 M NaBr). The equilibrium surface tension isotherm of the mixture β-C12G2:C12TAB = 1:50 (+0.1 M NaBr) was obtained by the Wilhelmy-plate method. Based on this isotherm and the isotherms of the individual surfactants, CMC of the other mixtures with electrolyte were determined theoretically, by the Ingramʹs approach. The molar fractions of the surfactants in the mixed adsorption layers were estimated. ilm thickness vs. time’ dependences were plotted for films with radii 0.05 mm or 0.1 mm. The results were compared to those obtained by different theoretical equations for the rate of film thinning. It was found that the small films from β-C12G2 or mixtures β-C12G2:C12TAB = 1:1; 50:1 (+0.1 M NaBr) exhibit regular thinning which is almost in line with the classical Reynolds equation (Re). It was confirmed by the values of fractal dimension: α = 2. In contrast, the small films of β-C12TAB or mixture β-C12G2:C12TAB = 1:50 without NaBr drained at slower rate than that predicted by the Reynolds equation. We assume that it is due to the effect of electrokinetic streaming potential, which decreases the velocity of the film drainage because of the reverse fluxes from the periphery toward the center of the film. Films with radii 0.09 mm thinned almost (or slower) according to Manev–Tsekov–Radoev (MTR) equation. The value of α obtained for these films was equal to 0.6–1.4 keeping in mind that α = 0.5 corresponds to MTR type of film drainage. This result confirms the influence of the film size on its non-homogeneity.