At sufficiently high electrolyte concentrations (at suppressed electrostatic repulsion), the free foam films thin gradually, until reaching a certain critical thickness, and then they break. The value of this critical thickness is sensitive to the magnitude of the attractive surface forces acting in the film. We experimentally investigated the rupture of the films formed from aqueous solutions of sodium dodecyl sulfate (SDS) in the presence of 0.3 M NaCl added. The theoretical fits of the data indicate that the van der Waals interaction, alone, is insufficient to explain the measured critical thickness, especially for the lower SDS concentrations. If the difference is attributed to the hydrophobic attraction, then a very good agreement between the theory and experiment is achieved. From the best fit, we determine that the decay length of the hydrophobic force is about 15.8 nm, which coincides with the value obtained by other authors for hydrophobized mica surfaces. The strength of the hydrophobic interaction increases with the decrease of the SDS concentration, which can be explained with the fact that between the adsorbed surfactant molecules greater areas of bare hydrophobic air-water interface are uncovered. In the investigated concentration range, the strength of the hydrophobic force is found to be inversely proportional to the surface density of the adsorbed ions.

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The surface tension isotherms of dodecyl acid diethanole amide (DADA), sodium dodecyl sulfate (SDS) and their mixtures at different molar ratios are measured by the Wilhelmy-plate method. The isotherms are analyzed by the theory of nonideal interactions in binary surfactant mixtures (NIBM) and by the nonideal multicomponent mixed micelle model (NMMM). The molecular interaction parameters (MIP) on the interface, β S , and in the micelles, β M , are determined by the NIBM method. By using the Ingram's modification of this method, we calculate the compositions of the mixed adsorption layers and of the micelles, as functions of DADA and SDS concentrations both below and above the critical micelle concentration (CMC). It is shown that the DADA molecules prevail on the surface and in the micelles (>50%) at concentrations around CMC even when DADA presents a small fraction in the surfactant mixture (5 mol%). This result is explained by the much higher surface activity of DADA in comparison with SDS. At concentrations, which are about 1 order of magnitude above the CMC, the composition of the micelles approaches the composition of the total surfactant mixture. The composition of the surfactant adsorption layer also becomes much closer to the composition of the total surfactant mixture at concentrations well above the CMC. The results for the micelles composition at moderate and high molar fractions of DADA in the surfactant mixture (above 10 mol%), obtained by NMMM and by NIBM methods, agree well with each other. A discrepancy between the results obtained by these two models is found at low DADA fraction in the mixture, and this discrepancy is explained by the presence of traces of dodecanol (DOH) in the used SDS sample.

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