Hartmut Dr. Schmidt-Lewerkühne

Adsorption kinetics of surfactant mixtures from micellar solutions as studied by maximum bubble pressure technique.

The adsorption kinetics of micellar solutions of anionic/cationic SDS/DATB mixtures with mixing ratios of 10/1 and 10/2, respectively, are studied experimentally by means of the maximum bubble pressure method. For long adsorption times the adsorption of the highly surface-active anionic/cationic complex leads to a decrease of dynamic surface tension in comparison to the single SDS system. However, the situation is the reverse for short adsorption times where the dynamic surface tension is increased by addition of the cationic surfactant, although the overall concentration is increased. This unexpected behavior is explained by partial solubilization of free SDS molecules into micelles formed by SDS/DTAB complexes. With increasing overall concentration, when eventually the CMC of SDS is reached, the anionic/cationic complex itself is solubilized by SDS micelles. Finally, no complex micelles, which for their part can solubilize an excess of SDS molecules, are present. Hence, the dynamic properties of the solution are no longer influenced by the depletion of SDS molecules and the mixture tends to behave like a pure SDS solution.

Analysis of dynamic surface tension data for SDS–DTAB mixed solutions

From dynamic surface tension data the micelle kinetics dissolution constants for SDS–DTAB mixed solutions of the two components and the highly surface active complex are determined. The experiments at different mixing ratios 1∶10, 1∶1 and 10∶1 allow conclusions about the competitive adsorption of the complex and the respective free component. It turns out that the micelle kinetics constant of dissolution for SDS is about one order of magnitude larger than the respective constant for DTAB, while the respective constant for the complex is again one order of magnitude smaller than that of DTAB. These constants are inversely proportional to the micelle relaxation times. Thus the complex micelles are most stable, while DTAB and SDS micelles are 10 and 100 times less stable, respectively.