Adsorption Process of Binary Surfactant Aqueous Solutions Studied by Dynamic Surface Tension Measurements

We investigated the dynamics of surface tension in binary nonionic surfactant mixtures in aqueous solutions using a custom-made sessile bubble tensiometer. When the bubble surface is initially saturated with tetraethyleneglycol monooctylether (C8E4), adding a second surfactant, octyl-β-D-maltopyranoside (OM), to the adsorbed film results in a straightforward, gradual decrease in surface tension. Conversely, if the surfactants are introduced in reverse order, the surface tension exhibits an overshoot or oscillation over time, depending on the relative concentrations of C8E4 and OM. This pattern mirrors those previously observed in C8E4 - dodecyltrimethylammonium bromide (DTAB) and tetradecyltrimethylammonium bromide (TTAB) - DTAB systems. The surface tension overshoot and oscillation phenomena are explained by the differences in the critical micelle concentrations (i.e., surface activity) of the surfactants. We also analyzed the observed surface tension behaviors using the theory developed for the adsorption kinetics of higher alcohols on lipid-insoluble monolayers. Comparing our findings with those in lipid-insoluble monolayers, it was suggested that the elasticity of the initially formed adsorbed film is another crucial factor in determining whether surface tension oscillation or overshoot occurs.