Interfacial Properties of a Catanionic Surfactant

Abstract

Interfacial properties of aqueous solutions containing the catanionic surfactant n-hexylammonium n-dodecyl sulfate (AM-DS) are reported. The cmc (1.0 mmol dm-3) and equilibrium surface tensions γeq were measured by DuNouy ring tensiometry. Dynamic surface tensions γ(t) were measured in the range 5 ms to 10 s using the maximum bubble pressure technique. Neutron reflection measurements were used to determine the area per molecule as and the surface excess Γ as a function of concentration, as well as the structure in the monolayer. Data from these experiments were combined to analyze the γ(t) decays in terms of asymptotic solutions to the Ward and Tordai equation. During the initial adsorption period the dynamic surface tensions were all consistent with this diffusion-controlled mechanism. However, toward the end of the process, for concentrations below the cmc, there is some evidence for a weak adsorption barrier. Using measured quantities, the barrier strength was estimated as between 5 and 15 kJ mol-1 at 25 °C.