Adsorption of dimethyldodecylamine-N-oxide at the mica-solution interface studied by ellipsometry

The surface excess of dimethyldodecylamine-N-oxide (DDAO) surfactant at the mica–water interface has been determined using phase modulated ellipsometry. Surface excess versus bulk surfactant concentration isotherms were constructed at different pH values. The plateau adsorbed amount reaches a maximum in the pH range 6–7.5, which includes the native pH of DDAO solutions at the cmc (pH 7.5). At low pH values, where the surfactant is essentially cationic, the surface excess is reduced due to decrease of the mica surface charge, and due to that electrostatic repulsion between charged headgroups prevents formation of tightly packed surfactant bilayers. Adsorption from solutions at pH 9 also renders a lower plateau surface excess than that obtained at neutral pH. In this case the surfactant–substrate affinity is greatly reduced due to complete deprotonation of the headgroup of the surfactant in bulk solution. The results suggest that the degree of protonation of DDAO in the adsorbed layer differs from its bulk value. Close to the negatively charged mica surface the protonation of the headgroup is increased in order to adjust its charge to that of the surface, whereas the uncharged form is expected to be enriched in the outer part of the bilayer.