Unexpected Effects of Surfactant Adsorption Upon Water/Vapor Surface Geometry and Fluctuations

Exploring the effects of adsorbed surfactants surface geometry has is fundamental to various disciplines ranging from separations to catalysis. This study examines the influence of surfactant adsorption on surface geometric descriptors relevant to capillary wave fluctuations at the water/air interface using a coarse-grained instantaneous representation of the surface. Unexpected relationships are observed relative to that found in liquid/liquid interfaces of pure solvents and the pure water/vapor surface. In the case of concentration dependent tributyl phosphate adsorption, the surface develops sharper and less broad waves with increased TBP until a critical concentration in agreement with TBP self-assembly is reached. Thereafter the convexity and concavity of the surface stop cancelling each other. As the alkyl groups on the phopsphate head group are shortened (to form triethyl, tripropyl and trimethyl phosphate), the surface activity necessarily decreases. Yet the ability of the surfactant to increase surface fluctuations is dramatically increased.