Molecular origins of capillary wave structure and interfacial viscosity at surfactant-laden oil–water interfaces

Capillary wave theory has been employed to study the concentration dependent effects of surfactants upon the spatial structure and temporal dynamics of surfactant laden oil-water interfaces. Surface concentrations up to a monolayer of trioctyl phosphine oxide and tributyl phosphate were examined at the water-hexane interface. We demonstrate that changes to capillary wave modes are consistent with the perturbations to the surface geometry (orientation and curvature) induced by surfactant-surfactant interactions and disruption of interfacial water hydrogen bond networks. The surfactant induced perturbations have the largest effect on the long wavelength capillary modes in terms of the amplitudes; however, curvature changes are due to the coupling of small wavelength modes, resulting in the increase in curvature and inducing long lived structures on the interface. As a result, the impact of the surfactants upon the characteristic relaxation times of the surface is pronounced for the short wavelength modes. Additionally, for each surfactant studied, there is a critical surface coverage value, related to the formation of water-bridged surfactant pairs, at which the interfacial dynamics dramatically slow down, leading to an increase in the interfacial surface viscosity.