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.