Understanding ion-surfactant interactions are important for many chemical processes, including separations. At aqueous interfaces, cationic surfactants can induce anion adsorption through electrostatic interactions. We investigate the structure of thiocyanate (SCN-) anions at an interface with a monolayer of the cationic surfactant 1,2-dipalmitoyl-3-trimethylammonium-propane. We combine surface-sensitive techniques, including sum frequency generation (SFG) spectroscopy, with molecular dynamics simulations to show how SCN- adsorption, and orientation depends on anion concentration. Simulations provide a microscopic picture of the interface, which can resolve the spatial and orientational contributions from ions in different interfacial layers to facilitate the interpretation of experimentally observed spectroscopic features. Understanding the balance between anion hydration and ion-ion interactions, including with both the surfactant and, at high concentration, other surface-adsorbed anions will help the future development of liquid-liquid separations processes of anionic species.

Probing anion adsorption at a charged surfactant monolayer interface with sum frequency generation spectroscopy and molecular dynamics simulations

ACS Spring 2022

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