Molecular dynamics simulation of surfactant effects on ion transport through a liquid-liquid interface between partially miscible liquids

Molecular dynamics (MD) simulations of water + 1-hexanol + NaCl mixtures with and without a surfactant (methanol) were performed to analyze the surfactant's effect on the transport of a sodium ion through the liquid-liquid interface. Without surfactant, the 1-hexanol forms a bilayer at the interface with OH groups directed outward toward the aqueous phase. Addition of the surfactant produces higher concentrations of the surfactant on the aqueous side of the interface without altering the organic bilayer structure. An electrical double-layer is created in both cases as chloride ion concentration is enhanced near the interface and sodium ion concentration is enhanced toward the center of the water phase. A potential of mean force (PMF) was calculated for the transfer of a sodium ion through the interface. Although the surfactant reduced the interfacial tension, the total work required for the ion transfer increased with the addition of the surfactant.