Anisotropic compressibility in inhomogeneous fluids

Abstract

Compressibility has a clear meaning in bulk liquids, but at the interface between two fluids, compression can alter local molecular packing differently along the surface normal and parallel to the interfacial plane. Here, we introduce a linear-response approach to measure this directional mechanical response in inhomogeneous fluids. The response has a scalar part that recovers the usual macroscopic compressibility and deviatoric components that reveal how compression is distributed between normal and lateral molecular environments. These fluctuation formulas provide a practical route to quantify interfacial softness and directional compressive modes directly from equilibrium simulations. We apply the method to the water/carbon tetrachloride interface in molecular dynamics simulations and show that the interface has a strong local anisotropic response together with a positive scalar surface excess response. The resulting interfacial softening can significantly modify the apparent compressive response of water confined by organic liquids at thicknesses relevant to thin films and multilamellar vesicles.