Density profiles at liquid–vapor and liquid–liquid interfaces: An integral equation study

Abstract

The structure of liquid–vapor and liquid–liquid interfaces in Lennard-Jones (LJ) fluids and mixtures is studied using integral equations. To obtain density distributions at interfaces between coexisting fluid phases we solve the Lovett–Mou–Buff–Wertheim equation. In this equation we approximate the direct correlation functions of the inhomogeneous fluid via interpolation between the direct correlation functions of the bulk phases. In the homogeneous bulk phases the system of the Ornstein–Zernike equation with the reference-hypernetted-chain closure is solved to obtain the direct correlation functions at coexisting densities. Density distributions and other interfacial properties are studied for a liquid–vapor interface in a pure LJ fluid, in an Ar–Kr mixture and for a liquid–liquid interface between two immiscible LJ fluids. The results are in good agreement with simulations and other theories. At low temperatures the liquid–vapor and liquid–liquid density profiles exhibit oscillating structures with periods near the diameters of the LJ spheres. Being quite weak at liquid–vapor interfaces these oscillations become very pronounced at a liquid–liquid interface between immiscible fluids.