Vapor-Liquid Interface in Ionic Fluids*

One of the foremost unsolved problems in the field of electrolyte solution theory is the accurate calculation of interfacial properties in situations where spatially varying dielectric permittivity, large concentration gradients, and strong ion-ion correlations are prevalent. A solution to this problem has applications ranging from prediction of surface tension of water-air/oil interface to double-layer structure inside nanopores and highly charged surfaces, among many others. Currently, such problems are handled using a phenomenological approach, non-local density functional theory, or liquid-state theory. All these methods require approximations that have not been verified. Here, through the example of the vapor-liquid interface in ionic fluids, we demonstrate the application of a new theory within the Gaussian-fluctuation framework which can be non-perturbatively and self-consistently solved for interfaces with a large gradient in ionic strength. The theory systematically captures the anisotropic electrostatic interactions and the spatially varying ion correlation in the vicinity of the interface. We show results for the interfacial structure for both symmetric and asymmetric electrolytes. For symmetric salts, the surface tension predicted by our theory is quantitatively in agreement with the simulation data. Furthermore, our work also provides the first calculation of the concentration distribution and electrostatic potential profiles between vapor and liquid phases for asymmetric salts without any approximations.