Binary Gaussian core model: Fluid-fluid phase separation and interfacial properties

Using a mean-field equation of state we calculate the density-concentration phase diagrams for a binary mixture of repulsive Gaussian core particles over a range of size ratios. A simple mean-field density functional (DFT) approach, equivalent to the random phase approximation, is used to calculate the surface tension and density profiles of the interface between the demixed fluid phases of the binary mixture. For certain coexisting states oscillations are found in the density profiles on both sides of the interface, i.e., approaching both bulk phases. The form of the oscillations is determined by the asymptotic decay of the bulk total pairwise correlations, and the onset of oscillations in the interfacial density profiles depends on the location of the crossover line (Fisher-Widom line) in the bulk phase diagram where the asymptotic decay changes from monotonic to damped oscillatory. For certain particle size ratios we find another crossover line that separates a region of the phase diagram where the longest-range decay of the pairwise correlations is damped oscillatory from a region where the longest-range decay is damped oscillatory but with a different wavelength. We argue that many of the predictions of the simple DFT approach should remain valid in more refined treatments.