Binary non-additive hard sphere mixtures: fluid demixing, asymptotic decay of correlationsand free fluid interfaces

Abstract

Using a fundamental measure density functional theory we investigate both bulk andinhomogeneous systems of the binary non-additive hard sphere model. For sufficiently large(positive) non-additivity the mixture phase separates into two fluid phases with differentcompositions. We calculate bulk fluid–fluid coexistence curves for a range of size ratios andnon-additivity parameters and find that they compare well to simulation results from theliterature. Using the Ornstein–Zernike equation, we investigate the asymptotic, , decay of the partial pair correlation functions,gij(r). At low densities a structural crossover occurs in the asymptotic decay between two differentdamped oscillatory modes with different wavelengths corresponding to the two intra-specieshard-core diameters. On approaching the fluid–fluid critical point there is a Fisher–Widomcrossover from exponentially damped oscillatory to monotonic asymptotic decay. Using thedensity functional we calculate the density profiles for the planar free fluid–fluid interfacebetween coexisting fluid phases. We show that the type of asymptotic decay ofgij(r) not only determines the asymptotic decay of the interface profiles, but is also relevant forintermediate and even short-ranged behaviour. We also determine the surface tension of thefree fluid interface, finding that it increases with non-additivity, and that on approachingthe critical point mean-field scaling holds.