Homo- and heterocoordination in nonadditive hard-sphere mixtures and a test of the van der Waals one-fluid model

Abstract

An equimolar mixture of hard spheres with equal diameter d, having an unequal collision diameter d12 between unlike species, favors a hetercoordinated arrangement if d12 lies between 0.65d and d, and a homocoordinated packing below d12=0.65d. At d12<0.65d, hard spheres of one type move increasingly unaffected by the presence of hard spheres of the other type. This conclusion was drawn from Monte Carlo (MC) simulations over large ranges of density and d12 and originates in the fact that collisions between like-species at d prevent collisions of unlike-species at shorter distance d12<d. The observed shift of hetero- to homocoordination affects equation-of-state data as well. Additionally, a test of the van der Waals one-fluid model of mixtures by the Monte Carlo data shows that this model significantly overestimates the pressure at high density, and hence, is less suitable for nonadditive hard sphere mixtures. As expected, the MC data suggest a fluid phase separation at d12≳d.