HOMOCOORDINATION AND HETEROCOORDINATION IN NONADDITIVE HARD-SPHERE MIXTURES AND A TEST OF THE VAN-DER-WAALS ONE-FLUID MODEL

An equimolar mixture of hard spheres with equal diameter d, having an unequal collision diameter d,2 between unlike species, favors a heterc oordinated arrangement if d12 lies between 0.65d and d, and a homocoor dinated packing below d12=0.65d. At d12<0.65d, hard spheres of one typ e move increasingly unaffected by the presence of hard spheres of the other type. This conclusion was drawn from Monte Carlo (MC) simulation s over large ranges of density and d12 and originates in the fact that collisions between like-species at d prevent collisions of unlike-spe cies at shorter distance d12<d. The observed shift of hetero- to homoc oordination affects equation-of-state data as well. Additionally, a te st of the van der Waals one-fluid model of mixtures by the Monte Carlo data shows that this model significantly overestimates the pressure a t high density, and hence, is less suitable for nonadditive hard spher e mixtures. As expected, the MC data suggest a fluid phase separation at d12>d.