ENTROPY-DRIVE DEMIXING IN BINARY HARD-CORE MIXTURES - FROM HARD SPHEROCYLINDERS TOWARDS HARD-SPHERES

We present a computer simulation study of a binary mixture of hard sph erocylinders with different diameters (D-1<D-2) and the same lengths ( L-1 = L-2 = L). We first study a mixture of spherocylinders with lengt hs L = 15D(2) and D-1 = 0, which can be regarded as a mixture of rodli ke colloids and ideal needles. We find clearly an entropy-driven isotr opic-isotropic (I-I) demixing transition in this mixture. In addition, we study a mixture of spherocylinders with diameter ratio D-1/D-2 = 0 .1 and we investigated the I-I demixing transition as a function of th e length L of the particles. We observe a stable I-I demixing for all values of L in the range of 3 less than or equal to L/D-2 less than or equal to 15, but we could not reach the limit L = 0, i.e., the hard-s phere mixture with diameter ratio of 0.1. Striking agreement is found for L/D-2 = 15 with the results that follow from the second virial the ory for infinitely elongated rods. For L/D-2 = 2, we did not find a de mixing transition till a total packing fraction of eta = 0.581, which is higher than the packing fraction at which freezing occurs for a pur e system of thick rods. Thus this result and the extrapolation of our finite-L data to L = 0 gives us a fingerprint that the fluid-fluid dem ixing transition in the binary hard-sphere mixture with a diameter rat io of 0.1 is metastable with respect to freezing or does not exist at all at densities below close packing.