Crowding effects in binary mixtures of rod-like and spherical particles

Crowding effects relevant to the phase stability of binary mixtures of rod-like and spherical particles are investigated by means of Monte Carlo simulations in the isobaric NPT ensemble. The two types of particles are represented, respectively, by freely rotating hard spherocylinders of a moderate aspect ratio (L/sigma = 5) and hard spheres of the same diameter sigma. Molar fractions of spheres ranging x(HS) = 0.00-0.37 are considered with the aim of characterizing the crowding effects on the liquid crystal phases of the hard spherocylinder fluid induced by the spherical component as depleting agent. We find that the addition of the spherical crowder is beneficial for the stabilization of the layers of the rod-like particles characteristic of the smectic phase. On the contrary, the addition of spheres has a negative impact upon the stability of the nematic phase, where the rod-like particles tend to align collectively parallel to each other. Interestingly, the spheres tend to arrange forming rod-like clusters in the nematic phase and lamellar structures in the smectic phase, which is compensated by the entropy gained by the spherocylinder particles in each phase. The main results are in qualitative agreement with recent experimental and theoretical studies and serve to test the prediction of current equations of state for these types of binary mixtures. Copyright (C) 2004 John Wiley Sons, Ltd.