Gas-liquid coexistence and demixing in systems with highly directional pair potentials

Abstract

Recent computer simulation studies strongly indicate that fluids of dipolar hard spheres do not display gas--isotropic liquid coexistence. In this paper we discuss a second example that also exhibits this rather unexpected behavior. This is a simple liquid-crystal model that we explore employing Gibbs ensemble Monte Carlo (GEMC) methods. It is shown that the system has clear gas--nematic liquid coexistence, but that the gas--isotropic liquid coexistence line is completely missing from the phase diagram. We attribute this to the highly directional nature of the attractive potential and argue that similar considerations are likely of relevance in the dipolar hard-sphere case as well. We also use GEMC techniques to investigate demixing in binary mixtures of neutral and dipolar hard spheres. For similar mixtures of neutral and charged hard spheres, it is known that demixing is essentially condensation of the Coulombic fluid weakly influenced by the background of neutral hard spheres. Therefore, given that dipolar hard spheres do not condense, whether or not the present mixtures demix is an interesting question. In fact, demixing is observed and, moreover, the transition temperatures are in reasonable agreement with those predicted by the same integral equation theories that incorrectly predict condensation of the pure dipolar fluid. The critical temperature decreases rapidly with decreasing diameter of the neutral species consistent with the lack of gas--isotropic liquid coexistence for pure dipolar hard spheres. Clearly, for the present model demixing and dipolar condensation are not closely related phenomena as they are in the Coulombic systems. The neutral species appears to reduce the formation of dipolar ``chains'' or ``clusters'' that inhibit condensation of the pure dipolar hard-sphere fluid.