GAS-LIQUID COEXISTENCE AND DEMIXING IN SYSTEMS WITH HIGHLY DIRECTIONAL PAIR POTENTIALS

Recent computer simulation studies strongly indicate that fluids of di polar 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 ex plore employing Gibbs ensemble Monte Carlo (GEMC) methods. It is shown that the system has clear gas-nematic liquid coexistence, but that th e 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 lik ely 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 have spheres, it is known that demixing is essentially condensation of the Coulombic fluid weakly influenced by the background of neutral ha rd spheres. Therefore, given that dipolar hard spheres do not condense , whether or not the present mixtures demix is an interesting question . In fact, demi?ring is observed and, moreover, the transition tempera tures are in reasonable agreement with those predicted by the same int egral equation theories that incorrectly predict condensation of the p ure dipolar fluid. The critical temperature decreases rapidly with dec reasing diameter of the neutral species consistent with the lack of ga s-isotropic liquid coexistence for pure dipolar hard spheres. Clearly, for the present model demixing and dipolar condensation are not close ly related phenomena as they are in the Coulombic systems. The neutral species appears to reduce the formation of dipolar ''chains'' or ''cl usters'' that inhibit condensation of the purl dipolar hard-sphere flu id.