Effects of elongation on the phase behavior of the Gay-Berne fluid

Abstract

In this paper we present a computer simulation study of the phase behavior of the Gay-Berne liquid crystal model, concentrating on the effects of varying the molecular elongation $\ensuremath{\kappa}.$ We study a range of length-to-width parameters $3<~\ensuremath{\kappa}<~4,$ using a variety of molecular dynamics and Monte Carlo techniques, obtaining a guide to the phase behavior for each shape studied. We observe vapor $(V),$ isotropic liquid $(I),$ nematic $(N),$ smectic-$A$ ${(S}_{A})$ and smectic-$B$ ${(S}_{B})$ liquid crystal phases. Within the small range of elongation studied, the phase diagram shows significant changes. On increasing $\ensuremath{\kappa},$ the liquid-vapor critical point moves to lower temperature until it falls below the ${I\ensuremath{-}S}_{B}$ coexistence line, around $\ensuremath{\kappa}=3.4,$ where liquid-vapor coexistence proves hard to establish. The liquid-vapor critical point seems to be completely absent at $\ensuremath{\kappa}=4.0.$ Another dramatic effect is the growth of a stable ${S}_{A}$ ``island'' in the phase diagram at elongations slightly above $\ensuremath{\kappa}=3.0.$ The ${S}_{A}$ range extends to both higher and lower temperatures as $\ensuremath{\kappa}$ is increased. Also as $\ensuremath{\kappa}$ is increased, the $I\ensuremath{-}N$ transition is seen to move to lower density (and pressure) at given temperature. The lowest temperature at which the nematic phase is stable does not vary dramatically with $\ensuremath{\kappa}.$ On cooling, no ${S}_{B}$-crystal transition can be identified in the equation of state for any of these elongations; we suggest that, on the basis of simulation evidence, ${S}_{B}$ and crystal are really the same phase for these models.