Computer Simulation of Nematic Reentrance in a Simple Molecular Model

We report a computer simulation study of a system of parallel hard ellipsoids with attractive interactions represented by a spherically symmetric square well of range $\lambda$. For suitable values of $\lambda$ the results are consistent with the sequence of phases nematic ($N$), smectic-$A$ ($\mathrm{Sm}A$), reentrant nematic ($N_{\mathrm{re}}$), and crystal (Cr) under pressure. Both the $N\mathrm{\text{−}}\mathrm{Sm}A$ and the $\mathrm{Sm}A\mathrm{\text{−}}{N}_{\mathrm{re}}$ transitions are found to be first order or continuous depending on temperature. Moreover, we show that a generalized van der Waals–type theory provides a qualitative description of the observed phase behavior.

Figure 1

Variation of the pressure $P$ with packing fraction $\eta$ in the $N\mathrm{\text{−}}\mathrm{Sm}A$ transition region for the generalized square-well model with $L/D=5$ and $\lambda /D=3.4$. Symbols correspond to simulation data along different isotherms (temperatures are labeled on the plot). The curves are the predictions from density functional theory.

Figure 2

Variation of the smectic order parameter $\tau$ at the $\mathrm{Sm}A\mathrm{\text{−}}{N}_{\mathrm{re}}$ transition at high densities for the generalized square-well model with $L/D=5$ and $\lambda /D=3.4$ as obtained from simulation along several isotherms. Lines are included as a guide to the eye.

Figure 3

Approximate phase diagram of the generalized square-well model with $L/D=5$, $\lambda /D=3.4$ as obtained from simulation showing crystal (Cr), smectic $A$ ($\mathrm{Sm}A$), nematic ($N$), and reentrant nematic ($N_{\mathrm{re}}$) phases. Solid lines indicate first-order transitions, and discontinuous lines indicate continuous transitions. Solid symbols represent tricritical points.

Figure 4

Variation of the internal energy per particle $U$ (in units of $ϵ$) with pressure $P$ at the $\mathrm{Sm}A\mathrm{\text{−}}{N}_{\mathrm{re}}$ transition as obtained from simulation along different isotherms (temperatures are labeled on the plot). Open symbols are for the $\mathrm{Sm}A$ phase and solid symbols are for the $N_{\mathrm{re}}$ phase. Lines are a guide to the eye.