Monte Carlo simulations of racemic liquid mixtures: thermodynamic properties and local structure

Abstract

NPT Monte Carlo simulations of racemic mixtures were performed on a simplified model for the enantiomers. The molecules were spheres of equal size, interacting via a modified LennardJones potential. The interaction strength and the optimum distance between heterochiral molecules (D and L) differed (by a factor e and a factor s, respectively) from the homochiral values (D and D or L and L). Chemical potentials, enthalpies and orientational bond order parameters Q _l were calculated for a lot of racemic liquid mixtures. Weak heterochiral interactions (e < 1) showed a tendency to phase separation. Extremely strong interactions (e ≈ 5) gave rise to a glassy phase. A correlation analysis of the measured Q _l values for e ≠ / 1 and s = 1 showed no similarity of the molecular scale structure with that of binary crystals. Mixtures with s < 1 and with s > 1 both had a negative excess entropy, showing ordering. This effect was outweighed by a large negative excess enthalpy, and in total they had a negative excess Gibbs energy. In these mixtures the molecular scale structure was similar to that of certain binary crystal structures. By comparing the measured Q _l values with those for ideal reference crystals an NaCl-like ordering was found for small s and various layered closepacked crystal structures for larger s.