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

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 Lennard-Jones 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 approximate to 5) gave rise to a glassy phase. A correlation analysis of the measured Q(l) Values for e not equal 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 close-packed crystal structures for larger s.