(318j) Geometric Surfactancy Probed By Molecular Dynamics Simulation of Lennard-Jones Rods, Spheres, and Dyads Thereof

Fullerene, octa(p-phenylene), and the dyad thereof are treated as Lennard-Jones, L-J, particles for coarse-grained molecular dynamics, MD, calculations of single-component spheres, binary rod/sphere, and ternary rod/dyad/sphere mixtures. The densities of benzene and fullerene are calculated with independently evaluated van der Waals radii. The equimolar molar rod/sphere binary mixture was found to undergo phase separation into isotropic spheres and rods in smectic phase. Doped at 0.05 mole fraction in the equimolar rod/sphere binary mixture, surfactancy is manifested by dyads preferentially locating themselves at the rod/sphere interface. It is important to note that the computed densities and phase behaviors have been verified to be independent of system size and pair potential cutoff distance over the tested ranges. The MD simulations of L-J particles accommodate both geometric effects and dispersion forces. The formation of microstructures as a result of geometric surfactancy with an increased interfacial area from the initial random distribution of rods, spheres, and dyads at constant T and P is favored by Gibbs energy depression contributed by enthalpy loss overriding the entropy loss in an L-J system. In a hard-particle system, the enthalpy loss consists only of volume reduction, which must outweigh entropy loss to support geometric surfactancy. The desired condition can be met in part by the first-order ordering transition of hard rods constituting half of the system from the initial isotropic to smectic phase accompanied by a volume reduction while hard spheres remain in the disordered state.