Mixtures of quadrupolar hard dumbells

Abstract

We report Monte Carlo (MC) simulation results for pure and mixed fluids of hard homonuclear diatomic molecules (dumbells) with added point charges arranged in a quadrupolar symmetry; for completeness we include the case of hard spheres with added point charge quadrupoles. We calculate the thermodynamic and structural properties. Included in the calculations are mixtures in which the two components have quadrupoles of opposite signs, as well as quadrupolar/non-polar mixtures; the molecules can have the same or differing core shapes. The point charges lead to a pronounced preference for molecular pairs to adopt ‘T’ configurations in the pure fluids; this is also true for the like pairs AA and BB in the mixtures. In mixtures of two quadrupoles of opposite sign the unlike pair AB structure is strikingly different from the AA and BB structures, because of the strong preference for AB pairs to adopt parallel or end-to-end configurations. Changing the shape of the molecular core from a sphere to a dumbell interferes with the ability of the molecules to adopt those configurations favoured by the quadrupoles, and leads generally to larger configurational energies and pressures.