Compositional and orientational ordering of CO-Ar and N2-Ar on graphite

The compositional and orientational ordering of mixtures of diatomic and spherically symmetric molecules such as CO-Ar and N2-Ar physisorbed on the surface of graphite at low temperatures is examined. Mean-field theory is used with simple Hamiltonians that include both isotropic and anisotropic interactions between the molecular species. It is found that stabilization of the experimentally observed pinwheel phase in these systems occurs with the inclusion of an anisotropic interaction between diatomic and spherically symmetric molecules in addition to the usual electric-quadrupole-quadrupole interaction between the diatomic species alone. Although isotropic interactions which favor mixing are also found to stabilize the pinwheel phase, it is argued from experimental three-dimensional phase diagrams that isotropic interactions probably have the wrong sign to explain the two-dimensional pinwheel phase.