Orientational phases of methane bilayers

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Abstract
Orientational phases of methane bilayers have been studied using a combination of molecular field calculations and Monte Carlo simulations. In the free standing bilayer, whose dynamics are determined entirely by internal octopole–octopole interactions, the MF method predicts that the high temperature disordered phase can branch into a partially ordered intermediate phase with either ferro or antiferrorotational combinations of the orientations of the molecules in the individual layers. Each of these structures is transformed into a low temperature structure by a second phase transition, in the ferro case to a stable structure, and in the antiferro case to a metastable one. The simulations find only the ferro branch. A close relationship is found between these phases and those of the three dimensional octopolar solid. The consistency of the two methods and their relative strengths and weaknesses are discussed. We have also studied the tripod bilayer system where the molecules of the lower layer are constrained to stand as stable tripods on a supporting surface free to rotate only about the axis normal to the surface. Although this system also has two branches, each with three transitions, the most significant aspect of its behavior is the extremely weak coupling between layers. Experimental data for the bilayer is discussed in the light of the structural predictions for the tripod bilayer.