Orientational transitions in a dimer system

A lattice fluid in which molecules can have orientation-dependent interactions is considered. It is found that a high density transition to an orientationally ordered state can be produced if a particular interaction between molecules in a specific relative orientation is dominant. The transition is second order and can be related to transitions found experimentally in lipid monolayers. If all interactions are attractive a separate low density condensation can coexist with the orientational transition.