MOLECULAR-MODEL FOR THE SIMULTANEOUS ORIENTATIONAL AND TRANSLATIONAL ORDERING IN A 2-DIMENSIONAL LIQUID

We present a molecular-statistical theory of the phase transition from the two-dimensional isotropic liquid to the phase which has the eleme nts of the orientational and translational order combined in a single order parameter. This phase possesses the glide symmetry, similar to t he herringbone order in crystals, and is related to the mesophases obs erved in Langmuir monolayers. The microscopic definition of the herrin gbone order parameter is presented and the transition temperatures fro m the isotropic to the nematic, smectic, and the herringbone phases ar e expressed in terms of the direct correlation function of the two-dim ensional isotropic fluid. The relative stability of these phases is di scussed. The transition temperature into the herringbone phase is esti mated using the simple model of hard discs interacting via the quadrup ole-quadrupole potential that promotes the herringbone order. (C) 1998 American Institute of Physics.