Two-dimensional bonded lattice fluids. II. Orientable molecule model

For pt. I see ibid. vol. 3, 427-41 (1970). A bonded fluid model on a plane triangular lattice is studied. Each molecule has three bonding directions at angles of 120 degrees to each other and two possible orientations in each of which its bonding directions point to three of the six nearest-neighbour sites. If the molecules of a nearest-neighbour pair have bonding directions pointing towards each other then a bond is formed and the pair has interaction energy -( epsilon +w), while an unbonded nearest-neighbour pair has interaction energy - epsilon ( epsilon >0, w>0). For epsilon /w<1/3 regions of open structure short- range order became important at low temperatures and pressures, with each molecule in such a region bonded to three others and one third of the sites vacant. At higher pressures the predominant low-temperature configuration is close-packed with all sites occupied. Calculations are performed, using a first-order approximation based on a triangle of sites, for epsilon /w=0 and epsilon /w=1/4. Critical points are deduced for separation into liquid and vapour phases, both without long-range order. The behaviour of the density as a function of pressure and temperature in the model resembles that found in fluid water, especially for epsilon /w=1/4. There is a supercritical region where curves of density against temperature at constant pressure show turning points, though at very high pressures the density decreases monotonically. For epsilon /w=1/4 these turning points are also found below the critical pressure in the liquid phase.