A lattice model for fluids with directional interactions

Here we analyze a lattice model for fluids with directional interactions in the framework of the Ono-Kondo theory. The free energy of the system is re presented as an explicit function of the temperature and bulk density. It i s shown that the model predicts both order-disorder and vapor-liquid phase transitions. This theory predicts a tricritical point where the vapor-liqui d and order-disorder phase transitions both disappear. Also, it predicts re trograde condensation where the boundary of phase stability becomes a multi valued function of concentration. In addition, predictions of the theory ar e compared with Monte Carlo simulation data. It is shown that the partition function cannot be factored to predict separately the contributions of, fo r example, dispersion and hydrogen bonding interactions. (C) 1999 American Institute of Physics. [S0021-9606(99)50829-5].