PRIMITIVE MODELS OF ASSOCIATED LIQUIDS - EQUATION OF STATE, LIQUID GAS-PHASE TRANSITION, AND PERCOLATION-THRESHOLD

An analytic equation of state of an arbitrary primitive model of associated liquids, based on Wertheim's thermodynamic perturbation theory, is given. The equations for the models of ammonia (spherical core) and methanol (non-spherical core) are then examined in detail, and it is shown that they give only one fluid phase. An attempt is made to link this result to the H-bond network: using the Cayley tree approximation no percolation threshold has been found for these two models, i.e. their molecules may form clusters of finite size only. This finding contrasts with the result for the model of water which may form infinite clusters and exhibits the liquid-gas phase transition.