Thermodynamics of simple models of associating fluids: primitive models of ammonia, methanol, ethanol and water

Thermodynamic P-V-T properties of primitive models that descend directly from realistic Hamiltonians and reproduce the structure of real fluids have been studied both by means of theory and computer simulations. Analytic expressions for the Helmholtz free energy of four typical associating fluids, ammonia, methanol, ethanol and water, have been derived using the thermodynamic perturbation theory. Whereas for the models which allow only single bonding of each site the first-order theory is sufficient, for models in which some sites may form simultaneously up to two bonds the theory has to be extended to the second order. Comparison with simulation data shows that the theory is very accurate and has therefore also been used to determine vapour-liquid equilibria. We have found fundamental differences in the behaviour of different models; these differences are linked to the properties of the hydrogen-bond network that are discussed in detail.