(620a) Molecular Based Modeling of Associating Fluids Via Calculation of Wertheim Cluster Integrals

Strong association interactions between particles reduce the efficiency of the virial equation of state (VEOS) to predict the true thermodynamic behavior of the short-ranged and strongly directional associating fluids. As an alternative to VEOS, we have applied Wertheim's multi-density formalism to associating systems. The main strength of Wertheim's formulation is that we can keep the molecular level detail, like VEOS, while explicitly accounting for the strong directional association between particles in the equation of state. We examine the Wertheim equation of state (WEOS) for one-, two-, and four-site models, respectively, based on Wertheim theory. The four-site model is considered to represent the behavior of water. The molecules are modeled as hard spheres or Lennard-Jones atoms with diameter σ having two hydrogen and two electron bonding sites modeled by square wells. Mayer Sampling Monte Carlo (MSMC) simulation is used to evaluate every cluster integral in VEOS and WEOS. It is shown that the thermodynamic properties of associating fluids computed by Wertheim's multi-density formalism agreeing better with NPT simulation data than the single-density virial formulation. More complex molecular models can be handled by the treatment, so the approach promises to provide thermodynamic models based on detailed molecular behaviors.