(207a) Modeling of Electrolyte Systems with an Equation of State and Comparison with Molecular Simulations

Modeling of electrolyte systems is important for many industrial processes, e.g. sour gas treatment and extractive distillation. Besides processes with small spherical ions there are also industrial relevant processes where large non-spherical ions are involved, e.g. ionic liquids, proteins and polyelectrolytes. The modeling of such systems asks for physically based theories, where the interactions between ions and polar substances are properly accounted for. The development of physically based fluid theories is closely connected to molecular simulations: firstly, to compare and evaluate the results of a theory with simulation data of model fluids and secondly, to simplify theories in adjusting model parameters to the molecular simulation data. However, for ion-dipole systems the simulation data of model fluids are scarce in literature. Therefore, this contribution presents molecular simulations results of mixtures of charged Lennard-Jones fluids and dipolar Lennard-Jones (Stockmayer) fluids. Monte Carlo simulations were conducted in the NPT ensemble with long-range interactions treated with the Ewald summation. The results will be used to develop a new contribution for the ion-dipole interaction. In this contribution we furthermore apply the non-primitive mean spherical approximation together with the PC-SAFT equation of state to (aqueous) electrolyte systems. In contrast to the so-called primitive models which treat the solvent as a dielectric continuum, the semirestricted non-primitive mean spherical approximation (npMSA) explicitly accounts for the molecular structure of the solvent. The theory contains three contributions, namely the ion-ion, the ion-dipole and the dipole-dipole interaction. This is beneficial because a temperature and concentration dependent dielectric constant is not needed. In this work we present calculated mean ionic activity coefficients, osmotic coefficients and densities of aqueous electrolyte solutions over the entire solubility range. One parameter needs to be adjusted to fit the experimental data of the mean ionic activity coefficients and the osmotic coefficients at a temperature of 25°C. We find the ion-dipole attraction in the npMSA to only be a crude approximation. In order to compensate for this deficiency and increase the ion solvation we used the Wertheim association term and fitted the association strength between the cation and water. Although correlations and extrapolations to conditions outside the range where parameters were adjusted are possible, we show that one profits from developing equation of state contributions with the aid of molecular simulations.