(692g) Problems of Molecular Theories of Fluid Mixtures: Intermolecular Potential Models and Combining Rules

To predict properties of fluids and fluid mixtures intermolecular potential models are required that, however, are available only for pure fluids. Consequently, certain combining rules must be used, typically the Lorentz-Berthelot (LB) ones. However, no satisfactory agreement with experiment can be achieved unless certain deviations from the LB rules are applied, particularly from the Berthelot rule. Recent studies on mixtures of realistic fluid models have shown that also deviations from the Lorentz rule for the size parameter must be considered. Moreover, we have recently shown that a deviation from the Lorentz rule may even lead to a qualitative change in the behavior of the mixture. On top of this, the use of pure fluid potential models themselves for mixtures must be questioned. Consequently, properties of mixtures are, in principle, unpredictable unless some a priori known experimental result is used.

One possibility to account more appropriately for the cross interactions is to include polarizability. Molecular simulations on polarizable models bring the results to at least qualitative agreement with experimental data but at the expense of considerably increased complexity and CPU time demands. An alternative is to maintain the pure fluid pair-wise additivity and to develop certain rules for the combining rules. Nonetheless, large discrepancies between results obtained for excess properties of the same mixture using different force fields indicate that good performance for pure fluids need not be sufficient for a good description of mixtures. All these findings will be presented and discussed along with an attempt to identify defects in pure fluid models when applied to mixtures.