(444c) Combining Intermolecular Potentials for the Prediction of Fluid Phase Behavior

Molecular simulation [1] can be used to determine many properties of interest to chemical engineers such as phase behavior, thermodynamic quantities and transport phenomenon. The key requirement for molecular simulation is the existence of a suitable intermolecular potential [2]. Many different potentials have been proposed [3] and the evaluation of candidate potentials is both time consuming and often not transferable, i.e., it is often specific to a particular atom or molecule. This invariably means that both the evaluation and testing needs to be repeated for each new situation. This work, demonstrates how knowledge of existing intermolecular potentials can be used to improve the prediction of fluid properties.

We report and evaluate a general method [4] of combining intermolecular for the prediction of vapor-liquid phase equilibria. The method is illustrated using combinations of different Mie potentials [5] that involve different exponents (nand m). The ability to combine different exponents enables the examination of additional repulsive and cohesive interactions on phase behavior. The 12-8-6 potential, obtained by adding a m = 6 contribution to the 12-8 potential significantly broadens the phase envelope, which remains inside of the 12-6 envelope. In contrast the 12+9-6 potential, which involves an additional n = 9 repulsive contribution lifts the phase envelope above the 12-6 values. Significantly, comparsion of vapor-liquid equilibria data for two-body only simulations for some systems indicates that there is very good agreement with the 12-8-6 data. That is, the 12-8-6 potential may provide a useful description of two-body only interactions, which would otherwise require a much more complicated intermolecular potential. Furthermore, additional contributions allow the relatively simple characterization of properties governed by two-body + three-body interactions. Comparison with results obtained from ab intio + three-body simulations [6] indicates very good agreement at modest computational cost.

1. R. J. Sadus, Molecular Simulation of Fluids: Theory, Algorithms and Object-Orientation (Elsevier, Amsterdam, 1999).

2. A. J. Stone, The Theory of Intermolecular Forces (Clarendon Press, Oxford, 1996).

3. G. C. Maitland, M. Rigby, E. B. Smith, and W. A. Wakeham, Intermolecular Forces: Their Origin and Determination(Clarendon Press, Oxford, 1981).

4. R. J. Sadus, J. Chem. Phys. 153, 214509 (2020).

5. J. R. Mick, M. S. Barhaghi, B. Jackman, K. Rushaidat, L. Schwiebert, and J. J. Potoff, J. Chem. Phys. 143, 114504 (2015).

6. U. K. Deiters and R. J. Sadus, J. Phys. Chem. B 125, 8522 (2021).