(331f) Predicting the Properties of Fluid Mixtures Using Ab Initio Potentials

Considerable progress has been achieved in calculating both the thermophysical properties of fluids and phase equilbria.¹ Were possible, molecular simulation² is arguably the preferred method for predicting thermophysical properties because it minimizes the number of assumptions. Recent improvements in computational chemistry³ also mean that accurate potentials are being increasingly developed from first principles. However, in common with equations of state¹ and other theoretical methods, the ‘state-of-the-art’ ab initio potentials are typically developed for pure fluids. To predict, rather than simply calculate the properties of mixtures requires knowledge of the interactions between the dissimilar molecules. Achieving accurate agreement with experiment usually requires the use of combining rules,¹ which almost invariably need interaction parameters, such as the k_ij value. These interaction parameters are commonly obtained empirically, often by fitting the property being investigated. This means that the scope of genuine predictions for mixtures is limited.

This work attempts to at least partially address this key limitation. We demonstrate how ab initio potentials can be transitioned to accurately predict the properties of mixtures involving noble gases, small molecular systems and water. This involves a simplified approach for generating ab initio potentials⁴ that both minimizes the mathematical complexity of the potential and substantially reduces the computational cost. Results for mixtures are presented for both common thermodynamic properties (e.g., heat capacities, thermal expansion, the Joule-Thomson coefficient etc) and vapor-liquid equilibria. The comparison with experiment demonstrates that ab initiopotentials can be systematically used to predict the properties of fluid mixtures.

1. Y. S. Wei and R.J. Sadus, 2000, Equations of state for the calculation of fluid-phase equilibria, AIChE J. 2000, 46, 169-196.
2. R. J. Sadus, Molecular Simulation of Fluids: Theory, Algorithms and Object-Orientation, Elsevier, Amsterdam, 1999.
3. R. Hellmann E. Bich, and E. Vogel, 2017, State-of-the-art ab initio potential energy curve for the xenon atom pair and related spectroscopic and thermophysical properties, J. Chem. Phys., 147, 034304.
4. U. K. Deiters and R. J. Sadus, 2019, Two-body interatomic potentials for He, Ne, Ar, Kr, and Xe from ab initio data. J. Chem. Phys., 150, 134504.