(364b) Developing SAFT-gamma Mie Models for Aqueous Electrolytes: Organic Salts and CO2-Brine Mixtures

Thermodynamic modelling of electrolyte solutions proves particularly challenging for complex systems characterized by the presence of organic electrolytes, which are generally non-spherical molecules. SAFT-γ Mie [1,2] is a successful statistical thermodynamic framework to model bulk thermodynamic properties. SAFT-γ Mie is essentially the SAFT-VR Mie equation of state casted in a group contribution formulation, where each component can be constructed using functional groups that represent various molecular moieties. SAFT-VR Mie has been recently used to study ionic solutions [3] by incorporating the Mean Spherical Approximation (MSA) approach to account for the Coulombic interactions between ions in the presence of solvent molecules; the polar-polar and ion-polar interactions are implicitly treated with a separate Born solvation energy term. However, both of these electrostatic terms are limited to spherical molecules and further approximations are required to treat non-spherical species.

Molecular simulations have been widely used to assess the necessary approximations to develop statistical mechanical theories of fluids. In this work we compare the electrostatic contributions to the chemical potential predicted using various primitive approaches (MSA, Debye-Huckel, Born) against simulation data for model systems that incorporate the solvent explicitly. We also compare two different methodologies that we developed to account for cases were some of the charged compounds are non-spherical.

Estimating the chemical potential of charged molecules using traditional molecular simulation techniques is challenging. Thus, we have used the expanded ensemble transition matrix method [4] to run these calculations robustly and efficiently. The theoretical predictions were compared with simulation data to provide a better understanding of the implications of the assumptions employed within the SAFT-γ Mie electrolyte framework.

Finally, the methodology is applied to real systems like brine-CO₂ mixtures and short chain carboxylate salt solutions. The fitted models for these mixtures are characterized by good accuracy and can be used to model properties like salt and CO₂ solubility, vapor pressures and densities.

References

[1] V. Papaioannou, T. Lafitte, C. Avendaño, C.S. Adjiman, G. Jackson, E.A. Müller and A. Galindo, J. Chem. Phys., 140, 054107 (2014)

[2] V. Papaioannou, F. Calado, T. La, S. Dufal, M. Sadeqzadeh, G. Jackson, C.S. Adjiman, A. Galindo, Fluid Phase Equilib., 416, 104 (2016)

[3] D.K. Eriksen, G. Lazarou, A. Galindo, G. Jackson, C.S. Adjiman, A.J. Haslam, Mol. Phys. 114, 2724 (2016)

[4] A.S. Paluch, S. Jayaraman, J.K. Shah, E.J. Maginn, J. Chem. Phys. 133 124504 (2010)