Importance of the relative static permittivity in electrolytic SAFT-VR Mie equations of state
Date:
The influence and importance of the relative static permittivity (RSP) in electrolyte equations of state is examined in-depth for the case of aqueous sodium chloride. The SAFT-VR Mie equation of state is used to model the dispersive and associative interactions, whilst the Debye-Hückel (DH) or Mean-Spherical Approximation (MSA) terms, as well as the Born-solvation term, are used to account for the presence of ions within the mixture. The RSP is obtained from a variety of models, each differing in their dependencies; we consider constant, temperature dependent, temperature-volume dependent and temperature-volume-composition (of both the ions and solvent) dependent models. For a fair comparison between different combinations of electroÍstatic terms and RSP models, all ion-related parameters are obtained a priori. A novel combining rule is proposed to obtain the unlike parameters between solvents and ions; its reliability is examined for a variety of electrolyte systems. We also compare its performance relative to parameterised electrolyte models developed by Eriksen et al.[1] and Selam et al.[2].
Both the DH and MSA terms yield similar results for almost all properties and conditions. The RSP models used have the more-significant impact—the extent of which depends on which property is considered. Liquid densities and solvent saturation pressures showed limited changes between RSP models. Osmotic coefficients, mean ionic activity coefficients and carbon dioxide solubilities observed drastically different behaviour where, in the case of the latter, the impact of the salt can change from a salting-in to a salting-out effect between RSP models. Analysing the contributions of the various terms to the activities of each species in an electrolyte mixture reveals an important balance between the Born-solvation and the DH or MSA terms where the RSP models have a significant impact on this balance, particularly when these carry a solvent- or ion-composition dependence.
[1] D.K. Eriksen, G. Lazarou, A. Galindo, G. Jackson, C.S. Adjiman and A.J. Haslam, 2016. Development of intermolecular potential models for electrolyte solutions using an electrolyte SAFT-VR Mie equation of state. Mol. Phys, 114, 2724-2749. [2] M.A. Selam, I.G. Economou and M. Castier, 2018. A thermodynamic model for strong aqueous electrolytes based on the eSAFT-VR Mie equation of state. Fluid Phase Equilib. 464, 47-63.
Speaker: Pierre Walker