Abstract
ABSTRACTWe present a theoretical framework and parameterisation of intermolecular potentials for aqueous electrolyte solutions using the statistical associating fluid theory based on the Mie interaction potential (SAFT-VR Mie), coupled with the primitive, non-restricted mean-spherical approximation (MSA) for electrolytes. In common with other SAFT approaches, water is modelled as a spherical molecule with four off-centre association sites to represent the hydrogen-bonding interactions; the repulsive and dispersive interactions between the molecular cores are represented with a potential of the Mie (generalised Lennard-Jones) form. The ionic species are modelled as fully dissociated, and each ion is treated as spherical: Coulombic ion–ion interactions are included at the centre of a Mie core; the ion–water interactions are also modelled with a Mie potential without an explicit treatment of ion–dipole interaction. A Born contribution to the Helmholtz free energy of the system is included to account for the process of charging the ions in the aqueous dielectric medium. The parameterisation of the ion potential models is simplified by representing the ion–ion dispersive interaction energies with a modified version of the London theory for the unlike attractions. By combining the Shannon estimates of the size of the ionic species with the Born cavity size reported by Rashin and Honig, the parameterisation of the model is reduced to the determination of a single ion–solvent attractive interaction parameter. The resulting SAFT-VRE Mie parameter sets allow one to accurately reproduce the densities, vapour pressures, and osmotic coefficients for a broad variety of aqueous electrolyte solutions; the activity coefficients of the ions, which are not used in the parameterisation of the models, are also found to be in good agreement with the experimental data. The models are shown to be reliable beyond the molality range considered during parameter estimation. The inclusion of the Born free-energy contribution, together with appropriate estimates for the size of the ionic cavity, allows for accurate predictions of the Gibbs free energy of solvation of the ionic species considered. The solubility limits are also predicted for a number of salts; in cases where reliable reference data are available the predictions are in good agreement with experiment.
Highlights
Aqueous electrolyte solutions are liquid mixtures containing charged species, typically stemming from the dissociation equilibrium of the constituent ions of salts in water
Our statistical associating fluid theory (SAFT)-VRE Mie methodology provides a good description of the thermodynamic properties used in the development of the ion models within the range of thermodynamic conditions of the experimental data points used for parameter estimation
The SAFT-VR Mie expression for the Helmholtz free energy of a mixture is combined with the mean-spherical approximation for a non-restricted primitive model of an electrolyte solution
Summary
Aqueous electrolyte solutions are liquid mixtures containing charged species, typically stemming from the dissociation equilibrium of the constituent ions of salts in water. The ubiquitous nature of electrolytes makes them relevant in many scientific and industrial. The presence of charged species (ionic components) in solution drastically alters the bulk thermodynamic properties of the mixture, usually leading to a lowering of the vapour pressure (with a corresponding increase in the boiling temperature), an increase in density, and changes in the viscosity and surface tension of the system. The development of a successful theory of electrolyte solutions relies on both the availability of accurate inter-particle potentials for all of the species in the system and the ability to resolve the statistical–mechanical relations that lead to the bulk properties of interest. Available theories predominantly incorporate the Coulombic nature of the ionic interactions with a much simplified representation of the neutral solvent molecules, which are often polar and, as in the case of water, can exhibit hydrogen-bonding interactions. The intricacy of the solvent–solvent, solvent–solute and solute–solute interactions and their corresponding interplay makes this a challenging area of research
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