(534b) New Generation SAFT Association for Electrolyte Systems

Understanding the thermodynamic properties of aqueous electrolyte solutions is required for many practical applications in biology, geology, oceanography, power generation, hydrometallurgy and oil recovery. The need for high temperature thermodynamic data becomes apparent when one recognizes that the chemistry of aqueous electrolyte solutions at high temperature differs considerably from that at room temperature, 298.15 K.  There are hundreds of electrolytes of interest for which thermodynamic properties at high temperature and high pressure are not available.  At present it is not feasible to attempt to make measurements on each and all chemically stable aqueous ions in the temperature range of interest.  It is important, however, to search for theoretical methods to reliably estimate the thermodynamic properties for a much larger set.

Statistical associating fluid theory (SAFT) is a chemical equilibrium based theory and hence has the potential to describe electrolyte systems over the temperature and pressure range of interest. The thermodynamic modeling of aqueous electrolyte solutions using the SAFT model has been applied by several research groups to ionic systems by combining SAFT with integral equation approaches – some using a primitive model and some a civilized approach. In primitive models, ions represented by hard spheres are present in a dielectric continuum of the solvent. In civilized or non-primitive models, a molecular representation of solvent as hard sphere is used along with ions. In general, in the SAFT based approaches for electrolyte systems, association between solvent and ion is used to account for solvation of the ions. In the SAFT association term, used with different versions of SAFT and CPA, the ions are modeled as segments with directional sites.  For ions where charge is uniform over the molecule such a representation may not be able to give the true picture. Recently, Marshall and Chapman^1–3 developed a theory, for self-assembling mixtures of particles with directional as well as spherically symmetric attractions. A comparison of the association scheme with spherically symmetric sites for ions versus SAFT based association with fixed number of sites will be presented. As an example, thermodynamic properties of aqueous solutions of alkali halides will be presented using the present modified association scheme. Extensions to higher temperatures will also be presented.

1. Marshall, B. D. & Chapman, W. G. Molecular theory for self assembling mixtures of patchy colloids and colloids with spherically symmetric attractions: The single patch case. J. Chem. Phys. 139, 104904 (2013).

2. Marshall, B. D. & Chapman, W. G. Thermodynamic perturbation theory for self assembling mixtures of multi-patch colloids and colloids with spherically symmetric attractions. Soft Matter 9, 11346–11356 (2013).

3. Wertheim, M. S. Fluids with highly directional attractive forces. III. Multiple attraction sites. J. Stat. Phys. 42, 459–476 (1986).