Thermodynamic modeling of aqueous electrolytes and ionic liquids (ILs) with electrolyte non random two liquid (eNRTL) model

Date

2018-08

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Abstract

Accurate and consistent thermodynamic model is essential to address the non-ideality of the aqueous electrolyte and ionic liquids (ILs). The necessity of thermodynamic modeling for aqueous electrolytes arises in the mass and energy balance in the industrial applications such as desalination of highly saline water, treatment of produced water from the hydraulic fracturing, membrane separation, solubility predictions, salt extraction from the salt-lake brine, etc. The most widely used thermodynamic models are the Pitzer model and the electrolyte nonrandom two liquid (eNRTL) model. The interaction parameters of the Pitzer model are nonlinear functions of concentration and temperature. On the other hand, the eNRTL model parameters are only a well-defined function of temperature. In this study, we first assess the concentration dependence of the interaction parameters of both Pitzer and eNRTL model for NaCl + H2O water system. The Pitzer model and the eNRTL model show a deviation in the extrapolation of the mean ionic activity coefficient beyond 6 molal concentration. We calculated the mean ionic activity coefficient in a wide range of concentration based on molecular simulations and Kirkwood-Buff theory. The eNRTL model shows the correct asymptotic behavior of the mean ionic activity coefficient which is supported by molecular dynamics simulations and supersaturation experimental data, while the Pitzer model diverges very quickly beyond the salt saturation. Then we expressed the interaction parameters of the eNRTL model as a Gibbs-Helmholtz type equation which has less adjustable parameters than the Pitzer model. We successfully applied the eNRTL model for aqueous Li+–Na+–K+–Mg2+−SO42− quinary system to correlate and predict the thermodynamic properties accurately from 273.15-573.15 K and concentration up to saturation. The applicability of the eNRTL model is extended to the aqueous ionic liquids (ILs). Ionic liquid has gained a lot of attention in the past few decades for their negligible volatility, which would make them “green solvent”. The dissociation behavior of the ionic liquids in water is a particular interest in this study. The eNRTL model and COSMO-SAC model fail to satisfy the experimental dissociation extent data of ionic liquids in water with conventional partial dissociation chemistry. We propose a new dissociation chemistry based on the finding from the molecular dynamics simulation. The eNRTL model, with new dissociation chemistry, correlates the dissociation extent and vapor-liquid equilibrium (VLE) data throughout the concentration range.

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Keywords

Pitzer model, eNRTL model, Aqueous NaCl, Mean ionic activity coefficient, Molecular dynamics simulation, Kirkwood-Buff theory, Electrolyte NRTL model, Binary interaction parameters, Temperature dependence, Excess gibbs energy, Heat capacity, Quinary system, Lithium sulfate, Thermodynamic properties, Ionic liquids, Molecular dynamics simulations, Partial dissociation, Symmetric electrolyte non-Random two liquid model, Excess enthalpy

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