Modeling Henry's law and phase separations of water-NaCl-organic mixtures with solvation and ion-pairing

Empirical measurements of solution vapor pressure of ternary acetonitrile (MeCN) H2O-NaCl-MeCN mixtures were recorded, with NaCl concentrations ranging from zero to the saturation limit, and MeCN concentrations ranging from zero to an absolute mole fraction of 0.64. After accounting for speciation, the variability of the Henry's law coefficient at vapor-liquid equilibrium (VLE) of MeCN ternary mixtures decreased from 107% to 5.1%. Solute speciation was modeled using a mass action solution model that incorporates solute solvation and ion-pairing phenomena. Two empirically determined equilibrium constants corresponding to solute dissociation and ion pairing were utilized for each solute. When speciation effects were considered, the solid-liquid equilibrium of H2O-NaCl-MeCN mixtures appear to be governed by a simple saturation equilibrium constant that is consistent with the binary H2O-NaCl saturation coefficient. Further, our results indicate that the precipitation of NaCl in the MeCN ternary mixtures was not governed by changes in the dielectric constant. Our model indicates that the compositions of the salt-induced liquid-liquid equilibrium (LLE) boundary of the H2O-NaCl-MeCN mixture correspond to the binary plateau activity of MeCN, a range of concentrations over which the activity remains largely invariant in the binary water-MeCN system. Broader comparisons with other ternary miscible organic solvent (MOS) mixtures suggest that salt-induced liquid-liquid equilibrium exists if: (1) the solution displays a positive deviation from the ideal limits governed by Raoult's law; and (2) the minimum of the mixing free energy profile for the binary water-MOS system is organic-rich. This work is one of the first applications of speciation-based solution models to a ternary system, and the first that includes an organic solute. We used a two-variable speciation-based solution model to correlate VLE (organic Henry's law coefficient and water activity), SLE (organic-induced salt crystallization), and salt-induced LLE separation of H2O-NaCl-MeCN mixtures.

Automated summary

This study measured the solution vapor pressure of ternary acetonitrile (MeCN) H2O-NaCl-MeCN mixtures and used a mass action solution model to characterize solute speciation. The results showed that accounting for speciation significantly reduced the variability of the vapor-liquid and solid-liquid equilibria. Additionally, the precipitation of NaCl in the mixtures was found to be independent of the dielectric constant but related to the solution activity. Comparison with other ternary mixtures revealed that salt-induced liquid-liquid equilibrium exists in solutions with positive deviations from Raoult's law and organic-rich mixing free energy profiles. This study is the first to apply a speciation-based solution model to a ternary system and contributes to a better understanding of multi-component solution equilibria.