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Ionic Liquids ( ILs) are organic salts characterized by low melting temperatures below 100℃. These materials offer a vast design space that is readily assessable through judicious anion and cation selection, thus enabling their thermophysical properties — including density, viscosity, conductivity, and surface tension — to be easily tuned. In recent years, aqueous IL systems have becoming increasingly important in fields such as biotechnology, fuel cell development, working fluids, pharmaceutical solvents, and protein stabilization. Accordingly, developing a comprehensive understanding of how aqueous IL systems behave across the entire IL-water composition range, i.e. from infinite dilution to pure IL is paramount for their design and implementation.

To date, various models have been proposed for calculating the activity coefficients of aqueous electrolytes with the vast majority based on the Debye-Huckel model for inorganic electrolytes in which ion-ion interactions are dominated by electrostatics. However, significantly less consideration has given to more complex organic ions (e.g., ILs) in which various intermolecular interactions including ion pairing, van der Waals forces, and hydrophobic self-assembly are known to significantly contribute to electrolyte behavior. Alternatively, the Non-Random Two Liquid (NRTL) model, is designed to capture IL behavior at higher ionic concentrations but is limited in its ability to capture the behavior of dilute IL systems.

In the present work, we leverage data obtained from the NIST ILThermo database in order to investigate water and IL activity coefficients in aqueous IL electrolytes. In addition to water activity coefficients directly obtained from the reported data, we calculate the ion activity coefficients of various ILs. In particular, we focus on the effectiveness of dilute electrolyte (i.e., Debye-Huckel) and concentrated electrolyte models (i.e. the NRTL model) in three concentration regimes: near infinite dilution, intermediate (i.e. bi-percolated concentrations), and high IL concentrations similar to those found in water in salt electrolytes in order to access the limitations and efficacy of these models and develop a more robust understanding of the intermolecular interactions underpinning these systems.