Protic Ionic Liquid-H₂O Mixtures─Structure, Interactions, and Structure-Property Relationships

A better understanding of the molecular interactions and, consequently, the structure-property relationships in protic ionic liquids (PIL) can help create more accurate property prediction models and enhance the efficiency of their various applications. This approach is especially relevant in understanding the PIL-H₂O mixtures. Depending on the application, water molecules can be considered as not only an impurity in PILs but also a dopant or even a cosolvent. Our study investigated density, viscosity, electrical conductivity, and derived properties like thermal expansion coefficients, excess molar volume, and excess viscosity of low-toxic alkanolammonium- and carboxylate-based PIL-H₂O mixtures. One- (1D) and two-dimensional (2D) nuclear magnetic resonance spectroscopy was employed to elucidate the distribution of water molecules within the PIL structure. In addition, critical aggregation concentrations (CAC) of PIL-H₂O mixtures were determined based on the physicochemical properties and ¹H longitudinal relaxation times. The results showed that the strength of PIL-H₂O interactions depends on the anion, while the cation affects the position of water in the PIL solvent network. Overall, our study provides valuable insight into the molecular modeling and property prediction of this type of PIL, a promising and currently underexplored subclass of ionic liquids.