Thermophysical and Transport Properties of Ethanolammonium Carboxylate Protic Ionic Liquids in Water and Dimethyl Sulfoxide

Volumetric and transport properties of protic ionic liquids (PILs) in molecular solvents provide essential insights into solute–solute and solute–solvent interactions in solution. In this study, the density and electrical conductivity of binary mixtures of ethanolammonium carboxylate PILs, ethanolammonium acetate, propionate, butyrate, and pentanoate with water and dimethyl sulfoxide (DMSO) were measured across the entire mole fraction range at 298 K. Excess molar volumes (VE) were derived from density (ρ) data and correlated using the Redlich–Kister polynomial. The VE values were negative over the full composition range for both PIL + water and PIL + DMSO systems, indicating strong ion–dipole interactions, extensive hydrogen bonding, and efficient molecular packing. Increasing the alkyl chain length of the PIL anion decreased the magnitude of the density and VE, highlighting the role of hydrophobicity in reducing packing efficiency and solvation. To account for differences in solvent size, the excess molar volumes were normalized with respect to the molar volume of the mixture (Vm). The normalized values (VE/Vm) indicate that the larger absolute contractions observed in aqueous systems are associated with both stronger interactions and the smaller molar volume of water relative to DMSO. In addition to specific conductivity (κ), molar conductivity (λm) was evaluated to better assess ionic dissociation independent of solvent molar volume effects. The λm analysis revealed enhanced dissociation in solvent-rich regions and a progressive decrease at higher PIL mole fractions due to increased ion association and reduced mobility. Overall, the results demonstrate that molecular interactions, volumetric behavior, and charge transport in PIL–solvent mixtures are governed by the combined effects of the solvent polarity, hydrogen-bonding ability, solvent molar volume, and anion hydrophobicity.