In the present study, to study the soluting effect as well as the applicability of the linear isopiestic relation (LIR) to the prediction of thermodynamic properties in ternary mixed ionic liquid aqueous solutions, isopiestic measurements were carried out for different IL1 + IL2 + water systems at 298.15 K. The imidazolium-based ionic liquids with the same cations and different anions, including 1-butyl-3-methylimidazolium chloride ([C4MIM]Cl), 1-butyl-3-methylimidazolium bromide ([C4MIM]Br), and 1-butyl-3-methylimidazolium iodide ([C4MIM]I), were selected to investigate the effect of anion on isopiestic systems. The constant water activity lines of the systems containing [C4MIM]Cl (with higher hydrophilicity) and [C4MIM]I (with higher hydrophobicity) had negative deviations from the LIR. However, the other investigated systems [C4MIM]Br + [C4MIM]Cl + H2O and [C4MIM]Br + [C4MIM]I + H2O behave like semi-ideal solutions. In the next section of the experimental study, the equations derived from the LIR were used to predict the thermodynamic properties (density, sound velocity, and surface tension) of the ternary aqueous solutions using the data of binary subsystems, while these solutions were under isopiestic equilibrium. The predictability of these equations for different systems was investigated according to their amount of deviation from the linear isopiestic relation.
