In this work, the excess Gibbs free energy models, i.e., non-random
two-liquid (NRTL) model, electrolyte NRTL model, and electrolyte NRTL
model including new strategies (association or hydration), were used to
describe the macroscale properties and interpret the microstructure,
clarifying the role of association and hydration in model development,
and the enthalpy of mixing of three imidazolium-based IL-H2O systems
containing the same cation but different sizes of anions, i.e., Cl−,
Br−, and I− were measured for the first time to provide systematic data
for model development. The models were developed and evaluated based on
the newly measured data and the osmotic coefficient from the literature.
The model reflecting the intrinsic mechanism of dissociation and
hydration competition gives the best modeling results. The real ionic
strength predicted from the identified model was quantitatively
correlated with the electrical conductivities.