Anion basicity orders hydration structure and dynamics in imidazolium ionic liquid-water mixtures: a systematic NMR...

Ionic liquid-water mixtures are widely used in electrochemical, separation, and process applications. However, how anion identity shapes the hydration response across the full composition range has not been systematically quantified. Here we report a systematic study of three 1-ethyl-3-methylimidazolium ([EMIM]⁺) ionic liquids paired with anions of distinct hydrogen-bond basicity: chloride (Cl^-), tetrafluoroborate (BF₄^-), and diethyl phosphate (DEP^-). Viscosity, conductivity, ¹H, ¹⁹F, and ³¹P NMR chemical shifts and linewidths were measured across the accessible composition range. Across the three systems studied here, anion basicity orders the measured observables consistently, from the neat-liquid imidazolium C2-H (H2) chemical shift and linewidth to the position of the water ¹H shift minimum, following the sequence DEP^- > Cl^- > BF₄^-. In [EMIM][Cl] and [EMIM][DEP], the water shift and linewidth minima occur at similar compositions, consistent with coupled structural and dynamic changes; in [EMIM][BF₄], the water shift plateaus while the linewidth minimum occurs at a distinctly higher water content, a linewidth-based observation consistent with the structural crossover independently reported by optical Kerr effect and neutron scattering studies in this composition range. [EMIM][DEP] additionally shows a V-shaped ³¹P response consistent with multi-stage phosphate hydration. These results identify anion basicity as a practical first-order descriptor for IL-water behaviour in the systems studied here, with NMR linewidth as a necessary complement to chemical shift.