Role of anion hydrophobicity: Water interactions in imidazolium ionic liquids

Ionic liquids (ILs) have been widely investigated as tunable solvents for gas capture, yet a molecular-level understanding of how their intrinsic nanostructure responds to small penetrant species remains incomplete. While previous studies have shown that nonpolar gases such as CO2 induce only localized perturbations in ILs, the presence of strongly interacting molecules can fundamentally modify the balance between polarity, connectivity, and nanoscale organization. Here, molecular dynamics simulations are employed to systematically investigate water-IL mixtures across 24 imidazolium-based ILs, combining multiple anions and cation alkyl chain lengths. Structural and energetic descriptors are analyzed at 300 K and 1 bar for water mole fractions of 0%, 10%, and 50%. The results demonstrate that the response of ILs to water is dominated by the nature of the anion, whereas variations in cation alkyl length primarily modulate the extent of nanosegregation. Hydrophilic anions promote reorganization of the polar network, leading to percolated hydrogen-bonded water-rich domains, while hydrophobic anions preserve the inherent nanosegregation of the liquid, confining water to localized pockets. In contrast to CO2, water, therefore, acts as a critical modulator of IL nanostructure rather than a passive occupant of pre-existing cavities. Interaction energies emerge as a unifying descriptor linking local coordination, hydrogen-bonding, and macroscopic water affinity across all systems. By contrasting penetrant-induced restructuring with the largely non-disruptive incorporation of CO2, this work provides fundamental insights into the molecular factors governing stability, adaptability, and selectivity in IL nanostructures, with direct implications for the design of ILs for greenhouse gas capture and separation.