Influence of Alkyl Chain Length in Cation of Imidazolium-Based Ionic Liquids in Restructuring Water

Abstract The effect of alkyl chain length of a series of ionic liquids (ILs), 1-alkyl-3-methylimidazolium chloride ([Cnmim]Cl, where n = 2, 4, 8, 12, and 14) on the ability to restructure water in aqueous media has been systematically investigated. When water and solutes interact, different types of clusters form, and the strength and number of hydrogen bonds in these clusters vary with the concentration of solutes. The hydrodynamic diameter, measured using dynamic light scattering, reveals monomodal distributions at low concentrations and temperatures (20–30 °C), while multimodal distributions at high concentrations, particularly for n ≥ 8. The limiting partial molar volume at infinite dilution increases significantly with alkyl chain length (∼130 cm3 mol–1 for [C2mim]Cl to ∼330 cm3 mol–1 for [C14mim]Cl). The density of the IL-water system increases with concentration for shorter alkyl chains, whereas an opposite trend is observed for longer chains. The asynchronous 2D NIR spectra reveal that short-chain ILs exhibit structure-making behavior driven by strong electrostatic hydration, whereas long-chain analogues induce structure-breaking through hydrophobic association. Solutions with high concentrations exhibit greater temperature sensitivity, and a distinct dichotomous behavior appears beyond 50 °C for all the IL solutions. This work enhances understanding of IL–water interactions and may contribute to the rational design of alternative solvents with reduced volatility compared to conventional organic solvents.