Low dilution hydration boundary in liquid–liquid phase equilibria of 1-methyl-3-alkyl (hexyl/octyl) imidazolium chloride ionic liquids

Abstract

We report observation of low dilution hydration boundary in room temperature liquids (RTILs), 1-methyl-3-octylimidazolium chloride and 1-methyl-3-hexylimidazolium chloride ([C8mim][Cl] and [C6mim][Cl]), homologous members differing by two methylene units, were diluted with water at very low concentration (xw<0.08). The liquid–liquid phase equilibria were probed systematically using data obtained from rheology and Raman studies. Viscoelastic studies revealed binding of water to ILs was predominantly driven by hydrogen bonding, and was concentration specific, with maximum IL–water interaction occurring at mole fractions of water xw=0.024 and 0.044 for [C6mim][Cl] and [C8mim][Cl] respectively. [C8mim][Cl], though more hydrophobic, was found to be more selectively hydrated as compared to [C6mim][Cl]. Change in packing fraction (ΔΦm) was estimated from Kreigher–Dougherty dispersion model, ηIL−w=ηw(1−ΦIL/Φm)−2 where ΦIL is the volume fraction of IL (ΦIL<maximum packing fraction, Φm). ηIL-w and ηw are viscosity of IL–water solution and water respectively. [C8mim][Cl] molecules experienced twice as much change in packing due to higher dilution as compared to [C6mim][Cl] molecules. No discernible blue shift in Raman bands was noticed indicating the presence of strong cation–anion pairs in the explored samples. The identified bands were: (CH2)asym, (2895cm−1), CH3–FR (2930cm−1), (CH3)asym (2956cm−1) and (C4–C5)asym (3086cm−1) modes. These bands selectively got affected in presence of water due to formation of H-bonds. Unique hydration phase diagrams depicting the liquid–liquid phase equilibria of the ILs are presented.