Effect of structural variations in cations of ionic liquids on the coexistence curve of isobutyric acid and water

Abstract

Interestingly, various behaviors of ionic liquids (ILs) having different cation chains with a common anion is observed in the coexistence curve of isobutyric acid–water (IBW). The critical region of IBW was found to increase in the presence of aliphatic chain 1-hexyl-3-methyl-imidazolium tetrafluoroborate [C₆mim][BF₄], (IL-1) whereas the critical region decreased in the presence of aromatic chain 1-benzyl-3-methylimidazolium tetrafluoroborate [Bnmim][BF₄], (IL-2). Adding 0.5–2.0 mg ml⁻¹ of IL-1 or IL-2 to IBW caused the critical temperature (T_c) to decrease by 0.553–2.898 K (for IL-1) and to increase by 0.502–1.664 K (for IL-2). The critical exponent (β) increases with increasing the IL concentrations, which are fully renormalized critical exponents. Further, our dynamic light scattering (DLS) results for the binary critical solution of IBW indicate that there exists a fluctuation in the local density that contributes significantly to the fluctuations in the scattered intensity at T_c. The variations in fluctuations are found to be pronounced in the presence of IL as an impurity. The reason for this behavior may be the formation of large water clusters with the neighboring molecules of ILs at T_c. This indicates that the IL entangles with the both coexisting phases, thereby, the IL strongly interacts with the neighboring solvent molecules. In addition, we report the fluorescence spectroscopy data for the critical mixture of IBW and its mixture with ILs as impurities using Nile Red as a fluorescent probe. Noticeable transitions of the two phases were detected at T_c through fluorescence technique. The observed variations in the fluorescence intensities with temperature predict unambiguously the formation of the solvation structure at T_c for the pure critical mixture as well as in presence of ILs.