Organic co-solvents mediated variation in anion-water hydrogen bonding in [Bmim][BF₄] ionic liquid through FTIR spectroscopy

Abstract

FTIR spectroscopy has been employed to investigate the variation of anion-water hydrogen bonding in 1-butyl 3-methyl imidazolium tetrafluoroborate ([Bmim] [BF 4 ]) ionic liquid caused by addition of organic co-solvents with various polarities. The variation was estimated by probing band shape and intensity of the OH stretching vibration of trace water present in ionic liquid at 3400-3800 cm -1. The presence of polar aprotic co-solvent in ionic liquid dramatically reduces the absorptivity of the OH stretch band, indicating that the co-solvent changes the nature of anion-water hydrogen bond drastically, which might be responsible for the reduction of the viscosity of ionic liquid in the presence of the cosolvent.For the last two decades ionic liquids have drawn significant attentions of the scientific and industrial community for possessing characteristics like low vapor pressure, wide solubility range, large electrochemical window, low flammability, extensive liquid range, and inertness etc., which are desirable properties in synthesizing the chemical product efficiently by emphasizing on toxicity reduction as well as waste minimization.1-4 Ionic liquids remain liquid at room temperature and are composed of organic cations and inorganic or organic anions. 1,4 Built in molecular asymmetry in the structure of ionic liquids diminishes the extent as well as the range of Coulombic interactions among the constituent ions, resulting in properties more akin to normal liquids. 3 Moreover, existence of hydrogen bonding network in the midst of cations and anions persuades co-operative effect and structural directionality (entropic effect) in the ionic liquids. 5,6Though ionic liquids possess most of characteristics propagated in 'green chemistry', still wide spread applicability of ionic liquids in the realm of chemical synthesis at large scale is limited due to its high viscosity which retards the chemical reactions by slowing the dynamics or movement of the reactants. 7 This challenge has been overcome by introducing co-solvents which help to reduce viscosity of the ionic liquids keeping other properties least altered to widen its applicability. 8 However, the molecular origin of the process by which these co-solvents reduce viscosity of ionic liquids is still illusive. All these ionic liquids possess some water which is practically impossible to remove. , the appearance of two isolated IR bands implies that a few water molecules in ionic liquids remain free or isolated rather than participating in hydrogen bonding with other water molecules.9 Most of the properties of ionic liquid depend on the characteristics of constituent anion. The IR band is significantly modified by the variation of anions rather than cations of the ionic liquids. The band position and its shape are the function of stretching vibration of hydrogen bond evolved due to formation of 'anion···HOH···anion' hydrogen bond. 9 We have concentrated on this specific region of IR spectra of water in ionic liquids since alteration of the position and...