Insight into the Intermolecular Interactions in [Bmim]BF4/[Amim]Cl−Ethanol−Water Mixtures by Near-Infrared Spectroscopy

Abstract

In this contribution, we examined the effect of ethanol on the structural organization of aqueous solution of ionic liquids (ILs) using the near-infrared (NIR) technique and two-dimensional (2D) correlation spectra. The ILs used here are 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim]BF(4)) and 1-allyl-3-methylimidazolium chloride ([Amim]Cl). It was easily found, from the distinct change tendency of NIR spectra for their aqueous solution, that the added ethanol exerted a different effect on the solution structure of [Bmim]BF(4) and [Amim]Cl. For [Amim]Cl/H(2)O, it was deduced that ethanol molecules prefer to compete with water by interacting with imidazolium C(2)-H rather than C(4,5)-H groups, accompanied by the formation of C(2)-H...O interactions with ethanol molecules, while ethanol molecules do not interact specifically with any imidazolium C-H groups for [Bmim]BF(4). Furthermore, it was shown that the nonpolar tail of [Amim]Cl is more sensitive to the decrease of polarity or dielectric constant of solvents than its polar head, whereas the converse is true for [Bmim]BF(4). However, for both ILs, ethanol molecules were capable of changing the interaction between cations and water, anions and water by introduction of C-H...O interactions with cations, as well as the strong hydrogen-bond interactions between the anions and the hydroxyls of the ethanol. This information is suggestive for recycling the hydrophilic ILs by distillation from their aqueous diluted solutions, as well as for purifying hygroscopic ILs.