Computational and experimental study of the interactions between ionic liquids and volatile organic compounds

Abstract

Computational chemistry calculations were performed to investigate the interactions of ionic liquids with different classes of volatile organic compounds (VOCs), including alcohols, aldehydes, ketones, alkanes, alkenes, alkynes and aromatic compounds. At least one VOC was studied to represent each class. Initially, 1-butyl-3-methylimindazolium chloride (abbreviated as C₄mimCl) was used as the test ionic liquid compound. Calculated interaction lengths between atoms in the ionic liquid and the VOC tested as well as thermodynamic data suggest that C₄mimCl preferentially interacts with alcohols as compared to other classes of volatile organic compounds. The interactions of methanol with different kinds of ionic liquids, specifically 1-butyl-3-methylimidazolium bromine (C₄mimBr) and 1-butyl-3-methylimidazolium tetrafluoroborate (C₄mimBF₄) were also studied. In comparing C₄mimCl, C₄mimBr, and C₄mimBF₄, the computational results suggest that C₄mimCl is more likely to interact with methanol. Laboratory experiments were performed to provide further evidence for the interaction between C₄mimCl and different classes of VOCs. Fourier transform infrared spectroscopy was used to probe the ionic liquid surface before and after exposure to the VOCs that were tested. New spectral features were detected after exposure of C₄mimCl to various alcohols. The new features are characteristic of the alcohols tested. No new IR features were detected after exposure of the C₄mimCl to the aldehyde, ketone, alkane, alkene, alkyne or aromatic compounds studied. In addition, after exposing the C₄mimCl to a multi-component mixture of various classes of compounds (including an alcohol), the only new peaks that were detected were characteristic of the alcohol that was tested. These experimental results demonstrated that C₄mimCl is selective to alcohols, even in complex mixtures. The findings in this work provide information for future gas-phase alcohol sensor design.