Optical measurements of impurities in room-temperature ionic liquids

•We measure quantitative UV to mid-IR absorption spectra of 1-methylimidazole and [EMIM][TFSI].•We report DFT calculations of UV and infrared line positions and modes.•We identify wavelengths for in-situ detection if 1-methylimidazole in [EMIM][TFSI].•We demonstrate optical measurement of 1-methylimidazole mole fraction in [EMIM][TFSI].•We observe and use DFT to verify intermolecular interactions between 1-methylimidazole and [EMIM][TFSI] in the UV near 240 nm.

The absorption spectra of 1-methylimidazole and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [EMIM][TFSI], mixtures from 0% to 20% mole fraction are reported at ultraviolet to infrared wavelengths. Quantitative measurement of 1-methylimidazole in [EMIM][TFSI] is reported using an isolated 1-methylimidazole absorption feature at 1520 cm−1. UV measurements reveal a 1-methylimidazole absorption feature at 270 nm and a 260 nm feature for [EMIM][TFSI]. Absorption at 280 nm is demonstrated to scale with 1-methylimidazole mole fraction in [EMIM][TFSI] and is used to measure 1-methylimidazole mole fractions. Absorption is shown to increase around 240 nm upon addition of 1-methylimidazole to [EMIM][TFSI], which preliminary time-dependent density functional theory (DFT) calculations identify as an intermolecular excitation between [EMIM][TFSI] and 1-methylimidazole. We report recommendations for UV and infrared detection schemes for the quantitative detection of 1-methylimidazole in [EMIM][TFSI].