Concentration measurements in ionic liquid-water mixtures by mid-infrared spectroscopy and indirect hard modeling

An analytical method for the quantitative characterization of binary mixtures of water and ionic liquids (ILs) is presented. Mid-infrared (mid-IR) spectroscopy in combination with indirect hard modeling (IHM) is employed to quantify the water content in 1-butyl-3-methylimidazolium chloride (BMIMCl), 1,3-dimethylimidazolium dimethylphosphate (DMIMDMP), and 1-ethyl-3-methylimidazolium acetate (EMIMAc). Despite significant nonlinear shifts of the spectral bands, a good spectral fit with calibration errors of less than 2.3 wt % can be achieved almost over the whole concentration range. A profound analysis of the spectral models including peak assignment substantiates the physico-chemical foundation of the spectral models. Furthermore, the shift of peak functions in the spectral models is shown to provide a measure of molecular interaction in IL-water mixtures, which can also be utilized quantitatively. The vibrational bands of the water dipole reveal differences in the strength of hydrogen bonding with water in the IL studied. These properties of the spectral hard models demonstrate their quantitative analytical potential and set the stage for multiway calibration in comprehensive reaction monitoring in these highly interacting mixtures of IL and water.