Evolutional mechanism of 1-ethyl-3-methyl-imdazolium acetate uptaking water from air detected with a new coupled method: Two-dimensional correlation difference spectroscopy

Abstract

A new concept by coupling two-dimensional correlation spectroscopy (2D-COS) (particularly synchronous perturbation-correlation moving-window 2D-COS (s-PCMW2D-COS)) and difference spectroscopy (DS), i.e., 2D-CODS (particularly s-PCMW2D-CODS), is proposed in this study. Four steps (i.e., original spectra, DS, s-PCMW2D-COS, and sliced spectra in sequence) are proposed to produce s-PCMW2D-CODS with the original data. Computational simulated spectra in the same/opposite direction with linear/nonlinear comparison are used to illustrate the s-PCMW2D-CODS. It is shown that the relationship for comparing intensity along the perturbation variable is clearer, more accurate and more direct by s-PCMW2D-CODS than by original spectra, DS, or s-PCMW2D-COS. Finally, a practical example of utilizing s-PCMW2D-CODS to investigate the dynamically atmospheric water sorption process in acetate-based ionic liquid (IL) 1-ethyl-3-methlyl-imidazolium acetate ([EMIM][Ac]) via in-situ infrared spectroscopy (IR) is given. It is found that OH of the atmospheric water is affected first due to their weaker hydrogen-bonding network than that of [EMIM][Ac]; then, the cation ring C4,5H is freed followed by C2H because of the weaker hydrogen-bonding interaction between C4,5H and anion than that between C2H and anion; finally, the alkyl H (i.e., CbH, C6,7H, C8H) is interrupted owning to their lower hydrogen-bonding donating ability than the cation ring H (i.e., C2H, C4,5H) with O of water and their higher hydrophobicity.