Insights into the Hydrogen Bond Interactions in Deep Eutectic Solvents Composed of Choline Chloride and Polyols

Deep eutectic solvents (DESs) consisting of cholinium chloride (ChCl) and alcohols have been widely applied in the purification of bioactive compounds, biodiesel, and flavonoids. However, an explicit and complete knowledge of the interactions between ChCl and alcohols is still lacking. In this work, the interactions between ChCl and polyols (1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 2,3-butanediol, 1,3-propanediol, glycerol, 1,5-pentanediol, 1,2,5-pentanetriol, and xylitol) have been investigated at different molar ratios of ChCI to polyols by Fourier transform middle infrared, far-infrared, H-1 and Cl-35 NMR spectroscopy as well as quantum chemistry calculation. It is shown that hydrogen bond interaction between Cl atom of ChCl and H atom of the O-H group in the polyols is predominant and its strength decreases with both the increase of carbon number between two hydroxyl groups in butanediol and the decrease of hydroxyl number in polyols. As butanediol content is increased in the mixture, the hydrogen bond interaction of choline cations with Cl is weakened, whereas that among butanediol molecules is enhanced. A molar ratio of ChCl to butanediol at 1:2 is indispensable for the formation of DES in a reasonable strength of hydrogen bond. The present results offer a possible explanation for viscosities of the DESs and may afford useful knowledge for the design and development of new DESs.