Raman and IR spectroscopy research on hydrogen bonding in water-ethanol systems

摘要

Vibrational spectroscopy provides invaluable information about hydrogen bonding in aqueous solutions. To study changes in H-bonding due to increase of ethanol concentration in water, we perform research on water-ethanol binary mixtures with various mixing ratios using a combination of Raman scattering and IR absorption techniques. We study Raman spectra from 200 to 4000 cm ~(-1) excited at 488 nm and IR spectra from 500 and 4000 cm~(-1) for solutions with different ethanol concentrations from pure water to pure ethanol. Using the intensity ratio of OH stretching band taken at 3200 and 3420 cm~(-1) for Raman spectra and at 3240 and 3360 cm~(-1) for IR spectra we evaluate the strength of H-bonding. Maximal strength of H-bonding in water-ethanol mixture corresponds to ethanol concentration 15-20% w/w. We explain it by the presence of transient ethanol hydrates similar in composition to gaseous clathrates with stoichiometric water/ethanol ratio 5:1. Further weakening of H-bonding with ethanol concentration is caused by the formation of chain aggregates from ethanol/water molecules. In addition, we apply other approaches, such as multivariate curve resolution-alternating least squares analysis, decomposition of water Raman stretching band, and comparison of water Raman stretching band in ethanol solutions to that of gas clathrates to support this hypothesis.