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

Abstract

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.

Keywords:

• Raman scattering
• IR absorption spectra
• stretching vibrations
• hydrogen bonding
• ethanol hydrates

Acknowledgements

The authors thank Prof. Dale W. Schaefer and Dr Naiping Hu, Department of Chemical and Materials Engineering, University of Cincinnati, USA for performing MCR-ALS analysis and for helpful discussion of results.