Low-frequency Raman spectroscopy of ethanol-water binary solution: Evidence for self-association of solute and solvent molecules

Low-frequency Raman spectra of ethanol-water binary solutions with varying mixing ratios are measured and presented in the form of the intrinsic Raman scattering activity, the R(<(nu)over bar>) spectre. They exhibit an isosbestic point at 135 cm(-1) between the pure water O-O stretching peak at 185 cm(-1) and that of pure ethanol at 110 cm(-1). The R(<(nu)over bar>) spectra of the binary solutions are found to be decomposed into linear combinations of the R(v) spectra of pure water and pure ethanol. The plot of the coefficients of linear combinations as a function of ethanol mole fraction x(E) has an inflection point at x(E) = 0.2, and the increasing fraction of the ethanol component is compensated with the decreasing fraction of the water component. This can be explained when ethanol-water binary solution does not get ideally mixed on the molecular level and the hydrogen bonds between water associates and ethanol associates become weak owing to the collective molecular motion or the sliding motion of the two units.