Low-frequency Raman spectroscopy of aqueous solutions of aliphatic alcohols

Abstract

Low-frequency Raman spectra have been measured at room temperature as functions of the alcohol mole fraction in aqueous solutions of methanol, ethanol, 1-propanol, 2-propanol, and tert-butylalcohol (TBA). Intrinsic Raman spectra R(v̄) were obtained from depolarized Rayleigh wing spectra. Isosbestic points have been observed in R(v̄) of the aqueous solutions of ethanol, 1-propanol, and 2-propanol, suggesting that the structure of the solutions is characterized by individual alcohol aggregates and water clusters without a significant amount of alcohol-water mixed aggregates. The R(v̄) spectra have been expressed as R(v̄, x) = wR(v̄, 0) + aR(v̄, 1), where R(v̄, 0) and R(v̄, 1) are those for pure water and pure alcohols, respectively, and x is the mole fraction of alcohols. The coefficients w and a show the inflection points at characteristic alcohol mole fractions, where microhetrogeneity and structural transition of the solvent clusters take place, as previously shown by X-ray diffraction. In the aqueous solutions of methanol, where no microhetrogeneity takes place, no clear isosbestic point in R(v̄) has been observed. For aqueous solutions of TBA, an isosbestic point in R(v̄) has appeared when xTBA > 0.05. Two inflections points in the coefficients have been observed at xTBA ≈ 0.1 and 0.35; the former composition corresponds to the transition composition from the TBA-TBA intermolecular contact to the TBA-water molecular association, as previously reported by neutron diffraction.