Low-frequency Raman spectroscopy of aqueous solutions of aliphatic alcohols

Low-frequency Raman spectra have been measured at room temperature as funct ions of the alcohol mole fraction in aqueous solutions of methanol, ethanol , 1-propanol, 2-propanol, and tert-butylalcohol (TBA). Intrinsic Raman spec tra R((v) over bar) were obtained from depolarized Rayleigh wing spectra. I sosbestic points have been observed in R((v) over bar) of the aqueous solut ions of ethanol, 1-propanol, and 2-propanol, suggesting that the structure of the solutions is characterized by individual alcohol aggregates and wate r clusters without a significant amount of alcohol-water mixed aggregates. The R((v) over bar) spectra have been expressed as R((v) over bar, x) = wR( (v) over bar, 0) + aR((v) over bar, 1), where R((v) over bar, 0) and R((v) over bar, 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 infle ction points at characteristic alcohol mole fractions, where microhetrogene ity and structural transition of the solvent clusters take place, as previo usly shown by X-ray diffraction. In the aqueous solutions of methanol, wher e no microhetrogeneity takes place, no clear isosbestic point in R((v) over bar) has been observed. For aqueous solutions of TBA, an isosbestic point in R((v) over bar) has appeared when x(TBA) > 0.05. Two inflections points in the coefficients have been observed at x(TBA) approximate to 0.1 and 0.3 5; the former composition corresponds to the transition composition from th e TBA-TBA intermolecular contact to the TBA-water molecular association, as previously reported by neutron diffraction.