¹⁷O NMR relaxation study of dynamics of water molecules in aqueous mixtures of methanol, ethanol, and 1-propanol over a temperature range of 283-403 K

¹⁷O NMR spin-lattice relaxation time (T₁) of water molecules was measured in aqueous mixtures of methanol, ethanol, and 1-propanol over a temperature range of 283-403 K. At temperatures below ∼ 303 K, the relaxation rate R₁ (= 1/T₁) in the aqueous mixtures of methanol and ethanol sharply increased with increasing alcohol concentration up to characteristic alcohol mole fractions x_Me and x_Et of ∼ 0.3 and ∼ 0.2, respectively, showing that the rotation of water molecules is gradually retarded in the water-rich region due to hydrophobic hydration. With increasing x_Me and x_Et up to ∼ 0.7 the R₁ values slightly decreased and/or remained almost constant, and then increased again when x_Et > 0.7. With increasing temperature, the degree of the retardation in the water-rich region became insignificant, and the R₁ values monotonously increased with increasing alcohol concentration. The relaxation rates in the 1-propanol-water mixtures monotonously increased with increasing alcohol content at all temperatures investigated. These results are discussed in view of microscopic structure of the alcohol-water mixtures previously obtained from X-ray diffraction measurements.