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 xMe and xEt 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 xMe and xEt up to ∼ 0.7 the R1 values slightly decreased and/or remained almost constant, and then increased again when xEt > 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.
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