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

Abstract

17O NMR spin-lattice relaxation time ( T 1) 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 R 1 (= 1/ T 1) 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 R 1 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 R 1 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.