Pressure and temperature effects on the molecular rotation in acetonitrile-water mixtures

摘要

NMRspin-lattice relaxation time (T-1) measurements were performed for N-14 of acetonitrile in acetonitrile (CH3CN)-H2O mixtures and for H-2 of heavy water in CH3CN-D2O mixtures at 30degreesC up to 294.2MPa together with those for H-2 in CH3CN-D2O mixtures at 10 and 20degreesC under atmospheric pressure over the whole composition range of the mixtures. IR absorption spectra for CH3CN-H2O and CH3CN-10 mol% HDO/D2O mixtures were obtained at 30degreesC under atmospheric pressure. Densities and viscosities of CH3CN-H2O mixtures were also measured under high pressure. The rotational correlation times for D2O [tau(c)(D)] and acetonitrile [tau(c)(N)] were determined from T-1 measurements. Under atmospheric pressure, tau(c)(D) exhibits a small maximum around 10 mol% of acetonitrile at each temperature, and the maximum position is almost independent of temperature. These results suggest that the dipole - dipole interaction between acetonitrile and water molecules plays an important role in determining the rotational motion of water molecules in the mixtures. This is supported by the variation of the peak for the bending vibration of water molecules with composition. The decreases in tau(c)(D) and tau(c)(N) at higher acetonitrile contents are ascribed to the formation of acetonitrile dimer, trimer, and oligomer aggregates. Except for tau(c)(D) in the water-rich region, the pressure coefficients of tau(c)(D) and tau(c)(N) are positive which is understood as a simple compression effect. Furthermore, the composition of mixture at which tau(c)(D) and tau(c)(N) show a maximum shifted to higher acetonitrile content with increasing pressure. These results are discussed in terms of the pressure effect on the equilibria of acetonitrile monomers with the aggregates of acetonitrile in the mixtures.