Pressure and Temperature Effects on the Molecular Rotation in Acetonitrile-Water Mixtures

Abstract

NMR spin-lattice relaxation time (T1) measurements were performed for 14N of acetonitrile in acetonitrile (CH3CN)—H2O mixtures and for 2H of heavy water in CH3CN—D2O mixtures at 30°C up to 294.2 MPa together with those for 2H in CH3CN—D2O mixtures at 10 and 20°C 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 30°C under atmospheric pressure. Densities and viscosities of CH3CN—H2O mixtures were also measured under high pressure. The rotational correlation times for D2O [τ c (D)] and acetonitrile [τ c (N)] were determined from T1 measurements. Under atmospheric pressure, τ 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 τ c (D) and τ c (N) at higher acetonitrile contents are ascribed to the formation of acetonitrile dimer, trimer, and oligomer aggregates. Except for τ c (D) in the water-rich region, the pressure coefficients of τ c (D) and τ c (N) are positive which is understood as a simple compression effect. Furthermore, the composition of mixture at which τ c (D) and τ 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.