MOLECULAR-DYNAMICS SIMULATION AND NMR-STUDY OF WATER ACETONITRILE MIXTURES

Pure water, pure acetonitrile, and biliary mixtures of these two liquids at 0. 1 2, 0.50, and 0.88 molar fractions were simulated by using the molecular dynamics technique with two different sets of effective pair potentials. The reorientational correlation times were deduced from the NMR relaxation of N-14 in acetonitrile and O-17 in water for comparison.. A manifold increase was observed in the amplitudes of the water-water radial distribution functions upon dilution with acetonitrile, while the acetonitrile arrangement remained relatively intact. The intercomponent interactions were also intensified as moving from the high to low water concentrations. The simulations reproduced the experimental trends in the self-diffusion and reorientation of acetonitrile but rendered too high a mobility for pure water, a feature known previously. At low water content, however, the agreement between the simulated and observed dynamics of water was better. The simulations indicate a slight initial enhancement of the hydrogen-bonded structure of water upon dilution, followed by a breakdown leading to heterogeneity at the molecular level. In the diluted aqueous solution, the presence of water dimers, trimers, tetramers, etc. leads to faster but correlated molecular motions.