Structural and Dynamical Insights into Acetonitrile–Water Mixtures: A Molecular Dynamics Study of Self-Diffusion, Reorientational Correlation Times, and Temperature-Dependent Behavior

The study investigates the structural and dynamical properties of acetonitrile-water mixtures using molecular dynamics simulations over a broad range of acetonitrile molar fractions (0.0 to 1.0) and temperatures (298-348 K). The dielectric constant, recognized as one of the slowest-converging dynamical properties, exhibited consistent stabilization across all systems after approximately 20 ns. Particular attention is given to the self-diffusion coefficients and reorientational correlation times of key molecular vectors in both solvents. Structural analysis based on radial distribution functions reveals that the nearest neighbor interactions between acetonitrile molecules occur within the 2.85-4.00 Å range. With increasing acetonitrile concentration, the self-diffusion coefficient of acetonitrile increases progressively, whereas that of water exhibits a nonlinear dependence on the acetonitrile content. Reorientational correlation times for the OH and dipole moment vectors of water molecules exhibit nonlinear trends as a function of acetonitrile molar fraction, whereas the correlation time for the acetonitrile dipole moment decreases progressively with increasing acetonitrile concentrations. For all systems, self-diffusion coefficients increase with temperature, while reorientational correlation times decrease. In these mixtures, water molecules undergo reorientation via a jump rotation mechanism.