An investigation of water dynamics in binary mixtures of water and dimethyl sulfoxide

The motion of water molecules in mixtures of water and d6-dimethyl sulfoxide (DMSO) has been explored through molecular dynamics (MD) simulations using the SPC/E water model (J. Chem. Phys. 1987, 91, 6269) and the P2 DMSO model (J. Chem. Phys. 1993, 98, 8160). We evaluate the self-intermediate scattering functions, FS(Q,t), which are related by a Fourier transform to the incoherent structure factors, S(Q,omega), measured in quasielastic neutron scattering (QNS) experiments. We compare our results to recent QNS experiments on these mixtures reported by Bordallo et al. (J. Chem. Phys. 2004, 121, 12457). In addition to comparing the MD data to the experimental signals, which correspond to a convolution of S(Q,omega) with a resolution function, we examine the rotational and translational components of FS(Q,t) and investigate to what extent simulation results for the single-molecule dynamics follow the dynamical models that are used in the analysis of the experimental data. We find that the agreement between the experimental signal and the MD data is quite good and that the portion of FS(Q,t) due to translational dynamics is well represented by the jump-diffusion model. The model parameters and their composition dependence are in reasonable agreement with experiment, exhibiting similar trends in water mobility with composition. Specifically, we find that water motion is less hindered in water-rich and water-poor mixtures than it is near equimolar composition. We find that the extent of coupling between rotational and translational motion contributing to FS(Q,t) increases as the equimolar composition of the mixture is approached. Thus, the decoupling approximation, which is used to extract information on rotational relaxation from QNS spectra at higher momentum transfer (Q) values, becomes less accurate than that in water-rich or DMSO-rich mixtures. We also find that rotational relaxation deviates quite strongly from the isotropic rotational diffusion model. We explore this issue further by investigating the behavior of orientational time correlations for different unit vectors and corresponding to Legendre polynomials of orders 1-4. We find that the rotational time correlations of water molecules behave in a way that is more consistent with the extended jump rotation model recently proposed by Laage and Hynes (Science 2006, 311, 832).