Diffusion in aqueous solutions of 1,2-dimethoxyethane: comparison of molecular dynamics simulations and quasielastic neutron scattering

Neutron quasielastic scattering experiments and molecular dynamics simulati ons have been used to probe the diffusive dynamics of the water in aqueous solutions of 1,2-dimethoxyethane (DME). This study focuses on the compariso n of the simulation and scattering results, from a variety of perspectives. The conventional method of analyzing the quasielastic scattering is a mode l based fit of the scattering law, assuming a jump translational motion of the center-of-mass in combination with an independent model for the rotatio nal diffusion of the proton on the surface of a sphere. The diffusion const ants derived from such an analysis is contrasted with the predictions of th e simulation model. At the next level, the simulation model is compared wit h the jump diffusion and rotational diffusion formalisms, followed by a dir ect comparison of the simulation and the scattering experiment at the level of the intermediate scattering function. From this approach, it is clear that the free rotation on the surface of a sphere model is adequate for the water rotational motion in the bulk and in dilute solutions of DME. However, for concentrated solutions of DME, the s imulation results are inconsistent with this model and there is a signature of a low-frequency librational mode. For the translational model, the simu lation and scattering experiments yield similar translational diffusion con stants, although the jump diffusion model is inconsistent with the simulati on results. The direct comparison of the intermediate scattering functions highlights the strong influence of the translational contribution to this f unction at longer times, which makes such a direct comparison a poor test o f the rotational model. The general. conclusion is that a detailed analysis at many levels of both the simulation and the scattering data is required to validate the analysis of the scattering data and the validity of the simulation model. (C) 2000 Published by Elsevier Science B.V.