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

Neutron quasielastic scattering experiments and molecular dynamics simulations have been used to probe the diffusive dynamics of the water in aqueous solutions of 1,2-dimethoxyethane (DME). This study focuses on the comparison of the simulation and scattering results, from a variety of perspectives. The conventional method of analyzing the quasielastic scattering is a model based fit of the scattering law, assuming a jump translational motion of the center-of-mass in combination with an independent model for the rotational diffusion of the proton on the surface of a sphere. The diffusion constants derived from such an analysis is contrasted with the predictions of the simulation model. At the next level, the simulation model is compared with the jump diffusion and rotational diffusion formalisms, followed by a direct 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 simulation results are inconsistent with this model and there is a signature of a low-frequency librational mode. For the translational model, the simulation and scattering experiments yield similar translational diffusion constants, although the jump diffusion model is inconsistent with the simulation results. The direct comparison of the intermediate scattering functions highlights the strong influence of the translational contribution to this function at longer times, which makes such a direct comparison a poor test of 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.