Quasi-Elastic Neutron Scattering Spectroscopy of the 1-Propanol/Water Solution by Molecular Dynamics Simulations

Quasi-elastic neutron scattering (QENS) spectroscopy as an important tool can be used to extract the molecular dynamic properties. However, the validity of the dynamical models and the decoupling approximation used in QENS spectral analysis is a topic of ongoing debate. In this paper, the self-intermediate scattering function F-s(Q, t) and the decoupling approximation function F-p(Q, t) of the hydroxyl hydrogen in pure water and in 1-propanol/water mixture, and certain dynamic properties predicted by three translation models, are derived from molecular dynamics simulations to assess their reasonability. The results suggest that the decoupling approximations for the water hydrogen in pure water and in mixture are reasonable at low momentum transfer Q. The contribution from the translation-rotation coupling term is small for the pure water. The coupling effect is strengthened for the water hydrogen when 1-propanol is added to the water. Under these conditions, the coupling and rotation terms both increase with the momentum transfer Q and largely cancel each other. For the hydroxyl hydrogen of 1-propanol in the mixture, the translational diffusion constant cannot be directly derived from the experimental spectrum, due to large deviation between F-s(Q, t) and the center-of-mass translational function F-CM(Q, t). The translational diffusion constants by the three translation models used in our current work are consistent with experimental results and a little higher than those predicted by the Einstein method. The jump rotation, as opposed to continuous rotation, is observed for the water molecule in both bulk water and mixture. For the 1-propanol molecule, rotations are anisotropic, being continuous along the axis from the hydroxyl hydrogen to the center-of-mass, and jumping along the hydroxyl bond vector. Simulations indicate that neither the rotational diffusion constant nor the relaxation time at high momentum transfer Q are adequately determined by the decoupling models, since the coupling effects become significant. Within the low momentum transfer range, the translation properties can be reasonably derived, due to the negligible contributions from the rotation and the coupling terms, as well as the canceling effect between them.