Diffusion of water in clays - microscopic simulation and neutron scattering

The dynamics of water in porous charged media (montmorillonite clay) is investigated on the picosecond-timescale by quasi-elastic neutron scattering (time-of-flight (TOF) and neutron spin echo (NSE) techniques) and classical molecular dynamics simulations. Correspondence is discussed not only in terms of integrated quantities such as diffusion coefficients but also more directly on the level of intermediate scattering functions. Both simulated and experimental water diffusion coefficients are of the order of 5-10 × 10−10 m2 s−1. Closer analysis suggests that, unlike NSE, TOF and simulation underestimate relaxation times in the low-Q region due to insufficiently large correlation times probed. Comparison between experimental and simulated dynamics is rendered difficult by the features of the real montmorillonite clay (interstratification, mesoporous water) omitted in the model. For the de-coupling of phenomena in a real clay, a more complete set of data for the montmorillonite clay (different ions, hydration states) or the use of other, in some respect more homogeneous clays (hectorite, vermiculite) is suggested.