Dynamics of Interfacial Water Molecules Inside a Mesoporous Material as Probed by NMR Relaxometry: A Multimodal Approach

Abstract

Probing molecular dynamics in porous media with nuclear magnetic resonance dispersion (NMRD) provides access to a unique time window that spans from nanoseconds to microseconds. The counterpart of this large window is the difficulty in reaching the uniqueness of the modeling. To overcome this problem, a multimodal approach is applied, combining small-angle X-ray scattering (SAXS), 3D reconstruction of the pore lattice from experimental images (previously done), and numerical simulations. This article is focused on the dynamics of water molecules confined inside boehmite, a mesoporous material, precursor of γ-alumina catalytic support. 1H and 2H R1(νI) are analyzed at different water uptakes. Their similar frequency dependence highlights the strong dynamical confinement of water near the pore surface and the important contribution of intradipolar interaction in the frequency dependence of R1(νI). 3D numerical simulations of water surface dynamics confirm this point. Good agreement with experimental NMRD data is achieved for a surface diffusion coefficient Ds = 10–10 m2/s, associated with a very long surface residence time (above 10 μs). Finally, this multimodal approach provides a quantitative link between NMRD and surface pore orientation properties through the definition of an orientation order function.