Effect of Exchange Dynamics on the NMR Relaxation of Water in Porous Silica

Abstract

The interaction of molecules, in particular, water, with solid interfaces has been studied by a multitude of methods, among them nuclear magnetic resonance spin relaxation. The frequency dependence of the relaxation times follows patterns that have been interpreted in terms of the molecular orientation and dynamics. Several different model approaches could successfully explain limiting cases of ¹H relaxation dispersion in systems with rigid surfaces such as silica gel or glass, but none of them can reproduce the relaxation of both ¹H and ²H nuclei, which differ in their respective relaxation mechanisms, dipolar vs quadrupolar. From detailed studies of the dynamics of hydration of water in biological materials, the importance of hydrogen and molecular exchange to the longitudinal relaxation time of T₁ was demonstrated. In this work, exchange times of both H₂O and D₂O in hydrophilic silica gel are varied in a controlled fashion in a wide range using disodium hydrogen phosphate, and the effect of physical exchange on spin relaxation is quantified for the first time in such systems using the exchange-mediated reorientation model.

Figure 1

²H (a) and ¹H (b) NMRD of D₂O and H₂O, respectively, at different concentrations of Na₂HPO₄ in fumed silica. The lines are fittings using eqs 3 and 5–8.

Figure 2

Exchange model of adsorbed water molecules on the silica surface includes hydroxyl groups on the silica surface (I), adsorbed water molecules (II), and bulk water (III) with respective exchange processes “EMOR1” and “EMOR2” described by the EMOR model (see eqs 3 and 5).