Water Dynamics on the Surface of MCM-41 via 2H Double Quantum Filtered NMR and Relaxation Measurements

Abstract

Dynamics of water adsorbed on MCM-41 has been investigated by line shape analyses of 2H double quantum filtered (DQF) NMR and T1 measurements. From temperature-dependent studies with various water loadings on MCM-41, it is found that there are three sites for water adsorption on MCM-41, namely, a “slow site”, a “fast site I”, and a “fast site II”. The signal of water molecules in the slow site dominates in the observed DQF NMR. Fast site I is adjacent to the slow site, and the water molecules are exchangeable in these two sites. The fast site II is next to the fast site I, and water in the fast site II is only exchangeable with that in the fast site I. The influence of silanol groups on the MCM-41 surface strongly affects the motion of water molecules in the slow site, resulting in a residual quadrupolar interaction. As the signal of water in the fast site I dominates that observed in single quantum spectra, it also appears in DQF spectra through an exchange process. We found that when the fast site I is saturated, it has no influence on the internal rotation, but the wobbling motion of adsorbed D2O in the slow site becomes faster, i.e., the translational diffusion of D2O in the fast site I may increase the wobbling motion of adsorbed D2O through collision.