QENS and NMR Study of Water Dynamics in SBA-15 with a Low Water Content

In this study, motions performed by water molecules adsorbed on the silica surface of SBA-15 material with 6.1% of water content (15% of pore filling) were investigated using NMR and Quasielastic Neutron Scattering (QENS) techniques. The results show no sign of translational diffusion of water, but two types of stochastic localized motions were identified, and both described using a model of proton jumps between two sites. For both motions, the characteristic jump distances and correlation times, as well as activation energies, have been extracted and found to differ significantly. On this basis, the faster motion was ascribed to jumps of water molecules between neighboring positions (d = 2.5 Å, τ = 4 ps at 300 K, and E_a = 5.2 ± 0.2 kJ/mol from NMR data, and 5.6 ± 1.1 kJ/mol from QENS), while the slower one exhibits a temperature dependent jump distance and was ascribed to jumps of water molecules between more spatially separated positions (d = 2.9–4.3 Å, τ = 25 ps at 300 K, and E_a = 16.1 ± 0.3 kJ/mol from NMR, and 17.3 ± 0.3 kJ/mol from QENS data).