Proton rotating frame spin-lattice relaxation study of slow motion of pore water

Proton rotating frame spin-lattice relaxation times of water in hydrated porous silica glass, with pore diameters of 7.4, 23.7, 49.1, and 54.8 nm, are reported. Experiments were limited to hydrations corresponding to more than one-third hydrated pore volumes. The frequency dependence of the relaxation rates reveals a slow motion of pore water with a characteristic correlation time of ∼10^-5 s. Within the limits given above, this correlation time depends on pore dimension and hydration only slightly. This indicates that the surface water molecules, which reorient with a characteristic time of ~∼10^-8 s, are changing their orientational axis with the slower correlation time ∼10^-5 s. This process, which disrupts the nearby water molecules, depends primarily on the environment in the vicinity of the molecule's original binding site. The analysis, based on the two-site fast exchange model, indicates that in small pores all the water may be affected by these slow jumps.