PROTON ROTATING-FRAME SPIN-LATTICE RELAXATION STUDY OF SLOW MOTION OFPORE-WATER

Proton rotating frame spin-lattice relaxation times of water in hydrat ed porous silica glass, with pore diameters of 7.4, 23.7, 49.1, and 54 .8 nm, are reported. Experiments were limited to hydrations correspond ing to more than one-third hydrated pore volumes. The frequency depend ence of the relaxation rates reveals a slow motion of pore water with a characteristic correlation time of similar to 10(-5) s. Within the l imits given above, this correlation time depends on pore dimension and hydration only slightly. This indicates that the surface water molecu les, which reorient with a characteristic time of similar to 10(-8) s, are changing their orientational axis with the slower correlation tim e similar to 10(-5) s. This process, which disrupts the nearby water m olecules, 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 b e affected by these slow jumps. (C) 1998 American Institute of Physics .