Two-dimensional NMR study of surface water dynamics in hydrated silica spheres

Partially hydrated LUDOX silica spheres (LUDOX colloidal silica HS-40, E.I. DuPont de Nemours & Co.) of 15 nm diameter represent a system where the wetting layer of water is quite well defined in the sense that the study of the structure and dynamics of this layered water is not obscured by the much stronger signal of bulk water. When placed in a homogeneous magnetic field, the magnetic susceptibility differences of the silica and water create a distribution of local magnetic fields at the solid-liquid boundary. These fields encode the NMR frequency of the resonant nuclei and label water molecules in the surface layer. The distribution of local fields was determined by the deuteron two-dimensional (2D) separation of interactions NMR technique. The characteristic autocorrelation time tau(c) for the motion of water molecules in the surface layer was studied by measuring the deuteron quadrupolar spin-lattice and spin-spin relaxation times, and taking their ratio. The tau(c) characterizing the fluctuations of the electric-field gradient tensor at the deuteron site was determined at room temperature for a set of hydrations corresponding to an estimated thickness of surface water from 2 to 22 water monolayers. The magnitude of tau(c), which is on the order of 10(-8) s, demonstrates that the quadrupolar relaxation mechanism is the intramolecular rotational diffusion. This dynamic process slows down only insignificantly with a decreasing thickness of the wetting layer. A search for slower motions was made by the 2D exchange NMR of deuterons in a 6% hydrated LUDOX. No slow molecular motions were detected within the observation window of the experiment, demonstrating that the motional frequencies are faster than 2 pi x 10(5) s(-1).