Reorientation of Deeply Cooled Water in Mesoporous Silica: NMR Studies of the Pore-Size Dependence.

We apply 2H NMR to investigate water reorientation in mesoporous silica. Using synthesized MCM-41 and purchased SBA-15 materials, we systematically study the dependence on the pore diameter. For a characterization of the properties of the silica matrices, scanning electron microscopy and nitrogen gas adsorption are employed. To ascertain the thermodynamic behavior of the confined water, we utilize differential scanning calorimetry. The dynamical properties are analyzed in a broad temperature range from the weakly to the deeply cooled regime using a combination of 2H spin-lattice relaxation and stimulated-echo experiments. We find that the reorientation of confined water is governed by prominent heterogeneity. The peak correlation time τ_p of the distribution is independent of the pore diameter in a range of ca. 2-6 nm. It shows a dynamical crossover at ~220 K in all studied confinements, including wide silica pores where the effect occurs below T_m and, hence, in the two-phase region with coexisting liquid and solid water fractions. Above and below the no-man's land of water, the pore-size independent correlation times τ_p of confined water are consistent with that of the bulk liquid, suggesting that we probe a water-characteristic dynamical process. We argue that none of the proposed origins of the dynamical crossover is fully consistent with all observations and discuss possible new directions.