Local and diffusive dynamics of LiCl aqueous solutions in pristine and modified silica nanopores.

We use 1H, 2H, and 7Li nuclear magnetic resonance to investigate local and diffusive dynamics of LiCl-7H2O and LiCl-7D2O solutions in pristine and functionalized silica nanopores in a component-selective manner. Recently, we showed that the solution dynamics become slower when the diameter of the pristine pores is reduced. Here, we determine the effects of (aminopropyl)triethoxysilane and dye surface functionalizations on the motions of the water molecules and lithium ions from ambient temperatures down to the glass transition. The local and diffusive solution dynamics are similar in both functionalized pores but, on average, slower than in pristine pores with comparable diameters. When the exchange between different confinement regions is sufficiently slow at reduced temperatures, bimodal water and lithium dynamics may be observed. We attribute this bimodality to bulk-like motion in the pore centers and slowed-down motion at the pore walls. For the lithium ions, a bimodality observed in the pristine pores is absent in the functionalized ones. We conjecture that the steric hindrance and electrostatic interactions associated with the grafted functional groups interfere with the formation of a defined electric double layer, while the enhanced surface roughness and unequal charge distribution result in overall slower dynamics. Thus, the nature of the walls is an important parameter for the solution dynamics. Thereby, in situ measurements of the pH value inside the silica pores using the grafted dye molecules reveal that observed changes in the pH value in response to the surface functionalization are of limited relevance for the water reorientation.

NMR studies on the influence of silica confinements on local and diffusive dynamics in LiCl aqueous solutions approaching their glass transitions.

We use 1H, 2H, and 7Li NMR to investigate the molecular dynamics of glass-forming LiCl-7H2O and LiCl-7D2O solutions confined to MCM-41 or SBA-15 silica pores with diameters in the range of d = 2.8 nm-5.4 nm. Specifically, it is exploited that NMR experiments in homogeneous and gradient magnetic fields provide access to local and diffusive motions, respectively, and that the isotope selectivity of the method allows us to characterize the dynamics of the water molecules and the lithium ions separately. We find that the silica confinements cause a slowdown of the dynamics on all length scales, which is stronger at lower temperatures and in narrower pores and is more prominent for the lithium ions than the water molecules. However, we do not observe a temperature-dependent decoupling of short-range and long-range dynamics inside the pores. 7Li NMR correlation functions show bimodal decays when the pores are sufficiently wide (d > 3 nm) so that bulk-like ion dynamics in the pore centers can be distinguished from significantly retarded ion dynamics at the pore walls, possibly in a Stern layer. However, we do not find evidence for truly immobile fractions of water molecules or lithium ions and, hence, for the existence of a static Stern layer in any of the studied silica pores.

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 τp 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 Tm 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 τp of confined water are consistent with those of the bulk liquid, suggesting that we probe a dynamical process that is a characteristic feature of water. We argue that none of the proposed origins of the dynamical crossover is fully consistent with all observations and discuss possible new directions.