Water structure and dynamics at a silica surface: Pake doublets in 1H NMR spectra

Identification of Proton Populations in Cherts as Natural Analogues of Pure Silica Materials by Means of Low Field NMR

Recent theories about the sources of silica in bedded and nodular cherts do not fit the origin of cherts from the Kraków-Częstochowa Upland. Since siliceous sponges as a single source of silica is questionable, assumptions about additional sources have to be verified. In order to do so, three samples of nodular cherts and one representative sample of bedded chert were studied by means of ¹H LF-NMR 1D and 2D relaxometry and complementary geochemical methods. The results were compared with the literature and standard silica materials which helped to identify five types of ¹H signal. The very distinct 1D-T ₂ spectra of the dried samples indicated the existence of closed pores which, after comprehensive analysis, were identified as inclusions filled with different types of siliceous materials. Saturation revealed the differences between nodular and bedded cherts that were visible mainly in the amount and size of open porosity. The principal component analysis of NMR parameters showed the excellent separation of these two groups of samples and this is well visible on the biplots. Additionally, the estimated pore size distribution revealed that the total porosity of around 2% consisted primarily of mesopores (2-50 nm in diameter) and macropores (diameter >50 nm). In bedded cherts, open porosity is dominated by macropores, while the share of mesopores and macropores is similar in nodular cherts.

Role of the hydrogen bond lifetimes and rotations at the water/amorphous silica interface on proton transport

Using a highly robust and reactive all-atom potential, molecular dynamics computer simulations have been used to provide detailed analysis of the behavior of water and protons at a large-scale amorphous silica surface that offers the heterogeneity of surface sites and water/silica interactions. Structural data of the H-O distances as a function of distance from the glass surface showed variation in hydrogen bond (H-bond) lengths to second and third nearest oxygen neighbors that play an important role in H-bond lifetimes, rotations, and proton transfer, especially at the glass surface. The higher density and inherently closer average spacing between oxygens in the glass surface (2.6 Å) in comparison to that in water (2.8 Å) create a significantly different environment for H-bond lifetimes and proton transfers. Continuous H-bond lifetime autocorrelation functions for water H-bonded to the surface are considerably shorter than those of bulk water, whereas the intermittent lifetime autocorrelation functions are longer. Such results affect proton transfers that are over an order of magnitude higher at the surface than farther from the surface or in bulk water. However, most of these transfers are rattling events between the participating oxygens, one of which is the newly formed H₃O⁺ ion adjacent to the interface. Such a H₃O⁺ ion has an extremely low barrier to proton transfer back to the surface site in comparison to a H₃O⁺ ion in bulk water. Nonetheless, the simulations showed that rotation of the H₃O⁺ ion away from the initial transfer site allowed for structural diffusion of an excess proton away from the surface. Proton conduction from such rotations could be enhanced by external forces.

Water immobilization by glass microspheres affects biological activity

We recently reported that the water holding capacity of myofibrillar protein hydrogels could be increased upon addition of small amounts of microparticles, particularly glass microspheres. Glass microspheres were found to decrease the spin-spin relaxation time (T₂) of water protons in the gels, which was interpreted as enhanced water binding by the glass. We were thus interested in determining whether the observed effects on water proton relaxation were a direct consequence of water-glass interactions. Here we show how glass microspheres reduce the mobility of pure water, reflected in large decreases in the T₂ of water protons, decreases in the self-diffusion coefficient of water molecules, a lower water activity, and strengthening of O-H bonds. Even though glass is considered an inert material, glass microspheres were shown to inhibit the growth of human embryonic kidney cells, and stimulate or inhibit the growth of leukemia and monocytic lymphoma cells in vitro, depending on dose and time. The germination of alfalfa seeds and the growth of E.coli cells were also inhibited upon exposure to glass microspheres. This work indicates that the properties and behavior of materials, even ones considered inert, can be affected by their size. These observations suggest possible toxicological consequences of exposure to microparticles, but also open us possibilities to affect cellular/organism function via modulation of macromolecular hydration.

In Situ Measurement of Surface Functional Groups on Silica Nanoparticles Using Solvent Relaxation Nuclear Magnetic Resonance

In situ analysis and study on the surface of nanoparticles (NPs) is a key to obtain their important physicochemical properties for the subsequent applications. Of them, most works focus on the qualitative characterization whereas quantitative analysis and measurement on the NPs under their storage and usage conditions is still a challenge. In order to cope with this challenge, solvation relaxation-based nuclear magnetic resonance (NMR) technology has been applied to measure the wet specific surface area and, therefore, determine the number of the bound water molecules on the surface of silica NPs in solution and the hydrophilic groups of various types grafted on the surface of the NPs. By changing the surface functional group on silica particles, the fine distinction for the solvent-particle interaction with different surface group can be quantitatively differentiated by measuring the number of water molecules absorbed on the surface. The results show that the number of the surface hydroxyl, amine, and carboxyl group per nm² is 4.0, 3.7, and 2.3, respectively, for the silica particles with a diameter of 203 nm. The method reported here is the first attempt to determine in situ the number of bound solvent molecules and any solvophilic groups grafted on nanoparticles.

Long-range surface-induced water structures and the effect of 1-butanol studied by 1H nuclear magnetic resonance

Thin films of water between glass plates were investigated in this study with regard to water structure and dynamics in the temperature range of 278-313 K. We further investigated how addition of 1-butanol (0.05 and 0.5 M) affects the range and properties of the surface-induced water structures. From the observation of two (1)H nuclear magnetic resonance (NMR) water resonances and two relaxation components, it was found that the interfacial water exists in a two-state mixture in dynamic equilibrium, with the respective structures interpreted as being high-density water (HDW) and low-density water (LDW). In the absence of 1-butanol, the LDW state is more pronounced, with a further shift in equilibrium toward the LDW state with an increase in temperature. However, in water film samples containing 1-butanol, the HDW state dominates at low temperatures while the LDW state becomes more visible at higher temperatures. Furthermore, the addition of 1-butanol significantly increased the extent of the surface-induced water structures. NMR relaxation shows that the dynamics of water in the HDW state is significantly affected by the presence of 1-butanol and further indicates that the distribution of values for the enthalpy of activation associated with translational motion of water molecules in the HDW state is narrower in the 0.05 M 1-butanol sample than in the 0.5 M 1-butanol sample.