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

Abstract

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.

Direct and Indirect DNP NMR Uncovers the Interplay of Surfactants with Their Mesoporous Host Material

“…Pore sizes obtained by nonlocal density functional theory (NLDFT) used a model for N 2 adsorption on silicon at 77 K. The model for the adsorption of nitrogen on silica surfaces with cylindrical pore geometries of the Advanced Data Processing (ADP) software (V 6.2.4) was used for evaluation. Interpretation of the results follow our previous reports. , …”

“…Interpretation of the results follow our previous reports. 34,58 2.4. Sample Preparation for DNP NMR and DSC Experiments.…”

Strong Enhancement of Nanoconfined Water Mobility by a Structure Breaking Salt

“…Physico-chemical properties of water in bulk, confined in various media, or in solutions are dramatically different. − These properties are critical for the performance of materials in applications, such as energy storage in electrical double-layer capacitors, − as well as for the ion transport in biological , and synthetic membranes. − Ion transport and electrochemical activities in energy storage devices are closely related to the structure and dynamics of confined hydrated ions. The strength and the number of molecules in a hydration shell, which is an envelope of water molecules formed around an ion during solvation, depends on the nature of the solvated ions. , It has been well demonstrated that the size of ions together with the electron densities on the surface of ions control the ion–water interactions, thereby impacting the mobility of water molecules. , Even in the absence of ions, water in confinement exhibits substantially varied characteristics, depending on the shape, size, and surfaces of the confining matrixes. , When water is confined together with metal ions, the structure and dynamics of water are impacted by not only the nature of metal ions but also the morphology of the confining media. − …”

“…Along with other observations, they found an optimum pore size of the matrixes and a concentration of those salts at which the dynamics of water molecules is independent of the salt concentration but depends only on the pore size. Even though there are studies on the structure and dynamics of water of aqueous salt solutions in bulk and in confinement, ,− a straight comparison of the dynamics of water in aqueous solutions with distinctively chaotropic and distinctively kosmotropic ions in confinement has never been attempted (e.g., all the cations studied in refs − were, to varying degree, kosmotropic). Here we explore the dynamics of water in aqueous solutions of NaCl and KCl of various concentrations confined in MCM41 mesoporous silica.…”

Effect of different soluble salt ions on deformation of desulfurization gypsum in high humidity environment: a statistically designed experimental analysis method

“…On the other hand, this phenomenon can also be explained from the perspective of solubility. The solubility of K SO 2 4 at 20 °C is 110 g l −1 , while the solubility of MgSO 4 at the same temperature is 255 g l −1 , the existence of + K ions will reduce the solubility of desulfurization gypsum, reducing the entry of water molecules, and finally reduce the sag and deflection of desulfurization gypsum board (Schneider et al 2020 and + K combination, this inhibition effect is more obvious. The corresponding sag deflection decreases from 2.924 mm before + K addition to 0.796 mm after + K addition, and the inhibition degree reaches 72.9%.…”