Important Roles of Water Clusters Confined in a Nanospace as Revealed by a Synchrotron X-ray Diffraction Study

Abstract

States of water molecules confined in a nanospace designed by montmorillonite (negatively charged silicate layer) and charge compensating benzyl­ammonium were investigated. Caffeine was used as a probe because of its compatibility for the fine structure of the interlayer water. Powder synchrotron radiation X-ray diffraction (SXRD) and adsorption isotherms of the water vapor revealed a metastable structure of bimolecular water layers (2WLs) in the interlayer space. Water molecules readily penetrated to expand the interlayer space to 0.56 nm. The interlayer space did not increase further even in the presence of excess water. According to the isosteric heat of water, the expansion was limited because of moderate hydration as forming 2WLs. Caffeine molecules replaced a part of the water molecules in the 2WLs to expand the interlayer space to 0.65 nm. Time-resolved SXRD with an accumulation time of 500 ms revealed that the interlayer expansion reached a steady state within a few minutes. The caffeine intercalation proceeded, involving a change in the molecular orientation that increased the contact area of the caffeine molecules. The interlayer expansion was limited in all the solvents examined (mixtures of water with methanol, ethanol, acetone, and tetrahydrofuran), while the packing density of the incorporated caffeine was maximized in the absence of an organic solvent. The water molecules confined in the interlayer space acted as an actuator to accommodate a large quantity of amphiphilic molecules by adapting the nanostructure, which was achieved by releasing the confined water molecules.

Acceleration of the Dehydrogenation of d-Glucose to 2-Keto-d-gluconate in Aqueous Amino Acid via Hydrated Stacked Clay Nanosheets

“…While no diffraction peak was observed up to 48 h, a peak ascribable to the 001 plane appeared at 72 h at around 2θ = 4.5° (Cu Kα) with the d value of ∼2.0 nm. Since the hydration energies of these organic molecules are smaller than those of interlayer Na + cations, the repulsive potential between the silicate layers decreases when these organic molecules penetrate the interlayer space . Therefore, we speculate that the coadsorption of Asp and Glc proceeds with the gradual formation of a periodic lamellar structure (intercalation compound) with the d 001 value of approximately 2.0 nm.…”

“…Fe (in the octahedral sheet) and Al (in the tetrahedral sheet) have been reported to facilitate photoinduced reactions mediated by interlayer photoactive cations. − The interlayer 2D nanospace expands in water depending on the interlayer hydrated molecules/cations, and spontaneous swelling (leading to delamination) occurs when the hydration energy of the interlayer cations (e.g., Na + ) overcomes the potential barrier imposed by the decrease in the electrostatic attractive forces between the silicate layers . In contrast, nanoconfined hydrated layers are formed when an organic cation/molecule is intercalated because of its relatively low hydration energy . Powder X-ray diffraction (XRD) measurements are applicable for analyzing the interlayer hydrated structures of organoclays in water since the periodic lamellar structures are maintained. − Hydrated amino acids are well-known guest species that can penetrate the interlayer space of smectites (intercalation). − At a high solution pH, various factors are involved in the intercalation of neutral and anionic amino acids, including ion–dipole interactions between the interlayer cation and/or the negatively charged siloxane surface, hydrogen bonding, and van der Waals force. , The initial concentration of an aqueous amino acid solution also affects the amount of amino acid intercalated on the smectite.…”

“…In contrast, nanoconfined hydrated layers are formed when an organic cation/molecule is intercalated because of its relatively low hydration energy . Powder X-ray diffraction (XRD) measurements are applicable for analyzing the interlayer hydrated structures of organoclays in water since the periodic lamellar structures are maintained. − Hydrated amino acids are well-known guest species that can penetrate the interlayer space of smectites (intercalation). − At a high solution pH, various factors are involved in the intercalation of neutral and anionic amino acids, including ion–dipole interactions between the interlayer cation and/or the negatively charged siloxane surface, hydrogen bonding, and van der Waals force. , The initial concentration of an aqueous amino acid solution also affects the amount of amino acid intercalated on the smectite. Therefore, the amount (packing density) of carboxylates/dicarboxylates in amino acids will serve as a controlling factor for the reactivity for Glc dehydrogenation.…”

“…30 In contrast, nanoconfined hydrated layers are formed when an organic cation/molecule is intercalated because of its relatively low hydration energy. 31 Powder X-ray diffraction (XRD) measurements are applicable for analyzing the interlayer hydrated structures of organoclays in water since the periodic lamellar structures are maintained. 31−33 Hydrated amino acids are well-known guest species that can penetrate the interlayer space of smectites (intercalation).…”

Specific lift-up behaviour of acetate-intercalated layered yttrium hydroxide interlayer in water: application for heterogeneous Brønsted base catalysts toward Knoevenagel reactions

“…Measurement of the t-SXRD for the anion-exchange reaction 41,42 The crystalline powder of Cl − /Y-LRH (0.05 g) was dispersed in Milli-Q water (0.5 mL) set on the sample stage with vigorous stirring, and then the SXRD pattern of the aqueous dispersion of the sample was collected. Five seconds after the first acquisition of the t-SXRD pattern, the aqueous CH 3 -COONa solution (1.25 M, 2 mL) was immediately added using the autotitrator (dosing rate: 2.0 mL s −1 ).…”