Kinetics of Interlayer Expansion of a Layered Silicate Driven by Caffeine Intercalation in the Water Phase Using Transmission X-ray Diffraction

Estimation of Adsorbed Amounts in Organoclay by Machine Learning

Adsorption properties of organoclay have been investigated for decades focusing on the morphology and physicochemical properties of two-dimensional interlayers. Experimental studies have previously revealed that the adsorption mechanisms depend on the molecular species of the organocation and adsorbate, making it difficult to estimate the adsorbed amount without experiments. Considering that the adsorption of aromatic compounds has been reported by using various clays, organocations, and adsorbates, machine learning is a promising method to overcome the difficulty. In the present study, we collected adsorption data from the literature and constructed models to estimate the adsorbed amount of the organoclay by random forest regression. The composition of the clay, molecular descriptors of the organocation and adsorbate obtained by the RDKit, and experimental conditions were used as the explanatory variables. Simple model construction by using all the experimental data resulted in low R ² and a mean absolute error. This problem was solved by the correction of the adsorbed amount data by the Langmuir or Freundlich equation and the following model construction at various equilibrium concentrations. The plots of the adsorbed amount estimated by the latter model were located close to the corresponding adsorption isotherm, while that by the former was not. Thus, it was revealed that the adsorbed amount was estimated quantitatively without understanding the adsorption mechanisms individually. Feature importance analysis also revealed that the combination of the organocation and adsorbate is important at high equilibrium concentrations, while the clay should be selected carefully as the concentration gets lower. Our results give an insight into the rational design of the organoclay including the synthesis and adsorption properties.

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

The assembly of discrete active species to form periodical nanostructures is essential in realizing low-cost artificial enzymes that mimic natural enzymatic functions in extraordinary bio(chemo)selective reactions. In this study, we developed artificial bifunctional glucose/gluconic acid dehydrogenase from naturally abundant resources: l-aspartic acid (Asp) and montmorillonite (a subgroup of smectite natural clay minerals). β-d-Glucose (Glc) was dehydrogenated to 2-keto-d-gluconate (2-KGA) at 25 and 30 °C in an aqueous acidic solution (pH = 3, 4, and 5). The reaction involved sequential steps that yielded d-gluconic acid (GA) as an intermediate. The second step of the dehydrogenation (GA to 2-KGA) occurred at a higher rate than the first (Glc to GA), which is comparable to the natural process. A negatively charged carboxylate in Asp was required for the dehydrogenation, which donates an electron pair (COO:^-) to the hydroxyl group bonded to the C(1)-position of Glc. The acidic sites in clay served as coenzymatic sites (electron acceptor), promoting the Glc dehydrogenation as the Glc reduced by Asp approached the clay coenzymatic sites. The active coenzymatic structures were developed in 48 h (induction period) through the rearrangement of the adsorbed Asp and Glc molecules on montmorillonite in water (intermediate structure). The spontaneous assembling of the intermediate structures facilitated the one-pot dehydrogenation of Glc to 2-KGA via periodic "hydrated stacked layers" comprising clay nanosheets, Asp, and Glc. The facile synthetic route proposed here is inexpensive and would be beneficial without using both GDH and GADH enzymes bound to a cell membrane.

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

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.