Kinetics of Oriented Attachment of Mica Crystals

Oriented attachment (OA), that is, the coalescence of crystals through attachment on coaligned crystal faces, is a nonclassical crystal growth process. Before attachment, a mesocrystal consisting of coaligned parallel crystals but with liquid separating them was observed. Fundamental questions such as why OA is kinetically favored and whether a mesocrystal stage is a prerequisite for OA are raised. Through combining brute-force molecular dynamics simulations and path samplings based on extensive umbrella simulations, we address these questions with a case study on the OA of a mica nanocrystal onto a mica crystal substrate in water. Brute-force simulations show that if two mica crystals are attached but largely misaligned, coalignment hardly appears. Thus, if OA is possible, then coalignment must appear before the attachment between crystals. Electrophoresis of the nanocrystal toward the substrate surface is spontaneous, but mesocrystal formation is occasional, also shown by brute-force simulations. Free energies along different pathways show that OA is spontaneous and kinetically favored over non-OA, and a mesocrystal formation is just a bifurcation in the pathway. OA is through a pathway in which the nanocrystal is tilted with respect to the substrate. Part of the nanocrystal is attached to the substrate first, and then, OA is gradually completed. Once a mesocrystal is occasionally formed, then a jump event is needed for the nanocrystal to get back to the OA pathway. The sampling technique here can hopefully guide the design of nanostructured materials facilitated by OA

Closest-Packing Water Monolayer Stably Intercalated in Phyllosilicate Minerals under High Pressure

The directional hydrogen-bond (HB) network and nondirectional van der Waals (vdW) interactions make up the specificity of water. Directional HBs could construct an ice-like monolayer in hydrophobic confinement even in the ambient regime. Here, we report a water monolayer dominated by vdW interactions confined in a phyllosilicate interlayer under high pressure. Surprisingly, it was in a thermodynamically stable state coupled with bulk water at the same pressure (P) and temperature (T), as revealed by the thermodynamic integration approach on the basis of molecular dynamics (MD) simulations. Both classical and ab initio MD simulations showed water O atoms were stably trapped and exhibited an ordered hexagonal closest-packing arrangement, but OH bonds of water reoriented frequently and exhibited a specific two-stage reorientation relaxation. Strikingly, hydration in the interlayer under high pressure had no relevance with surface hydrophilicity rationalized by the HB forming ability, which, however, determines wetting in the ambient regime. Intercalated water molecules were trapped by vdW interactions, which shaped the closest-packing arrangement and made hydration energetically available. The high pressure–volume term largely drives hydration, as it compensates the entropy penalty which is restricted by a relatively lower temperature. This vdW water monolayer should be ubiquitous in the high pressure but low-temperature regime

Closest-Packing Water Monolayer Stably Intercalated in Phyllosilicate Minerals under High Pressure

The directional hydrogen-bond (HB) network and nondirectional van der Waals (vdW) interactions make up the specificity of water. Directional HBs could construct an ice-like monolayer in hydrophobic confinement even in the ambient regime. Here, we report a water monolayer dominated by vdW interactions confined in a phyllosilicate interlayer under high pressure. Surprisingly, it was in a thermodynamically stable state coupled with bulk water at the same pressure (P) and temperature (T), as revealed by the thermodynamic integration approach on the basis of molecular dynamics (MD) simulations. Both classical and ab initio MD simulations showed water O atoms were stably trapped and exhibited an ordered hexagonal closest-packing arrangement, but OH bonds of water reoriented frequently and exhibited a specific two-stage reorientation relaxation. Strikingly, hydration in the interlayer under high pressure had no relevance with surface hydrophilicity rationalized by the HB forming ability, which, however, determines wetting in the ambient regime. Intercalated water molecules were trapped by vdW interactions, which shaped the closest-packing arrangement and made hydration energetically available. The high pressure–volume term largely drives hydration, as it compensates the entropy penalty which is restricted by a relatively lower temperature. This vdW water monolayer should be ubiquitous in the high pressure but low-temperature regime

Closest-Packing Water Monolayer Stably Intercalated in Phyllosilicate Minerals under High Pressure

The directional hydrogen-bond (HB) network and nondirectional van der Waals (vdW) interactions make up the specificity of water. Directional HBs could construct an ice-like monolayer in hydrophobic confinement even in the ambient regime. Here, we report a water monolayer dominated by vdW interactions confined in a phyllosilicate interlayer under high pressure. Surprisingly, it was in a thermodynamically stable state coupled with bulk water at the same pressure (P) and temperature (T), as revealed by the thermodynamic integration approach on the basis of molecular dynamics (MD) simulations. Both classical and ab initio MD simulations showed water O atoms were stably trapped and exhibited an ordered hexagonal closest-packing arrangement, but OH bonds of water reoriented frequently and exhibited a specific two-stage reorientation relaxation. Strikingly, hydration in the interlayer under high pressure had no relevance with surface hydrophilicity rationalized by the HB forming ability, which, however, determines wetting in the ambient regime. Intercalated water molecules were trapped by vdW interactions, which shaped the closest-packing arrangement and made hydration energetically available. The high pressure–volume term largely drives hydration, as it compensates the entropy penalty which is restricted by a relatively lower temperature. This vdW water monolayer should be ubiquitous in the high pressure but low-temperature regime

Two new monoterpenoid indole alkaloids from <i>Tabernaemontana divaricata</i>

Two new monoterpenoid indole alkaloids, tabervarines A (1) and B (2), along with seven known monoterpenoid indole alkaloids, were isolated from the methanol extract of the twigs and leaves of Tabernaemontana divaricata. The structures including the absolute configurations of the new alkaloids were elucidated based on MS, NMR, and ECD calculation. The in vitro cytotoxic activities of the isolated alkaloids against several human cancer cell lines were also evaluated.</p

Influences of Cation Ratio, Anion Type, and Water Content on Polytypism of Layered Double Hydroxides

Layered double hydroxides (LDHs) are a significant sink of anions (CO₃^2–, SO₄^2–, NO₃^–, Cl^–, etc.) and divalent transition-metal cations in soil. The anion exchange capacity gives rise to functional materials. The stability of LDHs is determined by the interaction between cation-bearing layers and intercalated water and anions, which is correlated with polytypism and coordination structure. A systematic investigation is performed to show the influence of cation ratio, anion type, and water content on polytypism, swelling behavior, and interlayer structure of Mg–Al-LDHs using molecular dynamics simulations. LDHs intercalated with NO₃^– ions exhibit a polytype transition from 3<i>R</i>₁ (three-layer rhombohedral polytype) to 1<i>T</i> (one-layer trigonal polytype) with increasing water content. NO₃^– ions exhibit a <i>D</i>_3<i>h</i> point group symmetry at low water contents. The polytype transition coincides with the complete transformation into tilted NO₃^– ion with a <i>C</i>_2<i>v</i> point group symmetry. The transition appears at a lower water content when the Mg/Al ratio is lower. LDHs with SO₄^2– ions exhibit a three-stage polytypism. The first and last stages are 3<i>R</i>₁. The intermediate stage could be 1<i>T</i> or a mixture of different <i>O</i>(octahedra)-type interlayers, which depends on the cation ratio. The relative popularity of SO₄^2– ions with a <i>C</i>_<i>s</i> point group symmetry is characteristic for the intermediate stage, while mostly SO₄^2– ions exhibit a <i>C</i>_3<i>v</i> symmetry. There is no clear relevance between cation ratio and water content at which a polytype transition happens. The configurational adjustments of NO₃^– and SO₄^2– ions facilitate the swelling behavior of LDHs. LDHs with CO₃^2– or Cl^– ions always maintain a 3<i>R</i>₁ polytype irrespective of water content and hardly swell. The configurations of anions and water reflect local coordination structure due to hydrogen bonds. The layer-stacking way influences long-ranged Coulombic interactions. Hydrogen-bonding structure and long-ranged Coulombic interactions collectively determine polytypism and stability of LDHs

Closest-Packing Water Monolayer Stably Intercalated in Phyllosilicate Minerals under High Pressure

The directional hydrogen-bond (HB) network and nondirectional van der Waals (vdW) interactions make up the specificity of water. Directional HBs could construct an ice-like monolayer in hydrophobic confinement even in the ambient regime. Here, we report a water monolayer dominated by vdW interactions confined in a phyllosilicate interlayer under high pressure. Surprisingly, it was in a thermodynamically stable state coupled with bulk water at the same pressure (P) and temperature (T), as revealed by the thermodynamic integration approach on the basis of molecular dynamics (MD) simulations. Both classical and ab initio MD simulations showed water O atoms were stably trapped and exhibited an ordered hexagonal closest-packing arrangement, but OH bonds of water reoriented frequently and exhibited a specific two-stage reorientation relaxation. Strikingly, hydration in the interlayer under high pressure had no relevance with surface hydrophilicity rationalized by the HB forming ability, which, however, determines wetting in the ambient regime. Intercalated water molecules were trapped by vdW interactions, which shaped the closest-packing arrangement and made hydration energetically available. The high pressure–volume term largely drives hydration, as it compensates the entropy penalty which is restricted by a relatively lower temperature. This vdW water monolayer should be ubiquitous in the high pressure but low-temperature regime

Closest-Packing Water Monolayer Stably Intercalated in Phyllosilicate Minerals under High Pressure

The directional hydrogen-bond (HB) network and nondirectional van der Waals (vdW) interactions make up the specificity of water. Directional HBs could construct an ice-like monolayer in hydrophobic confinement even in the ambient regime. Here, we report a water monolayer dominated by vdW interactions confined in a phyllosilicate interlayer under high pressure. Surprisingly, it was in a thermodynamically stable state coupled with bulk water at the same pressure (P) and temperature (T), as revealed by the thermodynamic integration approach on the basis of molecular dynamics (MD) simulations. Both classical and ab initio MD simulations showed water O atoms were stably trapped and exhibited an ordered hexagonal closest-packing arrangement, but OH bonds of water reoriented frequently and exhibited a specific two-stage reorientation relaxation. Strikingly, hydration in the interlayer under high pressure had no relevance with surface hydrophilicity rationalized by the HB forming ability, which, however, determines wetting in the ambient regime. Intercalated water molecules were trapped by vdW interactions, which shaped the closest-packing arrangement and made hydration energetically available. The high pressure–volume term largely drives hydration, as it compensates the entropy penalty which is restricted by a relatively lower temperature. This vdW water monolayer should be ubiquitous in the high pressure but low-temperature regime

Jumping Diffusion of Water Intercalated in Layered Double Hydroxides

Our molecular dynamics simulation study shows water in the nanoconfined monolayer in Cl^–-Mg₂Al-layered double hydroxides (Mg₂Al­(OH)₆Cl·<i>m</i>H₂O) diffuses in a similar way as atoms in solid lattice. A water molecule is mostly fixed in a hydroxyl group site, as an acceptor of hydrogen bonds donated by the upper and lower hydroxyl groups simultaneously. Because of exchange of acceptors, it loses hydrogen bonds from the two hydroxyl groups and accepts hydrogen bonds from another two groups in an adjacent site. Thus, a water molecule jumps from one site to another, which is rapid but rare. On average it takes ∼10⁴ ps for a jump to happen on a water molecule. The diffusion coefficient derived by the jump model is of the same order (∼10^–9 cm²/s) as that obtained by fitting the mean-square displacement, revealing water diffusion in the confined monolayer is largely contributed by a series of jump events

Surface Heterogeneity of SiO₂ Polymorphs: An XPS Investigation of α‑Quartz and α‑Cristobalite

Silica minerals, one of the most abundant mineral species on the earth, play important roles in geochemistry and environment processes. The diversity of the SiO₄ tetrahedron polymerization style might result in the heterogeneity of the surface microstructures and properties of SiO₂ polymorphs. The surface properties of two common crystalline SiO₂ polymorphs, i.e., α-quartz and α-cristobalite, have been investigated by the surface site density measurement, batch methylene blue (MB) adsorption, and X-ray photoelectron spectroscopy (XPS). The Langmuir adsorption isotherms suggest the formation of monolayer MB on both α-quartz and α-cristobalite surfaces. The adsorption capacity of α-quartz toward MB is larger than that of α-cristobalite, which positively correlates with the density of surface site. XPS spectra reveal that the adsorption takes place between the nitrogen atom of the dimethylamino groups in MB and silanols on α-quartz and α-cristobalite surfaces. The O/Si atom ratio related with adsorption of α-quartz is found to be about 1.8:1, which is higher than that of α-cristobalite (about 1.3:1). This suggests that there are two different silanol species (single and germinal) related to adsorption on the surface of α-quartz and α-cristobalite, and the higher O/Si ratio implies a larger proportion of germinal silanols in α-quartz. The N_low/N_high ratio (N_low stands for the N atoms with lower binding energy (399.2 eV), and N_high for the N atoms with higher binding energy (399.7 eV)) changes to about 2:1 with the adsorption saturation, implying that the space arrangement of MB adsorbed on the surface was adjusted with the increase of adsorption amount by lifting the average tilt angle between the long axis of the MB molecule and the sample surface. The higher surface site density of α-quartz leads to a larger average tilt angle, while α-cristobalite does conversely