Dynamic Vapor Adsorption Research Articles

Thermodynamic and Kinetic Mechanism of the Phase Transition from Aztreonam Dihydrate to Anhydrates

In this work, two anhydrates (form A and form B) and one dihydrate (form C) of aztreonam were found and were characterized by powder X-ray diffraction (PXRD), thermogravimetric analysis, differential thermal analysis, and Fourier transform infrared spectroscopy. Dynamic vapor adsorption and variable-temperature PXRD experiments were carried out to study their thermal stability and moisture absorption stability. Furthermore, the critical water activity of aztreonam at 10–45 °C was determined, and it was found that the water activity determines the dehydration process of form C. The solubility of form A and form B in methanol solvent was measured at 10–45 °C to decide the thermodynamic stability of the polymorphs, and it was found that form B is thermodynamically stable below 28 °C, while form A is thermodynamically stable above 28 °C. The competitive suspension experiments further proved that form A and form B are enantiotropic polymorphs. In addition, the solution-mediated phase transition (SMPT) process of aztreonam form C was in situ monitored using Raman spectroscopy. The results show that the SMPT process is jointly controlled by the dissolution of the dihydrate and the nucleation of anhydrates, in which temperature plays a very important role. Finally, the SMPT mechanism of the dihydrate form is proposed.

Photocatalytic Degradation of Selected Pharmaceuticals Using g-C3N4 and TiO2 Nanomaterials.

Exfoliated graphitic carbon nitride (g-C3N4) and two commercially available nanomaterials from titanium dioxide (P25 and CG300) were tested for the photocatalytic degradation of paracetamol (PAR), ibuprofen (IBU), and diclofenac (DIC). Prior to photocatalytic experiments, the nanomaterials were characterized by common methods, such as X-ray diffraction (XRD), UV–VIS diffuse reflectance spectroscopy (DRS), Fourier transformed infrared spectroscopy in attenuated total reflection mode (FTIR–ATR), transmission electron microscopy (TEM), physisorption of nitrogen, and dynamic vapor adsorption (DVS) of water. The sizes and specific surface area (SSA) of the TiO2 nanoparticles were 6 nm and 300 m2·g−1 for CG300 and 21 nm and 50 m2·g−1 for P25. The SSA of g-C3N4 was 140 m2·g−1. All photocatalytic experiments were performed under UV (368 nm), as well as VIS (446 nm) irradiation. TiO2 P25 was the most active photocatalyst under UV irradiation and g-C3N4 was the most active one under VIS irradiation. Photodegradation yields were evaluated by means of high performance liquid chromatography (HPLC) and reaction intermediates were identified using gas chromatography with mass detection (GC–MS). Paracetamol and ibuprofen were totally removed but the intermediates of diclofenac were observed even after 6 h of irradiation. Some intermediates, such as carbazole-1-acetic acid, 2,6-dichloraniline, and hydroxylated derivates of diclofenac were identified. This study showed that g-C3N4 is a promising photocatalyst for the degradation of pharmaceuticals in an aqueous environment, under visible light.

Reaction-diffusion approach to modeling water diffusion in glutinous rice flour particles during dynamic vapor adsorption.

Dynamic vapor sorption (DVS) method is now widely adopted to determine water diffusion properties in food materials using a sheet or bulk particles as test samples. The Fickian second law with an instant equilibrium boundary condition, although commonly used, can not accurately model the water adsorption kinetics during DVS tests. Dynamic water adsorption of glutinous rice flour was measured at 30°C and eleven relative humidity steps, and modeled using the Fickian second law with three kinds of boundary condition. Results indicated that the boundary conditions had great impacts on the predicted values especially for the initial section in the DVS curves, and that modifying boundary conditions could not improve the fitness of the final section which characterized a continuing slight increase of water concentration. A reaction-diffusion model, which assumes two diffusible water populations and describes water transport as a competition between diffusion and reversible adsorption on solid matrix, was developed and found to be able to capture the features of water diffusion in the whole adsorption duration. Implementation of the reaction-diffusion approach to glutinous rice flour indicated that diffusion of the Langmuir water became very slow when its adsorption reached equilibrium, while the diffusion of the non-Langmuir water slowed down when water clustering occurred, at the same time the rates of the surface adsorption and bulk adsorption began to decrease. The model developed in this work would help to deepen our mechanistic understanding of water diffusion during a isothermal adsorption.

Thin Water Films at Multifaceted Hematite Particle Surfaces.

Mineral surfaces exposed to moist air stabilize nanometer- to micrometer-thick water films. This study resolves the nature of thin water film formation at multifaceted hematite (α-Fe2O3) nanoparticle surfaces with crystallographic faces resolved by selected area electron diffraction. Dynamic vapor adsorption (DVA) in the 0-19 Torr range at 298 K showed that these particles stabilize water films consisting of up to 4-5 monolayers. Modeling of these data predicts water loadings in terms of an "adsorption regime" (up to 16 H2O/nm(2)) involving direct water binding to hematite surface sites, and of a "condensation regime" (up to 34 H2O/nm(2)) involving water binding to hematite-bound water nanoclusters. Vibration spectroscopy identified the predominant hematite surface hydroxo groups (-OH, μ-OH, μ3-OH) through which first layer water molecules formed hydrogen bonds, as well as surface iron sites directly coordinating water molecules (i.e., as geminal η-(OH2)2 sites). Chemometric analyses of the vibration spectra also revealed a strong correspondence in the response of hematite surface hydroxo groups to DVA-derived water loadings. These findings point to a near-saturation of the hydrogen-bonding environment of surface hydroxo groups at a partial water vapor pressure of ∼8 Torr (∼40% relative humidity). Classical molecular dynamics (MD) resolved the interfacial water structures and hydrogen bonding populations at five representative crystallographic faces expressed in these nanoparticles. Simulations of single oriented slabs underscored the individual roles of all (hydro)oxo groups in donating and accepting hydrogen bonds with first layer water in the "adsorption regime". These analyses pointed to the preponderance of hydrogen bond-donating -OH groups in the stabilization of thin water films. Contributions of μ-OH and μ3-OH groups are secondary, yet remain essential in the stabilization of thin water films. MD simulations also helped resolve crystallographic controls on water-water interactions occurring in the "condensation regime". Water-water hydrogen bond populations are greatest on the (001) face, and decrease in importance in the order (001) > (012) ≈ (110) > (014) ≫ (100). Simulations of a single (∼5 nm × ∼ 6 nm × ∼ 6 nm) nanometric hematite particle terminated by the (001), (110), (012), and (100) faces also highlighted the key roles that sites at particle edges play in interconnecting thin water films grown along contiguous crystallographic faces. Hydroxo-water hydrogen bond populations showed that edges were the preferential loci of binding. These simulations also suggested that equilibration times for water binding at edges were slower than on crystallographic faces. In this regard, edges, and by extension roughened surfaces, are expected to play commanding roles in the stabilization of thin water films. Thus, in focusing on the properties of nanometric-thick water layers at hematite surfaces, this study revealed the nature of interactions between water and multifaced particle surfaces. Our results pave the way for furthering our understanding of mineral-thin water film interfacial structure and reactivity on a broader range of materials.

Mapping the Cu-BTC metal–organic framework (HKUST-1) stability envelope in the presence of water vapour for CO2 adsorption from flue gases

Cu-BTC metal–organic framework (HKUST-1) was evaluated as the model material for CO2 capture from flue gas streams. This paper presents an optimised hydrothermal synthesis of HKUST-1 and an analysis of water stability of HKUST-1. Substantial improvements of the hydrothermal synthesis process of HKUST-1 are shown to increase the quantitative yield up to 89.4% at 100°C. Single-component adsorption experiments were carried out under conditions relevant for flue gases adsorption (45–60°C, 0–1barG) to evaluate the performance of HKUST-1 in terms of adsorption capacity, showing that the amount adsorbed of water can reach up to 21.7mmolg−1, about one order of magnitude higher than CO2 (1.75mmolg−1) and almost two orders of magnitude higher than N2 (0.17mmolg−1). The hydration process of HKUST-1 framework was investigated using dynamic vapour adsorption under the flue gas emitting conditions. HKUST-1 is sensitive to humid streams and dynamic deformation of its porous structure takes place at 40–50°C and various relative humidity values, leading to the irreversible decomposition of HKUST-1 framework and the consequent deterioration in its adsorption capacity. Under humid conditions, water displaces the organic linkers from the copper centres causing the collapse of HKUST-1 framework. These results provide fundamental knowledge to enable future material design for the modification of the hydrophilic nature of copper sites in HKUST-1 to improve its moisture stability.

The water binding behavior of κ-Carrageenan determined by three different methods

κ-Carrageenan is a biopolymer extracted from red seaweeds which has been in the focus of pharmaceutical development for many years. Most applications make use of the large water binding capacity of κ-carrageenan. The primary limitation of κ-carrageenan is the variation in the substance quality. Therefore, the water binding capacity of different κ-carrageenan products was investigated by dynamic vapor adsorption, freezing and non-freezing bound water and water retention value. The κ-carrageenans were observed to have a higher water binding capacity than microcrystalline cellulose (MCC) in all three methods. The amount of adsorbed water is similar for all carrageenans. Differences between the carrageenan types (κ, ι, and λ) were remarkable for the freezing bound water and centrifugation bound water as well as between the κ-carrageenans of different suppliers.