Homogeneous Organic Crystal Nucleation Rates in Solution from the Perspective of Chemical Reaction Kinetics

It is demonstrated for 11 different combinations of organic solutes and solvents that the supersaturation dependence of homogeneous organic crystal nucleation rates from solution can be predicted from the solubility, bar a single empirical rate constant, when it is assumed that nucleation takes place in reversible aggregates of solvated solutes formed in supersaturated solutions. Reversible solute aggregation represents natural solute density fluctuations that take place in any solute/solvent system. For thermodynamically ideal solutions, the steady state size distribution, and thus the population of reversible aggregates in supersaturated solution, can be predicted quantitatively from the overall solute concentration by a simple mathematical expression. Supersaturation creates an excess of reversible aggregates with sizes exceeding that of the largest aggregate in saturated solution. It is shown that the number of these excess aggregates is proportional to experimental homogeneous nucleation rates, suggesting a rate equation for homogeneous nucleation that has only one empirical parameter, namely, a rate constant specific to the solute/solvent combination. This rate constant can be determined from standard nucleation rate data. The system-specificity of homogeneous nucleation rates thus appears to be encoded solely in a rate constant for the transformation of the large excess aggregates into crystal nuclei. The driving force for triggering nucleation events in these aggregates is likely the extremely high local supersaturation, which provides the conditions for spatiotemporally aligned bond-breaking (e.g., de-solvation) and bond-forming (e.g., solute–solute bonding) events that create stable crystal nuclei. The possible influence of heterogeneous nucleation by solid impurities is considered

Auto-agglomeration of dry ibuprofen powder under mechanical vibration: Effect of particle morphology and surface properties

This study investigates the effect of ibuprofen particle morphology and surface properties on its tendency to auto-agglomerate. Four diverse particle morphologies were produced by recrystallisation of ibuprofen from solvents with different polarities. The aspect ratio and the polar component of the recrystallised particles decreased as follows: methanol > ethanol > acetonitrile > hexane. The auto-agglomeration process was induced by mechanical vibration and the agglomerate strength was assessed by the Dispersion and Agglomerate Strength indices. Relatively strong agglomerates were detected only in the case of ibuprofen recrystallised from ethanol and acetonitrile, whereas weaker and no agglomerates were formed during mechanical vibration of ibuprofen recrystallised from methanol and hexane, respectively. It was found that tendency of ibuprofen powder to auto-agglomerate was associated with lower tribo-electric charges during mechanical vibration, hypothetically linked to the competitive advantage of electrostatic attraction of high surface polarity particles to the walls of the vibration box which can hinder particle-particle interactions. Crystal surface polarity could be altered by the crystallisation protocol and/or by particle breakage (exposing more crystal facets of high polarity) during mechanical vibration

Continuous-Flow Laboratory SAXS for In Situ Determination of the Impact of Hydrophilic Block Length on Spherical Nano-Object Formation during Polymerization-Induced Self-Assembly

In situ small-angle X-ray scattering (SAXS) is a powerful technique for characterizing block-copolymer nano-object formation during polymerization-induced self-assembly. To work effectively in situ, it requires high intensity X-rays which enable the short acquisition times required for real-time measurements. However, routine access to synchrotron X-ray sources is expensive and highly competitive. Flow reactors provide an opportunity to obtain temporal resolution by operating at a consistent flow rate. Here, we equip a flow-reactor with an X-ray transparent flow-cell at the outlet which facilitates the use of a low-flux laboratory SAXS instrument for in situ monitoring. The formation and morphological evolution of spherical block copolymer nano-objects was characterized during reversible addition fragmentation chain transfer polymerization of diacetone acrylamide in the presence of a series of poly(dimethylacrylamide) (PDMAm) macromolecular chain transfer agents with varying degrees of polymerization. SAXS analysis indicated that during the polymerization, highly solvated, loosely defined aggregates form after approximately 100 s, followed by expulsion of solvent to form well-defined spherical particles with PDAAm cores and PDMAm stabilizer chains, which then grow as the polymerization proceeds. Analysis also indicates that the aggregation number (Nagg) increases during the reaction, likely due to collisions between swollen, growing nanoparticles. In situ SAXS conducted on PISA syntheses using different PDMAm DPs indicated a varying conformation of the chains in the particle cores, from collapsed chains for PDMAm47 to extended chains for PDMAm143. At high conversion, the final Nagg decreased as a function of increasing PDMAm DP, indicating increased steric stabilization afforded by the longer chains which is reflected by a decrease in both core diameter (from SAXS) and hydrodynamic diameter (from DLS) for a constant core DP of 400

Immobilisation of chromium in magnesium carbonate minerals

Hexavalent chromium (Cr6+) is a toxic carcinogenic pollutant that might be released by the mining and processing of ultramafic rocks and nickel laterites and which requires permanent removal from the contaminated biosphere. Ultramafic material can also serve as a feedstock for the sequestration of CO2 resulting from the growth of new minerals, raising the intriguing proposition of integrated sequestration of both pollutants, CO2 and chromium, into magnesium carbonates. Such a synergistic process downstream of ore recovery and mineral processing could be an elegant proposition for more sustainable utilisation and management of the Earth's resources. We have therefore carried out an experimental and microanalytical study to investigate potentially suitable carbonate minerals. Uptake of chromium in carbonate phases was determined, followed by identification of the crystalline phases and characterisation of the local structural environment around the incorporated chromium centres. The results suggest that neither nesquehonite nor hydromagnesite have the structural capacity to incorporate Cr6+ or Cr3+ significantly at room temperature. We therefore propose that further research into this technology should focus on laboratory assessments of other phases, such as layered double hyroxides, that have a natural structural capacity to uptake both chromium and CO2

Green Alternatives to Zinc Dialkyldithiophosphates: Vanadium Oxide-Based Additives

A functionalized vanadyl(IV) acetylacetonate (acac) complex has been found to be a superior and highly effective antiwear agent, affording remarkable wear protection, compared to the current industry standard, zinc dialkyldithiophosphates (ZDDPs). Analysis of vanadium speciation and the depth profile of the active tribofilms by a combination of X-ray absorption near-edge structure (XANES), X-ray photoelectron spectroscopy (XPS), and near-edge X-ray absorption fine structure (NEXAFS) analyses indicated a mixed-valence oxide composite, comprising V(III), V(IV), and V(V) species. A marked difference in composition between the bulk and the surfaces of the tribofilms was found. The vanadyl(VI) acac precursor has the potential to reduce or even replace ZDDP, which would represent a paradigm shift in the antiwear agent design. A major benefit relative to ZDDPs is the absence of S and P moieties, eliminating the potential for forming noxious and environmentally harmful byproducts of these elements

X-ray Absorption Spectroscopy as Process Analytical Technology: Reaction Studies for the Manufacture of Sulfonate-Stabilized Calcium Carbonate Particles

Process analytical technologies are widely used to inform process control by identifying relationships between reagents and products. Here, we present a novel process analytical technology system for operando XAS on multiphase multicomponent synthesis processes based on the combination of a conventional lab-scale agitated reactor with a liquid-jet cell. The preparation of sulfonate-stabilized CaCO3 particles from polyphasic Ca(OH)2 dispersions was monitored in real time by Ca K-edge XAS to identify changes in Ca speciation in the bulk solution/dispersion as a function of time and process conditions. Linear combination fitting of the spectra quantitatively resolved composition changes from the initial conversion of Ca(OH)2 to the Ca(R–SO3)2 surfactant to the ultimate formation of nCaCO3·mCa(R− SO3)2 particles. The system provides a novel tool with strong chemical specificity for probing multiphase synthesis processes at a molecular level, providing an avenue to establishing the relationships between critical quality attributes of a process and the quality and performance of the product

Probing Depth of Total Electron-Yield XAS: Monte-Carlo Simulations of Auger Electron Trajectories

The signal formation in total electron-yield (TEY) XAS has been modelled for keV absorption edges using a computationally fast Monte-Carlo algorithm for the simulation of the Auger electron trajectories. It is shown that a pure KLL Auger-yield model achieves good agreement with experimental data for the K-edge TEY attenuation in Al and Cu. Advantages of the Monte-Carlo simulation approach over empirical and analytical models for the TEY are pointed out

The Chemistry of (U,Pu)O2 Dissolution in Nitric Acid

AbstractThe focus of this paper is the chemistry of mixed uranium plutonium oxide (MOx,) in nitric acid. An overview of dissolution chemistry is discussed by comparing the differences in the dissolution characteristics of uranium and plutonium oxides. An overview of batch dissolution experiments, studying the dissolution chemistry of high surface area MOx powders and low surface area MOx pellets with reference to the effects of nitrous acid, nitric acid and temperature are described. The results are discussed in terms of the autocatalytic mechanism and mass transfer limited dissolution

VerSoX B07-B: a high-throughput XPS and ambient pressure NEXAFS beamline at Diamond Light Source

The beamline optics and endstations at branch B of the Versatile Soft X-ray (VerSoX) beamline B07 at Diamond Light Source are described. B07-B provides medium-flux X-rays in the range 45–2200 eV from a bending magnet source, giving access to local electronic structure for atoms of all elements from Li to Y. It has an endstation for high-throughput X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine-structure (NEXAFS) measurements under ultrahigh-vacuum (UHV) conditions. B07-B has a second endstation dedicated to NEXAFS at pressures from UHV to ambient pressure (1 atm). The combination of these endstations permits studies of a wide range of interfaces and materials. The beamline and endstation designs are discussed in detail, as well as their performance and the commissioning process