Nucleation: What Happens at the Initial Stage?

Crystallizing growth : The initial structure of crystal nuclei is supersaturation‐dependent. At low degrees of supersaturation, liquid‐like nuclei are formed initially, which undergo a continuous structure transition from liquid‐like to crystal‐like as the size N increases. This gradual structure evolution substantially lowers the nucleation barrier ΔG* and facilitates the nucleation relative to the formation of crystal‐like clusters from the beginning.

     

On the Mechanism of the Initial Stage of Titanium–Carbon Interaction

High-speed micro video recording and electron and atomic force microscopy have been used to obtain new experimental data at the initial stage of the titanium melt spreading along a carbon material (oriented pyrolytic graphite) and the formation of a reaction product. It has been shown that the melt spreading at the initial stages takes place along a framework of flat submicron crystalline grains of TiC with an open porosity. The grains form an island structure at the reagent contact boundary with gaps of about 100 nm in width between them, and these gaps are connected to each other to give a continuous network of channels along which the melt spreads. Thus, at the initial stage of the interaction, despite the formation of a solid product, direct contact of the reagents is retained, which ensures a high process speed.     

Microwave Effect in the Fast Synthesis of Microporous Materials: Which Stage Between Nucleation and Crystal Growth is Accelerated by Microwave Irradiation?

Microporous materials, such as silicalite-1 and VSB-5 molecular sieves, have been synthesized by both microwave irradiation (MW) and conventional electric heating (CE). The accelerated syntheses by microwave irradiation can be quantitatively investigated by various heating modes conducted in two steps such as MW-MW, MW-CE, CE-MW, and CE-CE (in the order of nucleation-crystal growth). In the case of synthesis by MW-CE or CE-MW, the heating modes were changed for the second step just after the appearance of X-ray diffraction peaks in the first step. We have quantitatively demonstrated that the microwave irradiation accelerates not only the nucleation but also crystal growth. However, the contribution to decrease the synthesis time by microwave irradiation is larger in the nucleation stage than in the step of crystal growth. The crystal size increases in the order of MW-MW相似文献   

Photodegradation of Riboflavin under Alkaline Conditions: What Can Gas-Phase Photolysis Tell Us about What Happens in Solution?

The application of electrospray ionisation mass spectrometry (ESI-MS) as a direct method for detecting reactive intermediates is a technique of developing importance in the routine monitoring of solution-phase reaction pathways. Here, we utilise a novel on-line photolysis ESI-MS approach to detect the photoproducts of riboflavin in aqueous solution under mildly alkaline conditions. Riboflavin is a constituent of many food products, so its breakdown processes are of wide interest. Our on-line photolysis setup allows for solution-phase photolysis to occur within a syringe using UVA LEDs, immediately prior to being introduced into the mass spectrometer via ESI. Gas-phase photofragmentation studies via laser-interfaced mass spectrometry of deprotonated riboflavin, [RF − H]⁻, the dominant solution-phase species under the conditions of our study, are presented alongside the solution-phase photolysis. The results obtained illustrate the extent to which gas-phase photolysis methods can inform our understanding of the corresponding solution-phase photochemistry. We determine that the solution-phase photofragmentation observed for [RF − H]⁻ closely mirrors the gas-phase photochemistry, with the dominant m/z 241 condensed-phase photoproduct also being observed in gas-phase photodissociation. Further gas-phase photoproducts are observed at m/z 255, 212, and 145. The value of exploring both the gas- and solution-phase photochemistry to characterise photochemical reactions is discussed.     

What is the best definition of a liquid cluster at the molecular scale?

We investigate the ability of different cluster definitions to serve as a good reaction coordinate in molecular simulations of nucleation. In particular, the most commonly used Stillinger criterion [J. Chem. Phys. 38, 1486 (1963)] is compared with the cluster definition introduced by ten Wolde and Frenkel [J. Chem. Phys. 109, 9901 (1998)]. The accuracy of these two different cluster definitions is tested by using molecular dynamics to study the vapor-liquid nucleation of Lennard-Jones argon as a model system. We are able to compare the size of the critical cluster identified by each cluster definition with a completely model-independent value provided by the nucleation theorem, aided by a recently introduced method that accurately extracts the location of the transition state directly from the kinetics. It is found that the Stillinger definition strongly overestimates the size of small molecular clusters by up to a factor of 2. A simple change of the Stillinger radius is unable to rectify this deficiency. On the contrary, the ten Wolde-Frenkel definition, while being only slightly more elaborate than a simple Stillinger criterion, is remarkably successful in identifying the correct molecular excess of the small clusters if the parameters are chosen adequately. The method described here can also be generalized to identify a proper reaction coordinate in other activated processes.     

Influence of Vapor–Gas Medium Heating due to the Condensation Heat on the Stage of Multicomponent Droplet Nucleation

Colloid Journal - The article describes the effect of phase transition heat on the temperature of a closed multicomponent vapor–gas metastable phase and on growing supercritical droplets and...     

Supramolecular chemistry: What is the commercialisation plan?

Opportunities for commercialisation along with personal views are provided in this essay.     

What is the mole?

The mole is a difficult concept. Surveys have shown that even many teachers do not have a proper understanding of it. To help to meet this problem, the SI/IUPAC formulation of the mole is carefully presented and explained. New SI proposals are also briefly discussed.     

What Happens to Hydrophobic Interactions during Transfer from the Solution to the Gas Phase? The Case of Electrospray-Based Soft Ionization Methods

The disappearance of the hydrophobic effect in the gas phase due to the absence of an aqueous surrounding raises a long-standing question: can noncovalent complexes that are exclusively bound by hydrophobic interactions in solution be preserved in the gas phase? Some reports of successful detection by mass spectrometry of complexes largely stabilized by hydrophobic effect are questionable by the presence of electrostatic forces that hold them together in the gas phase. Here, we report on the MS-based analysis of model supramolecular complexes with a purely hydrophobic association in solution, β-cyclodextrin, and synthetic adamantyl-containing ligands with several binding sites. The stability of these complexes in the gas phase is investigated by quantum chemical methods (DFT-M06). Compared with the free interaction partners, the inclusion complex between β-cyclodextrin and adamantyl-containing ligand is shown to be stabilized in the gas phase by ΔG = 9.6 kcal mol^–1. The host–guest association is mainly enthalpy-driven due to strong dispersion interactions caused by a large nonpolar interface and a high steric complementarity of the binding partners. Interference from other types of noncovalent binding forces is virtually absent. The complexes are successfully detected via electrospray ionization mass spectrometry, although a high dissociation yield is also observed. We attribute this pronounced dissociation of the complexes to the collisional activation of ions in the atmospheric interface of mass spectrometer. The comparison of several electrospray-based ionization methods reveals that cold spray ionization provides the softest ion generation conditions for these complexes.     

Native Mass Spectrometry: What is in the Name?

Electrospray ionization mass spectrometry (ESI-MS) is nowadays one of the cornerstones of biomolecular mass spectrometry and proteomics. Advances in sample preparation and mass analyzers have enabled researchers to extract much more information from biological samples than just the molecular weight. In particular, relevant for structural biology, noncovalent protein–protein and protein–ligand complexes can now also be analyzed by MS. For these types of analyses, assemblies need to be retained in their native quaternary state in the gas phase. This initial small niche of biomolecular mass spectrometry, nowadays often referred to as “native MS,” has come to maturation over the last two decades, with dozens of laboratories using it to study mostly protein assemblies, but also DNA and RNA-protein assemblies, with the goal to define structure–function relationships. In this perspective, we describe the origins of and (re)define the term native MS, portraying in detail what we meant by “native MS,” when the term was coined and also describing what it does (according to us) not entail. Additionally, we describe a few examples highlighting what native MS is, showing its successes to date while illustrating the wide scope this technology has in solving complex biological questions.

Open image in new window

Graphical Abstract ?

     

Ion Analysis at the Highest Stage – The Metrological Approach by National Metrological Institutes

National Metrological Institutes (NMIs) from fifteen different countries participated in interlaboratory comparisons where concentrations of about 1gkg^–1 in solutions of aluminium, copper, iron, magnesium, chloride and phosphate were measured. A very high comparability of the results irrespective of the analyte and the applied measurement technique was observed. The relative in-between laboratory standard deviations of the results as reported by the participating laboratories were between 0.13% for copper and chloride up to 0.33% for aluminium, and all of the 81 results were found in the range of ±1% with respect to the reference value. Due to the gravimetric preparation of the samples, a conventional true reference value was calculated, and no significant deviations of the reference values and the means of all results reported by the institutes were found.     

What is the structure of the RecA-DNA filament?

The bacterial RecA protein has been a model system for understanding how a protein can catalyze homologous genetic recombination. RecA-like proteins have now been characterized from many organisms, from bacteriophage to humans. Some of the RecA-like proteins, including human RAD51, appear to function as helical filaments formed on DNA. However, we currently have high resolution structures of inactive forms of the protein, and low resolution structures of the active complexes formed by RecA-like proteins on DNA in the presence of ATP or ATP analogs. Within a crystal of the E. coli RecA protein, a helical polymer exists, and it has been widely assumed that this polymer is quite similar to the active helical filament formed on DNA. Recent developments have suggested that this may not be the case.     

What are the slowest metastable-ion decompositions?

An upper limit on the lifetime of a metastable-ion decomposition is imposed by the competing relaxation of internal excitation by i.r. radiative cooling. Quantitative estimates of this limiting lifetime have been calculated for molecules in the size range of ∼ 20– ∼ 100 internal degrees of freedom. The lifetime of the most slowly decomposing metastable ions observable in significant abundance is predicted to be of the order of 30 ms for typical molecules, and is affected, but not strongly, by molecular size, dissociation threshold energy and nature of the activated complex. The largest factor in determining the slowest metastable-ion lifetimes is the i.r. luminosity, and for molecules of exceptionally low luminosity at low internal temperatures, metastable-ion lifetimes may approach 1 s⁻¹ under the most favorable circumstances.     

Nucleation of the β-modification of isotactic polypropylene

The effects of the organic pigments C.I.P. RED 177 and C.I.P. Yellow 83 as nucleating agents on the crystallization of polypropylene were studied by DSC. The anthraquinone pigment exerted a significant effect, resulting in structural modifications with lower melting point, and particularly the -modification. The DSC curves exhibit four transition regions, with the following temperature intervals: I. 415–417 K, II. 423–425 K, III. 430–432 K and IV. 438–439 K. For evaluation of the -nucleation effect of pigments, the ratio (H ₁+H ₂)/(H ₃+H ₄) was suggested.     

Nucleation and fractal growth of zirconium oxo-alkoxy nanoparticles at the induction stage of sol–gel process

Monodispersed zirconium oxo-alkoxy nanoparticles are synthesized via a sol?Cgel method in a rapid micromixing reactor with in situ particle size measurements. The nucleated nanoparticles of 4.7?nm diameter are free from impurities and conserve high chemical activity. They can be associated in 1D fractals. These nanoparticles can form high optical-quality coatings on hydrophilic substrates.