Rate Of Nucleus Formation Research Articles

Influence of denaturants on amyloid β42 aggregation kinetics.

Amyloid formation is linked to devastating neurodegenerative diseases, motivating detailed studies of the mechanisms of amyloid formation. For Aβ, the peptide associated with Alzheimer’s disease, the mechanism and rate of aggregation have been established for a range of variants and conditions in vitro and in bodily fluids. A key outstanding question is how the relative stabilities of monomers, fibrils and intermediates affect each step in the fibril formation process. By monitoring the kinetics of aggregation of Aβ42, in the presence of urea or guanidinium hydrochloride (GuHCl), we here determine the rates of the underlying microscopic steps and establish the importance of changes in relative stability induced by the presence of denaturant for each individual step. Denaturants shift the equilibrium towards the unfolded state of each species. We find that a non-ionic denaturant, urea, reduces the overall aggregation rate, and that the effect on nucleation is stronger than the effect on elongation. Urea reduces the rate of secondary nucleation by decreasing the coverage of fibril surfaces and the rate of nucleus formation. It also reduces the rate of primary nucleation, increasing its reaction order. The ionic denaturant, GuHCl, accelerates the aggregation at low denaturant concentrations and decelerates the aggregation at high denaturant concentrations. Below approximately 0.25 M GuHCl, the screening of repulsive electrostatic interactions between peptides by the charged denaturant dominates, leading to an increased aggregation rate. At higher GuHCl concentrations, the electrostatic repulsion is completely screened, and the denaturing effect dominates. The results illustrate how the differential effects of denaturants on stability of monomer, oligomer and fibril translate to differential effects on microscopic steps, with the rate of nucleation being most strongly reduced.

Successful development of HAN based green propellant

Research and development of Hydroxyl Ammonium Nitrate based liquid propellant; SHP-163 is reviewed. Starting from the composition selection of the propellant to the development of 1 ​N thruster of Green Propellant Reaction Control System (GPRCS) project and the successful demonstration test in space is summarized. Combustion behaviors of SHP-163 were investigated and the mechanism of the very high burning rate at high pressures was studied. The observation on HAN-aqueous solutions revealed that the apparent very high burning rate is triggered by hydrodynamic instability and controlled by water nuclei formation rate which is induced by the superheat by the exothermic reaction of HAN decomposition. The operation pressure of GPRCS was determined considering this high burning rate region. Other characteristics of SHP-163 are also presented.

Selective chemical vapor deposition of HfB2 on Al2O3 over SiO2 and the acceleration of nucleation on SiO2 by pretreatment with Hf[N(CH3)2

We show that growth of the metallic ceramic HfB2 by CVD from Hf(BH4)4 at 220 °C is inherently selective on Al2O3 over SiO2: a 10.4-nm film grows on Al2O3 in 16 min, whereas only 0.07 nm of HfB2 grows on SiO2 in 18 min. Nucleation occurs on both SiO2 and Al2O3; however, the Al2O3 surface has a much higher density of nuclei such that HfB2 islands quickly coalesce to form continuous films, followed by steady-state growth of HfB2. On SiO2, nucleation is sparse and coalescence of the islands takes much longer; as a result, the overall growth rate is slower. Sparse nucleation on SiO2 also leads to a rough layer with a broad height distribution function: for a deposit containing 1.6 × 1015 Hf atoms/cm2 (equivalent to a bulk thickness of 0.5 nm for HfB2), the rms roughness is 3.8 nm on SiO2 but only 1.3 nm on Al2O3. The difference in the formation rate of nuclei (and thus the area density of nuclei) is attributed to the different acid-base character of hydroxyl groups on these oxide surfaces. We also found that, when growth on SiO2 is desired, the surface can be modified by exposure to tetrakis(dimethylamido)hafnium, which adsorbs to saturation at ∼1 monolayer. Subsequent exposure of this pretreated surface leads to an increased density of HfB2 nuclei, a reduced coalescence time, and a smaller roughness of the resulting surface from 3.8 to 1.7 nm. By contrast, a similar pretreatment on Al2O3 has little effect on the roughness of subsequently grown HfB2 films, which are already relatively smooth when grown on untreated alumina surfaces.

Synthesis and characterization of NaX-type zeolites prepared by different silica and alumina sources and their CO2 adsorption properties

Abstract In this work, two pairs of silicon and aluminium sources (sodium silicate solution + Al(OH)3 and sodium silicate powder + sodium aluminate) were used for synthesizing zeolites which were labeled as Z1 and Z2, respectively. The results from XRD, BET and surface analysis indicated that sodium silicate and sodium aluminate sources yielded zeolite with high surface area and pore volume. Cetyltrimethylammonium bromide (CTAB) and heptane were used to modify the CO2 adsorption capacity and selectivity of zeolite NaX. The surface area and pore volume of zeolite products increased due to smaller crystalline size and then resulted in significant enhancement of CO2 uptake of the synthesized zeolite NaX. The role of additives on zeolite formation was to increase the nuclei formation rate and number which gave rise to smaller zeolite crystals. The best adsorbent exhibited high CO2 adsorption capacity (5.08 mmol g−1 at 303 K) and relatively high CO2 selectivity over N2 (116) which indicated the potential of adsorbent to separate CO2 from flue gas application.

Preparation of halloysite/polyvinylidene fluoride composite membrane by phase inversion method for lithium ion battery

In this paper, the composite membranes based on poly(vinylidene fluoride) (PVDF) with different amounts of halloysite (HNT) nano-filler were prepared by phase inversion method for the application of lithium ion batteries. The effect of the content of HNT on the crystallinity and morphology of the composite membranes were investigated. The results showed that the crystallization behaviors of the HNT/PVDF composite membranes were reduced with the increase of HNT filler content. Furthermore, the introduction of HNT into PVDF matrix remarkably affected the formation rate of nuclei which could evolve into pores to change surface morphology. The HNT/PVDF composite membranes showed the sufficient porosity and good affinity between the eletrolyte as well as the electrode, which led to superb electrolyte uptake and low interfacial impedance. Among HNT/PVDF composite membranes, in particular, the 4-HNT/PVDF composite membrane with 4 wt% HNT content had a great electrolyte uptake (430.2%), superior electrochemical stability window (4.5 V), high ionic conductivity (2.4 mS/cm), and low interfacial impedance (63Ω). The cells assembled with the 4-HNT/PVDF composite membranes obtain discharge capacities of 164.72 mAh/g (0.1C) and 134.88 mAh/g (2C), corresponding to 96.89% and 79.34% of the theoretical capacity of LiFePO4 (170 mAh/g), respectively, better cycle performance (with capacity retention 89.12% after 100 cycles) at 0.5C and excellent rate performance. It is concluded that the content of HNT deeply affects membrane performance. Therefore, the 4-HNT/PVDF composite membranes could be a good option of high performance lithium ion batteries.

Simulation of CP-Ti Recrystallization and Grain Growth by a Cellular Automata Algorithm: Simulated Versus Experimental Results

The application of cellular automata in materials science requires the conversion of the automata's rules and abstract general properties to rules and properties associated with the material and phenomena under study. In this paper we propose a model which uses cellular automata to simulate recrystallization and grain growth during isothermal and non-isothermal treatments of cold worked polycrystalline materials. The algorithm's spatial and temporal scaling is based on known experimental results for recrystallization and grain growth in highly cold-worked commercially pure titanium grade 2. In the recrystallization, the best agreement between experimental and computational results in terms of the process kinetics and the average diameter of recrystallized grains is obtained from a nucleation model that considers the temperature-dependent nuclei formation rate. In the simulation of grain growth after primary recrystallization, the results indicate the normal growth of an equiaxed grain structure whose kinetics and dimensions are comparable to those observed experimentally.

Viral Lysis of Photosynthesizing Microbes As a Mechanism for Calcium Carbonate Nucleation in Seawater.

Removal of carbon through the precipitation and burial of calcium carbonate in marine sediments constitutes over 70% of the total carbon on Earth and is partitioned between coastal and pelagic zones. The precipitation of authigenic calcium carbonate in seawater, however, has been hotly debated because despite being in a supersaturated state, there is an absence of persistent precipitation. One of the explanations for this paradox is the geochemical conditions in seawater cannot overcome the activation energy barrier for the first step in any precipitation reaction; nucleation. Here we show that virally induced rupturing of photosynthetic cyanobacterial cells releases cytoplasmic-associated bicarbonate at concentrations ~23-fold greater than in the surrounding seawater, thereby shifting the carbonate chemistry toward the homogenous nucleation of one or more of the calcium carbonate polymorphs. Using geochemical reaction energetics, we show the saturation states (Ω) in typical seawater for calcite (Ω = 4.3), aragonite (Ω = 3.1), and vaterite (Ω = 1.2) are significantly elevated following the release and diffusion of the cytoplasmic bicarbonate (Ωcalcite = 95.7; Ωaragonite = 68.5; Ωvaterite = 25.9). These increases in Ω significantly reduce the activation energy for nuclei formation thresholds for all three polymorphs, but only vaterite nucleation is energetically favored. In the post-lysis seawater, vaterite's nuclei formation activation energy is significantly reduced from 1.85 × 10−17 J to 3.85 × 10−20 J, which increases the nuclei formation rate from highly improbable (<<1.0 nuclei cm−3 s−1) to instantaneous (8.60 × 1025 nuclei cm−3 s−1). The proposed model for homogenous nucleation of calcium carbonate in seawater describes a mechanism through which the initial step in the production of carbonate sediments may proceed. It also presents an additional role of photosynthesizing microbes and their viruses in marine carbon cycles and reveals these microorganisms are a collective repository for concentrated and reactive dissolved inorganic carbon (DIC) that is currently not accounted for in global carbon budgets and carbonate sediment diagenesis models.

Effects of Pore Nucleation, Growth and Solidification Mode on Pore Structure and Distribution of Lotus Type Porous Copper

Effects of pore nucleation, growth and solidification mode of matrix on pore structure and distribution of lotus-type porous Cu fabricated by Gasar process were investigated. The results showed that it was difficult to obtain an ordered pore structure when the matrix contained equiaxed grain. An ordered pore structure, with increased pore length and circularity, could be obtained when the matrix consisted of columnar grains. The pore distribution moved from the grain boundaries to the interior of grain with increasing gas pressure. It was further noticed that the average pore diameter decreased and the pore density increased due to decreased activation energy and increased rate of nucleus formation. Due to slight depressions at grain boundaries of the solid/liquid interface, the pore nucleation was favored at grain boundaries and thus the average pore diameter in grain boundaries was larger than that in grains.

Calibrating Sickle Cell Disease

Sickle cell disease is fundamentally a kinetic disorder, in which cells containing the mutated hemoglobin (hemoglobin S; HbS) will cause occlusion if they sickle in the microvasculature, but have minimal (or no) consequences if they sickle in the venous return. Physiologically, sickling always occurs when some ligands are present; nonetheless, the kinetics in the presence of ligands are virtually unstudied. Sickling arises from nucleation-controlled polymer formation, triggered when the HbS loses ligands (e.g., oxygen). Thus, understanding how nucleation responds to the presence of oxygen is the key to understanding how sickling proceeds in a physiological context. We have measured the rate of nucleus formation in HbS partially liganded with NO or CO, which we find have equivalent effects in reducing the nucleation rates. We find that hemoglobin must be in the T (tense) quaternary structure for nucleation, but the presence of ligands inhibits nucleus formation even when the correct quaternary structure is present. From these results, we can predict the fraction of cells that will sickle at any given partial ligand saturations. The ability to make such predictions may prove especially useful in designing future therapies, particularly those where the oxygen affinity is perturbed.

Nucleation phenomena in electrochemical systems: kinetic models

The classical thermodynamics allow deriving theoretical expressions for the nucleation work and for the size of the critical nuclei in a supersaturated system. However, information on the rate of nucleus formation should be determined by means of kinetic considerations. This article sheds light on the nucleation kinetics in electrochemical systems at a constant thermodynamic supersaturation $$ \Delta \tilde{\mu } $$ . Theoretical expressions are derived for the stationary and non-stationary nucleation rate.

Calibrating Sickle Cell Disease

Sickle cell disease is fundamentally a kinetic disorder, in which cells containing the mutated hemoglobin (HbS) will cause occlusion if they sickle in the microvasculature, but have minimal (or no) consequences if they sickle in the venous return. Physiologically, sickling always occurs when some ligands are present; nonetheless, the kinetics in the presence of ligands are virtually unstudied. Sickling arises from nucleation-controlled polymer formation, triggered when the HbS loses ligands (e.g. oxygen). Thus, understanding how nucleation responds to the presence of oxygen is the key to understanding how sickling proceeds in a physiological context. We have measured the rate of nucleus formation in HbS partially liganded with NO or CO, which we find have equivalent effects in reducing the nucleation rates. We find that hemoglobin must be in the T (tense) quaternary structure for nucleation, but the presence of ligands inhibits nucleus formation even when the correct quaternary structure is present. From these results we can predict the fraction of cells that will sickle at any given partial ligand saturations. It appears that symptoms are ameliorated once fewer than 30% of the cells sickle in less than 1 sec. This conclusion is supported by analysis of exchange transfusions, and suggests how to interpret the mixed results of hydroxyurea therapy. The ability to make such predictions may prove especially useful in designing future therapies, particularly those where the oxygen affinity is perturbed.

Polyacrylate-Assisted Size Control of Silver Nanoparticles and Their Catalytic Activity

In this work, a simple one-step method of silver nanoparticle (NPs) preparation with controlled size is introduced. Silver NPs were prepared by reduction of [Ag(NH3)2]+ complex cation by d-maltose in the presence of low concentrations (1 × 10–10 mol/L to 1 × 10–7 mol/L) of high-molecular-weight poly(acrylic acid) (PAA). This modification of the reduction reaction results in managing of the prepared silver nanoparticles’ size in the range from 28 nm (the reaction system without PAA) to 77 nm (system with the highest used PAA concentration). The presence of poly(acrylic acid) influences both nucleation process and also the subsequent stage of nanoparticle growth. The rate of nuclei formation by homogeneous nucleation is affected by PAA due to the formation of the strong complex with silver ions which results in a prolongation of the initiation phase of the reduction process proportionally to PAA concentration. The formation of the adsorption layer of PAA on the surface of the emerging silver nuclei prevents...

Kinetics of Palladium Oxidation in the mbar Pressure Range: Ambient Pressure XPS Study

Palladium oxidation was studied by ambient-pressure X-ray photoelectron spectroscopy in the mbar pressure range on the Pd(111) and Pd(110) surfaces. The oxidation kinetics on both surfaces show an induction period when the oxidation rate was low at the beginning and then accelerated. The slow initial oxidation is governed by (a) the rate of nucleus formation, and (b) the rate of oxide nucleus growth. Depth profiling varying photon energy/kinetic photoelectron energy pointed to a 3D oxidation. It is remarkable that the oxidation of Pd(110) proceeds at ~100 K lower temperatures than on Pd(111). We suggest that at the high temperature required on Pd(111) nucleation is thermodynamically controlled, and therefore, the nucleation rate decreases with temperature. On Pd(110), nucleation is predominantly kinetically controlled and thus the oxidation rate increases with temperature.

The kinetics of redox reaction of gold(III) chloride complex ions with l-ascorbic acid

The synthesis of gold nanoparticles consists of several stages, i.e. reduction, nucleation and growth. The most important step in particles formation process is the reduction reaction of Au(III) ions to Au(I). The rate of such reaction determines the rate of nuclei formation, i.e. reduction reaction of Au(I) to Au(0) and thus the rate of nanoparticles formation. In this paper, the kinetics and mechanism of redox reaction between gold(III) chloride complex ions and l-ascorbic acid in the aqueous solution have been carried out using spectrophotometric technique. From the experimentally obtained kinetic curves the rate constants of reaction at different conditions of initial concentrations of reactants, pH, temperature, ionic strength and concentration of chloride ions, were determined. It was found that in strong acidic solution (pH 1.3), the values of enthalpy (ΔH‡) and entropy (ΔS‡) of activation were 38.5kJ/mol and −89.5kJ/molK, respectively and at higher pH (pH 2.5) the values of ΔH‡ and ΔS‡ were 42.0kJ/mol and −64.5kJ/molK, respectively. Using obtained results, the kinetic equation was determined and corresponding mechanism of reaction was suggested. Obtained data can be useful for calculating time of nuclei formation and consequently for better size control and size distribution of particles, e.g. in a microreactor.

Thickness-dependent structural transition in GaAs nanocrystals grown on Si (1 1 1) surface

Structural stabilities in GaAs nanocrystals grown on the Si (1 1 1) substrate have been studied by transmission electron microscopy in order to see the structure and growth mechanism. The GaAs nanocrystals grown epitaxially on the Si (1 1 1) surface kept at 573 K have thin shapes consisting of a flat surface which is parallel to the Si (1 1 1) surface. The crystalline structure of the initial growth layer approximately below 5 nm in thickness is the zincblend structure, but with increasing thickness the structure changes to the wurtzite structure by formation of orderly-arranged stacking faults. The small difference in the driving force between the wurtzite structure and the zincblende structure could bring about a situation, where the kinetic rate of nucleus formation is high for the wurtzite structure than for the zincblende structure. It would highly increase the probability that the wurtzite structure is formed as a non-equilibrium state.

Structural Stabilities in GaAs Nanocrystals Grown on Si (111) Surface

Structural stabilities in GaAs nanocrystals grown on the Si (111) substrate have been studied by transmission electron microscopy in order to see the structure and growth mechanism. The GaAs nanocrystals grown epitaxially on the Si (111) surface kept at 573 K have thin shapes consisting of a flat surface which is parallel to the Si (111) surface. The crystalline structure of the initial growth layer, below approximately 5 nm in thickness is zincblend, but with increasing thickness the structure changes to the wurtzite structure by formation of orderly-arranged stacking faults. The small difference in the driving force between wurtzite and the zincblende structure could lead to a situation where the kinetic rate of nucleus formation is higher for the wurtzite structure than for the zincblende structure. It would highly increase the probability that the wurtzite structure is formed as a non-equilibrium state.

Effect of deposition temperature on boron-doped carbon coatings deposited from a BCl 3–C 3H 6–H 2 mixture using low pressure chemical vapor deposition

A mixture of propylene, hydrogen and boron trichloride was used to fabricate boron-doped carbon coatings by using low pressure chemical vapor deposition ( LPCVD) technique. Effect of deposition temperature on deposition rate, morphologies, compositions and bonding states of boron-doped carbon coatings was investigated. Below 1273 K, the deposition rate is controlled by reaction dynamics. The deposition rate increases with increasing deposition temperature. The activation energy is 208.74 kJ/mol. Above 1273 K, the deposition rate decreases due to smaller critical radius r c and higher nuclei formation rate J with increasing temperature. Scanning electron microscopy shows that the structure changes from glass-like to nano-laminates with increasing deposition temperature. The boron concentration decreases with increasing deposition temperature, corresponding with increasing carbon concentration. The five types of bonding states are B–C, B-sub-C, BC 2O, BCO 2 and B–O. B-sub-C and BC 2O are the main bonding states. The reactions are dominant at all temperatures, in which the B-sub-C and PyC are formed.

Liquid superheat during nonequilibrium boiling

Heat and mass transfer processes in a pure liquid subject to intense heating is investigated. The temperature escalation rate in a heated pure liquid is controlled by two competing processes; the external power deposition and the rate of nuclei formation and growth in the liquid, which acts as a heat sink. A heat balance equation is developed and solved numerically to yield the liquid temperature curve and the evaporation rate up to the maximum attainable superheat point. The effect of heating rate on the liquid temperature curve is quantified.

Estudo da eletrocristalização de Ni e Ni-P sobre ultramicroeletrodo de platina

This work describes a comparative study of the electrocrystallization of Ni and Ni-P on Pt ultramicroelectrodes using chronoamperometric measurements. It was possible to confirm that in all cases a progressive nucleation was the predominant mechanism. Moreover, the application of the Atomistic Theory to the experimental rate of nuclei formation showed that the number of atoms in the critical nucleus was zero, except for Ni-P on Pt at low overpotentials were a value of one was observed. Furthermore, the physical characterisation of the different deposits on Pt by atomic force microscopy allowed observing the coalescence of the hemispherical nuclei of Ni and Ni-P at tmax thus confirming the results obtained from the current-time analysis.

The physical characteristics of sulfur aerosols

A review of the physical characteristics of sulfur-containing aerosols, with respect to size distribution of the physical distributions, sulfur distributions, distribution modal characteristics, nuclei formation rates, aerosol growth characteristics, and in situ measurement, has been made. Physical size distributions can be characterized well by a trimodal model consisting of three additive lognormal distributions. When atmospheric physical aerosol size distributions are characterized by the trimodal model, the following typical modal parameters are observed: 1. Nuclei mode – geometric mean size by volume, DGV n, from 0.015 to 0.04 μm. σ gn=1.6, nucler mode volumes from 0.0005 over the remote oceans to 9 μm 3 cm −3 on an urban freeway. 2. Accumulation mode – geometric mean size by volume, DGV a, from 0.15 to 0.5 μm, σ ga=1.6–2.2 and mode volume concentrations from 1 for very clean marine or continental backgrounds to as high as 300 μm 3 cm −3 under very polluted conditions in urban areas. 3. Coarse particle mode – geometric mean size by volume, DGV c, from 5 to 30 μm, σ gn=2–3, and mode volume concentrations from 2 to 1000 μm 3 cm −3. It has also been concluded that the fine particles ( D p <2 μm) are essentially independent in formation, transformation and removal from the coarse particles ( D p >2 μm). Modal characterization of impactor-measured sulfate size distributions from the literature shows that the sulfate is nearly all in the accumulation mode and has the same size distribution as the physical accumulation mode distribution. Average sulfate aerodynamic geometric mean dia. was found to be 0.48±0.1 μm (0.37±0.1 μm vol. dia.) and σ g=2.00±0.29. Concentrations range from a low of about 0.04 μg m −3 over the remote oceans to over 8 μg m −3 under polluted conditions over the continents. Review of the data on nucleation in smog chambers and in the atmosphere suggests that when SO 2, is present, SO 2-to-aerosol conversion dominates the Aitken nuclei count and, indirectly, through coagulation and condensation, the accumulation mode size and concentration. There are indications that nucleation is ubiquitous in the atmosphere, ranging from values as low as 2 cm −3 h −1 over the clean remote oceans to a high of 6×10 6 cm −3 h −1 in a power plant plume under sunny conditions. There is considerable theoretical and experimental evidence that even if most of the mass for the condensational growth of the accumulation mode comes from hydrocarbon conversion, sulfur conversion provides most of the nuclei.