Experimental analysis of non-isothermal transformations involving nucleation and growth

A critical examination is made of a recently suggested experimental technique which utilizes the Johnson-Mehl-Avrami transformation rate equation to describe nonisothermal transformations. It is shown that for transformations involving nucleation and growth the technique has limited applicability.     

Investigation of the influence of NaCl concentration on Halobacterium salinarum growth

Microcalorimetry was used to study the influence of NaCl concentration on Halobacterium salinarum growth. From the thermogenic curves and thermokinetic parameters of H. salinarum growth in different concentrations of NaCl, it was found that the optimum NaCl concentration for H. salinarum growth was not a wide range from 3.5 mol L^–1 to NaCl saturation (about 5.2 mol L^–1), as is generally acknowledged, but just around 230 g L^–1 (approximately 3.9 mol L^–1). And when external NaCl concentration was above 230 g L^–1, the growth metabolism of H. salinarum decreased constantly with the increasing of NaCl concentration. These have never been described before. Further investigation by transmission electron microscopy revealed that H. salinarum growing in approaching NaCl saturation underwent plasmolysis, which interpreted the novel finding of microcalorimetry perfectly. Our work shows that microcalorimetry may reveal more and newer details about microbial growth than the existing methods do. These details are significant to understand biological processes.     

Kinetic theory of binary nucleation based on a first passage time analysis

The binary classical nucleation theory (BCNT) is based on the Gibbsian thermodynamics and applies the macroscopic concept of surface tension to nanosize clusters. This leads to severe inconsistencies and large discrepancies between theoretical predictions and experimental results regarding the nucleation rate. We present an alternative approach to the kinetics of binary nucleation which avoids the use of classical thermodynamics for clusters. The new approach is an extension to binary mixtures of the kinetic theory previously developed by Narsimhan and Ruckenstein and Ruckenstein and Nowakowski [J. Colloid Interface Sci. 128, 549 (1989); 137, 583 (1990)] for unary nucleation which is based on molecular interactions and in which the rate of emission of molecules from a cluster is determined via a mean first passage time analysis. This time is calculated by solving the single-molecule master equation for the probability distribution of a "surface" molecule moving in a potential field created by the cluster. The starting master equation is a Fokker-Planck equation for the probability distribution of a surface molecule with respect to its phase coordinates. Owing to the hierarchy of characteristic time scales in the evolution of the molecule, this equation can be reduced to the Smoluchowski equation for the distribution function involving only the spatial coordinates. The new theory is combined with density functional theory methods to determine the density profiles. This is essential for nucleation in binary systems particularly when one of the components is surface active. Knowing these profiles, one can determine the potential fields created by the cluster, its rate of emission of molecules, and the nucleation rate more accurately than by using the uniform density approximation. The new theory is illustrated by numerical calculations for a model binary mixture of Lennard-Jones monomers and rigidly bonded dimers of Lennard-Jones atoms. The amphiphilic character of the dimer component (i.e., its surface activity) is induced by the asymmetry in the interaction between a monomer and the two different sites of a dimer. The inconsistencies of the BCNT are avoided in the new theory.     

Effects of reactant concentration on the internal diffusion retardation of irreversible heterogeneous catalytic reactions

A theoretical analysis is presented for these effects for reactions that are not retarded by products and of orders zero, one, and two. The retardation is found to increase for reactions of fractional order 0n<1 as the reactant is consumed, while first-order reactions are unaffected and higher-order ones show a decreasing effect. The results for zero order are compared with experiment for the catalytic hydrogenation of carbon monoxide.     

Interrelation between cluster formation time, cluster growth probability, and nucleation rate

Approximate expressions are derived for the mean time tau for formation of a cluster of n molecules in nucleation of single-component phases. The derivation elucidates the interrelation between tau, the cluster growth probability P, and the stationary nucleation rate. The extraction of both tau(n) and P(n) data from individual cluster growth curves obtained in experiments or simulations is discussed. It is shown that the analysis of tau(n) data allows a model-independent determination of the nucleus size, the Zeldovich factor, the stationary nucleation rate, the frequency with which molecules are attached to the nucleus, and the difference between the works to form the nucleus and the smallest "cluster" of one molecule.     

Electrochemical nucleation with diffusion-limited growth. Properties and analysis of transients

A general equation for the potentiostatic transient for the case of electrochemical nucleation with diffusion-controlled growth has been published recently [L. Heerman, A. Tarallo, J. Electroanal. Chem. 470 (1999) 70]. This equation essentially is a correction to the model of Scharifker and Mostany [J. Electroanal. Chem. 177 (1984) 13] but leads to quite different predictions. This paper discusses the differences between these two models with special emphasis on the analysis of experimental transients. Numerical data are presented which allow the determination of the nucleation parameters (nucleation rate constant A and site density N₀). Finally, it is shown that plots of I/I_max versus t/t_max, which are widely used in the literature for the graphical representation of experimental results, are not very sensitive and can easily lead to misinterpretation of experimental results.     

Spatially-resolved analysis of nanoparticle nucleation and growth in a microfluidic reactor

Microfluidic systems provide a unique platform for investigation of fundamental reaction processes, which is critical to understanding how to control nanostructure synthesis on a production scale. We have examined the synthesis of cysteine-capped CdS quantum dot nanocrystals (CdS-Cys) between two interdiffusing reagent streams in a continuous-flow microfluidic reactor. Using spatially resolved photoluminescence imaging and spectroscopy of the microreactor, we have acquired kinetic and mechanistic data on the CdS-Cys nanoparticle nucleation and growth, and observed a binary shift in the particle emission spectrum from a higher (2.9 eV) to lower (2.5 eV) energy emission peak within the first second of residence time. Several reactor models have been tested against the spatially and spectrally resolved signals, which suggest that homogeneous reaction and particle nucleation are diffusion-limited and occur only at the boundary between the two laminar streams, while a slower activation process occurs on a longer (seconds) time scale. The results provide direct insight into the rapid processes that occur during crystallization in microfluidic mixing channels, and demonstrate the potential of using controlled microfluidic environments with spatially resolved monitoring to conduct fundamental studies of nanocrystal nucleation and growth.     

Polymer phase separation in composite latex particles. 1. Considerations for the nucleation and growth mechanism

This study addresses the question of how polymer phase separation takes place during polymerization reactions within composite latex particles. Experiments resulted in acrylic/styrene latices with two-phase structures that were analyzed via TEM. Those that resulted from the use of semi-batch reactions allowed us to observe domains that likely did not undergo phase rearrangement after they were formed within the particles. We computed the critical size of the phase-separated domains by assuming that the nucleation and growth mechanism applied to such experiments. We also computed how much these domains would increase in size by subsequent polymerization within those domains. Comparisons of predicted and experimental domain sizes and distributions showed quite reasonable agreement. The domains formed in latex particles of about 350 nm were in the 30–50-nm range. Despite the close agreement between theory and experiment, we are not convinced that phase separation occurs by nucleation and growth, as it appears to us that given the relative rates of reaction and polymer diffusion, phase separation events will often be forced to occur within the spinodal region of the phase diagram. To cite this article: J.M. Stubbs, C. R. Chimie 6 (2003).     

无机原料体系合成TS-1影响因素的研究

以廉价的硅溶胶和三氯化钛分别作为硅源和钛源,四丙基溴化铵(TPABr)为模板剂,二乙胺、正丁胺、氨水等作为碱源,在无机体系中合成了TS-1分子筛。采用XRD,IR,UV-vis,SEM,元素分析和N2吸附/脱附等对合成的分子筛进行了表征。详细考察了碱源、硅钛比N2保护、晶种、模板剂用量、硅源及晶化时间等因素对分子筛合成的影响。结果表明,以无机原料合成的TS-1与用传统有机原料合成的TS-1具有同样的特征。碱度的控制是合成的关键,配胶时需N2保护,加入晶种对合成有明显的导向作用,模板剂最低用量有一临界值,硅溶胶作硅源合成的TS-1晶粒比较大。     

The influence of blood sample storage time on the propofol concentration in plasma and solid blood elements

In order to describe the changes of propofol concentration in whole blood and in its components during the blood storage we examined venous blood samples collected from patients anaesthetized either with or without propofol. Blood samples from patients anaesthetized without propofol were spike with propofol 45 min before analysis. Propofol concentration was examined in whole blood, plasma, rinsed formed elements and rinsed and lysed formed blood elements by means of HPLC after 1, 4, 7, 13, 21, 25 and 28 days of storage. There was significant decrease in plasma concentration of propofol during the first few days of sample storage followed by its increase during subsequent days. The opposite phenomenon was observed for formed blood elements. The findings support the hypothesis that propofol distribution between blood components changes in time.     

Secondary nucleation and growth of ZnO

Recently we discovered that under certain conditions new crystal growth (branch) can be induced on specific crystalline planes of the same material. This is a new phenomenon and is in sharp contrast to typical nucleation and growth in which a crystal will simply grow larger in preferred directions depending on the surface energy of the specific crystalline planes. Based on our observation, we developed a sequential nucleation and growth technique offering the power to assemble complex hierarchical crystals step-by-step. However, the key questions of when and how the secondary nucleation takes place have not been answered. Here we systematically study secondary ZnO crystal growth using organic diamine additives with a range of chain lengths and concentration. We found that ZnO branches form for a narrow diamine concentration range with a critical lower and upper critical nucleation concentration limit, which increases by about a factor of 5 for each additional carbon in the diaminoalkane chain. Our results suggest that the narrow window for secondary growth is dictated by the solubility of the ZnO crystals, where the low critical nucleation concentration is determined by slight etching of the surface to produce new nucleation sites, and the upper critical concentration is determined by the supersaturation concentration. Kinetic measurements show that the induction time and growth rate increase with increasing diamine concentration and follow classical nucleation and growth theory. Observations of branch morphological evolution reveal the mechanisms guiding the tunable crystal size and morphology.     

Kinetics of electrochemical nucleation and growth

A united scheme for the kinetics of electrochemical nucleation and the growth of a new phase is presented. The peculiarities of ion-transfer kinetics during electrochemical phase formation are analysed. The influence of the exchange current density at the electrolyte/cluster of the new phase interface on the nucleation rate, the nucleation induction time and the growth rate is reported.     

Kinetic theory of nucleation based on a first passage time analysis: improvement by the density-functional theory

A recent kinetic theory of nucleation [see, e.g., E. Ruckenstein and B. Nowakowski, J. Colloid Interface Sci. 137, 583 (1990)] is based on molecular interactions and avoids the traditional thermodynamics. The rate of emission of molecules from a cluster is found via a first passage time analysis. This time is calculated by solving the single-molecule master equation for the probability distribution function of a surface molecule located in the potential field created by the cluster. The liquid cluster was assumed to have sharp boundaries and uniform density. In the present paper, this assumption is removed by using the density-functional theory to find the density profiles. Thus, more accurate calculations of the potential field created by the cluster, its emission rate, and nucleation rate are obtained. The modified theory is illustrated by numerical calculations for a molecular pair interaction potential combining the dispersive attraction with the hard-sphere repulsion.     

Crystal growth and classical nucleation theory

Calculations were performed of the crystal growth rates in lithium disilicate glass in the low-temperature regime where homogeneous nucleation is observed. The computations were executed using the gain-loss (Becker–Doring) equations that form the framework of Classical Nucleation Theory (CNT). The growth rates were obtained in several different ways, using various choices for the kinetic model, the generalized diffusion coefficient, and the physical input data. The results of these calculations are compared with recently obtained experimental values of the growth rates.     

Study of the influence of concentration on the time for first appearance of crystallization of polyethylene from xylene

An experiment is reported in which a simple laser light-scattering technique is used to measure the time for a critical turbidity to appear during the crystallization of polyethylene in xylene. The effect is examined over the entire range of solution concentration, and it is found that the relation between this concentration and temperature is linear for all solutions, provided that the time for turbidity to develop is arranged to be the same in all cases. In the appendix it is reasoned that similar results could have been obtained if, instead, times had been measured to a given degree of crystallinity rather than of turbidity. Departures from this linearity at low concentrations are taken as indicating multimolecular nucleation for all concentrations greater than about 1%.     

Memory effect on the pressure-temperature condition and induction time of gas hydrate nucleation

The focus of this study is to investigate the influence of memory effect and the relation of its existence with the dissociation temperature, using gas hydrate formation and dissociation experiments. This is beneficial because memory effect is considered as an effective approach to promote the thermodynamic and dynamic conditions of gas hydrate nucleation. Seven experimental systems (twenty tests in total) were performed in a 1 L pressure cell. Three types of hydrate morphology, namely massive, whiskery and jelly crystals were present in the experiments. The pressures and temperatures at the time when visual hydrate crystals appeared were measured. Furthermore, the influence of memory effect was quantified in terms of pressure-temperature-time (p-T-t) relations. The results revealed that memory effect could promote the thermodynamic conditions and shorten the induction time when the dissociation temperature was not higher than 25 ℃. In this study, the nucleation superpressure and induction time decrease gradually with time of tests, when the earlier and the later tests are compared. It is assumed that the residual structure of hydrate dissociation, as the source of the memory effect, provides a site for mass transfer between host and guest molecules. Therefore, a driving force is created between the residual structures and its surrounding bulk phase to promote the hydrate nucleation. However, when the dissociation temperature was higher than 25 ℃, the memory effect vanished. These findings provide references for the application of memory effect in hydrate-based technology.     

Metadynamics simulations of ice nucleation and growth

The metadynamics method for accelerating rate events in molecular simulations is applied to the problem of ice freezing. We demonstrate homogeneous nucleation and growth of ice at 180 K in the isothermal-isobaric ensemble without the presence of external fields or surfaces. This result represents the first report of continuous and dynamic ice nucleation in a system of freely evolving density. Simulations are conducted using a variety of periodic simulation domains. In all cases the cubic polymorph ice I(c) is grown. The influence of boundary effects on estimates of the nucleation free energy barrier are discussed in relation to differences between this and earlier work.     

聚苯胺的成核及生长机理

本文通过恒电位阶跃法研究了聚苯胺在不同介质中的成核与膜的生长过程动力学。结果表明, 在硫酸介质中, 成核过程为扩散控制下的三维连续成核, 得到疏松、多孔的膜; 而在高氯酸介质中, 成核则是电化学动力学控制下的二维成核过程。在高电位时(E>1.02V, vs, SCE)为二维连续成核过程, 而在较低的电位时, 主要表现为二维瞬时成核, 膜层呈网状且致密。     

Real-time image analysis and numerical simulation of isothermal spherulite nucleation and growth in polyoxymethylene

A method of analysing nucleation and crystallization kinetics, based on real time image analysis and hot stage optical microscopy, has been used to investigate the isothermal crystallization of different grades polyoxymethylene. The data were compared with results from differential scanning calorimetry (DSC), using a simple numerical simulation to model the effects of finite smaple thickness on the form of the isothermal DSC curves. This simulation was then used to predict the microstructural development in a bulk sample for different boundary conditions, taking into account latent heat evolution and diffusion during crystallization.