Kinetic model for binary homogeneous nucleation in the H2O-H2SO4 system: comparison with experiments and classical theory of nucleation

A kinetic model to predict nucleation rates in the sulfuric acid-water system is presented. It allows calculating steady-state nucleation rates and the corresponding time lag, using a direct solution of a system of kinetic equations that describe the populations of sub- and near-critical clusters. This kinetic model takes into account cluster-cluster collisions and decay of clusters into smaller clusters. The model results are compared with some predictions obtained with the classical nucleation theory (CNT) and also with available measurement data obtained in smog chambers or flow tubes. It is shown that in the case of slow nucleation processes, the kinetic model and the CNT as used by Shugard et al. [J. Chem. Phys. 75, 5298 (1974)] give the same results. However, in the case of intensive nucleation, a large part of the nucleation flux is due to cluster-cluster collisions and the CNT underestimates the nucleation rates.     

Interpretation of the ultrasonic effect on induction time during BaSO4 homogeneous nucleation by a cluster coagulation model

The effects of power ultrasound on the induction time of BaSO4 are studied experimentally and theoretically. In the experiments, barium sulfate is precipitated by mixing aqueous BaCl2 solution and Na2SO4 solution. The induction time is identified and measured by recording the change of turbidity in solution. Various energy inputs are used to investigate the effect of energy on nucleation. The results show that the induction time decreases with increasing supersaturation and increasing energy input. Employing the classical nucleation theory, the interfacial tension is estimated. In addition, the ultrasonic effects on nucleation order (n) and the nucleation coefficient (kN) are also investigated. A cluster coagulation model, which brings together the current nucleation models and the theories describing the behavior of colloidal suspensions, was applied to estimate the induction time under various energy inputs. A comparison between the results of model and the results of experiments shows that the number of monomers in dominating clusters (g) in the solution remains constant with increasing of energy input.     

Nucleation of bulk phases in the HCl/H2O system

We report experimental results on the low-temperature uptake of HCl on H(2)O ice (ice). HCl was deposited on the surface at greater than monolayer amounts at 85 K, and the ice substrate was heated. The temperature dependence of the HCl vapor pressure from this phase was measured from 110 to 150 K, with the nucleation of a bulk hydrate phase observed at 150 K. Measurements were conducted in a closed system by simultaneous application of gas phase mass spectrometry and surface spectroscopy to characterize vapor/solid equilibrium and the nucleation of bulk hydrate phases. Combining the nucleation data reported here with data we reported previously (180 to 200 K) and data from two other laboratories (165 and 170 K), the thermodynamic boundaries for the nucleation of both the metastable bulk solution and bulk hydrate phases subsequent to monolayer adsorption of HCl have been determined. The nucleation of the metastable bulk solution phase occurs promptly at monolayer coverage at the ice/liquid coexistence boundary on the binary bulk phase diagram. The nucleation of the bulk hexahydrate occurs from this metastable solution along a locus of points defining a state of constant solution free energy. This measured free energy is -51.2 +/- 0.9 kJ/mol. Finally, the temperature dependence of the HCl vapor pressure from the low-temperature phase is reported here for the first time and is consistent with that of the metastable solution predicted by this thermodynamic model of uptake, extending the range of validity of this model of adsorption followed by bulk solution and hydrate nucleation to a lower bound in temperature of 110 K.     

过饱和铝酸钠溶液中氢氧化铝自发成核动力学规律的研究

用电导法和吸光光度法首次对苛性比相同的不同浓度过饱和铝酸钠溶液自发分解过程进行了实时跟踪研究,获得铝酸钠溶解氢氧化铝自发成核动力学方程,探索了H₂O和Na⁺对氢氧化铝自发成核过程的影响.结果表明,过饱和铝酸钠溶液分解为氢氧化铝属化学反应控制过程;H₂O参与了溶液分解过程控制步骤的反应;K⁺和Na⁺等阳离子参与了溶液的重构,但对溶液分解的控制步骤影响不大.     

Kinetics of ion-induced nucleation in a vapor-gas mixture

A general solution for the steady-state ion-induced nucleation kinetics has been derived, considering the differences between ion-induced nucleation and homogeneous nucleation. This solution includes a new effect for nucleation kinetics, the interaction of charged clusters with vapor molecules. Analytical expressions for the ion-induced nucleation rate have been obtained for the limiting cases of high and low thermodynamic barriers. The physical explanation of the so-called sign effect is proposed based on multipole expansion of an electric field of the cluster ion. This theory gives good agreement with experiments and is used to elucidate experimentally observed phenomena.     

3D Study of the Morphology and Dynamics of Zeolite Nucleation

The principle aspects and constraints of the dynamics and kinetics of zeolite nucleation in hydrogel systems are analyzed on the basis of a model Na‐rich aluminosilicate system. A detailed time‐series EMT‐type zeolite crystallization study in the model hydrogel system was performed to elucidate the topological and temporal aspects of zeolite nucleation. A comprehensive set of analytical tools and methods was employed to analyze the gel evolution and complement the primary methods of transmission electron microscopy (TEM) and nuclear magnetic resonance (NMR) spectroscopy. TEM tomography reveals that the initial gel particles exhibit a core–shell structure. Zeolite nucleation is topologically limited to this shell structure and the kinetics of nucleation is controlled by the shell integrity. The induction period extends to the moment when the shell is consumed and the bulk solution can react with the core of the gel particles. These new findings, in particular the importance of the gel particle shell in zeolite nucleation, can be used to control the growth process and properties of zeolites formed in hydrogels.     

冷冻过程中胞内溶液均相成核温度的确定

为了研究冷冻过程中胞内溶液均相成核温度下降值与平衡凝固点下降值的关系, 提出了一种求解变组元系统成核温度的方法, 并联合细胞脱水方程, 进行了数值求解. 研究发现, 甘油-氯化钠-水三元系统的均相成核温度下降与平衡凝固点下降存在如下的线性关系: ⊿Th=1.17⊿Tm, 表明水溶液均相成核温度与溶质有关.     

Atomistic theory of amyloid fibril nucleation

We consider the nucleation of amyloid fibrils at the molecular level when the process takes place by a direct polymerization of peptides or protein segments into β-sheets. Employing the atomistic nucleation theory (ANT), we derive a general expression for the work to form a nanosized amyloid fibril (protofilament) composed of successively layered β-sheets. The application of this expression to a recently studied peptide system allows us to determine the size of the fibril nucleus, the fibril nucleation work, and the fibril nucleation rate as functions of the supersaturation of the protein solution. Our analysis illustrates the unique feature of ANT that the size of the fibril nucleus is a constant integer in a given supersaturation range. We obtain the ANT nucleation rate and compare it with the rates determined previously in the scope of the classical nucleation theory (CNT) and the corrected classical nucleation theory (CCNT). We find that while the CNT nucleation rate is orders of magnitude greater than the ANT one, the CCNT and ANT nucleation rates are in very good quantitative agreement. The results obtained are applicable to homogeneous nucleation, which occurs when the protein solution is sufficiently pure and/or strongly supersaturated.     

A multigrid method for N-component nucleation

A multigrid algorithm has been developed enabling more efficient solution of the cluster size distribution for N-component nucleation from the Becker-D?ring equations. The theoretical derivation is valid for an arbitrary number of condensing components, making the simulation of many-component nucleating systems feasible. A steady state ternary nucleation problem is defined to demonstrate its efficiency. The results are used as a validation for existing nucleation theories. The non-steady state ternary problem provides useful insight into the initial stages of the nucleation process. We observe that for the ideal mixture the main nucleation flux bypasses the saddle point.     

A simulation test of the optical Kerr mechanism for laser-induced nucleation

Recent experiments have demonstrated that intense, nanosecond laser pulses can induce crystal nucleation from supersaturated solutions that are transparent at the incident wavelengths, a phenomenon termed nonphotochemical laser-induced nucleation (NPLIN). Previous work has proposed that this effect is due to the alignment of solute molecules in solution due to the electric field of the applied laser light, promoting crystalline order. We have used simulations of NPLIN to examine how an orientational bias in solution affects nucleation with Monte Carlo simulations of a Potts lattice gas model. We examine this effect within both a classical, one-step nucleation framework as well as in the context of two-step nucleation. Our results indicate that an orientational bias can reduce the free energy barrier to nucleation within the one-step picture as well as promote the crystallization of amorphous precritical nuclei (the rate-determining step in the two-step picture). However, these effects are only present with field strengths that are much greater than those used in experiments.     

Nanocluster nucleation and growth kinetic and mechanistic studies: a review emphasizing transition-metal nanoclusters

A review of the literature of kinetic and mechanistic studies of transition-metal nanocluster nucleation and growth is presented; the focus is on nucleation processes. A brief survey of nucleation theory is given first, with an emphasis on classical nucleation theory, as this is the logical starting point of transition-metal nanocluster nucleation and growth studies. The main experimental methods for following nanocluster formation are examined next--dynamic light scattering, UV-visible spectroscopy, electron microscopy, and X-ray spectroscopies--with special attention paid to their strengths and weaknesses. Several specific examples of transition-metal nanocluster formation are then given, beginning with LaMer's classic sulfur sol system and including the Finke-Watzky mechanism of slow continuous nucleation A-->B followed by fast autocatalytic surface growth A+B-->2B. Finally, brief overviews of semiconductor nanoparticle preparations, solid-state nucleation studies-emanating from Avrami's work--and protein agglomeration mechanistic studies are also provided, as these processes are relevant, conceptually and in a general sense, to the field of transition-metal nanocluster nucleation and growth mechanisms.     

Bubble nucleation in micellar solution: a density functional study

We explore the free energetics of bubble nucleation in the micellar solution subjected to a negative pressure using a density functional model of a non-ionic surfactant solution. In this two-component model, the solvent is represented by a single hard-core sphere and the surfactant is represented by two tangent hard-core spheres connected by a rigid bond. The attractive interactions between the particles are modeled by the simple 1/R(6) form. Under all conditions of pressure and interparticle interactions we studied, the free energy barrier of bubble nucleation is found to be lower in the binary surfactant solution than that in a pure solvent and to continue to decrease as the mole fraction of the surfactant in the solution increases. We analyze the free energy surface of the model system under the conditions where both the critical bubble nucleus and the stable micelle exist in equilibrium with the same metastable solution. Our study shows that at moderately low pressures, bubbles can nucleate from the stable micelle and that the resulting free energy barrier of bubble nucleation is expected to be lower than that in the absence of this mechanism. However, as the spinodal is approached at lower pressures, the mechanism of micelle-assisted bubble nucleation becomes less effective. The liquid-liquid miscibility of the model system correlates well with the mechanism of bubble nucleation from the stable micelle.     

Ion-induced nucleation in solution: promotion of solute nucleation in charged levitated droplets

We have investigated the nucleation and growth of sodium chloride in both single quiescent charged droplets and charged droplet populations that were levitated in an electrodynamic levitation trap (EDLT). In both cases, the magnitude of a droplet's net excess charge (ions(DNEC)) influenced NaCl nucleation and growth, albeit in different capacities. We have termed the phenomenon ion-induced nucleation in solution. For single quiescent levitated droplets, an increase in ions(DNEC) resulted in a significant promotion of NaCl nucleation, as determined by the number of crystals observed. For levitated droplet populations, a change in NaCl crystal habit, from regular cubic shapes to dome-shaped dendrites, was observed once a surface charge density threshold of -9 x 10(-4) e.nm(-2) was surpassed. Although promotion of NaCl nucleation was observed for droplet population experiments, this can be attributed in part to the increased rate of solvent evaporation observed for levitated droplet populations having a high net charge. Promotion of nucleation was also observed for two organic acids, 2,4,6-trihydroxyacetophenone monohydrate (THAP) and alpha-cyano-4-hydroxycinnamic acid (CHCA). These results are of direct relevance to processes that occur in both soft-ionization techniques for mass spectrometry and to a variety of industrial processes. To this end, we have demonstrated the use of ion-induced nucleation in solution to form ammonium nitrate particles from levitated droplets to be used in in vitro toxicology studies of ambient particle types.     

Site-specific nucleation and growth kinetics in hierarchical nanosyntheses of branched ZnO crystallites

Here we report a site-specific sequential nucleation and growth route to the systematic building of hierarchical, complex, and oriented ZnO micro/nanostructures in solution nanosynthesis. Structures and morphologies of the products were confirmed by results from X-ray diffraction and scanning electron microscopy studies. The organic structure-directing agents (SDAs), diaminopropane and citrate, are found to play different roles in controlling the evolution of these new morphologies. Through the selective adsorptions of SDAs on different crystal facets of the primary ZnO rods, we have alternated the hierarchical growth of secondary and tertiary new complex nanostructures. Roles of the SDA concentration, nucleation time, and growth kinetics in the solution hierarchical ZnO nanosyntheses have all been systematically investigated.     

纳米Al_2O_3颗粒对镍电结晶过程的影响

借助循环伏安(CV)和计时安培(CA)研究了在不同电位下,纳米Al2O3微粒对镍由硫酸盐混合溶液在铜基底上电结晶沉积的影响.结果表明,Ni-Al2O3镀液体系电沉积的起始电位约为-740 mV.随着阶跃电位负移,Ni-Al2O3镀液体系电沉积成核时间tm逐渐缩短.与纯Ni镀液体系电沉积的tm相比,在-740~-830mV较低阶跃电位下,Ni-Al2O3镀液体系电沉积的成核时间tm明显缩短,表明Al2O3微粒有助于镍的电结晶成核.在-890 mV阶跃电位下,Ni-Al2O3镀液体系电沉积初始阶段的成核过程满足Scharifker-Hills三维瞬时成核模型.     

Steady-state nucleation rate and flux of composite nucleus at saddle point

The steady-state nucleation rate and flux of composite nucleus at the saddle point is studied by extending the theory of binary nucleation. The Fokker-Planck equation that describes the nucleation flux is derived using the Master equation for the growth of the composite nucleus, which consists of the core of the final stable phase surrounded by a wetting layer of the intermediate metastable phase nucleated from a metastable parent phase recently evaluated by Iwamatsu [J. Chem. Phys. 134, 164508 (2011)]. The Fokker-Planck equation is similar to that used in the theory of binary nucleation, but the non-diagonal elements exist in the reaction rate matrix. First, the general solution for the steady-state nucleation rate and the direction of nucleation flux is derived. Next, this information is then used to study the nucleation of composite nucleus at the saddle point. The dependence of steady-state nucleation rate as well as the direction of nucleation flux on the reaction rate in addition to the free-energy surface is studied using a model free-energy surface. The direction of nucleation current deviates from the steepest-descent direction of the free-energy surface. The results show the importance of two reaction rate constants: one from the metastable environment to the intermediate metastable phase and the other from the metastable intermediate phase to the stable new phase. On the other hand, the gradient of the potential Φ or the Kramers crossover function (the commitment or splitting probability) is relatively insensitive to reaction rates or free-energy surface.     

Molecular mechanism of crystal nucleation from solution

Nucleation from solution is fundamental to many natural and industrial processes. The understanding of molecular mechanism of nucleation from solution is conducive to predict crystal structure, control polymorph and design desired crystal materials. In this review, the nucleation theories, including classical nucleation theory(CNT), nonclassical nucleation theory, as well as other new proposed theories, were reprised, and the molecular mechanism of these theories was compared. Then, the molecular process of nucleation, including the current study techniques, the effect of molecular self-assembly in solutions, desolvation process, as well as the properties of solvent and crystal structure on nucleation from solution were summarized. Furthermore, the relationship of molecular conformation in solution and in crystal, and the effect of solute molecular flexibility on nucleation were discussed.Finally, the current challenges and future scopes of crystal nucleation from solution were discussed.     

Using microfluidics to observe the effect of mixing on nucleation of protein crystals

This paper analyzes the effect of mixing on nucleation of protein crystals. The mixing of protein and precipitant was controlled by changing the flow rate in a plug-based microfluidic system. The nucleation rate inversely depended on the flow rate, and flow rate could be used to control nucleation. For example, at higher supersaturations, precipitation happened at low flow rates while large crystals grew at high flow rates. Mixing at low flow velocities in a winding channel induces nucleation more effectively than mixing in straight channels. A qualitative scaling argument that relies on a number of assumptions is presented to understand the experimental results. In addition to helping fundamental understanding, this result may be used to control nucleation, using rapid chaotic mixing to eliminate formation of precipitates at high supersaturation and using slow chaotic mixing to induce nucleation at lower supersaturation.     

An algorithm for semi-empirical design of nucleation rate surface

During the last half of century, Classical Nucleation Theory (CNT) has been developed and there have been advances in the molecular theory of nucleation. Most of these efforts have been directed towards small molecule system modeling using intermolecular potentials. Summarizing the nucleation theory, it can be concluded that the current theory is far from complete. Agreement is generally not obtained between experimental and theoretical results. In practical applications, parametric theories can be used for the systems of interest. However, experimental measurements are still the best source of information on nucleation. Experiments are labor intensive and costly, and thus, it is useful to extend the value of limited experimental measurements to a broader range of nucleation conditions. The available nucleation parameters represent only small regions of possible nucleation conditions over the range from the critical temperature to absolute zero. Thus, it is useful to develop better tools to use the data to estimate semi-empirical nucleation rate surfaces. Following our published approach, the nucleation rate surface for any system can be constructed over its phase diagram. This concept involves using the phase equilibrium diagram to establish lines of zero nucleation rates. Nucleation rate surfaces arise from equilibrium lines and their extensions that are representing unstable equilibria. Only limited experimental data is available for use in normalizing the slopes of the linearized nucleation rate surfaces. The nucleation rate surface is described in terms of steady-state nucleation rates. To design the surfaces of nucleation rates, several assumptions are presented. In the present study, an algorithm for the semi-empirical design of nucleation rate surfaces is introduced. The topology of the nucleation rate surface for a unary system using the example of water vapor nucleation is created semi-empirically. The nucleation of two concurrent (stable and unstable) phase states of critical embryos is considered in the context of multi-surface nucleation rates. Only one phase transition (melting) in the condensed state of water is considered for simplicity. The nucleation rate surface is constructed numerically using the available experimental results for vapor nucleation and phase diagram for water. The nucleation rate for water vapor is developed for the full temperature interval, i.e. from critical point to absolute zero. The results help to suggest a new direction for experimental nucleation research.     

Te,CdTe,HgCdTe的电沉积及其成核机理

应用自装微机联用恒电位系统研究了在玻璃碳或铁电极上Te、CdTe、HgCdTe的电沉积及其电结晶成核机理。结果表明,在酸性溶液中,HTeO_2~+的阴极还原符合4电子还原机理,其电结晶生长表现为由HTeO_2~+扩散控制的三维瞬时成核机理;CdTe沉积层的形成是亚碲酸还原的延续,其电结晶成核机理因电位阶跃值、沉积温度及溶液pH值的改变而由HTeO_2~+扩散控制的三维瞬时成核转变为二维瞬时成核机理;对HgCdTe,其电沉积过程的动力学步骤可设想为: Hg~(2+)+2e—→Hg,HTeO_2~++3H~++4e—→Te+2H_2O xHg+Te—→Hg_2Te,Hg_2Te+(1—x)Cd~(2+)+2(1—x)e—→Hg_2Cd_(1-2)Te 相关的结晶生长除受各种实验因素影响外,还与CdTe的成核过程有关。在本文实验条件下,大体遵循二维瞬时成核机理。