Crisis of stability of hydration shell of Na<Superscript>+</Superscript> ion in condensing water vapor

The Gibbs energy and equilibrium work of the formation of nuclei of the condensed phase on sodium ions are calculated on the molecular level by a Monte Carlo simulation using a detailed interaction model. The stationary rate of nucleation is estimated based on the data obtained. The presence of ionic impurities only substantially affects the rate of nucleation at strong vapor supersaturation. The nucleus losses its thermodynamic stability with an increase in the size of the nucleus and the barrier is formed depending on the work of formation on the size of the nucleus. An abrupt loss of stability is accompanied by pushing the ion off of the microdroplet surface and the restoration of the network of hydrogen bonds. The effect of pushing an ion to the surface of a cluster greatly depends on many-particle polarization interactions.     

Molecular nuclei of condensation. Conditions for observance and physicochemical features

The formation of aerosol particles on molecular nuclei of condensation (MoNC) is considered. An interpretation of this phenomenon, consistent with the physicochemical features of MoNC and with the observed regularities, is proposed. The nucleation is induced by the addition of two or several vapor molecules of a developing agent to MoNC to give clusters that are more stable than spontaneous heterogeneous fluctuations. The physical properties of the supersaturated vapors of empirically selected developing agents conform to the condition of low concentrations of heterogeneous fluctuations, which is needed to observe MoNC. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 208–213, February, 1998.     

Water-vapor clustering on the surface of β-AgI crystal in the field of defects with a disordered structure

The Monte Carlo method has been employed to simulate the nucleation of condensed water phase from vapor at 260 K on a crystalline silver-iodide surface containing a defect in the form of a nanoscopic spot with a random distribution of ions. The free energy and work of formation of a nucleus have been calculated in the bicanonical ensemble at the molecular level as functions of nucleus size; computer images and spatial correlation functions of molecules have been obtained. The presence of a defect with a disordered (amorphous) structure, on the one hand, entails local destructions of a monomolecular film, but, on the other hand, shifts the onset of the adsorption process toward lower vapor pressures by several orders of magnitude. Under the conditions of a growing condensate film, the defect leads to its thermodynamic stabilization and a decrease in the barrier of the formation of subsequent layers, thereby weakening the known effect of the hydrophobicity of monomolecular films on crystalline surfaces with hexagonal structures. The factors that predetermine the abnormally high efficiency of silver-iodide particles as stimulators for atmospheric-moisture nucleation at negative Celsius temperatures seem to be the presence of extended defects on the surface of aerosol particles in combination with the hexagonal structure of their crystal lattice, the optimum magnitude of direct interactions between water molecules with ions of the crystal surface layer, and the collective domain-formation effects that result from a relatively high polarizability of iodine ions.     

Nucleation of water vapor in microcracks on the surface of β-AgI aerosol particles: 2. Thermodynamics of nucleation

Free energy, entropy, and the work of formation of condensation nuclei at 260 K in microcracks of β-AgI crystal structure at the initial stage of nucleation preceding crystallization are calculated by the Monte Carlo method. Unlike ideal crystal surface, nuclei in microcracks are thermodynamically stable and the barrier of free energy of nucleation is absent. Conditions of microcrack are favorable for the crystallization that qualitatively changes the regime and rate of nucleation. Stable size of nuclei at the humidity corresponding to natural atmosphere is sufficient for the filling of nanoscopic microcracks and the attainment of substrate surface. The probability of nucleus formation in microcracks by the fluctuation mechanism is incomparably higher than the probability of their formation on the defect-free surface. High crystallization ability of the particles of βAgI aerosol is ensured by multiple surface microcracks acting as active sites in combination with its complementary crystal structure. The efficiency of aerosols as stimulants of the nucleation of water vapor at negative Celsius temperatures is determined by the surface density and geometry of nanoscopic cracks and fissures on the particle surface.     

Monte Carlo simulations of critical cluster sizes and nucleation rates of water

We have calculated the critical cluster sizes and homogeneous nucleation rates of water at temperatures and vapor densities corresponding to experiments by Wolk and Strey [J. Phys. Chem B 105, 11683 (2001)]. The calculations have been done with an expanded version of a Monte Carlo method originally developed by Vehkamaki and Ford [J. Chem. Phys. 112, 4193 (2000)]. Their method calculates the statistical growth and decay probabilities of molecular clusters. We have derived a connection between these probabilities and kinetic condensation and evaporation rates, and introduce a new way for the calculation of the work of formation of clusters. Three different interaction potential models of water have been used in the simulations. These include the unpolarizable SPC/E [J. Phys. Chem. 91, 6269 (1987)] and TIP4P [J. Chem. Phys. 79, 926 (1983)] models and a polarizable model by Guillot and Guissani [J. Chem. Phys. 114, 6720 (2001)]. We show that TIP4P produces critical cluster sizes and a temperature and vapor density dependence for the nucleation rate that agree well with the experimental data, although the magnitude of nucleation rate is constantly overestimated by a factor of 2 x 10(4). Guissani and Guillot's model is somewhat less successful, but both the TIP4P and Guillot and Guissani models are able to reproduce a much better experimental temperature dependency of the nucleation rate than the classical nucleation theory. Using SPC/E results in dramatically too small critical clusters and high nucleation rates. The water models give different average binding energies for clusters. We show that stronger binding between cluster molecules suppresses the decay probability of a cluster, while the growth probability is not affected. This explains the differences in results from different water models.     

Implementation of dynamical nucleation theory effective fragment potentials method for modeling aerosol chemistry

In this work, the dynamical nucleation theory (DNT) model using the ab initio based effective fragment potential (EFP) is implemented for evaluating the evaporation rate constant and molecular properties of molecular clusters. Predicting the nucleation rates of aerosol particles in different chemical environments is a key step toward understanding the dynamics of complex aerosol chemistry. Therefore, molecular scale models of nanoclusters are required to understand the macroscopic nucleation process. On the basis of variational transition state theory, DNT provides an efficient approach to predict nucleation kinetics. While most DNT Monte Carlo simulations use analytic potentials to model critical sized clusters, or use ab initio potentials to model very small clusters, the DNTEFP Monte Carlo method presented here can treat up to critical sized clusters using the effective fragment potential (EFP), a rigorous nonempirical intermolecular model potential based on ab initio electronic structure theory calculations, improvable in a systematic manner. The DNTEFP method is applied to study the evaporation rates, energetics, and structure factors of multicomponent clusters containing water and isoprene. The most probable topology of the transition state characterizing the evaporation of one water molecule from a water hexamer at 243 K is predicted to be a conformer that contains six hydrogen bonds, with a topology that corresponds to two water molecules stacked on top of a quadrangular (H(2)O)(4) cluster. For the water hexamer in the presence of isoprene, an increase in the cluster size and a 3-fold increase in the evaporation rate are predicted relative to the reaction in which one water molecule evaporates from a water hexamer cluster.     

The Effect of Temperature on Nucleation of Condensed Water Phase on the Surface of a β-AgI Crystal. 2. Formation Work

In the condensation mechanism of heterogeneous ice formation, water crystallization occurs after a necessary amount of the liquid phase has accumulated on a substrate surface. In this way, the ice-forming activity of the surface is governed by its adsorption ability with respect to water vapor. The Monte Carlo canonical statistical ensemble method has been used to calculate the free energy, entropy, and work of nucleation of a disordered condensed water phase on the surface of crystalline silver iodide and to determine the surface tension. Comparative calculations have been performed at 260 and 320 K for the defect-free surface of a basal face of a crystal. The surface of a β-AgI crystal is completely covered with a monomolecular film even in unsaturated water vapors. The surface tension at the growing nucleus–substrate interface substantially increases due to the formation of the underlying film, and the growth of the nucleus becomes possible only in a supersaturated vapor. As the vapor density increases, the thickness of the condensed water layer grows, and, at negative Celsius temperatures, conditions are created for its crystallization. The underlying film with pronounced hydrophobic properties hinders nucleation, thereby decreasing the ice-forming activity of the surface in the condensation process. Under these conditions, the observed abnormally high ice-forming activity of silver-iodide aerosol particles may be explained by the presence of numerous crystal defects on the particle surface, with these defects representing channels that provide overcoming the hindering action of the film.     

The role of natural and man-made ice-forming nuclei in the atmosphere

Water-soluble and insoluble, organic and inorganic, natural and man-made aerosol particles participate in vapor-liquid, vapor-solid (ice), and liquid-solid phase transitions in the atmosphere. Hydrosol particles (aerosol particles that have been transferred into water droplets) nucleate ice through freezing. A small without scavenging or being scavenged by another aerosol particle. It is also difficult to imagine that pure mineral particles can be lifted from soil surfaces. In view of this, an ice-nucleating site may be a much smaller particle attached to a larger clay particle. To this category belong, e.g., silver iodide-clay mixed particles. Limited studies indicate that decaying leaves and forest litter under the surface of soils are a potential source of biogenic ice-forming nuclei but that their contribution to the atmosphere is very limited. Research should be directed to study possible relations between cloud condensation nuclei and ice-forming nuclei derived from natural organic compounds (terpenes, leaf-derived nuclei, bacteria, etc.).

A balance must be maintained between large cloud chambers, in which duplication of in-cloud processes is possible, and the special instrumentation which provides information about the modes of ice nucleation on aerosol particles. The two modes of instrumentation should supplement each other.

The greatest difficulty in attempting to make a comparison between the number of ice-forming nuclei estimated in the laboratory and the number in a cloud is the lack of knowledge of the time-temperature-humidity history of the aerosol particles. In nature, the ability of an aerosol particle to nucleate ice may be destroyed or“poisoned“ in the presence of pollutants. An aerosol particle may, on the other hand, become an activated or warmer ice-forming nucleus, e.g.,after the sublimation of ice once formed on it. The temperature of ice nucleation is not a singular property of a particle; the warmest temperatures of ice nucleation of, e.g., particles of a certain soil 10cm in diameter are-15°C,-10°C, and-8°C for nucleation through freezing, condensation followed by freezing and contact, respectively (ref.26). The progress made in instrumentation permits studies of the modes of ice nucleation. Understanding the physical and chemical processes taking place in clouds makes estimates of the rates of ice particle formation more realistic (Young [ref.157]).

The reader should examine two previous reviews written by Mossop (1963) and Montefinale . (1971) for a more complete list of references.     

Molecular dynamics simulations of nucleation from vapor to solid composed of Lennard-Jones molecules

We performed molecular dynamics (MD) simulations of nucleation from vapor at temperatures below the triple point for systems consisting of 10(4)-10(5) Lennard-Jones (L-J) type molecules in order to test nucleation theories at relatively low temperatures. Simulations are performed for a wide range of initial supersaturation ratio (S(0) ? 10-10(8)) and temperature (kT = 0.2-0.6ε), where ε and k are the depth of the L-J potential and the Boltzmann constant, respectively. Clusters are nucleated as supercooled liquid droplets because of their small size. Crystallization of the supercooled liquid nuclei is observed after their growth slows. The classical nucleation theory (CNT) significantly underestimates the nucleation rates (or the number density of critical clusters) in the low-T region. The semi-phenomenological (SP) model, which corrects the CNT prediction of the formation energy of clusters using the second virial coefficient of a vapor, reproduces the nucleation rate and the cluster size distributions with good accuracy in the low-T region, as well as in the higher-T cases considered in our previous study. The sticking probability of vapor molecules onto the clusters is also obtained in the present MD simulations. Using the obtained values of sticking probability in the SP model, we can further refine the accuracy of the SP model.     

Molecular dynamic simulation of dicarboxylic acid coated aqueous aerosol: structure and processing of water vapor

Organic monolayers at the surfaces of aqueous aerosols play an important role in determining the mass, heat transfer rate and surface reactivity of atmospheric aerosols. They can potentially contribute to the formation of cloud condensation nuclei (CCN) and are involved in a series of chemical reactions occurring in atmosphere. Recent studies even suggest that organic-coated interfaces could have played some role in prebiotic biochemistry and the origin of life. However, creating reproducible, well-characterized aqueous aerosol particles coated with organic films is an experimental challenge. This opens the opportunity for computer simulations and modeling of these complex structures. In this work, molecular dynamics simulation was used to probe the structure and the interfacial properties of the dicarboxylic acid coated aqueous aerosol. Low molecular weight dicarboxylic acids of various chain lengths and water solubility were chosen to coat a water droplet consisting of 2440 water molecules. For malonic acid coated aerosol, the surface acid molecules dissolved into the water core and formed an ordered structure due to the hydrophobic interactions. The acid and the water are separated inside the aerosol. For other nanoaerosols coated with low solubility acids, phase separation between water and acid molecules was observed on the surface of the particle. To study the water processing of the coated aerosols, the water vapor accommodation factors were calculated.     

Heterogeneous condensation of the Lennard-Jones vapor onto a nanoscale seed particle

The heterogeneous condensation of a Lennard-Jones vapor onto a nanoscale seed particle is studied using molecular dynamics simulations. Measuring the nucleation rate and the height of the free energy barrier using the mean first passage time method shows that the presence of a weakly interacting seed has little effect on the work of forming very small cluster embryos but accelerates the rate by lowering the barrier for larger clusters. We suggest that this results from a competition between the energetic and entropic features of cluster formation in the bulk and at the heterogeneity. As the interaction is increased, the free energy of formation is reduced for all cluster sizes. We also develop a simple phenomenological model of film formation on a small seed that captures the general features of the nucleation process for small heterogeneities. A comparison of our simulation results with the model shows that heterogeneous classical nucleation theory provides a good estimate of the critical size of the film but significantly overestimates the size of the barrier.     

Experimental studies of the vapor phase nucleation of refractory compounds. VI. The condensation of sodium

In this paper we discuss the condensation of sodium vapor and the formation of a sodium aerosol as it occurs in a gas evaporation condensation chamber. A one-dimensional model describing the vapor transport to the vapor/aerosol interface was employed to determine the onset supersaturation, in which we assume the observed location of the interface is coincident with a nucleation rate maximum. We then present and discuss the resulting nucleation onset supersaturation data within the context of nucleation theory based on the liquid droplet model. Nucleation results appear to be consistent with a cesium vapor-to-liquid nucleation study performed in a thermal diffusion cloud chamber.     

Molecular-dynamics study of the density scaling of inert gas condensation

The initial stages of vapor condensation of Ge in the presence of a cold Ar atmosphere were studied by molecular-dynamics simulations. The state variables of interest included the densities of condensing vapor and gas, the density of clusters, and the average cluster size, while the temperatures of the vapor and the clusters were separately monitored with time. Three condensation processes were explicitly identified: nucleation, monomeric growth, and cluster aggregation. Our principal finding is that both the average cluster size and the number of clusters scale with the linear dimension of the computation cell, L, and Ln, with the scaling parameter n approximately 4, corresponding to a reaction order of nu approximately 2.33. This small value of n is explained by an unexpected nucleation path involving the formation of Ge dimers via two-body collisions.     

Multicomponent dynamical nucleation theory and sensitivity analysis

Vapor to liquid multicomponent nucleation is a dynamical process governed by a delicate interplay between condensation and evaporation. Since the population of the vapor phase is dominated by monomers at reasonable supersaturations, the formation of clusters is governed by monomer association and dissociation reactions. Although there is no intrinsic barrier in the interaction potential along the minimum energy path for the association process, the formation of a cluster is impeded by a free energy barrier. Dynamical nucleation theory provides a framework in which equilibrium evaporation rate constants can be calculated and the corresponding condensation rate constants determined from detailed balance. The nucleation rate can then be obtained by solving the kinetic equations. The rate constants governing the multistep kinetics of multicomponent nucleation including sensitivity analysis and the potential influence of contaminants will be presented and discussed.     

A study of sulfur homogeneous nucleation from supersaturated vapor. Determination of surface tension of sulfur nanoparticles

Homogeneous nucleation in sulfur vapor is studied in a laminar-flow chamber. Concentration and size distribution of resulting aerosol particles are measured with a diffusion spectrometer of aerosols and a PK.GTA-0,3-002 photoelectric particle counter. The crystal structure of the formed particles is studied by X-ray diffraction analysis. The rate of sulfur evaporation from a boat and the profile of a deposit on the chamber wall along the axial coordinate are determined by gravimetry. Axial and radial temperature profiles are measured using a chromel-alumel thermocouple. The vapor concentration distribution in the chamber is found and the supersaturation is calculated from the solution of the mass-transfer problem. An experimental low-laborious method is developed for the supersaturation cutoff. This method enables one to rapidly deter-mine the position of the zone in which the nucleation proceeds at the highest rate. The position of the zone of nucleation found by this method is in good agreement with the results of calculations based on experimental data and theoretical calculation of the rate of nucleation by an exact formula that has been recently derived based on the works by Kusaka and Reiss, as well as the Frenkel liquid kinetics theory. The surface tension of critical sulfur nuclei resulting from the nucleation is calculated based on this formula and experimental data on the nucleation. It is established that, in a temperature range of 312–319 K, the critical nuclei have tension surface radius R _s ～ 10.6 Å and surface tension σ = 72.5 ± 1.1 dyn/cm. The surface tension of critical sulfur nuclei in this temperature range is constant and approximately 5% higher than that of a planar surface.     

Computer simulation of two-step atomization in graphite furnaces for analytical atomic spectrometry

The processes of sample fractionation by two-step atomization with the intermediate condensation of the analyte on a cold surface in graphite furnaces were theoretically studied. The transfer equation was solved for the atoms, molecules, and condensed particles of the sample from a flow of argon directed along this surface. The spatial distributions of vapor and the condensate formed were calculated depending on the composition and flow rate. It was found that a cold surface section with a length of 6 mm is sufficient for the complete trapping of atomic analyte vapor from an argon layer having a velocity of about 1 m/sec and a thickness of 5 mm. In this case, the molecules and clusters condensation coefficients smaller than unity were deposited insignificantly; that is, they were fractionally separated. The results of the shadow spectral visualization of the process of sample fractionation on a cold probe surface of in commercial HGA and THGA atomizers were interpreted. The advantages of analytical signals upon the evaporation of a sample condensate from the probe in these atomizers and inductively coupled plasma were demonstrated.     

Experimental study of homogeneous nucleation from the bismuth supersaturated vapor: evaluation of the surface tension of critical nucleus

The homogeneous nucleation of bismuth supersaturated vapor is studied in a laminar flow quartz tube nucleation chamber. The concentration, size, and morphology of outcoming aerosol particles are analyzed by a transmission electron microscope (TEM) and an automatic diffusion battery (ADB). The wall deposit morphology is studied by scanning electron microscopy. The rate of wall deposition is measured by the light absorption technique and direct weighting of the wall deposits. The confines of the nucleation region are determined in the "supersaturation cut-off" measurements inserting a metal grid into the nucleation zone and monitoring the outlet aerosol concentration response. Using the above experimental techniques, the nucleation rate, supersaturation, and nucleation temperature are measured. The surface tension of the critical nucleus and the radius of the surface of tension are determined from the measured nucleation parameters. To this aim an analytical formula for the nucleation rate is used, derived from author's previous papers based on the Gibbs formula for the work of formation of critical nucleus and the translation-rotation correction. A more accurate approach is also applied to determine the surface tension of critical drop from the experimentally measured bismuth mass flow, temperature profiles, ADB, and TEM data solving an inverse problem by numerical simulation. The simulation of the vapor to particles conversion is carried out in the framework of the explicit finite difference scheme accounting the nucleation, vapor to particles and vapor to wall deposition, and particle to wall deposition, coagulation. The nucleation rate is determined from simulations to be in the range of 10(9)-10(11) cm(-3) s(-1) for the supersaturation of Bi(2) dimers being 10(17)-10(7) and the nucleation temperature 330-570 K, respectively. The surface tension σ(S) of the bismuth critical nucleus is found to be in the range of 455-487 mN/m for the radius of the surface of tension from 0.36 to 0.48 nm. The function σ(S) changes weakly with the radius of critical nucleus. The value of σ(S) is from 14% to 24% higher than the surface tension of a flat surface.     

New approach to the kinetics of heterogeneous unary nucleation on liquid aerosols of a binary solution

The formation of a droplet on a hygroscopic center may occur either in a barrierless way via Kohler activation or via nucleation by overcoming a free energy barrier. Unlike the former, the latter mechanism of this process has been studied very little and only in the framework of the classical nucleation theory based on the capillarity approximation whereby a nucleating droplet behaves like a bulk liquid. In this paper the authors apply another approach to the kinetics of heterogeneous nucleation on liquid binary aerosols, based on a first passage time analysis which avoids the concept of surface tension for tiny droplets involved in nucleation. Liquid aerosols of a binary solution containing a nonvolatile solute are considered. In addition to modeling aerosols formed through the deliquescence of solid soluble particles, the considered aerosols constitute a rough model of "processed" marine aerosols. The theoretical results are illustrated by numerical calculations for the condensation of water vapor on binary aqueous aerosols with nonvolatile nondissociating solute molecules using Lennard-Jones potentials for the molecular interactions.     

Comparison between the classical theory predictions and molecular simulation results for heterogeneous nucleation of argon

We have performed Monte Carlo simulations of homogeneous and heterogeneous nucleations of Lennard-Jones argon clusters. The simulation results were interpreted using the major concept posing a difference between the homogeneous and heterogeneous classical nucleation theories-the contact parameter. Our results show that the multiplication concept of the classical heterogeneous nucleation theory describes the cluster-substrate interaction surprisingly well even for small molecular clusters. However, in the case of argon nucleating on a rigid monolayer of fcc(111) substrate at T=60 K, the argon-substrate atom interaction being approximately one-third as strong as the argon-argon interaction, the use of the classical theory concept results in an underestimation of the heterogeneous nucleation rate by two to three orders of magnitude even for large clusters. The main contribution to this discrepancy is induced by the failure of the classical theory of homogeneous nucleation to predict the energy involved in bringing one molecule from the vapor to the cluster for clusters containing less than approximately 15 molecules.     

Molecular Dynamics Studies of the Kinetics of Phase Changes in Clusters IV： Crystal Nucleation from Molten （NaCl）256 and （NaCl）500 Clusters

Molecular dynamics computer simulation based on the Born-Mayer-Huggins potential function has been carried out to study the effects of duster size and temperature on the nucleation rate of sodium chloride dusters in the temperature range of 580 K to 630 K. Clusters with 256 and 500 NaCl molecules have been studied and the results have been compared with those obtained from 108 molecule dusters. The melting point (MP) of the clusters were observed to increase with the size of the clusters and can be well described by a linear equation MP =1107(37)-1229(23)N^-1/3(N is the number of molecules in the duster).The nucleation rate was found to decrease with increasing the duster size or temperature. Various nucleation theories have been used to interpret the nucleation rates obtained from this molecular dynamics simulation. It is possible to use a constant diffuse interface thickness to interpret the nucleation rate from the diffuse interface theory in the temperature range of this study. However, the interfacinl free energy estimated from classical nucleation theory and diffuse interface theory increases too fast with increasing the temperature while that from Gran-Gunton theory does not change with changing temperatures.The sizes of critical nuclei estimated from all the theories are smaller than those estimated from our simulations.