Numerical simulation of water vapor nucleation on electrically neutral nanoparticles

Atomic-level Monte Carlo simulations are performed to calculate the free energy, entropy, and work of nucleation for clusters of more than 6 × 10³ water molecules growing on silver iodide crystalline particles of size up to 4 nm at a temperature of 260 K. The Hamiltonian of the system includes explicit expressions for hydrogen bonding energy and Coulomb, dispersion, exchange, and polarization interactions. The work of nucleation exhibits complex behavior depending on the nucleation-site size. With increasing nanoparticle size, clusters become less stable and the probability of crystallization increases. Mutual polarization enhances the bonding between a cluster and a crystalline particle. Cluster growth on relatively large nanoparticles involves two stages characterized by two critical sizes: monolayer growth on the surface and growth normal to the surface. Spontaneous microdroplet polarization involving domain formation is found to occur at the crystal surface. The dependence of the ice-forming activity of an aerosol on particulate size observed in experiments is explained by combined effects of several competing factors, the dominant ones being the stabilizing and destabilizing effects of the nanoparticle electric field.     

Stimulation of vapor nucleation on perfect and imperfect hexagonal lattice surfaces

Monte Carlo simulations of water vapor nucleation on a perfect crystal surface and on a surface with defects are performed. Mass exchange with the vapor phase is modeled by using an open ensemble. Cluster-substrate interaction is described in terms of conventional atom-atom potentials. The Hamiltonian of the system includes expressions for electrostatic, polarization, exchange, and dispersion interactions. The Gibbs free energy and work of adsorption are calculated by Monte Carlo simulation in the bicano?nical ensemble. The microscopic structure of nuclei is analyzed in terms of pair correlation functions. Periodic boundary conditions are used to simulate an infinite substrate surface. Molecule-substrate and molecule-molecule long-range electrostatic interactions are calculated by summing the Fourier harmonics of the electrostatic potential. Dispersion interactions are calculated by direct summation over layers of unit cells. Nucleation on a surface with matching structure follows a layer-by-layer mechanism. The work of adsorption per molecule of a monolayer on the substrate surface has a maximum as a function of nucleus size. The steady rate of nucleation of islands of supercritical size is evaluated. The work of adsorption per molecule for layer-by-layer film growth is an oscillating function of cluster size. As a function of layer number, it has a minimum depending on the vapor pressure. The electric field generated by a microscopic surface protrusion destroys the layered structure of the condensate and eliminates free-energy nucleation barriers. However, point lattice defects do not stimulate explosive nucleation.     

The Energy Barrier to Recombination in Hydrated Plasma

An abnormally high potential barrier that separates the H₃O⁺ and Cl^? ions in a cluster of water molecules was revealed. The profile of the barrier was calculated by computer simulation. The calculation was based on a detailed model of intermolecular interactions developed on the basis of experimental data on the free energy and entropy for the addition reactions of water molecules in the vapor phase to the hydration shell of ions in conjunction with the results of quantum-chemical calculations.     

Water vapor nucleation on an infinite substrate with complementary structure: 2. The influence of crystal defects

The simulation of the adsorption of water vapor on the infinite surface of a silver iodide crystal with regularly arranged crystal defects is performed by the Monte Carlo method. Long-range electrostatic and polarization interactions are taken into account by the Ewald method. The work of nucleus formation in the field of crystal defects is calculated by the method of bicanonical statistical ensemble. The interaction with a defect decreases the internal energy, the Gibbs energy, and the entropy of the nucleus. The rough surface of the silver iodide crystal exerts a larger stimulating effect on vapor nucleation than an ideally smooth surface. Saturation vapor pressure above the condensate layer formed on the rough surface can be several times lower than the pressure above the smooth surface. This effect is caused by the cooperative action of surface crystal defects. It may be expected that the surface with a characteristic size of rough elements of 15–20 Å is the most efficient for the formation of condensed phase nuclei. Single point crystal defects with extremely small sizes on the substrate surface do not exert a stimulating effect on the formation of the macroscopic condensed phase. Single defects with moderate sizes can lower the barrier of monomolecular film formation; however, such a route of the stimulation of ice-forming activity of the surface is less efficient than the cooperative action of the defects of rough surface.     

Computer Simulation of the Initial Stage of Water Vapor Nucleation on a Silver Iodide Crystal Surface: 1. Microstructure

The nucleation of water vapors on the surface of a fragment of silver iodide crystal is simulated by the Monte Carlo method under the conditions similar to natural conditions in a humid atmosphere. A stable monolayer island of water molecules with clearly pronounced features of hexagonal symmetry and low orientational order is formed at the initial stage, when the vapor pressure is still lower than the saturating pressure. The island readily grows over the surface and, in the unsaturated vapor, does not grow in the direction perpendicular to the surface. The formed monolayer represents a substrate for further growth of a condensed phase and, eventually, is responsible for the mechanism of nucleation on the crystal surface. Water molecules are held by the substrate mainly owing to the directional electrostatic interaction between the negatively charged oxygen atoms and positively charged silver ions. The interaction with iodine ions lowers the binding of the island (nucleus) and the substrate. A point defect in the form of an extra ion on the surface does not change the planar shape of the nucleus and virtually does not distort its hexagonal structure. Indirect experimental data supporting the formation of a water monolayer at the stage preceding nucleation, as well as the data of observations indicating the important role of defects on a crystal surface, are reported.__________Translated from Kolloidnyi Zhurnal, Vol. 67, No. 4, 2005, pp. 548–560.Original Russian Text Copyright © 2005 by Shevkunov.     

Water vapor nucleation on an infinite substrate with complementary structure: 1. The mechanism of nucleation

The nucleation of water vapor on the infinite surface of a silver iodide crystal at 260 K is simulated. Long-range electrostatic and polarization interactions are taken into account by the Ewald method. The free energy and work of equilibrium formation of nuclei are calculated at the molecular level by the method of bicanonical statistical ensemble. It is shown that, at the initial stage, the substrate is completely covered with a water monolayer. The substrate tends to decrease by two orders of magnitude the vapor pressure required to form the critical nucleus of a monomolecular film with a size of 10² molecules, the nucleation rate being increased by tens of orders of magnitude as compared to homogeneous nucleation. The saturation pressure above the adsorbed monomolecular film is 12 times lower than that above the flat ice surface. The free energy at the edges of “spots” per unit length is 1.4 × 10^?11 J/m. The critical size of the spot increases with a decrease in vapor pressure as the inverse second power of the logarithm of pressure.     

Computer Simulation of the Initial Stage of the Nucleation of Water Vapors on the Silver Iodide Crystal Surface: 2. Thermodynamics

The formation work of the condensed phase nucleus from the vapor on the surface of silver iodide at 273 K is calculated by the method of bicanonical statistical ensemble. The energy barrier of the formation of a nucleus of a monolayer on the surface is located in the region of extremely small sizes and its height does not exceed the energy of thermal motion k _B T. Such a barrier cannot markedly decelerate the nucleation. A point crystal defect in the form of an extra ion on the surface qualitatively changes the pattern of the formation work curve: the minimum with a depth of about 100k _B T corresponding to a thermodynamically stable nucleus appears on this curve. As the vapor pressure increases, “spots” of water molecules are formed and grow on the substrate surface around point crystal defects. These spots tend to coalesce and cover the entire surface as a monolayer; however, the high free energy barrier prevents the formation of further layers.__________Translated from Kolloidnyi Zhurnal, Vol. 67, No. 4, 2005, pp. 561–572.Original Russian Text Copyright © 2005 by Shevkunov.     

Hydration of Cl<Superscript>–</Superscript> ion in a planar nanopore with hydrophilic walls. 1. Molecular structure

Computer simulation has been employed to obtain equilibrium molecular configurations, as well as spatial and angular distributions of water molecules, under the action of the field of a single-charged chlorine anion in a model planar nanopore with structureless walls at room temperature. A detailed many-body model of intermolecular interactions calibrated in accordance with experimental data relative to the free energy of hydration in water vapor has been used. The effect of the hydrophilicity of the walls on the ion hydration shell consists in its disintegration into two parts, i.e., molecules retained exclusively due to the interactions with the ion and those adsorbed on the walls. In the regime of strong interactions with the walls, two relatively stable states arise with asymmetric distribution of molecules between opposite walls. The existence of the two metastable states destabilizes the position of ions inside a pore and is expected to accelerate their adsorption on the walls.     

The Effect of Temperature on Nucleation of Condensed Water Phase on the Surface of a β-AgI Crystal. 1. Structure

The Monte Carlo method has been employed to study the effect of temperature on the structure and the mechanism of retaining condensed water phase nuclei on the surface of the basal face of a silver iodide crystal. Comparative calculations of spatial correlation functions and computer images of vapors being condensed at 260 and 320 K have indicated an increased stability of monomolecular water-film spots with respect to thermal fluctuations. The disturbances of the regular “honeycomb” structure have a collective character and occur according to the “domino principle”; i.e., the rupture of a hydrogen bond between neighboring molecules releases enhanced libration motions of the latter, which, in turn, provoke the rupture of bonds with other neighbors. In accordance with this scenario, the distortion of the hexagonal structure of the film under the action of thermal fluctuations develops with the formation of growing spots of destruction. The thermal fluctuations significantly affect the orientational molecular order and the degree of clustering on the surface. The positions of molecules relative to the ions of the surface crystallographic layer of a substrate weakly depend on temperature.