S. V. Shevkunov

Effect of chlorine ions on the stability of nucleation cores in condensing water vapors

The Gibbs energy and the equilibrium work of nucleation of condensed-phase nuclei in water vapors in the presence of chlorine anions are calculated at the molecular level using the Monte Carlo method at a temperature of 273 K for sizes of up to 200 molecules. It is found that the growth of a nucleus leads to the displacement of a chloride ion onto its surface, which is accompanied by a loss of the thermodynamic stability of the microdroplet. It is found that the size dependence of the work of nucleation exhibits a minimum and a maximum with an inflection point that separates the region of stable and unstable equilibrium sizes.

Nonpair interactions in Na+(H2O)n clusters under thermal fluctuation conditions

The influence of many-particle interactions on the structure of Na+(H2O)n clusters at 298 K was studied by the Monte Carlo method. The interaction parameters were reproduced from the experimental data on the Gibbs energy of hydration in water vapor. The interaction of induced dipoles results in the displacement of part of molecules through large distances from the ion. Covalent interactions strengthen the bond with the first attached molecule and weaken bonds with the other molecules.

Structure of the hydration shell of the Na + ion in a planar nanopore with hydrophobic walls

The effect of steric hindrances in extremely narrow planar pores on the structure of the hydration shell of the single-charged sodium cation in water vapors at room temperature was studied by computer simulation. The deficiency of empty space for the motion in the slit-like pore was shown to slightly affect the radial distribution of molecules around the ion. The integrated (over the directions) numbers of ion-oxygen atom bonds of molecules in the ion’s hydration shell did not change despite the change in the shape of the hydration cluster from three- to two-dimensional. It was concluded that the changes in the positions of molecules relative to the ion were mainly reduced to azimuthal displacements; as a result, the local bulk density of molecules in the pore was higher than at the same distances outside the pore for the same total number of molecules. The distribution of molecules over layers inside the pore demonstrates the effect of molecules spread over the walls. The effect of ion displacement from its own hydration shell found earlier for the free chloride ion is steadily reproduced under the pore conditions. An alternative explanation to this effect was proposed that does not suggest high ion polarizability.

Layer-by-Layer Adsorption of Water Molecules on the Surface of a Silver Iodide Crystal

Computer simulations at the molecular level were used to analyze the mechanism of the nucleation of water condensate from the vapor phase on the surface of a silver iodide crystal at 260 K. The initial stage of the condensation process is the sequential growth of monolayers on the substrate surface without formation of a compact microdroplet. The dependence of the equilibrium work of formation of the condensate film on its thickness exhibits oscillations. The formation of layers close to the substrate surface involves the overcoming of a Gibbs energy barrier.

Thermodynamic characteristics of the hydrate shell of a Na+ ion in a plane nanopore with hydrophobic walls

The chemical potential, free energy, and work of hydration of a single-charged sodium cation are calculated using the Monte Carlo method for a bicanonic statistic ensemble at the molecular level at 298 K in plane model nanopores 0.5 and 0.7 nm wide. It is shown that the nanopores have a stabilizing effect on the hydrate shell of an ion. It is concluded that the crisis of stability that occurs outside a pore is transformed into an abrupt acceleration of growth with the conservation of a stable equilibrium with vapor under the conditions of plane nanopores. It is established that the mechanism of the threshold acceleration of growth inside a pore is associated with an ion being displaced from its own hydrate shell.

Supramolecular Mechanisms Responsible for the High Activity of Aerosol Stimulants of Atmospheric Moisture Nucleation

Water vapor nucleation preceding crystal nucleation was studied by computer simulation; the role of crystal defects on the surface with a structure complementary to the structure of ice was investigated. The Gibbs energy and microdrop formation energy were calculated at the molecular level. The structure and thermodynamic stability of nuclei in the fields of crystal defects of different types were analyzed. It was found that crystal point defects could not stimulate the formation of large nuclei. Extended defects such as wedge-shaped fractures were shown to be the most effective stimulants. The stage of monomolecular film formation in fractures was preceded by the formation of three-dimensional microdrops. The conclusion was drawn that microcrystallization was favored by a relatively large surface of contact with microfracture walls, the three-dimensional structure of nuclei, and their high thermodynamic stability in the transverse microfracture field; these factors drastically accelerated the growth of nuclei and their coalescence on the surface of aerosol particles.

Structure of water in microscopic fractures of a silver iodide crystal

The results of a computer simulation of flat fractures with widths of 0.63, 1.25, and 2.5 nm filled with vapor molecules in a silver iodide crystals at 260 K were presented. The two-dimensional gas of molecules adsorbed on the walls was found to be strongly clustered. Before the pore was filled, its walls had been covered with a monomolecular water film with a characteristic hexagonal structure. The perpendicular growth of the film was hindered by the hydrophobicity of its surface; the adsorbed molecules were bonded with the walls by interactions with the ions of the second crystal layer to form a specific orientational molecular order in the region of contact with the wall. On the wall with silver cations, the molecular energy was lower and the entropy higher than on the wall with iodide anions; on the wall with a lower energy, the adsorption started earlier and was more active. In an extremely narrow pore having room for only one molecular layer, the monomolecular film consists of spots held on opposite walls; in each spot, the orientational molecular order is the one characteristic of the wall with which the spot is in contact.

Domain nucleation in the contact layer at an interface of water and a polarizable substrate

The growth of a molecular water film on the basic plane of a silver iodide monocrystal is studied through computer simulation. Decomposition into domains with spontaneous polarization is observed in the contact layer of the film at the interface with the substrate. The formation of domains is found to be sharply enhanced on a model substrate with the double polarizability of iodine ions; heteropolarization interactions caused by the formation of domain structures increase the film’s coupling with the substrate. It is demonstrated that the vapor pressure needed for molecular film growth is reduced appreciably via heteropolarization interactions.

The Structure of the Hydration Shell of the Ionized HCl Molecule in Water Vapor

The H3O+(H2O)nCl− clusters were simulated by the Monte Carlo method in a grand canonical ensemble in thermal and material contact with water vapor under the conditions close to the natural conditions in the stratosphere. A detailed model including nonpair polarization and covalent interactions was used. The correlation functions, density distributions, and free energy and entropy as functions of the interionic distance were calculated. The mechanism of ionized HCl state stabilization was determined by the formation of a special structure of the hydrate cluster component with low Gibbs energy and entropy.

Effect of hydrophilic walls on the hydration of sodium cations in planar nanopores

A computer simulation of the structure of Na+ ion hydration shells with sizes in the range of 1 to 100 molecules in a planar model nanopore 0.7 nm wide with structureless hydrophilic walls is performed using the Monte Carlo method at a temperature of 298 K. A detailed model of many-body intermolecular interactions, calibrated with reference to experimental data on the free energy and enthalpy of reactions after gaseous water molecules are added to a hydration shell, is used. It is found that perturbations produced by hydrophilic walls cause the hydration shell to decay into two components that differ in their spatial arrangement and molecular orientational order.