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.     

Water in extremely narrow planar pores with crystalline walls. 2. Thermodynamics

The free energy, entropy, and work of water vapor adsorption in planar pores with widths of 0.62 and 1.25 nm located in a silver iodide crystal parallel to its basal face have been computed at the molecular level. In contrast to adsorption on a free surface, the adsorption in the pores proceeds in three stages, i.e., the formation of molecular films on the walls, coalescence of the films, and densification of the fluid in the pore volume. At the second stage, the equilibrium between the fluid in the pore and the vapor over the pore at temperatures corresponding to normal conditions is thermodynamically unstable and accompanied by the development of a free energy barrier and the existence of metastable states. As temperature is elevated, the instability is gradually evened out; however, its signs remain preserved even at the boiling temperature of water. Extremely narrow pores with widths smaller than 1 nm are always filled with water under conditions of even a rather dry natural atmosphere. The filling of pores several nanometers wide in strongly unsaturated water vapors overcomes the free-energy barrier; however, the fluid that has filled the pore remains stable with respect to evaporation in vapors with densities lower than the density of saturated vapor by several orders of magnitude. The existence of the free-energy barrier and metastable states in nanosized breaks in crystals creates conditions for hysteresis of adsorption-desorption cycles.     

Destruction of the ozone protecting layer: Charge-separation mechanism in clusters

A model is constructed that explains the intense accumulation of chlorine in stratospheric ice microclusters. The model is approved in detail for quantitative consistency using Monte-Carlo computer simulation. A complex interparticle-interaction model is used that includes nonpair forces, charge transfer, and corrections for the quantum-statistical uncertainty in nuclear positions. In the presence of thermal fluctuations, two thermodynamically stable states of the HCl molecule—bound and dissociated—occur in water clusters. In clusters containing more than 35 water molecules, molecular dissociation into ions becomes thermodynamically favorable under the conditions of the polar stratosphere.     

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.     

Computer simulation of the hydration of a chloride anion in a nanopore with hydrophilic walls

The hydration of a single-charged chloride anion Cl^- in a model plane nanopore with structureless hydrophilic walls in water vapor at room temperature is simulated using the Monte Carlo method. It is established that the adsorption of a fraction of associate molecules Cl^-(H₂O)_N on the walls enhances its thermodynamic stability and simulates the hydration of the ion at low vapor pressures. It is shown that a second stability crisis forms on the curve of the hydration work function in the mode of weak wall hydrophilicity.     

Phenomenon of the ousting of a monatomic ion from its hydration shell in flat nanopores

The structure and stability of hydrate shells of singly charged sodium and chlorine ions are studied by computer simulations under the conditions of nanoscopic flat pores with the use of the previously proposed detailed force field model containing polarization interactions, transferring charge effects as well as manybody interactions of covalent type. It is found that the effect of ousting a monatomic ion from its hydration shell, which has previously been observed by independent authors in bulk vapor, is also reproduced persistently in nanoscopic pores. Whereas the ousting of the ion from its hydration shell in bulk vapor is accompanied by the loss of thermodynamic stability of the system and at sufficiently high vapor pressure causes avalanche-like condensation, under the conditions of a nanoscopic pore the thermodynamic stability is retained. The obtained data show that the ousting of the ion from its hydration shell is a universal phenomenon covering the majority, if not all, of monatomic and, possibly, some of molecular ions.     

Ionization-recombination equilibrium in cold cluster plasma under conditions of retardation by high energy barrier

The kinetics of charge separation in a cold plasma was studied with the degradation reaction in molecular clusters HCl(H₂O) _n + m(H₂O) ? H₃O⁺(H₂O) _{n + m ?1}Cl^?, taken as an example, which precedes chlorine adsorption on the ice surface in the stratosphere. The formation of a vast population of H⁺, Cl^? ion pairs stabilized in water clusters ensures the intense binding of chlorine in ice microcrystals that occur in stratospheric clouds. The accumulation of chlorine in the stratosphere is recognized as the main cause of the destruction of the protective ozone layer. The ion buildup effect is a result of the balance between opposite ionization and recombination processes in the presence of a high energy barrier that retards ion recombination in water clusters. A kinetic equation for the process was obtained and its solution was analyzed. The parameters of the barrier were calculated by computer simulation.     

Stability of molecular form of HCl in water vapor: A computer simulation

The free energy and entropy of the dissociation of HCl molecule into ions in water vapor, HCl(H₂O) _n + mH₂O → H₃O + (H₂O) _n+m -1Cl^?, were calculated. The dependences of various parameters on the interionic distance at 273 K and various vapor pressures were obtained. A detailed model taking into account unpaired covalent-type interactions, polarization interactions, charge transfer effect, and hydrogen bonds was applied. The numerical values of the parameters were reconstructed from the experimental data on the free energy and enthalpy of the first reactions of addition of vapor molecules to ions, and also from the results of quantum-chemical calculations of the energy and geometry of locally stable configurations of clusters HCl(H₂O) _n . Despite lower internal energy of the dissociated state, the molecular form is absolutely stable in clusters of water molecules. The dissociated state is relatively stable. Accumulation of unrecombined ion pairs in clusters is possible with a decrease in the temperature to 200 K.     

Collective interactions in the mechanism of adhesion of condensed phase nuclei to a crystal surface. 2. Thermodynamic stability

The Monte Carlo method is used to calculate, at the molecular level, the free energy, entropy, and the work of formation at an initial stage of nucleation of a condensed phase from water vapor on the surface of a solid crystalline silver iodide substrate. The pattern of the obtained dependences confirms the pronounced layer-by-layer character of the growth of nuclei and the thermodynamic stability of a molecular film formed at the contact with the substrate. An increased hydrophilicity of the substrate surface with respect to the first monomolecular layer is enhanced by the formation of regions of spontaneous polarization in the latter. The reasons for the thermodynamic advantage of the separation of the nucleus contact layer on the substrate into domains with different types of polarization are analyzed in terms of a lattice model. Computer simulation within the framework of the lattice model demonstrates that a rise in the polarizability of the substrate is accompanied by a continuous increase in the equilibrium sizes of the domains; moreover, the model predicts their strongly nonlinear dependence on both temperature and the polarizability of the substrate.