Description of the influence of intermolecular interaction on IR spectra's frequencies of simple dense fluids by Monte-Carlo simulation

The solvent-induced shift in IR spectrum of liquid CO is investigated by Monte-Carlo computer simulation. The intramolecular potential is Morse function, pair intermolecular potential includes short-range and dispertion parts. The calculated shift is 5 cm^?1 while the experimental value is 4 cm^?1.     

Phase transformations of a condensate on a crystal surface in a strong electric field

The evolution of the phase state of the water vapor condensate of the silver iodide crystal surface in an applied electric field up to 10 V/nm is studied by computer simulation. The previously found domain structure of the contact layer is stable against the external field and remains up to the complete break of the molecular film. In a strong electric field, the film condensation mode is changed by the formation of a new phase consisting of molecular nanothreads growing in the direction of the electric field lines. The transition to the new state is sharp. The presence of a phase transition is likely analogous to that accompanying the transformation of water microdrops to the superpolarized state under the action of an external electric field at stratospheric temperatures.     

Water vapor clustering in the field of Na<Superscript>+</Superscript> cation inside a nanopore with hydrophilic walls. 1. Spatial organization

Computer simulation has been employed to study the structure of a hydration shell of a Na⁺ ion under the conditions of a planar nanopore with structureless hydrophilic walls at 298 K. Intermolecular interactions have been described in terms of a detailed model calibrated with respect to experimental data on the free energy and enthalpy of the initial reactions of vapor molecule attachment to the ion. In the field of hydrophilic walls, the hydration shell is disrupted into an enveloping part and that spread over the surface of the walls. At the final stage of hydration, states with asymmetric distribution of molecules on opposite walls survive and the phenomenon of ion displacement out of its shell is stably reproduced. The orientational molecular order in the system strongly depends on the degree of wall hydrophilicity. The hydration shell of a sodium ion is less stable with respect to disturbances generated by the field of hydrophilic walls than the shell of a chlorine ion is.     

Water vapor nucleation on a crystal surface in a strong electric field

Water vapor nucleation at 260 K in a transverse electric field has been simulated by the Monte Carlo method under conditions corresponding to an internal wall of a spatially extended microcrack in a silver iodide crystal. The bicanonical statistical ensemble method has been employed to calculate, at the molecular level, the free energy of addition and the work of formation of dense phase nuclei in fields with different strengths. In a moderate field, the film mechanism of nucleation characterized by intense distortions on the film surface remains preserved. A domain structure of a film layer in contact with the surface exhibits a high stability with respect to an external field and remains preserved until the film is completely destroyed. In a strong electric field, the nucleation mechanism is fundamentally changed; i.e., the film is destroyed to yield threadlike structures. Therewith, the area of the contact with the surface drastically decreases. The orientation of nanothreads along the electric field lines overcomes a low free energy barrier. The point of equilibrium of nanothreads with vapor depends on the presence of hydrogen bonds, while their stability is determined by longer-range dipole-dipole interactions. The observed form of existence of the condensate as polarized nanothreads seems to be analogous to the superpolarized state previously revealed for water microdroplets, the transition to which has the character of the first-order phase transition.     

Computer simulation of dissociative equilibrium in aqueous NaCl electrolyte with account for polarization and ion recharging. Ionization mechanism

The Monte Carlo method in a system with periodic boundary conditions was used within the model with explicit account for many-bod interactions to calculate ion-water correlation functions and the mean force ion-ion potential for extremely dilute aqueous electrolyte. Many-body interactions result in a decrease in the first coordination number of ions by approximately one molecule. The same effect is observed in the case of hydration in water vapors. Partial displacement of molecules from the lower layer into the higher hydrate layers occurs mainly by means of interactions of dipoles induced on molecules. Many-body interactions enhance the stability of unrecombined ion pairs separated by solvent molecules (SSIP states). The depth of the minimum in the dependence of the ion-ion mean force potential with account for many-body interaction forces is several times higher than in primitive interaction models. The value of effective relative dielectric permeability of the solvent at short distances from the ions grows faster than 1/R. Due to solvent polarization, counterions are strongly repelled at distances corresponding to overlapping of their hydrate shells and are weakly attracted at large distances. Stability of ion pair SSIP states in liquid electrolyte is due to rearrangement of the molecular structure of the solvent in the interion space and is an entropy effect. This mechanism differs qualitatively from that observed under hydration in water vapor and the depth of the minimum corresponding to SSIP states is by an order of magnitude lower in liquid electrolyte as compared to that in saturated water vapor.     

Water vapor clustering in the field of Na<Superscript>+</Superscript> cation inside a nanopore with hydrophilic walls. 2. Thermodynamic properties

The bicanonical statistical ensemble method has been used to calculate at the molecular level the free energy, entropy, and work of hydration of single-charged sodium cation in a model planar nanopore with structureless hydrophilic walls. The calculations have been performed in terms of a detailed many-particle model of intermolecular interactions calibrated with respect to experimental data on the free energy and enthalpy of the initial reactions of attachment in water vapor. In contrast to chlorine anion, at initial stages of formation, the hydration shell of sodium cation has a loose chain structure, which is reflected in the character of the interaction with pore walls and the behavior of entropy. Under the conditions of weakly hydrophilic walls, the system loses its stability; however, the stability remains preserved in a pore with strongly hydrophilic walls. Hydrophilic walls stabilize the system and shift the onset of hydration toward lower vapor pressures by several orders of magnitude.     

The structure of the hydration shell of the ionized HCl molecule in water vapor

The H₃O⁺(H₂O) _n Cl^? 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.     

Structure of a Na<Superscript>+</Superscript> cation hydration shell on heating in a planar nanopore

Resistance to heating above the boiling point of water of the molecular structure of a single-charged sodium cation hydration shell growing under the conditions of a model planar nanopore with a width of 5 Å is studied by computer simulation. Monte Carlo calculations of spatial correlation functions are performed in a detailed model with regard to many-body interactions between the ion and water molecules. The system demonstrates an increased resistance to thermal fluctuations along the pore plane and a decreased one in the transverse direction. The heating is accompanied by an enhanced coating effect of molecules around the ion and a diminished effect of extruding the ion out of its own hydration shell. The orientational molecular order due to strong spatial anisotropy inside the nanopore is much more stable than the hydrogen bonds between the molecules.     

Computer Simulation of Water Clusterization on Chlorine Ions: 1. Thermodynamic Properties

Cl^–(H₂O) _n clusters, n = 1–60, in equilibrium with vapor were simulated using the Monte Carlo method. Free energy and the work of clusters formation at room temperature and temperature corresponding to polar stratosphere were calculated. Clusters retain their stability over the entire investigated size range even at multiple vapor supersaturation; however, when supersaturation increases further, the cluster grows in an avalanche-like manner. In clusters with n > 20, the effect of ion field on the free energy of added molecules diminishes dramatically retaining, however, its stabilizing function.