Water vapor clustering in the field of a chlorine anion occurring in a planar nanopore with structureless walls

The Monte Carlo bicanonical statistical ensemble method has been employed to calculate the free energy, entropy, and work of Cl^? ion hydration in model planar pores 0.5 and 0.7 nm wide at 298 and 400 K. A detailed model of many-body interactions with the ion has been used, the model being matched to experimental data with respect to the free energy and enthalpy of attachment reaction in water vapor. Under the conditions of a restricted volume, the equilibrium size of a hydration shell substantially decreases, with the effect becoming stronger in the range of moderate and large sizes. In moderately supersaturated vapors, under the conditions of a nanopore, the ion loses its hydration shell as the temperature is decreased. In supersaturated vapors, the hydration shell formed on the ion is thermodynamically stable, while the stability crisis shifts to the region of larger sizes. The enhancement of the thermodynamic stability in the pore results from a rise in the chemical potential of molecules due to the deficiency of closet neighbors and a reduction in the entropy under the conditions of the restricted volume. As the temperature is elevated, the effect of ion displacement out of its hydration shell is leveled. The regularities derived in terms of the estimation model based on the capillary approximation are in qualitative agreement with the results of computer simulation.     

Hydration of Cl<Superscript>–</Superscript> ion in a planar nanopore with hydrophilic walls. 2. Thermodynamic stability

The Monte Carlo bicanonical statistical ensemble method has been employed to calculate the dependences of the Gibbs free energy, formation work, and entropy on the size of a hydration shell grown from water vapor on single-charged chlorine anion in a model planar nanopore with hydrophilic structureless walls at 298 K. A refined model comprising many-particle polarization interactions and calibrated with respect to experimental data on the free energy and enthalpy of the initial reactions of attachment of water molecules to the ion has been used. It has been found that a weak hydrophilicity of pore walls leads to destabilization of the hydration shell, while a strong one, on the contrary, causes its stabilization. The physical reason for the instability in the field of hydrophilic walls qualitatively differs from that under the conditions of hydration in bulk water vapor.     

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.     

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.     

Water vapor nucleation on ion pairs under the conditions of a planar nanopore

Computer simulation has been employed to study the effect of a confined space of a planar model pore with structureless hydrophobic walls on the hydration of Na⁺Cl^– ion pairs in water vapor at room temperature. A detailed many-body model of intermolecular interactions has been used. The model has been calibrated relative to experimental data on the free energy and enthalpy of the initial reactions of water molecule attachment to ions and the results of quantum-chemical calculations of the geometry and energy of Na⁺Cl^– (H₂O)_N clusters in stable configurations, as well as spectroscopic data on Na⁺Cl^– dimer vibration frequencies. The free energy and work of hydration, as well as the adsorption curve, have been calculated from the first principles by the bicanonical statistical ensemble method. The dependence of hydration shell size on interionic distance has been calculated by the method of compensation potential. The transition between the states of a contact (CIP) and a solvent-separated ion pair (SSIP) has been reproduced under the conditions of a nanopore. The influence of the pore increases with the hydration shell size and leads to the stabilization of the SSIP states, which are only conditionally stable in bulk water vapor.     

Ion permeation dynamics in carbon nanotubes

Molecular dynamics simulations are carried out to investigate the permeation of ions and water in a membrane consisting of single wall carbon nanotubes possessing no surface charges connecting two reservoirs. Our simulations reveal that there are changes in the first hydration shell of the ions upon confinement in tubes of 0.82 or 0.90 nm effective internal diameter. Although the first minimum in the g(r) is barely changed in the nanotube compared to in the bulk solution, the hydration number of Na(+) ion is reduced by 1.0 (from 4.5 in bulk to 3.5 in the 0.90 nm tube) and the hydration number is reduced further in the 0.82 nm tube. The changes in the hydration shell of Cl(-) ion are negligible, within statistical errors. The water molecules of the first hydration shell of both ions exchange less frequently inside the tube than in the bulk solution. We compare ion trajectories for ions in the same tube under identical reservoir conditions but with different numbers of ions in the tubes. This permits investigation of changes in structure and dynamics which arise from multiple ion occupancy in a carbon nanotube possessing no surface charges. We also investigated the effects of tube flexibility. Ions enter the tubes so as to form a train of ion pairs. We find that the radial distribution profiles of Na(+) ions broaden significantly systematically with increasing number of ion pairs in the tube. The radial distribution profiles of Cl(-) ions change only slightly with increasing number of ions in the tube. Trajectories reveal that Na(+) ions do not pass each other in 0.90 nm tubes, while Cl(-) ions pass each other, as do ions of opposite charge. An ion entering the tube causes the like-charged ions preceding it in the tube to be displaced along the tube axis and positive or negative ions will exit the tube only when one or two other ions of the same charge are present in the tube. Thus, the permeation mechanism involves multiple ions and Coulomb repulsion among the ions plays an essential role.     

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.     

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.     

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.     

Mechanisms for ion retention in molecular water clusters in a planar nanopore against the background of thermal fluctuations

The Monte Carlo method has been used to calculate the potential of mean force for Na⁺ and Cl^? ions interacting in model planar nanopores with structureless walls under the conditions of the material contact with water vapor at room temperature and above water boiling point. The interactions have been described using a detailed many-body model calibrated with respect to experimental data on the free energy of attachment reactions and the results of quantum-chemical calculations. Dissociation becomes possible when the vapor density increases as a sufficient number of molecules are pulled into the field of the ions. The dissociation proceeds sooner under the conditions of the nanopore than in bulk water vapor. Hydration decreases the energy of the dissociated state; however, the entropy component of the free energy partly compensates for the decrease in the internal energy, thereby increasing the stability of a contact ion pair. After the dissociation of a contact ion pair (CIP), ions are retained within a cluster in the state of a solvent-separated ion pair (SSIP). Fluctuations in the number of pulled-in vapor molecules, which are correlated with fluctuations in the interionic distance, stabilize the SSIP states with respect to recombination, while a decrease in the screening of the field of ions under the conditions of the nanopore stabilize the SSIP states with respect to cluster decay. The conditions of the nanopore stimulate the passage of an ion pair from the CIP to the SSIP state due to the rearrangement of the statistical weights in favor of molecules being located in the interionic gap. Thus, under the conditions of the nanopore, the stability of the SSIP states increases with respect to both the recombination of the ions and the decay of the ion-molecular associate.     

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.     

Quantifying barriers to monovalent anion transport in narrow non-polar pores

The transport of anionic drinking water contaminants (fluoride, chloride, nitrate and nitrite) through narrow pores ranging in effective radius from 2.5 to 6.5 ? was systematically evaluated using molecular dynamics simulations to elucidate the magnitude and origin of energetic barriers encountered in nanofiltration. Free energy profiles for ion transport through the pores show that energy barriers depend on pore size and ion properties and that there are three key regimes that affect transport. The first is where the ion can fit in the pore with its full inner hydration shell, the second is where the pore size is between the bare ion and hydrated radius, and the third is where the ion size approaches that of the pore. Energy barriers in the first regime are relatively small and due to rearrangement of the inner hydration shell and/or displacement of further hydration shells. Energy barriers in the second regime are due to partial dehydration and are larger than barriers seen in the first regime. In the third regime, the pore becomes too small for bare ions to fit regardless of hydration and thus energy barriers are very high. In the second regime where partial dehydration controls transport, the trend in the slopes of the change in energy barrier with pore size corresponds to the hydration strength of the anions.     

Formation and interaction of hydrated alkali metal ions at the graphite-water interface

Ion hydration at a solid surface ubiquitously exists in nature and plays important roles in many natural processes and technological applications. Aiming at obtaining a microscopic insight into the formation of such systems and interactions therein, we have investigated the hydration of alkali metal ions at a prototype surface-graphite (0001), using first-principles molecular dynamics simulations. At low water coverage, the alkali metal ions form two-dimensional hydration shells accommodating at most four (Li, Na) and three (K, Rb, Cs) waters in the first shell. These two-dimensional shells generally evolve into three-dimensional structures at higher water coverage, due to the competition between hydration and ion-surface interactions. Exceptionally K was found to reside at the graphite-water interface for water coverages up to bulk water limit, where it forms an "umbrellalike" surface hydration shell with an average water-ion-surface angle of 115 degrees . Interactions between the hydrated K and Na ions at the interface have also been studied. Water molecules seem to mediate an effective ion-ion interaction, which favors the aggregation of Na ions but prevents nucleation of K. These results agree with experimental observations in electron energy loss spectroscopy, desorption spectroscopy, and work function measurement. In addition, the sensitive dependence of charge transfer on dynamical structure evolution during the hydration process, implies the necessity to describe surface ion hydration from electronic structure calculations.     

Ion pairs in aqueous electrolyte microdrops under conditions of a flat nanopore

The molecular mechanisms of aqueous solvent penetration into a flat nanopore with hydrophobic structureless walls containing a Na⁺Cl^? ion pair with nonfixed distance between ions is studied by computer simulations. A detailed many-body polycenter model of intermolecular interactions calibrated with respect to experimental data for the free energy of attachment of water vapor molecules and quantum-chemical calculations in clusters is used. The ion pair hydration results in its decomposition. Drawing the molecules into the gap between ions makes easier penetration of solvent and filling of the nanopore with electrolyte. The ion-pair dissociation is accompanied by dramatic changes in the chemical potential of molecules and electric properties of the whole system. The thermodynamic characteristics of decomposition are stable as regards variations in the pore width. The post-decomposition electric polarizability demonstrates strong anisotropy associated with the nanopore flatness.     

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.     

High Temperature Stability of Hydrated Ion Pairs Na<Superscript>+</Superscript>Cl<Superscript>–</Superscript>(H<Subscript>2</Subscript>O)<Subscript><Emphasis Type="Italic">N</Emphasis></Subscript> under Conditions of a Flat Nanopore

The high-temperature stability of hydrated ion pairs under conditions of a nanoscopic flat pore with hydrophobic structureless walls is studied by computer simulations. The limited space of the nanopore stimulates dissociation of the contact ion pair (CIP) with its transition to the state of the solvent-separated ion pair (SSIP); moreover, the ion pair demonstrates a high degree of stability on heating. The inverse temperature effect where the heating renders a moderate consolidating effect on the state of a hydrated contact ion pair is observed: when heated to the electrolyte boiling point, the free energy barrier that separates the CIP and SSIP states shifts by 2 molecules towards the larger hydration shells. On the pressure scale, the boundary between CIP and SSIP states shifts at the same rate as the saturating pressure with the increase in the temperature.     

Molecular structure of finely disperse Na+Cl−(H2O) n aerosol particles in water vapor

The thermodynamic states corresponding to solvent separated (SSIP) and contacting (CIP) Na⁺Cl^? ion pairs in molecular water clusters have been obtained by random walks in a configurational space with an equilibrium distribution function at 273 and 150 K. The transition to the SSIP state begins in a thresh-old-type manner in clusters containing 10–12 molecules, with the interionic distance increasing continuously up to disintegration into two hydrated ions with the growth of a hydration shell. As the cluster size increases, the hydration shell shifts from sodium ion to chlorine ion. In the first hydration layer, the electric field of the ions ruptures as many as 50% of hydrogen bonds.     

高温及超临界条件下 MgCl2与CaCl2水溶液中离子水化与缔合的量子化学计算

对NaCl等碱金属水溶液的研究表明,室温条件下,离子在溶液中以水合形式存在,而在高温及超临界时,阴阳离子将结合成为离子对．采用量子化学计算,研究了MgCl2与CaCl2水溶液中水化与缔合的情况．通过Gaussian98软件包计算了阳离子的水化自由能以及离子对的生成能,从而获得水合离子与离子对的热力学稳定性及其随温度、压力的变化情况．通过比较热力学稳定性,考察了两种溶液中水化与缔合的变化情况．研究结果表明,MgCl2与CaCl2水溶液中离子水化与缔合的变化趋势与碱金属溶液基本一致,但是存在一个过渡区域,该区域内离子对与水合离子共存,因此需要采用不同于碱金属溶液的方法处理MgCl2与CaCl2水溶液．