Ion Solvation Free Energies Research Articles

Continuum Corrections to the Polarization and Thermodynamic Properties of Ewald Sum Simulations for Ions and Ion Pairs at Infinite Dilution

Ewald sum simulations of the solvation thermodynamics of charged solutes employ an unphysical uniform neutralizing background charge to ensure convergence of the periodic lattice sum. Continuum corrections to the solvation free energy, entropy, and volume of single ions and ion pairs for the effect of the neutralizing background and periodic boundary conditions at finite cell size are derived in order to allow efficient calculations of the ionic properties at infinite dilution. The derivation presented in this paper shows the physical origin of the effects and can be easily extended to multiple charge sites. Corrections are small for high dielectric constant solvents but become increasingly important as the ion size is increased, the dielectric constant is decreased, or the unit cell size is decreased. An alternative way of calculating the thermodynamic properties from Ewald sum simulations is proposed for which the corrections are small for low dielectric constant solvents. Tests for low and high dielectric constant water show that, after the appropriate continuum corrections are applied, the free energy of charging an ion using Ewald sum simulations agrees with the results for potential truncation simulations and spherical boundary simulations (when corrected for truncation effects). The corrected free energy of hydration is not sensitive to the system size even for low dielectric constants. The continuum model correctly predicts that Ewald sum simulations yield a solvent polarization at large distance from an ion that is smaller than the polarization of a truly infinite system.

Connecting Cluster Anion Properties to Bulk: Ion Solvation Free Energy Trends with Cluster Size and the Surface vs Internal Nature of Iodide in Water Clusters

There is disagreement in the literature on whether small water clusters (up to ∼60 waters) accommodate an excess iodide anion at the surface, in the interior, or in states intermediate between the two extremes. Small cluster solvation data, the results of ion simulations in model polar solvent clusters of intermediate size, and limiting continuum dielectric trends at large cluster size have been combined to illustrate the progression of the free energy of ion solvation from the smallest cluster size to bulk, establishing a set of general expectations for cluster ion solvation properties vs cluster size. The surface vs internal state issue for iodide in water clusters is examined in the context of the cluster ion properties of solvation thermochemistry, electron vertical detachment thresholds, and vertical detachment energies with specific regard to variation with cluster size and extrapolation to bulk. In aqueous systems, the anionic cluster property of vertical detachment threshold is shown to bear a bet...

Accurate Solvation Free Energies of Acetate and Methylammonium Ions Calculated with a Polarizable Water Model

Accurate Solvation Free Energies of Acetate and Methylammonium Ions Calculated with a Polarizable Water Model

Water orientation and ion solvation effects during multicomponent salt partitioning in a Nafion cation exchange membrane

A molecular-level equilibrium partition coefficient model has been formulated to describe single and multicomponent ion uptake by a Nafion 117 cation exchange membrane. The model presumes a cylindrical pore structure for the membrane and considers ion solvation free energy changes which occur during solute partitioning and the orientation of solvent dipoles inside a membrane pore due to the strong electric field generated by the membrane's fixed-charge groups. Membrane structure parameters (radius of a membrane pore and the pore wall density of fixed charge sites) which are needed to obtain a model solution are determined from experimental membrane porosities and X-ray diffraction data in the literature. Without the use of adjustable parameters, the model predicts to within 6% equilibrium cation concentrations in Nafion for a variety of aqueous single salt and binary and ternary alkali metal chloride mixtures at a total external salt concentration of between 0.1 and 1.0 M. A close inspection of the computed electrostatic and hydration forces acting on membrane-phase cations in a mixed salt system indicates that the univalent cation with the highest surface charge density (smallest hard sphere ion radius for like-charged ions) is excluded from the pore wall region in a manner qualitatively similar to that observed for coions.

Application of the mean spherical approximation to the estimation of single ion thermodynamic quantities of salvation for monoatomic monovalent ions in aqueous solutions

Abstract : The expression for the Gibbs energy of solvation for simple 1-1 electrolytes within the mean spherical approximation has been fitted to data for 20 alkali metal halides in water. In performing this fit, one needs to assume a radius for each ion, the Pauling values often having been chosen in the past. The MSA parameters appear as corrections to these radii and are different for cations and anions. Two sets of ionic radii were considered, the Pauling values from crystallographic data and values based on X-ray diffraction results obtained in aqueous solutions. Excellent fits between theory and experiment were obtained, the results with the latter set of radii being preferred. This analysis provides an extrathermodynamic route to single ion solvation free energies and yields results which are very close to the real free energy of solvation measured by Randles.

Quantum-chemical calculations of the hydration energies of organic cations and anions in the framework of a continuum solvent approximation

We investigate the method of calculation of solvation energies using the continuum solvent model and the semiemperical NDDO quantum-chemical background. The electrostatic part of the calculation is based on the Miertuš-Scrocco-Tomasi procedure treating the solvent as a dielectric continuum with a cavity and numerically solving the integral equation for the charge distribution on the cavity surface. Its efficiency was significantly increased by accelerating the convergence of the surface integration procedure and modifying the iterative scheme. The calculated solvation free energies of numerous organic ions agree with the experimental estimates within 2–3 kcal/mol. The specific solute-solvent interactions are shown to be important in several cases and a consistent procedure of calculating their contribution to the solvation energy is elaborated.

General parameterized SCF model for free energies of solvation in aqueous solution

We present a new general parameterization for aqueous solvation free energies of molecules and ions in aqueous solution. It is obtained by extending a semianalytic treatment of solvation recently proposed for use with molecular mechanics and liquid simulations by Still et al. As extended here, the solvation terms are included in a Fock operator. The model incorporates reaction field polarization effects through the generalized Born functional with charges obtained by AMI molecular orbital calculations, and it includes cavitation, dispersion, and hydrophobic effects through an empirical function of solvent-accessible surface area. A general parameter set, including parameters for H, C, N, 0, F, S, CI, Br, and I, has been obtained by considering a data set consisting of 141 neutral molecules, 10 cations, and 17 anions. The neutral molecules include alkanes, cycloalkanes, alkenes, arenes, alkynes, ethers, heterocycles, carboxylic acids, esters, nitriles, aldehydes, ketones, alcohols, amines, nitro compounds, sulfides, thiols, halides, and polyfunctional compounds. The general parameterization is called Solvation Model 1, and it is particularly well suited for chemical reaction dynamics and reaction intermediates. We also discuss how the model may be refined for solvation free energies for stable neutral molecules.

Charge Dependence in Ionic Solvation

Systems consisting of a single electrically neutral or ionic species in interaction with a large number of solvent molecules are treated in general quantum-statistical-mechanical terms. Such systems are assumed to have Hamiltonians, for each electronic state, which exhibit a parametric dependence upon charge not exceeding quadratic. Changes in the Helmholtz free-energy, energy, and entropy functions are determined formally for the process of ionic solvation due to Born. On the basis of a general argument that an electrically neutral species acquires a nonzero potential upon solvation, the changes in the cited thermodynamic functions are shown to be capable of taking on positive values for a range of values of the charge parameter. This behavior is in direct contrast with expectations from the Born expression for such changes. Semiempirical treatment of data available for solvation free energies of aqueous ions permits an estimate, of 1 V, to be made of this potential. A possible experimental test of this result is described.