First Solvation Shell Of Ions Research Articles

Ion association in binary mixtures of water-CO2 in supercritical conditions through classical molecular dynamics simulations

The structural and thermodynamic properties of Na+ - Cl− ion-pair association in water-CO2 binary mixtures in supercritical conditions for infinitely dilute solutions are studied using constrained molecular dynamics simulations over a wide range of compositions. It is found that solvation structure varies dramatically with the solvent composition. Contact ion pairs (CIPs) are found to be more stable than all other configurations as seen from the potentials of mean force (PMFs). PMFs of the NaCl ion pair in pure CO2 look almost like the pair potential between the ion pair. Stabilities of CIPs increase with increase in the mole fraction of CO2. An increment in the average number of hydrogen bonds with an increase in the mole fraction of H2O in the bulk as well as in the solvation shell of the ions is observed. Ion-pair association in aqueous CO2 mixtures in supercritical conditions is found to be endothermic and driven by entropy. Preferential solvation analysis shows that both Na+ and Cl− ions are preferentially solvated by water and even a small percentage of water in the mixture prevents CO2 molecules from entering the first solvation shell of ions due to the strong hydrophilicity of the ions.

Influence of a Counterion on the Ion Atmosphere of an Anion: A Molecular Dynamics Study of LiX and CsX (X = F(-), Cl(-), I(-)) in Methanol.

We report molecular dynamics (MD) simulations to explore the influence of a counterion on the structure and dynamics of cationic and anionic solvation shells for various ions in methanol at 298 K. We show that the variation in ionic size of either the cation or the anion in an ion pair influences the solvation structure of the other ion as well as the diffusivity in an electrolyte solution of methanol. The extent of ionic association between the cation and its counteranion of different ionic sizes has been investigated by analyzing the radial distribution functions (RDFs) and the orientation of methanol molecules in the first solvation shell (FSS) of ions. It is shown that the methanol in the FSS of the anion as well the cation exhibit quite different radial and orientational structures as compared to methanol which lie in the FSS of either the anion or the cation but not both. We find that the coordination number (CN) of F(-), Cl(-), and I(-) ions decreases with increasing size of the anion which is contrary to the trend reported for the anions in H2O. The mean residence time (MRT) of methanol molecules in the FSS of ions has been calculated using the stable states picture (SSP) approach. It is seen that the ion-counterion interaction has a considerable influence on the MRT of methanol molecules in the FSS of ions. We also discuss the stability order of the ion-counterion using the potentials of mean force (PMFs) for ion pairs with ions of different sizes. The PMF plots reveal that the Li(+)-F(-) pair (small-small) is highly stable and the Li(+)-I(-) pair is least stable (small-large) in electrolyte solutions.

Solvation Structures and Dynamics of the Magnesium Chloride (Mg2+–Cl–) Ion Pair in Water–Ethanol Mixtures

We have performed constrained molecular dynamics simulations of magnesium chloride in water-ethanol mixtures. From the potentials of mean force (PMFs) of the Mg(2+)-Cl(-) ion pair, we notice that, as the mole fraction of ethanol increases, the depths of the minima of the contact ion pair (CIP) and solvent assisted ion pair (SAIP) increase, but the depth of the CIP minimum increases more in comparison to the SAIP minimum. This shows that ion pairing becomes more favorable with an increase in the mole fraction of ethanol. Significant differences in the PMFs between the Mg(2+) and the Cl(-) ion (depending upon whether the second Cl(-) ion is present in the first coordination shell of the Mg(2+) ion or not) seem to have been reported for the first time in this work. The local mole fraction of water molecules in the first solvation shell of ions is generally greater than in the bulk. The diffusional behavior of solvent molecules in solvation shells of the ion-pair indicates that the ions as well as the first solvation shells of the ions diffuse at much slower rates. Also, the diffusion constant of bulk water in the mixtures is greatly reduced compared to the pure solvent value.

The dynamics of ions in dimethylformamide

Time autocorrelation functions were calculated from the results of molecular dynamics simulations of ions in liquid N,N-dimethylformamide at 233, 298, and 318 K. The mobilities and diffusion coefficients of ions determined by molecular dynamics simulations and from conductometric data are compared. The influence of the temperature and size of ions on the mobility of solvent molecules in the first solvation shells of ions is analyzed.

Theoretical Calculation of pKa Using the Cluster−Continuum Model

The pKa's of 17 species from −10 to 50 were calculated using the ab initio MP2/6-311+G(2df,2p) level of theory and inclusion of solvent effects by the cluster−continuum model, a hybrid approach that combines gas-phase clustering by explicit solvent molecules and solvation of the cluster by the dielectric continuum. In addition, the pure continuum methods SM5.42R and PCM were also used for comparison purposes. Species such as alcohols, carboxylic acids, phenol, acetaldehyde and its hydrate, thiols, hydrochloric acid, amines, and ethane were included. Our results show that the cluster−continuum model yields much better agreement with experiment than do the above-mentioned pure continuum methods, with a rms error of 2.2 pKa units as opposed to 7 pKa units for the SM5.42R and PCM methods. The good performance of the cluster−continuum model can be attributed to the introduction of strong and specific solute−solvent interactions with the molecules in the first solvation shell of ions. This feature decreases the...