Solvation Shell Of Ions Research Articles

Microscopic structure of liquid dimethyl sulphoxide and its electrolyte solutions: molecular dynamics simulations

Molecular dynamics (MD) simulations of pure dimethyl sulphoxide (DMSO) and solutions of Na+, Ca2+, Cl−, NaCl and CaCl2 in DMSO have been performed at 298.15 K and 398.15 K in NVT ensembles by using a four-interaction-site model of DMSO and reaction field method for Coulombic interactions. The structure of solvent, ion-solvation shells and ion-pairs have been analysed by employing a concept of coordination centres and characteristic vectors of the solvent molecule. Results are given for atom-atom (corresponding to DMSO), ion-atom and ion-ion radial distribution functions (RDFs), orientation of the DMSO molecules and their geometrical arrangements in the first solvation shells of the ions (Na+, Ca2+, Cl−). A preferential formation of cyclic dimers with antiparallel alignment between dipole moments of nearest-neighbour molecules in the pure solvent is found. Geometrical models of the first coordination shells of the ions in ‘infinitely dilute solutions’ are proposed. Ion-ion RDFs in NaCl-DMSO and CaCl2-DMSO solutions reveal the presence of both solvent separated (SSIP) and contact (CIP) ion pairs. The structures of the solvation shells of such ion pairs are also discussed.

Computer simulation of Na+ and Cl− ions solvation in aqueous mixtures of formamide

Abstract Molecular dynamics computer simulations have been performed for Na+/Cl−-water-formamide mixtures over the whole concentration range of the mixed solvent. It is shown that both Na+ cation and Cl− anion are preferentially solvated by formamide even if only small amounts of formamide are present in the mixture. The Walden products for Na+ and Cl− ions in binary mixture are calculated. It is concluded that the attempts of the direct correlation between the Walden product function shape and the specific ion solvation shell structure might not be successful.

Ionic and hydrogen bonding dynamics coupled to optical and thermal electron transfer: temperature, pressure and viscosity effects

Abstract Electron transfer processes, along with the coupling to the solvent continuum, can also be essentially coupled to, and controlled by the ionic and specific hydrogen bonding dynamics, occuring within the vicinity of the reaction zone. The hexacyanoferrate species, Fe(CN) 6 4−/3− , and related cyanometalate couples lend credence to their consideration as model redox systems suitable for studying the role of such medium dynamics. Recent experimental findings for the temperature, pressure and solvent composition control of three physically related paths for electron transfer (thermal homogeneous, thermal electrochemical, and optically induced homogeneous) detected for this system, allow for the discrimination of microscopic mechanisms responsible for different catalytic effects of electrolyte cations and the solvent control effect of added dextrose. It is argued that for an adequate description of the overall catalytic effect of electrolyte cations, three different factors, viz., the preequilibrium effect of ionic atmosphere, the dynamic effect of stoichiometric ion reorganization, and the superexchange effect of a bridging counter ion should be taken into consideration. The latter two effects may also be “inhibiting”, depending on the composition and geometry of the reactive ionic associate in the precursor and transition states, and the symmetry requirements for the latter. Variation of these three factors in the cases of three different electron exchange paths, leads to the observed interplay of kinetic constants and activation parameters (as well as the CT-band parameters). Participation of the specifically hydrogen bonded water in the first solvation shell of cyanometalate ions, detectable through the differential O-D overtone spectroscopy of heavy water, may be responsible for other peculiarities of the considered redox couple, viz., the solvent isotope effects detected for the homogeneous thermal kinetic constant, the CT-band maximum shift for the optically induced process, and the solvent friction mechanism for the thermal electrochemical process.

Detailed Study of Potassium Solvation Using Molecular Dynamics Techniques

Molecular dynamics simulations are carried out to examine the solvation properties and the ion-solvation shell exchange process of the K+ ion in liquid water, chloroform, and carbon tetrachloride. The solvent molecules are found to form well-defined solvation shells around the K+ ion and show a preferred orientational order toward the ion. The induced dipole moment distribution of K+ becomes broader and shifts to a larger average value from chloroform to carbon tetrachloride to water. It is observed that the K+ ion diffuses more rapidly in the aqueous phase than in liquid chloroform and carbon tetrachloride. We have also evaluated both ion and first solvent shell velocity autocorrelation functions and the residence time autocorrelation functions for the ion in water, chloroform, and carbon tetrachloride. The residence time is found to be 9.4 ps for water and about 30 ps for both chloroform and carbon tetrachloride. By use of a constrained molecular dynamics technique, the first solvation shell exchange pr...

Molecular dynamics studies of NaCl solutions in methanol–water mixtures.: An effect of NaCl on hydrogen bonded network

Hydrogen bonds of methanol and water molecules in binary solvents and in ternary solutions containing NaCl have been investigated by molecular dynamics simulation. Methanol and water molecules have been described using 3-site flexible models. Average numbers of the H-bonds were computed using geometric and energetic definitions. Special attention has been paid to the influence of the mixture composition and addition of NaCl on H-bond statistics and energies. Increase of the methanol concentration causes a decrease of the average H-bond numbers and an increase of the H-bond strength. The influence of NaCl on the H-bonded network is limited. An NaCl effect is evident only on the molecules in the solvation shells of ions.

Properties of 1-1 electrolytes solutions in ethylene glycol at temperatures from 5 to 175°C Part 2Limiting ion conductances and ion–molecule interactions

From previous experimental data (A. V. Lebed, O. N. Kalugin and I. N. Vyunnik, J. Chem. Soc., FaradayTrans., 1998, 94, 2097.), the limiting conductances of Li+, Na+, K+, Rb+, Cs+, Me4N+, Et4N+, Bu4N+, F−, Cl−, Br− and BPh4− ions and the structure-dynamical parameters of ion–molecule interaction were evaluated in the framework of the model proposed by the authors. The thicknesses of the ion solvation shell (Gurney co-sphere) have also been obtained. It was shown that single-charged ions with the smallest radii (Li+, Na+, F−), moderate radii (K+, Rb+, Cs+, Cl−, Br−) and large complex ions (Me4N+, Et4N+, Bu4N+, BPh4−), have different mechanisms of influence on the solvent structure and dynamics. It is concluded that the temperature dependence of near-solvation parameters is caused by a significant change in the solvent hydrogen-bond net.

Molecular dynamics simulations of Na + and Cl − ions solvation in aqueous mixtures of formamide

Molecular dynamics computer simulations have been performed for Na +/Cl −-water-formamide mixtures over the whole concentration range of the mixed solvent. It is shown that both Na + cation and Cl − anion are preferentially solvated by formamide even if only small amounts of formamide are present in the mixture. Translational diffusion and vibrational motions of ion solvation shell molecules correlate strongly with the local composition of solvent near the ions. Librational motion of solvate molecules is determined rather by H-bonding than spatial solvent arrangements. The ionic preferential solvation phenomena manifest themselves in the reduction of the average number of H-bonds of the solvation shell molecules compared to bulk solvent molecules. The mobility of the Na + cation depends strongly on the immediate environment of the ion, whereas for Cl − mainly solvation shell-bulk processes control the dynamics of ion movement. It is concluded that the ionic mobility is determined to a large extent by first shell processes and by molecular motions in the time scale below the time of Debye relaxation.

Thermochemical investigations of the systems useful for lithium battery construction

Abstract Solution enthalpies of LiClO 4 in 1,2-dimethoxyethane + γ-butyrolactone (1,2-DME + γBL) mixtures and of LiClO 4 , NaI and N,N-dimethylacetamide in 1,2-DME + propylene carbonate (PC) mixtures have been measured. The molar enthalpies of solution, Δ sol H m of the examined electrolytes as a function of the mixed solvent composition exhibit a distinct minimum within the range of high 1,2-DME content, similar to those observed earlier for LiBF 4 and LiPF 6 solutions in 1,2-DME + γ-BL mixtures while the dissolution enthalpy of DMA runs monotonously within the same range of the mixed solvent compositions. The presence of the Δ sol H m minimum of the salts is explained as a resultant of energetic effects of interactions in the solution, connected with the ionic association of the salt, the 1,2-DME + cation complex formation and changes in a composition of the ionic solvation shells

High pressure NMR and electrical conductivity studies of gel electrolytes based on poly(acrylonitrile)

The effect of high pressure on electrical conductivity and NMR in gels prepared from lithium or sodium perchlorate, ethylene carbonate, dimethyl carbonate and poly(acrylonitrile) (PAN) has been determined. The corresponding liquids were also studied. Complex impedance studies at frequencies from 10 to 10 8 Hz and NMR measurements of T 1 were carried out as a function of pressure up to 0.25 GPa. Activation volumes for NMR relaxation and ionic conductivity were calculated from the variable pressure data. Both activation volumes were found to be approximately the same in the liquids. For the gels, however, the NMR activation volumes are the same or lower than for the liquid while the electrical conductivity pressure dependences are larger. The implications of these results are discussed in terms of possible effects of the PAN on the ionic solvation shell.

A thermodynamic analysis of the first solvation shells of alkali and halide ions in liquid water and in the gas phase

Abstract Let n denote the number of water molecules in the nearest-neighbour shell (NS) of an ion J± in liquid water, and denote J± ·nH2O in the gas phase by J± · NSG (g). The standard free energy of hydration ΔG°hyd (J± ·NSG) can then be deduced by a thermodynamic cycle involving ΔG°n for the formation of J± · NSG (g) and ΔG°hyd for the transfer of J± (g) to water. Values of ΔG°hyd (J± ·NSG) for alkali and halide ions are substantial, ranging from 48 % to 86 % of ΔG0 hyd. The values of ΔG0 hyd(J± ·NSG) can be accounted for largely by the calculated work—electrostatic (ΔWelec) and surface (ΔWsurt)—in the process J± ·NSG (g) → J± (aq). ΔWelec is the major contributor. ΔWsurf depends on whether (i) J± ·NSG (g) can be represented by a cluster consisting of the ion and n separate water molecules, or (ii) there is some molecular complex formation within that cluster. In fact, ΔWsurt < 20 kJ mol−1 for the alkali ions and of the order of 100 kJ mol−1 for the halide ions. A reasonable case can be built that the alkali ions and some of the n water molecules form molecular complexes while the anions are better represented by case (i).

The first solvation shell of magnesium and calcium ions in a model nucleic acid environment: an ab initio study.

The interaction of organophosphate anions with divalent metal ions is central to many biological catalytic events. While experimental structural studies can give insight into the likely geometries that can be adopted, quantum mechanics allows for a more complete exploration of the competing forms. Ab initio quantum mechanical calculations have been performed on a series of complexes comprised of dimethyl phosphate, a divalent metal ion (either Mg(II) or Ca(II)) and water of hydration. An additional series of complexes were studied that included a Cl(I) ion to provide for charge neutrality. The most stable orientation of the hydrated metal ion complexed with the phosphate anion occurs when the metal ion is in a unidentate, rather than bidentate, orientation. The question of whether the divalent metal ion is located in the phosphinyl (-PO2(-)-) plane depends on the identity of the divalent metal ion and on the charge state of the complex.

Molecular dynamics studies on the structure of methanol-water solutions of NaCl

Molecular dynamics simulations of NaCl solutions in water and in two methanol-water mixtures with methanol concentrations of 10 and 90 mol% at room temperature have been performed. Three-site flexible models of water and of methanol have been applied. The structural properties of the systems have been discussed on the basis of radial and angular distribution functions, the orientation of the solvent molecules and their geometrical arrangement in the solvation shells of ions. A preferential solvation of ions has been found. In the methanol-deficit mixture cations and anions are preferentially solvated by methanol molecules. In the methanol-rich system no preferential solvation of cations has been found, but the anions are solvated only by the methanol molecules.

Influence of the hydration shell structure on the electric field close to an ion

The influence of the ionic solvation shell structure on the ionic electric field of F − at infinite dilution in aqueous solution is studied by means of NpT Monte Carlo simulation data performed at 1.0 atm and 298 K on TIP4P modeled water. The first step of the study is the determination of the solvation shell structure that was performed using two, three and four particle distribution functions or frequency distributions. The solvation shell structure just established was then used to determine the energetic profiles of proof charges in the neighborhood of the ion. Its structure does not seem to present a definite influence on the electrostatic interactions that are governed by collective and indirect interactions between the electrostatic charge of the ion and the proof charges.

From ionic aqueous solvation shell to bulk fluid: A structural-energetic stability problem

The aqueous solvation shell structures for two ions, Li+ and F−, were obtained through Monte Carlo simulations of systems consisting of one ion immersed in water, using the four points transferable intermolecular potential (TIP4P). The clear definition of the tridimensional structure of the solvation shells of these ions, both constituted by four water molecules, permits the calculation of the solvent molecules’ energy in the immediate vicinity of the central ion. A pseudopotential of mean force and the mean energy per molecule were also calculated. The energy profiles are easily related to the formation and the stability of the interface ion–solvent molecules. The stability of the solvation shell is due to (i) an energy difference between the molecules in the solvation shell and in the bulk phase greater, in absolute value, than the thermal energy, and (ii) an energy barrier that separates the solvation shell from the bulk. In general, most of the cationic aqueous solvation shells are similar because the cations are small ions and because the water molecule dipole vector is oriented to the cation’s center. On the other hand, the anionic aqueous solvation shells are complex structures due to the larger size of the anions and due to the fact that the typical approximate collinearity anion–H–O perturbs strongly the solvent immediate neighborhood. These facts result in monolayer cationic and bilayers anionic solvation shells. These observations were confirmed by the structure of the solvation shells of several different ions and their energies.

Structural characteristics of the chlorine ion solvation shell in infinitely dilute aqueous solution

The chlorine ion aqueous solvation shell structure was found, by Monte Carlo simulations, to be constituted by three to six molecules. Three more expressive structures were identified: one with azimuthal angles near to 120° and the other two with angles equal to 90° and 180°. The radius of the solvation shell and the angle oxygen-hydrogen-chlorine ions are in good agreement with experimental diffraction results.

Solvation of lithium ions in mixtures of tetraethylene glycol dimethyl ether and propylene carbonate

The effect of propylene carbonate (PC) on the solvation shell of lithium ions in solutions of lithium perchlorate and lithium picrate in tetraethylene glycol dimethyl ether (TEGDME) was studied as a function of PC content and temperature using 13C NMR, conductance, and optical and infrared spectroscopy. The results show that while PC promotes the dissociation of the electrolyte into free ions, it is at the same time a relatively poor cation chelating agent, and most of the cations remain bound to TEGDME even up to 80 mass % (90 mol%) PC. The ramifications of these observations for PC-plasticized lithium—polymer electrolytes are discussed.

Investigation of very fast motions in electrolyte solutions by far infrared spectroscopy

The far infrared spectra of solutions of a number of alkali halide salts in protic (methanol) and dipolar aprotic solvents (acetonitrile, acetone, dimethyl sulfoxide) have been investigated in the frequency range 25–650 cm−1. Special attention has been paid to detect weak bands in making the difference between the absorption spectrum of the solution and that of the pure solvent. These spectra are characterized by several absorption peaks spread all over the FIR spectral range. Introducing a chemical model of the electrolyte solution, it is shown that the time correlation function at the origin of the absorption phenomenon is governed essentially by three processes, namely, (i) the ion-induced dipole mechanism, (ii) the ionic association, and (iii) the relaxation of the solvent molecules within the ionic solvation shell. A band shape analysis, based upon the Mori theory, permits us to identify the molecular motions which give rise to specific frequency modes on the absorption spectrum. In particular, the role played by the time scale separation between the fast dynamics occuring within the first solvation shell of ionic species and the time spent to interconvert inner and outer solvation shells, is emphasized.

Ionic Mobilities at Infinite Dilution: Structural Aspects

AbstractEmpirical correlations between ionic mobilities and parameters characteristic of the solvent molecules motions for a variety of single ions and 1‐1 electrolytes at infinite dilution in 19 one‐component solvents and 8 aqueous‐nonaqueous mixed solvents are found, presented and discussed. The parameters are the self‐diffusion coefficient, the dipole relaxation time, the rotational correlation time of various NMR vectors, and the residence time of solvent molecules in the first ion solvation shell. The residual friction coefficients are considered in a more theoretical vein.

Temperature dependence of conductance of the Li+, Cs+, and Cl− ions in water: Molecular dynamics simulation

The continuum theory of Hubbard–Onsager predicts for the friction coefficient Δζ (defined as a difference between friction coefficient and the slip Stokes friction coefficient) the following behavior: Δζ&amp;gt;0 and ∂Δζ/∂T&amp;lt;0. In case of the Li+ ion in water these predictions are confirmed by experiment. In contrast, experimental observations on Cs+ and Cl− ions in water show that at low temperatures Δζ&amp;lt;0 and ∂Δζ/∂T&amp;gt;0. To explain the observed behavior of Δζ(Cs+), the passage through cavities (PTC) mechanism was proposed by Nakahara et al. The molecular dynamics computer simulation technique was used to find out if the PTC mechanism is responsible for the nonclassical behavior of Δζ. No passage through cavities was observed in the computer simulations. Computer simulations were performed on the Li+ ion in water, the Cs+ ion in water, and the Cl− ion in water at temperatures T=268 K and T=298 K. Our results indicate that the observed behavior of Δζ for Li+, Cl−, and Cs+ ions is related to the difference in the reorientation times of water molecules in the ion solvation shell and in the bulk.

Composition of the solvation shell of paramagnetic ions of metals in aqueous-organic mixtures according to data from NMR and some other methods

Experimental data on the cobalt(II), nickel(II), and some other ions in mixed aqueous-organic solvents obtained with the aid of PMR, optical spectroscopy, and other methods 9 been systematized. The dependence of the composition of the solvation shell of the ions indicated on the composition of the solvent has been analyzed. A review of the PMR shifts and the lifetimes of the ligands in mixedligand solvento complexes of cobalt(II) and nickel(II) has been given.