Structure of solvated ions

This distinction is meaningful if the resultant distribution function is of the type shown in Figure 4.7 (Szwarc, 1965). This figure shows that there is a high probability that the cation and anion are either in contact, separated by a solvent molecule or far apart (Szwarc, 1965). Intermediate positions are improbable. The structure of solvated ion-pairs has been studied by Grunwald (1979) using dipole measurements. 

Grunwald, E. (1979). Structure of solvated ion pairs from electric dipole moments. Journal of Pure and Applied Chemistry, 51, 53-61. 

The dissolution of ionic or polar solutes is explained. by the polarity and the high dielectric constant of water. The solvation of ions allows their separation in solution. The structure of solvated ions is investigated using spectroscopy techniques (EXAFS, XANES, NMR, infrared, Raman), or diffraction and diffusion techniques (X-ray, neutrons) [11,12]. Such techniques have shown that solvated cations and anions are linked to water molecules through the oxygen and hydrogen atoms respectively (Figure 1.4a). 

The structure of the ions, where the bulky phenyl groups surround the central ion in a tetrahedron, lends validity to the assumption that the interaction of the shell of the ions with the environment is van der Waals in nature and identical for both ions, while the interaction of the ionic charge with the environment can be described by the Born approximation (see Section 1.2), leading to identical solvation energies for the anion and cation. 

Figure 2.2 (a) The structure of the electrode/electrolyte interface, assuming a single layer of solvated ions adjacent to the electrode. The distance of closest approach of the ions to the electrode is a, and the ion sheet forms the outer Helmholtz plane (OHP). (b) The variation of the potential as a function of the distance from the metal surface for the interface shown in (a). 

J. M. Lisy, Spectroscopy and structure of solvates alkali metal ions. Int. Rev. Phys. Chem. 16, 267 289 (1997). 

Note Added in Proof After we sent the manuscript to the publishers we became aware of CNDO studies on alkali ion solvation performed by Gupta and Rao 270> and Balasubramanian et al.271 >, which might be of some importance for readers interested in cation solvation by water and various amides. Another CNDO model investigation on the structure of hydrated ions was published very recently by Cremaschi and Simonetta 272> They studied CH5 and CH5 surrounded by a first shell of water molecules in order to discuss solvation effects on structure and stability of these organic intermediates or transition states respectively. 

These points indicate that the continuum theory expression of the free energy of activation, which is based on the Born solvation equation, has no relevance to the process of activation of ions in solution. The activation of ions in solution should involve the interaction energy with the solvent molecules, which depends on the structure of the ions, the solvent, and their orientation, and not on the Born charging energy in solvents of high dielectric constant (e.g., water). Consequently, the continuum theory of activation, which depends on the Born equation,fails to correlate (see Fig. 1) with experimental results. Inverse correlations were also found between the experimental values of the rate constant for an ET reaction in solvents having different dielectric constants with those computed from the continuum theory expression. Continuum theory also fails to explain the well-known Tafel linearity of current density at a metal electrode. ... 

Despite the fact that the structure of the interface between a metal and an electrolyte solution has been the subject of numerous experimental and theoretical studies since the early days of physical chemistry," our understanding of this important system is still incomplete. One problem has been the unavailability (until recently) of experimental data that can provide direct structural information at the interface. For example, despite the fact that much is known about the structure of the ion s solvation shell from experimental and theoretical studies in bulk electrolyte solutions, " information about the structure of the adsorbed ion solvation shell has been mainly inferred from the measured capacity of the interface. The interface between a metal and an electrolyte solution is also very complex. One needs to consider simultaneously the electronic structure of the metal and the molecular structure of the water and the solvated ions in the inhomogeneous surface region. The availability of more direct experimental information through methods that are sensitive to the microscopic... 

The main goal of the molecular dynamics computer simulation of ionic solvation and adsorption on a metal surface has been to test the above model and to provide more quantitative information about the different factors that influence the structure of hydrated ions at the interface. Unfortunately, most of the experimental information about these issues has been obtained from indirect measurements such as capacity and current-potential plots, although in recent years in situ experimental techniques have begun to provide an accurate test of the above model. For a recent review of experimental techniques and the theory of ionic adsorption at the water/metal interface, see the excellent paper by Philpott. ... 

Although the potential energy functions can be made to reproduce thermodynamic solvation data quite well, they are not without problems. In some cases, the structure of the ion solvation shell, and in particular the coordination number, deviates from experimental data. The marked sensitivity of calculated thermodynamic data for ion pairs on the potential parameters is also a problem. Attempts to alleviate these problems by introducing polarizable ion-water potentials (which take into account the induced dipole on the water caused by the ion strong electric field) have been made, and this is still an active area of research. 

The structure of the adsorbed ion coordination shell is determined by the competition between the water-ion and the metal-ion interactions, and by the constraints imposed on the water by the metal surface. This structure can be characterized by water-ion radial distribution functions and water-ion orientational probability distribution functions. Much is known about this structure from X-ray and neutron scattering measurements performed in bulk solutions, and these are generally in agreement with computer simulations. The goal of molecular dynamics simulations of ions at the metal/water interface has been to examine to what degree the structure of the ion solvation shell is modified at the interface. 

The gas-phase model would then be tested on condensed phases. In the case of the carbonate ion, the parameters can be used to examine the structure of C02(aq), C032-(aq), and HC03 (aq) as well as the structure of, for example, siderite FeC03 and nahcolite Na(HC03). For the aqueous species, the most instructive comparisons are with the results of ab initio molecular dynamics studies of solvated ions, where the radial distribution functions can be used to check the extent of solvation. Fig. 2, for... 

Fig. 2.6 Typical model of solvated ions in structured solvents such as water and alcohols.

X-ray and neutron diffraction methods and EXAFS spectroscopy are very useful in getting structural information of solvated ions. These methods, combined with molecular dynamics and Monte Carlo simulations, have been used extensively to study the structures of hydrated ions in water. Detailed results can be found in the review by Ohtaki and Radnai [17]. The structural study of solvated ions in lion-aqueous solvents has not been as extensive, partly because the low solubility of electrolytes in 11011-aqueous solvents limits the use of X-ray and neutron diffraction methods that need electrolyte of -1 M. However, this situation has been improved by EXAFS (applicable at -0.1 M), at least for ions of the elements with large atomic numbers, and the amount of data on ion-coordinating atom distances and solvation numbers for ions in non-aqueous solvents are growing [15 a, 18]. For example, according to the X-ray diffraction method, the lithium ion in for-mamide (FA) has, on average, 5.4 FA molecules as nearest neighbors with an... 

The association of a cation that is surrounded by a tight solvation shell with an anion proceeds smoothly until the solvent shell comes into contact with the anion. At this stage either the structure of the ion pair, separated by solvent molecules, is preserved (Figure 7.1a) or the solvation shell is squeezed out in a process that leads to a contact pair. This implies that at least two types of ion pair may coexist in solution, each having its own physical and chemical properties such two-step associations have been revealed by various relaxation experiments. However, ions that weakly interact with the solvent and do not surround themselves with tight solvation shells form contact pairs only. This situation is encountered in poorly solvated liquids and for bulky ions. Those cations that interact strongly with solvent molecules tend to form solvent-separated pairs, especially when combined with large anions. 

The reasons for preferential solvation of Ag ions by acetonitrile in acetonitrile/water mixtures and the solvation shell structure of silver ions have been discussed [251]. 

Structure and dynamics of solvated ions new tendencies of research Tamas Radnai... 

Some recent developments in the research of the structure and dynamics of solvated ions are discussed. The solvate structure of lithium ion in dimethyl formamide and preliminary results on the structure of sodium chloride aqueous solutions under high pressures are presented to demonstrate the capabilities of the traditional X-ray diffiraction method at new conditions. Perspectives of solution chemistry studies by combined methods as e.g. diffraction results with reverse Monte Carlo simulations, are also shown. 

From these data it is suspected that the molecules of the solvate structure of lithium ion might be largely effected by the solvent molecules. Since the solubility of some lithium salts is relatively high in MN-dimethyl formamide (DMF), concentrated solutions can also be examined. In a previous study the solvate structure of lithium has been described in an 1.5 mol dm LiNCS solution in DMF [38]. A new XD measurement has been carried out for a LiCl solution of the same concentration. Table 1 hows the structural parameters for the lithium solvates in both solutions. The structural parameters were determined by a least-squares fitting method (LSQ). After the subtraction of the contributions ascribed to the intramolecular stmcture of the DMF molecules and to the assumed structure around the anions from the total structure function of the solution, the resulted difference curve was approximated by calculated model curves. The result is shown in Figure 1. 

One of the most exciting questions in solution chemistry is concerned with the effect of the pressure on the solvation shell of ions. From conductance data it was suggested that the pressure breaks up the structure in the bulk water and also in the local water structure near the ions. Opposite opinions also exist according to which the increase in pressure leads to an enhancement of the close hydration. MD simulation studies on a concentrated aqueous NaCl solution [23] showed almost no changes in the hydrate structures of the ions even at 10 kbar pressure. On the other hand, the H-bonded network of the water molecules is distorted in a similar way as it was found in pure water [40]. Because of extreme technical difficulties, only a few attempts were to determine the structure of pure solvents at elevated pressures and only one ND experiment is reported on aqueous solutions of a LiCl and a NiClj aqueous solution [24], leading to contradictory results. No XD diffraction studies were reported. 

Solvation structures of Br ion dissolved in 23 solvents have been studied at the Br K-edge using extended X-ray absorption fine structure (EXAFS). The results are summarized... 

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