Ion solvation thermodynamics

Grossfield A, Ren PY, Ponder JW (2003) Ion solvation thermodynamics from simulation with a polarizable force field. J Am Chem Soc 125(50) 15671—15682... 

Grossfield A, Ren P, and Ponder JW. Ion Solvation Thermodynamics for Simulation with a Polarizable Force Field./Am Chem Soc 2003 125 15671-15682. 

At present, intercalation compounds are used widely in various electrochemical devices (batteries, fuel cells, electrochromic devices, etc.). At the same time, many fundamental problems in this field do not yet have an explanation (e.g., the influence of ion solvation, the influence of defects in the host structure and/or in the host stoichiometry on the kinetic and thermodynamic properties of intercalation compounds). Optimization of the host stoichiometry of high-voltage intercalation compounds into oxide host materials is of prime importance for their practical application. Intercalation processes into organic polymer host materials are discussed in Chapter 26. 

The complexation of the alkaline earth metals is reminiscent of the behaviour of several of the naturally occurring antibiotics and, like the latter, the crown often exhibits remarkable selectivity for particular ions. The thermodynamic factors underlying the selectivity of many of the crowns have been studied in some depth and the results related to such parameters as cavity size, number of donor atoms present, possible ring conformations on complex formation and the solvation energies of the various species involved. 

For molecules and molecular ions, such as the cations of 8-methyl-N5-deazapterin and 8-methyl-pterin, the charge distribution (which is represented in MD simulations by a set of discrete atomic charges) will be dependent on the chosen quantum chemical model. Differences in the charge distributions of these cations may influence both the relative binding and solvation thermodynamics. Consequently, we studied the relative solvation thermodynamics of similar DHFR-binding molecular ions.30 Atomic charges... 

To round off the discussion of solvation enthalpies, reference is made here to the articles by Case 38> and by Friedman and Krishnan 27>, who have reviewed the thermodynamic aspects of ion solvation extensively. 

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. 

Various potentiometric indicator electrodes work as sensors for ion solvation. Metal and metal amalgam electrodes, in principle, respond in a thermodynamic way to the solvation energy of the relevant metal ions. Some ion-selective electrodes can also respond almost thermodynamically to the solvation energies of the ions to which they are sensitive. Thus, the main difficulty in the potentiometric study of ion solvation arises from having to compare the potentials in different solvents, even though there is no thermodynamic way of doing it. In order to overcome this difficulty, we have to employ a method based on an extra-thermodynamic assumption. For example, we can use (1) or (2) below ... 

First, immediately after ionization, contact ion pairs are formed, in which no solvent molecules intervene between the two ions that are in close contact. The contact ion pair constitutes an electric dipole having only one common primary solvation shell. The ion pair separated by the thickness of only one solvent molecule is called a solvent-shared ion pair In solvent-shared ion pairs, the two ions already have their own primary solvation shells. These, however, interpenetrate each other. Contact and solvent-shared ion pairs are separated by an energy barrier which corresponds to the necessity of creating a void between the ions that grows to molecular size before a solvent molecule can occupy it. Further dissociation leads to solvent-separated ion pairs Here, the primary solvation shells of the two ions are in contact, so that some overlap of secondary and further solvation shells takes place. Increase in ion-solvating power and relative permittivity of the solvent favours solvent-shared and solvent-separated ion pairs. However, a clear experimental distinction between solvent-shared and solvent-separated ion pairs is not easily obtainable. Therefore, the designations solvent-shared and solvent-separated ion pairs are sometimes interchangeable. Eventually, further dissociation of the two ions leads to free, i.e. unpaired solvated ions with independent primary and secondary solvation shells. The circumstances under which contact, solvent-shared, and solvent-separated ion pairs can exist as thermodynamically distinct species in solution have been reviewed by Swarcz [138] and by Marcus [241],... 

The slopes of these lines provide partial molar enthalpies of solution of 97.5 4.7 and 89.2 2.6 kJ mol1 for NaBr and Nal, respectively, where the thermodynamic reference state is the solid halide. The solvation enthalpies derived from these values are —265.6 9.9 and —225.0 7.9 kJ mol-1 for bromide and iodide ion, respectively. The salts are considered to dissolve in the metal as the dissociated ions, solvated by liquid metal, and the solutions show large deviations from ideal but small deviations from regular behaviour.16 The solubilities of potassium chloride in liquid potassium and in... 

Aqueous Solvation.—A review deals with ion solvation in both aqueous and non-aqueous media and covers thermodynamic aspects, electrical conductivity, viscosity, transport coefficients, and n.m.r. spectra. Factors affecting the... 

In the early 50 s, an ion pair model was introduced by Winstein to rationalize the mechanism and stereochemistry of solvolysis of sulfonates72). This research of carbocationic intermediates and the role of ion solvation equilibrium in reaction mechanisms represents a landmark in the study of charged species. These thermodynamically different ionic species were coined as free ions, contact ion-pairs (c.i.p.), and solvent-separated ion pairs (s.s.i.p.). The ion pair situation can be described as an equilibrium between thermodynamically distinct contact (c.i.p.) and solvent-separated ion pairs (s.s.i.p.) 2-l3 16 The situation should be represented by a continuum of ion-solvation equilibria states in which the two extreme states are the c.i.p. and the s.s.i.p. 2 76) (Eq. 12)... 

Thermodynamic data for electrolyte solvation have been found experimentally for many different electrolytes. Ultimately, one would like to be able to analyze these results further to obtain separate contributions from the cation and anion. However, that is not possible without making an extrathermodynamic assumption. As a result, a scale of single ion solvation parameters has been defined relative to those for the H" " ion. For example, the enthalpy associated with the process... 

Having obtained absolute values of the thermodynamic properties describing ion solvation it is interesting to examine various theories for ion solvation. The best known of these is the Born model, which is described in the following section. 

Initially, some relevant thermodynamic and molecular properties of polar solvents are considered. Then, their dielectric properties are considered in detail. Ion solvation in these solvents is also discussed with emphasis on some non-thermo-dynamic methods of dividing experimentally measured data for electrolytes into contributions for the cation and anion. Finally, the important characteristics of the solvent in its direct interaction with the solute, namely, its acidity and basicity, are also described. 

There are other methods of studying PS which do not depend on the concepts of solvation thermodynamics. Perhaps the earliest treatment of PS of ions in a two-component solvent was presented by Grunwald et al. (1960). This was followed by Covington and Newman (1976, 1988). For a review see Engberts (1979). 

G.A. Krestov, Thermodynamics of Ion Solvation, Ellis Horwood, New York, 1991. 

While the relatively wide energy gap between the participating Ca and H2O orbitals plays an important role in the driving force of this reaction, the thermodynamics associated with ion solvation and gas evolution are vital as well. 

In considering the thermodynamics of ion solvation, two equilibria of great importance are... 

An alternative description of a molecular solvent in contact with a solute of arbitrary shape is provided by the 3D generalization of the RfSM theory (3D-RISM) which yields the 3D correlation functions of interaction sites of solvent molecules near the solute. It was first proposed in a general form by Chandler, McCoy, and Singer [22] and recently developed by several authors for various systems by Cortis, Rossky, and Friesner [23] for a one-component dipolar molecular liquid, by Beglov and Roux [24, 25] for water and a number of organic molecules in water, and by Hirata and co-workers for water [26, 27], metal-water [26, 28] and metal oxide-water [31] interfaces, orientationally dependent potentials of mean force between molecular ions in a polar molecular solvent [29], ion pairs in aqueous electrolyte [30], and hydration of hydrophobic and hydrophilic solutes alkanes [32], polar molecule of carbon monoxide [33], simple ions [34], protein [35], amino acids and polypeptides [36, 37]. It should be noted that accurate calculation of the solvation thermodynamics for ionic and polar solutes in a polar molecular liquid requires special corrections to the 3D-RISM equations to eliminate the electrostatic artifacts of the supercell treatment employed in the 3D-RISM approach [30, 34]. 

Heydweiler A (1925) Optical research on electrolytic aqueous solutions. Phys Z 26 526-556 Hiinenberger P, Reif M (2010) Single-ion Solvation experimented emd theoretical approaches to elusive thermodynamic quantities. Royal Society of Chemistry, Cambridge Jenkins HBD, Thakur KP (1979) Reappraisal of thermochemiced radii for complex ions. J Chem Educ 56 576-577... 

Marcus Y (2012) The viscosity B-coeffricient of the thiocyanate anion. J Chem Eng Data Marcus Y, Ben-Naim A (1985) A study of the structure of water and its dependence on solutes, based on the isotope effects on solvation thermodynamics in water. J Chem Phys 83 4744-4759 Marcus Y, Loewenschuss A (1985) Standard entropies of hydration of ions Annu Rep Part C (Royal Soc Chem, London) 1984, 81-135... 

A somewhat more interesting problem is the implicit solvent potential of mean force (PMF) between two Bom ions. Unlike the single ion solvation, no closed-form analytic solution is available for this system. Instead, it must be either modeled with one of the numerous published series solutions or with numerical calculations. Figure 5 presents a thermodynamic cycle illustrating a typical PB calculation of an ion—ion PMF. The energy of bringing the ion to a distance R in solution is... 

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