Hydrate ions

Structural investigations of metal-ion hydration have been carried out by spectroscopic, scadering and diffraction teclmiques, but these teclmiques do not always give identical results since they measure in different timescales. There are tliree distinct types of measurement ... 

Rashin A A and B Honig 1985. Reevaluation of the Bom Model of Ion Hydration. Journal of Physici Chemistry 89 5588-5593. 

If a substance is to be dissolved, its ions or molecules must first move apart and then force their way between the solvent molecules which interact with the solute particles. If an ionic crystal is dissolved, electrostatic interaction forces must be overcome between the ions. The higher the dielectric constant of the solvent, the more effective this process is. The solvent-solute interaction is termed ion solvation (ion hydration in aqueous solutions). The importance of this phenomenon follows from comparison of the energy changes accompanying solvation of ions and uncharged molecules for monovalent ions, the enthalpy of hydration is about 400 kJ mol-1, and equals about 12 kJ mol-1 for simple non-polar species such as argon or methane. 

The ionic mobility and diffusion coefficient are also affected by the ion hydration. The particle dimensions calculated from these values by using Stokes law (Eq. 2.6.2) do not correspond to the ionic dimensions found, for example, from the crystal structure, and hydration numbers can be calculated from them. In the absence of further assumptions, diffusion measurements again yield only the sum of the hydration numbers of the cation and the anion. 

Grossfield A (2005) Dependence of ion hydration on the sign of the ion s charge. J Chem Phys... 

B. Thermochemistry of Ion-Molecule Complexes and Specifically Ion-Hydrates Obtained from Equilibrium Determinations... 

Pratt, L. R., Rempe, S. B., Topol, I. A., and Burt, S. K. (2000). Alkali metal ion hydration and energetics of selectivity by ion-channels. Biophys.J. 78, P2057-P2057. 

Hummer, G., Fast-growth thermodynamic integration results for sodium ion hydration, Mol. Simul. 2002, 28, 81-90... 

With M as Be, Mg, Ca, Sr, or Ba, the reaction has a positive free energy change due to the overcompensating effect of ion hydration energy relative to electron affinity. Therefore, these ions do not react with eh. ... 

In the virial methods, therefore, the activity coefficients account implicitly for the reduction in the free ion s activity due to the formation of whatever ion pairs and complex species are not included in the formulation. As such, they describe not only the factors traditionally accounted for by activity coefficient models, such as the effects of electrostatic interaction and ion hydration, but also the distribution of species in solution. There is no provision in the method for separating the traditional part of the coefficients from the portion attributable to speciation. For this reason, the coefficients differ (even in the absence of error) in meaning and value from activity coefficients given by other methods. It might be more accurate and less confusing to refer to the virial methods as activity models rather than as activity coefficient models. 

The enthalpies of solution and solubilities reviewed here provide much of the experimental information required in the derivation of single-ion hydration and solvation enthalpies, Gibbs free energies, and entropies for scandium, yttrium, and lanthanide 3+ cations. 

An interesting combined use of discrete molecular and continuum techniques was demonstrated by Floris et al.181,182 They used the PCM to develop effective pair potentials and then applied these to molecular dynamics simulations of metal ion hydration. Another approach to such systems is to do an ab initio cluster calculation for the first hydration shell, which would typically involve four to eight water molecules, and then to depict the remainder of the solvent as a continuum. This was done by Sanchez Marcos et al. for a group of five cations 183 the continuum model was that developed by Rivail, Rinaldi et al.14,108-112 (Section III.2.ii). Their results are compared in Table 14 with those of Floris et al.,139 who used a similar procedure but PCM-based. In... 

The inconvenience of the convention Af//°(H+, ao) = 0 is, of course, that it does not provide absolute values of the ion hydration enthalpies—only relative numbers can be evaluated. In other words, it tells nothing about the real size of the difference between the energetics of gas-phase and solution ions. Yet given the uncertainty that still affects the value of Af/7°(H+, ao) [50], the convention is a sensible option. 

Within the general framework of relating solution and gas-phase behaviour, many theoretical approaches have been developed to evaluate ion hydration ... 

---