Solvation shells

The term solvation shell only describes a region of solvent around the ion. Simply being part of the shell does not imply anything about the interactions between the ion and the water molecules of the shell, or the strength of these interactions. Likewise it says nothing about the state and stmcture of the water. 

This is the region of the solvent in which the molecules are immediate nearest neighbours of the ion, and it is almost certain that they will have properties which are distinct from the molecules of bulk water. 

The next region of the solvent which lies further out is described as consisting of next nearest neighbours. This shell is also likely to be distinct from bulk water. 

These are water molecules further out from the ion, and they may or may not have properties different from bulk water. 

This constitutes regions of the water which are totally unmodified by the ions of the electrolyte. 

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The FI2O molecules of these aquo-complexes constitute the iimer solvation shell of the ions, which are, in turn, surrounded by an external solvation shell of more or less uncoordinated water molecules fomiing part of the water continuum, as described in section A2.4.2 above. Owing to the difference in the solvation energies,... 

Similarly, changes must take place in the outer solvation shell diirmg electron transfer, all of which implies that the solvation shells themselves inliibit electron transfer. This inliibition by the surrounding solvent molecules in the iimer and outer solvation shells can be characterized by an activation free energy AG. ... 

Consider now the aquo-complexes above, and let v be the distance of the centre of mass of the water molecules constituting the iimer solvation shell from the central ion. The binding mteraction of these molecules leads to vibrations... 

In our simple model, the expression in A2.4.135 corresponds to the activation energy for a redox process in which only the interaction between the central ion and the ligands in the primary solvation shell is considered, and this only in the fonn of the totally synnnetrical vibration. In reality, the rate of the electron transfer reaction is also infiuenced by the motion of molecules in the outer solvation shell, as well as by other... 

As with SCRF-PCM only macroscopic electrostatic contribntions to the Gibbs free energy of solvation are taken into account, short-range effects which are limited predominantly to the first solvation shell have to be considered by adding additional tenns. These correct for the neglect of effects caused by solnte-solvent electron correlation inclnding dispersion forces, hydrophobic interactions, dielectric saturation in the case of... 

Specific solute-solvent interactions involving the first solvation shell only can be treated in detail by discrete solvent models. The various approaches like point charge models, siipennoleciilar calculations, quantum theories of reactions in solution, and their implementations in Monte Carlo methods and molecular dynamics simulations like the Car-Parrinello method are discussed elsewhere in this encyclopedia. Here only some points will be briefly mentioned that seem of relevance for later sections. 

In either case, the structure of the solvation shell has to be calculated by otiier methods supplied or introduced ad hoc by some fiirther model assumptions, while charge distributions of the solute and within solvent molecules are obtained from quantum chemistry. 

The quality of the results that can be obtained with point charge or dipole models depends critically on the input solvation shell structure. In view of the computer power available today, taking the most rigorous route... 

R), i.e. there is no effect due to caging of the encounter complex in the common solvation shell. There exist numerous modifications and extensions of this basic theory that not only involve different initial and boundary conditions, but also the inclusion of microscopic structural aspects [31]. Among these are hydrodynamic repulsion at short distances that may be modelled, for example, by a distance-dependent diffiision coefficient... 

The analysis of recent measurements of the density dependence of has shown, however, that considering only the variation of solvent structure in the vicinity of the atom pair as a fiinction of density is entirely sufficient to understand tire observed changes in with pressure and also with size of the solvent molecules [38]. Assuming that iodine atoms colliding with a solvent molecule of the first solvation shell under an angle a less than (the value of is solvent dependent and has to be found by simulations) are reflected back onto each other in the solvent cage, is given by... 

Chambers C C, G D Hawkins, C J Cramer and D G Tmlilar 1996. Model for Aqueous Solvation Ba sed on Class IC Atomic Charges and First Solvation Shell Effects. Journal of Physical Chemistry 100 16385-16398. 

The energy of solvation can be further broken down into terms that are a function of the bulk solvent and terms that are specifically associated with the first solvation shell. The bulk solvent contribution is primarily the result of dielectric shielding of electrostatic charge interactions. In the simplest form, this can be included in electrostatic interactions by including a dielectric constant k, as in the following Coulombic interaction equation ... 

Modem understanding of the hydrophobic effect attributes it primarily to a decrease in the number of hydrogen bonds that can be achieved by the water molecules when they are near a nonpolar surface. This view is confirmed by computer simulations of nonpolar solutes in water [15]. To a first approximation, the magnimde of the free energy associated with the nonpolar contribution can thus be considered to be proportional to the number of solvent molecules in the first solvation shell. This idea leads to a convenient and attractive approximation that is used extensively in biophysical applications [9,16-18]. It consists in assuming that the nonpolar free energy contribution is directly related to the SASA [9],... 

Finally, we want to describe two examples of those isolated polymer chains in a sea of solvent molecules. Polymer chains relax considerably faster in a low-molecular-weight solvent than in melts or glasses. Yet it is still almost impossible to study the conformational relaxation of a polymer chain in solvent using atomistic simulations. However, in many cases it is not the polymer dynamics that is of interest but the structure and dynamics of the solvent around the chain. Often, the first and maybe second solvation shells dominate the solvation. Two recent examples of aqueous and non-aqueous polymer solutions should illustrate this poly(ethylene oxide) (PEO) [31]... 

The coordination number is the number of solvent molecules in the primary solvation shell. This quantity can be estimated (for ions) by conductance measurements and by... 

Because the key operation in studying solvent effects on rates is to vary the solvent, evidently the nature of the solvation shell will vary as the solvent is changed. A distinction is often made between general and specific solvent effects, general effects being associated (by hypothesis) with some appropriate physical property such as dielectric constant, and specific effects with particular solute-solvent interactions in the solvation shell. In this context the idea of preferential solvation (or selective solvation) is often invoked. If a reaction is studied in a mixed solvent. 

Solvatochromic shifts are rationalized with the aid of the Franck-Condon principle, which states that during the electronic transition the nuclei are essentially immobile because of their relatively great masses. The solvation shell about the solute molecule minimizes the total energy of the ground state by means of dipole-dipole, dipole-induced dipole, and dispersion forces. Upon transition to the excited state, the solute has a different electronic configuration, yet it is still surrounded by a solvation shell optimized for the ground state. There are two possibilities to consider ... 

Let (Xgr and jXex be the dipole moments of the ground and excited states. Then if iXgr > the less polar excited state is surrounded by a solvation shell... 

The mixed solvent models, where the first solvation sphere is accounted for by including a number of solvent molecules, implicitly include the solute-solvent cavity/ dispersion terms, although the corresponding tenns between the solvent molecules and the continuum are usually neglected. Once discrete solvent molecules are included, however, the problem of configuration sampling arises. Nevertheless, in many cases the first solvation shell is by far the most important, and mixed models may yield substantially better results than pure continuum models, at the price of an increase in computational cost. 

The substituent effects on the H-bonding in an adenine-uracil (A-U) base pair were studied for a series of common functional groups [99JPC(A)8516]. Substitutions in the 5 position of uracil are of particular importance because they are located toward the major groove and can easily be introduced by several chemical methods. Based on DFT calculation with a basis set including diffuse functions, variations of about 1 kcal/mol were found for the two H-bonds. The solvent effects on three different Watson-Crick A-U base pairs (Scheme 100) have been modeled by seven water molecules creating the first solvation shell [98JPC(A)6167]. 

Micelles and reversed micelles are able to solubilize substances which are insoluble in the bulk phase of the system considered. This solubilization is due to a solvation by the amphiphile and concomitantly a change in the order of the solubilized molecules may occur as a consequence of its modified solvation shell. In this sense reversed micelles of detergents in hydrophobic solvents with solubilized water in the core are... 

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