Solvation shells, outer

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. ... 

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... 

Cations and anions with a strong solvation shell retain their solvation shell and thus interact with the electrode surface only through electrostatic forces. Since the interaction is exclusively electrostatic, the amount of these ions at the interface is defined by the electrostatic bias between the sample and the counter electrodes and independent from the chemical properties of the electrode surface non-specific adsorption. Considering the size effect of their hydration shell, these ions are able to approach the electrode to a distance limited by the size of the solvation shell of the ion. The center of these ions at a distance of closest approach defined by the size of the solvation shell is called the outer Helmholtz layer. The electrode surface and the outer Helmholtz layer have charges of equal magnitude but opposite sign, resulting in the formation of an equivalent of a plate condenser on a scale of a molecular layer. Helmholtz proposed such a plate condenser on such a molecular scale for the first time in the middle of the nineteenth century. 

The equilibria considered up to now have all involved inner sphere complexes. There is the possibility that an inner sphere complex may react with free ligands in solution this includes the solvent itself, to give an outer sphere complex where the ligand enters the secondary solvation shell of the inner sphere complex. If the two species involved in this type of interaction are of opposite sign, which is the situation where this type of complex formation is expected to be most effective, the outer sphere complex is called an ion pair. Fuoss65 has derived an expression (equation 38) for the ion pair formation constant, XIP, from electrostatic arguments ... 

To summarize, it seems likely that enzymes can stabilize ion pairs and other charge distributions more effectively than water can because the enzyme has dipoles that are kept oriented toward the charge, whereas water dipoles are randomized by outer solvation shells interacting with bulk solvent.28... 

Pratt and co-workers have proposed a quasichemical theory [118-122] in which the solvent is partitioned into inner-shell and outer-shell domains with the outer shell treated by a continuum electrostatic method. The cluster-continuum model, mixed discrete-continuum models, and the quasichemical theory are essentially three different names for the same approach to the problem [123], The quasichemical theory, the cluster-continuum model, other mixed discrete-continuum approaches, and the use of geometry-dependent atomic surface tensions provide different ways to account for the fact that the solvent does not retain its bulk properties right up to the solute-solvent boundary. Experience has shown that deviations from bulk behavior are mainly localized in the first solvation shell. Although these first-solvation-shell effects are sometimes classified into cavitation energy, dispersion, hydrophobic effects, hydrogen bonding, repulsion, and so forth, they clearly must also include the fact that the local dielectric constant (to the extent that such a quantity may even be defined) of the solvent is different near the solute than in the bulk (or near a different kind of solute or near a different part of the same solute). Furthermore... 

Figure 4.27 Charges representing the ASEP generated by the outer solvation shell.

It is conventional to classify electrochemical reactions as outer-sphere and inner-sphere. The former involve the outer coordination sphere of a reacting ion. Thus, little if any change inside the ion solvate shell occurs they proceed without breaking-up intramolecular bonds. But in the latter, involving the inner coordination sphere, electron transfer is accompanied by breaking up or formation of such bonds. Often the inner-sphere reactions are complicated by adsorption of reactants and/or reaction products on the electrode surface. The electron transfer in the Fc(CN)62 /4 system is example of an outer-sphere reaction (with due reservation for some complications... 

For the [Li(NH3)5]+ cluster, the stabilisation gained in forming the bipyramidal complex is smaller than that for the structure on which one molecule is placed out of the first solvation shell. In principle, the fifth molecule, which cannot interact directly with the ion, should have a considerably less marked stabilizing effect. However, because the molecules in the coordination shell are strongly polarized and distorted by the ion, the hydrogen-bonding with the outer molecule is stronger. 

The role of the outer solvation shell in mixed solvents was also studied using the CoEn system as a model [302] (En = ethylenediamine). In this system the inner sphere of the substrate (CoEn ) and the product (CoEnl ) was not changed in the course of the one-electron electrode reaction. Therefore, the changes in the rate constant (determined by chronocoulometric method), observed when the composi-... 

The terms inner sphere and outer sphere are sometimes used to distinguish between the contributions to the solvent response associated with motions close to the charge transfer centers (e.g. the first solvation shell) and the bulk of the solvent, respectively. Intramolecular motions are in this sense part of the inner sphere response. 

This equation actually describes the interaction between the ion and the first solvation shell. Ions frequently form a very stable "aquo complex with H2O molecules, for example an Fe(H2O)6 complex. Besides this inner sphere interaction there is also an outer sphere one, leading to a corresponding arrangement of the H2O dipoles around the ions. The outer sphere interaction is given by the energy required if an ion with the inner solvation shell (radius /j -1- z-soi) is transferred from a vacuum into the solution, as derived by Born using the continuum model ... 

Outer Helmholtz Plane (OHP). Later Grahame [8] distinguished between specifically adsorbed ions (those who loose their solvation shell) and non-specifically adsorbed ions. The double layer consists of three different regions, separated by the Inner Helmholtz Plane (IHP), which passes through the location of specifically adsorbed... 

The influence of an ion in an aqueous electrolyte solution on the structure of liquid water can be pictured spatially as a localized perturbation of the tetrahedral configuration shown in Fig. 2.2. In the region of the liquid nearest the ion the water molecules are dominated by SagiZ ectro-stricted mass called the primary solvation shell. In the next outer region, the water molecules intefaCT v ldy with the ion and form a structure known as the secondary solvation shell or second-zone structure. Beyond the second zone, the structure of the liquid is indistinguishable from that in the pure bulk phase. 

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