Hydration spheres

Roberts J E and Schnitker J 1993 Ionic quadrupolar relaxation in aqueous solution—dynamics of the hydration sphered. Rhys. Chem. 97 5410-17... 

For many practically relevant material/environment combinations, thennodynamic stability is not provided, since E > E. Hence, a key consideration is how fast the corrosion reaction proceeds. As for other electrochemical reactions, a variety of factors can influence the rate detennining step. In the most straightforward case the reaction is activation energy controlled i.e. the ion transfer tlrrough the surface Helmholtz double layer involving migration and the adjustment of the hydration sphere to electron uptake or donation is rate detennining. The transition state is... 

The reaction corresponds to a proton transfer and not to a net formation of ions, and thus the AS is of minor importance in the whole series, especially for the two t-Bu derivatives. This last effect is believed to be due to a structure-promoting effect of the bulky alkyl groups in the disordered region outside the primary hydration sphere of the thiazolium ion (322). 

In the first step the hydrated ion and ligand form a solvent-separated complex this step is believed to be relatively fast. The second, slow, step involves the readjustment of the hydration sphere about the complex. The measured rate constants can be approximately related to the constants in Scheme IX by applying the fast preequilibrium assumption the result is k = Koko and k = k Q. However, the situation can be more complicated than this. - °... 

A. Hydration energy profile, using the Bom formalism (Eqn. 1), shows the drop of ion self energy as a function of the radius of a hydration sphere. Note that even with a hydration shell of 10 A radius not all of the hydration energy is obtained. 

Hydration and Pu oxidation states. 216-29 Hydration sphere, actinide cation and... 

The water molecules in the inner hydration sphere can undergo dissociation reactions just as water molecules far from a dissolved metal ion... 

Fig. 15-2 Comparison of water dissociation in bulk solution (a) and in the hydration sphere of a metal ion (b). Exchange of water of hydration for a chloride ion (c) forms the Me-Cl complex (from Manahan, 1979).

On this basis = 0.0170 sec , = 0.645 sec , and K = 0.739 mole.P at 25 °C. The corresponding activation parameters were determined also by Es-penson. By a method involving extrapolation of the first-order rate plots at various wavelengths to zero time, the absorption spectrum of the intermediate was revealed (Fig. 1). Furthermore, the value of K obtained from the kinetics was compatible with that derived from measurements on the acid dependence of the spectrum of the intermediate. Rate data for a number of binuclear intermediates are collected in Table 2. Espenson shows there to be a correlation between the rate of decomposition of the dimer and the substitution lability of the more labile metal ion component. The latter is assessed in terms of the rate of substitution of SCN in the hydration sphere of the more labile hydrated metal ion. 

By contrast, anions have larger radii and tend to be more weakly hydrated. In addition, they are able to form relatively strong ionic/covalent bonds to the surface of the metal electrode and, as a result, frequently find it energetically feasible to shed their inner hydration sphere, or at least part of it, and adsorh directly on the surface. The plane formed hy the nuclei of anions directly adsorbed on the metal surface is termed the inner Helmholtz plane (IHP). 

Figure 4.1 Copper sulfate pentaquo complex. In solution, CuS04 exists as a Cu2 + ion in octahedral co-ordination surrounded by the S042- ion and five water molecules orientated so that the oxygen atom points towards the copper ion. It is the effect of this hydration sphere on the electronic orbital structure of the copper which gives rise to d-d band transitions, and hence the blue color of the solution.

In contrast to the other three cations, Mg2+ has a much slower exchange rate of water in its hydration sphere (Table 10.1). Mg2+ often participates in structures, for example in ATP-binding catalytic pockets of kinases and other phosphoryl-transferase enzymes, where... 

As an example, infrared spectroscopy has shown that the lowest stable hydration state for a Li-hectorite has a structure in which the lithium cation is partially keyed into the ditrigonal hole of the hectorite and has 3 water molecules coordinating the exposed part of the cation in a triangular arrangement (17), as proposed in the model of Mamy (J2.) The water molecules exhibit two kinds of motion a slow rotation of the whole hydration sphere about an axis through the triangle of the water molecules, and a faster rotation of each water molecule about its own C axis ( l8). A similar structure for adsorbed water at low water contents has been observed for Cu-hectorite, Ca-bentonite, and Ca-vermiculite (17). 

Our model for the adsorption of water on silicates was developed for a system with few if any interlayer cations. However, it strongly resembles the model proposed by Mamy (12.) for smectites with monovalent interlayer cations. The presence of divalent interlayer cations, as shown by studies of smectites and vermiculites, should result in a strong structuring of their primary hydration sphere and probably the next nearest neighbor water molecules as well. If the concentration of the divalent cations is low, then the water in interlayer space between the divalent cations will correspond to the present model. On the other hand, if the concentration of divalent cations approaches the number of ditrigonal sites, this model will not be applicable. Such a situation would only be found in concentrated electrolyte solutions. 

The influence of hydration on the reactivity of anions is much more evident in the case of OH. In the chlorobenzene-aqueous NaOH system the hydration sphere of tetrahexylammonium hydroxide dissolved in the organic phase progressively decreases from 11 to 3.3 water molecules when the base concentration is raised from 15 to 63%. This leads to an enhanced reactivity of OH which was measured in the Hofmann elimination (Equation 3). In the examined ranges of NaOH concentrations the reactivity increased up to more than four orders of magnitude (Table I). Although the dehydration of OH is... 

Since sialic acid is dissociated at normal vaginal pH, it contains a large hydration sphere that strongly attracts cationic substances. Thus, cationic polymers were designed, synthesized, and screened to bioadhere to these sites. 

Often, it is difficult to distinguish definitely between inner sphere and outer sphere complexes in the same system. Based on the preceding discussion of the thermodynamic parameters, AH and AS values can be used, with cation, to obtain insight into the outer vs. inner sphere nature of metal complexes. For inner sphere complexation, the hydration sphere is disrupted more extensively and the net entropy and enthalpy changes are usually positive. In outer sphere complexes, the dehydration sphere is less disrupted. The net enthalpy and entropy changes are negative due to the complexation with its decrease in randomness without a compensatory disruption of the hydration spheres. 

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