Primary hydration sphere

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). 

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. 

There is a conceptual model of hydrated ions that includes the primary hydration shell as discussed above, secondary hydration sphere consists of water molecules that are hydrogen bonded to those in the primary shell and experience some electrostatic attraction from the central ion. This secondary shell merges with the bulk liquid water. A diagram of the model is shown in Figure 2.3. X-ray diffraction measurements and NMR spectroscopy have revealed only two different environments for water molecules in solution of ions. These are associated with the primary hydration shell and water molecules in the bulk solution. Both methods are subject to deficiencies, because of the generally very rapid exchange of water molecules between various positions around ions and in the bulk liquid. Evidence from studies of the electrical conductivities of ions shows that when ions move under the influence of an electrical gradient they tow with them as many as 40 water molecules, in dilute solutions. 

The hydration of anions is regarded as being electrostatic with additional hydrogen bonding. The number of water molecules in the primary hydration sphere of an anion depends upon the size, charge and nature of the species. Monatomic anions such as the halide ions are expected to have primary hydration spheres similar to those of monatomic cations. Many aqueous anions consist of a central ion in a... 

Figure 2.3 A diagram showing lhe primary and secondary hydration spheres around an ion...

The above ideas are not limited to species with central metal ions. They apply to the higher oxidation states of non-metallic elements. Many simple anions do exist with primary hydration spheres in which the positive ends of dipoles are attracted to the central negative charge. Table 3.8 gives examples of ions that may be thought about in terms of the hydrolysis of parent hypothetical hydrated ions. 

Figure 4.6 Clustering of water molecules which has significant consequences on the hydration of ions in water. When salts (e.g., NaCl) are added to pure water, ionic bonds of NaCl are broken due to a primary hydration sphere that develops around each ion (hydration)—as shown here for Na+. (From Degens, 1989, with permission.)...

Experimentally the single relaxation observed was attributed to step 3 in the mechanism because (a) theoretical calculation of k 2 and k2 predicted an estimated relaxation frequency inaccessible to the equipment used and (b) step 2 is excluded since the rate of displacement of water from the primary hydration sphere of an anion is independent of the cation. The relaxation data were analyzed by the equation... 

The nature of ions in aqueous solution has been studied using a wide variety of techniques, including X ray and neutron diffraction, and quasielastic neutron scattering, NMR, IR, and UV spectroscopies. The ions are generally considered to have primary and secondary hydration spheres, although there is relatively little quantitative information available concerning the second sphere in solntion. The rate of exchange of the... 

H2O molecules in the hydration spheres with the bnlk H2O solvent varies by many orders of magnitude depending on the metal ion. This, combined with the use of many different experimental techniqnes, has resulted in discrepancies in the literatnre, particnlarly for metal ions with a weakly bonnd, rapidly exchanging hydration sphere. The Na+ cation, for example, has had qnoted values for the primary solvation number ranging from 2 to 13, although a value close to six is generally accepted." ... 

Concentrated salt solutions are a class of solvent whose properties have hardly begun to be appreciated. The ratio of water to salt in these media is so low that the primary hydration number (the number of water molecules about each ion) must be far lower than in dilute solutions. A saturated D2O solution of KF, for example, which is 12.4M with a density of 1.563, has only 3.4 moles of D2O per mole of KF while perhaps eight moles of water per mole of KF (4) are required for the primary hydration sphere in a dilute solution. The enhanced complex formation which necessarily results may lead to a chemistry different from aqueous chemistry. Asprey and Penneman (3) have reported that Am " is both stable and soluble in saturated NH4F, RbF, and KF. 

In outer-sphere electron transfer, the primary hydration spheres remain intact the metal ions are separated by at least two water molecules (or other ligands), and only the electron moves between them. No breaking or formation of chemical bonds occurs, such mechanisms are explained by the Franck-Condon principle. This principle states that, because nuclei are more massive than electrons, an electronic transition takes place, while the nuclei in a molecule are effectively stationary. The principle governs the probabilities of transitions between the vibrational levels of different molecular electronic states. 

The results of ion and water sorption measurements for the two polymers under these solution conditions help to explain this difference. Table 5 " lists the concentrations of various sorbed species and the mole ratio of water to cation/anion in the polymer phase for NaCl and NaOH solution environments. This ratio decreases both in the polymers and in solution with increasing concentration. In solution, the ratio varies from 10.8 to 4.0 over the concentration range of 5-12.5 M NaOH, so that ions in the polymer phase exist in a significantly less aqueous environment compared to the solution phase. As noted by Mauritz and co-workers for perfluorosulfonate membranes, these water contents are insufficient to provide even primary hydration spheres for sodium ions, sorbed anions, and exchange sites, and the likelihood... 

Marcus LFER. Oxidation-reduction reactions involving metal ions occur by (wo types of mechanisms inner- and outer-sphere electron transfer. In the former, the oxidant and reductant approach intimately and share a common primary hydration sphere so that the activated complex has a bridging ligand between the two metal ions (M—L—M ). Inner-sphere redox reactions thus involve bond forming and breaking processes like other group transfer and substitution rcaclions, and transition-state theory applies directly to them. In outer-sphere electron transfer, the primary hydration spheres remain intact. The... 

Already Ostwald (6 ) distinguished primary hydration spheres around organic solutes and a second diffuse hydrate sphere. Bungenberg de Jong assumed coacervate formation as combination of... 

The primary hydration spheres of the cations of the third period, Na, Mg and probably consist of six water molecules, octahedrally... 

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