Ion-pairs hydration

Figure 10 shows the 23Na MAS NMR spectra of some sodium salt ethylene ionomers at room temperature. A broad peak at about —10 to 12 ppm is clearly observed for all samples. The 23Na NMR spectra of the styrene ionomers have been reported44 (details are given in the next section, see Fig. 14). Peaks are observed at about 7, 0, and -12 to -23 ppm, which are assigned to isolated ion pairs, hydrated ions and aggregated ions respectively. The sodium cations in ethylene ionomers, therefore, are almost in ionic aggregates. The isolated ion pairs...

Figure 12. Similarity between the model of ion-pair hydration and hydrates in a...

The Raman bands of very concentrated A1(N03)3 solutions are consistent with [Al(OH2)6] and NOj in the form of solvent-separated ion pairs.Hydration-shell and bulk-water protons can be distinguished by NMR. Al NMR signals distinguish [Al(OH2)s] from other species, such as [A1(0H2)5(S04)] in sulfate solutions, or [Al2(OH)2(OH2)8] and the more highly polymerized complexes which are produced on the ageing of aqueous aluminate solutions. According to Raman and NMR data, - the cation... 

The solubilities of the ionic halides are determined by a variety of factors, especially the lattice enthalpy and enthalpy of hydration. There is a delicate balance between the two factors, with the lattice enthalpy usually being the determining one. Lattice enthalpies decrease from chloride to iodide, so water molecules can more readily separate the ions in the latter. Less ionic halides, such as the silver halides, generally have a much lower solubility, and the trend in solubility is the reverse of the more ionic halides. For the less ionic halides, the covalent character of the bond allows the ion pairs to persist in water. The ions are not easily hydrated, making them less soluble. The polarizability of the halide ions and the covalency of their bonding increases down the group. 

It has been suggested by Ikegami (1968) that the carboxylate groups of a polyacrylate chain are each surrounded by a primary local sphere of oriented water molecules, and that the polyacrylate chain itself is surrounded by a secondary sheath of water molecules. This secondary sheath is maintained as a result of the cooperative action of the charged functional groups on the backbone of the molecule. The monovalent ions Li", Na and are able to penetrate only this secondary hydration sheath, and thereby form a solvent-separated ion-pair, rather than a contact ion-pair. Divalent ions, such as Mg " or Ba +, cause a much greater disruption to the secondary hydration sheath. 

The solvation (hydration) and desolvation of ions is important to the gelation process in AB cement chemistry. The large dipole moment of ion-pairs causes them to interact with polar molecules, including those of the solvent. This interaction can be appreciable. Much depends on whether the solvent molecule or molecules can intrude themselves between the two ions of the ion-pair. Thus, hydration states can affect the magnitude of the interaction. The process leading to separation of ions by solvent molecules was perceived by Winstein et al. (1954) and Grunwald (1954). 

Counterions can affect the strocture of hydration regions, and conversely hydration regions can affect ion binding. We have already touched on this subject in discussing contact and solvent-separated ion pairs in Section 4.2.8. 

It is well known that lyophilic sols are coagulated by the removal of a stabilizing hydration region. In this case, conversion of a sol to a gel occurs when bound cations destroy the hydration regions about the polyanion, and solvated ion-pairs are converted into contact ion-pairs. Desolvation depends on the degree of ionization, a, of the polyacid, and the nature of the cation. Ba ions form contact ion-pairs and precipitate PAA when a is low (0-25), whereas the strongly hydrated Mg + ion disrupts the hydration region only when a > 0-60. 

The authors relate this effect to ion pairing for strongly hydrated cations such as Li which results in a decreased activity of the active (poly) sulfide at the electrode. Using Cs instead of Li not only the electrochemical kinetics but also the stability would be considerably improved ... 

Recently, the abilities of primary to tertiary alkylammonium ions with Me, Et, and -Bu groups to transport water to NB have been studied [48]. As the result of careful consideration of the ion-pair formation, it has been shown that the hydration numbers (w ) of the ammonium ions in NB, being little affected by the alkyl chain length, are simply dependent on the class of the ammonium ion % = 1.64, 1.04, 0.66, respectively, for the primary, secondary, and tertiary ammonium ions. 

Fig. 1.7 Possible hydration modes of an ion pair (A) contact of primary hydration shells, (B) sharing of primary hydration shells, (C) direct contact of ions...

Liu, W. B. Wood, R. H. Doren, D. J., Hydration free energy and potential of mean force for a model of the sodium chloride ion pair in supercritical water with ab initio solute-solvent interactions, 7. Chem. Phys. 2003,118, 2837-2844... 

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. 

Surface complexation models attempt to represent on a molecular level realistic surface complexes e.g., models attempt to distinguish between inner- or outer-sphere surface complexes, i.e., those that lose portions of or retain their primary hydration sheath, respectively, in forming surface complexes. The type of bonding is also used to characterize different types of surface complexes e.g., a distinction between coordinative (sharing of electrons) or ionic bonding is often made. While surface coordination complexes are always inner-sphere, ion-pair complexes can be either inner- or outer-sphere. Representing model analogues to surface complexes has two parts stoichiometry and closeness of approach of metal ion to... 

The change in selectivity on changing the catalyst is ascribed to lower lipophilicity and greater hydration of the tight Q+ CCI3 ion pair, which reduces the activity of the trichloromethyl anion, allowing more time for decomposition to the carbene. The effect of catalyst structure may be summarized as follows [47] ... 

Table 5.4 Relative reactivities (normalized to iodide) for a series of nucleophiles under phase transfer, homogeneous dipolar aprotic, and homogeneous protic conditions, and the hydration number of the quaternary onium-anion ion pair [43]...

During the last year two Monte Carlo calculations and one molecular dynamics study 276> on ion hydration were published. Watts et al.274> extended previous Monte Carlo calculations 162> to the Li+F- ion pair, which was surrounded in this model calculation by 50 water molecules. In the more recent work of this group Fromm et at.275 ) increased the number of solvent molecules to 200 and coordination numbers for the ions at room temperature were evaluated ( li+ 6,... 

The thermodynamics of the extraction mechanism is extremely complex. In the initial equilibration of the ion pairs (Scheme 1.6) account has to be taken not only of the relative stabilities of the ion-pairs but also of the relative hydration of the anionic species. Assuming the complete non-solvation of the ion-pairs, the formation of the ion-pair [Q+Y] will generally be favoured when the relative hydration of X- is greater than that of Y. However, in many cases, the anion of the ion-pair is hydrated [8-11] (Table 1.1) and this has a significant effect both on equilibrium between the ion-pairs in the aqueous phase and the relative values of the partition coefficients of the two ion-pairs [Q+X ] and [Q+Y ] between the two phases. 

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