Secondary hydration sphere

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. 

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

In aqueous solutions, in which the most probable ligand is the water molecule, most of the lower oxid ation states (i.e. + 2, + 3 and some of the + 4 states) of transition metal ions are best regarded as hexaaqua complex ions, e.g. [Feu(H20)6]2 +. In these ions the six coordinated water molecules are those that constitute the first hydration sphere, and it is normally accepted that such ions would have a secondary hydration sphere of water molecules that would be electrostatically attracted to the positive central ion. The following discussion includes only the aqua cations that do not, at pH = 0, undergo hydrolysis. For example, the iron(III) ion is considered quite correctly as [Fe(H20)6]3 +, but at pH values higher than 1.8 the ion participates in several hydrolysis reactions, which lead to the formation of polymers and the eventual precipitation of the iron(III) as an insoluble compound as the pH value increases, e.g. ... 

He estimated a thickness of about 25 A of the bond water layer. This value is similar to the extension of the H-bonds in bulk water and would mean in a simple model that the biopolymer surface prevents the flickering of the defects in water. Garlid has found too that the bound water on mitochondria is a solvent of H-bonding organic molecules275. If his interpretation is correct, solutes in the secondary hydrate sphere would get more immobile. The time for transfer in cells is increased. —... 

Bonding Forces Between Dye and Fiber. Dye anions can participate in ionic interactions with fibers that possess cationic groups. However, the formation of ionic bonds is not sufficient to explain dye binding, because compounds that can dissociate are cleaved in the presence of water. Secondary bonds (dispersion, polar bonds, and hydrogen bonds) are additionally formed between dye and fiber [47], Close proximity between the two is a prerequisite for bond formation. However, this is counteracted by the hydration spheres of the dye and of wool keratin. On approach, these spheres are disturbed, especially at higher temperature, and common hydration spheres are formed. The entropy of the water molecules involved is increased in this process (hydrophobic bonding). In addition, coordinate and covalent bonds can be superimposed on secondary and ionic bonds. 

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... 

Incorporation of metal ions into porphyrins is affected by other compounds in solution. Lowe and Phillips (25) found that copper(II) ions were chelated with dimethyl protoporphyrin ester 20,000 times faster in 2.5% sodium dodecylsulfate (SDS) than in 5% cetyl trimethyl ammonium bromide (CATB). The increased activity of SDS treated porphyrin was attributed to electrostatic attraction between anionic micelles formed around the tetrapyrrole nucleus and the metal cation. The authors also reported the influence of certain chelating agents on the rate of copper complex formation. Equimolar concentrations of copper and 8-hydroxyquinoline or sodium diethylthiocarbamate in 2.5% SDS increased the reaction rate 38 and 165 times, respectively, above the control. Secondary chelators may act by removing the hydration sphere on the metal ion increasing its attraction to pyrrole nitrogens (26). 

With the help of multinuclear solid state NMR experiments, the promoter (phosphate) induced acceleration of zeolite formation is proved unambiguously. It is proposed that the hydration spheres of the silicate units formed by the hydrolysis of the tetraethyl ortho silicate (TEOS) will be modified as part of the water molecules will be taken away by the promoter. This will speed up the process of the association of Qi, Q2, and Q3 units to form more Q4 units in the gel, leading to the formation of secondary building units at a faster rate. The promoter can also enhance the assembling of the SBU s in the similar way. 

The peak in the X-ray RDF at 5 A (fig. 4) was assigned to Ln-O distances for water molecules in the second (outer) hydration sphere. Curve B in fig. 6 shows the dependency of the position of this peak on the lanthanide radius. Both ion pair interactions [Ln(H20) (] -Cl and secondary solvation [Ln(H20) ] -H20 are expected to be responsive to differences in ionic radii of the lanthanide ions as well as to changes in the inner-sphere hydration. The decrease in the Ln H2O distance between La " and Lu " " (including the hydration change offset) is 0.24 A (fig. 6, curve A), in good agreement with the difference of 0.22 A for the peak at ca. 5 A. 

The electrostatic effects of an ion, however, can extend far beyond the primary hydration sphere. This accounts for the very large so-called hydration numbers that have been reported for some ions (up to 700 for Na+, for example). There is clearly a much larger region around the ion which contains loosely bound, but probably non-orientated, water. This assembly constitutes the secondary hydration sphere . 

Hydration of ions is due to the dipole nature of water. In the case of a cation in water, the negative (oxygen) end of the neighboring water molecules will be oriented toward the ion, and a sheet of oriented water molecules will be formed around the cation. This sheet is called the primary hydration sphere. The water molecules in the primary hydration sphere will furthermore attract other water molecules in a secondary hydration sphere, which will not be as rigorous as die primary sphere. Several sheets may likewise be involved until at a certain distance the behavior of the water molecules will not be influenced by the ion. 

Fig. 1. Hydration zones of a cation in solution. Metal cation Zone A, the primary hydration sphere zone B, the secondary hydration sphere zone C, the disordered water zone D, the bulk water.

Assuming that the interaction between the primary hydration sphere and the secondary water molecules of hydration are strongly electrostatic for both f-element series, Fourest et al. (1984) were able to estimate the inner-sphere hydration numbers of the trivalent actinide ions by interpolation using the values for the lanthanide elements (Habenschuss and Spedding 1979a, b, 1980). The interpolated values are listed in the last column of table 3 and shown in fig. 3. 

Electrostatic forces hold a small number of H2O molecules around a Li ion in a primary hydration sphere. These molecules, in turn, hold other molecules, but more weakly, in a secondary hydration sphere. 

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. 

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