Water hydration spheres

Solvation of cyclen 36 was studied using Monte Carlo (MC) simulations <1996JPC17655, 1997JCF3045>. Potentials for cyclen were calculated by an ab initio method. It was found that the water hydration sphere is composed of three layers two water molecules are strongly bound in close vicinity, six molecules form the inner hydration sphere, 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).

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... 

To explain this different fractionation behavior, Taube (1954) postulated different isotope effects between the isotopic properties of water in the hydration sphere of the cation and the remaining bulk water. The hydration sphere is highly ordered, whereas the outer layer is poorly ordered. The relative sizes of the two layers are dependent upon the magnitude of the electric field around the dissolved ions. The strength of the interaction between the dissolved ion and water molecules is also dependent upon the atomic mass of the atom to which the ion is bonded. 

Contributions to (T )e include effects of and rs. A plot of log [(T2)eTctl-— C/tJ vs. 1/T gives 8.4 kcal/mole for the activation energy of proton exchange between water and the hydration sphere of the manganous ion. For more recent data on exchange involving Mn(H20)6 H, the interested reader is directed to the O17 NMR studies of Connick and Swift (16a). 

A One dm3 of a 1 mol dm 3 solution of magnesium chloride has a mass of 10 3 x 1066 kg= 1066 g. It contains 24.3 + 70.9 = 95.2 g of the salt, leaving 1066 — 95.2 = 970.8 g of water. This amount of water contains 970.8/18.015 = 53.89 moles. There are three moles of ions in the solution, so each ion has a maximum of 53,89/3= 18.0 water molecules that could be in its hydration sphere. It would be expected that the doubly charged Mg2 1 ion would affect more water molecules than the singly charged chloride ions. 

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... 

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