Alkali ion hydration

Six alkali halide salts KF, RbF, CsF, CsCl, CsBr, and Csl-have an alkali ion radius close to that of the water molecule and a ratio of hydration numbers of alkali ion to halide ion of less than 2. These alkali halides were classified into one group, and calculations based on steric and hydration effects were made to analyze their surface charge. The steric effect, which is related to the size of the alkali ion, halide ion, and water molecule, has Htde influence on the partial hydration of these six alkali halides due to the fact that the radii of alkali ion, halide ion, and water molecule are very close to one another. The other effect, the hydration effect, mainly depends on the ratio of alkali ion hydration number to halide ion hydration number. In this case, since the ratio is less than 2, the influence of hydration is also negligible. The situation for case A is summarized in Table 6. 

Pratt, L. R., Rempe, S. B., Topol, I. A., and Burt, S. K. (2000). Alkali metal ion hydration and energetics of selectivity by ion-channels. Biophys.J. 78, P2057-P2057. 

The spiro-derivative (206) forms anhydrous complexes with larger alkali ions (such as K+). Such complexes appear to have a 2 1 structure of type (207) (Weber, 1979). For Li+, the hydrated complex [Li2L(H20)4]l2 [where L = (206)] is formed. The X-ray structure of this species reveals that it is essentially of type (207) except that the Li+ in each macro-ring coordinates to only three ether oxygens of the ring. Each Li+... 

EHM has also been applied for the calculations of optimum coordination numbers 226> in hydrated alkali ions (Table 20). The affinities of alkali ions to NN -dimethylacetamide and methylacetate were estimated by Kostetsky et al. 227>, using the CNDO/2 procedure (Table 21). As long as the ions are constrained to lie in the peptide- or ester planes, correct trends are obtained, but relaxation of this constraint reveals serious discrepancies to ab initio calculations. The CNDO procedure artificially stabilizes structures with nonpolar bonding geometries,... 

Note Added in Proof After we sent the manuscript to the publishers we became aware of CNDO studies on alkali ion solvation performed by Gupta and Rao 270> and Balasubramanian et al.271 >, which might be of some importance for readers interested in cation solvation by water and various amides. Another CNDO model investigation on the structure of hydrated ions was published very recently by Cremaschi and Simonetta 272> They studied CH5 and CH5 surrounded by a first shell of water molecules in order to discuss solvation effects on structure and stability of these organic intermediates or transition states respectively. 

There are only few main group metal ion hydrates open to detailed mechanistic study of water exchange by NMR Be , Mg , Al" , Ga" and to a less extent, In" . They provide the opportunity to study the influence of size and charge on exchange rate constant and mechanism without the complicating effects of the variation of the electronic occupancy of the d-orbitals. All of the alkali ions as well as Ca , Sr, and Ba are very labile as a consequence of their relatively low surface charge density. However, indications on water exchange on Sr " can be obtained from... 

Dosing with alkali (usually hydrated lime (Ca(OH)2), and less frequently caustic soda (NaOH)), which serve both to raise the pH (thus lowering the solubility of most problematic metals) and to supply OH ions for the rapid precipitation of metal hydroxide solids. 

Thus for example the differences in hydration energy (from lattice energy and heat of solution of salts with the same anion) of the alkali ions compared with those of Li+ and likewise of the anions compared with F, vary proportionally to 1 /r, from which by extrapolation to i/r =0 the hydration energy of the... 

Fig. 9. Sequential hydration energies in kj/mol of the singly charged alkali ions, lithium (open circles) and sodium (open diamonds), the alkaline earth, magnesium (open triangles), and two first row transition metal ions, manganese (solid triangles) and copper (closed circles). All data taken from Table 3...

An ND study of Ag" shows a coordination (Fig. 5) intermediate between that of Li+ and with a value of Ago 4 (Table II). Based on the assumption that the coordination of monovalent ions scales with hare-ion size, a recent XD experiment has been used to demonstrate that Ag (aq) and Na+(aq) are the same to a first approximation (71, 73). Consequently, difference methods have confirmed the trend that Li+ > Na+ > K in terms of the stability of their hydration shells. The lability of the aquaions in the alkali series is further confirmed in QENS experiments (31b), in which results show that the translational dynamics of the protons of the water molecules are not appreciably perturbed for alkali metal cations other than Li+. Results of computer simulation studies of models in which alkali ion-water potential is based on ab initio calculations give good overall agreement with the... 

These results provide an overview of the structure of aquaions and show that a broad classification into labile and stable species can be made. Those of the former category can be represented by a relatively weak and variable hydration shell and include the alkali ions (other than Li+), Ag+, Ca, and ND4+. On the other hand, cations of the transition metals, the rare earths, and small, highly charged ions such as Be +, Mg, and AP, which have well-defined hydration shells, form stable aquaions. Cations such as Cu + and Li+ are intermediate, having exchange times in the range lO -lO sec. 

Maximum swelling concentrations of different alkalies depend on the degree of hydration of the alkali ion [9, 10]. Table 9.1 contains data of different kinds of... 

In the case of dilute acid solutions (pH >2), the alkalis and basic oxides are dissolved preferentially, but the amount of silica dissolved is less than that removed by water alone ( 7, , ). This can be explained by the fact that the acid neutralizes the leached alkalis, so that the pH does not rise to higher values where silica dissolution becomes important. The preferential leaching produces an alkali-depleted layer that can be twice as thick as the one obtained by neutral water (10). Because of the replacement of the alkali ions by the smaller hydrogen ions, stresses will be induced in this layer which can cause it to crack. Further shrinkage can also occur if this hydrated silica layer loses water (10,11). 

Fig. 1. Free energy (-AG) of ligand binding (dashed lines) and of hydration (solid line) of alkali ions as a function of their reciprocal radius...

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