The Structure of Aquaions

In the following section, diffraction results are presented and discussed for various groups of ions. The basis for discussing these ions is a selection of results from the neutron and X-ray diffraction difference methods (ND and XD), total X-ray and neutron diffraction methods (TX and TD), and EXAFS (EX). A selection of results are contained in Tables II-VII and, where appropriate, will be contrasted with results derived from other methods. A more complete tabulation of TX results is available in table 2 of ref. (53) or in ref. (47). 

The first-order difference method of neutron diffraction (ND) can be carried out on Li+, K+, Ag, and T1+ (Table I). The sodium ion can be investigated at an approximate level by total X-ray diffraction and 

An ND study of K+ hydration gives a Gk(/ ) that shows little structure (Fig. 4), a result in marked contrast to that for Li. Clearly the larger K+ ion coordinates water molecules relatively weakly and so forms a labile aquaion. One can show from Gk(t) that nKo is 5.5 0.5 in the range 2.3 r 3.4. However, such a value cannot be taken as a hydration number in the same sense as for a stable species. Indeed, the value from computer simulation of 10 for tiko can be equally useful (7). 

Preliminary EXAFS and anomalous scattering results for a 4.6-molar aqueous solution of rubidium bromide (43) suggest that the Rb -water interaction is relatively weak, in good agreement with thermo- 

Ion Counter- ion Concentration (molality) Ion-water distances (A) Hydration number ( MO Tilt angle 0 (deg) Method / reference 

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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. 

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