Alkali metal cations, hydrated

Electrodes based on solutions of cyclic polyethers in hydrocarbons show a selective response to alkali metal cations. The cyclic structure and physical dimensions of these compounds enable them to surround and replace the hydration shell of the cations and carry them into the membrane phase. Conduction occurs by diffusion of these charged complexes, which constitute a space charge within the membrane. Electrodes with a high selectivity for potassium over sodium (> 1000 1) have been produced. 

Yeager and Steck derived diffusion coefficients for water in totally hydrated Nafion 120 membranes that were exchanged with alkali metal cations, using a radiotracer technique. At 25 °C, 77for the Na+ form was 2.65 X 10 cm /s and the values for the K+ and Cs+ forms were somewhat smaller, which would seem to reflect the lower maximal degree of hydration of these forms. 

The earliest of these studies was on PbS. PbS can have either p- or n-type conductivity, although CD PbS is usually p-type. Based on the belief that the p-type conductivity may be due to alkali metal cations from the deposition solution, an alkali metal—free deposition, using lead acetate, thiourea, and hydrazine hydrate was used [33]. While initially n-type, the film converted to p-type in air. Attempts to stabilize the p-type material by adding trivalent cations to the deposition solution were unsuccessful. However, deposition of the PbS on a trivalent metal, such as Al, did stabilize the n-PbS, at least for a time. In this way, p-n junctions were made (the PbS close to the trivalent metal was n-type, while the rest of the film was p-type). Photovoltages up to 100 mV were obtained from these junctions at room temperature and almost 300 mV at low temperatures (90 K). 

For anionic monolayers, the reversal of the tt-A isotherms can be explained in terms of a competition between the anionic head groups and the alkali metal cations for molecules of water. If a modified Stern-type model of the plane interface is assumed, this interface will be composed of distinct adsorption sites, with counterions (cations) of finite size that can adsorb on these sites if the standard free energies of adsorption are favorable. If the anionic head group is more polarizable than water, as with carboxylic acids or phosphates, the hydration shell of the cation is incompletely filled, and the order of cation sizes near the interface is K+ > Na+ > Li+. When the polarizability of the anionic group is less than that of water, as with the sulfates, the lithium cation becomes the most hydrated one, and the order of cation sizes becomes Li+ > Na+ > K+. 

Dissolution of alkali metal cations such as Cs+ results in short-range liquid order in water as a primary solvation shell of about eight water molecules is established about the metal cation. Lithium, however, exerts a much greater polarising power and is capable of organising a first- and second-coordination sphere of about 12 water molecules about itself, resulting in a much larger hydrated radius for the ion and hence decreased ionic mobility. 

In support of this hypothesis alkali metal cations, especially the strongly hydrated Li , favor propionitrile formation under otherwise identical conditions80-. ... 

Table 5. Conventional enthalpies of hydration of alkali metal cations at 298° K (heal mole-1)...

Nevertheless one might have expected that the kinetic data would reflect clearly the hydration properties of the added salt. This is not usually the case because charge-charge interactions are dominant. Moreover, structural effects often compensate and their influence on rate constants is minimised (p. 247). If, however, the reactants are neutral solutes, it might be anticipated that the kinetic parameters would be markedly sensitive to the particular salt added to the solution (p. 272). This prediction is borne out in practice, striking differences often being observed between the effects of added alkali metal cations and alkylammonium ions as the following examples show. 

But it was not to be. Try as we might, the difference in scattering lengths between the 6Li and 7Li isotopes was too small to permit us to measure the lithium ion distribution in the swollen state. We had to content ourselves with the results for the crystalline phase, where the behavior of the lithium ions is different from that of the larger alkali metal cations [27], Potassium and cesium ions bind directly to vermiculite clay surfaces rather than hydrating fully. Because only lithium-substituted vermiculites of the alkali metal series will swell macroscopically when soaked in water, it seems that interlayer cations must form fully hydrated ion-water complexes if the particles are to expand colloidally. This conclusion has since been supported... 

The alkali metal cations Li, Na, K, Rb and Cs cannot be reduced by hydrated electrons because they have a more negative reduction potential. 

The latter process seems to be very selective against alkali metal cations in the initial stages of exchange and is critically dependent on alkali metal solution phase concentration. Sometimes ion-exchange processes can be described as ion sieving when a particular cation (hydrated or unhydrated) is too large to enter all, or part of, the zeolite internal structure. 

Alkali metal cations and alkaline earth metal cations are not reduced by the hydrated electron as the redox potential of the or couple is lower than the redox potential of the hydrated... 

Yeager et al. [00] reported diffusion coefficients of water in Nafion 120 membranes containing various alkali-metal cations, determined by radiotracer measurements. In these studies, the diffusion coefficient of water was measured for fully hydrated (i.e., immersed) membranes. The water diffusion coefficient was found to be only slightly dependent on the cation present in the immersed membrane, with a... 

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... 

Solids corresponding to nearly all of the solution species (the is one exception) have been prepared as microcrystalline powders via precipitation. " The structures of compounds MNp02(C03) and M3An02(C03)2, where M is an alkali metal or ammonium, have been described in detail." These compounds show interesting structural changes due to the alkali metal cation present (size), the size similarity of hydrated ions such as K and Np02", and the extent of hydration. 

Flanigen believed that the components of alkali metal cations of the synthetic system would play an important role in two aspects in the synthesis of zeolites [55] (1) supply of base, i.e., OH and (2) structure directing. Recently, Dutta and colleagues concluded that the hydrated cations can stabilize aluminosilicate ions via static electric and steric interactions.[56] In a word, alkali metal cations or alkaline earth metal cations mainly play the roles of structure direction, framework charge balancing, and pH adjustment in the synthesis of aluminosilicate zeolites. 

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