Sodium cation hydration shell

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. 

According to a number of authors (28- 1). the Interaction of cations such as Na with a poly-anlon In water can occur under two different modes either a true contact Ion pair Is formed, as dictated by the mass action law and so that the two hydration shells are perturbed or the sodium cation, as the hexaquo species. Is attracted Into the electrostatic potential In the vicinity of the charged counter-Ion It Is like a free Ion In every respect except for being constrjdned not to diffuse outside of a certain volume surrounding the polyelectrolyte. The former Interaction type Is called site binding. The latter, which Is reminiscent of formation of a loose, solvent-separated. Ion pair Is referred to as atmospheric condensation. (Figures 1-2). 

The somewhat low value of the limiting relaxation rate for the bound state (260 Hz) Is a little Intriguing It may correspond to the sodium cation retaining Its full hydration shell In the bound state and/or to a rather short correlation time In this bound state (47). Both these criteria evoke atmospheric condensation rather than true site binding. These phosphatldylserlne vesicles appear to this writer as an Intermediate case between the two extremes of site binding and of atmospheric condensation. 

Because the only variable changed in this dissolution study was the type of alkali metal hydroxide, differences in dissolution rate must be attributed to differences in adsorption behavior of the alkali metal cations. The affinity for alkali metal cations to adsorb on silica is reported [8] to increase in a continuous way from Cs to Li" ", so the discontinuous behavior of dissolution rate cannot simply be related to the adsorption behavior of the alkah metal cations. We ascribe the differences in dissolution rate to a promoting effect of the cations in the transport of hydroxyl anions toward the surface of the silica gel. Because differences in hydration properties of the cations contribute to differences in water bonding to the alkali metal cations, differences in local transport phenomena and water structure can be expected, especially when the silica surface is largely covered by cations. Lithium and sodium cations are known as water structure formers and thus have a large tendency to construct a coherent network of water molecules in which water molecules closest to the central cation are very strongly bonded slow exchange (compared to normal water diffusion) will take place between water molecules in the nearest hydration shell and the bulk water molecules. 

Relevant properties of the sodium and potassium ions, gleaned from Chap. 2 and elsewhere (Marcus 1997) are shown in Table 5.6. The bare potassium ion is larger than the sodium one and is more polarizable, as their crystal ion radii n (valid also in solutions) and molar refractivity 7 di show. However, the K+ cations move faster in aqueous solutions as their mobilities u and diffusion coefficients D show, the K+ ions having a smaller Stokes radius, rist. The K+ cations carry along when moving less of their hydration shells that are more loosely bound, the residence times of water molecules near K+ being about one half that near Na+. Also, the hydration number h of K+ is smaller than that of Na+, as derived from the compressibility of the solutions as well as other measures. Such numbers are smaller than the coordination numbers CN in solution, which are governed only by the sizes... 

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