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Concept
Ionic potential
Résumé
Ionic potential is the ratio of the electrical charge (z) to the radius (r) of an ion.
As such, this ratio is a measure of the charge density at the surface of the ion; usually the denser the charge, the stronger the bond formed by the ion with ions of opposite charge.
The ionic potential gives an indication of how strongly, or weakly, the ion will be electrostatically attracted by ions of opposite charge; and to what extent the ion will be repelled by ions of the same charge.
Victor Moritz Goldschmidt, the father of modern geochemistry found that the behavior of an element in its environment could be predicted from its ionic potential and illustrated this with a diagram (plot of the bare ionic radius as a function of the ionic charge). For instance, the solubility of dissolved iron is highly dependent on its redox state. Fe2+ with a lower ionic potential than Fe3+ is much more soluble because it exerts a weaker interaction force with OH- ion present in water and exhibits a less pronounced trend to hydrolysis and precipitation. Under reducing conditions Fe(II) can be present at relatively high concentration in anoxic water, similar to these encountered for other divalent species such as Ca2+ and Mg2+. However, once anoxic ground water is pumped from a deep well and is discharged to the surface, it enters in contact with atmospheric oxygen. Then Fe2+ is easily oxidized to Fe3+ and this latter rapidly hydrolyzes and precipitates because of its lower solubility due to a higher z/r ratio.
Millot (1970) also illustrated the importance of the ionic potential of cations to explain the high, or the low, solubility of minerals and the expansive behaviour (swelling/shrinking) of clay materials.
The ionic potential of the different cations (Na+, K+, Mg2+ and Ca2+) present in the interlayer of clay minerals also contribute to explain their swelling/shrinking properties. The more hydrated cations such as Na+ and Mg2+ are responsible for the swelling of smectite while the less hydrated K+ and Ca2+ cause the collapse of the interlayer.
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