Electrostatic Aspects on Ion Binding Specificity

Micellar solutions of anionic amphiphiles are usually not stable with respect to the addition of di- or multivalent cations since a precipitation occurs (hard water). In exceptional cases, where precipitation does not occur, the question arises as to how the uni-, di- and multivalent ions compete for binding to the micelles. Due to the high value of I i l close to the micellar surface a counterion of high charge will be strongly favored and there is a discrimination between the different types of ions299. For example for SDS close to the CMC, -e J (r[)/kT — 7 and if the presence of small amounts of calcium ions does not affect h(rj), Eq. (6.2) can be used to calculate the ratio 

An increase in salt and/or micelle concentration leads to a decrease in l4 (rj) l and the discrimination between univalent and divalent ions is decreased. The same effects should occur for cationic amphiphiles but they have been even less studied. 

For ions of the same charge, the only parameter in the model presented above that can give rise to an ion specificity is the distance of closest approach rs — rm. 

In the model, the smaller ion gives the stronger ion binding. However, it seems that in most cases other factors, e.g., ion hydration and ion polarizability, dominate over the ion size effect, particularly for anions, (cf. Sect. 4). 

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