Abstract
The onset in the micellization of ionic surfactants and the binding of counterions onto ionic micelles can be related to ionic pair effects using Bjerrum's correlation. Two magnitudes are defined to calculate such effects. One is a characteristic dimension for the volume element occupied by a single ion devS=0.5x(10(24) nm3 l(-1)/(NA[S]))1/3, and the other is an "interaction control volume" ratio Phii=XSi(zSi devSi)3/[Sigmaj=1,nXSj zSj devSj)3]. It is shown that in both the case of the onset of micellization and the binding of counterion onto micelles there are two situations: when the concentration and charge of a polyvalent cation i present is such that Phii>0.5 and otherwise. When Phii is above 0.5, the critical micelle concentration (CMC) and the binding of ions onto micelles are dependent on devS of the polyvalent cation. Below Phii=0.5, both the CMC and the binding ratios are related to the contributions of both monovalent and polyvalent species weighed by their "interaction control volume" ratio. As a consequence, it is proposed that ionic groups formed by a counterion and three or four surfactant ions are the building blocks of ionic micelles.
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