Abstract
Surfactant induced shifts of micellar equilibria provided some of the earliest examples of micellar effects on chemical reactions. As with micellar catalyzed bimolecular reactions, much of the observed effect on indicator deprotonation is caused by concentrating the reactants in the small volume of the micellar pseudophase. However, a complete treatment must also account for micellar effects on the distribution of hydrophilic ions participating in the reaction, in this case OH−. We measured the apparent basicity constant, KB, of benzimidazole in solutions of CTAX (X = NO3, Br, Cl) and NaOH, with and without added salt. In surfactant alone, Kg passes through minima (i.e., KA through maxima) with increasing [CTAX], but for constant total [X−], Kg decreases smoothly. At constant [CTAX] added NaX increases Kg, and the salt order in Kg follows the usual series: NO3 − > Br− > Cl−. All these observations can be reasonably accounted for by the pseudophase ion exchange model developed earlier for micellar effects on reaction rates, because added X− displaces OH− from the micellar surface, increasing KB, without significantly altering the distribution of the two forms of the indicator. The concentration and ion exchange effects are combined into a quantitative treatment based on independently verifiable parameters. The results are used to estimate an intrinsic micellar basicity constant for benzimidazole whose value is similar to that in water.
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© 1982 Plenum Press, New York
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Bunton, C.A., Hong, YS., Romsted, L.S. (1982). A Quantitative Treatment of the Deprotonation Equilibria of Benzimidazole in Basic Solutions of Cetyltrimethylammonium Ion (CTAX) Surfactants. In: Mittal, K.L., Fendler, E.J. (eds) Solution Behavior of Surfactants. Springer, Boston, MA. https://doi.org/10.1007/978-1-4613-3494-1_26
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DOI: https://doi.org/10.1007/978-1-4613-3494-1_26
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