Effect of ethanol on micellization and on decarboxylation of 6-nitrobenzisoxazole-3-carboxylate in acqueous cationic micelles

The effects of ethanol on the critical micellar concentration (cmc) and the rates of decarboxylation of 6-nitrobenzisoxazole-3- carboxylate (6-NBIC) have been investigated in aqueous cationic surfactants of the cetyltrialkylammonium family with bromide [CT(R)ABr], chloride [CT(R)ACl], and nitrate [CT(R)ANO3] counterions, and methyl (CTAX), n-propyl (CTPAX), and n-butyl (CTBAX) as the head group alkyl moieties. Effects upon cmc and reactivity are similar, featuring a break at the ethanol mole fraction, xEtOH, of ca. 0.055; these effects have been related to changes in solvent structure, with formation of a clathrate at xEtOH = 0.055. Rate data in CTBABr were further investigated and fitting of raw kinetic data to the pseudophase model is possible up to xEtOH = 0.1, showing an unexpected decrease of rate constant values in the micellar pseudophase, k'M, as ethanol content increases: a significant variation of micellar ionization degree could account for this kinetic effect.