Effect of ethanol on micellization and on decarboxylation of 6-nitrobenzisoxazole-3-carboxylate in aqueous 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)ACI], and nitrate [CT(R)ANO(3)] 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, X-EtOH, of ca. 0.055; these effects have been related to changes in solvent structure, with formation of a clathrate at X-EtOH = 0.055. Rate data in CTBABr were further investigated and fitting of raw kinetic data to the pseudophase model is possible up to X-EtOH = 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. (C) 2002 Elsevier Science (USA).