Journal of Colloid and Interface Science 262 (2003) 290–293 www.elsevier.com/locate/jcis Note Decarboxylation of 6-nitrobenzisoxazole-3-carboxylate in aqueous cationic micelles: kinetic evidence of microinterface property changes Lucia Brinchi, Raimondo Germani, Gianfranco Savelli, ∗ and Luisa Marte Dipartimento di Chimica, Università di Perugia, Via Elce di Sotto, 8, 06123 Perugia, Italy Received 24 June 2002; accepted 16 October 2002 Abstract We studied decarboxylation of 6-nitrobenzisoxazole-3-carboxylate, 1, as a kinetic probe to investigate microinterface properties of aqueous micelles formed by cationic surfactants of increasing head group bulk, i.e., cetyltrialkylammonium bromide, with alkyl = Me (CTABr), Et (CTEABr), n-Pr (CTPABr), n-Bu (CTBABr) and p-octyloxybenzyltrialkylammonium bromide surfactants with alkyl = Me (pOOTABr), n-Pr (pOOTPABr), and n-Bu (pOOTBABr), and the longer p-dodecyloxybenzyltrimethylammonium bromide (pDoTABr) at concentrations higher than 0.05 M. The pseudophase kinetic treatment fails to fit the data that show anomalies with abrupt increases in k obs for CTPABr and CTBABr (but not for CTEABr) and with smooth and continuos increase of k obs for all p-alkyloxybenzyltrilakylammonium bromides. Abrupt and successive modifications of the micellar interface properties, undergoing only when the polar head or the alkyl chain have some covalent structure, account for the observed kinetic behavior.  2003 Elsevier Science (USA). All rights reserved. Keywords: Micelles; Decarboxylation; Microinterface 1. Introduction Decarboxylation of 6-NBIC, 1, is a well-studied reac- tion whose reactivity is strongly dependent upon solvent properties [1], due to the charge dispersion in going from a carboxylate ion in the initial state to the charge-delocalized transition state (Scheme 1). The reaction is catalyzed by a variety of colloidal assemblies. For example, cationic and zwitterionic micelles speed reaction in water by incorporat- ing 1 and providing a favorable reaction environment [2–4]. The sensitivity of rate of 6-NBIC decarboxylation to medium effects makes this probe an attractive system for studying the kinetic role of interfaces and it has been exten- sively used to study the properties of micellar assemblies in water [3,4], in organic solvents [5], and recently in mixtures of water ethanol [6]. For instance, rate effects by cationic mi- celles on the decarboxylation of 1 are sensitive to the nature of counterion and head group bulk [4]. In this paper we used this kinetic probe to investigate the properties of the microinterface of aqueous assemblies formed by cationic surfactants of increasing head group * Corresponding author. E-mail address: savelli@unipg.it (G. Savelli). bulk, i.e., cetyltrialkylammonium bromide, with alkyl = Me (CTABr), Et (CTEABr), n-Pr (CTPABr), n-Bu (CTBABr), and p-octyloxybenzyltrialkylammonium bromide surfac- tants with alkyl = Me (pOOTABr), n-Pr (pOOTPABr), and n-Bu (pOOTBABr) and the longer p-dodecyloxybenzyltri- methylammonium bromide (pDoTABr). Decarboxylation of 1 has already been studied in these surfactants (a part the last one), but only in the dilute region [4], whereas we enlarged the investigation because we are interested in studying the structure of the aggregates at relatively high surfactant con- centrations, higher than 0.05 M. In fact we have recently ob- served anomalous kinetic behavior for the bimolecular S N 2 reaction of bromide with alkyl naphthalene-2-sulfonates in big-head cationic surfactant, CTBABr, at concentration higher than 0.05 M: kinetic rate constants appear to level off when substrate is completely associated, as predicted by the pseudophase kinetic treatment, but then trends upward, in particular with NaBr added [7,8]. Kinetic treatments of bi- molecular reactions involve several assumptions and several factors could give deviations from the simple treatment, in- cluding a change in the micellar structure or a change in the local concentration of bromide ion at high [surfactant] [7,8]. Therefore we used decarboxylation of 6-NBIC as a powerful kinetic tool to investigate properties of micellar interfaces, 0021-9797/03/$ – see front matter  2003 Elsevier Science (USA). All rights reserved. doi:10.1016/S0021-9797(02)00086-3