Decarboxylation of 6-nitrobenzisoxazole-3-carboxylate as kinetic probe for piperazinium-based cationic micelles

A new class of cationic surfactants containing the heterocyclic piperazinium ring in their covalent structure was prepared; cetyldialkylpiperazinium halides, CRPX, with alkyl=Me (CMPX), Et (CEPX), n-Pr (CPPX), and with halide=Br and Cl. They were characterized by measures of critical micellar concentration, cmc, and ionization degree, alpha, and also by use of the decarboxylation of 6-nitrobenzisoxazole-3-carboxylate as a kinetic probe to investigate the properties of the microinterface they provide in aqueous solutions. The pseudophase kinetic treatment fails to fit the data at high [surfactant], which show anomalies with abrupt increase in k(obs), especially for CEPX and CPPX. Data can be fitted up to [surfactant] ca. 0.1 M, and 0.2 M in some cases. Compared with cetyltrialkylammonium halides, values of k'(M) indicate a water-richer microenvironment and less important interface property changes with increasing head group bulk. The reaction could be studied both in the presence and in the absence of NaOH; comparison shows that NaOH affects only the shape of the kinetic profile at low [surfactant], without affecting the microenvironment provided by mature micelles.