Water-N,N-dimethylformamide alkyltrimethylammonium bromide micellar solutions: thermodynamic, structural, and kinetic studies

Various amounts of N,N-dimethylformamide (DMF) with the weight percentage of DMF varying within the range 0-20, were added to aqueous micellar solutions of hexadecyl-, tetradecyl-, and dodecyltrimethylammonium bromides (CTAB, TTAB, and DTAB, respectively). Information about changes in the critical micelle concentrations, in the micellar ionization degrees, in the aggregation numbers, and in the polarity of the interfacial region of micelles upon changing the weight percent of DMF was obtained through conductivity and fluorescence measurements. Surface tension measurements permitted the estimation of the Gordon parameter of the water-DMF mixtures. The thermodynamic and structural changes provoked by the addition of DMF to the cationic micellar solutions were evidenced through the micellar kinetic effects observed in the reaction methyl 4-nitrobenzenesulfonate + Br-, investigated in the water-DMF cationic micellar solutions. The pseudophase kinetic model was adequate to quantitatively rationalize the dependence of the observed rate constant on surfactant concentration as well as on the weight percent of DMF.