A novel combined chemical kinetic and trapping method for probing the relationships between chemical reactivity and interfacial H 2 O, Br - and H + ion molarities in CTAB/C 12 E 6 mixed micelles.

A delicate balance-of-forces governs the interactions responsible for surfactant self-assembly and chemical reactivity within them. Chemical reactions in micellar media generally occur in the interfacial region of micelles that is a complex mixture of: water, headgroups, counterions, co-ions, acids or bases, organic solvents, and the reactants themselves. We have carried out a detailed study of a complex chemical reaction in mixed CTAB/C12 E6 micelles by using the chemical kinetic (CK) and chemical trapping (CT) methods. The results provide a detailed quantitative treatment of the reaction of the anion of the antioxidant t-butylhydroquinone, TBHQ- , with 4-hexadecylbenzenediazonium, 16-ArN2 + , within the interfacial region of the mixed micelles in the C12 E6 mole fraction range of 0 to 1 at three different total surfactant concentrations. CK experiments showed that this reaction is monophasic in C12 E6 micelles, but biphasic in mixed micelles. The results were fully consistent with a complex mechanism in which TBHQ- reacts with 16-ArN2 + to give a transient diazoether intermediate that competitively breaks down into products and or reverts to starting materials. The kinetics are the same in mixed micelles of CTAB/C12 E6 (grow) and CTAB/C12 E8 (don't grow) showing that the rates only depend on micelle composition, not shape. CT results provided estimates of interfacial molarities of H2 O are approximately constant at ca. 39 and Br- decreases from ca. 2.75 to 0.05 moles per liter of interfacial volume as C12 E6 mole fraction increases from 0 to 1. Combined CK/CT results provided values for interfacial pH, ranging from ca. 4.25 in cationic micelles to 1.5 in nonionic micelles despite a constant bulk pH of 1.5 and the TBHQ interfacial pKa = 3.8 at all C12 E6 molar fractions. In totality, these results yielded an extraordinary amount of quantitative information about the relationships between the chemical reactivity and interfacial compositions of the mixed micelles.