A novel combined chemical kinetic and trapping method for probing the relationships between chemical reactivity and interfacial HO, Br and H ion molarities in CTAB/CE 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/CE 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-ArN, within the interfacial region of the mixed micelles in the CE mole fraction range of 0 to 1 at three different total surfactant concentrations. CK experiments showed that this reaction is monophasic in CE micelles, but biphasic in mixed micelles. The results were fully consistent with a complex mechanism in which TBHQ reacts with 16-ArN 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/CE (grow) and CTAB/CE (don't grow) showing that the rates only depend on micelle composition, not shape. CT results provided estimates of interfacial molarities of HO are approximately constant at ca. 39 and Br decreases from ca. 2.75 to 0.05 moles per liter of interfacial volume as CE 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 pK = 3.8 at all CE 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.