A novel combined chemical kinetic and trapping method for probing the relationships between chemical reactivity and interfacial H₂O, Br^- and H⁺ ion molarities in CTAB/C₁₂E₆ 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/C₁₂E₆ 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 C₁₂E₆ mole fraction range of 0 to 1 at three different total surfactant concentrations. CK experiments showed that this reaction is monophasic in C₁₂E₆ 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/C₁₂E₆ (grow) and CTAB/C₁₂E₈ (don't grow) showing that the rates only depend on micelle composition, not shape. CT results provided estimates of interfacial molarities of H₂O are approximately constant at ca. 39 and Br^- decreases from ca. 2.75 to 0.05 moles per liter of interfacial volume as C₁₂E₆ 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_a = 3.8 at all C₁₂E₆ 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.