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

Abstract

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.

pH Changes in the Micelle–Water Interface of Surface-Active Ionic Liquids Dictate the Stability of Encapsulated Curcumin: An Insight Through a Unique Interfacial Reaction between Arenediazonium Ions and t-Butyl Hydroquinone

“…As clearly seen from the figure, k obs av changes negligibly with the concentration of SDS. It has been reported 4,7,10 that the increase in the concentration of the surfactant leads to the increase in the interfacial volume (due to the increase in the number of micelles and not due to However, any change in the interfacial pH of the micelles changes the amount of the TBHQ − anion due to the shift of protonation−deprotonation equilibrium of TBHQ and hence affects the k obs values. In this context, no change in k obs av values with the SDS concentration can be attributed to the balance of these two opposing factors.…”

“…The kinetic data fitted very well to the biexponential decay given in eq 9 as clearly shown in the case of the 3 mM OmimDS surfactant system in the Supporting Information as a prototype (Figure S2). 10 From the fitted values of k obs , A i , and % k obs i , the weighted average observed rate constant ( k obs av ) was calculated by using eq 12 . 10 We use variation of k obs av to discuss the effect of changing concentration of amphiphiles (SDS and SAILs) on the kinetics of 16-ArN 2 + with TBHQ occurring within the interfacial region of the micelles.…”

“… 10 From the fitted values of k obs , A i , and % k obs i , the weighted average observed rate constant ( k obs av ) was calculated by using eq 12 . 10 We use variation of k obs av to discuss the effect of changing concentration of amphiphiles (SDS and SAILs) on the kinetics of 16-ArN 2 + with TBHQ occurring within the interfacial region of the micelles. This procedure makes the comparison of kinetics between solutions of different concentrations easier in contrast to using k obs 1 and k obs 2 separately, but without affecting the data analysis.…”

“…To apply this method to the association colloids to study the interface, 4-hexadecylbenzenediazonium ion, 16-ArN 2 + is used as a probe, which undergoes the reduction reaction with t -butylhydroquinone (TBHQ) through a similar mechanism. 10 The long alkyl chain of 16-ArN 2 + gets solubilized in the hydrophobic region of the micelle, and its charged headgroup is exposed to the interfacial region of the micelle. The reactions are generally carried out at low pH to monitor the rate of reaction on a large time scale.…”

“…We have recently shown the biphasic mechanism for the reduction of 16-ArN 2 + by TBHQ in the mixed micelles of CTAB/C 12 E 6 , leading to the formation of products 16-ArH and t -butylquinone via the formation of the diazoether intermediate ( Scheme 4 b). 10 Such a mechanism follows the following rate equation for the loss of the arenediazonium ion where k 1 and k 2 are the second-order rate constants for the reaction between TBHQ – and 16-ArN 2 + and the first-order rate constant of conversion of the diazoether adduct back to starting materials, respectively, in the interfacial region. The (diazoether) term is the concentration of the intermediate in mol L –1 of the interfacial volume.…”