Two important and unsolved problems in the food industry and also fundamental questions in colloid chemistry are how to measure molecular distributions, especially antioxidants (AOs), and how to model chemical reactivity, including AO efficiency in opaque emulsions. The key to understanding reactivity in organized surfactant media is that reaction mechanisms are consistent with a discrete structures-separate continuous regions duality. Aggregate structures in emulsions are determined by highly cooperative but weak organizing forces that allow reactants to diffuse at rates approaching their diffusion-controlled limit. Reactant distributions for slow thermal bimolecular reactions are in dynamic equilibrium, and their distributions are proportional to their relative solubilities in the oil, interfacial, and aqueous regions. Our chemical kinetic method is grounded in thermodynamics and combines a pseudophase model with methods for monitoring the reactions of AOs with a hydrophobic arenediazonium ion probe in opaque emulsions. We introduce (a) the logic and basic assumptions of the pseudophase model used to define the distributions of AOs among the oil, interfacial, and aqueous regions in microemulsions and emulsions and (b) the dye derivatization and linear sweep voltammetry methods for monitoring the rates of reaction in opaque emulsions. Our results show that this approach provides a unique, versatile, and robust method for obtaining quantitative estimates of AO partition coefficients or partition constants and distributions and interfacial rate constants in emulsions. The examples provided illustrate the effects of various emulsion properties on AO distributions such as oil hydrophobicity, emulsifier structure and HLB, temperature, droplet size, surfactant charge, and acidity on reactant distributions. Finally, we show that the chemical kinetic method provides a natural explanation for the cut-off effect, a maximum followed by a sharp reduction in AO efficiency with increasing alkyl chain length of a particular AO. We conclude with perspectives and prospects.

Added salts induce micelle-to-vesicle transitions at specific cation concentrations in Hofmeister order by forming polar headgroup–counterion pairs that release water.

Long-chain alcohol induced micelle-to-vesicle transition is accompanied with concurrent increase of interfacial water molarity and decrease of interfacial counterion molarity.

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A new series of gemini quaternary ammonium surfactants containing amide groups with the formula C n H2n+1CONH­(CH2)2N+(CH3)2(CH2)2 N+(CH3)2(CH2)2NHCOC n H2n+1·2CH3CO3 – (n = 11, 13, 15, 17) have been synthesized by the using green reagent dimethyl carbonate. The structures of these surfactants were confirmed by Fourier transform infrared spectroscopy, 1H NMR, 13C NMR, and mass spectra. Their surface activities and aggregation properties were investigated by surface tension, conductivity, steady-state fluorescence, dynamic light scattering, and transmission electron microscope measurements. The results show that these synthesized gemini quaternary ammonium surfactants reduce the surface tension of water to a minimum value of 28.21 mN·m–1 at a concentration of 0.256 mmol·kg–1 and self-assemble spontaneously into double layer aggregates which are mostly single-room vesicles and multilamellar vesicles. Furthermore, with increasing alkyl chain length, their critical micelle concentration values and the degree of counterion binding (β) decrease; and the spontaneous tendency of vesicle formation and the micellar aggregation numbers (N m) increase. They are also found to be effective corrosion inhibitors for A3 steel in acid solution.

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.

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An efficient protocol for [2,3]-sigmatropic rearrangement of allyl and propargyl thioethers is reported.