Using the pseudophase kinetic model to interpret chemical reactivity in ionic emulsions: Determining antioxidant partition constants and interfacial rate constants

Kinetic results obtained in cationic and anionic emulsions show for the first time that pseudophase kinetic models give reasonable estimates of the partition constants of reactants, here t  -butylhydroquinone (TBHQ) between the oil and interfacial region, POI, and the water and interfacial region, PWI, and of the interfacial rate constant, kI, for the reaction with an arenediazonium ion in emulsions containing a 1:1 volume ratio of a medium chain length triglyceride, MCT, and aqueous acid or buffer. The results provide: (a) an explanation for the large difference in pH, >4 pH units, required to run the reaction in CTAB (pH 1.54, added HBr) and SDS (pH 5.71, acetate buffer) emulsions; (b) reasonable estimates of POI and kI in the CTAB emulsions; (c) a sensible interpretation of added counterion effects based on ion exchange in SDS emulsions (Na+/H3O+ ion exchange in the interfacial region) and Donnan equilibrium in CTAB emulsions (Br− increasing the interfacial H3O+); and (d) the significance of the effect of the much greater solubility of TBHQ in MCT versus octane, 1000/1, as the oil. These results should aid in interpreting the effects of ionic surfactants on chemical reactivity in emulsions in general and in selecting the most efficient antioxidant for particular food applications.

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► Pseudophase ion exchange models work with both cationic and anionic emulsions.
► Determined partition constants for t-butylhydroquinone, TBHQ.
► Determined the rate constant, kI, in the interfacial region of the emulsions.
► Added NaBr decreases the observed rate constant, kobs, in cationic emulsions and increases it in anionic emulsions.
► The polarity of the oil region has a major effect on the values of partition constants.