To Model Chemical Reactivity in Heterogeneous Emulsions, Think Homogeneous Microemulsions

Abstract

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.

Physical evidence that the variations in the efficiency of homologous series of antioxidants in emulsions are a result of differences in their distribution

“…The classical analytical methods employed to determine the distributions of AOs in oil–water mixtures (no added surfactant) cannot be employed because it is physically impossible to separate the oil and aqueous regions from the interfacial one without disrupting the existing equilibria. Therefore, the distributions of AOs need to be assessed in intact emulsions …”

“…Our laboratory developed a methodology for determining the distributions of AOs in intact emulsions by exploiting the power of the pseudophase kinetic model, which was previously employed to model chemical reactions in surfactant‐based systems (micelles, microemulsions, liposomes, etc.) .…”

“…The oxidation/reduction reaction of 16‐ArN 2 + with AOs usually takes place through the monoanionic form of the phenolic AO (phenoxide), leading to the formation of a highly reactive diazo ether, which rapidly decomposes to yield reduction products. The reaction is significantly faster than its spontaneous heterolytic dediazoniation in different solvents, association colloids and emulsions …”

“…The method is robust, does not require any knowledge about the size of the droplets and provides reliable results provided that the emulsions are sufficiently stable over a time longer than that required to complete the chemical reaction …”

“…is part of the interfacial region and not part of the aqueous pseudophase. Hence, any micelles that are present in emulsions are part of the emulsion pseudophase and not part of the aqueous pseudophase …”