title: 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.
authors: Universidade de Vigo , Facultad de Química, Departamento Química Física, 36200, Vigo, Spain.
authors: Carlos
authors: C
authors: Bravo-Díaz
authors: Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey , New Brunswick, New Jersey 08854, United States.
authors: Laurence Stuart
authors: LS
authors: Romsted
authors: Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey , New Brunswick, New Jersey 08854, United States.
authors: Changyao
authors: C
authors: Liu
authors: Universidade de Vigo , Facultad de Química, Departamento Química Física, 36200, Vigo, Spain.
authors: Sonia
authors: S
authors: Losada-Barreiro
authors: Departamento Física Aplicada, Facultad Química, Universidad de Vigo , 36200 Vigo, Spain.
authors: Maria José
authors: MJ
authors: Pastoriza-Gallego
authors: Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey , New Brunswick, New Jersey 08854, United States.
authors: Xiang
authors: X
authors: Gao
authors: Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey , New Brunswick, New Jersey 08854, United States.
authors: Qing
authors: Q
authors: Gu
authors: Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey , New Brunswick, New Jersey 08854, United States.
authors: Gunaseelan
authors: G
authors: Krishnan
authors: Universidade de Vigo , Facultad de Química, Departamento Química Física, 36200, Vigo, Spain.
authors: Verónica
authors: V
authors: Sánchez-Paz
authors: Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey , New Brunswick, New Jersey 08854, United States.
authors: Yongliang
authors: Y
authors: Zhang
authors: Department of Chemistry, University of Kashmir , Hazratbal, Srinagar, J&K India.
authors: Aijaz Ahmad
authors: AA
authors: Dar