Pseudophase kinetic models that were originally developed for interpreting chemical reactivity in homogeneous association colloids such as micelles, microemulsions, and vesicles, also work in stirred emulsions. All these surfactant-based systems have the same basic properties as media for chemical reactions—but one—droplet size. In both thermodynamically stable, single-phase surfactant solutions and in kinetically stable, stirred two-phase emulsions, the reactive components are in dynamic equilibrium because the diffusivities of molecules and ions are orders of magnitude faster than the rates of the thermal chemical reaction of interest. In pseudophase models both association colloids and emulsions are conceptually divided into three separate reaction regions in which the observed rate constant, k, is proportional to reactant concentrations in the total volume of each region (sums of all oil droplets and of all interfacial regions around each droplet) and the rate constant in each region.
The effect of surfactant on k for reduction of a hydrophobic arenediazonium ion probe by an uncharged antioxidant, AO is modeled by assuming that the AO distributes between the oil, interfacial and aqueous regions of an emulsion, but the reactive group of the surfactant like cationic arenediazonium ion is located only in the interfacial region. Values of k are obtained by electrochemical and spectrometric methods and physical separation of the phases is not required. The data are fit with a kinetic model that provides estimates of the partition constants, between oil/interfacial and aqueous/interfacial regions for the distribution of the AO and for reaction rate constant in the interfacial region.
This talk will describe the logic and application of pseudophase models to emulsions, show that AOs are located primarily in the interfacial region, that their distributions depend on both surfactant concentration and oil polarity, are applicable to cationic, anionic, and zwitterionic emulsions, and provide a natural explanation for observed maximum (cut-off effect) in AO efficiency with increasing AO hydrophobicity. Future work will focus on developing a new scale of AO efficiency.
Professor Laurence S. Romsted romsted@rutchem.rutgers.edu
