Using a pseudophase model to determine AO distributions in emulsions: why dynamic equilibrium matters

Abstract

For decades, explorations with ground state, thermal reactions combined with pseudophase kinetic models and methods for interpreting the results have provided insights into the properties of the different regions of homogeneous association colloids. More recent successful determination of antioxidant (AO) distributions by this approach is providing new insights into AO efficiency in opaque, well-mixed two-phase intact emulsions and eliminating the need to separate the phases. A special probe, 4-hexadecylbenzenediazonium, reacts with AOs exclusively in the interfacial region of the emulsion, permitting simplification of the kinetic treatment and determining its distribution between the oil, interfacial and aqueous regions. AO distributions are obtained from the two partition constants, PO^I and PW^I, of the AOs between the oil-interfacial and aqueous-interfacial regions, respectively. PO^I and PW^I values are obtained by fitting the observed rate constant, kobs, versus surfactant concentration profiles with an overall kinetic approach or model we call the "pseudophase chemical kinetic method." However, because emulsions break up and reform, and reactants and other components diffuse at various time scales within and between the oil, interfacial and water regions, k_obs could also depend on reactant diffusion coefficients. Here we demonstrate that reactant diffusion is generally orders of magnitude faster than most thermal reactions and reactant distributions between the multiple oil, aqueous and interfacial droplets and regions of emulsions are in dynamic equilibrium throughout the multiphase systems during the time course of the reaction. Thus, kinetic probes are powerful tools for determining structure-reactivity, e.g., the HLB, relationships governing AO distributions and efficiencies in emulsions.

Practical applications: The analysis presented here demonstrates that one of the basic assumptions of the pseudophase chemical kinetic model that we have developed has a solid foundation in the properties of emulsions. That is, we can determine the distributions of reactants between oil (O), interfacial (I) and aqueous (W) regions of the emulsions because the diffusivity coefficients of reactants within emulsions are orders of magnitude greater than the rate of the reaction between the antioxidant and the 4-hexadecylbenzenediazonium probe. Consequently, we can use the same kinetic model in emulsions as we have used in homogeneous microemulsions. The method permits determination of the partition constants of many antioxidants between the O-I and W-I regions of the emulsions and from them their distributions. The method provides new insights into the relationships between antioxidant hydrophobic-lipophilic balance (HLB) and its efficiency in emulsions and a natural explanation for the cut-off effect observed with increasing antioxidant HLB.

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