Reactivity of a model lipophilic ingredient in surfactant-stabilized emulsions: Effect of droplet surface charge and ingredient location

The aim of this work was to investigate the location and reactivity of a lipophilic spin probe, 4-phenyl-2,2,5,5-tetramethyl-3-imidazoline-1-oxyl nitroxide (PTMIO) in emulsion systems stabilized with anionic (sodium dodecyl sulfate, SDS) or cationic (dodecyl trimethylammonium bromide, DTAB) surfactants. The analysis of electron paramagnetic resonance (EPR) spectra of PTMIO in emulsion systems showed that probe molecules partitioned between three environments: the aqueous phase, the lipid droplet core and the surfactant micellar pseudophase. The rate of the reduction of the nitroxide group of PTMIO by ascorbate anions was much faster in DTAB-stabilized emulsions than in SDS-stabilized emulsions, showing that the droplet surface charge controlled to a large extent the probe reactivity with charged aqueous compounds. When excess surfactant was added to the emulsion aqueous phase, a displacement of a fraction of PTMIO molecules from the lipid droplet core to the micellar pseudophase was observed. The subsequent change in the probe partitioning was found to affect the probe's reduction rate, which confirms that aqueous phase micelles contribute substantially to the reactivity of lipophilic ingredients incorporated in emulsion systems.