Cut-off effect of radical TEMPO derivatives in olive oil-in-water emulsions

•Three stable oil-in-water emulsions of different drop sizes were prepared.•Radical-probe reactivities with Trolox and pyrene were evaluated in the emulsions.•A cut-off effect was observed for the pyrene quenching by the amphiphobic probes.•No cut-off effect was observed for the hydrogen-abstracting reaction with Trolox.•A cut-off effect requires partitioning of reagents between distinct pseudophases.

Three oil-in-water emulsions were prepared from mixtures of olive oil and Tween 20 in water. The effectiveness of a series of radical 2,2,6,6-tetramethylpiperidinoxyl (TEMPO) derivatives of variable lipophilicity in reactions with antioxidant Trolox, and as pyrene-fluorescence quenchers, was compared in the three emulsions. A “cut-off” effect was observed for the pyrene quenching by the probes, but not for their reaction with Trolox. The results were rationalized in terms of the amphiphobic nature of the probes, and the different locations of probe, pyrene and Trolox in the three-phase microheterogeneous systems.

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