The distribution and reactivity of the lipophilic spin probe 4-phenyl-2,2,5,5-tetramethyl-3-imidazoline-1-oxyl nitroxide (PTMIO) in tetradecane (C14)- and eicosane (C20)-in-water emulsions and solid lipid nanoparticles (SLN) respectively, were investigated by electron paramagnetic resonance (EPR) spectroscopy. The lipid phase (10 wt% C14 or C20) was emulsified into either caseinate solutions (I wt%) or lecithin + bile salt dispersions (2.4 wt% + -0.6 wt%) at 70-75 degrees C. In C14 emulsions stabilized with lecithin + bile salt, three populations of PTMIO were observed: a population in the lipid phase (similar to 60%, a(N) similar to 13.9 G), an aqueous phase population (similar to 20%, a(N) similar to 15.4 G) with high mobility, and an immobilized surface layer population (similar to 20%, a(N) similar to 14.2 G) with low mobility. However, in C14 emulsions stabilized by caseinate, only two distinct populations of PTMIO were seen: a lipid phase population (similar to 70%, a(N) similar to 13.8 G) and an aqueous phase population (similar to 30%, a(N) similar to 15.5 G) with high mobility. In C20 SLN stabilized with either lecithin + bile salt or caseinate, PTMIO was excluded from the lipid phase. In lecithin + bile salt-stabilized C20 SLN, the majority of the probe (similar to 77%) was in the interfacial layer. For both surfactant systems the rate of PTMIO reduction by aqueous iron/ascorbate was greater for C20 SLN than C14 emulsions. Lecithin affects the properties of emulsions and SLN as delivery systems by providing a distinct environment for small molecules. (C) 2013 Elsevier Inc. All rights reserved.