Localization and reactivity of a hydrophobic solute in lecithin and caseinate stabilized solid lipid nanoparticles and nanoemulsions

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 (10wt% C14 or C20) was emulsified into either caseinate solutions (1wt%) or lecithin+bile salt dispersions (2.4wt%+0.6wt%) at 70-75°C. In C14 emulsions stabilized with lecithin+bile salt, three populations of PTMIO were observed: a population in the lipid phase (∼60%, a_N∼13.9G), an aqueous phase population (∼20%, a_N∼15.4G) with high mobility, and an immobilized surface layer population (∼20%, a_N∼14.2G) with low mobility. However, in C14 emulsions stabilized by caseinate, only two distinct populations of PTMIO were seen: a lipid phase population (∼70%, a_N∼13.8G) and an aqueous phase population (∼30%, a_N∼15.5G) 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 (∼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.