Effect of the lipophilicity of model ingredients on their location and reactivity in emulsions and solid lipid nanoparticles

• Spin probes with various lipophilicities were studied in emulsions and solid lipid nanoparticles.
• The location and reactivity of spin probes were determined by EPR.
• Spin probes partitioned between lipid, interfacial and aqueous environments.
• Oil phase crystallization expelled spin probes from the droplet core.
• Probe's adsorption at the interface had a protective effect against reaction with aqueous components.

The aim of this study was to use electron paramagnetic resonance (EPR) to investigate the distribution and chemical reactivity of small molecules with varying lipophilicities [lipophilic: 4-phenyl-2,2,5,5-tetramethyl-3-imidazoline-1-oxyl nitroxide (PTMIO), amphiphilic 16-doxyl-stearic acid (16-DS) and hydrophilic 4-hydroxy-2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPOL)] in protein-stabilized emulsions and solid lipid nanoparticles (SLN). PTMIO in emulsions was distributed between the lipid and aqueous phases but in SLN was excluded from the droplets to the aqueous phase. 16-DS in emulsions was distributed between the lipid phase and interface but in SLN was excluded from the droplets to the interface. TEMPOL was present exclusively in the aqueous phase in both emulsions and SLN. The probes were all highly mobile except for the 16-DS at the interface. The rate of reduction of the nitroxide group of the probes by ascorbate anions was much faster when the probe molecules were mainly located in the aqueous whereas the anchoring of 16-DS molecules at the interface resulted in the highest chemical stability, which was independent of the oil's physical state. This work shows that the location and mobility of small molecules is determined by their structure and the physical state of the lipid in emulsions, and thus greatly affects their chemical stability.

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