Effect of lipophilization on the distribution and reactivity of ingredients in emulsions

• A homologous series of lipophilized spin probes were synthesized.
• Probes distributed between phases of an emulsion according to their lipophilicity.
• The rate of reduction by ascorbate was greater for more hydrophilic probes.
• Larger probes precipitated when added to the emulsion as ethanolic solution.
• Precipitated probes slowly diffused to lipid droplets.

HypothesisThe reactivity of small molecules in emulsions is believed to depend on their partitioning between phases, yet this is hard to verify experimentally in situ. In the present work, we use electron paramagnetic resonance (EPR) spectroscopy to simultaneously measure the distribution and reactivity of a homologous series of lipophilized spin probes in an emulsion.Experiments4-Hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPOL) was derivatized with saturated fatty acids to create a series of spin probes with increasing lipophilicity (C4-, C8-, C12-, and C16-TEMPO). The probes were added to a 10 wt.% tetradecane-in water emulsions (d32 ∼ 190 nm) stabilized with sodium caseinate (1 wt.% in the aqueous phase, pH 7). The distribution of the probes between phases was measured by electron paramagnetic resonance (EPR) spectroscopy.FindingsTEMPOL partitioned into the aqueous phase, C4-TEMPO distributed between the lipid and aqueous phases (69% and 31% respectively) while the more lipophilic probes dissolved exclusively within the lipid droplets. Interestingly, the more lipophilic probes initially precipitated upon their addition to the emulsion, and only slowly redistributed to the droplets over hours or days, the rate of which was dependent on their carbon chain length. The reactivity of the probes with aqueous an aqueous phase reductant (ascorbate) generally depended on the proportion in the aqueous phase (i.e., TEMPOL > C4-TEMPO > C8-TEMPO ∼ C12-TEMPO ∼ C16-TEMPO).

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