In Situ Measuring Partition Coefficient at Intact Nanoemulsions: A New Application of Single-Entity Electrochemistry

We report a new application of the single-entity electrochemistry (SEE) to in situ measure a partition coefficient at intact nanoemulsions (NEs). The partition coefficient at intact NEs is the most crucial physicochemical property to determine the uptake of delivery molecules inside NEs. It, however, has not been unequivocally elucidated by currently existing techniques based on ex situ measurements. Herein, we apply the single-entity electrochemistry (SEE) to directly and quantitatively measure the partition coefficient at NEs in situ. In this work, we use NEs featured with amphiphilic triblock copolymer (Pluronic F-127) as a model system to extract/preconcentrate 2-aminobiphenyl (2-ABP) dissolved in the water and demonstrate a new application of SEE to in situ quantitatively estimate the amounts of 2-ABP distributed into each intact NE. Our SEE measurements reveal that the partitioning is governed by extraction of 2-ABP inside NEs rather than its adsorption on the NE surface, and this extraction is remarkably efficient with up to ∼8 orders of magnitude of the preconcentration factor, thus leading to the unprecedentedly large partition coefficient of 1.9 (±1.4) × 10¹⁰. This result implies that not only the thermodynamic distribution but also the intermolecular interaction of extracted compounds inside NEs could play a significant role in the apparent partition coefficient (P = 1.9 (±1.4) × 10¹⁰). The experimentally determined partition coefficient was validated by molecular dynamics (MD) simulations with showing a stabilizing role of intermolecular interaction in the partitioned system. We further verified our methodology with other compounds exhibiting aromatic properties, e.g., ferrocenemethanol. Significantly, our new approach can be readily applicable to investigate practical NEs commercially marketed for drug, food, and cosmetics.