Partitioning behavior of hydrophobic bioactives in vesicles

Vesicles can be effective as carrier particles for poorly water-soluble bioactives within an aqueous medium. Advancing the performance of such carriers relies on knowledge of equilibrium solute partitioning between vesicles and the outside continuum. This partitioning behavior determines the maximum capacity of the vesicles, their ability to protect and retain the solute for prolonged periods, and the mechanisms and rate of release at a target. The objective of this research was to quantify solute partition coefficients between phosphatidylcholine (PL) vesicles and water, for solute–to–PL ratios (r) ranging over two orders of magnitude: from highly dilute to values near saturation. Results were compared with fundamental theories for the solute chemical potential, in order to advance our conceptual understanding of the role of composition, phospholipid structure, vesicle size, and temperature on partitioning. Partition coefficients for limonene, defined as the ratio of solute mole fraction within the vesicle bilayers to concentrations of dissolved aqueous solute, were measured using solid phase microextraction (HS–SPME) via short-time sampling of the vapor headspace above the equilibrated vesicle dispersion, and quantified using GC/MS. Resulting vapor phase concentrations were analyzed using equilibrium and mass balance relationships to determine the partition coefficient K_lipw. K_lipw values as high as 12 mM^–1 were obtained near 25˚C, and up to ~10 solutes per PL could be solubilized by the vesicles. K_lipw was only weakly dependent on the phospholipid structure, vesicle size, and temperature, as long as bilayers were in a fluid phase, and could be treated theoretically as an ideal–dilute mixture. For DMPC at 15˚C, similar theory could be used to predict formation and partitioning of limonene into gel, fluid, and gel/fluid coexisting bilayer phases, as shown in the figure below.