Abstract
Stable aqueous dispersions of α-tocopherol acetate (α-TA) were obtained by cosonication with dipalmitoylphosphatidylcholine (DPPC) in the α-TA mole fraction range of 0.1-0.8. In order to clarify the dispersal mechanism, the dispersed particles were characterized and the interaction between α-TA and DPPC was investigated using several physicochemical techniques. Dynamic light scattering measurements showed that the diameter of the dispersed particles was 50-75 nm. The trapped aqueous volume inside the particles was determined fluorometrically using the aqueous space marker calcein. The trapped volume in the α-TA/DPPC particles decreased remarkably with the addition of α-TA into small unilamellar vesicles of DPPC. The decline in the fraction of vesicular particles was also confirmed by fluorescence quenching of N-dansylhexadecylamine in the DPPC membrane by the addition of the quencher CuSO4. These results indicate that the excess α-TA separated from the DPPC bilayers is stabilized as emulsion particles by the DPPC surface monolayer. The monolayer-bilayer equilibrium of α-TA/DPPC mixtures was estimated by measurement of spreading and collapse pressures. The results showed that the coexistence of emulsion particles (surface monolayer of DPPC+core of α-TA) with vesicular particles (bilayer) was critically important for the formation of the stably dispersed particles of the lipid mixture.
