Abstract

The biogenesis oftriglyceride (TG) Lipid Droplets (LD) and  the encapsulation of apolar substances, suchas vaseline (V), in bilayers involve a dewetting process and the formation of blisterscontaining a bulk phase of the hydrophobic molecule (HM). To gain insight intothe factors affecting the stability of the bulk structure in contact with phospholipidswe studied Langmuir films of PC with HM which are forming (TG) and non-forming(V) monolayers. Then, we evaluate TG blisters formation using microscopy,calorimetry (DSC) and Fluorescence Anisotropy (FA). 

EPC/TG and EPC/V monolayerscollapse at a composition-dependent surface pressure (SP) producing micrometriclenses (observed by BAM and fluorescence with Nile red probe) interpreted asbulk HM phase. EPC/TG compression isotherms are reversible after collapse (lensesdisappeared at SP below the collapse pressure). On the contrary, in EPC/V, onceformed, V lenses remain on the surface. To analyze the monolayer wettability, HMlenses height was calculated assuming refractive indexes similar to those for bulkTG and V. We found that TG lenses were thicker than V lenses. This allowedestimating contact angles, interfacial tensions and spreading/enteringcoefficients whose values and wettability arguments indicated that seggregated TGwould not spread along the intrabilayer space but would form discretestructures, whereas V would tend to spread along the intrabilayer space (as observedwith oher HM, such as n-decane and squalene).

Free standing bilayers ofPC/TG did not exhibit microscopic blisters. However, DSC of dpPC/TG MLVs atcompositions where TG remains in the bilayer showed a thermotropic behavior similarto that of pure PC, suggesting a phase separation inside a global bilayerstructure. FA of dpPC/MLVs using DPH and TMA-DPH (hycrocarbon chain and polarhead regions probes) are coherent with X_TG~TG solubility defined inmonolayers and suggest TG segregation inside the bilayer (blisters) up to X_TG=0.25.