CONICET | Buscador de Institutos y Recursos Humanos

Lipid microemulsions (ME), commonly used to encapsulate molecules of pharmacological interest, are oil in water dispersions stabilized by an interfacial layer of a surfactant. Previously, studies on monomolecular layers (ML) of phospholipids at the Vaseline (VAS)/water interface (MLV/W) used as an experimental model of ME, allow us to define a surface pressure () vs. composition phase diagram which indicated that MLV/W consists of a EPC/VAS mixture. In turn, EPC/VAS mixed ML at the air-water interface (MLa/w) can be used as a model of MLV/W to allow topographic analysis by different microscopies. Thus, in the present work we could evidence the appearance of surface droplets along compression that did not disappeared upon decompression. Spectral confocal microscopy of VAS/DPPC mixed MLa/w using Nile Red (NR), confirmed that droplets were composed of VAS since this probe partitioned in the droplets exhibiting a fluorescence emission spectra (ES) similar to that in VAS bulk phase. However, NR also partitioned in the liquid expanded PC monolayer phase with an ES similar to that in VAS droplets but different from that reported in bilayers. On the other hand, epifluorescence microscopy revealed that droplets were not marked by NBD-PE probe which appeared as dark points. Brewster Angle Microscopy data (assuming the liquid VAS refractive index) revealed that despite of the micrometer radii, these lenses have a nanometer (3-6 nm) thickness. Then, by geometrical analyses of these lenses, estimation of contact angles can be performed. Taken together, these experimental results indicate that VAS collapsed structures correspond to isotropic VAS (lenses) and are consistent with the null spreading coefficient (resultant from the interfacial tension (IT) values for VAS/W and VAS/A (372 and 332 mN/m, respectively) which also predicts that VAS against an A/W interface (IT=72 mN/m) would not form stable monolayers.