CONICET | Buscador de Institutos y Recursos Humanos

Lipid microemulsions (ME), commonly used to encapsulate moleculesof pharmacological interest, are oil in water dispersions stabilizedby an interfacial layer of a surfactant. Previously, studies onmonomolecular layers (ML) of phospholipids at the Vaseline (VAS)/water interface (MLV/W) used as an experimental model of ME, allowus to define a surface pressure (p) vs. composition phase diagramwhich 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 usedas a model of MLV/W to allow topographic analysis by different microscopies.Thus, in the present work we could evidenced the appearanceof surface droplets along compression that did not disappearedupon decompression. Spectral confocal microscopy of VAS/DPPC mixed MLa/w using Nile Red (NR), confirmed that dropletswere composed of VAS since this probe partitioned in the dropletsexhibiting a fluorescence emission spectra (ES) similar to that inVAS bulk phase. However, NR also partitioned in the liquid expandedPC monolayer phase with an ES similar to that in VAS dropletsbut different from that reported in bilayers. On the other hand, epifluorescencemicroscopy revealed that droplets were not markedby NBD-PE probe which appeared as dark points. Brewster AngleMicroscopy data (assuming the liquid VAS refractive index) revealedthat inspite 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 structurescorrespond to isotropic VAS (lenses) and are consistent with thenull 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.