A molecular, rheological and thermodynamic study of lipid-vaseline mixtures

MOTTOLA M; PERILLO MA

Tucumán

Congreso; III LAFeBS- IX IberoAmerican Congress of Biophysics ? SAB2016; 2016

Lipidmicroemulsions (ME), commonly used to encapsulate molecules of pharmacologicalinterest, are oil in water dispersions stabilized by an interfacial layer of asurfactant. Previously we defined monomolecular layers of phospholipids at theVaseline 80 SSU (VAS)/water interface as an experimental model of ME, and reporteddata of their composition and thermal behavior using a Langmuir interfacialtrough. Results suggested that molecules from the oil phase were incorporatedin the monolayer transforming it in a VAS/DPPC mixture. In the present work we confirmedthis hypothesis by studying the interfacial behavior of VAS/DPPC pseudobinary mixturesat the air/water. According to 1H-NMR and GC-MS analysis, VAS was amixture of alkanes with carbon lengths (CL) ranging from 19 to 29 (mean CL=25±3)and a weighted mean molecular mass of 346,89 g/mol. It is noteworthy that VASwas unstable at the air-water interface (it did not form monolayers) so, at XVAS=1,the value for the mean molecular area (Mma) was 0 nm2. Thus, the Mmafor all mixtures was proportional to the molecular area of DPPC. At all the assayed compositionsthe p?A isotherms wereshifted to higher molecular areas with respect to pure DPPC. The p value for bidimensional phase transition (pT) increased from ~8.5for pure DPPC to 15.5 mN/m for mixtures containing a molar fraction of VAS (XVAS) within the 0.05-0.6 range. Up onXVAS increased, the phase transition became less cooperative. At XVAS=0.8it disappeared and the monolayer acquired a smooth liquid-expanded behavior. TheMma vs. XVAS plot revealed that DPPC/VAS mixtures exhibited huge positivedeviations from ideality at all compositions, indicating repulsiveintermolecular interactions. The p-XVASphase diagram allowed predicting a phase separation at high pand XVAS This conclusion was supported by BAMimages which exhibited the emergence of bright dots at XVAS=0.9,possibly due to collapsed multilayered structures.