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

MOTTOLA, MILAGRO; PERILLO, MARÍA ANGÉLICA

San Miguel de Tucuman

Congreso; III Latin American Federation of Biophysical Societies (LAFeBS), IX IberoAmerican Congress of Biophysics, XLV Reunion Anual SAB; 2016

Sociedad Argentina de Biofísica

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 we defined monomolecular layers of phospholipids at Vaseline 80 SSU (VAS)/water interface as an experimental model of ME, and reported data of their composition and thermal behavior using a Langmuir interfacial trough. Results suggested that molecules from oil phase were incorporated in the monolayer transforming it in a VAS/DPPC mixture. In the present work we confirmed this hypothesis by studying the interfacial behavior of VAS/DPPC pseudobinary mixtures at the air/water. According to 1H-NMR and GC-MS analysis, VAS was a mixture 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 VAS was 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 Å2. Thus, the Mma for all mixtures was proportional to the molecular area of DPPC. At all the assayed compositions the ?A isotherms were shifted to higher molecular areas with respect to pure DPPC. The  value for bidimensional phase transition (T) increased from 8.5 for pure DPPC to 15.5 mN/m for mixtures containing a molar fraction of VAS (XVAS) within the 0.05-0.6 range. Up on XVAS increased, the phase transition became less cooperative. At XVAS=0.8 it disappeared and the monolayer acquired a smooth liquid-expanded behavior. The Mma vs. XVAS plot revealed that DPPC/VAS mixtures exhibited huge positive deviations from ideality at all compositions, indicating repulsive intermolecular interactions. The -XVAS phase diagram allowed predicting a phase separation at high  and XVAS This conclusion was supported by BAM images which exhibited the emergence of bright dots at XVAS=0.9, possibly due to collapsed multilayered structures. Acknowledgements: CONICET, FONCYT, SeCyT-UNC.