MAESTRI Damian Modesto
congresos y reuniones científicas
Bilayer vs o/w emulsions stability prediction from surface activity data of phospholipid-oil mixtures at the air-water interface
Congreso; XXXV Reunión Anual de la Sociedad Argentina de Biofísica; 2006
Institución organizadora:
Sociedad Argentina de Biofísica
The aim of the present work was to predict the stability of the self-assembled structures spontaneously formed when mixtures composed of triglycerides (TG) from vegetable oils (olive, O; soybean, S; and nut, N) and phospholipids (PL) (EPC, egg phosphatidilcholine and dpPC, dipalmytoilphosphatidilcholine) are dispersed in water. Oils were extracted and purified as described previously [1]. Langmuir films were prepared from chloroformic solutions of the TG-PL pseudo binary mixtures at TG mole fractions (xTG) from 0 to 1. From the surface pressure-area isotherms (p-A), we evaluated the miscibility properties of the compounds in the mixtures from the analysis of collapse (pc) and transition points (pT), constructed the bidimensional phase diagrams (mean molecular area (MMA) vs xTG) and calculated the compressional modulus (K), critical packing parameter (Pc) and excess free energy of the mixture (DGex). At xTG£0.125 the film showed no phase separation and decreasing K@ 260-150 mN/m and 100-70 mN/m for dpPC-TG and EPC-TG, compatible with a liquid condensed and liquid expanded type bidimensional phases, respectively. At xTG>0.2, the phase diagram exhibited two regions separated by a line joining the p transition points, which decreased with increasing xTG, The upper limit of the upper region was a p value similar to the PC pc (either EPC or dpPC according to the mixture). This behavior is compatible with that of an eutectic mixture. Alternatively, it can be interpreted that a single mixed monolayer phase is stable at p bellow the pT´s joining line and above this p values it collapsed forming a non-monolayer phase that coexists with a pure PC monolayer. At p<pc,TG TGs and PCs were miscible at all proportions. This is supported by the xTG dependent partial collapse. The MMA-xTG plot showed negative deviations from ideality for  dpPC-O and positive for EPC-O and no deviations for the rest of the studied mixtures. The compression from 0 to 10 mN/m required a DGex@+300 and -200 J/mol for dpPC-O and EPC-O at 0.25. Vesicle type self-aggregates are predicted from 0.5<Pc<1 values for water dispersion of dpPC-O within the p values compatible with mixtures stable at the interface. Assuming a lipid-water surface tension at the critical emulsion concentration gcmc=gmw @ pc=25 mN/m (at xTG=0.875; pc=10mN/m; MMA=1.4 nm2), 1 mm radius particles would required a surface bending energy equivalent to @3kT at 21ºC (kT=2.4 kJ/mol). EPC-SO nanoparticles of r=1 mm were experimentally obtained but coalesced reaching a 8 mm radius after 1 h.