Organization and dynamics of phospholipids at the liquid-liquid interface

PERILLO, MARÍA ANGÉLICA; MOTTOLA, MILAGRO

Buenos Aires

Congreso; Humboldt Kolleg - Frontiers in Physical Sciences.; 2016

IInternational Center for Advanced Studies

Lipid microemulsions are systems commonly used to encapsulate, maintain and release molecules of pharmacological interest. In order to gain insights on interfaces within microemulsions, at the molecular level1, we have characterized the composition and thermal behavior of monomolecular layers of DPPC at the Vaseline 80 SSU /water interface (O/W) by using a Langmuir interfacial trough. For the film preparation, two different techniques were assayed to make both adsorbed (AM) or spread (SM) monolayers 2. AMs were formed by the adsorption of DPPC to the O/W interface after the lipid solution was spread over the upper phase. Since the distance from the point of spreading to the Wilhelmy plate affected the lag time to start sensing molecules adsorbed at the O/W interface, at least 30 min were required for the evaporation of the solvent and the initial surface pressure (πi) stabilization. In contrast, to form SM, the DPPC solution was spread over an air/water interface (A/W), and then the oil was poured over it to obtain the O/W. Similar results were obtained with both techniques but the second method was easiest to perform. The apparent mean molecular areas (MMAapp) of DPPC measured at the O/W were higher than those measured at the A/W at the same lateral pressure. Moreover, the bidimensional phase transition of DPPC at the O/W occurred at π values higher than those observed at the A/W at the same T. Since there are technical difficulties to observe a lipid monolayer by epifluorescence microscopy at O/W, monolayers of DPPC/Vaseline mixtures at A/W were imaged by Brewster Angle Microscopy (BAM). The shape of the condensed domains, varied slightly with the mixture composition. However, in conditions of Vaseline excess (XVAS=0.9), we observed the emergence of bright structures which were associated to multilayered structures, due to their relatively high reflectivity. Our results indicate that alkane molecules from the upper phase insert between the hydrocarbon chains of the phospholipid, and are not squeezed out even at the highest compressions achieved before the collapse point.