Lipid monolayers at the oil/water interface as characterization tools for structure-composition relationship in lipid self-assembled emulsion particles.

PERILLO MA; MOTTOLA M

Buenos Aires

Workshop; Frontiers in Physical Sciences; 2016

Max Plank

Lipid microemulsions are systems commonly used toencapsulate, maintain and release molecules of pharmacological interest. Inorder to gain insights on interfaces within microemulsions, at the molecularlevel1, we have characterized the composition and thermal behavior ofmonomolecular layers of DPPC at the Vaseline 80 SSU /water interface (O/W) byusing a Langmuir interfacial trough. For the film preparation, two differenttechniques were assayed to make both adsorbed (AM) or spread (SM) monolayers 2.AMs were formed by the adsorption of DPPC to the OW interface after the lipidsolution was spread over the upper phase. Since the distance from the point ofspreading to the Wilhelmy plate affected the lag time to start sensingmolecules adsorbed at the OW interface, at least 30 min were required for theevaporation of the solvent and the initial surface pressure (πi) stabilization.In contrast, to form SM, the DPPC solution was spread over an air/waterinterface (A/W), and then the oil was poured over it to obtain the O/W. Similarresults were obtained with both techniques but the second method was easiest toperform. The apparent mean molecular areas (MMAapp) of DPPC measured at the O/Wwere 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 valueshigher than those observed at the A/W at the same T. Since there are technicaldifficulties to observe a lipid monolayer by epifluorescence microscopy at O/W,monolayers of DPPC/Vaseline mixtures at A/W which were imaged by Brewster AngleMicroscopy (BAM). The shape of the condensed domains, varied slightly with themixture composition. However, in conditions of Vaseline excess (XVAS=0.9),we observed the emergence of bright structures which were associated tomultilayered structures, due to their relatively high reflectivity. Our resultsindicate that alkane molecules from the upper phase insert between thehydrocarbon chains of the phospholipid, and are not squeezed out even at thehighest compressions achieved before the collapse point.