Structure and wettability of heterogeneous monomolecular films of phospholipids with cholesterol or lauryl gallate

The mica supported Langmuir-Blodgett (LB) monolayers composed of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) were used to investigate the effect of cholesterol (Chol) or lauryl gallate (LG) on physicochemical properties of the phospholipid DPPC/DOPC monomolecular film treated as a model leaflet of the bilayer structured membrane. First the surface topography studies of these systems were carried out by means of atomic force microscopy (AFM). Then wettability was investigated via the contact angle measurements and the surface free energy determination. Finally, the time-offlight secondary ion mass spectrometry (TOF-SIMS) was applied for the surface chemical analysis. Wettability of the monolayers was found to be closely related to the surface chemistry, molecular packing and ordering, miscibility and co-localization of molecules rather than nanoscale roughness. The comparison of the surface polarity determined from the values of contact angle and secondary ion signal intensity provided consistent results. The correlation between the changes of the electron-donor parameter and the 184/57 ratio revealed significance of using the latter quantity as a polarity indicator. The carried out studies show preferential association of components providing a framework for understanding the mechanism of heterogeneities formation within the membrane which reflect the specialization of various regions.