Surface free energy and topography of mixed lipid layers on mica

The Langmuir–Blodgett/Schaefer (LB/LS) method, AFM technique and contact angle measurements were used to investigate the formation, structure and energetic properties of lipid films, which were constituted of single-component or binary systems formed by phospholipids: saturated 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), unsaturated 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and triglyceride: glyceryl trioleate (GTO) or their mixture DPPC/DOPC and DPPC/GTO with a different DOPC or GTO molar ratio. The surface free energy and its components (apolar and polar interaction) for single and binary lipid mono- and bilayer systems were evaluated from the advancing and receding contact angles of water, formamide and diiodomethane, using two different models of interfacial interactions, namely, van Oss et al. (LWAB) and the contact angle hysteresis (CAH). The molecular structure of these layers was studied by means of atomic force microscopy (AFM). The thermodynamic analysis of the excess area and excess free energy of DPPC/DOPC mixture indicated that lateral phase separation took place for these two components. The AFM images also confirmed the phase separation, which was visualized as condensed DPPC-rich domains and a fluid DOPC matrix. Saturated DPPC mono- and bilayers exhibited much lower surface free energy than unsaturated DOPC ones, probably due to the differences in the packing of molecules. In the DPPC/DOPC mixed layers the energy increased with increasing mole fraction of DOPC in the layer. For DPPC/GTO layers a similar trend in the energy changes as a function of GTO amount was observed. These studies provided a fundamental framework for better understanding of the interactions occurring in the used lipid systems.