Interaction energy of model lipid membranes with water and diiodomethane

The Langmuir–Blodgett/Schaefer (LB/LS) method and contact angle measurements were applied to study interaction energy of lipid films with water and diiodomethane. The membranes were formed on mica and constituted of single-component saturated phospholipids 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) or unsaturated 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and cholesterol (Chol), or they constituted binary systems of DPPC/DOPC or DPPC/Chol with varying DOPC and Chol molar ratio. The surface free energy of single and binary lipid mono- and bilayers was evaluated from the advancing and receding contact angles of water and diiodomethane using the contact angle hysteresis (CAH) model. Taking into account the areas occupied by one molecule in the packed monolayers determined from π–A isotherms and the calculated surface free energy of the layers, the molar interaction energies of one lipid mole with water or diiodomethane were evaluated. Thus obtained values of molar interaction energy of DPPC and DOPC agree well with some literature data available. It is believed that this procedure of the energy evaluation provided a framework for better understanding of the interactions occurring in the biological lipid systems and their wettability.