Surface Gibbs energy interaction of phospholipid/cholesterol monolayers deposited on mica with probe liquids

The mica supported binary monolayers containing phospholipids: 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), 2-oleoyl-1-palmitoyl-sn-glycero-3-phosphocholine (POPC), 1,2-dipalmitoyl-sn-glycero-3-phospho-rac-(1-glycerol) (DPPG), and cholesterol (Chol), mixed at different molar fractions, were investigated by measurements of the contact angles of water, formamide and diiodomethane. This allowed calculation of apparent surface Gibbs energy (further in the paper termed as 'surface free energy') of the monolayers according to the theoretical approach developed by Chibowski (contact angle hysteresis model, CAH). Then, based on the surface free energy values, the molar interaction Gibbs energy of the lipid molecules with the given probe liquid was evaluated. These values correlate with the values of excess area, interpreted as an indicator of the condensing effect of cholesterol on phospholipid monolayers at the air-water interface. The results indicate that the thermodynamic parameters of interactions depend on the monolayer composition and the probe liquid used to their determination. Changes of the parameters are discussed in relation to the monolayer packing, ordering, tilting of the molecules, and properties of the probe liquids as well.