Characteristics of a phospholipid DOPC/cholesterol bilayer based on surface free energy and its components

The effect of the cholesterol (Chol) content (x(Chol) = 0.25; 0.5; 0.75) on the wettability changes of solid supported bilayers of unsaturated 1,2-dioleoyl-sn-glycero-3-phosphatidylcholine (DOPC) was investigated at 20 degrees C and 37 degrees C. The effect was determined via the changes of the film's apparent surface free energy and its components. The energy changes were calculated from the measured apparent advancing contact angles. The obtained results demonstrate that the amount of cholesterol in the DOPC bilayers influences the film's surface free energy and especially the electron-donor component. The changes depend on the system temperature but are not linear relative to the cholesterol content. Also the surface roughness of the DOPC/Chol surface was determined by atomic force microscopy (AFM). The average roughness, root-mean-square roughness and average height parameters showed that up to x(Chol) = 0.5 the surfaces were very smooth (subnanometer roughness). At x(Chol) = 0.75 phase separation occurred with formation of visible cholesterol islands (domains) and the roughness parameters increased to 3-4.8 nm. This allows preparation of film surfaces with targeted properties which would potentially have practical applications.