X-ray reflectivity and BAM studies on the LB film of mixed DPPC/DC-cholesterol monolayer

A synthetic charged lipid, DC-Chol (3β-[N-(dimethylaminoethane)-carbamoyl]-cholesterol), was used to mix with the DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine) to form monolayers at the air–liquid interface. The mixed DPPC/DC-Chol monolayer at the air–water interface was studied by measuring the surface pressure versus molecular area isotherms and BAM. The pressure–area isotherm shows that typical plateau of DPPC disappears by adding DC-Chol more than 30%. It was also found that the DPPC/DC-Chol system has only one minimum excess area point at XDC-Chol = 0.2. The absence of the minimum excess area point at higher DC-Chol mixing ratio should be due to the charge interaction of DC-Chol. It was observed by BAM that the domains formed in the plateau region of the isotherm become smaller by adding small amounts of DC-Chol. These domains as well as the plateau region disappear at XDC-Chol ≧ 0.3. The characteristics of the mixed monolayer seems to be dominated by the charged DC-Chol at XDC-Chol ≧ 0.3. The results of X-ray reflectivity measurements of the mixed DPPC-d62/DC-Chol LB monolayer supported on silicon wafer show that the DC-Chol molecules are positioned in the DPPC-d62 hydrophobic layer.