Studies on behaviors of dipalmitoylposphatidylcholine and bilirubin in mixed monolayer at the air/water interface

Abstract Mixed monolayers of dipalmitoylposphatidylcholine (DPPC) and bilirubin (BR) were prepared on different subphases. The properties of DPPC/BR monolayer, such as collapse pressure ( π coll ), limiting area per molecule ( A lim ), surface compressibility modulus, free energy (Δ G mix ) and excess free energy (Δ G ex ), were investigated based on the analysis of the surface pressure–area isotherms on pure water. The results showed that DPPC and BR were miscible and formed non-ideal mixed monolayers at the air/water interface. With the molar fraction of BR ( X BR ) increasing, the LE–LC coexistence region of DPPC monolayer was eliminated gradually. The DPPC/BR complex (M D–B ) of 1:2 stoichiometry formed as a result of the strong hydrogen bonds between the polar groups of DPPC and BR. The studies of effects of pH values and calcium ions in subphase on the DPPC/BR monolayers showed that the mixed monolayer became expanded on alkali aqueous solution and on 1 mmol/L CaCl 2 aqueous solution. The orientation of DPPC and BR at air/water interface was also discussed.