The dimyristoylphosphatidylcholine （DMPC） and a newly isolated surfactin analogue, [Asp1, Glus] surfactin-C16 in mixed monolayer was studied. Surface pressure-molecular area isotherms combined with atomic force microscopy （AFM） were used to probe the interfacial behavior of DMPC and surfactin-C16. The results show that the miscibility between DMPC and surfactin-C 16 is dependent on surfactin molar fraction （Xs） and surface pressure The two components are partially miscible on pure water （pH= 6） when Xs is 0.5, indicating that there exists mutual interactions between two components. AFM images showed similar pattern in surfactin-C16 monolayer and DMPC/surfactin-C 16 （Xs = 0.5） mixed monolayer. Based on the finding it is believed that DMPC would coalescent into surfactin-C 16 surface clusters in the mixed monolayer. Furthermore, the excess free energy of mixing （△ G^（ex）m ） was calculated to investigate the effect of subphase pH on the miscibility of two components in the mixed monolayer. It is found that the △ G^（ex）m was negative on alkaline subphase （pH= 10） when Xs equals to 0.75, indicating the molecular interactions between surfactin and DMPC are attractive at the condition. The results presented here suggest that hydrophobic interactions between surfactin-C 16 and DMPC play an important role in mutual interactions, and the interfacial activity was influenced by the ionization state of polar groups of surfactin-C 16.
