Nanometer scale organization of mixed surfactin/phosphatidylcholine monolayers

Mixed monolayers of the surface-active lipopeptide surfactin-C-15 and of di palmitoyl phosphatidylcholine (DPPC) were deposited on mica and their nanom eter scale organization was investigated using atomic force microscopy (AFM ) and x-ray photoelectron spectroscopy (XPS). AFM topographic images reveal ed phase separation for mixed monolayers prepared at 0.1, 0.25, and 0.5 sur factin molar ratios. This was in agreement with the monolayer properties at the air-water interface indicating a tendency of the two compounds to form bidimensional domains in the mixed systems. The step height measured betwe en the surfactin and the DPPC domains was 1.2 +/- 0.1 nm, pointing to a dif ference in molecular orientation: while DPPC had a vertical orientation, th e large peptide ring of surfactin was lying on the mica surface. The N/C at om concentration ratios obtained by XPS for pure monolayers were compatible with two distinct geometric models: a random layer for surfactin and for D PPC, a layer of vertically-oriented molecules in which the polar headgroups are in contact with mica. XPS data for mixed systems were accounted for by a combination of the two pure monolayers, considering respective surface c overages that were in excellent agreement with those measured by AFM. These results illustrate the complementarity of AFM and XPS to directly probe th e molecular organization of multicomponent monolayers.