Surfactin, one of the most surface-active microbial lipopeptides, can readily form an insoluble monolayer at the air/water interface. Consecutive compression-expansion cycles of surfactin with a β-hydroxyl fatty acid chain consisting of 14 carbon atoms were studied by a Langmuir film balance. A larger hysteresis loop was observed when the compression isotherm reached a plateau compared with that expanded at a lower surface pressure (20 mN•m -1 ). The 2nd cycle was shifted towards smaller

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... ar areas compared with the 1st cycle. We also studied the hysteresis cycles of the surfactin monolayer on subphase of different pH values. With a decrease in the subphase pH the hysteresis loop became smaller and the expansion isotherm curve underwent a longer pseudo plateau. Furthermore, the morphologies of the surfactin monolayers in the plateau region, which were transferred onto a mica surface, were characterized by atomic force microscopy (AFM) and scanning electron microscopy (SEM). Both AFM and SEM images gave three-dimensional surface aggregates with heights ranging from tens to hundreds of nanometers. The above results suggest that the formation of three-dimensional surface aggregates at the plateau region induces a large hysteresis loop in the surfactin monolayer, which can also be attributed to the submergence of molecules into the subphase when the peptide loop in the surfactin molecule is ionized.