Hysteresis Behavior of Surfactin Monolayer at the Air/Water Interface

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 beta-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 molecular 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.