Aggregation behavior and surface morphology studies of surfactin in Langmuir–Blodgett films

Surfactin monolayer, which was transferred from the air–water interface onto mica substrate by Langmuir–Blodgett (LB) technique, was characterized with atomic force microscopy (AFM). The effect of deposition pressure on morphologies of SuC14 (surfactin analogue with a β-hydroxyl fatty acid chain of 14 carbon atoms) LB films was investigated. AFM topographic image reveals phase separation at lower deposition pressure, indicating the existence of surfactin domains. At higher deposition pressure, the mica substrate was fully covered with surfactin monolayer, and spherical aggregates were formed atop the monolayer. Surfactin analogues (same polar head, different alkyl chain length) were applied to study the influence of the hydrophobic chain length on the aggregation behavior. On the basis of Fourier transform infrared (FTIR) measurements, the hydrogen bonding is supposed to be involved in the aggregates. These results, concerning the aggregates formation in surfactin LB films, provide new insights into the interfacial behavior of the amphiphilic lipopeptide at the air–water interface.