Self-assembly of a surfactin nanolayer at solid-liquid and air-liquid interfaces

Surfactin, a sustainable and environmentally friendly surface active agent, is used as a model to study the adsorption of biosurfactants at hydrophobic and hydrophilic solid-liquid interfaces as well as the air-liquid interface. Surfactin adsorption was monitored as a function of time and concentration using surface plasmon resonance (SPR) technique in the case of the solid-liquid interfaces or the drop shape analysis (DSA) technique in the case of the air-liquid interface. The results obtained in this study showed that surfactin adsorption at the hard hydrophobic (functionalized with octadecanethiol) solid-liquid and the soft air-liquid interface were 1.12 +/- A 0.01 mg m(-2) (area per molecule of 157 +/- A 2 (2)) and 1.11 +/- A 0.05 mg m(-2) (area per molecule of 159 +/- A 7 (2)), respectively, demonstrating the negligible effect of the interface hardness on surfactin adsorption. The adsorption of surfactin at the hydrophilic (functionalized with beta-mercaptoethanol) solid-liquid interface was about threefold lower than its adsorption at the hydrophobic-liquid interfaces, revealing the importance of hydrophobic interaction in surfactin adsorption process. The affinity constant of surfactin for the investigated interfaces follows the following order: air > octadecanethiol > beta-mercaptoethanol. Biosurfactants, such as surfactin, are expected to replace the conventional fossil-based surfactants in several applications, and therefore the current study is a contribution towards the fundamental understanding of biosurfactant behavior, on a molecular level, at hydrophobic and hydrophilic solid-liquid interfaces in addition to the air-liquid interface. Such understanding might aid further optimization of the utilization of surfactin in a number of industrial applications such as enhanced oil recovery, bioremediation, and detergency.