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 ± 0.01 mg m⁻² (area per molecule of 157 ± 2 Å²) and 1.11 ± 0.05 mg m⁻² (area per molecule of 159 ± 7 Å²), respectively, demonstrating the negligible effect of the interface “hardness” on surfactin adsorption. The adsorption of surfactin at the hydrophilic (functionalized with β-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 > β-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.