Dynamic surface tension and adsorption mechanism of surfactin biosurfactant at the air–water interface

The dynamic adsorption of the anionic biosurfactant, surfactin, at the air–water interface has been investigated in this work and compared to those of two synthetic surfactants: the anionic sodium dodecylbenzenesulfonate (SDBS) and the nonionic octaethylene glycol monotetradecyl ether (C₁₄E₈). The results revealed that surfactin adsorption at the air–water interface is purely controlled by diffusion mechanism at the initial stage of the adsorption process (i.e., t→ 0), but shifts towards a mixed diffusion-barrier mechanism when surface tension approaches equilibrium (i.e., t→ ∞) due to the development of an energy barrier for adsorption. Such energy barrier has been found to be a function of the surfactin bulk concentration (increases with increasing surfactin concentration) and it is estimated to be in the range of 1.8–9.5 kJ/mol. Interestingly, such a trend (pure diffusion-controlled mechanism at t→ 0 and mixed diffusion-barrier mechanism at t→ ∞) has been also observed for the nonionic C₁₄E₈ surfactant. Unlike the pure diffusion-controlled mechanism of the initial surfactin adsorption, which was the case in the presence and the absence of the sodium ion (Na⁺), SDBS showed a mixed diffusion-barrier controlled at both short and long time, with an energy barrier of 3.0–9.0 and 3.8–18.0 kJ/mol, respectively. Such finding highlights the nonionic-like adsorption mechanism of surfactin despite its negative charge.