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 -> infinity) 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 -> infinity) 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.