MICELLAR CHARACTERISTICS OF THE BIOLOGICAL SURFACTANT - SURFACTIN

Biological surfactants produced by bacteria and living organisms are used in the food, agricultural, pharmaceutical and medical, cosmetic, oil and gas industries, for soil remediation, and purification of industrial wastewater. They are highly biodegradable, low toxic, low cost, and are a "green" alternative to synthetic surfactants. The paper presents the results of studies of the micellar properties of the biosurfactant surfactin. The critical concentration of micelle formation of surfactin in aqueous solutions at different pH values was measured using tensometry, refractometry, and NMR diffusometry methods. Studies carried out using the tensometry method showed that the CMC of surfactin is weakly dependent on pH, and has exceptionally low values of CMC = 0.02 g/l compared to synthetic surfactants. At the same time, measurements performed by refractometry and NMR diffusometry methods give CMC equal to 0.65 g/l and 0.28 g/l, respectively. Using the Stokes-Einstein relationship, based on the self-diffusion coefficients obtained by NMR diffusometry, the effective hydrodynamic radii of surfactin molecules and micelles, as well as the aggregation numbers of micelles, were calculated depending on the concentration of surfactin in solutions. At surfactin concentrations Csurf≈ 0.02 g/l, a monolayer of surfactin molecules is formed at the air-water interface, which is observed by a sharp drop in the surface tension coefficient. The formation of premicelles and spherical micelles in solution at Csurf≈ 0.28 g/l leads to a decrease in the mobility of surfactin molecules and is manifested in changes in the refractive indices and self-diffusion coefficients. The aggregation numbers of micelles change from Nagr = 2-4 (pre-micellar region of dimers, trimers, etc.) to 10-50 (in the region of spherical, spherocylindrical micelles) and sharply increase to Nagr ≈ 7000 with the formation of vesicles at Csurf = 200 g/l.