Thermodynamic properties of rhamnolipid micellization and adsorption

Abstract

Measurementsof the surface tension, density, viscosity and conductivity of aqueous solutions of rhamnolipid at natural and controlled pH were made at 293K. On the basis of the obtained results the critical micelle concentration of rhamnolipid and its Gibbs surface excess concentration at the water–air interface were determined. The maximal surface excess concentration was considered in the light of the size of rhamnolipid molecule. Next the Gibbs standard free energy of rhamnolipid adsorption at this interface was determined on the basis of the different approaches to this energy. The standard free energy of adsorption was also deduced on the basis of the surface tension of n-hexane and water-n-hexane interface tension. Standard free energy obtained in this way was close to those determined by using the Langmuir, Szyszkowski, Aronson and Rosen, Gu and Zhu as well as modified Gamboa and Olea equations. The standard free energy of rhamnolipid adsorption at the water–air interface was compared to its standard free energy of micellization which was determined from the Philips equation taking into account the degree of rhamnolipid dissociation in the micelles.