Thermodynamic parameters of some biosurfactants and surfactants adsorption at water-air interface

The measurements of surface tension of aqueous solutions of rhamnolipid (RL) and surfactin (SF) at different temperatures were made. On the basis of the obtained results Gibbs surface excess concentration was determined at the water-air interface which next was applied for determination of the surface coverage by surfactant molecules as well as for calculation of the standard Gibbs free energy, enthalpy and entropy of adsorption. For determination of the standard Gibbs free energy of adsorption different known methods were analyzed and next applied. The obtained results were compared to those determined by the method proposed in this paper. Good agreement between the values of Gibbs standard free energy of adsorption determined from the proposed equation and the linear form of Langmuir adsorption isotherm as well as from the Langmuir equation modified by de Boer was obtained. To check the applitability of the proposed equation, the Gibbs standard free energy of adsorption was calculated for many other ionic and nonionic surfactants. The results of the calculations were compared to those determined using other methods. It appeared that there is the best agreement between the values obtained using the proposed equation and those calculated by the modified Langmuir equation. (C) 2017 Elsevier B.V. All rights reserved.