Surface and volumetric properties of n-octyl-β-d-glucopyranoside and rhamnolipid mixture

Abstract

The measurements of surface tension, density and viscosity of aqueous solutions of n-octyl-β-d-glucopyranoside (OGP) and rhamnolipid (RL) were performed at constant concentration of OGP or RL. The measured values of the surface tension were compared to those calculated from the Joos equation as well as to the hypothetical one. The obtained results were analyzed with regard to adsorption at the water-air interface and micellization of OGP in the presence of RL and vice versa. On the basis of this analysis, the Gibbs surface excess concentration of the particular surfactants at the water-air interface, the Gibbs surface free energy of adsorption and micellization, the composition of the surface layer, the apparent and partial molar volumes of OGP and RL were determined. The Gibbs surface free energy of adsorption was calculated on the basis of the critical micelle concentration (CMC) and the surface tension values at this concentration as well as by using the Langmuir equation. The composition of the surface layer was compared to that of hypothetical one. In turn, apparent and partial molar volumes were discussed in the light of the volume of surfactant molecules on the basis of the length and angle of the bonds. From the obtained results it was deduced that there is a strong synergism in the reduction of water surface tension by the OGP and RL mixture as well as in the CMC. The adsorption of RL increases in the presence of OGP but OGP decreases in the presence of RL.