Critical micelle concentration, composition and thermodynamic properties of n-octyl-β-d-glucopyranoside and sodium dodecylsulfate mixed micelles

Abstract

The measurements of density and viscosity of the aqueous solution of n-octyl-β-d-glucopyranoside (OGP) and sodium dodecylsulfate (SDDS) mixtures were made. On the basis of the results and those of surface tension obtained earlier the critical micelle concentration (CMC) as well as values of apparent and partial molar volume of OGP and SDDS were determined. Their partial molar volume was compared to that calculated using the bond length between the atoms in the OGP and SDDS molecules, the angle between the bonds and the average distance between the surfactant molecules and those of water. From the analysis of the CMC values it appeared that there is no ideal mixing of OGP and SDDS in the micelle and therefore the composition of mixed micelles, the activity coefficient of their components and the parameter of intermolecular interactions were determined. On the basis of the parameter of intermolecular interactions and CMC of SDDS and OGP, the synergetic effect in the mixed micelle formation was analyzed. In turn, using the mole fraction of OGP and SDDS in the micelle and their activity coefficient the standard Gibbs free energy of OGP and SDDS micellization was calculated. Next, the values obtained from this calculation were used for determination of the standard Gibbs free energy of SDDS and OGP mixture micellization. This energy was compared to that evaluated directly based on CMC of the SDDS and OGP mixture. It proved that there are differences between the values of the standard Gibbs free energy of SDDS and OGP mixture micellization calculated in these two ways which at the first approximation were close to the Gibbs free energy of OGP and SDDS mixing in the micelles determined based on their mole fraction and activity coefficient in the micelles.