Thermodynamic properties of surfactant sodium n-heptyl sulfonate in water and in aqueous solutions of poly(ethylene glycol) at different temperatures

The volumetric, compressibility and electrical conductivity properties of sodium n-heptyl sulfonate (C7SO3Na) in pure water and in aqueous poly(ethylene glycol) (PEG) solutions were determined at different temperatures below and above the micellar composition range. At each temperature, the infinite dilution apparent molar volumes of the monomer and micellar state of C7SO3Na in aqueous PEG solutions respectively are smaller and larger than those in pure water. However, the values of the infinite dilution apparent molar isentropic compressibility of both monomer and micellar states of C7SO3Na in aqueous PEG solutions are larger than those in pure water. Thermodynamic parameters of micellization of investigated surfactant in water and in aqueous solutions of PEG at different temperatures were estimated and it was found that the micelle formation process is endothermic and therefore, this process must be driven by entropy increase. The calculated Gibbs free energies of micellization for aqueous PEG solutions are more negative than those for pure water and become more negative by increasing temperature. The variation of the critical micelle concentration (CMC) of C7SO3Na in water and in aqueous PEG solutions with temperature was obtained and a comparison between the CMC of C7SO3Na obtained from different thermodynamic properties was also made.