Surface and micellar properties of sodium 1-alkyl decane sulfonates

Abstract

Surface and thermodynamic properties of well-purified sodium 1-alkyl decane sulfonates in aqueous solutions and in sodium chloride solutions were measured at 45°C. The solubility, critical micelle concentration (CMC), minimum area per molecule, and standard thermodynamic parameters of adsorption and micellization were studied. The minimum area per molecule at the air-aqueous interface increases from 0.57 to 1.04 nm 2 when the side-chain length is increased from 0 to 9 carbon atoms. The CMC value of the ethyl and butyl decane sulfonates is somewhat higher in comparison with their straight chain homologues. The other terms obey the usual relationship log CMC = − an + b. The micelle ionization degree α increases as the side chain grows; this fact could be correlated with the changes of areas per head group at the micelle surface. Standard free energy change of adsorption per methylene group is constant in the series when calculated from the values obtained at the surface of solutions containing sodium chloride. The first term containing ethyl substituent shows a standard free energy of micellization value which is less negative than the value observed for the straight isomer. From the 1-butyl decane sulfonate a constant standard free energy increment per CH 2 is observed.