INTERFACIAL-TENSION AND SURFACTANT DISTRIBUTION IN WATER OIL NACL SYSTEMS CONTAINING DOUBLE-TAILED SULFONATES

Interfacial tensions have been measured using the spinning drop technique in oil-water-NaCl systems containing branched alkyl sodium sulfonates: R (R)CHSO3Na with R going from 7 to 11 carbon atoms. Five normal alkanes were used: heptane, nonane, undecane, tetradecane and hexadecane. At 45°C, with addition of NACl, the interfacial tension values were found to pass through a minimum for the sulfonates with R=7,8,9. The optimum salinity required to produce a minimum depends on surfactant carbon number. A good linearity is obtained by plotting log CNaCl (min) versus surfactant carbon number. For a given surfactant, the shorter alkanes need less salt to achieve a minimum. Tetradecane and hexadecane do not lead to ultralow interfacial tensions. These measurements were performed in a very low surfactant concentration range: five times the c.m.c. at a given salinity. Surfactant distribution was determined by hyamine titration in each phase after equilibrium. It was found that the minimum in interfacial tension corresponds to the salinity for which the amphiphile is changing9 from hydrophile to lipophile for the considered oil. The alkane affinity for the surfactant decreases when increasing the alkane carbon number and thus explains the difference in salinity required to obtain the interfacial tension minima and also the absence of such minima with long-chain hydrocarbons. © 1990.