Conductometric study of the mixing behaviour of sodium benzoate and salicylate with an anionic surfactant sodium lauroylsarcosinate

Effects of two anionic hydrotropes – sodium benzoate (NaBz) and sodium salicylate (NaSal) – on the mixed-micelle formation with an amino-acid-based surfactant – sodium lauroylsarcosinate (SLS) – in water were investigated by the conductometric method. Specific conductivity was measured for SLS/NaBz/water and SLS/NaSal/water systems to determine the critical micelle concentration (cmc). Using the regular solution theory for non-ideal mixing, the pairwise interaction parameter, β₁₂, and micellar composition, χ, were estimated in the mixed micelle. The cmc values of the surfactant–hydrotropes mixtures were generally lower than those predicted from the ideal mixing theory. The β₁₂ values are generally negative for the two systems at all mole fractions with an average value of ?2.83 for the SLS/NaBz and ?3.31 for SLS/NaSal systems, respectively, indicative of a strong attractive interaction between the SLS/NaBz and SLS/NaSal mixed micelle. The calculated thermodynamic parameters of micellisation all indicated spontaneity in mixed-micelle formation for the systems studied.