Thermodynamic analysis of the interaction between trivalent metal ions and sodium dodecyl sulfate: An electrical conductance study

The interaction between the trivalent cations chromium(III), lanthanum(III) and gadolinium(III) and the anionic surfactant sodium dodecyl sulfate (SDS) has been studied in aqueous solutions using electrical conductivity. This data, together with previous results for aluminium(III) and SDS, are considered in terms of interaction of the metal ions and dodecyl sulfate at a critical aggregation concentration (cac) considerably below the surfactant cmc. The aggregates have a stoichiometry somewhat greater than that expected from simple charge neutralization, possibly due to formation of lamellar structures. Aggregates redissolve upon increasing SDS concentration, and at higher surfactant concentrations formation of surfactant micelles is observed. From detailed thermodynamic analysis of this process considerations are given on the major factors responsible for differences in behavior between the four trivalent cations.