On the flocculation and re-dissolution of trivalent lanthanide metal ions by sodium dodecyl sulfate in aqueous solutions

Abstract

The interaction between aqueous solutions of trivalent lanthanide ions (M 3+: La(III) and Gd(III) and Tb (III)) at fixed (1 mM) concentrations and various concentrations of sodium dodecyl sulfate (SDS), ranging from pre- to post-micellar, has been investigated by ICP-AES (La(III) and Gd(III)), luminescence spectra (Gd(III)) and lifetimes (Tb(III)) and 139La NMR spectroscopy. It has been found that at concentration ratios, r = [SDS]/[M 3+], around the charge neutralization value (ca. 3), dodecyl sulfate (DS −) anion interacts with the metal ions to form insoluble aggregates. The metal ion–DS − complexes remain flocculated for r values below 5–6 (Gd(III) and La(III), respectively), while at higher r values, re-dissolution takes place. The flocculated aggregates have been characterized by X-ray powder diffraction, and show a lamellar structure. Job plot method indicates that a complex with a 1:3 (M 3+:DS −) stoichiometry is formed. From ICP-AES analysis, a model based on a three-step mechanism has been developed and association constants calculated. For all systems the interaction between DS − and metal ions follows an associative process with K values ranging between K 1 = 10 and K 3 = 10 4. These data are discussed on the basis of the physical–chemical characteristics of the metal ions. Re-dissolution with increasing surfactant concentration is attributed to formation of mixed lanthanide/sodium dodecyl sulfate aggregates, with the relative lanthanide fraction in these species decreasing with increasing SDS concentration.