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

The interaction between aqueous solutions of trivalent lanthanide ions (M3+: 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 La-139 NMR spectroscopy. It has been found that at concentration ratios, r = [SDS]/[M+], 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 (M3+: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. (C) 2010 Elsevier Inc. All rights reserved.