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 139La NMR spectroscopy. It has been found that at concentration ratios, r = [SDS]/[M3+], 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 K1 = 10 and K3 = 104. 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.

The addition of SDS, in its unimer form, to trivalent lanthanide ion solutions leads to a flocculation process followed by re-dissolution, as seen by ICP-AES.Figure optionsDownload high-quality image (110 K)Download as PowerPoint slideResearch highlights
► Trivalent lanthanide ions form lamellar flocs with SDS.
► These redissolve in excess surfactant to form mixed aggregates.
► At higher concentrations normal micelles form, but the cmc decreases.