title: 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 (1mM) 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 (139)La 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.
authors.affiliation: Dept. of Chemistry, University of Coimbra, 3004-535 Coimbra, Portugal.
authors.forename: Rui F P
authors.initials: RF
authors.lastname: Pereira
authors.forename: María J
authors.initials: MJ
authors.lastname: Tapia
authors.forename: Artur J M
authors.initials: AJ
authors.lastname: Valente
authors.forename: Rachel C
authors.initials: RC
authors.lastname: Evans
authors.forename: Hugh D
authors.initials: HD
authors.lastname: Burrows
authors.forename: Rui A
authors.initials: RA
authors.lastname: Carvalho
mesh_terms: D005427:Flocculation; D007477:Ions / Q000737:chemistry; D028581:Lanthanoid Series Elements / Q000737:chemistry*; D049449:Luminescence; D009682:Magnetic Resonance Spectroscopy; D033362:Powder Diffraction; D012967:Sodium Dodecyl Sulfate / Q000737:chemistry*; D012995:Solubility; D013054:Spectrophotometry, Atomic; D013501:Surface-Active Agents / Q000737:chemistry*; D014867:Water / Q000737:chemistry; D014961:X-Ray Diffraction
chemical_list: D007477:Ions; D028581:Lanthanoid Series Elements; D013501:Surface-Active Agents; D014867:Water; D012967:Sodium Dodecyl Sulfate