Structural characterization of solid trivalent metal dodecyl sulfates: from aqueous solution to lamellar superstructures

Abstract

Metal dodecyl sulfates of trivalent aluminium, chromium, lanthanum and gadolinium were prepared by addition of the corresponding salts to aqueous solutions of sodium dodecyl sulfate at the natural pH (ca. 6). Using X-ray diffraction, FT-IR and NMR spectroscopy, DSC, thermogravimetry and polarizing light thermomicroscopy it is shown that metal dodecyl sulfates are formed with lamellar structures. These have different degrees of hydration, which depend upon the metal ion. In some cases there is evidence for coexistence of different lamellar phases. The metal is strongly bound electrostatically to the sulfate group, and although the alkyl chain is in an extended conformation, there are suggestions of local disordering of the methylene groups adjacent to the anionic head group. SEM with an EDS probe on the trivalent metal complexes shows a 1 : 3 ratio at the surface identical to the bulk structure. Studies by SEM and AFM provide evidence of periodicity, which is likely to be induced by the lamellar, layered structures. Differences are observed in the thermal behavior, which appear to reflect both the coordination behavior of the metal ion and the degree of hydration.