Phase stability of Keplerate-type polyoxomolybdates controlled by added cationic surfactant

Abstract

Phase stability of two nanometer-scale Keplerate-type polyoxomolybdates, (NH 4) 42[Mo VI 72Mo V 60O 372(CH 3COO) 30(H 2O) 72]⋅300H 2O⋅10CH 3COONH 4 ({Mo 132}) and Mo VI 72Fe III 30O 252L 102⋅ca. 180H 2O with L = H 2O/CH 3COO −/Mo 2O n− 8/9 ({Mo 72Fe 30}), can be easily achieved by controlling the concentration of a cationic surfactant, tetradecyltrimethylammonium bromide (TTABr), in aqueous solution. Precipitates and floccules were observed when the stoichiometric ratios of r TTA + / { Mo 132 } and r TTA + / { Mo 72 Fe 30 } were 40:1 and 90:1, respectively, which were determined by zeta potential measurements. The surface charge properties and structure morphologies of {Mo 132} and {Mo 72Fe 30} induced by controlling cationic TTABr in aqueous solution were determined by zeta potential measurements and transmission electron microscopy (TEM) observations. {Mo 132} and {Mo 72Fe 30} can self-assemble into supramolecular “Blackberry” structures and exist at compositions less than the stoichiometric ratios of r TTA + / { Mo 132 } and r TTA + / { Mo 72 Fe 30 } in aqueous solution. Above the 1:1 stoichiometric ratio of TTABr/{Mo 132} or TTABr/{Mo 72Fe 30}, the precipitates and floccules dissolve. Dynamic laser light scattering (DLS) measurements clearly demonstrated that the R h values have essentially no angular dependence at excess amounts of TTABr, suggesting the presence of spherically symmetric aggregates of {Mo 132} and {Mo 72Fe 30}. Bilayer-like structures in aqueous solution were also demonstrated by TEM images. The interesting phase transition observed in our model systems of {Mo 132} and {Mo 72Fe 30} macroanions with high chemical stability, similar shape, and masses could provide models for the understanding of more complex polyelectrolyte solutions and self-assembled soft magnetic materials and in bioapplications for highly selective adsorbents of proteins with different molecular sizes and charges.