Phase stability of two nanometer-scale Keplerate-type polyoxomolybdates, (NH4)(42)[(Mo72Mo60O372)-Mo-VI-O-V(CH3-COO)(30)(H2O)(72)]center dot 300H(2)O center dot 10CH(3)COONH(4) ({Mo-132}) and (Mo72Fe30O252L102)-Fe-VI-O-III center dot ca. 180H(2)O with L = H2O/CH3-COO-/Mo2O8/9n- ({Mo72Fe30}), call 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+/{Mo132}) and r(TTA+/{Mo72Fe30}) 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 {Mo72Fe30} induced by controlling cationic TTABr in aqueous solution were determined by zeta potential measurements and transmission electron microscopy (TEM) observations. {Mo-132} and {Mo72Fe30} can self-assemble into supramolecular "Blackberry" structures and exist at compositions less than the stoichiometric ratios of r(TTA+/{Mo132}) and r(TTA+/{Mo72Fe30}) in aqueous solution. Above the 1:1 stoichiometric ratio of TTABr/{Mo-132} Or TTABr/{Mo72Fe30}, 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 {Mo72Fe30}. 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 {Mo72Fe30} 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. (C) 2009 Elsevier Inc. All rights reserved.