Automatic and Subsequent Dissolution and Precipitation Process in Inorganic Macroionic Solutions

We report an interesting phenomenon in the NaCl-containing aqueous solution of {Mo72Fe30} macroions, where dissolution and precipitation processes of hydrophilic macroions automatically and subsequently occur without changing external conditions or chemical reactions. Our previous work indicates that {Mo72Fe30} macroions tend to slowly self-assemble into single-layer, vesicle-like "blackberries". Such macroions have two solute states in solutions: the entropy-favored general state (homogeneous distribution) and the free-energy favored second solute state (blackberries). With additional salts, the originally stable blackberries become less stable due to their shortened screening length, and they tend to further aggregate and precipitate at much lower concentrations. Therefore, in such a solution, we can observe a subsequent process: crystal solids --> homogeneous single macroion solution --> homogeneous blackberry solution --> precipitates containing noncrystalline solids. In other words, we observed the behaviors of both soluble inorganic ions and colloids in the same solution due to the unique features of the macroions. Static and dynamic laser light scattering, as well as AFM measurements, were used to characterize both the macroionic solutions and the precipitates.