Hydrophilic Inorganic Macro-Ions in Solution: Unprecedented Self-Assembly Emerging from Historical "Blue Waters"

For more than 200 years, the beautiful “molybdenum blue solutions†have been a puzzle for chemists because they could not determine the molecular structures of the solutes while experiments showing the Tyndall effect proved the presence of “giant speciesâ€. This problem was finally solved in Bielefeld. As a result of this discovery, novel inorganic clusters belonging to the family of polyoxomolybdates (POMs)–or, generally speaking polyoxometalates–could be synthesized and characterized. These species, usually existing as giant ions in solutions, are fully hydrophilic but still tend to self-associate into supramolecular structures, based on complicated driving forces other than the usual hydrophobic interactions, bringing new progress in the physical chemistry of solutions. Such fascinating solution behaviors are different from those of other well-categorized solutes and suspensions, such as simple ions, polymers, surfactant micelles in solution, and colloidal suspensions. The new results may change some of our common knowledge on inorganic ionic solutions. For example, our fundamental principles on concepts like soluble inorganic ions distribute homogeneously in dilute solutions and soluble ions reach an equilibrium very quickly in dilute solutions might not be valid for large ions. In the future, with additional evidence being accumulated and summarized, macro-ionic solutions might become a separate topic of the chemistry curriculum.