Hydrophilic Macroionic Solutions: What Happens When Soluble Ions Reach the Size of Nanometer Scale?

Abstract

Large, hydrophilic inorganic ions (mostly polyoxometalate macroions and cationic metal−organic hybrid nanocages) with high solubility in water and/or other polar solvents demonstrate unique solution behaviors. In dilute solutions, they behave significantly different from small simple ions (as described by the Debye−Hückel theory) because the macroions cannot be treated as point charges or large, insoluble colloidal suspensions (usually described by the DLVO theory) because the macroions form homogeneous, stable “real solutions”. The size disparity between the macroions and their counterions results in complex macroion−counterion interaction and leads to the self-assembly of macroions into single-layered, hollow, spherical “blackberry” structures. The blackberries, with robust and very stable structures mimicking biological membranes, can adjust their size accurately and reversibly in response to the change of solvent content, charge density on the macroions, or in some cases merely solution pH. The blackberry membrane is permeable to small cations. The inorganic macroions with well-defined size, shape, mass, charge density (even accurately tunable within certain range), and no intramolecular interaction can be treated as simple model systems to understand the intermolecular interaction in polyelectrolyte solutions. The blackberry structures show certain similarities to the spherical virus capsids, from the overall structure to the kinetic properties of formation.