The effect of specific non-covalent interactions on the assembly behavior of macroions in solution

Ions possess an interesting behavior in solution when they reach the size of the nanometer scale. When carrying moderate amount of charge on their surface, these macroions, mainly polyoxometalate (POM) clusters, tend to self-assemble into hollow spherical blackberry-like structures in aqueous solution, with the counterion mediated attraction as the major driving force. Interestingly, macroions can undergo the self-recognition phenomena by recognizing their own type when mixed with other macroionic species in solution. This self-recognition phenomenon is shown to be universal and exist when almost identical macroions carry very high and very small amount of charges on their surface. Moreover, macroions with anisotropic surface charge density distributions tend to deviate from the normal spherical blackberry structures and assemble into rod-like morphologies as a result of the directionality of the interaction when the effect of hydrogen bonding is weaker. Furthermore, specific hydrogen bonding and hydrophobic interactions cause a reverse trend of blackberry size vs. solvent polarity compared to the normal charge regulated self-assembly process of macroions into blackberries. Finally, the counterion mediated attraction is shown to be extended to polymer systems with POM clusters in the main chain. Scattering techniques (DLS, SLS and SAXS) along with electron microscopy were the major techniques used in understanding the complex macroionic solutions.