Gelation and Multiple Macrophase Transitions in Dilute Aqueous Solutions of Nanoscale Hydrophilic Macroions

Nanoscale (1-5 nm) hydrophilic macroions demonstrate different solution behaviors in polar solvents from simple ions or colloidal suspensions. We observe a series of macrophase transitions in their dilute aqueous solutions (as low as 0.5 mM) involving solution, gel, coacervate and precipitation phases in the presence of di- or trivalent cations (Y³⁺, Sr²⁺…), by using two uranyl peroxide (U₆₀^60- and U₂₄Pp₁₂^48-) molecular clusters as models, suggesting that soluble ions could possess unexpected rich phasal behaviors. The gelation mechanism could be attributed to the size disparity between macroions and counterions, the consequent moderate counterion association around macroions and then tunable counterion-mediated attraction. In many cases, 2-D nanosheets are formed, followed by enclosing into self-assembled hollow, spherical, single-layered blackberry-type structures with sizes accurately tunable depending on the strength of the attraction. Gelation is the result of very tough nanosheets formed by strong counterions which cannot easily bend therefore staying as large, open structures in solution. The dilute macroion solution will first transfer into hydrogels and then turn into coacervates with increasing multivalent cation concentration. We will discuss the critical requirements of such interesting phase transitions and the common features of these unique phase behaviors of macroion solutions, as well as the rheological and thermodynamical features of the gels.