Complex phase transitions and phase engineering in the aqueous solution of an isopolyoxometalate cluster

Inorganic salts usually demonstrate simple phasal behaviors in dilute aqueous solution mainly involving soluble (homogeneous) and insoluble (macrophase separation) scenarios. Herein, we report the discovery of complex phase behavior involving multiple phase transitions of clear solution - macrophase separation - gelation - solution - macrophase separation in the dilute aqueous solutions of a structurally well-defined molecular cluster [Mo₇O₂₄]^6- macroanions with the continuous addition of Fe³⁺. No chemical reaction was involved. The transitions are closely related to the strong electrostatic interaction between [Mo₇O₂₄]^6- and their Fe³⁺ counterions, the counterion-mediated attraction and the consequent charge inversion, leading to the formation of linear/branched supramolecular structures, as confirmed by experimental results and molecular dynamics simulations. The rich phase behavior demonstrated by the inorganic cluster [Mo₇O₂₄]^6- expands our understanding of nanoscale ions in solution.