The counterion-mediated controllable coacervation of nano-ions with polyelectrolytes.

Nano-ions can complex with polyelectrolytes for coacervates with hierarchical structures; however, the rational design of functional coacervations is still rare due to the poor understanding of their structure-property relationship from their complex interaction. Herein, 1 nm anionic metal oxide clusters, PWO, with well-defined, mono-disperse structures are applied to complex with cationic polyelectrolyte and the system shows tunable coacervation via the alternation of counterions (H and Na) of PWO. Suggested from Fourier transform infrared spectroscopy (FT-IR) and isothermal titration studies, the interaction between PWO and cationic polyelectrolytes can be modulated by the bridging effect of counterions via hydrogen bonding or ion-dipole interaction to carbonyl groups of polyelectrolytes. The condensed structures of the complexed coacervates are explored by small angle X-ray and neutron scattering techniques, respectively. The coacervate with H as counterions shows both crystallized and discrete PWO clusters, with a loose polymer-cluster network in comparison to the system of Na which shows a dense packing structure with aggregated nano-ions filling the meshes of polyelectrolyte networks. The bridging effect of counterions helps understand the super-chaotropic effect observed in nano-ion system and provides avenues for the design of metal oxide cluster-based functional coacervates.