Mineral Deposition from Liquid Precursors Triggered by Complementary Polyelectrolytes

Polymer-induced liquid precursors are hydrated, metastable, amorphous droplets rich in insoluble ionic compounds; they form in aqueous solutions that are supersaturated with mineral ions in the presence of charged polymers that delay the onset of crystal nucleation. This work reports that rapid crystallization of a range of inorganic mineral salts can be triggered by the addition of polycations to polyanion-stabilized precursor solutions that would otherwise be stably amorphous for over ten days. In complex coacervate systems, entropic gains drive the competitive displacement of small (e.g.) cationic species from anionic binding partners by larger molecules with more positive charges. We build evidence that the constituents of the stabilized mineral salt are displaced from the stabilizing polyanion by the added polycation in a similar manner. By exposing liquid precursor solutions to arrays of polycationic hydrogels, we demonstrate the ability to deposit crystals in a site-selective manner, enabling spatial control over mineral formation.