Stabilization of Polymer-Polyoxometalate Coacervate Droplets by Divalent Cations

Polymeric coacervates are two-aqueous phase separating complexes with polymer-rich dense coacervate droplets dispersed in a polymer-poor supernatant aqueous solution, which can be formed with two or more distinct polymers, including charged biomolecules and nanoclusters. Due to their ultralow interfacial tension, such coacervate droplets are inherently dynamic and unstable with a high tendency to coalescence over time. In this work, we have surprisingly found that highly concentrated divalent salts, such as CaCl₂ and SrCl₂, can significantly enhance the stability of dense coacervate droplets formed between a neutral polymer, poly(ethylene glycol) (PEG) and anionic polyoxometalate (POM) nanoclusters. Dense PEG-POM coacervate droplets dispersed in CaCl₂-added aqueous solution exhibit a robust spherical shape over a long period of time of more than two years. In comparison to the coalescent PEG-POM coacervates, the stable coacervate droplets formed in CaCl₂ solution exhibit considerably enhanced mechanical strength with increasing POM concentration. The segregation of anionic POM nanoclusters to the outer perimetric region of the droplet with depleted PEG is observed microscopically and accounts for the strong interaction between POM and divalent cations to achieve the droplet stabilization. Such stabilized polymer-POM coacervate droplets in a simple divalent salted solution can be explored to develop a functional stable nanocolloidal dispersion with tunable compartments for applications ranging from catalysts to nanomedicines.