# Stabilization of Polymer-Polyoxometalate Coacervate Droplets by Divalent Cations

## TL;DR

Adding divalent salts like CaCl2 stabilizes coacervate droplets formed from PEG and POM nanoclusters, making them robust and long-lasting.

## Contribution

Discovery that divalent cations significantly enhance the stability of polymer-polyoxometalate coacervate droplets.

## Key findings

- PEG–POM coacervate droplets in CaCl2 solution remain stable for over two years.
- Droplet stability increases with higher POM concentration due to mechanical strength enhancement.
- Anionic POM nanoclusters segregate to the droplet periphery, interacting strongly with divalent cations.

## Abstract

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.