Polymer-polyoxometalate (POM) systems represent an emergent class of functional hybrid materials. However, the pH-dependent stability of POMs limits their scope in water. We show here that the water-mediated, so-called superchaotropic binding of α-Keggin POMs to a non-ionic biopolymer, hydroxypropylcellulose (HPC), (i) selectively stabilizes superchaotropic POMs in water, and (ii) enables pH-responsive HPC solutions and hydrogels. Raman and NMR spectroscopy revealed that binding to HPC protects the superchaotropic PW12O403- and SiW12O404- against hydrolysis, extending their stability from acidic to near-neutral pH. In turn, POMs with higher charge, stronger hydration, and thus without the ability to bind to HPC, such as H2W12O406-, PW11O397- and SiW11O398-, do not get stabilized. Cloud points, small-angle neutron scattering, and rotational rheology showed that pH-induced conversion from superchaotropic PW12O403- to non-superchaotropic PW11O397- switches HPC from a bound, crosslinked to an unbound, non-crosslinked state, enabling pH-switchable viscosity and gel-sol transitions. Superchaotropic stabilization and pH-switching are proposed as general phenomena in superchaotropic POM/solute systems, highlighting the potential of superchaotropicity in aqueous soft materials.
Lokare et al. (2026) studied this question.