Hydrogels Under Superchaotropic Control: Polyoxometalate Stabilization and pH‐Responsive Crosslinking in Cellulose Ether Solutions

Abstract

ABSTRACT 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 [PW 12 O 40 ] 3− and [SiW 12 O 40 ] 4− 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 [H 2 W 12 O 40 ] 6− , [PW 11 O 39 ] 7− and [SiW 11 O 39 ] 8− , do not get stabilized. Cloud points, small‐angle neutron scattering, and rotational rheology showed that pH‐induced conversion from superchaotropic [PW 12 O 40 ] 3− to non‐superchaotropic [PW 11 O 39 ] 7− 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.