Hydrogels Under Superchaotropic Control: Polyoxometalate Stabilization and pH-Responsive Crosslinking in Cellulose...

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 [PW₁₂O₄₀]^3- and [SiW₁₂O₄₀]^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 [H₂W₁₂O₄₀]^6-, [PW₁₁O₃₉]^7- and [SiW₁₁O₃₉]^8-, do not get stabilized. Cloud points, small-angle neutron scattering, and rotational rheology showed that pH-induced conversion from superchaotropic [PW₁₂O₄₀]^3- to non-superchaotropic [PW₁₁O₃₉]^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.