Dataset Structural Changes of Flexible, Small-Sized Polycationic Microgels upon Interaction with Hexacyanoferrate(III) Viewed at an Atomic and a Particle Scale

Charged colloid systems pose challenges and offer unexpected chances in their dispersion/solution behaviour because of the interdependence of structural reorganizations, electrostatics and molecular ion binding. Here, strong polyelectrolytic P(NIPAM-co-MAPTAC) microgels/nanogels consisting of N-isopropylacrylamide and methacrylamidopropyltrimethylammonium chloride (quaternized dimethylaminopropylmethacrylamide DMAPMA) with ultrasmall hydrodynamic radius (41 nm, 20°C, 0.1 M KCl; minimum: 25 nm) are prepared and investigated with varying concentrations of multivalent counterions as ionic cargo (i.e. hexacyanoferrate(III)). We combine paramagnetic relaxation enhancement NMR (PRE-NMR), dynamic light scattering DLS, and small angle X-ray scattering SAXS (fuzzy-sphere extended model) with quantum chemical calculations (r²SCAN-3c) to probe binding geometries and structural transitions across varying salt concentrations and temperatures under flooding versus Donnan equilibrium conditions. PRE-NMR reveals selective [Fe(CN)6]3- binding to MAPTAC via secondary amide-hexacyanoferrate coordination. SAXS quantifies ion-induced reorganizations: core contraction, fuzzy shell expansion, and changes in the correlation length upon hexacyanoferrate addition accompanied by a reentrant swelling based on the preferential interaction of hexacyanoferrate with the charged microgel outskirts. This multiscale approach demonstrates that peripheral charge localization enables reversible switching between intra- and interparticle crosslinking, where the thermal history above 55°C can be recorded. Our results could have implications on applications like sensing, controlled release and nanoparticle-templated catalysis.