Structural Changes of Flexible, Small‐Sized Polycationic Microgels Upon Interaction With Hexacyanoferrate(III) Viewed at an Atomic and a Particle Scale

ABSTRACT 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 (NIPAM) and 3‐(methacryloylamino)propyltrimethylammonium chloride (MAPTAC, quaternized from N ‐(3‐dimethylaminopropyl)methacrylamide, 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 (r 2 SCAN‐3c) to probe binding geometries and structural transitions across varying salt concentrations and temperatures under flooding (/nondialyzed) versus dialyzed conditions. PRE‐NMR reveals selective Fe(CN) 6 3− binding to MAPTAC, assisted with secondary amide‐hexacyanoferrate coordination according to quantum chemical calculations. 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 domains. 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.