Compression and Interpenetration of Ionic Microgels in Electrostatically Self-Assembled Clusters

Colloidal self-assembly is a powerful strategy to obtain materials with desired softness, porosity and biocompatibility. When soft building blocks are used, these properties can change dramatically, compared to materials produced using hard particles. However, the fate of individual colloids inside these assemblies is not fully understood. Here, asymmetric mixtures of soft microgels with opposite electrical charges are used as model building blocks to assemble colloidal clusters. The changes in the form factor of individual microgels due to cluster formation are analyzed using small-angle neutron scattering with contrast variation, complemented with small-angle X-ray scattering and molecular dynamics simulations. A strong compression of the fuzzy shell of a microgel is observed, which results in a peculiar core-shell architecture of the microgel. In contrast, a reference system of similarly charged microgels shows osmotic deswelling in both the dense core and fuzzy shell, as expected. Molecular dynamics simulations reveal that the polymer density at contact between the oppositely charged microgels increases due to the entropically favorable counterion release. To maximize the number of released counterions, the microgels compress their shells or interpenetrate each other. The results show that the structure of individual soft colloids may be significantly altered during the assembly of clusters, which can influence the larger hierarchical assembly.