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Tuning at will the properties of gel-forming systems is of key relevance for many biotechnological, agricultural, and biomedical applications. For polyelectrolyte-based gels, ion-specific effects can be an attractive way for this purpose. This study investigates the counterion-specific effect on the microscopic structure and the rheological properties of a physical hydrogel formed of ionene-type cationic polyelectrolytes. The focus is on two monovalent halide counterions (F– and Cl–) and a divalent counterion (SO4 2–). A strong counterion-specific effect appears within ionene-based gels. In the case of halide counterions, gelation is more effective for more weakly hydrated counterions. Indeed, strongly hydrated counterions maintain electrostatic repulsions between the chains and as a consequence gel formation is shifted toward higher concentrations (higher critical gelation concentration, CGC). The combination of the complementary small-angle X-ray and neutron scattering (SAXS and SANS) techniques reveals a strong contribution of ion–ion correlations in the structure of the gel network. Contrary to chloride gels, which present a single correlation length characterizing the distance between the cross-linking nodes, fluoride gels present an additional network of nodes. This is accompanied by a very rapid increase of the elastic modulus of fluoride gels, once CGC is reached. With divalent counterions, the gelation is even more remarkable with a lower CGC and a higher elastic modulus at equivalent polyelectrolyte concentrations. The presence of divalent counterions favors the association of chains, probably by a bridging effect. This evokes the “egg-box” model, and the characteristic scaling of the elastic modulus with reduced gel concentration confirms this. However, only a narrow concentration window for gel-forming exists for divalent counterions before precipitation takes over due to too strong attractive chain–chain interactions.

Depletion‐induced self‐assembly is routinely used to separate plasmonic nanoparticles (NPs) of different shapes, but less often for its ability to create supercrystals (SCs) in suspension. Therefore, these plasmonic assemblies have not yet reached a high level of maturity and their in‐depth characterization by a combination of in situ techniques is still very much needed. In this work, gold triangles (AuNTs) and silver nanorods (AgNRs) are assembled by depletion‐induced self‐assembly. Small Angle X‐ray Scattering (SAXS) and scanning electron microscopy (SEM) analysis shows that the AuNTs and AgNRs form 3D and 2D hexagonal lattices in bulk, respectively. The colloidal crystals are also imaged by in situ Liquid‐Cell Transmission Electron Microscopy. Under confinement, the affinity of the NPs for the liquid cell windows reduces their ability to stack perpendicularly to the membrane and lead to SCs with a lower dimensionality than their bulk counterparts. Moreover, extended beam irradiation leads to disassembly of the lattices, which is well described by a model accounting for the desorption kinetics highlighting the key role of the NP‐membrane interaction in the structural properties of SCs in the liquid‐cell. The results shed light on the reconfigurability of NP superlattices obtained by depletion‐induced self‐assembly, which can rearrange under confinement.