Tuning Structure and Rheological Properties of Polyelectrolyte- Based Hydrogels through Counterion-Specific Effects

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 (SO42-). 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.

Automated summary

Tuning the properties of gel-forming systems is crucial for various applications. This study investigates the effect of ion-specific counterions on the structure and rheological properties of ionene-based hydrogels. The results show that gelation is more effective for weakly hydrated counterions, and divalent counterions enhance gelation by promoting chain association. The combined techniques of SAXS and SANS reveal the contribution of ion-ion correlations in the gel network.