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

Abstract

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.

Rheo-SAXS study of a shear-thinning hydrogel containing clay nanoplatelets

“…This hydrogel is formed from cationic polyelectrolytes based on ionenes, a class of polyelectrolytes containing quaternary ammonium parts in the main chain [8,9,10]. Additionally, the presence in the main chain of aromatic rings and amide moities allows gelation through a combination of electrostatic repulsions, π-π stacking, hydrogen bonding and anion-π and cation-π interactions that can be tune according to the nature and the valence of the couterions [11]. In a previous study, we investigated the organisation of clay nanoplatelets in this 25 ionene-based hydrogel [10].…”

Ion-specific effects in polyelectrolyte solutions: chain-chain interactions, chain rigidity and dynamics

“…It has been observed that ion-specific effects manifest themselves more strongly for anions than cations [28,29]. Solutions and gels based on cationic chains with compensating anions, such as ionenes [20,21,30], show indeed stronger ion specific effects than anionic PEs, such as the widely studied polystyrene sulfonate [16,[31][32][33][34]. Needless to say, any purely electrostatic theory, such as the scaling-approach of de Gennes, Manning theory of counterion condensation [35,36] or indeed the Poisson-Boltzmann approach [32] cannot account for these effects, as hydration properties of solvated ions do not come into consideration.…”