Kinetically Frozen Rheology of Agar-agar Hydrogels Upon a Change of Anions in the Hofmeister Series

Hydrogels are materials in which water is present as the major component in a network of macromolecules, inorganic colloids or low molecular weight organogelators. To tune their viscoelastic properties, they can be crosslinked, making use of their chemical moieties, but most often with cytotoxicity issues. Another approach consists in modifying the intermolecular interactions of the gelator or its interactions with water. In this framework, playing with the nature of the used electrolyte becomes a popular approach. Even if many reports describe the influence of anions or cations in the Hofmeister series on the mechanical properties of hydrogels, very few investigate the possibility of modifying those properties in a reversible manner between two states characteristic of each electrolyte. Herein, it is shown that agar-agar hydrogels prepared in the presence of high ionic strength NaCl or NaSCN display strongly different mechanical properties and that a change in the nature of the electrolyte in the presence of the gel does not significantly change its storage and loss moduli. This lack of reversibility in the time scale of one day or more is shown to be related to the high inaccessibility of the water and ions present in the porous volume after the gel formation.