Effect of anions, concentration and mechanical stimulation on the gelation of agarose

In this contribution, we report the effect of two anions with different degrees of hydration, namely sulphate (SO₄^2-) and chloride (Cl^-), on the gelation of a low hydrophobic agarose (residual methylation = 7.4%). Using a combined approach of oscillatory rheology, low-field NMR, small-angle X-ray scattering (SAXS) and cryo-SEM, we propose that the addition of anions affects water molecules around agarose, causing general dehydration of the biopolymer. This results in earlier temperature of gelation, T_gel, less water-biopolymer interactions and greater elasticity of the network at the end of the gelation stage. Mechanical differences level off at the end of the pseudo-equilibrium stage, that is, after 24 h of incubation of the hydrogels at T = 37 °C. This causes also low sensitivity of the hydrogels to high ionic strength, which can be explained mainly by the residual amounts of hydrophobic domains of the agarose used in this study. In addition, this contribution provides insights into the role of the agarose concentration, C_p, during the gelation stage. We demonstrate that the established correlation between C_p, network elasticity, and T_gel can be related to the extent of interactions between water and agarose. Finally, we discuss the mechanical stimulation of agarose during the gelation stage in the presence of the highly hydrated SO₄^2- anion. We report that increasing the oscillatory strain during temperature sweep experiments has negligible effects on T_gel, but alters the evolution of the storage modulus on cooling.