Physicochemical Properties of Egg-Box-Mediated Hydrogels with Transiently Decreased pH Employing Carbonated Water

[Image: see text] Anionic polysaccharides, including low-methoxy (LM) pectin, are extensively used in biomaterial applications owing to their safety, biocompatibility, and feasibility in constructing supramolecular assemblies by forming egg-box structures with divalent cations. Mixing an LM pectin solution with CaCO(3) spontaneously forms a hydrogel. The gelation behavior can be controlled by adding an acidic compound to change the solubility of CaCO(3). CO(2) is used as the acidic agent and can be easily removed after gelation, thereby reducing the acidity of the final hydrogel. However, CO(2) addition has been controlled under varied thermodynamical conditions; therefore, specific CO(2) effects on gelation are not necessarily visualized. To evaluate the CO(2) impact on the final hydrogel, which would be extended to control hydrogel properties further, we utilized carbonated water to supply CO(2) into the gelation mixture without changing its thermodynamic conditions. The addition of the carbonated water accelerated gelation and significantly increased the mechanical strength, promoting cross-linking. However, the CO(2) volatilized into the atmosphere, and the final hydrogel became more alkaline than that without the carbonated water, probably because a considerable amount of the carboxy group was consumed for cross-linking. Moreover, when aerogels were prepared from the hydrogels with carbonated water, they exhibited highly ordered networks of elongated porosity in scanning electron microscopy, proposing an intrinsic structural change by CO(2) in the carbonated water. We also controlled the pH and strength of the final hydrogels by changing the CO(2) amounts in the carbonated water added, thereby validating the significant effect of CO(2) on hydrogel properties and the feasibility of using carbonated water.