Aluminum Cross-Linked Carboxylated Nanocellulose Hydrogels with Enhanced Antifreezing Properties

Carboxylated cellulose nanofibers (CNFs) can be cross-linked by multivalent cations through ionic or coordination interactions, forming robust hydrogels containing a large amount (>99%) of water or aqueous solution. In this study, hydrogels based on TEMPO-oxidized CNF scaffolds cross-linked with different amounts of aluminum cations (Al³⁺) were prepared, where the excess Al³⁺ cations bound with a large amount of water molecules. Consequently, the resulting CNF-Al hydrogels could suppress ice crystallization well below the typical freezing point of water. These antifreezing properties of CNF-Al hydrogels were studied by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and synchrotron wide-angle X-ray diffraction (WAXD) during cooling from 25 °C to -90 °C. The mechanical properties of these hydrogels were investigated by rheological measurements. The freezing-point depression and fractions of freezable and nonfreezable bound water as a function of Al³⁺ concentration were determined by DSC and TGA, where the ice formation within the CNF scaffold during freezing was monitored by in situ WAXD. Based on these results, the antifreezing behavior of the CNF-Al hydrogel can be explained by the existence of excess Al³⁺ cations creating bound water that significantly decreases the nominal freezing point. In addition, the 3D cross-linked CNF network appears to hinder the leaching of Al³⁺ cations, thus sustaining the antifreezing properties.