Alkane Chain Length of the Hydrogen-Bond Donor in Deep Eutectic Solvents Induces Nonmonotonic Changes in the Innate Structure and Properties of Hydrogels

The precise tuning of hydrogel properties using deep eutectic solvents (DESs) is fundamentally constrained by an unclear understanding of how the molecular structure of DES dictates hydrogel performance. Here, we demonstrated that the hydroxy-substituted alkane chain length of the hydrogen-bond donor in DESs nonmonotonically regulates the cross-linking density, water confinement, and mechanical properties of gelatin/carboxymethyl chitosan (CMCS) hydrogels. Systematic variation in the hydroxy-substituted alkane chain length from C 2(carbon number of two) to C 5 revealed that an optimal chain length (C 3 ) maximized favorable noncovalent interactions, leading to a densely cross-linked network in DES-GC-3. The extensive hydrogen bonding between water and hydrogel constituents, coupled with this tightly cross-linked polymer network, collectively endowed DES-GC-3 with better mechanical strength and water retention. Due to the restriction of water mobility, DES-GC-3 also possessed a low freezing temperature of −62.9 °C, while maintaining excellent mechanical performance at −40 °C. Our work establishes the alkane chain length in DES as a fundamental, programmable parameter for the rational design of adaptive hydrogels through the precise modulation of polymer–solvent interactions.