Stoichiometry-Defined Phase Selection and Hydrophobic Molecular Sealing in Bioderived Deep Eutectic Solvents

Deep eutectic solvents (DESs) are promising ionic media owing to their abundance, ionic conductivity, and wide potential windows. Bioderived DESs based on choline chloride are attractive due to their biodegradability and biocompatibility; however, their strong hygroscopicity leads to severe moisture uptake and instability. Here, we report a bioderived moisture-resistant DES of acetylcholine chloride as a hydrogen bond acceptor (HBA) and 3-phenylpropionic acid as a hydrophobic hydrogen bond donor. The hydrophobic component acts as a molecular seal, shielding the hygroscopic HBA from moisture through hydrogen-bond network reconfiguration. Systematic variation of the molar ratio reveals four distinct regimes: immiscible mixtures (1:1), saturated states (1:1.5), a homogeneous glass-forming liquid at the eutectic composition (1:2, T _g = -57.9 °C), and crystallizable phases at higher HBD contents (1:3 and 1:4). Only the eutectic composition stabilizes a uniform liquid without observable melting transitions, exhibiting over 75% reduction in moisture uptake relative to the parent salt at 58% relative humidity. The DES demonstrated a high ionic conductivity (197 μS cm^-1) and a wide potential window (2.43 V). Moreover, the DES exhibited high biodegradability (>90%), exceeding the OECD criterion for ready biodegradability (>60% within 28 days). These findings establish stoichiometry as a governing parameter coupling phase stability with moisture affinity in robust, biodegradable DESs.