Dual-component thermoresponsive anionic-cationic hydrogel networks: molecular interactions governing lithium ion adsorption and desorption

A dual-component hydrogel system was developed to elucidate the molecular-level mechanisms governing lithium-ion (Li + ) adsorption and desorption within thermoresponsive polymer networks. The primary anionic hydrogel was synthesized from 2-acrylamido-2-methylpropanesulfonic acid and N-isopropylacrylamide (AMPS-NIPAM), while a complementary cationic hydrogel was prepared from N,N -dimethylaminopropyl acrylamide and NIPAM (DMAPAA-NIPAM). When combined, these gels form an interacting polymer network within the solution, whose temperature-driven protonation-deprotonation equilibria and volume phase transitions modulate ion uptake and release. Coupling the two gels markedly intensified internal electrostatic repulsion within the anionic network, resulting in more than 75% and 540% increases in Li + adsorption and desorption, respectively, relative to AMPS-NIPAM alone. To probe how functional group acidity influences intermolecular forces and ion-binding dynamics, an additional anionic hydrogel containing acrylic acid (AA-NIPAM) was synthesized. Its weaker carboxylate-Li + interaction produced ~20% lower adsorption but ~14% higher desorption than AMPS-NIPAM, demonstrating that acid strength governs both ion-binding affinity and desorption reversibility at the molecular scale. Optimal performance was achieved at a 1:4 anionic-to-cationic gel ratio and at a pH of 3 in the desorption solution. Repeated temperature swing cycles (10 °C/65 °C) revealed stable ion-exchange dynamics, with AA-NIPAM showing ~38% greater long-term efficiency. These results deepen the molecular understanding of ion transport, hydration-mediated binding, and charge-regulated structural dynamics in polymer-rich liquid phases, providing fundamental insights relevant to polymer-solvent interaction systems and responsive materials. • Dual-component thermoresponsive hydrogels enable molecular-level control of Li + exchange. • Protonation-deprotonation equilibria regulate ion interactions and network dynamics. • Sulfonic vs. carboxylic groups show distinct Li + binding strengths and desorption behavior. • AA-NIPAM exhibits ~20% lower adsorption but ~14% higher desorption than AMPS-NIPAM. • Temperature swing cycles reveal stable ion transport and enhanced long-term efficiency.