Hydrogels typically deteriorate under high-salinity conditions because electrostatic screening and reduced polymer-solvent affinity suppress swelling and weaken load-bearing network connectivity. Here, we report a double-network hydrogel that strengthens while swelling in brine. Compared with its behavior in water, the network undergoes an ion-triggered topological reconfiguration upon salt exposure. The intrachain zwitterionic ion pairs open and reform as inter-network SBVI+-AMPS− bridges, increasing effective bridge density at lower polymer fraction. The hydrogel exhibits 1.63-fold tensile strength and 1.21-fold swelling ratio in 200 g/L NaCl compared to deionized water. SAXS and XPS confirm salt-induced structural homogenization and charge redistribution. Density functional theory calculations support strengthened ionic association under saline conditions between SBVI and AMPS. Free energy analysis reveals that reduced loop fraction and increased connectivity enable associative stabilization to compensate for elastic swelling penalties. Core-flooding demonstrates robust injectability in high-salinity porous rock. This mechanism provides design rules for salt-adaptive hydrogels. Hydrogels have potential for applications in a range of environments, but tend to be unstable in high-saline conditions. Here, the authors report the development of a double-network hydrogel that strengthens with swelling in brine, due to the behavior of zwitterionic ion pairs.
Ren et al. (Tue,) studied this question.