Engineering Charge Distribution in Polyzwitterionic Janus Interfaces for Solar Evaporation of Saline Water with Charged Organic Contaminants

Abstract Solar-driven interfacial evaporation offers a promising route for freshwater production, yet its efficiency in saline media is limited by strong ion hydration and electrostatic screening, which suppress charge accessibility at the evaporation interface. Although polyzwitterions can modulate interfacial water via coupled charges, their functionalities are largely embedded within bulk matrices, leading to limited interfacial utilization. Here, we report a surface-localized polyzwitterionic layer with high-density accessible sites. Preorganization of amphiphilic block copolymers into micellar structures before zwitterionic functionalization enables a controlled and comparable distribution of interfacial sites, allowing direct evaluation of molecular structure effects. Molecular engineering of betaine-like polyzwitterions through alkyl substitution and spacer-length tuning enhances electrostatic potential by reducing intramolecular charge shielding under strong ionic screening. When assembled on a hydrogel surface, the engineered interface activates interfacial water, enlarges the effective evaporation area, and promotes directional water transport. The Janus evaporator achieves an evaporation rate of 7.53 kg m–2 h–1 under one-sun illumination with 99.9% salt rejection and enables stable freshwater production (∼13 kg m–2 day–1) from complex saline water. This work establishes a structure-property relationship between polyzwitterionic charge distribution and interfacial water behavior, providing a chemically grounded strategy for efficient solar evaporation in saline environments.