Salt rejecting hydrogel design targets long life solar desalination

by Riko Seibo
Tokyo, Japan (SPX) Dec 18, 2025

Researchers from Hebei University of Technology and Tsinghua University have developed a zwitterionic hydrogel evaporator that maintains high solar desalination performance while resisting salt crystallization and biological fouling. The material mimics saltwater fish gill environments by using tightly paired N+ - O- groups in a poly(trimethylamine N-oxide) (PTMAO) network to hold a thick hydration layer that screens out ions, proteins, bacteria, and algae while supporting rapid water transport. The work is presented as a news article associated with the journal Nano-Micro Letters and focuses on solar-driven interfacial desalination as a low-carbon pathway to convert seawater into freshwater.

The team synthesized a superhydrated PTMAO/polyacrylamide (PAAm)/polypyrrole (PPy) hydrogel, referred to as PTAP, that reaches an evaporation rate of 2.35 kg m-2 h-1 under 1 kW m-2 illumination in 10 wt percent sodium chloride solution. PTAP operates continuously for 100 hours under these high-salinity conditions without visible salt accumulation on the surface, in contrast to a commercial PSBMA-based control that loses 22 percent of its evaporation flux under the same brine. In outdoor field tests, a three-stage distillation device using PTAP and fed with Bohai Sea water yields 25.6 kg m-2 day-1 of freshwater over 60 consecutive days without wiping, flushing, or performance loss.

Antifouling performance is a central feature of the hydrogel. Surface tests show bovine serum albumin adsorption of only 0.61 mg cm-2 on PTAP compared with 1.81 mg cm-2 on a PAAm hydrogel, indicating much lower protein fouling. Microbiological evaluations report that PTAP approaches sterile conditions when challenged with Escherichia coli, Staphylococcus aureus, and three common algal species, supporting long-term operation in natural waters with mixed contaminants. The hydration shell anchored by the zwitterionic PTMAO chains is identified as the key to resisting both organic and microbial attachment.

Molecular dynamics simulations help explain how the hydrogel combines high water flux with ion rejection. The PTMAO - water hydrogen bond energy is calculated at -54.9 kJ mol-1, approximately twice the energy of water - water hydrogen bonds, which stabilizes a dense hydration layer. This structure raises the potential of mean force for sodium and chloride ions attempting to enter the hydrogel while lowering the energy barrier for water molecules leaving the network, consistent with rapid evaporation and suppressed salt ingress. These simulations thus support the experimental observations of durable, salt-free operation.

The PTAP hydrogel is produced through an aqueous, low-temperature process. The TMAO monomer is obtained by oxidizing DMAPA with an 85 percent yield, followed by UV co-polymerization with acrylamide to form the zwitterionic-PAAm network. PPy nano-networks are then formed in situ on the hydrogel, creating a photothermal layer that absorbs more than 98 percent of solar radiation from 250 to 2500 nm and converts it into localized heat for interfacial evaporation. All reactions occur in water at temperatures no higher than 60 degrees Celsius, and the team reports kilogram-scale monomer batches suitable for roll-to-roll coating.

Thermal and structural characterization show how PTAP manages water states to enhance evaporation. Differential scanning calorimetry and Raman spectroscopy resolve bound, intermediate, and free water, with an optimized formulation designated PTAP2 displaying the highest intermediate-to-bound water ratio. This composition lowers the effective evaporation enthalpy to 1450 J g-1, corresponding to an energy efficiency of about 91 percent under standard one-sun illumination. Mechanical tests reveal that interpenetrating PAAm chains raise compressive stress in seawater to 88.8 kPa, roughly nine times higher than PTMAO alone, enabling 20 mm by 20 mm films to tolerate folding and twisting during handling and deployment.

The researchers highlight potential uses in off-grid and integrated desalination systems. Modular foam-cotton distillers equipped with PTAP are envisioned for life-raft and disaster-relief kits, where they could deliver around 50 liters m-2 week-1 of drinking water without maintenance. The group is also working on laminating PTAP onto the backsheets of photovoltaic panels to capture waste heat for combined electricity and freshwater production. Looking ahead, they indicate that the hydrogel can be recycled through solvent exchange and are exploring redox-active TMAO variants that could self-clean when subjected to nighttime voltage pulses.

Research Report:Superhydrated Zwitterionic Hydrogel with Dedicated Water Channels Enables Nonfouling Solar Desalination