Regulating Water States in Poly(sodium acrylate- co -acrylic acid) Hydrogels for Controlled Release/Retention: From Fundamental Design to Solar Desalination Application

Hydrogels are polymeric materials characterized by their unique 3D network structures, which enable them to absorb substantial amounts of water. The water in the hydrogels can be classified into three states: free, intermediate, and bound water. Notably, intermediate water exhibits a particularly low evaporation enthalpy due to its distinct molecular interactions. By adjusting the proportions of these three states of water, the evaporation performance of hydrogels can be effectively regulated, showcasing significant potential for efficient water resource management. In this study, poly(sodium acrylate-co-acrylic acid) (PAA-PAANa) was employed as the matrix material. Various strategies (including chemical grafting, surface cross-linking, and physical doping) were utilized to introduce both hydrophilic and hydrophobic components into the hydrogel system. These methodologies systematically modulate the molecular state of water and its associated evaporation properties. The experimental results demonstrate that the incorporation of hydrophilic monomers increases the free water content in the hydrogel, reducing the intermediate-to-free water ratio from 0.5004 to a minimum of 0.3067. This change elevates the evaporation enthalpy to 2964 J g^-1 and improves the water retention rate by 15-20%. In contrast, hydrophobic and amphiphilic materials enhance the intermediate water content, raising this ratio to a maximum of 0.8440. Consequently, the dark evaporation enthalpy decreases to 2177 J g^-1, while the water evaporation rate increases from 0.77 kg m^-2 h^-1 to 1.39 kg m^-2 h^-1. Building upon this enhanced evaporation performance, we integrated the hydrogel exhibiting optimal evaporation rates into poly(vinyl alcohol) (PVA) sponges to develop a novel high-efficiency solar desalination evaporator. Under one sun irradiation (1 kW m^-2), the seawater evaporation rate reached 2.80 kg m^-2 h^-1. Even at a high salt concentration of 15 wt %, the evaporation rate remained at 1.92 kg m^-2 h^-1 with no salt deposition observed during one-week cycling. This study provides new insights into material design for efficient solar desalination.