Water vapor formation from hydrogels via photothermal heating of nanoparticles*

Hydrogels are proposed to decrease the energy input needed to form water vapor in processes such as solar desalination. These hypotheses relate to the interactions between water and the gel, such as formation of surfaces or water clusters where evaporation is facilitated or changes in internal gel hydrophilicity. In addition, focusing energy at regions where vapor is most likely to form (rather than heating the bulk) also increases efficiency. Incorporation of larger (50-200 nm diameter) nanoparticles into/onto the hydrogel aids both processes - potentially creating additional internal disorder and, if the nanoparticle is photothermally active, providing a means to transition solar light into local heat. We present results on changes in vapor formation efficiency in sodium acrylate and N-isopropylamide hybrids, embedded with either melanin (MNP) or gold nanoparticles (GNP). Photothermal conversion is well established for GNP but impractical for applications. The photothermal properties of melanin are nascent, but MNPs ability to be mass produced and their innately biocompatible nature are significant advantages.