Modeling of weak polyelectrolyte hydrogels under compression - Implications for water desalination

Hydrogels immersed in salty solutions induce partitioning of small ions that can be exploited for water desalination. This partitioning also affects the H+ ions, leading to a decrease of the pH in the gel as compared to supernatant solution. When using weak polyelectrolyte gels, this lower pH may decrease their ionization degree, and hence diminish their desalination capacity. Using the recently developed grand-reaction ensemble, we performed particle-based computer simulations of a gel in contact with a reservoir of salt solution at pH similar or equal to 7. We showed that the effect of ion partitioning on the ionization degree is negligible, as long as the salinity of the reservoir is high. However, when the reservoir salinity approaches that of fresh water, the ionization degree of weak polyelectrolyte gel is significantly decreased, thus making it a less efficient desalination agent. Furthermore, we demonstrated that the ionization degree of the gel is decreased upon compression. However, up to pressures of approximately 30 bar this does not significantly affect the ion partitioning. Our result predict that weak polyelectrolyte hydrogels, such as the often employed poly(acrylic acid), can be efficiently used to lower the salinity of sea- or brackish water, while they will be inefficient in desalinating water of low salinity.