The thermodynamic model of improved desalination using hydrophobic polyelectrolyte gels

We study a weak polyelectrolyte hydrogel and its application as a desalination agent. We model the desalination process as a four-step reversible thermodynamic cycle transferring ions from a low salinity solution to a high salinity one. As a driving force for ions movement, we use the fact that compression of the gel decreases its ionization degree, and therefore releases ions entrapped in the gel. We showed that the use of hydrophobic monomer units in the hydrogel may significantly increase the number of transferred ions. The interplay between the repulsive electrostatic and attractive hydrophobic interactions shows up in a first-order phase transition. At certain applied pressure the gel collapses abruptly changing its volume almost to that of a dry state and releasing almost all ions collected inside. The huge changes of the gel volume taking place at constant pressure allowed us to construct desalination cycles transferring a much larger number of ions and working at rather low pressures, <10 bar. The desalination cycle implies reversibility at any stage, so, in principle, the method can achieve the maximum thermodynamic efficiency, comparable to reverse osmosis.