Modeling polyelectrolyte hydrogels
The lack of potable water is becoming an increasing problem for almost a quarter of the world’s population, according to the World Health Organization (WHO, 2017). However, many of them have access to seawater or brackish water. Modern desalination techniques are technologically demanding. Therefore, they are mostly used in developed countries with limited access to freshwater. Water desalination using polyelectrolyte hydrogels might become an inexpensive and low-tech alternative that could also be used in developing countries. The young team of Peter Košovan, part of the Soft Matter research group at the Department of Physical and Macromolecular Chemistry, has been developing theoretical models of polyelectrolyte gels in salt solutions and studying their potential applications for desalination.
Polyelectrolyte hydrogels are networks of charged polymers - polyelectrolytes. Their ability to swell is used in many applications: in aqueous solution, hydrogels can swell up to 1000 times the dry gel volume. The concentration of solutes inside the gel usually differs from that in the solution in contact with the gel. Using this knowledge, our colleagues at the Karlsruhe Institut für Technologie (Germany) have proposed using polyelectrolyte hydrogels for water desalination and experimentally demonstrated that their concept works.
When we place a polyelectrolyte gel in saltwater, the salt concentration inside the gel is always lower than that outside, which is the key to desalination. When we remove the gel from the solution, and squeeze it like a sponge, we obtain a solution with a lower salt concentration than the original one. We can reduce the salt concentration to the levels of potable water through a series of swelling and compression cycles, similarly to the fractional distillation process.
Fig.1: Schematic representation of desalination using polyelectrolyte hydrogels. Left: different salt concentrations at the hydrogel/solution interface. Key steps of the desalination cycle: (1) placing the gel in salt solution, (2) swelling of the gel, which leads to the difference in salt concentrations inside and outside the gel, (3) gel compression and desalinated water collection. Reproduced from Ref. with permission from Springer.
The desalination cycle, which consists of repeated gel swelling and compression steps (Fig.1), can be compared to the Carnot cycle for the heat engine from a thermodynamic standpoint. However, the experimental cycle provides less desalinated water than we would expect from the idealized theoretical model. To improve the desalination efficiency under real conditions, we must test different parameters of the desalination cycle. However, we must ensure that the search for optimal parameters is not a mere trial-and-error procedure, which requires further understanding the gel compression processes at the molecular level. Such understanding can be achieved by applying a more realistic theoretical model.
An improved theoretical model is useful, particularly if its predictions are faster and cheaper than performing experiments. Therefore, Dr. Košovan’s team develops simple computational models that provide reliable predictions within seconds just using a desktop PC. Initially, they used very simple models which only qualitatively described the experimentally observed effects . A subsequently augmented model yielded good agreement with simulations  and could be used to study different parameters of the desalination cycle . The latest improvements include ionization equilibria, and the knowledge gained from them has lead to a new design of the desalination cycle. In theory, this new cycle can achieve the maximum possible efficiency (publication in preparation). “After five years of effort, we can eventually test parameters of the desalination cycles purely based on theory, providing rather fast and reliable predictions without requiring time-consuming experiments or molecular simulations.” says Peter Košovan
Fig.2 Schematic representation of various models of the gel: (A) two-dimensional view of the polymer network with one highlighted node; (B) self-consistent field model for one node of the network( the positions of individual particles in this model are replaced by the average particle density); (C) molecular simulation model which uses explicit positions of individual particles. Reproduced from Ref. with permission of The Royal Society of Chemistry.
The most recent publication of Dr. Košovan’s team team, featured on the title page of the journal Soft Matter of The Royal Society of Chemistry , is based on the self-consistent field approach (Fig.2). “In this article, with a major contribution from our Russian postdoc Oleg Rud, we reported our research on weak polyelectrolyte gels. These gels consist of weak acids, whose ionization depends on pH, similarly to acetic acid. Our work shows that the ionization of these gels decreases upon compression, which may explain why compression experiments yield less desalinated water than expected. Furthermore, we have shown that the concentration of oxonium cations inside the gel is much lower than outside. Therefore, the local pH inside the gel is up to two units lower than outside.” explains Peter Košovan.
 A self-consistent mean-field model for polyelectrolyte gels, Soft Matter (2017) (www)
 Seawater Desalination via Hydrogels: Practical Realisation and First Coarse Grained Simulations, Colloid and Polymer Science (2013) (www)
 On the efficiency of a hydrogel-based desalination cycle Desalination (2017) (www)
 Modeling of Polyelectrolyte Gels in Equilibrium with Salt Solutions Macromolecules (2015) (www)