Distribution of low-molecular weight ions in charged hydrogelsin equilibrium and under the application of external stimuli

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Polyelectrolyte hydrogels, often referred to as superabsorbent polymers, consist of interconnected charged polymer strands which form a three-dimensional network. They exhibit a high affinity to water and are widely used in hygiene products, pharmaceutics, agriculture and food protection, mainly due to their high swelling capabilities. In the previous funding period we demonstrated an additional feature: Swollen polyelectrolyte hydrogels release salt depleted water when compressed by an external mechanical force. We exploited this behavior for a potential usage in a membrane-free desalination process and designed a novel experimental procedure to study the distribution of salt ions between a highly charged poly(sodium acrylate) gel phase and the external salt solution. The fundamental principle of the salt partitioning and consequently of the separation mechanism is based on the Donnan equilibrium where the fixed charges of the polyelectrolyte backbone cause an asymmetric distribution of mobile ions between the gel and supernatant phase, enabling the recovery of salt depleted water by compressing the gel. Based on our experimental results and hybrid Monte-Carlo and molecular dynamics simulations the Donnan theory was further extended to predict the salt partitioning and the desalination efficiency as the function of pressure and salt concentration. The energy demand for the desalination of a 35 g L-1 sodium chloride solution was estimated to be 8.9 kWh m-3, where the theoretical limit is 0.7 kWh m-3. Previous work indicated that a high charge density combined with low mechanical moduli are beneficial for the desalination efficiency, but the balance between swelling capability and mechanical strength for the compression procedure has not been optimized yet. Hence, in the second funding period the relation between swelling capacity and mechanical strength will be systematically studied. The charge density will be increased by incorporating non-elastic polymer components into the network, such as dangling ends or macromolecular objects (e.g. high-molecular weight polyelectrolyte chains, stars and dendrimers). Since the characterization of these multicomponent network structures on the molecular level is challenging, we will introduce 1H-NMR relaxometry as a characterization technique to determine the network structure by correlating topological features, e.g. dangling ends and crosslinking points, to their unique relaxation behavior. In combination with simulations that calculate the 1H homonuclear residual dipolar coupling constant by modulating the distance and relative orientation of protons, this technique will lead to a consistent fundamental understanding of the influence of molecular structure on macroscopic properties and the Donnan equilibrium. A description will emerge that can be utilized for the design and characterization of polyelectrolyte hydrogel materials with superior charge density and their application as separation medium.