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

聚电解质水凝胶，通常被称为高吸水性聚合物，由相互连接的带电聚合物链组成，形成三维网络。它们对水具有高亲和力，主要由于其高溶胀能力而广泛用于卫生用品，制药，农业和食品保护。在之前的资助期间，我们展示了一个额外的特性：膨胀的聚电解质水凝胶在受到外部机械力压缩时释放出盐耗尽的水。我们利用这种行为在无膜脱盐过程中的潜在应用，并设计了一种新的实验程序来研究高电荷聚丙烯酸钠凝胶相和外部盐溶液之间盐离子的分布。盐分配的基本原理和分离机制是基于Donnan平衡，其中聚电解质主链的固定电荷导致凝胶和上清相之间的移动离子不对称分布，从而通过压缩凝胶来回收盐耗尽的水。在实验结果的基础上，结合Monte-Carlo和分子动力学模拟，进一步扩展了Donnan理论，预测了盐的分配和脱盐效率随压力和盐浓度的变化。35克L-1氯化钠溶液脱盐所需的能源估计为8.9千瓦时m-3，而理论极限为0.7千瓦时m-3。以往的研究表明，高电荷密度和低机械模量有利于海水淡化效率的提高，但压缩过程中膨胀能力和机械强度之间的平衡尚未得到优化。因此，在第二个资助期内，将系统地研究膨胀能力与机械强度之间的关系。通过在网络中加入非弹性聚合物组分，如悬空末端或大分子物体（如高分子量聚电解质链、星形和树状大分子），电荷密度将会增加。由于在分子水平上表征这些多组分网络结构具有挑战性，因此我们将引入1H-NMR弛豫测量作为表征技术，通过将悬垂端和交联点等拓扑特征与其独特的弛豫行为相关联来确定网络结构。结合通过调制质子的距离和相对取向计算1H同核残余偶极耦合常数的模拟，该技术将导致对分子结构对宏观性质和Donnan平衡的影响的一致的基本理解。将出现一种描述，可用于设计和表征具有优越电荷密度的聚电解质水凝胶材料及其作为分离介质的应用。

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.