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

聚电解质水凝胶，通常被称为超吸收聚合物，由形成三维网络的相互连接的带电聚合物链组成。它们表现出对水的高亲和力，并广泛用于卫生产品、制药、农业和食品保护，主要是由于它们的高溶胀能力。在前一个资助期，我们证明了一个额外的功能：Swedish水凝胶在受到外部机械力压缩时释放出贫盐水。我们利用这种行为在无膜脱盐过程中的潜在用途，并设计了一种新的实验程序来研究高电荷的聚（丙烯酸钠）凝胶相和外部盐溶液之间的盐离子分布。盐分配的基本原理以及因此分离机制的基本原理是基于唐南平衡，其中凝胶骨架的固定电荷导致凝胶和上清液相之间的移动的离子的不对称分布，使得能够通过压缩凝胶来回收盐耗尽的水。基于我们的实验结果和混合蒙特-卡罗和分子动力学模拟的Donnan理论进一步扩展到预测盐分配和脱盐效率作为压力和盐浓度的函数。35 g L-1氯化钠溶液脱盐的能量需求估计为8.9 kWh m-3，其中理论极限为0.7 kWh m-3。以前的工作表明，高电荷密度与低机械模量相结合有利于脱盐效率，但压缩过程的溶胀能力和机械强度之间的平衡尚未优化。因此，在第二个资助期内，将系统地研究膨胀能力和机械强度之间的关系。电荷密度将通过将非弹性聚合物组分引入网络中而增加，例如悬挂末端或大分子物体（例如高分子量的双链、星形和树枝状聚合物）。由于在分子水平上表征这些多组分网络结构具有挑战性，我们将引入1H-NMR弛豫法作为表征技术，通过将拓扑特征（例如悬挂末端和交联点）与其独特的弛豫行为相关联来确定网络结构。结合模拟计算的1H homomethane剩余偶极耦合常数通过调制质子的距离和相对取向，这种技术将导致一个一致的基本理解的影响分子结构的宏观性质和Donnan平衡。将出现的描述，可用于设计和表征的水凝胶材料与上级电荷密度和它们的应用作为分离介质。

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.