解耦支撑依赖的传输剖面与分子水运动

中文摘要 AI 辅助

非平衡分子动力学（NEMD）模拟中的压力和浓度剖面常被用于推断致密且溶胀的聚合物膜中的水传输机制。然而，为稳定模拟膜所需的机械约束可能会改变这些剖面及其解释。在此，我们检验约束引起的压力和浓度剖面差异是否伴随分子水动力学的变化。使用三种支撑策略模拟了交联聚乙二醇二丙烯酸酯（PEGDA）膜：石墨烯支撑晶格、靠近渗透物界面的冻结膜原子，以及均匀分布的冻结膜原子。这些约束产生了显著不同的剖面，范围从类似溶液扩散到类似孔流，而总体水通量保持相当。然而，分子水动力学变化很小。界面交换强烈双向，且远超净渗透通量，膜内水扩散率相似，完全穿越在两个方向均发生。局部水关联寿命短暂，定向速度相关性随时间与距离迅速衰减。因此，尽管分子水动力学相似，压力与浓度剖面可能显著不同。因此，仅凭这些剖面不足以识别机械约束NEMD模拟中的传输机制。在PEGDA中，水运动由随机扩散主导，仅带有较小的净定向偏差。

英文摘要

Pressure and concentration profiles from non-equilibrium molecular dynamics (NEMD) simulations are often used to infer water transport mechanisms in dense and swollen polymer membranes. However, the mechanical restraints needed to stabilize simulated membranes can alter these profiles and their interpretation. Here, we test whether restraint-induced differences in pressure and water concentration profiles are accompanied by changes in molecular water dynamics. Crosslinked poly(ethylene glycol) diacrylate (PEGDA) membranes were simulated using three support strategies: a graphene support lattice, frozen membrane atoms near the permeate interface, and homogeneously distributed frozen membrane atoms. The restraints produced markedly different profiles, ranging from solution-diffusion-like to pore-flow-like, while overall water fluxes remained comparable. Molecular water dynamics, however, changed little. Interfacial exchange was strongly bidirectional and far exceeded the net permeation flux, membrane water diffusivities were similar, and complete crossings occurred in both directions. Local water associations were short-lived, and directional velocity correlations decayed rapidly with time and distance. Thus, substantially different pressure and concentration profiles can arise despite similar molecular water dynamics. Such profiles alone are therefore insufficient to identify transport mechanisms in mechanically restrained NEMD simulations. In PEGDA, water motion is dominated by stochastic diffusion with a small net directional bias.

发表机构

• RWTH Aachen University（亚琛工业大学）
• DWI - Leibniz Institute for Interactive Materials e.V（德国互动材料莱布尼茨研究所）

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