Materials Science and Engineering Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA.
J Chem Phys. 2018 Oct 28;149(16):163305. doi: 10.1063/1.5030530.
There has been significant interest in the tendency of highly charged particles having the same charge to form dynamic clusters in solution, but an accepted theoretical framework that can account for this ubiquitous phenomenon has been slow to develop. The theoretical difficulties are especially great for flexible polyelectrolytes due to the additional complex coupling between the polyelectrolyte chain configurations and the spatial distribution of the ionic species in solution. For highly charged polyelectrolytes, this leads to the formation of a diffuse "polarizable" cloud of counter-ions around these polymers, an effect having significant implications for the function of proteins and other natural occurring polyelectrolytes, as emphasized long ago by Kirkwood and co-workers. To investigate this phenomenon, we perform molecular dynamics simulations of a minimal model of polyelectrolyte solutions that includes an explicit solvent and counter-ions, where the relative affinity of the counter-ions and the polymer for the solvent is tunable through the variation of the relative strength of the dispersion interactions of the polymer and ions. In particular, we find that these dispersion interactions can greatly influence the nature of the association between the polyelectrolyte chains under salt-free conditions. We calculate static and dynamic correlation functions to quantify the equilibrium structure and dynamics of these complex liquids. Based on our coarse-grained model of polyelectrolyte solutions, we identify conditions in which three distinct types of polyelectrolyte association arise. We rationalize these types of polyelectrolyte association based on the impact of the selective solvent affinity on the charge distribution and polymer solvation in these solutions. Our findings demonstrate the essential role of the solvent in the description of the polyelectrolyte solutions, as well as providing a guideline for the development of a more predictive theory of the properties of the thermodynamic and transport properties of these complex fluids.
在溶液中，具有相同电荷的高电荷粒子形成动态簇的趋势引起了人们的极大兴趣，但能够解释这一普遍现象的公认理论框架的发展一直较为缓慢。由于聚电解质链构象与溶液中离子物种的空间分布之间存在额外的复杂耦合，因此对于柔性聚电解质来说，理论上的困难尤其大。对于高电荷聚电解质，这导致在这些聚合物周围形成一个弥散的“可极化”反离子云，这一效应对蛋白质和其他天然聚电解质的功能有重要影响，正如 Kirkwood 及其同事很久以前所强调的那样。为了研究这一现象，我们对包括溶剂和反离子的聚电解质溶液的最小模型进行了分子动力学模拟，其中反离子和聚合物对溶剂的相对亲和力可以通过改变聚合物和离子的色散相互作用的相对强度来调节。特别是，我们发现这些色散相互作用可以极大地影响无盐条件下聚电解质链之间的缔合性质。我们计算静态和动态相关函数来量化这些复杂液体的平衡结构和动力学。基于我们的聚电解质溶液粗粒化模型，我们确定了三种不同类型的聚电解质缔合出现的条件。我们根据选择性溶剂亲和力对这些溶液中电荷分布和聚合物溶剂化的影响，对这些聚电解质缔合类型进行了合理化。我们的研究结果表明了溶剂在描述聚电解质溶液中的重要作用，同时为开发更具预测性的热力学和输运性质理论提供了指导。
