Jackson Nicholas E, Brettmann Blair K, Vishwanath Venkatram, Tirrell Matthew, de Pablo Juan J
The Institute for Molecular Engineering, The University of Chicago, Chicago, Illinois 60637, United States.
ACS Macro Lett. 2017 Feb 21;6(2):155-160. doi: 10.1021/acsmacrolett.6b00837. Epub 2017 Feb 3.
Coarse-grained molecular dynamics enhanced by free-energy sampling methods is used to examine the roles of solvophobicity and multivalent salts on polyelectrolyte brush collapse. Specifically, we demonstrate that while ostensibly similar, solvophobic collapsed brushes and multivalent-ion collapsed brushes exhibit distinct mechanistic and structural features. Notably, multivalent-induced heterogeneous brush collapse is observed under good solvent polymer backbone conditions, demonstrating that the mechanism of multivalent collapse is not contingent upon a solvophobic backbone. Umbrella sampling of the potential of mean-force (PMF) between two individual brush strands confirms this analysis, revealing starkly different PMFs under solvophobic and multivalent conditions, suggesting the role of multivalent "bridging" as the discriminating feature in trivalent collapse. Structurally, multivalent ions show a propensity for nucleating order within collapsed brushes, whereas poor-solvent collapsed brushes are more disordered; this difference is traced to the existence of a metastable PMF minimum for poor solvent conditions, and a global PMF minimum for trivalent systems, under experimentally relevant conditions.
通过自由能采样方法增强的粗粒度分子动力学被用于研究疏溶剂性和多价盐对聚电解质刷塌缩的作用。具体而言，我们证明，虽然表面上相似，但疏溶剂塌缩刷和多价离子塌缩刷表现出不同的机理和结构特征。值得注意的是，在良溶剂聚合物主链条件下观察到多价诱导的非均匀刷塌缩，这表明多价塌缩的机制并不取决于疏溶剂主链。对两条单独刷链之间平均力势（PMF）的伞形采样证实了这一分析，揭示了在疏溶剂和多价条件下截然不同的PMF，表明多价“桥连”在三价塌缩中作为区分特征的作用。在结构上，多价离子在塌缩刷内有形成有序核的倾向，而不良溶剂塌缩刷则更无序；这种差异可追溯到在实验相关条件下，不良溶剂条件下存在亚稳的PMF最小值，而三价体系存在全局PMF最小值。
