Rathee Vikramjit S, Sidky Hythem, Sikora Benjamin J, Whitmer Jonathan K
Department of Chemical and Biomolecular Engineering , University of Notre Dame , Notre Dame , Indiana 46556 , United States.
J Am Chem Soc. 2018 Nov 14;140(45):15319-15328. doi: 10.1021/jacs.8b08649. Epub 2018 Oct 30.
Polyelectrolytes may be classified into two primary categories (strong and weak) depending on how their charge state responds to the local environment. Both of these find use in many applications, including drug delivery, gene therapy, layer-by-layer films, and fabrication of ion filtration membranes. The mechanism of polyelectrolyte complexation is, however, still not completely understood, though experimental investigations suggest that entropy gain due to release of counterions is the key driving force for strong polyelectrolyte complexation. Here we perform a comprehensive thermodynamic investigation through coarse-grained molecular simulations permitting us to calculate the free energy of complex formation. Importantly, our expanded-ensemble methods permit the explicit separation of energetic and entropic contributions to the free energy. Our investigations indicate that entropic contributions indeed dominate the free energy of complex formation for strong polyelectrolytes, but are less important than energetic contributions when weak electrostatic coupling or weak polyelectrolytes are present. Our results provide a new view of the free energy of polyelectrolyte complex formation driven by polymer association, which should also arise in systems with large charge spacings or bulky counterions, both of which act to weaken ion-polymer binding.
根据其电荷状态对局部环境的响应方式，聚电解质可分为两大类（强聚电解质和弱聚电解质）。这两类聚电解质在许多应用中都有使用，包括药物递送、基因治疗、层层组装膜以及离子过滤膜的制备。然而，尽管实验研究表明，抗衡离子释放导致的熵增是强聚电解质络合的关键驱动力，但聚电解质络合的机制仍未被完全理解。在这里，我们通过粗粒度分子模拟进行了全面的热力学研究，从而能够计算络合物形成的自由能。重要的是，我们的扩展系综方法能够明确分离出对自由能的能量贡献和熵贡献。我们的研究表明，对于强聚电解质，熵贡献确实主导了络合物形成的自由能，但在存在弱静电耦合或弱聚电解质时，熵贡献不如能量贡献重要。我们的结果提供了由聚合物缔合驱动的聚电解质络合物形成自由能的新观点，在具有大电荷间距或大体积抗衡离子的系统中也会出现这种情况，这两种情况都会削弱离子与聚合物的结合。
