Evaluation of the Electrostatic Effect on Ionic Reactions in Charged Polyion Systems

由于反离子集中在聚合物骨架周围而产生的静电效应使多离子-反离子的结合平衡变得复杂。本研究项目的目的是考察Gibbs-Donnan概念在评价线性和交联型线性多离子(多离子凝胶)结合平衡中的静电效应的适用性。葡聚糖硫酸盐(DxS)和羧甲基葡聚糖(CMDX)被用作线性聚离子的例子，磺丙基葡聚糖凝胶和羧甲基葡聚糖凝胶被用作交联型线性多离子凝胶的例子。引入反离子浓缩区的体积术语允许定义多离子骨架附近的离子物种的平均浓度。DXS和CMDX的反离子结合平衡分析结果表明，反离子浓集区的Donnan势项和体积项都可以简单地表示为多离子的线性电荷分离和加盐浓度的函数。多离子区域中的离子大气被认为对应于由固定离子、反离子和共离子组成的均匀的浓缩电解质溶液。通过将每个相的标准状态定义为相同，有助于有效地洞察多离子区域中各种物种的测量性质。用该方法测定的CMDX及其凝胶类似物的单齿络合物的内禀结合常数值与其单体类似物的相应金属络合物的内禀结合常数值一致，证明了该方法的价值。目前的系统工作已经证明，Gibbs-Donnan概念可以应用于任何多离子的反离子结合平衡的热力学表达式，而与它们的形状、外观和固定离子的性质无关。

An electrostatic effect attributable to counterions being concentrated around the polymer skeleton makes the binding equilibria of polyion-counterions complicated. The objective of the present research project is to examine the applicability of a Gibbs-Donnan concept to evaluate the electrostatic effect in the binding equilibria of linear and cross-linked linear polyions (polyion gels). Dextran sulfate (DxS) and carboxymethyldextran (CmDx) have been used as examples of linear polyions, and sulfopropyl dextran gel and carboxymethyldextran gel have been usesd as examples of cross-linked linear polyion gels. Introduction of a volume term for the counterion concentrating region permits definition of the averaged concentrations of the ionic species in the vicinity of the polyion skeleton. Results of the counterion binding equilibrium analysis of DxS and CmDx revealed that both the Donnan potential term and the volume term for the counterion concentrating region can be expressed simply as a function of the linear charge separation of the polyion and the added salt concentration. The ionic atmosphere in the polyion domain is considered to correspond to a homogeneous concentrated electrolyte solution composed of fixed ions, counterions, and co-ions. By defining the standard state of each phase to be the same, valid insights with respect to the measured properties of the various species in the polyion domain are facilitated. Agreement of the values of the intrinsic binding constants of the monodentate complexes of CmDx and its gel analogs determined with this approach, with those of the corresponding metal complexes of their monomer analogs demonstrate the value of this approach. It has been proved by the present systematic work that the Gibbs-Donnan concept can be applied to the thermodynamic expression of the counterion binding equilibria of any polyions, irrespective of their shapes, appearances, and the nature of the fixed ions.