Department of Chemistry, University of Pittsburgh, 219 Parkman Avenue, Pittsburgh, Pennsylvania 15260, USA.
J Am Chem Soc. 2011 Oct 12;133(40):16300-8. doi: 10.1021/ja207297q. Epub 2011 Sep 20.
Here, we report on the first electrochemical study that reveals the kinetics and molecular level mechanism of heterogeneous ion-ionophore recognition at plasticized polymer membrane/water interfaces. The new kinetic data provide greater understanding of this important ion-transfer (IT) process, which determines various dynamic characteristics of the current technologies that enable highly selective ion sensing and separation. The theoretical assessment of the reliable voltammetric data confirms that the dynamics of the ionophore-facilitated IT follows the one-step electrochemical (E) mechanism controlled by ion-ionophore complexation at the very interface in contrast to the thermodynamically equivalent two-step electrochemical-chemical (EC) mechanism based on the simple transfer of an aqueous ion followed by its complexation in the bulk membrane. Specifically, cyclic voltammograms of Ag(+), K(+), Ca(2+), Ba(2+), and Pb(2+) transfers facilitated by highly selective ionophores are measured and analyzed numerically using the E mechanism to obtain standard IT rate constants in the range of 10(-2) to 10(-3) cm/s at both plasticized poly(vinyl chloride) membrane/water and 1,2-dichloroethane/water interfaces. We demonstrate that these strongly facilitated IT processes are too fast to be ascribed to the EC mechanism. Moreover, the little effect of the viscosity of nonaqueous media on the IT kinetics excludes the EC mechanism, where the kinetics of simple IT is viscosity-dependent. Finally, we employ molecular level models for the E mechanism to propose three-dimensional ion-ionophore complexation at the two-dimensional interface as the unique kinetic requirement for the thermodynamically facilitated IT.
在这里，我们报告了第一项电化学研究，该研究揭示了在增塑聚合物膜/水界面处异质离子-离子载体识别的动力学和分子水平机制。新的动力学数据提供了对这一重要离子转移（IT）过程的更深入理解，该过程决定了各种高选择性离子传感和分离技术的各种动态特性。对可靠伏安数据的理论评估证实，离子载体促进的 IT 动力学遵循一步电化学（E）机制，该机制由界面处的离子-离子载体络合控制，与基于简单转移的热力学等效两步电化学-化学（EC）机制形成对比在本体膜中随后络合的水合离子。具体而言，通过高度选择性离子载体测量和分析 Ag(+)、K(+)、Ca(2+)、Ba(2+) 和 Pb(2+)转移的循环伏安图，并使用 E 机制进行数值分析，以在增塑聚（氯乙烯）膜/水和 1,2-二氯乙烷/水界面处获得标准 IT 速率常数在 10(-2) 到 10(-3) cm/s 的范围内。我们证明，这些强烈促进的 IT 过程太快，不能归因于 EC 机制。此外，非水介质的粘度对 IT 动力学的影响很小，排除了 EC 机制，其中简单 IT 的动力学是粘度依赖性的。最后，我们采用 E 机制的分子水平模型来提出二维界面处的三维离子-离子载体络合作为热力学促进 IT 的独特动力学要求。
