Department of Chemistry, Indian Institute of Technology Patna, Bihar 801106, India.
J Phys Chem B. 2022 Mar 31;126(12):2430-2440. doi: 10.1021/acs.jpcb.2c00240. Epub 2022 Mar 16.
Earlier, and reactive force-field-based molecular dynamics (MD) simulation studies suggested an overwhelming contribution of the vehicular diffusion in the total diffusion of hydroxide ions rather than structural diffusion. But does the vehicular diffusion occur via small-step displacement? This question is important to have an understanding of the real characteristics of vehicular diffusion. To answer this question, we perform a classical molecular dynamics simulation of a system containing a hydroxide ion exchange membrane polymer and hydroxide ion at different hydration levels and temperatures using the same molecular force field (Dubey, V. 2020, 755, 137802), which successfully captured the microscopic structure and dynamics of the system. We use the translational jump-diffusion approach, used previously in supercooled water for understanding the origin of breakdown of the Stokes-Einstein relation, to calculate the jump-diffusion coefficient of hydroxide ion and water in the anion exchange membrane. We have seen a significant role of hydration level and temperature in the mechanism of vehicular diffusion. In overhydrated membrane, both hydroxide ions and water molecules diffuse via both small- and large-step displacement. With decreasing hydration level and temperature, the diffusion is increasingly governed by the jump-diffusion mechanism. The larger contribution of jump-diffusion comes from the stronger caging of the diffusing species by the solvent at lower hydration levels and temperature. These results, therefore, suggest that the hydration level and temperature of the hydroxide ion exchange membrane determine the detailed mechanism of the vehicular diffusion of hydroxide ion, especially whether the diffusion follows hydrodynamics or not.
早些时候，基于作用力场的分子动力学（MD）模拟研究表明，氢氧根离子的总扩散中，载体扩散的贡献远远超过结构扩散。但是，载体扩散是否通过小步位移发生？这个问题对于理解载体扩散的真实特性很重要。为了回答这个问题，我们在不同水合水平和温度下，使用相同的分子力场（Dubey, V. 2020, 755, 137802），对含有氢氧根离子交换膜聚合物和氢氧根离子的系统进行了经典分子动力学模拟，该力场成功地捕获了系统的微观结构和动力学。我们使用了以前在过冷水中用于理解斯托克斯-爱因斯坦关系破裂起源的平移跳跃扩散方法，来计算阴离子交换膜中氢氧根离子和水的跳跃扩散系数。我们发现水合水平和温度在载体扩散机制中起着重要作用。在过饱和的膜中，氢氧根离子和水分子都通过小步和大步位移进行扩散。随着水合水平和温度的降低，扩散越来越受到跳跃扩散机制的控制。跳跃扩散的较大贡献来自于在较低水合水平和温度下，溶剂对扩散物质的更强束缚。因此，这些结果表明，氢氧根离子交换膜的水合水平和温度决定了氢氧根离子载体扩散的详细机制，特别是扩散是否遵循流体动力学。
