Department of Chemical, Biological and Macromolecular Sciences, S. N. Bose National Centre for Basic Sciences, Block JD, Sector III, Salt Lake, Kolkata-700 098, India.
J Chem Phys. 2012 Jan 7;136(1):014505. doi: 10.1063/1.3672508.
Here, we develop a framework for a molecular level understanding of the celebrated Stokes-Einstein-Debye (SED) formula. In particular, we explore reasons behind the surprising success of the SED model in describing dipolar solute rotation in complex polar media. Relative importance of solvent viscosity and solute-solvent dipolar interaction is quantified via a self-consistent treatment for the total friction on a rotating solute where the hydrodynamic contribution is modified by the friction arising from the longer ranged solute-solvent dipolar interaction. Although the solute-solvent dipolar coupling is obtained via the Mori-Zwanzig formalism, the inclusion of solvent structure via the wave vector dependent viscosity in the hydrodynamic contribution incorporates solvent molecularity in the present theory. This approach satisfactorily describes the experimental rotation times measured using a dipolar solute, coumarin 153 (C153), in protic and aprotic polar liquids, and more importantly, provides microscopic explanation for insignificant contribution of electrical interactions on solute rotation, in contrast to the substantial role played by the translational dielectric friction in the context of ionic mobility. It is also discussed on how the present theory can be suitably extended to study the rotation of a realistic solute in media other than dipolar solvents.
在这里，我们开发了一个分子水平理解著名的 Stokes-Einstein-Debye（SED）公式的框架。特别是，我们探索了 SED 模型在描述复杂极性介质中偶极溶质旋转方面惊人成功的背后原因。通过对旋转溶质的总摩擦进行自洽处理，定量评估了溶剂粘度和溶质-溶剂偶极相互作用的相对重要性，其中流体动力贡献受到来自长程溶质-溶剂偶极相互作用的摩擦的修正。尽管通过 Mori-Zwanzig 形式主义获得了溶质-溶剂偶极耦合，但通过在流体动力贡献中包含依赖波矢的粘度来包含溶剂结构，将溶剂分子数纳入了本理论中。这种方法令人满意地描述了使用偶极溶质香豆素 153（C153）在质子和非质子极性液体中测量的实验旋转时间，更重要的是，提供了微观解释，说明电相互作用对溶质旋转的贡献微不足道，与在离子迁移率的情况下，平移介电摩擦所起的重要作用形成对比。还讨论了如何适当扩展本理论以研究除偶极溶剂以外的介质中实际溶质的旋转。
