Kumar Shubham, Sarkar Sarmistha, Bagchi Biman
Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India.
J Chem Phys. 2020 Apr 30;152(16):164507. doi: 10.1063/5.0004725.
Aqueous binary mixtures often exhibit dramatic departure from the predicted hydrodynamic behavior when transport properties are plotted against composition. We show by inherent structure (IS) analysis that this sharp composition dependent breakdown of the Stokes-Einstein relation can be attributed to the non-monotonic variation in the average inherent structure energy of these mixtures. Further IS analysis reveals the existence of a unique ground state, stabilized by both the formation of an optimum number of H-bonds and a favorable hydrophobic interaction at this composition. The surprisingly sharp turnaround behavior observed in the effective hydrodynamic radius also owes its origin to the same combination of these two factors. Interestingly, the temperature dependence of isothermal compressibility shows a minimum at the particular composition. Extensive studies on water-dimethyl sulfoxide and water-ethanol mixtures using two different force-fields of water reveal many features that are nearly universal. A justification of this quasi-universal behavior is provided in terms of a mode-coupling theory (MCT) of viscosity, which can serve as the starting point of a remarkable correlation observed with the nearest neighbor structure, as captured by the first peaks of the radial distribution function, and the slowdown in the intermediate scattering function at intermediate wavenumbers. Therefore, the formation of the local structure captured through IS analysis can be correlated with the MCT.
当将输运性质与组成关系作图时，二元水混合体系常常表现出与预测的流体动力学行为有显著偏差。我们通过固有结构（IS）分析表明，斯托克斯 - 爱因斯坦关系这种与组成密切相关的急剧破坏可归因于这些混合物平均固有结构能量的非单调变化。进一步的IS分析揭示了存在一种独特的基态，它通过形成最佳数量的氢键以及在该组成下有利的疏水相互作用而得以稳定。在有效流体动力学半径中观察到的惊人急剧转变行为同样源于这两个因素的共同作用。有趣的是，等温压缩率的温度依赖性在特定组成处出现最小值。使用两种不同的水的力场对水 - 二甲基亚砜和水 - 乙醇混合体系进行的广泛研究揭示了许多几乎普遍存在的特征。根据粘度的模式耦合理论（MCT）对这种准普遍行为进行了论证，该理论可作为与最近邻结构（由径向分布函数的第一个峰所捕获）以及中间波数下中间散射函数的减慢所观察到的显著相关性的起点。因此，通过IS分析捕获的局部结构的形成可以与MCT相关联。
