Ghorai Pradip Kr, Yashonath S
Solid State and Structural Chemistry Unit and Center for Condensed Matter Theory, Indian Institute of Science, Bangalore 560012, India.
J Phys Chem B. 2005 Mar 31;109(12):5824-35. doi: 10.1021/jp046312w.
We report a molecular dynamics study of a binary mixture consisting of a large (host) particle and a smaller (guest) particle whose radius is varied over a range. These simulations investigate the possible existence of a diffusion anomaly or levitation effect in dense fluids, previously seen for guest molecules diffusing within porous solids. The voids in the larger component have been characterized in terms of void and neck distributions by means of Voronoi polyhedral analysis. Four different mixtures with differing ratios of guest to host diffusivities (D) have been studied. The results suggest that the diffusion anomaly is seen in both close-packed solids with disorder and dense fluids. In the latter, the void network is constantly and dynamically changing and possesses a considerable degree of disorder. The two regimes, viz., the linear regime (LR) and the anomalous regime (AR), found for porous solids are shown to exist for a dense medium as well. The linear regime is characterized by D(g) proportional to 1/sigma(gg)(2), where sigma(gg) is the diameter of the guest. The anomalous regime exhibits a maximum in D up to rather high temperatures (T = 1.663), even though in porous solids the maximum disappears at higher temperatures. In agreement with previous studies on porous solids, a particle in the AR is associated with lower activation energy, lower friction, and less backscattering in the velocity autocorrelation function when compared to a particle in the LR. Wavevector dependent self-diffusivity, Delta, and decay of the intermediate scattering function, F(s)(k, t), exhibit contrasting behaviors for the LR and AR. For LR, Delta exhibits a minimum at values of k at which there are spatial correlations in S(k) while a smooth decrease with k is seen for AR. For LR, F(s)(k, t) shows a biexponential decay corresponding to two different time scales of motion. Probably, the fast decay is associated with motion within the first shell of solvent neighbors and the slow decay with motion past these shells. For AR, a single-exponential decay is seen. The results indicate a breakdown of the Stokes-Einstein (SE) relationship. The relevant quantity that determines the validity of the SE relationship is the levitation parameter which is indirectly related to the solute/solvent radius ratio and not either the size of the solute or the solvent alone.
我们报告了一项分子动力学研究，该研究涉及由一个大的（主体）粒子和一个较小的（客体）粒子组成的二元混合物，其中客体粒子的半径在一定范围内变化。这些模拟研究了在致密流体中是否可能存在扩散异常或悬浮效应，此前在多孔固体中扩散的客体分子中曾观察到这种现象。通过Voronoi多面体分析，根据孔隙和颈部分布对较大组分中的孔隙进行了表征。研究了四种不同的混合物，其客体与主体扩散率（D）的比例不同。结果表明，在具有无序结构的密堆积固体和致密流体中均观察到了扩散异常。在后者中，孔隙网络不断动态变化，且具有相当程度的无序性。在多孔固体中发现的两种状态，即线性状态（LR）和异常状态（AR），在致密介质中也同样存在。线性状态的特征是D(g)与1/sigma(gg)(2)成正比，其中sigma(gg)是客体的直径。异常状态在相当高的温度（T = 1.663）下D会出现最大值，尽管在多孔固体中该最大值在较高温度下会消失。与之前对多孔固体的研究一致，与处于线性状态的粒子相比，处于异常状态的粒子在速度自相关函数中具有较低的活化能、较低的摩擦力和较少的反向散射。波矢相关的自扩散系数Delta以及中间散射函数F(s)(k, t)的衰减，对于线性状态和异常状态表现出不同的行为。对于线性状态，Delta在S(k)存在空间相关性的k值处出现最小值，而对于异常状态，Delta随k呈平滑下降。对于线性状态，F(s)(k, t)表现出双指数衰减，对应于两种不同的运动时间尺度。可能快速衰减与溶剂相邻分子第一壳层内的运动有关，而缓慢衰减与越过这些壳层的运动有关。对于异常状态，观察到单指数衰减。结果表明斯托克斯 - 爱因斯坦（SE）关系失效。决定SE关系有效性的相关量是悬浮参数，它与溶质/溶剂半径比间接相关，而不是仅与溶质或溶剂的大小相关。
