Sharma Manju, Yashonath S
Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore-560012, India.
J Phys Chem B. 2006 Aug 31;110(34):17207-11. doi: 10.1021/jp064364a.
Einstein and others derived the reciprocal dependence of the self-diffusivity D on the solute radius r(u) for large solutes based on kinetic theory. We examine here (a) the range of r(u) over which Stokes-Einstein (SE) dependence is valid and (b) the precise dependence for small solutes outside of the SE regime. We show through molecular dynamics simulations that there are two distinct regimes for smaller solutes: (i) the interaction or Levitation effect (LE) regime for solutes of intermediate size and (ii) the D proportional, variant 1/r(u)(2) for still smaller solutes. We show that as the solute-solvent size ratio decreases, the breakdown in the Stokes-Einstein relationship leading to the LE regime has its origin in dispersion interaction between the solute and the solvent. These results explain reports of enhanced solute diffusion in solvents existing in the literature seen for small solutes for which no explanation exists.
爱因斯坦等人基于动力学理论推导出了大溶质的自扩散系数D与溶质半径r(u)的相互依赖关系。在此，我们研究：(a)斯托克斯 - 爱因斯坦（SE）依赖关系有效的r(u)范围；(b)在SE范围之外的小溶质的精确依赖关系。我们通过分子动力学模拟表明，对于较小溶质存在两种不同的情况：(i)中等大小溶质的相互作用或悬浮效应（LE）情况；(ii)更小溶质的D与1/r(u)(2)成正比。我们表明，随着溶质 - 溶剂尺寸比的减小，导致LE情况的斯托克斯 - 爱因斯坦关系的破坏源于溶质与溶剂之间的色散相互作用。这些结果解释了文献中关于小溶质在溶剂中溶质扩散增强的报道，而此前对此并无解释。
