Berkowicz Sharon, Perakis Fivos
Department of Physics, AlbaNova University Center, Stockholm University, SE-10691 Stockholm, Sweden.
Phys Chem Chem Phys. 2021 Nov 24;23(45):25490-25499. doi: 10.1039/d1cp02866a.
The breakdown of Stokes-Einstein relation in liquid water is one of the many anomalies that take place upon cooling and indicates the decoupling of diffusion and viscosity. It is hypothesized that these anomalies manifest due to the appearance of nanometer-scale spatial fluctuations, which become increasingly pronounced in the supercooled regime. Here, we explore the validity of the Stokes-Einstein relation in supercooled water using nanomolecular probes. We capture the diffusive dynamics of the probes using dynamic light scattering and target dynamics at different length scales by varying the probe size, from ≈100 nm silica spheres to molecular-sized polyhydroxylated fullerenes (≈1 nm). We find that all the studied probes, independent of size, display similar diffusive dynamics with an Arrhenius activation energy of ≈23 kJ mol. Analysis of the diffusion coefficient further indicates that the probes, independent of their size, experience similar dynamic environment, which coincides with the macroscopic viscosity, while single water molecules effectively experience a comparatively lower viscosity. Finally, we conclude that our results indicate that the Stokes-Einstein relation is preserved for diffusion of probes in supercooled water ≥ 260 K with size as small as ≈1 nm.
液态水中斯托克斯 - 爱因斯坦关系的失效是冷却过程中出现的众多反常现象之一，这表明扩散与粘度发生了解耦。据推测，这些反常现象是由于纳米尺度空间涨落的出现而表现出来的，在过冷状态下这种涨落会愈发明显。在此，我们使用纳米分子探针来探究过冷水里斯托克斯 - 爱因斯坦关系的有效性。我们通过动态光散射捕捉探针的扩散动力学，并通过改变探针尺寸（从约100纳米的二氧化硅球到分子尺寸的多羟基化富勒烯（约1纳米））来研究不同长度尺度下的动力学。我们发现，所有研究的探针，无论尺寸大小，都表现出相似的扩散动力学，其阿仑尼乌斯活化能约为23千焦/摩尔。对扩散系数的分析进一步表明，无论探针尺寸如何，它们所处的动态环境相似，这与宏观粘度相符，而单个水分子实际经历的粘度相对较低。最后，我们得出结论，我们的结果表明，对于尺寸小至约1纳米的探针在过冷水（≥260 K）中的扩散，斯托克斯 - 爱因斯坦关系仍然成立。
