Fogarty Aoife C, Duboué-Dijon Elise, Laage Damien, Thompson Ward H
Ecole Normale Supérieure - PSL Research University, Département de Chimie, Sorbonne Universités - UPMC Univ Paris 06, CNRS UMR 8640 PASTEUR, 24 rue Lhomond, 75005 Paris, France.
Department of Chemistry, University of Kansas, Lawrence, Kansas 66045, USA.
J Chem Phys. 2014 Nov 14;141(18):18C523. doi: 10.1063/1.4896983.
The dynamics of water are dramatically modified upon confinement in nanoscale hydrophilic silica pores. In particular, the OH reorientation dynamics of the interfacial water are non-exponential and dramatically slowed relative to the bulk liquid. A detailed analysis of molecular dynamics simulations is carried out to elucidate the microscopic origins of this behavior. The results are analyzed in the context of the extended jump model for water that describes the reorientation as a combination of hydrogen-bond exchanges, or jumps, and rotation of intact hydrogen bonds, with the former representing the dominant contribution. Within this model, the roles of surface and dynamical heterogeneities are considered by spatially resolving the hydrogen-bond jump dynamics into individual sites on the silica pore surface. For each site the dynamics is nearly mono-exponential, indicating that dynamical heterogeneity is at most a minor influence, while the distribution of these individual site jump times is broad. The non-exponential dynamics can also not be attributed to enthalpic contributions to the barriers to hydrogen-bond exchanges. Two entropic effects related to the surface roughness are found to explain the retarded and diverse dynamics: those associated with the approach of a new hydrogen-bond acceptor and with the breaking of the initial hydrogen-bond.
当水被限制在纳米级亲水性二氧化硅孔隙中时，其动力学特性会发生显著改变。特别是，界面水的OH重排动力学是非指数型的，并且相对于本体液体而言显著减慢。为了阐明这种行为的微观起源，我们进行了分子动力学模拟的详细分析。在扩展的水跳跃模型的背景下对结果进行了分析，该模型将重排描述为氢键交换（即跳跃）和完整氢键旋转的组合，其中前者占主导贡献。在这个模型中，通过将氢键跳跃动力学在空间上解析到二氧化硅孔隙表面的各个位点，来考虑表面和动力学非均质性的作用。对于每个位点，动力学几乎是单指数型的，这表明动力学非均质性至多是一个次要影响，而这些单个位点跳跃时间的分布很宽。非指数动力学也不能归因于氢键交换势垒的焓贡献。发现与表面粗糙度相关的两种熵效应可以解释这种延迟和多样的动力学：一种与新氢键受体的接近有关，另一种与初始氢键的断裂有关。
