Zaragoza A, Gonzalez M A, Joly L, López-Montero I, Canales M A, Benavides A L, Valeriani C
Departamento de Estructura de la Materia, Facultad de Ciencias Físicas, Física Térmica y Electrónica, Universidad Complutense de Madrid, 28040 Madrid, Spain.
Departamento de Química Física, Facultad de Ciencias Químicas, Universidad Complutense de Madrid, 28040 Madrid, Spain.
Phys Chem Chem Phys. 2019 Jul 7;21(25):13653-13667. doi: 10.1039/c9cp02485a. Epub 2019 Jun 13.
In the past few decades great effort has been devoted to the study of water confined in hydrophobic geometries at the nanoscale (tubes and slit pores) due to the multiple technological applications of such systems, ranging from drug delivery to water desalination devices. To our knowledge, neither numerical/theoretical nor experimental approaches have so far reached a consensual understanding of structural and transport properties of water under these conditions. In this work, we present molecular dynamics simulations of TIP4P/2005 water under different nanoconfinements (slit pores or nanotubes, with two degrees of hydrophobicity) within a wide temperature range. It has been found that water is more structured near the less hydrophobic walls, independently of the confining geometries. Meanwhile, we observe an enhanced diffusion coefficient of water in both hydrophobic nanotubes. Finally, we propose a confined Stokes-Einstein relation to obtain the viscosity from diffusivity, whose result strongly differs from the Green-Kubo expression that has been used in previous works. While viscosity computed with the Green-Kubo formula (applied for anisotropic and confined systems) strongly differs from that of the bulk, viscosity computed with the confined Stokes-Einstein relation is not so much affected by the confinement, independently of its geometry. We discuss the shortcomings of both approaches, which could explain this discrepancy.
在过去几十年里，由于此类系统在从药物输送到水淡化装置等多种技术应用，人们致力于研究纳米尺度下（管和狭缝孔）限制在疏水几何结构中的水。据我们所知，到目前为止，无论是数值/理论方法还是实验方法，都尚未对这些条件下水的结构和传输性质达成共识性理解。在这项工作中，我们展示了在较宽温度范围内，不同纳米限制条件下（狭缝孔或纳米管，具有两种疏水性程度）TIP4P/2005水的分子动力学模拟。已经发现，在疏水性较弱的壁附近，水的结构更有序，这与限制几何结构无关。同时，我们观察到在两种疏水纳米管中，水的扩散系数都有所增强。最后，我们提出了一种受限的斯托克斯 - 爱因斯坦关系，以便从扩散率获得粘度，其结果与先前工作中使用的格林 - 久保表达式有很大不同。虽然用格林 - 久保公式（应用于各向异性和受限系统）计算的粘度与本体粘度有很大差异，但用受限的斯托克斯 - 爱因斯坦关系计算的粘度受限制的影响不大，与限制几何结构无关。我们讨论了这两种方法的缺点，这可以解释这种差异。
