Olivieri Jean-François, Hynes James T, Laage Damien
PASTEUR, Department of Chemistry, École normale supérieure, PSL University, Sorbonne Université, CNRS, 75005 Paris, France.
Department of Chemistry, University of Colorado, Boulder, Colorado 80309, United States.
J Phys Chem Lett. 2021 May 6;12(17):4319-4326. doi: 10.1021/acs.jpclett.1c00447. Epub 2021 Apr 29.
Liquid water confined within nanometer-sized channels exhibits a surprisingly low dielectric constant along the direction orthogonal to the channel walls. This is typically assumed to result from a pronounced heterogeneity across the sample: the dielectric constant would be bulk-like everywhere except at the interface, where it would be dramatically reduced by strong restrictions on interfacial molecules. Here we study the dielectric properties of water confined within graphene slit channels via classical molecular dynamics simulations. We show that the permittivity reduction is not due to any important alignment of interfacial water molecules, but instead to the long-ranged anisotropic dipole correlations combined with an excluded-volume effect of the low-dielectric confining material. The bulk permittivity is gradually recovered only over several nanometers due to the impact of long-range electrostatics, rather than structural features. This has important consequences for the control of, e.g., ion transport and chemical reactivity in nanoscopic channels and droplets.
限制在纳米级通道内的液态水在垂直于通道壁的方向上表现出出奇低的介电常数。通常认为这是由于样品中明显的不均匀性导致的：除了在界面处，介电常数在其他各处都类似体相，而在界面处，由于对界面分子的强烈限制，介电常数会大幅降低。在此，我们通过经典分子动力学模拟研究了限制在石墨烯狭缝通道内的水的介电性质。我们表明，电容率降低并非由于界面水分子的任何重要排列，而是由于长程各向异性偶极相关性以及低介电常数限制材料的排除体积效应。由于长程静电作用而非结构特征的影响，体相电容率仅在几纳米的范围内逐渐恢复。这对于控制例如纳米通道和液滴中的离子传输及化学反应性具有重要意义。
