Department of Civil and Environmental Engineering, Princeton University, Princeton, New Jersey 08544, United States.
High Meadows Environmental Institute, Princeton University, Princeton, New Jersey 08544, United States.
J Phys Chem B. 2022 Apr 14;126(14):2688-2698. doi: 10.1021/acs.jpcb.1c09688. Epub 2022 Apr 1.
In this study, we examine the spectral dielectric properties of liquid water in charged nanopores over a wide range of frequencies (0.3 GHz to 30 THz) and pore widths (0.3 to 5 nm). This has been achieved using classical molecular dynamics simulations of hydrated Na-smectite, the prototypical swelling clay mineral. We observe a drastic (20-fold) and anisotropic decrease in the static relative permittivity of the system as the pore width decreases. This large decrement in static permittivity reflects a strong attenuation of the main Debye relaxation mode of liquid water. Remarkably, this strong attenuation entails very little change in the time scale of the collective relaxation. Our results indicate that water confined in charged nanopores is a distinct solvent with a much weaker collective nature than bulk liquid water, in agreement with recent observations of water in uncharged nanopores. Finally, we observe remarkable agreement between the dielectric properties of the simulated clay system against a compiled set of soil samples at various volumetric water contents. This implies that saturation may not be the sole property dictating the dielectric properties of soil samples, rather that the pore-size distribution of fully saturated nanopores may also play a critically important role.
在这项研究中，我们通过对水合钠蒙脱石（典型的膨胀粘土矿物）的经典分子动力学模拟，在很宽的频率范围（0.3GHz 至 30THz）和孔径范围（0.3nm 至 5nm）内研究了带电纳米孔中液态水的光谱介电特性。我们观察到，随着孔径的减小，体系的静态相对介电常数急剧（20 倍）各向异性减小。这种静态介电常数的大幅降低反映了液态水主要德拜弛豫模式的强烈衰减。值得注意的是，这种强烈的衰减几乎没有改变集体弛豫的时间尺度。我们的结果表明，受限在带电纳米孔中的水是一种独特的溶剂，其集体性质比体相液态水弱得多，这与最近对不带电纳米孔中水分子的观察结果一致。最后，我们观察到模拟粘土系统的介电特性与各种体积含水量的土壤样本的一组汇编样本之间存在显著的一致性。这意味着饱和度可能不是决定土壤样本介电特性的唯一因素，而是完全饱和的纳米孔的孔径分布也可能起着至关重要的作用。
