Shahbabaei Majid, Kim Daejoong
Department of Mechanical Engineering, Sogang University, 35 Baekbeom-ro, Mapo-gu, Seoul 121-742, Republic of Korea.
Phys Chem Chem Phys. 2017 Aug 9;19(31):20749-20759. doi: 10.1039/c7cp03981f.
In this study, molecular dynamics (MD) simulations are used to examine the water transport properties through asymmetric hourglass-shaped pores in multilayer nanoporous graphene with a constant interlayer separation of 6 Å. The properties of the tested asymmetric hourglass-shaped pores [with the models having long cone (l, -P) and short cone (l, +P) entrances] are compared to a symmetric pore model. The study findings indicate that the water occupancy increases across the asymmetric pore (l, -P) compared to (l, +P), because of the length effect. The asymmetric pore, (l, -P), yields higher flux compared to (l, +P) and even the symmetric model, which can be attributed to the increase in the hydrogen bonds. In addition, the single-file water molecules across the narrowest pore diameter inside the (l, +P) pore exhibit higher viscosity compared to those in the (l, -P) pore because of the increase in the water layering effect. Moreover, it is found that the permeability inside the multilayer hourglass-shaped pore depends on the length of the flow path of the water molecules before approaching the layer with the smallest pore diameter. The probability of dipole orientation exhibits wider distribution inside the (l, -P) system compared to (l, +P), implying an enhanced formation of hydrogen bonding of water molecules. This results in the fast flow of water molecules. The MD trajectory shows that the dipole orientation across the single-layer graphene has frequently flipped compared to the dipole orientation across the pores in multilayer graphene, which is maintained during the whole simulation time (although the dipole orientation has flipped for a few picoseconds at the beginning of the simulation). This can be attributed to the energy barrier induced by the individual layer. The diffusion coefficient of water molecules inside the (l, +P) system increases with pressure difference, however, it decreases inside the (l, -P) system because of the increase in the number of collisions. It was found that the velocity in the axial direction (z-direction) has a significant impact on the permeation ability of water molecules across the asymmetric nanopores examined in this study. Finally, the study results suggest that the appropriate design of an asymmetric hourglass-shaped nanopore in multilayer graphene can significantly improve the water permeation rate even compared to a symmetric structure.
在本研究中，采用分子动力学（MD）模拟来研究在层间距恒定为6 Å的多层纳米多孔石墨烯中，水通过不对称沙漏形孔的传输特性。将测试的不对称沙漏形孔（模型具有长锥（l, -P）和短锥（l, +P）入口）的特性与对称孔模型进行比较。研究结果表明，由于长度效应，与（l, +P）相比，水在不对称孔（l, -P）中的占有率增加。与（l, +P）甚至对称模型相比，不对称孔（l, -P）产生更高的通量，这可归因于氢键的增加。此外，由于水层效应的增加，与（l, -P）孔中的水分子相比，穿过（l, +P）孔内最窄孔径的单列水分子表现出更高的粘度。此外，发现多层沙漏形孔内的渗透率取决于水分子在接近最小孔径层之前的流动路径长度。与（l, +P）相比，（l, -P）系统内偶极取向的概率分布更宽，这意味着水分子间氢键形成增强。这导致水分子快速流动。MD轨迹表明，与多层石墨烯孔中的偶极取向相比，单层石墨烯上的偶极取向频繁翻转，而多层石墨烯孔中的偶极取向在整个模拟时间内保持不变（尽管在模拟开始时偶极取向有几皮秒的翻转）。这可归因于各层引起的能垒。（l, +P）系统内水分子的扩散系数随压差增加而增大，然而，由于碰撞次数增加，（l, -P）系统内扩散系数减小。研究发现，轴向（z方向）速度对本研究中所考察的水分子通过不对称纳米孔的渗透能力有显著影响。最后，研究结果表明，即使与对称结构相比，多层石墨烯中不对称沙漏形纳米孔的适当设计也能显著提高水的渗透速率。
