A molecular simulation study into the stability of hydrated graphene nanochannels used in nanofluidics devices.
A molecular simulation study into the stability of hydrated graphene nanochannels used in nanofluidics devices.
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DOI：
10.1039/d1nr08275b
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发表时间：
2022-02
期刊：
影响因子：
6.7
通讯作者：
C. D. Williams;Zixuan Wei;Mohd Rafie Bin Shaharudin;P. Carbone
中科院分区：
文献类型：
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作者：
C. D. Williams;Zixuan Wei;Mohd Rafie Bin Shaharudin;P. Carbone
基于石墨烯的纳米通道由于其可调谐和纳米尺度的通道而成为新兴纳米流体应用的热门选择。在这项工作中，分子动力学（MD）模拟被用于(i)评估干燥和水合石墨烯纳米通道的稳定性，（ii）阐明这些通道中水的性质，使用通道高度为0.66-2.38 nm的复制尺度模型。柔性纳米通道壁的使用允许纳米通道高度随封闭流体产生的溶剂化力和封闭表面之间的力而放松，而无需施加任意高的外部压力。研究发现，如果初始纳米通道高度小于2纳米，由于石墨烯表面之间的范德华相互作用，干燥的纳米通道会完全坍塌。然而，由于水合斥力与吸引的范德华力相反，研究人员发现水的存在可以防止纳米通道的坍塌。对于高度小于~ 1.7 nm的纳米通道，必须放松围水表面以获得准确的水化压力和水扩散系数，从而确保围水层数与通道高度之间的可通约性。对于非常小（~ 0.7 nm）的水合通道，由于范德华力的作用，获得了231 MPa的压力。在同一体系中，承压水形成流动的液体单层，其扩散系数为4.0 × 10-5 cm2 s-1，远高于散装液态水。尽管这一发现与大多数经典的MD模拟相冲突，这些模拟预测平面内顺序和捕获动力学，但它得到了实验和最近发表的第一原理MD模拟的支持。因此，经典模拟可以用来预测亚纳米石墨烯通道中水的性质，提供足够真实的分子模型和精确的分子间电位。
Graphene-based nanochannels are a popular choice in emerging nanofluidics applications because of their tunable and nanometer-scale channels. In this work, molecular dynamics (MD) simulations were employed both to (i) assess the stability of dry and hydrated graphene nanochannels and (ii) elucidate the properties of water confined in these channels, using replica-scale models with 0.66-2.38 nm channel heights. The use of flexible nanochannel walls allows the nanochannel height to relax in response to the solvation forces arising from the confined fluid and the forces between the confining surfaces, without the need for application of arbitrarily high external pressures. Dry nanochannels were found to completely collapse if the initial nanochannel height was less than 2 nm, due to attractive van der Waals interactions between the confining graphene surfaces. However, the presence of water was found to prevent total nanochannel collapse, due to repulsive hydration forces opposing the attractive van der Waals force. For nanochannel heights less than ∼1.7 nm, the confining surfaces must be relaxed to obtain accurate hydration pressures and water diffusion coefficients, by ensuring commensurability between the number of confined water layers and the channel height. For very small (∼0.7 nm), hydrated channels a pressure of 231 MPa due to the van der Waals forces was obtained. In the same system, the confined water forms a mobile, liquid monolayer with a diffusion coefficient of 4.0 × 10-5 cm2 s-1, much higher than bulk liquid water. Although this finding conflicts with most classical MD simulations, which predict in-plane order and arrested dynamics, it is supported by experiments and recently published first-principles MD simulations. Classical simulations can therefore be used to predict the properties of water confined in sub-nanometre graphene channels, providing sufficiently realistic molecular models and accurate intermolecular potentials are employed.