Borthakur Manash Pratim, Bandyopadhyay Dipankar, Biswas Gautam
Department of Mechanical Engineering, Indian Institute of Technology Guwahati, India.
Faraday Discuss. 2018 Sep 28;209(0):259-271. doi: 10.1039/c8fd00027a.
We investigate the influence of an applied electric field on the separation of a water-ethanol solution inside a carbon nanotube (CNT) using a series of molecular dynamics simulations. The electric field is applied at an angle θ with respect to the axis of the CNT. The study uncovers that with the application of a 'small-angle' electric field (e.g. smaller θ), the water molecules exhibit preferential occupancy inside the CNT, whereas the application of the same electric field at a 'wide-angle' mode (e.g. higher θ) fills the CNT with ethanol molecules in place of water. Remarkably, the direction of the electric field plays a pivotal role because the field exerts a contrasting influence on the behaviours of the water and ethanol molecules. The water dipoles are favourably aligned at small values of θ creating an ordered water structure inside the CNT. Increasing θ disrupts the water dipole orientation and leads to the preferential occupancy of the CNT by ethanol molecules. An in-depth analysis on the simulated systems unveil that, at lower values of θ, multiple layers of water molecules are physically adsorbed near the CNT walls, which is found to diminish as θ is increased. In comparison, at higher magnitudes of θ, the ethanol molecules are preferentially adsorbed inside the CNT. The average interaction energy per ethanol (water) molecule is found to increase (reduce) when θ is monotonically increased, which can be ascribed to the increase (decrease) in the intermolecular hydrogen bonding capacity of the ethanol (water) molecules at larger values of θ. Consequently, inside the CNT, the average occupancy of water molecules decreases and ethanol molecules increases, as θ is monotonically increased, leading to the separation of the ethanol-water mixture. The proposed methodology can convert an equimolar mixture (1 : 1) of ethanol-water into a concentrated one (14 : 1) when the electric field is applied orthogonal to the axis of the CNT. The separation efficiency is found to improve with an increase in the intensity of the externally applied electric field.
我们通过一系列分子动力学模拟研究了外加电场对碳纳米管（CNT）内水 - 乙醇溶液分离的影响。电场相对于碳纳米管的轴以角度θ施加。研究发现，施加“小角度”电场（例如较小的θ）时，水分子在碳纳米管内表现出优先占据，而以“广角”模式（例如较大的θ）施加相同电场时，乙醇分子会取代水分子填充碳纳米管。值得注意的是，电场方向起着关键作用，因为电场对水和乙醇分子的行为产生相反的影响。在θ值较小时，水偶极子有利地排列，在碳纳米管内形成有序的水结构。增大θ会破坏水偶极子的取向，并导致乙醇分子优先占据碳纳米管。对模拟系统的深入分析表明，在较低的θ值下，多层水分子物理吸附在碳纳米管壁附近，随着θ的增加，这种吸附会减少。相比之下，在较高的θ值下，乙醇分子优先吸附在碳纳米管内。当θ单调增加时，发现每个乙醇（水）分子的平均相互作用能增加（减少），这可归因于在较大θ值时乙醇（水）分子分子间氢键能力的增加（减少）。因此，在碳纳米管内，随着θ单调增加，水分子的平均占据率降低，乙醇分子的平均占据率增加，导致乙醇 - 水混合物的分离。当电场垂直于碳纳米管的轴施加时，所提出的方法可以将等摩尔的乙醇 - 水混合物（1∶1）转化为浓缩混合物（14∶1）。发现分离效率随着外加电场强度的增加而提高。
