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Since the discovery of carbon nanotubes (CNTs), they have attracted extensive attention from scholars in various fields because of their excellent properties. The hollow-structured CNTs are often regarded as conduits and containers, which can act as nano-channels for various molecular substances in the membrane structure. As a source of life, water is indispensable to any living organism. In the application of carbon nanotubes as nanochannels, the most important is the ability of carbon nanotubes to store and transport liquids, especially nanoscaled aqueous solutions. Water molecular clusters in confined spaces exhibit unusual structures and properties. The study of special water structures in carbon nanotubes is of great theoretical importance in chemistry, biology and materials science. There are great difficulties in making the experiment on a nanoscale, but molecular dynamic simulation enables us to better study and analyze the structure and properties of water in confined space of CNT on a nanoscale. One has also studied the influence of temperature on the structure of water, but there are few studies focusing on the effect of temperature on the structure of water in confined space. Therefore, molecular dynamics simulation is used to investigate the effects of CNT diameter, CNT chirality and temperature on the water structure and distribution in a confined space. The simulation calculation is completed by GROMACS, the SPE/C water model is used for water molecules, and GROMOS96 54a7 force field is used. Because of the presence of carbon nanotubes, water molecules tend to line up against the walls of the tubes, both inside and outside. In addition, water molecules tend to form highly ordered multi-ring structures in the carbon nanotubes with a size of 1.018–1.253 nm at a certain temperature. It is difficult to form the ordered structure of water in the outer carbon nanotubes. In the above range, with the increase of pipe diameter, the structure of multi-element ring water changes from three-element ring to six-element ring. On the one hand, the ordered structure depends on the diameter of the carbon nanotube, but the chirality of the carbon nanotube does not have a great influence on it. On the other hand, the stability of the ordered structure is temperature-dependent, and the ordered structure of multiple ring water in the carbon nanotube with a larger diameter is more likely to disappear with the increase of temperature. The van der Waals potential distribution is calculated by Multiwfn, and it is concluded that the van der Waals potential inside the tube is extremely low, resulting in a very large dispersion effect, and molecules can spontaneously move from the outer area to the tube. The van der Waals potential can also be negative outside the tube. This explains why water molecules tend to line up against the wall of the tube.
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Keywords:
- confined space /
- single-wall carbon nanotube /
- water /
- van der Waals potential
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图 1 模型初始结构(CNT完全浸在水中, 内部中空)
Figure 1. Initial structure of the model. The CNT is completely immersed in the water and are hollow inside.
图 2 在0, 50, 100和200 ps时刻水分子的分布(红色球表示氧原子, 白色球表示氢原子, 蓝色球表示碳原子)
Figure 2. Distribution of water molecules in CNT (blue sphere) at 0, 50, 100 and 200 ps. Water molecules are shown by red (O) and white (H) spheres.
图 3 管内水分子排布俯视图和侧视图, 其中碳纳米管手性从左到右分别为(5, 5), (6, 6), (7, 7), (8, 8), (9, 9), (10, 10), (12, 12)
Figure 3. Distribution of water molecules in CNTs (top view and side view). The chirality of CNTs are (5, 5), (6, 6), (7, 7), (8, 8), (9, 9), (10, 10), (12, 12) from left to right.
图 4 298 K下, 管径为1.085 nm的纳米管内水分子的结构及氢键分布(黑色虚线表示氢键)
Figure 4. Structure and hydrogen bonding of water molecules inside the CNT with an inner diameter of 1.085 nm at 298 K. The black dashed lines represent hydrogen bonds.
图 5 水分子分布图 (a)水分子的轨迹叠加图; (b)水分子平均密度分布图
Figure 5. Distribution of water molecules: (a) Trajectory superposition of water molecules; (b) mean density distribution of water.
图 6 具有不同的手性参数和管径的碳纳米管内水的密度分布 (a) (13, 0), 1.018 nm; (b) (8, 8), 1.085 nm; (c) (14, 0), 1.096 nm; (d) (15, 0), 1.174 nm; (e) (9, 9), 1.220 nm; (f) (16, 0), 1.253 nm
Figure 6. Density distributions of the water in CNTs with different diameters and chiralities: (a) (13, 0), 1.018 nm; (b) (8, 8), 1.085 nm; (c) (14, 0), 1.096 nm; (d) (15, 0), 1.174 nm; (e) (9, 9), 1.220 nm; (f) (16, 0), 1.253 nm.
图 7 有水(a)和无水(b)环境下(9, 9)碳纳米管内水的密度分布
Figure 7. Density distribution of water molecules in (9, 9) CNT with (a) and without (b) water outside.
图 8 范德瓦耳斯势分布 (a) (8, 8)型CNT; (b) (10, 10)型CNT
Figure 8. van der Waals (VdW) potential distribution of CNTs: (a) (8, 8) CNT; (b) (10, 10) CNT.
图 9 298和373 K下(10, 10)碳纳米管内水的密度分布
Figure 9. Density distribution of water molecules in (10, 10) CNT at 298 and 373 K.
图 10 不同温度下管内水分子的密度分布, 其中管径为1.085 nm的是(8, 8)管, 管径为1.174 nm的是(15, 0)管, 管径为1.253 nm的是(16, 0)管
Figure 10. Density distribution of water molecules in CNTs at different temperatures. The diameter of (8, 8) CNT is 1.085 nm, the diameter of (15, 0) CNT is 1.174 nm, and the diameter of (16, 0) CNT is 1.253 nm.
表 1 碳纳米管类型
Table 1. Types of CNTs.
手性参数
(n, m)管径
D/nm手性参数
(n, m)管径
D/nm(5, 5) 0.678 (6, 6) 0.814 (7, 7) 0.949 (8, 8) 1.085 (9, 9) 1.220 (10, 10) 1.356 (12, 12) 1.627 (13, 0) 1.018 (14, 0) 1.096 (15, 0) 1.174 (16, 0) 1.253 
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[1] Iijima S 1991 Nature 354 56
Google Scholar
[2] Gumhye J, Seung Y Y, Jin K K 2012 J. Mater. Chem. 22 14814
Google Scholar
[3] Joshi R K, Carbone P, Wang F C, Kravets V G, Su Y, Grigorieva I V, Wu H A, Geim A K, Nair R R 2014 Science 343 752
Google Scholar
[4] Marc G, Golovchenko J A 2007 Nat. Nanotechnol. 2 775
Google Scholar
[5] David C T, Jeffrey C C 2012 Nano Lett. 12 3602
Google Scholar
[6] Hansong C, Alan C C, Guido P P, Milen K K, Pamela P, Steve J S 2005 J. Phys. Chem. B 109 3780
Google Scholar
[7] Eansom M S P, Kerr I D, Breed J, Sankararamakrishnan R 1996 Biophys. J. 70 693
Google Scholar
[8] Koga K, Gao G T, Tanaka H, Zeng X C 2001 Nature 412 802
Google Scholar
[9] Takaiwa D, Hatano I, Koga K, Tanaka H 2008 Proc. Natl. Acad. Sci. U.S.A. 105 39
Google Scholar
[10] Boynton P, Di Ventra M 2013 Phys. Rev. Lett. 111 216804
Google Scholar
[11] Ostmeyer J, Chakrapani S, Pan A C, Perozo E, Roux B 2013 Nature 501 121
Google Scholar
[12] Dong H, Fiorin G, Carnevale V, Treptow W, Klein M L 2013 Proc. Natl. Acad. Sci. U.S.A. 110 17332
Google Scholar
[13] Ohba T 2014 Angew Chem. 126 8170
Google Scholar
[14] Barati Farimani A, Aluru N R 2011 J. Phys. Chem. B 115 12145
Google Scholar
[15] Striolo A 2006 Nano Lett. 6 633
Google Scholar
[16] Alexiadis A, Kassinos S 2008 Chem. Rev. 108 5014
Google Scholar
[17] Spoel D V D, Lindahl E, Hess B, Groenhof G, Mark A E, Berendsen H J C 2005 J. Comput. Chem. 26 1701
Google Scholar
[18] Berendsen H J C, Grigera J R, Straatsma T P 1987 J. Phys. Chem. 91 6269
Google Scholar
[19] Mahoney M W, Jorgensen W L 2001 J. Chem. Phys. 114 363
Google Scholar
[20] Mahoney M W, Jorgensen W L 2000 J. Chem. Phys. 112 8910
Google Scholar
[21] Essmann U, Perera L, Berkowitz M L Darden T, Lee H, Pedersen L G 1995 J. Chem. Phys. 103 8577
Google Scholar
[22] 韩典荣, 朱兴凤, 戴亚飞, 程承平, 罗成林 2015 64 230201
Google Scholar
Han D R, Zhu X F, Dai Y F, Cheng C P, Luo C L 2015 Acta Phys. Sin. 64 230201
Google Scholar
[23] Bussi G, Donadio D, Parrinello M 2007 J. Chem. Phys. 126 014101
Google Scholar
[24] Lu T, Chen Q X 2020 ChemRxiv DOI: 10.26434/chemrxiv.12148572.v1
[25] Lu T, Chen F W 2012 J. Comput. Chem. 33 580
Google Scholar
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