Manipulation of Long-Range Water Ordering in Less Confined

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C: Physical Processes in Nanomaterials and Nanostructures

Manipulation of Long-Range Water Ordering in Less Confined Nanotubes Chun Shen, and Wanlin Guo J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 27 Mar 2019 Downloaded from http://pubs.acs.org on March 27, 2019

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The Journal of Physical Chemistry

Manipulation of Long-range Water Ordering in Less Confined Nanotubes Chun Shen, Wanlin Guo* Key Laboratory for Intelligent Nano Materials and Devices of the Ministry of Education, and Institute of Nanoscience, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China *Correspondence: Tel: +86 25 84891896; Fax: +86 25 84895827; Email: [email protected]

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ABSTRACT It is well known that the short-range correlation among water molecules can extend up to no more than three hydration layers in bulk, while the long-range water-water correlation was only observed under extreme confinement. Here, we report an unexpected long-range orientational correlation of water molecules confined in relatively wider carbon nanotubes for the first time. Molecular dynamics simulations, including both charge-dipole and dipole-dipole interactions, show that the ordering correlation can be dramatically enhanced by an external dipole, although the enhancement decreases with increasing tube width. The external dipole, namely the imposed point charges, can trigger the reorientation of the less confined water molecules, and the manipulation does not decay along the axial direction, at least in the simulated range. The long-range correlation and reorientation of water molecules in less confined nanotubes should have wide implications in both water-related biological systems and molecular scale signaling.

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INTRODUCTION The dipole-charge interactions between water molecules and charged groups of biomolecules or ions in cytoplasm have been widely recognized as the key to many physiological processes, including the transport of ions through ion channels1 and the conformational change of proteins.2 However, the thermally orientationally averaged interaction of a dipole and an ion or a charged group is limited to a very short range due to the sharp decay with a rate of 1/R4, where R is the oxygen-ion distance. The short-range correlations among water molecules with or without charged solutes have been deterministically observed in extensive experiments by using x-ray absorption spectroscopy,3 Raman spectroscopy,4 dielectric spectroscopy,5 x-ray scattering,6 neutron scattering,7-8 femtosecond elastic second harmonic scattering9 and vibrational dynamics measurements.10-13 Molecular dynamics (MD) simulations also showed the detailed interactions and dynamics between water molecules and ions.14-15 It was shown that a significant correlation between water-water or water-ion in bulk is mainly within three hydration shells. Although a long-range orientational water correlation, extending beyond 8 nm, has been observed in dilute aqueous salt solution,9,

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the

correlation became extremely weaker in the distance larger than 1 nm compared to that in the closest three water layers. The correlation among water molecules in the long range, typically 1 nm away from the first few hydrogen layers, was only observed in the extremely confined 3



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single-file water chain17 or monolayer square ice.18 In nanotubes with small diameters, the water molecules can arrange themselves into single-file chains through hydrogen bonds, form ordered correlation oscillation19, show high rate transport of water molecules through a carbon nanotube (CNT) driven by osmolality17, exhibit exotic thermodynamic and transport behavior inside different nanotubes,20-21 mediate proton transfer with high mobilities,22-23 and even transport signals at molecular scale by modulating the input dipole orientation.24 However, such exotic long-range ordering dynamics has never been reported in less confined environment such as wider nanotubes. Here, we report for the first time the long-range water correlations under non-extreme confinement, namely bulk-like water confined in relatively wide nanotubes, and the reorientation of water dipoles induced by the imposed charge. Using MD simulations, we investigated the correlation and reorientation of water molecules in the CNTs with diameters ranging from 0.82 nm to 3.4 nm, which provide the ideal constraint environment. The water correlations were found undiminished in the 100 nm long nanotube. The water molecules showed very quick reorientation triggered by the local electrostatic interaction between the imposed charge and water molecules around. Although the reorientation degree decreases with increasing widths of the CNTs, it is a peculiar and robust long-range correlated behavior of water molecules. 4


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METHODS The simulations were carried out in the NPT ensemble with the software NAMD 2.8.25 The Langevin dynamics was used to control the temperature at 300 K, and the Nosé– Hoover Langevin piston method was employed to maintain the pressure at 1 atm.26-27 A time step of 2 fs was used. Periodic boundaries were set in all directions with dimensions (Lx, Ly, Lz), where Lx = 4.3 nm, Ly = 4.3 nm, and Lz = 130 nm, and the cutoff was 1.2 nm for non-bonded interactions. The TIP3P water model was used.28 The carbon atoms were treated as uncharged Lennard-Jones particles with parameters from ref

29.

All the carbon atoms were fixed, which had no effect on the

structure of water molecules inside the CNTs.30 An equilibrium simulation of 100 ns for an initial system without assigned charges was carried out to obtain an equilibrated water chain. In the subsequent simulations, the reorientation of water molecules was induced by the external dipole, which was generated by a positive charge and a negative charge. A charge of +e was fixed in the middle of the CNT at z-axis with a radial distance of 1 Å from the CNT wall. A negative countercharge was assigned to a chloride ion in the water reservoir with a distance of 56 nm from +e to keep the whole system electrically neutral. Thus, a dipole was generated across the long distance. To test the effect of the resulted dipole, a series of simulations were carried out. If the negative charge was set in the vacuum, with a distance of 45 nm from the positive charge, the water molecules also show similar reorientation (Fig. S1). However, if the 5



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negative charge was set close to the positive charge with a small distance of 0.5 nm, the electrostatic interaction on water dipoles was offset, no reorientation was observed (Fig. S1). Moreover, if a negative charge was introduced to the background, the reorientation of water molecules with a weakened tendency can be observed (Fig. S1). The applied dipole has influence on the orientation and behavior of the confined water dipoles. Here in our study, we mainly focus on the effect of a nearby +e and a faraway -e on the confined water molecules. RESULTS AND DISCUSSION In our simulations, the water molecules were confined in 0.82 (6,6), 0.95 (7,7), 1.10 (8,8), 1.23 (9,9), 1.36 (10,10), 1.63 (12,12), 2.03 (15,15) and 3.4 (25,25) nm-diameter single-walled armchair CNTs. A snapshot of a typical simulation system, which consists of a water-filled CNT fragment connected to two water-filled reservoirs, is presented in Fig. 1a. A positively charged dummy atom, namely a point with positive charge of specific value, is set outside the CNT with a distance of 1 Å from the CNT wall, the specific position is shown in Fig. 1b. The length of the CNT is 100 nm.

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(a)

z

-

+

+

-

(b)

1Å

+

+

Figure 1. Sketch maps and snapshots of the simulation system. (a) Sketch maps of the side (left) and top view (right) of the simulation system. The water is shown partially transparent. The point charges are marked by “+” and “-”. In the top view, the water molecules in the tube are emphasized in licorice and the water layer near the CNT wall is marked in purple. (b) Partial snapshots of the simulation system, showing the relative position of the imposed charge to the CNT wall. To quantify the variation of water structure with the tube diameter, we calculated the density distribution of water molecules along the radii of CNTs. Dimensionless water density, divided by the maximum is shown in Fig. 2a. Water molecules form a single-file chain in the (6, 6) CNT with a sharp unimodal distribution. As the diameter increases, the number of density peaks increases. In the (7,7), (8,8) and (9,9) CNTs, there are two distribution peaks, and it’s a gradual fission from a single peak in the (6,6) CNT to two completely separate peaks in the (9,9) CNT, where the water molecules attach to the nanotube wall. For the larger 1.36 nm wide (10,10) CNT, the 7



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third peak arises in the center region, indicating the appearance of the second water layer within the nanotube. As the CNTs become wider and wider, more water layers emerge inside the tubes with a weakened stratification, ending in a platform in the density profile. At this moment, bulk-like water was finally obtained in the relatively wide CNTs. Figure 2b shows the axial distribution of water molecules in the CNTs, which was defined as the normalized water density at a distance of L from a specific position, and

L is the distance between any two water molecules along z-axis. Inside the CNTs with diameters less than 1 nm, the axial positions of the water molecules show obvious long-range correlations and oscillations (Fig. 2b), which is a distinguishing feature of subcontinuum liquids in highly confined environment.30 In the CNTs with diameters larger than 1.6 nm, the axial distribution of water molecules has a small peak at L = 0.3 nm, which is similar to that in the bulk water, suggesting the insignificant structure changes of water confined in the wide CNTs.

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(a) 0.5 0.0 0.5 0.0 0.5 0.0 0.5 0.0 0.5 0.0 0.5 0.0 0.5 0.0 0.5 0.0 -20 -15 -10 -5

CNT(6,6)

(b) 1.0

CNT(7,7)

0.8 CNT(8,8) CNT(9,9) CNT(10,10) CNT(12,12) CNT(15,15)

Normalized distribution

Water density

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.6

0.4

0.2

CNT(25, 25)

0 5 r (Å)

10 15 20

0.0

CNT (25,25)

0

2

4 6 L (nm)

8

10

Figure 2． Water density distribution inside CNTs. (a) Normalized water density along the radii of CNTs. The gray areas correspond to the axial section of the CNTs. (b) Axial distribution of water molecules in the CNTs. L is the distance between any two water molecules along z-axis. Figure 3a shows the water−water orientational correlation function |〈𝑐𝑜𝑠𝜑〉|, namely Cosine Similarity, among all pairs of water molecules near the nanotube wall (corresponding to the first density peak in Fig. 2a) as a function of distance, where 𝜑 is the angle between the dipoles, and R is the interoxygen distance. For small interoxygen distance (R < 1 nm), the oscillating water-water correlation corresponds to the axial water distribution in Fig. 2b. With R > 1 nm in the (6,6) CNT, the water-water orientation correlation is ~0.73, indicating a high correlation among water molecules, and this correlation increases to ~0.75 with the imposed charge. In the case of (7,7) 9



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and (8,8) CNTs, water molecules have a similar radial distribution (Fig. 2a) and the water-water orientation correlations are ~0.62 and ~0.60 at the distance beyond 1 nm, respectively. The correlations can be further enlarged to ~0.65 and ~0.61 with the external charges. Although the density distribution of water in the (9,9) CNT (Fig. 2a) is similar to that in the previously mentioned two smaller CNTs, the water cylinder formed in the tube has a relatively large hollow, the water-water orientation correlation within the water layer adjacent to the nanotube wall is only ~0.13. However, in this case, the correlation has a prominent increasement of ~0.37 with the imposed charge, implying a more ordered structure of the confined water molecules. For the water molecules in the CNTs with larger diameters, where there is more than a single chain or layer of water inside the nanotubes, the orientational correlation beyond the distance of 1 nm is nearly 0. That is to say, there is no correlation among the water molecules far away from each other in the layer near the nanotube wall. It is surprising that, once a charge is assigned near the nanotube wall, the water molecules become orientationally correlated with correlation values of ~0.19, ~0.08 and ~0.03 in the (10,10), (12,12) and (15,15) CNTs, respectively. The short-range dipole-charge interaction dramatically results in the long-range orientational correlations among water molecules.

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(b)

(a) 0.8

With charge CNT (6,6) CNT (7,7) CNT (8,8) CNT (9,9) CNT (10,10) CNT (12,12) CNT (15,15)

0.4

φ With charge CNT (6,6) CNT (7,7) CNT (8,8) CNT (9,9) CNT (10,10) CNT (12,12) CNT (15,15)

0.4

0.2

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20 R (Å)

Without charge CNT (6,6) CNT (7,7) CNT (8,8) CNT (9,9) CNT (10,10) CNT (12,12) CNT (15,15)

30

40

Probability density

0.6

||

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.3

Without charge CNT (6,6) CNT (7,7) CNT (8,8) CNT (9,9) CNT (10,10) CNT (12,12) CNT (15,15)

0.2

0.1

0.0

0

30

60 90 120 Anglez ()

150

180

Figure 3. Long-range changes of water molecules in CNTs. (a) Water-water orientational correlations of water molecules in the first layer near the CNT wall with (solid lines) and without (dash lines) the external charge. (b) Probability density of Anglez of water molecules near the CNT wall with (solid lines) and without (dash lines) the external charge. Furthermore, we calculated the probability density distribution of the Anglez of the water molecules in the first water layer, which is the angle between the water dipole and the z-axis. As shown in Fig. 3b, water molecules confined in the (6,6), (7,7) and (8,8) CNTs have uniform dipole orientation, connected by hydrogen bonds. Once an external charge +e was imposed near the CNT wall, the water molecules on one side of the tube, with the dipole initially orientated to the charge, flipped to the opposite direction. A representative snapshot for this flipping was shown in Fig. S2.

For the 11



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water molecules in the (9,9) CNT and without the external charge, Anglez has a relatively wide distribution compared to that in the smaller CNTs. Although water molecules show different orientations, there are two distribution peaks for Anglez with one of them more prominent, indicating a preference of water orientation. As mentioned before, an external charge can remarkably enlarge the water−water orientational correlation in the (9,9) CNT, thus the orientation preference is much more distinct with the imposed charge. For the water molecules in wider (10,10), (12,12) and (15,15) CNTs, when there is no external charge imposed, the water molecules in the distance beyond 1 nm show no correlation among them (Fig. 3a). The distributions of Anglez do not show preference to either end of the CNTs, although the distribution curvature is not straight due to the confinement of the nanotubes (Fig. 3b). Also, the imposed charges result in the preference of orientation of the water molecules near the nanotube wall in the long range. To verify the reliability of this phenomenon, SPC/E water model31 was also used in the (15, 15) CNT, and similar water reorientation was observed (Fig. S3). We also calculated the probability density distribution of Anglez of water molecules in the center region of the (10,10), (12,12) and (15,15) CNTs (Fig. S4). There is no orientational preference of water molecules beyond the first water layer. Similarly, the imposed charges result in slight preference of the water dipoles’

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orientation, although it is a much weaker tendency compared with that on the nanotube wall. To figure out the distinctiveness of water molecules in the first water layer near the CNT internal wall, the water−water orientational correlation along the circumference for the water near the (15, 15) CNT internal surface was calculated (Fig. S5), showing a strong correlation of ~0.64 among water molecules on the CNT internal surface, which is irrespective of the external charge. Owing to this strong correlation along the circumference, the point charge would have an overall effect on a ring of water. We split the charge +e into four charges of +0.25e and arranged them symmetrically around the (15, 15) CNT. As shown in Fig. 4, the water reorientation didn’t show any difference with the splitting of charge. For comparison, four +e charges were arranged symmetrically around the CNT, and as expected, the reorientation was enhanced with stronger electrostatic interaction. Therefore, only a point charge can affect all the water molecules with strong water-water correlation near the CNT wall.

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0.8

Probability density

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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with four charges of +0.25e with four charges of +e with one charge of +e without charge

0.6

+

0.4

+

+

0.2

0.0

+

0

30

60

90

120

150

180

Anglez (°)

Figure 4. Probability density of Anglez of water molecules near the (15, 15) CNT wall with one +e, four +0.25e, four +e or without the external charge. Inset is the cross-section snapshot for the simulation system containing four external charged points around the CNT. As the water-water correlation among water molecules confined in the CNTs can extend to a long range, the dipole reorientation induced by the imposed charge doesn’t decay along the axis of the nanotubes. Figure 5 shows the average Anglez of water molecules along the (15,15) CNT axis (Fig. 5a) and along the circumference (Fig. 5b) after equilibrium, the position of the imposed charge is set to the origin of

z-coordinate. The average Anglez of water molecules in the system without the external charge is ~90° for all areas inside the (15,15) CNT. However, with the imposed charge, the Anglez of water molecules near the nanotube wall becomes ~78° and the value increases towards the center of the CNT, indicating the reorientation of 14


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the water dipoles, especially those near the CNT wall. Most importantly, this is a long-range correlated reorientation of water dipoles under less confinement in the relatively wide nanotube. In the smaller CNTs, the reorientation is certainly more distinct. As mentioned above, the water molecules confined in the (6,6), (7,7) and (8,8) CNTs have uniform orientation, and they show completely reorientation after a charge is assigned near the nanotubes. The situation for water molecules in the (9,9), (10,10) and (12,12) CNTs is similar to that in the (15,15) CNT (Fig. S6).

(a)

Anglez (°)

(b) 10

90

Without charge

Anglez (°) 90

5

y(Å)

r (Å)

0 -5

85

With charge

85

5 80

y(Å)

10 CNT (15, 15) 8 6 4 2 0 ++ 10 8 6 4 2 0 0 10

r (Å)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80

0 -5

20 30 Z (nm)

40

50

75

-10 -10 -5

+

5

10

75

x(Å)

Figure 5. Reorientation of water molecules confined in the (15,15) CNT induced by the imposed charge. (a) The Anglez of water molecules along the axial direction with (bottom panel) and without (top panel) the external charge was shown by the 2D map. (b) The Anglez of water molecules on the cross section with (bottom panel) and

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without (top panel) the external charge was shown by the 2D map. The grey lines or circles represent the position of the CNT wall. The charge was marked at the corresponding position by a blue circle. The interactions between the carbon atoms and the water molecules make the water dipoles tilted, and these dipoles show long-range correlations which can be enhanced by the imposed nearby charges, resulting in the dipoles reorientation. The effect of the imposed charges on the water molecules varies with the CNT diameter, which is related to the confined water structures. If the diameter of the CNT is larger than ~2 nm, the water molecules in the center area of the CNT appear in bulk-like continuous structure (Fig. 2). Furthermore, it is reported that water confined in CNTs with diameter larger than 1.22 nm has similar axial diffusion properties with the bulk water.32 We also performed simulations based on a (25,25) CNT system to test the effect of a single point charge on the orientation of water molecules inside the wide nanotube with a diameter of 3.4 nm, where the water density inside is ~0.987 g/mL. In the control group where there was no imposed charge, the average Anglez of water molecules along the radii of the CNT was always 90° (Fig. S7a). Similarly, the charge was then imposed outside the CNT, 1 Å away from the CNT wall. In this case, the average Anglez decreased to ~88° in the bulk-like area and ~86° in the area corresponding to the first density distribution peak near the CNT wall after equilibrium. We then moved the charge from outside CNT to the inside area and found that, the 16


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water molecules showed reorientation only when the imposed charge was near the layered water on the CNT wall (Fig. S7a). Although the reorientation of water dipoles in the (25,25) CNT is slight, it remains to be a long-range behavior as shown by the 2D map of the average Anglez (Fig. S7b). However, on a flat graphene sheet, the water molecules in the first layer won’t have such long-range reorientation with the imposed charge (Fig. S8). Thus, the connection of water molecules along the circumferential direction plays an important role in the axial long-range behavior. CONCLUSIONS The properties of water molecules under strict confinement have always been related to the behavior of confined water in biosystem. However, the confinements in biosystem are more than the tightly constraint, such as the interstitial space among different components. Besides, the strong constraint case always has its particularity. Here we focus on the orientational correlation of water dipoles under less confinement. We found the long-range correlation among water molecules confined in the CNTs with diameters ranging from 0.82 – 3.4 nm, and it can be enlarged by an external charge near the CNT wall, especially in the relatively wide CNTs. With the imposed charge, water molecules show long-range reorientation. In the wide CNTs, the water molecules near the nanotube wall have the most significant reorientation. Our findings show that, apart from the extreme confinement, the layered water molecules on the circled surface also exhibit long-range correlated behavior, which 17



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can be enhanced by a single charge nearby. This novel finding should have wide implications in both biological systems and signal processing at molecular scale. ASSOCIATED CONTENT Supporting Information Available: Supporting figures S1-S8. AUTHOR INFORMATION Corresponding Author *Wanlin Guo, [email protected] ACKNOWLEDGEMENTS This work was supported by National Natural Science Foundation of China (51535005, 51472117), the Fundamental Research Funds for the Central Universities (NP2017101, NC2018001), and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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TOC Graphic

Long-range water-water correlation in bulk-like water

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Manipulation of Long-Range Water Ordering in Less Confined

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