Molecular Dynamics Study of Pore Inner Wall Modification Effect in

Dec 29, 2008 - The behavior of water molecules under nanoscale confinement has received considerable attention, especially for the influence caused by...
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J. Phys. Chem. C 2009, 113, 882–889

Molecular Dynamics Study of Pore Inner Wall Modification Effect in Structure of Water Molecules Confined in Single-Walled Carbon Nanotubes Yudan Zhu,† Mingjie Wei,† Qing Shao,† Linghong Lu,† Xiaohua Lu,*,† and Wenfeng Shen‡ State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing UniVersity of Technology, Nanjing 210009, China, School of Computer Engineering and Science, Shanghai UniVersity, Shanghai, 200070, P. R. China ReceiVed: October 8, 2008; ReVised Manuscript ReceiVed: NoVember 26, 2008

The behavior of water molecules under nanoscale confinement has received considerable attention, especially for the influence caused by the modified groups of pores. To better design bionic nanodevices for future research, we anchored carboxyl acid (-COOH) groups onto the inner wall of a single-walled armchair carbon nanotube’s (CNT’s) central region to model the pore shape of aquaporin-1 and investigated the effect of modified groups on the structure of water molecules. The orientations and density distributions of water molecules in the CNTs and near the tube mouths have been studied by molecular dynamics simulation. The results indicate that water molecules confined inside the two unmodified regions have opposite and steady preferential dipole orientations pointing toward the -COOH groups on the central region of the CNT. Meanwhile the orientations of water molecules near the tube mouths which are certain distances away from the -COOH groups are also affected. This phenomenon becomes stronger as the number of -COOH groups increases and the CNT diameter decreases. In addition, the results show that the -COOH groups on the inner wall of the central region have a slight effect on the axial density distribution of the water molecules near the tube mouths, but a strong impact on that of water molecules inside the CNTs. Different distances between the -COOH groups and tube mouths can create diverse axial density distributions of water molecules. 1. Introduction The behavior of water molecules under nanoscale confinement (for example, in cavities of biomolecules,1,2 water channels in cell membranes,3 and nanoporous catalysts,4-8) has received considerable attention. Theoretical9,10 and experimental11-13 studies have shown that the properties of confined water molecules are different from their bulk counterparts, and these properties affect the function of biological macromolecules14-16 and nanomaterials.17,18 To better understand the properties of confined water molecules, we can use molecular simulation as an effective tool. A number of molecular simulation studies adopted a CNT as a cylindrical pore model to explore the confinement effect on the structure of water molecules, and many special structures of water molecules have been reported.19-25 One special structure of water molecules, called n-gonal ring structure, formed under high pressure (50-500 MPa) was found by Koga et al.20 when they studied the phase transformation of water molecules. Wrapped-around icelike structures of water molecules were found in the physiological condition (300 K and 1 atm) by Noon et al.23 Wang et al.25 estimated the optimum diameter of a CNT that can accommodate n layers of water molecules and pointed out that single-file water chains can be formed only in narrow CNTs (0.676-0.811 nm diameter). However, the inner wall of pristine CNTs with homogeneous hydrophobic character is unable to reproduce some important features of real pore systems. For example, on the protein channel surface, there are various polar and nonpolar residues.26,27 Many metal oxide materials display partially hydrophilic * To whom correspondence should be addressed. Phone: +86-2583588063. Fax: +86-25-83588063. E-mail: [email protected]. † Nanjing University of Technology. ‡ Shanghai University.

character when their pore walls are hydroxylated.28,29 There are many oxygenated sites on the porous carbon surface.30,31 To understand the effect of the functional groups on the pore wall on the properties of confined water molecules, several theoretical research projects have been performed. They found these functional groups on the pore wall can affect the behavior of confined water molecules, leading to many intriguing phenomena in the orientation,32,33 adsorption,34 and transportation35 of water molecules. As reported,32,33 the channel of aquaporin-1 has a constriction in the center of the membrane and wide openings at the membrane surfaces, and the orientations of the water chain could be affected by the residues near the constriction. Striolo et al.34 found that the location of the oxygenated sites (modeled as carbonyl groups) on CNTs had a pronounced effect on the adsorption of water molecules. Zheng et al.35 anchored -COOH groups onto the inner wall of CNTs to alter the whole hydrophobic wall into a hydrophilic one. They investigated the transport behavior of a water and methanol mixture under a fixed concentration gradient through the modified CNTs. Huang et al.36 found that the incurvature or excurvature configurations of -COOH groups on the mouths of CNT could control the flow direction of water molecules. In 2001, Hummer et al.19 studied water molecules confined in a (6, 6) CNT by MD simulation and found that the water flowing through a (6, 6) CNT was similar to that flowing through aquaporin-1. This interesting finding prompted special attention to the structure of confined water molecules. To better understand the CNTs’ potential biological applications, a series of studies have been carried on in our group.25,36,37 A systematic simulation study25 had been performed to evaluate the diameter and helicity effects on static properties of water molecules confined in CNTs. Further studies indicated that the -COOH groups on the mouths of CNTs could strongly affect the structure

10.1021/jp8089006 CCC: $40.75  2009 American Chemical Society Published on Web 12/29/2008

MD Study of H2O Molecules in CNTs

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Figure 1. (a) A model sketch of modified CNT (M3_9). (b, c) A periodic simulation system sketch of modified CNT (M3_9) immersed in water: (b) lateral view; (c) side view. Only a fraction of the water molecules in the simulation box are shown to provide a better view of the CNT.

TABLE 1: The Details of Simulated Systems

of the water chain.37 To better design bionic nanodevices in future research, we anchored the -COOH groups onto the inner wall of the CNTs’ central region to model the pore shape of aquaporin-1 in this study. Though the channel sizes of the CNTs are larger than that of aquaporin-1, the pore shapes of both are similar because the size of the central region of the CNTs are constricted by anchoring the -COOH groups. The aim of this study is to investigate the structural characteristics of water molecules confined in the modified CNTs. Since it has been proven that water molecules’ properties strongly depend on the CNT diameter,22 we evaluate the diameter effect of these modified CNTs on the structure of water molecules. We are also interested in the effect of diverse inner wall modifications on the structure of water molecules. Moreover, modified groups on the tube mouth can affect the water molecules near the tube mouth. We keep the distance between the -COOH groups and the tube mouths to see if it is possible that the inner wall modification can also affect the molecules near the tube mouths. The following text is divided into three sections. Section 2 describes the simulation model and the details of method. The simulation results are discussed in Section 3. Based on the discussions, the main conclusions are drawn in Section 4. 2. Simulation Model and Methodology The CNT with a length of 3.6 nm was solvated in the center of a periodic bulk water box with density of 1.0 g · cm-3. The CNT was placed along the z axis (tube axis). No water molecule

label

(n, n)

M3_8 P_9 M1_9 M2_9 M3_9 M3_10 M3_11 M3_12

(8, 8) (9, 9) (9, 9) (9, 9) (9, 9) (10, 10) (11, 11) (12, 12)

Da/nm db/nm

NWc

N-COOHd

box sizee/nm3

1.085 1.221 1.221 1.221 1.221 1.356 1.492 1.628

3734 3703 3703 3703 3703 3895 4064 4324

8×3 0 9×1 9×2 9×3 10 × 3 11 × 3 12 × 3

4.4 × 4.4 × 6.8 4.4 × 4.4 × 6.8 4.4 × 4.4 × 6.8 4.4 × 4.4 × 6.8 4.4 × 4.4 × 6.8 4.5 × 4.5 × 6.8 4.7 × 4.7 × 6.8 4.8 × 4.8 × 6.8

0.433 1.221 0.569 0.569 0.569 0.704 0.839 0.976

a D: Diameters of CNTs. D)3n · acc/π, where acc is the nearest-neighbor carbon atom distance of 0.142 nm, ref..38 b d: Actual inner diameters of central regions for modified CNTs, which exclude the length of -COOH groups. c NW: Number of water molecules in the simulated case. d N-COOH: Total number of -COOH groups modified on CNT’s inner wall; Number of -COOH groups for each modified loops ×Number of modified loops. e Box size: Initial simulated water box size.

was inside the CNT initially to ensure that every water molecule entered the CNT spontaneously. The CNT was more than 1.5 nm away from the edges of the box to make sure that there was no interaction between the tube and its mirror. The -COOH groups were anchored onto the inner wall of the CNTs’ central region. One of the simulation frameworks is shown in Figure 1. All the simulated cases are listed in Table 1. Each CNT is represented by a specific label. The letters (“P” and “M”) in the label stand for the pristine and modified CNT, respectively. The numbers in the label stand for the CNT type and diverse modification. For instance, the label “M3_9” represents the

884 J. Phys. Chem. C, Vol. 113, No. 3, 2009 single-walled armchair (9, 9) CNT whose central region has three modified loops, and each loop is composed of nine -COOH groups with the same position in the tube axis. Totally, there are 27 -COOH groups homogeneous on the central interior of M3_9. We chose five armchair-type CNTs with three modified loops (M3_8, M3_9, M3_10, M3_11, and M3_12) to investigate the pore diameter effect on structural properties of confined water molecules. The water molecules confined in the same type CNTs (P_9, M1_9, M2_9, and M3_9) with a different number of modified loops on inner wall were also compared. The OPLS-AA force field39 was adopted to the CNTs and -COOH groups. We used the extended simple point charge model (SPC/E)40 for water molecules. The potential energy of intermolecular interactions was described as a combination of Lennard-Jones 12-6 potential and a Coulombic potential. Details can be found in our previous publications.36,37 The molecular simulations were performed in an isobaricisothermal ensemble (NPT) with NAMD (Version 2.6).41 The system was maintained at constant temperature (300 K) with a Langevin thermal bath42 and pressure (1 atm) using the Berendsen method.43 The particle mesh Ewald method44 was used for full electrostatic interactions, and the short-range Van der Waals interactions were calculated within a cutoff distance of 1.0 nm. During the simulation, the carbon atoms in the CNTs were kept rigid and fixed in the box center, whereas the -COOH groups were allowed to relax, and water molecules could move freely. For each of the systems described above, after the energy minimization and 1.9 ns MD run with integral step of 1.0 fs for equilibrium, another 1.0 ns run was carried out with coordinates recorded every 50 fs. The last 1.0 ns trajectory was used for analysis. During the production stage, the potential energy fluctuation of the ensemble was around 0.5%. 3. Results and Discussion 3.1. Preferential Orientation of Confined Water Molecules. Confined water molecules might have preferred orientations under some specific conditions. In this study, the orientation analysis of confined water molecules was explored. The emphasis of this research was on the water molecules confined within the tube and the ones near the tube mouths. At first, we studied the orientation angle distributions of water molecules in the modified CNTs with different diameters to determine the regions within the tube where water molecules have preferential orientation. Then, to find out whether there is any association between the inner wall modification and the preferential orientation of water molecules, we calculated ensemble average axial dipole moments 〈µz〉 of water molecules inside CNTs with the same diameter and diverse inner wall modifications. Finally, we compared the 〈µz〉 of water molecules in different regions as a function of time for modified CNTs with that for pristine CNTs to evaluate the stability of the preferential orientation of the water molecules. 3.1.1. Orientation Angle Distributions of Water Molecules in CNTs. We quantified the orientation of confined water molecules by defining two characteristic angles denoted by θ and φ, respectively. As illustrated in Figure 2f, θ is between a water molecule dipole moment and the positive direction of the z-axis (tube axis), and φ is between the OH bond of a water molecule and the positive direction of the z-axis. We divided each tube into three regions along the z-axis to better distinguish the characteristic the orientation of water molecules in different regions of the CNT. The central region (1.663 nm in length) was modified with -COOH groups, whereas the regions in the CNT denoted by S1 and S2 (0.903 and 1.034 nm in length,

Zhu et al. respectively) were not modified by -COOH groups. The orientation angle (θ, φ) distributions of water molecules in five CNTs (M3_8, M3_9, M3_10, M3_11, and M3_12) with the same number of modified loops on the inner wall were investigated to explore the diameter effect on the orientation, as shown in Figure 2a-e. For M3_9, as shown in Figure 2b, we can observe that, in the S1 region, the θ distribution peak is around 40°, and the φ’s of the water molecules are mostly obtuse angles. In contrast, in the S2 region, the θ distribution peak is around 140°, and most φ’s of the water molecules are acute angles. This observation indicates that the water molecules in the S1 and S2 regions have opposite orientations along the tube axial direction, as illustrated in the sketch of Figure 2f. That is to say, in both unmodified regions (S1, S2 regions), the hydrogen atoms of the water molecules prefer pointing to the central region, which is modified with -COOH groups. Wang et al.25 reported that the water molecules confined in pristine (9, 9) CNT oriented randomly to a certain degree and did not have preferential orientation, since the confinement effect on the water molecules weakens in a CNT with a diameter larger than 1.0 nm. In this study, the diameters of the S1 and S2 regions for M3_9 were identical to the diameter of pristine (9, 9) CNT. It is the -COOH groups modified on the central region that create the preferential orientations of the water molecules in the S1 and S2 regions. These phenomena reveal that proper inner wall modification for CNTs can control the orientation of the water molecules. For the other four CNTs (M3_8, M3_10, M3_11, and M3_12), as shown in Figure 2a,c-e, the positions of the θ distribution peaks are similar to the ones in the M3_9 CNT (see Figure 2b), which indicates that water molecules in the S1 and S2 regions of the five CNTs have similar preferential orientations. The θ distribution peak (see Figure 2a-e) becomes sharper as the CNT diameter decreases, probably due to the enhancement of the interactions between the modified CNT and water molecules. From Figure 2a-e, we can find that, for the five CNTs, the water molecules in the unmodified regions (S1 and S2 regions) have similar orientation distributions but are different in the modified regions (central regions). The diameter of the central regions decreases, since the inner space is taken by the modified -COOH groups. For the five CNTs, the actual inner diameters of the central regions are 0.433, 0.569, 0.704, 0.839, and 0.976 nm, respectively. Previous studies25 demonstrated that water molecules could form single-file water chains with the same orientation in unaltered narrow CNTs with diameterd of less than 1.0 nm. Nevertheless, for the water molecules in the central regions of the five modified CNTs (see Figure 2a-d), the peaks in θ have a relatively broad distribution, suggesting that the orientations of water molecules are relatively disordered and cannot form single-file water chains. The reason for these observations might be that the hydrogen bond network of the water molecules was broken by -COOH groups. Zheng et al.35 also found that the interactions between -COOH groups and water molecules could hamper the hydrogen bond formation of the water molecules among themselves. 3.1.2. Axial Distribution of Dipole Moments for Water Molecules. A water molecule described by the SPC/E model has a dipole moment, µ, of 2.35 D.40 In the bulk phase, the random orientation of water molecules leads the vector sum of their dipole moments to be close to zero in any direction. However, once confined water molecules exhibit preferential orientations, this average value might not be close to zero. In this study, 〈µz〉 was defined as the ensemble average axial dipole

MD Study of H2O Molecules in CNTs

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Figure 2. (a-e) The orientation angle (θ, φ) distributions of water molecules in CNTs (M3_8, M3_9, M3_10, M3_11, and M3_12). θ is the dipole angle illustrated by a solid square; φ is the OH bond angle illustrated by an open circle. (f) Schematic image of orientation angles and three regions of a CNT.

moments of water molecules. The analysis of 〈µz〉 can better reflect orientations of confined water molecules and the function of the modified tube. Due to the effect of modified -COOH groups on the central region, water molecules in both S1 and S2 regions have preferential orientations, which can induce a unique distribution of 〈µz〉 along the tube axis. To further reflect the effect of -COOH groups on the 〈µz〉, we analyzed the 〈µz〉 of water molecules along the tube axial distribution in four CNTs (M3_9, M2_9, M1_9, and P_9). Each 〈µz〉 was the average over coaxial cylindrical shells (dz ) 0.1 nm) in the z axis (tube axis) direction. The diameters of coaxial cylindrical shells were set to be the diameters of the CNTs. The four CNTs had the same

diameter, but the loops of -COOH groups modified on the inner wall were different (see simulated detail in Table 1 and sketch in Figure 3). Figure 3a-d shows 〈µz〉 of water molecules along the tube axial distribution. In P_9 (see Figure 3d), the values of 〈µz〉 along tube axis are close to zero, indicating that the confinement impact of this pristine CNT on the dipole moments of water molecules is weak. In the M3_9 CNT (see Figure 3a), the values of 〈µz〉 in the S1 region are more than zero, whereas the ones in S2 region are less than zero. It reveals that inner wall modifications such as M3_9 can induce bipolar ordering of water molecules along the tube axis, which is consistent with the result

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Figure 3. The axial distribution of 〈µz〉 for water molecules. Dotted lines indicate the location where decorated with -COOH groups; solid lines indicate the location of tube mouths.

Figure 4. The maximum and minimum values of 〈µz〉 for water molecules along the tube axis. Histograms in the left of the dash line indicate the maximum and minimum values inside the tube. The ones in the right of the dash line indicate the maximum and minimum values outside the tube.

obtained from dipole angle analysis in Figure 2b. Water molecules in the three modified CNTs (M3_9, M2_9, and M1_9) have similar 〈µz〉 axial distributions, as represented in Figure 3a-c. The effect of -COOH groups on the values of 〈µz〉 can persist to the vicinity of tube mouths. Water molecules outside a CNT approximately 0.45 nm away from the CNT mouths can be affected; then the values of 〈µz〉 become zero as the distance away from the tube mouth increases. These results suggest that modification on the inner wall also have an impact on the fluid structure near the pore mouth, even if there is a certain distance away from modified groups. On the basis of Figure 3, the maximum and minimum values of 〈µz〉 along tube axis in M3_9, M2_9, and M1_9 are shown in Figure 4 so as to clearly evaluate the effect of the -COOH groups on the values of 〈µz〉. As shown in Figure 3a-c, for the water molecules inside the modified CNTs (M3_9, M2_9, and M1_9), the maximum and minimum values of 〈µz〉 along the tube axis all appear beside the regions modified with -COOH

groups. By comparing data in Figure 4, we find that the absolute values |〈µz〉| of the maximum and minimum values decrease as the number of -COOH groups reduce. For the water molecules outside the modified CNTs (M3_9, M2_9, and M1_9), the maximum and minimum values of 〈µz〉 along the tube axis appear near the tube mouths, as plotted in Figure 3a-c. Figure 4 shows that as the number of -COOH loops modified on the CNT reduces, |〈µz〉| of the maximum values outside tube will decreases. It is noticeable that the |〈µz〉| of the minimum value for water molecules outside M3_9 is similar to that outside M2_9. This special finding might be due to the identical distance between the location of the modified loop and the tube mouth for both CNTs. From the comparison analysis above, we can deduce that the effect of the -COOH groups on the values of |〈µz〉| will be enhanced as the number of -COOH groups increases and as the distance away from the -COOH groups decreases. In other words, both factors will affect preferential orientations of the water molecules. 3.1.3. 〈µz〉 with Time EWolution of Confined Water Molecules in Different Regions. We adopted the 〈µz〉 of water molecules in different regions of the CNTs with time evolution to explore the orientations of the water molecules, which can reflect their orientation stability. Two CNTs (P_9 and M3_9) were chosen for comparison. Each CNT was divided into five regions along the z-axis (tube axis) direction; namely, five coaxial cylinders, as shown in Figure 5b. The five cylinder diameters were set as the same as the diameter of P_9. The 〈µz〉 of the water molecules was the average over the region and was recorded every 0.05 ns, as shown in Figure 5a. It is clear to find that, for P_9, with the time evolution, the values of 〈µz〉 for water molecules in the five regions all fluctuate around zero with large amplitude (from -1 to 1 D), implying that the orientation of the water molecules can be changed freely. In contrast, for M3_9, the values of 〈µz〉 for the water molecules in the S1 and S2 regions are close to 1.64 and -1.65 D, respectively, and the values of 〈µz〉 for the water molecules in the P1 and P2 regions are around 0.72 and -0.73 D, respectively. With the time evolution, the fluctuation amplitude of the 〈µz〉 values for the water molecules in M3_9’s five regions is

MD Study of H2O Molecules in CNTs

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Figure 5. (a) The 〈µz〉 of water molecules in different regions with time evolution. (black solid square, P1 region; red solid circle, S1 region; green up-pointing triangle, central region; blue down-pointing triangle, S2 region; light blue diamond, P2 region). (b) Schematic image of the divided regions.

smaller than that in a pristine CNT. From the comparison, it is proven that -COOH groups in the central region can lead water molecules which are not only in unmodified regions (S1 and S2 regions) but also in the vicinity of tube mouths (P1 and P2 regions) to have steady dipole moments. In other words, water molecules in the S1, S2, P1, and P2 regions of M3_9 can keep steady preferential orientations due to the effect of the -COOH groups. Similar results45,46 have been observed in the vicinity of a bimolecular surface, where the water molecules also exhibit steady preferential orientations compared with bulk water. From the analysis about the orientation of the confined water molecules above, we can summarize briefly as follows: When the inner wall of the CNTs’ central region was modified with -COOH groups, such as M3_9, water molecules in the S1 and S2 regions will have opposite and steady preferential orientations that the hydrogen atoms in a water molecule point to the -COOH groups. The preferential orientations of water molecules can induce bipolar ordering of water molecules along the tube axis. The effect can persist to the water molecules in the vicinity of tube mouths. We could attribute these intriguing phenomena to the electrostatic interaction between the water molecules and the -COOH groups and could also attribute these phenomena to the block effect of -COOH groups on the inner wall. Zhu et al.47 found similar preferential orientation in (6, 6) CNTs with charges distributed inhomogeneously, of which the middle part was modified with negative charges, the two mouths with positive charges. Raghunathan et al.48 reported that in the negative pore, water molecules penetrated through the pore with hydrogen atoms entering first. Although our modified CNTs are electrically neutral, the polarity of -COOH groups can determine the water molecular dipole orientations. On the other hand, the central diameters of CNTs constricted by -COOH groups will slow down the speed of water flowing through the tube, which also was proved by Striolo.49 Since many water molecules

accumulated in the S1 and S2 regions, their uniform dipole moments can induce the dipole moments of water molecules near the tube mouths. As a result, the preferential orientation of water molecules can extend into the tube mouths, although a certain distance away from -COOH groups. 3.2. Spatial Density Distribution of Confined Water Molecules. The spatial density distribution of confined water molecules usually adopts the density profile of oxygen atom to characterize, because the oxygen atom is quite near the mass center of a water molecule.37 In this study, first, we used the radial density profile of oxygen atoms (RDPO)s to study the distributions of water molecules in five types of CNTs (M3_8, M3_9, M3_10, M3_11, and M3_12), as shown in Figure 6. The reason is that the confinement analysis in the radial direction can clearly reflect the diameter effect of the CNTs’ diverse regions (unmodified regions, S1; S2 regions and modified regions, central region; see Figure 2f). Second, the axial density profile of oxygen atoms (ADPO)s of water molecules are plotted in Figure 7 to compare the axial distributions of the water molecules in four CNTs (M3_9, M2_9, M1_9, and P_9) because the impacts of different inner wall modifications on water molecule distributions are mainly presented in the axial direction. The definitions of RDPO and ADPO are identical with those of Huang et al.36 3.2.1. Radial Density Profile of Oxygen Atoms. The RDPOs of water molecules inside the five CNTs with different diameters have two features in common, as shown in Figure 6. One is that the RDPOs of water molecules in the S1 and S2 regions are quite similar. It can be attributed to the fact that both regions of a CNT are symmetrically arranged beside the region anchored by -COOH groups. As a result, the effects of -COOH the groups on the radial distribution of water molecules in the S1 and S2 regions are similar. Another feature is that the first maxima of the profiles in S1 and S2 regions are about 0.3 nm

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Figure 6. RDPOs of water molecules of various CNTs (M3_8, M3_9, M3_10, M3_11, and M3_12). Black, central region; red, S1 region; green, S2 region. Solid arrows indicate the inner diameters of the central regions which exclude the length of -COOH groups. Dotted arrows indicate the locations of the tube wall.

Figure 7. ADPOs of water molecules in various CNTs (M3_9, M2_9, M1_9, and P_9); Dotted lines indicate the locations where decorated with -COOH groups; solid lines indicate the locations of tube mouths.

away from the tube wall, and are larger than the ones in the central region away from the -COOH groups. The difference can be explained in that the -COOH groups alter the hydrophobic central region into a hydrophilic one so that the water molecules are closer to the -COOH groups. Moreover, from Figure 6, we observe the second maxima for the RDPOs of water molecules in S1 and S2 regions of M3_9, M3_10, M3_11, and M3_12. Correspondingly, for the water molecules in the central region of the large sized CNT (M3_12), the second peak in the RDPO appears 0.35 nm away from the first maximum. It reveals that the layered distribution of water molecules could be formed in both hydrophobic

unmodified regions (S1 and S2 regions) and hydrophilic modified regions (central region) as the CNTs’ diameter increases. For each CNT, as shown in Figure 6, the RDPOs of water molecules outside CNTs are identical, indicating that such an inner wall modification could not affect the water molecules outside CNTs along the radial distribution. 3.2.2. Axial Density Profile of Oxygen Atoms. As shown in Figure 7, the ADPOs of water molecules outside the four CNTs are similar. The peak positions in ADPOs of modified CNTs (M3_9, M2_9, and M1_9) are identical to the ones of a pristine CNT (P_9). This indicates that the -COOH groups have little effect on the axial density distribution of water molecules outside the CNTs. The 〈µz〉 analysis in Figure 3 has shown that the water molecules near the tube mouth regions (0.45 nm away from the tube mouths) have a preferential orientation, since the central region inner wall is modified by -COOH groups. In other words, the -COOH groups away from the tube mouths have a great effect on the orientation of water molecules but little impact on their spatial density distribution. These findings offer insights into the structure of water molecules near the tube mouth, which is affected by tube inner modification. Moreover, from Figure 7, we can find that once the pristine CNTs are modified with -COOH groups on the inner wall, the ADPOs of water molecules confined in the modified CNTs (M3_9, M2_9, and M1_9) exhibit a heterogeneous spatial density distribution along the tube axis that is different from the ADPOs of water molecules in a pristine CNT (P_9). There are some features of water molecules inside modified CNTs (M3_9, M2_9, and M1_9). First, for every two loops of -COOH groups, there is a small peak in the ADPOs of water molecules, as seen in M3_9 and M2_9, indicating that the water molecules are mostly distributed between the loops in the region modified by -COOH groups. Second, for water molecules in

MD Study of H2O Molecules in CNTs M3_9, M2_9, and M1_9, there is always a high peak of ADPOs between the loops of -COOH groups and tube mouths, which is about 0.35nm away from the z-axis position where decorated with -COOH groups. This observation might be caused by the hydrophilicity of the -COOH groups. On the other hands, many water molecules accumulate there, since the central inner diameter of a CNT is constricted by anchoring -COOH groups. Third, for water molecules in M3_9 and M1_9, the ADPOs are symmetrical along the tube axis, but for water molecules in M2_9, the ADPOs are asymmetrical. The reason for the difference is that the position of the modified region is not in the center of the tube, suggesting that the distances between the -COOH groups and tube mouths can affect the axial density distribution of water molecules, which is clearly reflected in M2_9. 4. Conclusions In this study, we have anchored -COOH groups onto the inner wall of CNTs’ central regions to investigate the effect of modified groups on the structural properties of water molecules. We have undertaken the orientation and spatial density distribution of confined water molecules. Not only water molecules in the CNTs but also the ones near the tube mouths have been studied. We observe that water molecules confined in the S1 and S2 regions have opposite and steady preferential orientations with their dipoles pointing toward -COOH groups. The water molecule dipole moments near the tube mouths are also induced so as to exhibit preferential orientations, although there is a certain distance away from modified groups. The inducement effect get stronger as the number of -COOH groups increases and the diameter of the CNT decreases. In addition, the -COOH groups on the inner wall of the central region have little effect on the axial density distribution of water molecules near the tube mouth but a strong impact on that inside the CNTs. The distance between the -COOH groups and the tube mouths can contribute to the axial density distribution of water molecules. We believe that these findings give us more insight into the inner wall modification to the water molecules confined in the pore and the ones near the pore mouths. These results might provide some information for future design of bionic nanodevices for controlling the orientation of water molecules. Acknowledgment. This study was supported by the Joint Research Fund for Young Scholars in Hong Kong and Abroad (no. 20428606), the National Natural Science Foundation of China (Grants nos. 20246002, 20236010, 20376032, 20676062, 20706029, and 20736002), the National Basic Research Program of China (nos. 2003CB615700 and 2009CB623400), the National High Technology Research and Development Program of China (nos. 2003AA333010 and 2006AA03Z455), the Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT 0732), and the Key Science Foundation of Jiangsu Province, China (BK 2004215). The authors also acknowledge computer time provided by the College of Computer Engineering and Science, Shanghai University, which is supported by the Shanghai Leading Academic Discipline Project (J50103). References and Notes (1) Yu, B.; Blaber, M.; Gronenborn, A. M.; Clore, G. M.; Caspar, D. L. D. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 103. (2) Pan, Y. S.; Birkedal, H.; Pattison, P.; Brown, D.; Chapuis, G. J. Phys. Chem. B 2004, 108, 6458. (3) Borgnia, M.; Nielsen, S.; Engel, A.; Agre, P. Annu. ReV. Biochem. 1999, 68, 425. (4) Gallo, P.; Rovere, M.; Spohr, E. J. Chem. Phys. 2000, 113, 11324.

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