Investigation of Transport Properties of Water ... - ACS Publications

Article pubs.acs.org/JPCB

Investigation of Transport Properties of Water−Methanol Solution through a CNT with Oscillating Electric Field Honglei Wang,† Jin Shi,‡ Guokui Liu,§ Yongqin Zhang,† Jingjing Zhang,† and Shenmin Li*,† †

College of Environmental and Chemical Engineering, Dalian University, Dalian 116622, China Department of Environmental Science & Engineering, Fudan University, Shanghai 200433, China § Key laboratory of Colloid and Interface Chemistry, Shandong University, Jinan 250100, China ‡

S Supporting Information *

ABSTRACT: Molecular dynamics simulations were used to investigate the transport properties of water−methanol solution getting through a carbon nanotube (CNT) with an oscillating electric field. Eight alternating electric fields with different oscillation periods were used in this work. Under the oscillating electric field, water molecules have the advantage of occupying a CNT over methanol molecules. Meanwhile, the space occupancy of water−methanol solution in the CNT increases as the oscillating period increases. More importantly, we found that the oscillating period of electric field affects the van der Waals interaction of the solution inside the CNT and the shell of the CNT, which results in the change in the number of hydrogen bonds in the water−methanol solution confined in the CNT. And the change in the hydrogen-bond network leads to the change in transport properties of water−methanol solution.

1. INTRODUCTION Methanol has been widely used in the chemical industry, in fuel cells, and in other fields. Due to hydrophilicity, methanol is miscible with water in any proportion, and it is very hard to separate it from its aqueous solution. Moreover, the traditional distillation technique used to separate methanol from mixed solution1 has shown such problems as high cost, low efficiency, and high energy consumption.2,3 Therefore, it is crucial to explore a new scientific method for high efficiency and low energy consumption to purify methanol from its aqueous solution. In recent decades, carbon nanotubes (CNTs) emerged as new materials4 and were suggested to be used as a separating machine for a gas mixture,5 for desalination,6−9 and as a water− organics system.10 In 2001, Hummer et al.11 used molecular dynamics (MD) simulations to investigate the filling of a (6, 6) CNT with water molecules. They found that water could conduct through the hydrophobic inner space of a CNT. Since then, some researchers started to study the reason for the fast transportation properties of water molecules in single-walled CNTs (SWCNTs). These researchers found that the length,12 diameter,13 chirality,14 shape,15 surfactant structures,16 and external charge17,18 in CNTs affect water transport properties. From the microcosmic perspective, Moskowitz et al.19 systematically investigated the hydrophilic effect on water flow by changing the hydrophilicity of nanotube from fully hydrophobic to fully hydrophilic. They found that the flux of water has an approximately linear increase, whereas f (the © XXXX American Chemical Society

fraction of hydrophilic atoms) is smaller than 0.4, and it remains almost unchanged for higher values. In addition to water molecules, the CNT could also affect the properties of methanol molecules inside it, as Liu reported20 that methanol molecules show an immiscible phenomenon near the inner surface of CNTs due to the strong van der Waals (vdW) interactions between methyl groups and CNTs. Zhao et al.21 also reported the selectivity for methanol. They found that almost 100% selectivity can be achieved for the armchair (6, 6) CNT. Thus, a CNT has a potential application in the separation of solute from its mixture. In addition, dispersion interaction22 is another key factor to explain the preferential attraction between CNTs and methanol molecules. However, it is difficult to quickly separate methanol from its aqueous solution only by self-diffusion at room temperature. To enhance the flux of liquid, researchers23−25 designed some effective models. For example, a CNT was submerged into pure water under the effect of an external force.26−28 But there has been no clear explanation or further research done so far. It is found that adding an electric field (including the static electric field,29,30 gradient electric (GE) field31 and alternating current (AC) electric field32,33) to the system could promote the flow rate of water inside the CNT. Rinne et al.33 reported that water can be pumped through a CNT with an AC electric Received: June 28, 2016 Revised: January 4, 2017 Published: January 9, 2017 A

DOI: 10.1021/acs.jpcb.6b06509 J. Phys. Chem. B XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry B

Figure 1. Snapshots of the MD system: (a) the distance between the molecules and the center of the CNT; (b) the angle between the dipole moment and solvent molecule; and (c) our system for MD simulation.

field. They also elaborated a microscopic theory based on a polarization-dragging mechanism to calculate the dielectric relaxation time of water. Unidirectional flow of water molecules across the nanotube is important for achieving water purification or desalination. Su and Guo13 studied the effect of nanotube length on the transport of single-file water molecules in a static electric field. They found that the flow of water is strongly dependent on the length of nanotube for short CNTs; however, it remains almost the same for long CNTs. Meanwhile, Huang et al.31 designed and investigated the rapid motion of water through SWCNTs with a GE or uniform electric (UE) field. They found that the maximum speed of water in the GE field is larger than that in the UE field. They also found that inverse transportation of water occurs in the UE field but it does not exist in the presence of the GE field. Winarto et al.34 investigated the transport of the mixture through the (8, 8) CNTs in an electric field. The result indicated that CNTs attract water molecules more easily than methanol molecules under the electric field and the structures of water molecules are not disrupted by the methanol molecules from reservoirs, which results in separation for water. According to the results of the previous research, it could be concluded that an electric field can lead to fast transport of water and separation of methanol in aqueous solutions. However, it is important to investigate new mechanisms of transport of water−methanol solutions through CNTs and separation of the mixture by CNTs. In this research, our aim is to find a method for the separation and purification of the mixture. In this study, the transportation and separation of the water−methanol solution confined in CNTs were investigated by MD simulation with an alternating electric field. More importantly, we aimed to give more detailed and reasonable explanations on the changes in the interaction between CNTs and water−methanol molecules from a new perspective.

2. METHODS The simulation system consists of two water−methanol reservoirs connected by a CNT (12, 12) with a diameter of 1.63 nm. The water−methanol reservoirs are bounded by a graphene wall on one side as shown in Figure 1. For MD simulations, the CNT and the graphene wall are treated as rigid. The force-field parameters of the CNT are given in Table 1. Table 1. Nonbonding Potential Parameters Used in this Work type water methanol

CNT and graphene

atom symbol

σ (nm)

ξ (kJ mol−1)

q/e

O H CH3 O H C

0.3151 0.0000 0.3775 0.3070 0.0000 0.3400

0.6362 0.0000 0.8661 0.7113 0.0000 0.3598

−0.834 0.417 0.265 −0.700 0.435 0.000

The TIP3P model35 is applied for water, and the united atom potentials36,37 are adopted for methanol. Moreover, the ratio of the number density of water to that of methanol is 1:1 in this system. For simulations, we chose an oscillating electric field (as shown in Figure S1). The oscillation period (T) is the key factor for this type of electric field. Thus, eight different oscillation periods (Group 1) are chosen from T = 0.5−100 ps (0.5, 1.0, 5.0, 20.0, 40.0, 60.0, 80.0, 100.0 ps), corresponding to frequencies of 200 THz to 10 GHz. In addition, a simulation without the electric field is carried out as reference. To improve the precision of the fitting for the dielectric relaxation time, we added five (0.05, 0.1, 7.0, 12.0, 15.0 ps) additional electric field periods (Group 2) to evaluate the dielectric relaxation time by the dragging theory.34 Table 2 shows the relationship between the job serial numbers and their oscillation periods. To generate an alternating electric field, the TCL script plugin of NAMD is used. The oscillating electric field is added B


Article


interactions between the CNT and water or methanol molecules upon two position coordinates (R, θ) for the molecules inside the tube, where R is the distance between the mass center of the molecule and the CNT axis and θ is the angle between the molecular dipole vector and the axis. The schematic of these two variables R and θ is shown in Figure 1a,b, respectively. As shown in Figure 2, the color changes from red to black, which means that the affinity changes from repulsion to attraction. It was found that the black zone of methanol molecules is wider than that of water molecules, which means that a CNT is more likely to attract methanol molecules than water molecules. It is worth noting that the attraction decreases with the increase in distance, R, between the solvent molecules and the CNT surface, until the distance is equal to 0.4 nm, and was hardly affected by the angle distribution in this distance range. In this article, we termed the surface that is near to the inner surface of a CNT as NIS-surface, where R is larger than 0.4 nm. In this distance range, nonbonded interaction becomes complexity, and the space occupancy of water−methanol solution is much larger than that in the other region of CNT. It is also found that the angle has a significant effect on the interaction, especially for methanol molecules. The interaction is quite sensitive to the change in the angle when methanol molecules were near the NIS-surface of the nanotube. When R is around 5.5 nm and θ is between 45 and 140°, there is an obvious attractive phenomenon between the water molecules and CNT. All of the information is represented in Figure 2 (Figure S2), and these properties are used to illustrate the changes in interactions between the water or methanol molecules with the CNT. 3.2. Spatial Distribution of Solvent Inside the CNT. The distribution of the angle between the dipole vector of water or methanol molecule and the Z axis of the CNT was investigated (Figure 3). When the electric field is applied at the same period, similar angle distributions for water and methanol inside the CNT could be seen. For Job 0, without the electric field, the probability for water and methanol distributions shows similar trends. No obvious peaks appear in this phase same as that in the bulk phase, whereas other jobs (except Jobs 1 and 2) reveal a bimodal distribution. The dielectric relaxation

Table 2. Relationship of the Job Serial Numbers and Their Oscillation Periods serial number

oscillation periods (ps)

serial number

oscillation periods (ps)

0 1 2 3 4

without electric field 0.5 1.0 5.0 20.0

5 6 7 8

40.0 60.0 80.0 100.0

to the MD system in the area between two CNT walls along the Z axis. The electric field intensity is defined as E(t) = E0 sin(2πωt), and its snapshot is shown in Figure S1, where ω = 2π/T denotes the oscillation frequency, t is the moment during the MD run, and E0 is the maximum intensity of the oscillating electric field. In this article, the effect of E0 is not considered; thus E0 is set to a constant value (2.0 V/nm) for all MD simulations. All MD simulations are performed by the NAMD2.9 package38 and carried out in the NVT ensemble with periodic boundary conditions in all three dimensions (3D). The temperature is maintained at 300 K by the Nose−Hoover thermostat with 0.1 ps time constant. The SHAKE algorithm39 is used to freeze the O−H bond of water and methanol. The vdW interactions are calculated with 1.2 nm cutoff. Meanwhile, the particle mesh Ewald40 method is implemented to compute the long-range electrostatic interactions. The time step is 1 fs and the trajectory data is collected every 0.1 ps. These simulations of Group 1 are run for 102 ns and of Group 2 for 42 ns, where the first 2 ns is used for pre-equilibrium and the rest is used for data analysis.

3. RESULTS AND DISCUSSION 3.1. vdW Interaction between the CNT and Aqueous Solution Inside the CNT. It is evident that the vdW energy is the only factor to measure the interactions between the CNT and aqueous solution as the CNT is uncharged. Feng and Ma also reported14 that the vdW interactions can dominate the pumping behaviors. However, the results in this study show that when an electric field is applied, the change in affinity takes place between the tube and water and between the tube and methanol. Figure 2 shows the dependence of the vdW

Figure 2. Snapshots of distribution of vdW interaction between the CNT and water molecules (A) and methanol molecules (B). C


Article


Figure 3. Probability distributions of the angle between the dipole moment of water (A) or methanol (B) molecule and the Z axis.

Figure 4. Dielectric relaxation time fitting for water (A) and methanol (B).

is the main factor that causes the anomalous occurrence of Jobs 1 and 2. The dragging theory was used to calculate the dielectric relaxation time, and its parameters are fitted to the simulation results (refer to Figure 4). The relaxation times of water and methanol are 6.3983 and 5.1476 ps, respectively. For Jobs 1 and 2, the period of oscillating electric field is less than the relaxation time and the molecules inside the CNT do not have enough time to adjust their positions to the quickly changing environment. Thus, their distributions are identical to those in the bulk phase. From Jobs 3−8, with the increase in the period, the distribution is more concentrated. The above results imply that the degrees of alignment of molecules were affected by the value of oscillating period. Interactions and steric hindrance exist between contiguous molecules, which affect the axial movement of the molecules inside the tube. Thus, it is important to profile the transport property by the axial distribution for the molecules confined in the CNT. This study explored the static but statistical properties of the mixture passing through the CNT in the presence of oscillating electric fields. Figure 5 unveils the distributions of the ratio of the number of water/methanol molecules (W/M) to determine the ability to occupy the CNT. As the figure shows, the ratios listed in the four jobs (Jobs 1−3

Figure 5. Ratio distribution of water/methanol molecules confined in the CNT.

and 0) are less than 1. In other words, the number of water molecules confined in the CNT is less than the number of methanol molecules for these jobs. The ratios are greater than 1 D


Article


Figure 6. Number distributions of water (A) and methanol (B) molecules confined in the CNT.

If the oscillating period is larger than TJob3, the number of water and methanol molecules inside the CNT increase as T increases and water molecules have the advantage to occupy the center of the CNT instead of methanol. The stronger ability of methanol to occupy the NIS-surface as mentioned above is contributed by its hydrophobic methyl group. It is clear that the abundance of water molecules near the NIS-surface is increased when the electric field is introduced. In addition, there exists a significant increasing trend for the abundance of water molecules with the increasing period of electric field except for Job 3. In contrast, the abundance of methanol molecules close to the NIS-surface has a decreasing tendency when the electric field is introduced. The change of abundance for these two different molecules indicates a potential separation capability. To present a more clear result of the abundance of water and methanol molecules inside the CNT, the difference of number density distributions (DNDDs) is calculated between water and methanol molecules in the XY plane. For each point in the XY plane, its DNDD was defined as DNDD(x, y) = Pwater(x, y) − Pmethanol(x, y), where “P” is the number density for water or methanol. The positive value of DNDD means that the abundance of water inside the tube is larger than that of methanol; on the contrary, a negative value indicates that the abundance of methanol is larger than that of water. As Figure 7c shows, the nine distributions can be divided into three categories according to their values. 1: Jobs 1−2, 2: Job 3, 3: Jobs 4−8. Job 3 is like a “transition state” of the nine distributions. The dielectric relaxation time has a direct relation with the average number of hydrogen bonds,41 whose network also affects the preferential ability of water or methanol to occupy the NIS-surface.34 As a result, it is an advantage for the methanol molecules to occupy the NIS-surface when the oscillating period (T) is less than TJob3. On the other hand, when T is greater, water has the advantage to occupy the NISsurface. The distributions of water and methanol molecules along the Z axis of CNT are investigated and shown in Figure 8a. As the figure shows, for Job 3, the number of water (or methanol) molecules inside the CNT is less when compared to that in the other jobs. Job 4 divides all of the distributions of water molecules into two parts. When the oscillating period is below the value for Job 4 (20 ps), their distributions are similar to those for Job 0 (without the electric field). However, when the oscillating period is higher than 20 ps, the distribution values

in Jobs 5−8. More interestingly, the result of Job 4 is almost equal to 1, which means that the abundance of water and methanol in the CNT is homogeneous under this case. These results indicate that the abilities of water and methanol molecules to occupy the CNT are different. The difference between the ratio value and the standard value (=1.0) represents the ability of Water (Methanol) molecules occupy the CNT. Therefore, higher frequency (small oscillating period) is more applicable to separate water and methanol molecules. To figure out more detailed information on water (or methanol) molecules inside the CNT under oscillating electric fields, their number distributions were also investigated in this study. There is a particular individual job (Job 3) in Figure 6, the oscillating period of which is near the dielectric relaxation time of methanol molecule. For Job 3, the number of methanol molecules is dramatically decreased when compared to that for Job 0. The dielectric relaxation time has a significant effect on the number distribution of methanol molecules. For Job 3, the value of W/M is larger than that for Jobs 4−8 but the number of methanol molecules inside the CNT is less than that for Jobs 4−8. For other jobs, the number distribution of methanol molecules inside the CNT is in accordance with the value of W/M. As it was stated previously, the interaction between water (methanol) molecules (inside the CNT) and the CNT is important, which affects the radial distribution of water− methanol molecules inside the tube and reflects the affinity of a CNT to the molecules. The 2D number density maps of water and methanol were calculated to determine the preferential ability to occupy the NIS-surface. Figure 7 shows that the abundance of methanol molecules is higher than that of water molecules near the tube, which is obvious. Meanwhile, there is a strong−weak alternating change for the distribution of water (or methanol) molecules inside the CNT along the radial direction of the CNT. The abundance of water molecules inside the CNT is increased when the electric field is introduced. This result is consistent with other studies.34 Moreover, the methanol molecules, without the electric field, show stronger ability of occupying the NIS-surface than that of the water molecules (Figures 7a and 8b). If the oscillating period (T) is less than TJob3 (the period for Job 3), methanol molecules occupy the CNT prior to water molecules. Here, Job 3 is a special case of all the jobs. For Job 3, the number of water and methanol molecules inside the CNT is the minimum one. E


Article


Figure 7. continued

F


Article


Figure 7. continued

G


Article


Figure 7. Two-dimensional number density maps of (a) water, (b) methanol, and (c) the difference of number density distributions between water and methanol, an average over the Z axis direction.

3.3. Distribution of Hydrogen-Bond Number. To better understand the properties of the molecules confined in CNTs, the distribution of hydrogen-bond (H-bond) number per water and methanol molecules was investigated along the Z axis of CNT (Figure 9). From Figure 9a, it could be observed that in most jobs the H-bond number per water molecule is between 2.5 and 3.0, whereas the average H-bond number per methanol molecule is about 2.0. The result of H-bond formation and vdW interaction is highly cooperative, and this result is in agreement with the previous research results.21 However, in Job 3, the H-bond number of water and methanol molecules inside

increase with the increasing period. In Figure 8b, all of the distributions for methanol (except Job 3) were divided into two obvious regions. One region in Figure 8b has a relatively higher density, which consists of Jobs 0, 1, and 2, and the other region consists of Jobs 4−8. The result is consistent with the previous results in Figure 2, which shows that the abundance of the methanol molecules inside the CNT decreases as the period increases. This result is in accordance with that in Figure 5, which reveals that the ratio of the numbers of methanol/water molecules inside the CNT decreases as the period increases. H


Article


Figure 8. Number density distributions of water (A) and methanol (B) along the Z axis. (a) Number density distributions of solvent molecules within the system; (b) number density distributions of solvent molecules inside the CNT.

special case in which the period of electric field is closed to the relaxation time of methanol molecule. Furthermore, the calculation of the average number of hydrogen bonds of the mixture inside the CNT (Table S1) indicates a significant decrease of H-bond numbers for both water and methanol molecules for Job 3 when compared with those for the other jobs. The above results show that the period of the oscillating electric field affects the formation of H-bonds. It is beneficial for a methanol molecule to form H-bond with water molecules under a long oscillating period of electric field, whereas it is beneficial for a water or methanol molecule to form H-bond with the same type of molecules, that is, to form water−water or methanol−methanol H-bonds under a short period of electric field. Moreover, when the oscillating period is approximately equal to the relaxation time, the number of Hbonds will have a significantly decreased influence on both water and methanol molecules. 3.4. Flux of Aqueous Solution Inside the CNT. Because of the difference between the dielectric relaxation times of water and methanol molecules, the flux of these molecules is affected by the oscillation period. Here, we shall define the flow rate as the number of molecules passing through the CNT per picosecond. The flow rates of water and methanol molecules

the CNT is about 1 lower than that in the other jobs. The dielectric relaxation time has a direct relation with the average H-bond number.41 The oscillating period for Job 3 is 5.0 ps, whereas the relaxation times for water and methanol are about 6.4 and 5.1 ps, respectively. Both of the relaxation times for water and methanol are approximately equal to the period of the electric field implies that the movement of water and methanol molecules is likely in resonance with the oscillating electric field, resulting in the breaking of hydrogen bonds. Therefore, in Job 3, the water molecules have high mobility via the CNT compared to that in the other jobs. Figure 9b shows the distributions of H-bond numbers per water molecule with its surrounding molecules, including Hbond types of water to water (style 1) and water to methanol (style 2). The “style 1” is on the left of Figure 9b, and “style 2” is on the right. It is observed that when the oscillating period is less than 5.0 ps, style 2 is the main contributor to the total Hbond number. But when the value becomes larger than 5.0 ps, style 1 is the main contributor. Figure 9c unveils the distribution of H-bond numbers per methanol molecule involved; this figure shows the trend of distribution for Hbond numbers with the surrounding methanol (Figure 9c (B)) is approximately contrary to the trend with the surrounding water (Figure 9c (A)). However, it is noted that Job 3 is a I


Article


Figure 9. Hydrogen-bond number distributions along the Z axis per water and methanol molecules: (a) total H-bond numbers per water (A) and methanol molecules (B); (b) H-bond numbers per water molecule with the surrounding water molecules (A) and the surrounding methanol molecules (B); (c) H-bond numbers per methanol molecule with the surrounding water molecules (A) and the surrounding methanol molecules (B). J


Article

The Journal of Physical Chemistry B Table 3. Flow Rates of Water and Methanol Molecules for Each Joba,b,c Water_P Water_N MeOH_P MeOH_N PPValue

Job 0

Job 1

Job 2

Job 3

Job 4

Job 5

Job 6

Job 7

Job 8

4.560 4.858 9.514 9.661 0.151

4.918 4.712 10.276 8.712 −1.358

4.098 5.798 8.128 11.118 −1.290

42.32 44.32 59.67 60.47 1.200

24.128 26.470 20.606 22.872 0.072

20.592 18.400 14.152 13.214 1.254

15.412 17.742 9.712 11.68 0.362

14.522 13.828 9.070 8.964 0.588

14.294 12.810 9.022 7.712 0.174

a Water_P (Water_N): The flow rate of water along the positive (negative) direction. bMeOH_P (MeOH_N): The flow rate of methanol along the positive (negative) direction. cPPValue = |Water_P − Water_N| − |MeOH_P − MeOH_N|.

Figure 10. Flow numbers of (a) water molecules and (b) methanol molecules as a function of MD time for each jobs, in the negative (A), and positive direction (B) of the Z axis.

of “Water_P − Water_N”, and the net flow rate of methanol is defined as “|MeOH_P − MeOH_N|”. There is a common feature of bidirectional flows for water molecules and methanol molecules in the CNTs. As it was explained previously, because of the influence of dielectric property, in Job 3, water and methanol molecules have the largest flow rates in both positive and negative directions. As it is seen, when the period of the electric field is longer than 20 ps, the flow rates of water and methanol molecules are in a relatively stable range. Furthermore, it is found that the flow rate of water molecules decreases when the oscillating period of the electric field is far off from its dielectric relaxation time.

are given in Table 3, which are the slopes of corresponding lines for flow numbers vs MD times shown Figure 10. “PPValue” means the difference between the net flow rates of water molecules and methanol molecules, and its expression is provided in the footnote of Table 3. In this study, water and methanol show the same flow direction. For instance, in Job 1, the value of Water_P is larger than that of Water_N; meanwhile, the value of MeOH_P is also larger than MeOH_N. There is no effect of flow direction when the “PPValue” is calculated. Therefore, we define the net flow rate of water as “|Water_P − Water_N|”, which is the absolute value K


Article

The Journal of Physical Chemistry B Notes

Meanwhile, the abundance of water molecules inside the tube increases as the oscillating period increases (Figure 7a). For methanol molecules, the abundance is decreased when the oscillation period is far off from its dielectric relaxation time as shown in Figure 7b, which is similar to its flux inside the CNT. The above results suggest that the flux of water or methanol molecules through the CNT depends not only on their ability to occupy the tube but also on their dielectric property. “PPValue” is the flux difference of water and methanol molecules. Table 3 describes that under the oscillating electric field there is an obvious disparity between the flux of water and methanol molecules in Jobs 1−3 and 5. In these jobs, the flux difference is one order of magnitude larger than that in Job 0 (reference job), which means that CNTs have the potential to separate methanol from its aqueous solution under an oscillating electric field.

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The project was funded by National Natural Science Foundation of China (21133005).

■

(1) Remy, T.; Remi, J. C. S.; Denayer, J. F. M Adsorption and Separation of C1−C8 Alcohols on Sapo-34. J. Phys. Chem. C 2011, 115, 8117−8125. (2) Huang, Y.; Baker, R. W.; Vane, L. M. Low-Energy DistillationMembrane Separation Process. Ind. Eng. Chem. Res. 2010, 49, 3760− 3768. (3) Liang, K.; Li, W.; Luo, H.; Xia, M.; Xu, C. Energy-Efficient Extractive Distillation Process by Combining Preconcentration Column and Entrainer Recovery Column. Ind. Eng. Chem. Res. 2014, 53, 7121−7131. (4) Zhang, Z.-Q.; Ye, H.-F.; Zheng, Y.-G.; Cheng, G.-G.; Ding, J.-N.; Ling, Z.-Y. Loading, Charging and Thermal Effects on the Mechanism of Water−Carbon Nanotube Transmission. Int. J. Comput. Mater. Sci. Eng. 2013, 02, No. 1350017. (5) Arora, G.; Sandler, S. I. Molecular Sieving Using Single Wall Carbon Nanotubes. Nano Lett. 2007, 7, 565−569. (6) Garcia-Fandino, R.; Sansom, M. S. P. Designing Biomimetic Pores Based on Carbon Nanotubes. Proc. Natl. Acad. Sci. U.S.A. 2012, 109, 6939−6944. (7) Zhao, K.; Wu, H. Fast Water Thermo-Pumping Flow across Nanotube Membranes for Desalination. Nano Lett. 2015, 15, 3664−8. (8) Kalra, A.; Garde, S.; Hummer, G. Osmotic Water Transport through Carbon Nanotube Membranes. Proc. Natl. Acad. Sci. U.S.A. 2003, 100, 10175−10180. (9) Chan, W.-F.; Chen, H.-y.; Marand, E.; Karl Johnson, J. Zwitterion Functionalized Carbon Nanotube/Polyamide Nanocomposite Membranes for Water Desalination. ACS Nano 2013, 7, 5308−5319. (10) Sae-Khow, O.; Mitra, S. Carbon Nanotube Immobilized Composite Hollow Fiber Membranes for Pervaporative Removal of Volatile Organics from Water. J. Phys. Chem. C 2010, 114, 16351− 16356. (11) Hummer, G.; Rasaiah, J. C.; Noworyta, J. P. Water Conduction through the Hydrophobic Channel of a Carbon Nanotube. nature 2001, 414, 188−190. (12) Nicholls, W. D.; Borg, M. K.; Lockerby, D. A.; Reese, J. M. Water Transport through (7,7) Carbon Nanotubes of Different Lengths Using Molecular Dynamics. Microfluid. Nanofluid. 2011, 12, 257−264. (13) Su, J.; Guo, H. Effect of Nanochannel Dimension on the Transport of Water Molecules. J. Phys. Chem. B 2012, 116, 5925− 5932. (14) Feng, J. W.; Ding, H. M.; Ren, C. L.; Ma, Y. Q. Pumping of Water by Rotating Chiral Carbon Nanotube. Nanoscale 2014, 6, 13606−13612. (15) Qiu, T.; Meng, X. W.; Huang, J. P. Nonstraight Nanochannels Transfer Water Faster Than Straight Nanochannels. J. Phys. Chem. B 2015, 119, 1496−1502. (16) Goldsmith, J.; Hinds, B. J. Simulation of Steady-State Methanol Flux through a Model Carbon Nanotube Catalyst Support. J. Phys. Chem. C 2011, 115, 19158−19164. (17) Li, J.; Gong, X.; Lu, H.; Li, D.; Fang, H.; Zhou, R. Electrostatic Gating of a Nanometer Water Channel. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 3687−3692. (18) Zhou, X.; Wu, F.; Kou, J.; Nie, X.; Liu, Y.; Lu, H. VibratingCharge-Driven Water Pump Controlled by the Deformation of the Carbon Nanotube. J. Phys. Chem. B 2013, 117, 11681−11686. (19) Moskowitz, I.; Snyder, M. A.; Mittal, J. Water Transport through Functionalized Nanotubes with Tunable Hydrophobicity. J. Chem. Phys. 2014, 141, No. 18C532.

4. CONCLUSIONS In summary, this study investigated 1:1 water−methanol solution across a (12, 12) SWCNT under an oscillating electric field. Without the electric field, methanol molecules preferentially occupy the CNT compared with water molecules. Under the oscillating electric field, water molecules occupy the CNT prior to methanol molecules, the reason for which is that the electric field affects the arrangement of molecules inside the CNT, thereby causing a change in vdW interactions between the CNT and the molecules inside it. For this reason, the occupancy of water molecules in the CNT increases as the oscillating period increases. The unidirectional fluxes are related to the ability of methanol and water to occupy the CNT. Remarkably, the oscillating period of the electric field significantly affects the flow rate of water and methanol molecules because of the difference between the dielectric relaxation times of water and methanol molecules. When the period of an oscillating electric field is equal to 5.0 ps (Job 3), the molecules inside the CNT show a higher mobility than that in other systems because of the decrease in the H-bond number. In this situation, the dielectric properties mainly contribute to enhance the flux of the molecules. In addition, there appears a peak value of a unidirectional flux of methanol molecules and water molecules, and the difference of flux between them is the most obvious at this period of electric field. Because of the obvious difference of fluxes between methanol and water molecules, Jobs 1−3 and 5 have potential applications in the separation. The effect of the CNT chirality on the separation under an oscillating electric field will be investigated in our future research.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.6b06509. Average number of hydrogen bonds; snapshot of distribution of vdW interaction; probability distributions; information of the dragging theory (PDF)

■

REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +86-0411-87403949. ORCID

Honglei Wang: 0000-0001-6802-0102 L


Article

The Journal of Physical Chemistry B (20) Liu, Y.; Consta, S.; Goddard, W. A. Nanoimmiscibility: Selective Absorption of Liquid Methanol−Water Mixtures in Carbon Nanotubes. J. Nanosci. Nanotechnol. 2010, 10, 3834−3843. (21) Zhao, W. H.; Shang, B.; Du, S. P.; Yuan, L. F.; Yang, J.; Zeng, X. C. Highly Selective Adsorption of Methanol in Carbon Nanotubes Immersed in Methanol-Water Solution. J. Chem. Phys. 2012, 137, No. 034501. (22) Tian, X.; Yang, Z.; Zhou, B.; Xiu, P.; Tu, Y. Alcohol-Induced Drying of Carbon Nanotubes and Its Implications for Alcohol/Water Separation: A Molecular Dynamics Study. J. Chem. Phys. 2013, 138, No. 204711. (23) Meng, L.; Li, Q.; Shuai, Z. Effects of Size Constraint on Water Filling Process in Nanotube. J. Chem. Phys. 2008, 128, No. 134703. (24) Köfinger, J.; Hummer, G.; Dellago, C. Single-File Water in Nanopores. Phys. Chem. Chem. Phys. 2011, 13, 15403−15417. (25) Kumar, H.; Dasgupta, C.; Maiti, P. K. Driving Force of Water Entry into Hydrophobic Channels of Carbon Nanotubes: Entropy or Energy? Mol. Simul. 2015, 41, 504−511. (26) Vaitheeswaran, S.; Rasaiah, J. C.; Hummer, G. Electric Field and Temperature Effects on Water in the Narrow Nonpolar Pores of Carbon Nanotubes. J. Chem. Phys. 2004, 121, 7955−7965. (27) Fu, Z.; Luo, Y.; Ma, J.; Wei, G. Phase Transition of NanotubeConfined Water Driven by Electric Field. J. Chem. Phys. 2011, 134, No. 154507. (28) He, Y.; Sun, G.; Koga, K.; Xu, L. Electrostatic Field-Exposed Water in Nanotube at Constant Axial Pressure. Sci. Rep. 2014, 4, 6596. (29) Joseph, S.; Aluru, N. R. Pumping of Confined Water in Carbon Nanotubes by Rotation-Translation Coupling. Phys. Rev. Lett. 2008, 101, No. 064502. (30) Bonthuis, D. J.; Rinne, K. F.; Falk, K.; Nadir Kaplan, C.; Horinek, D.; Nihat Berker, A.; Bocquet, L.; Netz, R. R. Theory and Simulations of Water Flow through Carbon Nanotubes: Prospects and Pitfalls. J. Phys.: Condens. Matter 2011, 23, No. 184110. (31) Wang, Y.; Zhao, Y. J.; Huang, J. P. Giant Pumping of Single-File Water Molecules in a Carbon Nanotube. J. Phys. Chem. B 2011, 115, 13275−13279. (32) Chen, Z.; Yang, Y.; Chen, F.; Qing, Q.; Wu, Z.; Liu, Z. Controllable Interconnection of Single-Walled Carbon Nanotubes under Ac Electric Field. J. Phys. Chem. B 2005, 109, 11420−11423. (33) Rinne, K. F.; Gekle, S.; Bonthuis, D. J.; Netz, R. R. Nanoscale Pumping of Water by Ac Electric Fields. Nano Lett. 2012, 12, 1780− 1783. (34) Winarto, W.; Takaiwa, D.; Yamamoto, E.; Yasuoka, K. Water− Methanol Separation with Carbon Nanotubes and Electric Fields. Nanoscale 2015, 7, 12659. (35) Jorgensen, W. L.; Chandrasekhar, J.; Madura, J. D.; Impey, R. W.; Klein, M. L. Comparison of Simple Potential Functions for Simulating Liquid Water. J. Chem. Phys. 1983, 79, 926−935. (36) Jorgensen, W. L. Optimized Intermolecular Potential Functions for Liquid Alcohols. J. Phys. Chem. 1986, 90, 1276−1284. (37) Jorgensen, W. L.; Briggs, J. M.; Contreras, M. L. Relative Partition Coefficients for Organic Solutes from Fluid Simulations. J. Phys. Chem. 1990, 94, 1683−1686. (38) Phillips, J. C.; Braun, R.; Wang, W.; Gumbart, J.; Tajkhorshid, E.; Villa, E.; Chipot, C.; Skeel, R. D.; Kalé, L.; Schulten, K. Scalable Molecular Dynamics with NAMD. J. Comput. Chem. 2005, 26, 1781− 1802. (39) Ryckaert, J.-P.; Ciccotti, G.; Berendsen, H. J. C. Numerical Integration of the Cartesian Equations of Motion of a System with Constraints: Molecular Dynamics of N-Alkanes. J. Comput. Chem. 1977, 23, 327−341. (40) Darden, T.; York, D.; Pedersen, L. Particle Mesh Ewald: An N.Log(N) Method for Ewald Sums in Large Systems. J. Chem. Phys. 1993, 98, 10089−10092. (41) Buchner, R.; Barthel, J.; Stauber, J. The Dielectric Relaxation of Water between 0 °C and 35 °C. Chem. Phys. Lett. 1999, 306, 57−63.

M


Investigation of Transport Properties of Water ... - ACS Publications

Recommend Documents