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Diffusion behaviors of ethanol and water through g–C3N4–based membranes: Insights from molecular dynamics simulation
Journal of Membrane Science 585 (2019) 81–89
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Diffusion behaviors of ethanol and water through g–C3N4–based membranes: Insights from molecular dynamics simulation
T
Xiuyang Zoua,b, Meisheng Lia,∗, Shouyong Zhoua, Chenglung Chena,c, Jing Zhongb, Ailian Xuea, Yan Zhanga, Yijiang Zhaoa,∗∗ a
School of Chemistry and Chemical Engineering, Huaiyin Normal University, Jiangsu Engineering Laboratory for Environment Functional Materials, Jiangsu Key Lab for Chemistry of Low-Dimensional Materials, Huaian, 223300, Jiangsu Province, PR China b College of Chemistry and Chemical Engineering, Changzhou University, Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, Changzhou, 213164, Jiangsu Province, PR China c Department of Chemistry, National Sun Yat-sen University, Kaohsiung, 80424, Taiwan, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Molecular dynamics simulation g-C3N4 Separation of water and ethanol
Graphitic carbon nitride (g-C3N4) with a layered lamellar structure was successively used to fabricate highly water-selective membranes via hybrid or free-standing methods. Molecular dynamics (MD) simulations were carried out to study the diffusion behaviors of water and ethanol molecules through pristine g-C3N4 (Pri-g-C3N4) and benzene ring-doped g-C3N4 (Ben-g-C3N4) membranes. MD results indicated that hydrogen bonds formed between water molecules and Pri-g-C3N4 or Ben-g-C3N4 nanosheets affected the permeation of water across these nanosheets. Interestingly, the simulation analysis showed that the lamellar structures of Pri-g-C3N4 nanosheets played a significant role in the separation of ethanol and water. The diffusion coefficients of water and ethanol through Pri-g-C3N4 lamellae were 2.24 × 10−5 cm2 s−1 and 1.04 × 10−5 cm2 s−1, respectively. Besides, the movement behaviors of water and ethanol molecules between lamellae of the sheet-shaped g-C3N4 (pristine gC3N4, Pri-g-C3N4) and distorted-type carbon nitride g-C3N4, i.e., benzene ring-doped into g-C3N4 (Ben-g-C3N4) were studied in detail. For the different configurations of g-C3N4, pure water flow was slower in Pri-g-C3N4 confinement compared to Ben-g-C3N4, as the lifetime of hydrogen bonds between water molecules in Ben-g-C3N4 was shorter than that in Pri-g-C3N4. Importantly, for ethanol/water binary mixtures, both Pri-g-C3N4 and Ben-gC3N4 surfaces preferentially adsorb ethanol, where the hydroxyl group of ethanol is closer to the membrane surface. The observed interesting feature suggested that the lamellar structures of Pri-g-C3N4 (or Ben-g-C3N4) nanosheets were efficient in the separation of ethanol and water.
1. Introduction Two dimensional (2D) materials have been explored as promising candidates in the field of membrane separation techniques. The reported 2D membranes include graphene [1,2], graphene oxide (GO) [3–5], boron nitride (BN) [6], molybdenum disulfide (MoS2) [7], stanene [8–11], etc. All of these materials exhibit some special electronic structures [12,13] and wide surface area [14,15]. Among these materials, graphitic carbon nitride (g-C3N4), a layered material with a πconjugated system consisting of tri-s-triazine as a basic structural unit, has attracted intense attention owing to its inherent lattice defect (triangular nanopores) and unique layered structure [16–22]. Interestingly, both the sizes of lattice defect [23] and inter-planar distance [24]
∗
in g-C3N4 are between 3.1 Å and 3.4 Å, respectively, which assists in the separation of ethanol (C2H5OH, kinetic diameter of 4.5 Å [25]) and water (H2O, kinetic diameter of 2.8 Å [26]). Thus, some researchers suggested that the separation of ethanol and water by g-C3N4 could be due to the sieving effect. For example, Cao et al. [23] prepared sodium alginate (SA) membrane doped with g-C3N4 nanosheets for the separation of water and ethanol and showed that both flux and separation factor enhanced significantly. In comparison, SA membrane incorporated with nonporous g-C3N4 (polydopamine-coated) nanosheets became more permeable and less selective. It has been predicted that the kinetic diameter of ethanol molecules was larger than the size of the inherent lattice defect in g-C3N4. Thus, ethanol molecules could only be transported between g-C3N4 layers, whereas water molecules move both in inner-layer and lattice defect. Longer and more tortuous
Corresponding author. Corresponding author. E-mail addresses: [email protected] (M. Li), [email protected] (Y. Zhao).
∗∗
https://doi.org/10.1016/j.memsci.2019.05.031 Received 25 March 2019; Received in revised form 7 May 2019; Accepted 11 May 2019 Available online 13 May 2019 0376-7388/ © 2019 Elsevier B.V. All rights reserved.
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conditions in x, y, and z directions were applied to all studied systems. Ewald summation method was used to calculate the long-range electrostatic interactions with an accuracy of 0.0001 kcal/mol and Atom based summation method was used for van der Waals interactions with the cutoff distance of 12.5 Å. Water molecules and ethanol molecules were optimized by COMPASSII force field. The bond length and bond angle of the optimized water molecule were 0.957 Å and 104.553°, respectively. It should be noted that the bond length and bond angle of the water molecules and ethanol molecules were not fixed during the simulation. According to the literature, the structure of Pri-g-C3N4 [40] and Ben-g-C3N4 [37] were made by using Visualizer tools in Materials Studio and were also optimized by COMPASSII force field. In this study, two simulation processes were carried out, where in the first case, the diffusion behaviors of different solvent molecules (water, ethanol, hydrogen, and neon) across the triangular nanopores (lattice defects of Pri-g-C3N4) were simulated. As shown in Fig. 2-a, the atoms of Pri-g-C3N4 were frozen in the simulation models, as the nanosheet is rather rigid in a real case [40]. The solvent (water, hydrogen (kinetic diameter of 2.89 Å [41]), or neon (kinetic diameter of 2.79 Å [42])) was filled into a simulation box (Lx = 49 Å, Ly = 49 Å, Lz = 74 Å) with the Amorphous Cell Modules [43] on the Pri-g-C3N4 membrane, and a vacuum layer of 20 Å was present under the membrane. The number of solvent molecules was 4203. This simulation process was subjected to 1 ns NVT simulation (time step = 1 fs, frame output for every 5000 steps, T = 298 K). In the other simulation, the dynamic properties of water molecules between two nanosheet layers were calculated. The distance between two layers was 20 Å, where each layer contains three Pri-g-C3N4 or Ben-g-C3N4 (42.84 Å × 51.97 Å) nanosheets (the distance between each nanosheet: 3.38 Å [44,45]), and there were two reservoirs on each side along the Z-axis. The number of water molecules in the bulk phase was 4000, and the density was 1.0 g/ cm3. This system was subjected to the steepest-descent energy minimization of 2000 steps, followed by a 2 ns NVT equilibrium simulation (time step = 1 fs, frame output for every 10000 steps, T = 298 K). Then, the production NVT simulation was performed for 1 ns (time step = 1 fs, frame output for every 5000 steps, T = 298 K). The velocity-scale thermostat with a temperature difference of 1.0 K was employed. To start the simulation the molecules were randomly distributed in the box, and all of the initial configurations were constructed by the Amorphous Cell Modules.
diffusion paths improved the separation factor of these molecules. On the other hand, some researchers insisted that g-C3N4 separated ethanol and water due to different solvent-interface interactions between ethanol/water (the phenomenon of ultralow friction) and g-C3N4 [27]. Experimental evidence confirmed that the tri-s-triazine units in gC3N4 connected with nitrogen (N) atoms by the single bond (N-(C)3) were eliminated easily to adjust the pore size of the membrane. For example, Papailias et al. [28] exfoliated g-C3N4 via chemical treatment using concentrated sulfuric acid. The X-ray photoelectron spectroscopy (XPS) results demonstrated the breaking of a single bond (N-(C)3), which led to the cleavage of the g-C3N4 network. Simultaneously, Wang et al. [17] reported on molecular dynamics (MD) simulations of water and hexane through g-C3N4 nanosheets with artificial nanopores. Their results showed that the velocities of water molecules through twolayered g-C3N4 nanosheets were faster than that of n-hexane molecules despite higher viscosity of water compared to n-hexane. This indicates the ultralow friction between g-C3N4 and water. Different from graphene [29] (thinnest 2D material), the N atoms in g-C3N4 have possibilities to form hydrogen bonds with water molecules or other hydroxyl group-containing solvent molecules of inter-layer. Therefore, the mass diffusion mechanism of water or ethanol molecules in the g-C3N4 layer may present different behavior than in the graphene layer. It is known that the structural [30] and dynamical properties [31] of water in a confined space [32] are different from those in bulk water. It was found that hydrogen bond (H-bond) networks between water molecules were destroyed and new solvent-interface interactions [33] were discovered because of confinement. Chakra borty et al. [34] explored the behavior of water in confined space inside the carbon nanotubes (CNTs) and found out that the relaxation time of the orientational dynamics of water molecules (150 fs) was shorter than that in bulk water (2.5 ps). They concluded this as only one hydrogen bond was formed for each water molecule and rotation about CNTs axis was relatively free. In addition to the experimental supports, molecular dynamics (MD) simulations are widely applied to investigate water permeation in confinement and notably reports are based on the simulation results. Wei et al. [35] concluded that the fast flow of water transport across GO membranes might be primarily due to the porous microstructure of GO, including wrinkles (leading to wide channels), holes, and interlayer spaces. Jordan et al. [36] showed that the permeability of water in GO depends not only on the size of the inlet, but also on the diffusion path in GO. Moreover, water was well mixed when transported between the layers of the graphene membrane. In this investigation, MD simulations were performed to explore the diffusion of water and ethanol molecules through sheet-typed g-C3N4 (pristine g-C3N4, Pri-g-C3N4) and benzene-doped distorted g-C3N4 (Beng-C3N4) free-standing membranes and Fig. 1-a and 1-b exhibit the structures of these two membranes. According to Kim et al. [37], the nitrogen of tertiary amine in g-C3N4, connected by three heptazine rings, is partly substituted (25%) with a benzene molecule in Ben-gC3N4. The permeability of water, ethanol, hydrogen and neon, was first investigated by employing MD simulations. Then, the diffusion mechanisms of water and ethanol molecules in Pri-g-C3N4 were obtained by analyzing the density profile, the residence time, and the diffusion coefficient of water molecule, etc. For comparison, the diffusion of water and ethanol molecules through Ben-g-C3N4 free-standing membranes was also investigated.
3. Results and discussion 3.1. Diffusion behaviors of molecules across the Pri-g-C3N4 intrinsic lattice defect At first, the permeability of different solvent molecules (water, ethanol, hydrogen and neon) across the lattice defect of Pri-g-C3N4 was investigated. As shown in Fig. 3, the simulation results showed that water molecules could hardly pass across the defect of the Pri-g-C3N4 membrane, while hydrogen could easily pass through. Meanwhile, there were 77 Ne atoms across the Pri-g-C3N4 membrane after 1 ns NVT simulation, although the diameter of the neon atom is similar to that of water molecule. The simulation box was filled with the same amount of water molecules, hydrogen molecules or neon atoms. In other words, the partial pressure of hydrogen or neon was much larger than water. Therefore, an external force above the water box was applied along the Z-axis to check the effect of pressure. When the external force was 50 kcal/mol/Å (≈20 MPa), no H2O molecule was found out across the Pri-g-C3N4 membrane. As shown in Fig. 4-c, the water molecules still accumulated in the intrinsic defects of Pri-g-C3N4, and the hydrogen atoms of water pointed to the nitrogen atoms of Pri-g-C3N4. It is implied that the hydrogen bonds may form between water molecules and nitrogen atoms in Pri-g-C3N4, which hinder water molecules passing across the Pri-g-C3N4 membrane. To confirm that such a hindrance is undoubtedly due to the hydrogen bonding, the permeability of a
2. Models and simulation methods All molecular dynamics simulations were performed using the Materials Studio software package. The force field of simulation systems was COMPASSII [38], and the force field gave charges of all the particles. COMPASSII [39] force field was a significant extension to the COMPASS force field, which was an ab initio force field. The parameters of COMPASSII force field were derived from quantum mechanics calculations and optimized to fit experimental data. Periodic boundary 82
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Fig. 1. Perspective views of Pri-g-C3N4 (a) and Ben-g-C3N4 (b). Fig. 2. Snapshots of the MD simulation systems. (a) Water across Pri-g-C3N4 membrane; (b) 5 wt% HCN across Pri-g-C3N4 membrane(The red and white atoms represent the oxygen and hydrogen atoms in water molecules, respectively. The blue and gray atoms represent the carbon and nitrogen atoms in HCN molecules, respectively.). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
3.2. Dynamic properties of different molecules between two nanosheet layers
straight rod-shaped molecule, i.e., hydrogen cyanide (HCN) [46] was simulated for comparison. Different from the V-shaped water, the rodshaped HCN may be quickly passed across the membrane due to its structure. In this run, the simulation box was filled with 5 wt% HCN (solvent is water) as shown in Fig. 2-b. Table 1 illustrate the simulation results and it could be seen that neither HCN nor H2O was found to pass across the Pri-g-C3N4 membrane after 1ns NVT simulation. Moreover, the permeability of pure HCN across the Pri-g-C3N4 membrane was simulated. As shown in Table 1, HCN molecules could hardly pass across the defects of the Pri-g-C3N4 membrane. It is verified that both water and HCN molecules can form hydrogen bonds with nitrogen atoms in Pri-g-C3N4. Thus, no water or HCN molecules can pass across the Pri-gC3N4 intrinsic lattice defect. However, several investigations [23,47,48] reported on the incorporation of g-C3N4 into dense polymeric membranes, such as polyvinyl alcohol (PVA) or SA membrane, to increase the permeability of solvent and selectivity of organic compounds. The experimental results showed that incorporating nonporous g-C3N4 into polymer only increased the flux of the membrane but not the separation factor. Thus, it becomes necessary to understand the diffusion mechanism of the molecules across Pri-g-C3N4 free-standing or composite membranes.
Fig. 4 shows the equilibrium configurations of the sheet-typed Pri-gC3N4 free-standing membranes. According to an earlier report [49], the inter lamellar distances of Pri-g-C3N4 and Ben-g-C3N4 confinement were all set at a width of 20 Å to ensure free movement of ethanol molecules in confinement. As shown in Fig. 4-b, the ethanol molecules move from the reservoirs into the confinement and preferentially adsorb onto the Pri-g-C3N4 nanosheets. Fig. 5 exhibits the equilibrium configurations of distorted-typed carbon nitride (Ben-g-C3N4) free-standing membranes which shows a similar phenomenon of preferential adsorption of ethanol molecules onto the Ben-g-C3N4 nanosheets. As shown in Fig. 5-c and 5-d, the doped benzene ring prevents the adsorption of water molecules onto the membrane. 3.2.1. Density distributions of water and ethanol molecules Detailed investigations were carried out on the density distribution profiles of water molecules confined in Pri-g-C3N4/Ben-g-C3N4 layers along the Y-axis, which were perpendicular to the Pri-g-C3N4/Ben-gC3N4 surface. Fig. 6 shows the density profiles of water and ethanol molecules inside a confined region of 20 Å for pure water or 5 wt% 83
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repelling water molecules. Fig. 6-c indicates that hydroxyl groups of ethanol are closer to the surface of Pri-g-C3N4 than methyl groups of ethanol. This is because that hydrogen atom of the hydroxyl group of ethanol forms hydrogen bonds with nitrogen atoms in Pri-g-C3N4. This result displays the differences in the absorption of ethanol in Pri-g-C3N4 and graphene. Gao et al. [50] reported on the simulation of alcohols absorbed by the graphene layer, presents that methyl groups in ethanol are closer to graphene than hydroxyl groups in ethanol. This is due to the absence in the formation of hydrogen bonds between ethanol and graphene surface. Fig. 6-e and 6-f indicate the density distributions of water in 5 wt% ethanol solution in the confinement of Pri-g-C3N4. It could be noted from these figures that the density distributions of water molecules in the mixture are similar to that of pure water. Since the hydroxyl groups of ethanol form hydrogen bonds with the surface of Pri-g-C3N4, the methyl groups on the other part of ethanol have less influence on the density distributions of water molecules. 3.2.2. Normalized distributions of orientation angle To analyze the configuration of ethanol in confinement, angles α and β are defined as the orientation angles of CH3–CH2 and O–H bonds of ethanol, respectively to the Y-axis. Fig. 7-a and 7-b are the distributions of these angles. In 20 Å Pri-g-C3N4 confinement, both α and β in Fig. 7-a show a maximum peak at 90°. This indicates that the CH3–CH2 and O–H bonds prefer to be parallel to the surface of Pri-gC3N4 membrane. Compared to β, it could be noted that the angle α has a broader distribution, which may be due to the formation of the hydrogen bond between ethanol and membrane. This indicates that the interaction between Pri-g-C3N4 membrane and the hydroxyl group is stronger than that of the methyl group. Fig. 7-a exhibits broad distributions of the orientation angles, α and β. This indicates that ethanol molecule is affected not only by Pri-g-C3N4 but also by the surrounding water molecules. In 20 Å Ben-g-C3N4 confinement, Fig. 7-b presents that the distributions of orientation angles of ethanol molecules are to a certain extent different than that in Fig. 7-a. It could be noticed that the intensity of the maximum peak around 90° of the orientation angle β was less for Ben-g-C3N4 than that in Pri-g-C3N4 confinement. Two other intense distributions of angle β appear around 54° and 126° (Fig. 7-b). These results confirm that the distorted triazine-ring (Ben-g-C3N4) affects the configuration of ethanol molecules, as the protuberant benzene in Ben-g-C3N4 causes triazine-ring to protrude from the surface of the membrane (inset snapshots of Fig. 7-b).
Fig. 3. MD simulations of water, hydrogen, neon, etc. across Pri-g-C3N4 membrane as a function of time; the y coordinate is the number of molecules across the membrane and t = 0 corresponds to appearance of solvent molecules on the permeated side.
ethanol solution. For pure water, Fig. 6-a indicates that the density distribution of water is symmetric about the center of the confinement which signifies a distinct layered structure in Pri-g-C3N4. A peak of maximum density peak for water appears around ∼0.36 nm away from the surface of Pri-g-C3N4, indicating that water molecules form a high adsorption layer. On the other hand, Fig. 6-b shows a different distribution of water molecules on the surface of Ben-g-C3N4 (Fig. 6-b). The position of maximum density peak also appears around ∼0.36 nm, but the maximum intensities are significantly reduced than that in Prig-C3N4. This could be due to that the protuberant benzene ring destroys the adsorption of water molecules. Fig. 6-c and 6-d demonstrate the density distributions of –CH3 and –OH groups of an ethanol solution of 5 wt% (Nethanol/Nwater = 150/ 7525) in Pri-g-C3N4 confinement. A peak of maximum density peak for hydroxyl groups of ethanol appears around ∼0.27 nm away from the surface of Pri-g-C3N4, while that of methyl groups of ethanol appears around ∼0.36 nm. Compared to water molecules (∼3.6 nm), hydroxyl groups of ethanol is closer to the surface of the Pri-g-C3N4. It's because that ethanol molecules move from the reservoirs into the confinement and preferentially adsorb onto the Pri-g-C3N4 nanosheets, thereby
3.2.3. Movement of solvent molecules between nanosheet layers To further understand the fluidic behavior of water in Pri-g-C3N4 confinement, the residence time of water molecules was calculated. Compared with water molecules in the center of confinement, which
Fig. 4. Snapshots of equilibrium configurations (side views) for (a) pure water in Pri-g-C3N4, (b) 5 wt% ethanol in Pri-g-C3N4, and (c) the configuration of confined water inside Pri-g-C3N4. 84
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remains inside that region. As shown in Fig. 8, the CA(t) of water molecules in Ben-g-C3N4 decays faster than that in Pri-g-C3N4. It denotes that the residence time is shorter in Ben-g-C3N4 and the mobility of water molecules is larger. Table 2 lists the calculated diffusion coefficients (D), mean residence times of water (τwater) and mean lifetimes of hydrogen bond (τHB) for water and ethanol molecules. Based on the slope of the mean square displacement (MSD), the calculated diffusion coefficient of bulk water was 5.51 × 10−5 cm2 s−1, which agrees well with the reported literature value [53,54]. The data in Table 2 shows that in both pure water and 5 wt% ethanol systems the diffusion coefficient of water in Ben-g-C3N4 is larger than that in Pri-g-C3N4. This result is consistent with the results of residence time. Thus, water molecules in Ben-g-C3N4 are more mobile than in Pri-g-C3N4. Therefore, it could be concluded that the pure water flux of Ben-g-C3N4 system is larger than that of Prig-C3N4 system. Both the density distributions (Fig. 6-b) and orientation angle distributions (Fig. 7-a) of water and ethanol molecules indicated that hydroxyl groups of ethanol was closer to the surface of the Pri-g-C3N4 surface. Compared to hydroxyl groups of ethanol, methyl groups in ethanol can not form hydrogen bonds with water molecules, thus the methyl groups of ethanol have less influence on the movement of water molecules. Table 2 shows that the diffusion coefficient of water molecules decreased with the addition of ethanol. This may be due to that the addition of ethanol reduces the movement of water molecules in space. On the other hand, Table 2 demonstrates that the diffusion coefficients of water are significantly larger than that of ethanol. This reveals that with incorporation of Pri-g-C3N4 into dense polymeric membranes, the permeability and separation ability were improved. The separation of ethanol and water was mainly due to the interactions between water/ethanol molecules and Pri-g-C3N4 but not due to the size effect.
Table 1 Table of External Force (Fexternal), Kinetic Diameter (Dk), Initial number of molecules (N0), and Number of molecules across the membrane (Nt) for water, hydrogen, neon, hydrogen cyanide, and ethanol, etc. molecules.
H2 Ne H2Oa H2Ob H2Oc H2Od HCNe HCNf C2H5OH a b c d e f g h
Fexternal/kcal/mol/Å
Dk/Å
N0
Nt
Referencesg
0 0 0 50 0 0 0 0 0
2.89 2.79 2.80 2.80 2.80 2.80 2.92h 2.92h 4.50
4203 4203 4203 4203 4065 4067 143 4203 83
1060 77 0 0 0 0 0 0 0
[39] [42] [26] [26] [26] [26] [46] [46] [25]
Pure water. Pure water under pressure. Water in 5 wt % HCN mixture system. Water in 5 wt % ethanol mixture system. HCN in 5 wt % HCN mixture system. Pure HCN. Molecular kinetic diameter references. Section diameter of HCN.
were similar to bulk water, the high adsorption confined water layer may have different movement behaviors. Referring to Fig. 6-a and 6-b, four particular regions along Y direction were selected: (−9.0677 to −5.3240 Å) and (5.2035–8.9734 Å) for Pri-g-C3N4 system; (−9.8558 to −4.5255 Å) and (6.5740–9.7931 Å) for Ben-g-C3N4 system, respectively. The normalized residence autocorrelation function was calculated using equation (1), where A(t) was unity when water was inside a particular region at time t and A(t) was zero otherwise. Also, the angular brackets denote ensemble average. The mean residence time was calculated by the integration of the residence autocorrelation function (equation (2)) [51,52].
CA (t ) =
τ=
∫0
t
〈A (t )⋅A (0) 〉 〈A (0)⋅A (0) 〉
(1)
CA (t ) dt
(2)
3.2.4. Microstructural analysis for analyzing the capacity of molecule mobility Related to the mobility of water, the H-bond network was considered to have a significant influence, and thus the amount of H-bond between water molecules in different membrane systems was calculated. In this system, the H-bond was determined when the distance between the hydrogen atom and the acceptor was less than or equal to 2.5 Å and the angle formed by the donor, hydrogen and acceptor atoms was larger than 139° [50,55]. The length of the hydrogen bond is defined as the distance between the hydrogen atom of donor and the acceptor atom.
The time correlation functions and residence times of water molecules in the particular region are shown in Fig. 8. The CA(t) decays from 1 to 0 with an increase in the simulation time, indicating that water molecules continuously entering and moving out of the particular region. The faster CA(t) decays to zero, the shorter is the time that water
Fig. 5. Snapshots of equilibrium configurations (side views) for (a) pure water in Ben-g-C3N4, (b) 5 wt% ethanol in Ben-g-C3N4; the configuration of confined water inside (c) Protuberant benzene, (d) depressed benzene. 85
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Fig. 6. Density profiles of molecules in 20 Å confined region: (a) pure water in Pri-g-C3N4, (b) pure water in Ben-g-C3N4, (c) methyl (green line) and hydroxyl groups (blue line) in ethanol in Pri-g-C3N4, (d) methyl (green line) and hydroxyl groups (blue line) in ethanol in Ben-g-C3N4, (e) water in the mixture in Pri-g-C3N4, and (f) water in the mixture in Ben-g-C3N4. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
averages. As shown in Table 3, it could be noted that the total number of Hbonds in Ben-g-C3N4 is 150, which is larger than that of 103 in case of Pri-g-C3N4. The calculated mean numbers of H-bonds (number of hydrogen bond/number of N atoms), however are in the reverse order: 0.96 in Pri-g-C3N4 and 0.88 in Ben-g-C3N4. The hydrogen bonds formed between water molecules and membrane lead to the reduction in the mobility of water. The smaller mean number of H-bonds indicates larger mobility of water. An analysis in the hydrogen bond suggestes that water moves faster in Ben-g-C3N4 than in Pri-g-C3N4. This agrees
Then, the lifetime autocorrelation function was calculated via equation (3) and the value of the lifetime autocorrelation function was integrated to acquire mean lifetime (equation (4)) [51,55].
CHB (t ) =
τ=
∫0
t
〈H (t )⋅H (0) 〉 〈H (0)⋅H (0) 〉
CHB (t ) dt
(3) (4)
Where H(t) was unity when H-bond was formed as time t and H(t) was zero otherwise. Also, the angular brackets denote ensemble 86
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Fig. 7. Normalized orientation angle (α, β) distributions of confined ethanol molecules within the 20 Å Pri-g-C3N4 (a) and Ben-g-C3N4 (b) for 5 wt% ethanol−water mixture. The inset snapshots show the structure of α and β. Table 3 Table of total number of H-bond formed by water molecules and different membrane surfaces(NT), number of N atoms forming hydrogen bonds on the membrane(NN), mean number of H-bond formed by water molecules and different membrane surfaces(NM) and mean value of H-bond length formed by water molecules and different membrane surfaces(LM).
g-C3N4 Ben-g-C3N4
NT
NN
NM
LM
103 150
107 170
0.96 0.88
2.20 2.25
(n = 1, 2, 3, 4) represents the formation of the number of hydrogen bonds for a single water molecule to its surrounding water molecules. From Fig. 9-b, it is seen that water molecules in Ben-g-C3N4 have the largest proportion of forming one hydrogen bond. On the other hand, the confined water in Pri-g-C3N4 has the largest proportion of forming three hydrogen bonds. This result shows that a water molecule is less restricted by the surrounding water molecules in Ben-g-C3N4 than that in Pri-g-C3N4 confinement. In addition, the analysis of H-bonds length suggested that H-bonds formed by water molecules and water molecules in Pri-g-C3N4 confinement were most stable (the data in Table 4).
Fig. 8. Intermittent time correlation functions CA(t) of water molecules and corresponding residence times in particular region.
4. Conclusions
well with the trends of the calculated diffusion coefficients of water in these two systems. Fig. 9-a exhibits the calculated correlation functions and their corresponding lifetimes of H-bonds. It shows that the lifetime in Pri-g-C3N4 confinement is larger than that in Ben-g-C3N4. Both the lifetimes and mean length (the data in Table 3) of the H-bond suggests that H-bonds formed by water molecules and Pri-g-C3N4 are more stable, thereby causing water moves slower through Pri-g-C3N4 layers. As shown in Table 4, the mean number of H-bonds formed by water molecules and water molecules in Ben-g-C3N4/Pri-g-C3N4 confinement is less than that of in bulk water. To explain the reason for the decrease in the number of H-bonds, the distribution of H-bonds of water molecules in bulk phase, Pri-g-C3N4 and Ben-g-C3N4 were investigated (Fig. 9-b). Fn represents the percentage of water molecules while n
In this study, MD simulations were successfully performed to explore the diffusion mechanisms of H2O and C2H5OH molecules in sheet-type Pri-g-C3N4 and distorted-type carbon nitride (Ben-g-C3N4) free-standing membranes. For g-C3N4 free-standing membranes, although Pri-g-C3N4 has intrinsic lattice defect, which is higher than the kinetic diameter of water, the hydrogen bonds formed between water and Pri-g-C3N4 hinder the passage of water molecules across the Pri-g-C3N4 sheet. Based on the diffusion coefficient analysis of water and ethanol molecules in Pri-gC3N4 interlayers, an improvement in the permeability and selectivity with the incorporation of Pri-g-C3N4 into dense polymeric membranes is mainly attributed to the separation of interlayer diffusion. Thus, it
Table 2 Table of Diffusion Coefficient (D), Mean Residence Times (τwater), and Mean Hydrogen Bond Lifetimes (τHB) for water molecules and ethanol molecules.
pure water 5 wt % ethanol
g-C3N4 Ben-g-C3N4 g-C3N4 Ben-g-C3N4
Dwater/( × 10−5 cm2 s−1)
Detanol/( × 10−5 cm2 s−1)
τwater/ps
τHB/ps
4.46 4.87 2.24 2.41
/ / 1.04 0.68
25.05 18.23 32.30 26.92
2.57 2.39 2.90 2.72
87
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Fig. 9. (a) Intermittent time correlation function CHB(t) of H-bond and corresponding lifetime in confinement. (b) Percentage of water molecules (Fn) with n (n = 1, 2, 3, 4) hydrogen bonds.
Chemistry of Low-Dimensional Materials (JSKC17005). Also, the authors would like to express their gratitude to EditSprings (https://www. editsprings.com/) for the expert linguistic services provided.
Table 4 Table of mean number of H-bond formed by water molecules and water molecules (NW), Maximum value of H-bond length (Lmax) formed by water molecules and water molecules, Minimum value of H-bond length (Lmin) formed by water molecules and water molecules and Mean value of H-bond length (Lmean) formed by water molecules and water molecules.
g-C3N4 Ben-g-C3N4 H2O
NW
Lmax
Lmin
Lmean
1878a 1849a 2198a
1.94 1.94 1.92
1.89 1.90 1.81
1.92 1.93 1.89
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a
Keep the number of water molecules used to calculate hydrogen bonds consistent.
could be concluded that Pri-g-C3N4 could be selected to separate ethanol and water mainly owing to the separation of interlayer diffusion and not due to the size of inherent lattice defect via sieve effect. For different configurations of g-C3N4, pure water in sheet-typed Pri-g-C3N4 confinement flows slower than that in distorted-type g-C3N4 (Ben-g-C3N4), which is based on the diffusion coefficient as well as the residence time of water molecules. More specifically, the residence time of water molecules in confinement decays in the order of, Pri-gC3N4 > Ben-g-C3N4, whereas the diffusion coefficient of water molecules decays in the order of, Ben-g-C3N4 > Pri-g-C3N4. Also, the lifetime of H-bonds in Ben-g-C3N4 is shorter than that in Pri-g-C3N4, thereby the hydrogen bonds between water molecules in Ben-g-C3N4 confinement are unstable. Due to this, the water molecules in Ben-gC3N4 have the greatest capacity in mobility. For ethanol/water binary mixtures, Pri-g-C3N4/Ben-g-C3N4 surfaces preferentially adsorb ethanol, and the hydroxyl group of ethanol is closer to the membrane surface. More specifically, the addition of ethanol squeezes the water molecules in space, making the actual confinement size to become smaller, as hydroxyl groups in ethanol destroy the hydrogen bonding network of water molecules rarely.
Acknowledgments The authors are grateful for the financial support of the National Natural Science Foundation of China (No. 21878118, 21406082, 21476094), Natural Science Foundation of the Jiangsu Higher Education Institutions of China (18KJA530003), Natural Science Foundation of Jiangsu Province (BK20171268), Jiangsu Province Qing Lan Project, and the open project program of Jiangsu Key Lab for 88
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Diffusion behaviors of ethanol and water through g–C3N4–based membranes: Insights from molecular dynamics simulation
T
Xiuyang Zoua,b, Meisheng Lia,∗, Shouyong Zhoua, Chenglung Chena,c, Jing Zhongb, Ailian Xuea, Yan Zhanga, Yijiang Zhaoa,∗∗ a
School of Chemistry and Chemical Engineering, Huaiyin Normal University, Jiangsu Engineering Laboratory for Environment Functional Materials, Jiangsu Key Lab for Chemistry of Low-Dimensional Materials, Huaian, 223300, Jiangsu Province, PR China b College of Chemistry and Chemical Engineering, Changzhou University, Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, Changzhou, 213164, Jiangsu Province, PR China c Department of Chemistry, National Sun Yat-sen University, Kaohsiung, 80424, Taiwan, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Molecular dynamics simulation g-C3N4 Separation of water and ethanol
Graphitic carbon nitride (g-C3N4) with a layered lamellar structure was successively used to fabricate highly water-selective membranes via hybrid or free-standing methods. Molecular dynamics (MD) simulations were carried out to study the diffusion behaviors of water and ethanol molecules through pristine g-C3N4 (Pri-g-C3N4) and benzene ring-doped g-C3N4 (Ben-g-C3N4) membranes. MD results indicated that hydrogen bonds formed between water molecules and Pri-g-C3N4 or Ben-g-C3N4 nanosheets affected the permeation of water across these nanosheets. Interestingly, the simulation analysis showed that the lamellar structures of Pri-g-C3N4 nanosheets played a significant role in the separation of ethanol and water. The diffusion coefficients of water and ethanol through Pri-g-C3N4 lamellae were 2.24 × 10−5 cm2 s−1 and 1.04 × 10−5 cm2 s−1, respectively. Besides, the movement behaviors of water and ethanol molecules between lamellae of the sheet-shaped g-C3N4 (pristine gC3N4, Pri-g-C3N4) and distorted-type carbon nitride g-C3N4, i.e., benzene ring-doped into g-C3N4 (Ben-g-C3N4) were studied in detail. For the different configurations of g-C3N4, pure water flow was slower in Pri-g-C3N4 confinement compared to Ben-g-C3N4, as the lifetime of hydrogen bonds between water molecules in Ben-g-C3N4 was shorter than that in Pri-g-C3N4. Importantly, for ethanol/water binary mixtures, both Pri-g-C3N4 and Ben-gC3N4 surfaces preferentially adsorb ethanol, where the hydroxyl group of ethanol is closer to the membrane surface. The observed interesting feature suggested that the lamellar structures of Pri-g-C3N4 (or Ben-g-C3N4) nanosheets were efficient in the separation of ethanol and water.
1. Introduction Two dimensional (2D) materials have been explored as promising candidates in the field of membrane separation techniques. The reported 2D membranes include graphene [1,2], graphene oxide (GO) [3–5], boron nitride (BN) [6], molybdenum disulfide (MoS2) [7], stanene [8–11], etc. All of these materials exhibit some special electronic structures [12,13] and wide surface area [14,15]. Among these materials, graphitic carbon nitride (g-C3N4), a layered material with a πconjugated system consisting of tri-s-triazine as a basic structural unit, has attracted intense attention owing to its inherent lattice defect (triangular nanopores) and unique layered structure [16–22]. Interestingly, both the sizes of lattice defect [23] and inter-planar distance [24]
∗
in g-C3N4 are between 3.1 Å and 3.4 Å, respectively, which assists in the separation of ethanol (C2H5OH, kinetic diameter of 4.5 Å [25]) and water (H2O, kinetic diameter of 2.8 Å [26]). Thus, some researchers suggested that the separation of ethanol and water by g-C3N4 could be due to the sieving effect. For example, Cao et al. [23] prepared sodium alginate (SA) membrane doped with g-C3N4 nanosheets for the separation of water and ethanol and showed that both flux and separation factor enhanced significantly. In comparison, SA membrane incorporated with nonporous g-C3N4 (polydopamine-coated) nanosheets became more permeable and less selective. It has been predicted that the kinetic diameter of ethanol molecules was larger than the size of the inherent lattice defect in g-C3N4. Thus, ethanol molecules could only be transported between g-C3N4 layers, whereas water molecules move both in inner-layer and lattice defect. Longer and more tortuous
Corresponding author. Corresponding author. E-mail addresses: [email protected] (M. Li), [email protected] (Y. Zhao).
∗∗
https://doi.org/10.1016/j.memsci.2019.05.031 Received 25 March 2019; Received in revised form 7 May 2019; Accepted 11 May 2019 Available online 13 May 2019 0376-7388/ © 2019 Elsevier B.V. All rights reserved.
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conditions in x, y, and z directions were applied to all studied systems. Ewald summation method was used to calculate the long-range electrostatic interactions with an accuracy of 0.0001 kcal/mol and Atom based summation method was used for van der Waals interactions with the cutoff distance of 12.5 Å. Water molecules and ethanol molecules were optimized by COMPASSII force field. The bond length and bond angle of the optimized water molecule were 0.957 Å and 104.553°, respectively. It should be noted that the bond length and bond angle of the water molecules and ethanol molecules were not fixed during the simulation. According to the literature, the structure of Pri-g-C3N4 [40] and Ben-g-C3N4 [37] were made by using Visualizer tools in Materials Studio and were also optimized by COMPASSII force field. In this study, two simulation processes were carried out, where in the first case, the diffusion behaviors of different solvent molecules (water, ethanol, hydrogen, and neon) across the triangular nanopores (lattice defects of Pri-g-C3N4) were simulated. As shown in Fig. 2-a, the atoms of Pri-g-C3N4 were frozen in the simulation models, as the nanosheet is rather rigid in a real case [40]. The solvent (water, hydrogen (kinetic diameter of 2.89 Å [41]), or neon (kinetic diameter of 2.79 Å [42])) was filled into a simulation box (Lx = 49 Å, Ly = 49 Å, Lz = 74 Å) with the Amorphous Cell Modules [43] on the Pri-g-C3N4 membrane, and a vacuum layer of 20 Å was present under the membrane. The number of solvent molecules was 4203. This simulation process was subjected to 1 ns NVT simulation (time step = 1 fs, frame output for every 5000 steps, T = 298 K). In the other simulation, the dynamic properties of water molecules between two nanosheet layers were calculated. The distance between two layers was 20 Å, where each layer contains three Pri-g-C3N4 or Ben-g-C3N4 (42.84 Å × 51.97 Å) nanosheets (the distance between each nanosheet: 3.38 Å [44,45]), and there were two reservoirs on each side along the Z-axis. The number of water molecules in the bulk phase was 4000, and the density was 1.0 g/ cm3. This system was subjected to the steepest-descent energy minimization of 2000 steps, followed by a 2 ns NVT equilibrium simulation (time step = 1 fs, frame output for every 10000 steps, T = 298 K). Then, the production NVT simulation was performed for 1 ns (time step = 1 fs, frame output for every 5000 steps, T = 298 K). The velocity-scale thermostat with a temperature difference of 1.0 K was employed. To start the simulation the molecules were randomly distributed in the box, and all of the initial configurations were constructed by the Amorphous Cell Modules.
diffusion paths improved the separation factor of these molecules. On the other hand, some researchers insisted that g-C3N4 separated ethanol and water due to different solvent-interface interactions between ethanol/water (the phenomenon of ultralow friction) and g-C3N4 [27]. Experimental evidence confirmed that the tri-s-triazine units in gC3N4 connected with nitrogen (N) atoms by the single bond (N-(C)3) were eliminated easily to adjust the pore size of the membrane. For example, Papailias et al. [28] exfoliated g-C3N4 via chemical treatment using concentrated sulfuric acid. The X-ray photoelectron spectroscopy (XPS) results demonstrated the breaking of a single bond (N-(C)3), which led to the cleavage of the g-C3N4 network. Simultaneously, Wang et al. [17] reported on molecular dynamics (MD) simulations of water and hexane through g-C3N4 nanosheets with artificial nanopores. Their results showed that the velocities of water molecules through twolayered g-C3N4 nanosheets were faster than that of n-hexane molecules despite higher viscosity of water compared to n-hexane. This indicates the ultralow friction between g-C3N4 and water. Different from graphene [29] (thinnest 2D material), the N atoms in g-C3N4 have possibilities to form hydrogen bonds with water molecules or other hydroxyl group-containing solvent molecules of inter-layer. Therefore, the mass diffusion mechanism of water or ethanol molecules in the g-C3N4 layer may present different behavior than in the graphene layer. It is known that the structural [30] and dynamical properties [31] of water in a confined space [32] are different from those in bulk water. It was found that hydrogen bond (H-bond) networks between water molecules were destroyed and new solvent-interface interactions [33] were discovered because of confinement. Chakra borty et al. [34] explored the behavior of water in confined space inside the carbon nanotubes (CNTs) and found out that the relaxation time of the orientational dynamics of water molecules (150 fs) was shorter than that in bulk water (2.5 ps). They concluded this as only one hydrogen bond was formed for each water molecule and rotation about CNTs axis was relatively free. In addition to the experimental supports, molecular dynamics (MD) simulations are widely applied to investigate water permeation in confinement and notably reports are based on the simulation results. Wei et al. [35] concluded that the fast flow of water transport across GO membranes might be primarily due to the porous microstructure of GO, including wrinkles (leading to wide channels), holes, and interlayer spaces. Jordan et al. [36] showed that the permeability of water in GO depends not only on the size of the inlet, but also on the diffusion path in GO. Moreover, water was well mixed when transported between the layers of the graphene membrane. In this investigation, MD simulations were performed to explore the diffusion of water and ethanol molecules through sheet-typed g-C3N4 (pristine g-C3N4, Pri-g-C3N4) and benzene-doped distorted g-C3N4 (Beng-C3N4) free-standing membranes and Fig. 1-a and 1-b exhibit the structures of these two membranes. According to Kim et al. [37], the nitrogen of tertiary amine in g-C3N4, connected by three heptazine rings, is partly substituted (25%) with a benzene molecule in Ben-gC3N4. The permeability of water, ethanol, hydrogen and neon, was first investigated by employing MD simulations. Then, the diffusion mechanisms of water and ethanol molecules in Pri-g-C3N4 were obtained by analyzing the density profile, the residence time, and the diffusion coefficient of water molecule, etc. For comparison, the diffusion of water and ethanol molecules through Ben-g-C3N4 free-standing membranes was also investigated.
3. Results and discussion 3.1. Diffusion behaviors of molecules across the Pri-g-C3N4 intrinsic lattice defect At first, the permeability of different solvent molecules (water, ethanol, hydrogen and neon) across the lattice defect of Pri-g-C3N4 was investigated. As shown in Fig. 3, the simulation results showed that water molecules could hardly pass across the defect of the Pri-g-C3N4 membrane, while hydrogen could easily pass through. Meanwhile, there were 77 Ne atoms across the Pri-g-C3N4 membrane after 1 ns NVT simulation, although the diameter of the neon atom is similar to that of water molecule. The simulation box was filled with the same amount of water molecules, hydrogen molecules or neon atoms. In other words, the partial pressure of hydrogen or neon was much larger than water. Therefore, an external force above the water box was applied along the Z-axis to check the effect of pressure. When the external force was 50 kcal/mol/Å (≈20 MPa), no H2O molecule was found out across the Pri-g-C3N4 membrane. As shown in Fig. 4-c, the water molecules still accumulated in the intrinsic defects of Pri-g-C3N4, and the hydrogen atoms of water pointed to the nitrogen atoms of Pri-g-C3N4. It is implied that the hydrogen bonds may form between water molecules and nitrogen atoms in Pri-g-C3N4, which hinder water molecules passing across the Pri-g-C3N4 membrane. To confirm that such a hindrance is undoubtedly due to the hydrogen bonding, the permeability of a
2. Models and simulation methods All molecular dynamics simulations were performed using the Materials Studio software package. The force field of simulation systems was COMPASSII [38], and the force field gave charges of all the particles. COMPASSII [39] force field was a significant extension to the COMPASS force field, which was an ab initio force field. The parameters of COMPASSII force field were derived from quantum mechanics calculations and optimized to fit experimental data. Periodic boundary 82
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Fig. 1. Perspective views of Pri-g-C3N4 (a) and Ben-g-C3N4 (b). Fig. 2. Snapshots of the MD simulation systems. (a) Water across Pri-g-C3N4 membrane; (b) 5 wt% HCN across Pri-g-C3N4 membrane(The red and white atoms represent the oxygen and hydrogen atoms in water molecules, respectively. The blue and gray atoms represent the carbon and nitrogen atoms in HCN molecules, respectively.). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
3.2. Dynamic properties of different molecules between two nanosheet layers
straight rod-shaped molecule, i.e., hydrogen cyanide (HCN) [46] was simulated for comparison. Different from the V-shaped water, the rodshaped HCN may be quickly passed across the membrane due to its structure. In this run, the simulation box was filled with 5 wt% HCN (solvent is water) as shown in Fig. 2-b. Table 1 illustrate the simulation results and it could be seen that neither HCN nor H2O was found to pass across the Pri-g-C3N4 membrane after 1ns NVT simulation. Moreover, the permeability of pure HCN across the Pri-g-C3N4 membrane was simulated. As shown in Table 1, HCN molecules could hardly pass across the defects of the Pri-g-C3N4 membrane. It is verified that both water and HCN molecules can form hydrogen bonds with nitrogen atoms in Pri-g-C3N4. Thus, no water or HCN molecules can pass across the Pri-gC3N4 intrinsic lattice defect. However, several investigations [23,47,48] reported on the incorporation of g-C3N4 into dense polymeric membranes, such as polyvinyl alcohol (PVA) or SA membrane, to increase the permeability of solvent and selectivity of organic compounds. The experimental results showed that incorporating nonporous g-C3N4 into polymer only increased the flux of the membrane but not the separation factor. Thus, it becomes necessary to understand the diffusion mechanism of the molecules across Pri-g-C3N4 free-standing or composite membranes.
Fig. 4 shows the equilibrium configurations of the sheet-typed Pri-gC3N4 free-standing membranes. According to an earlier report [49], the inter lamellar distances of Pri-g-C3N4 and Ben-g-C3N4 confinement were all set at a width of 20 Å to ensure free movement of ethanol molecules in confinement. As shown in Fig. 4-b, the ethanol molecules move from the reservoirs into the confinement and preferentially adsorb onto the Pri-g-C3N4 nanosheets. Fig. 5 exhibits the equilibrium configurations of distorted-typed carbon nitride (Ben-g-C3N4) free-standing membranes which shows a similar phenomenon of preferential adsorption of ethanol molecules onto the Ben-g-C3N4 nanosheets. As shown in Fig. 5-c and 5-d, the doped benzene ring prevents the adsorption of water molecules onto the membrane. 3.2.1. Density distributions of water and ethanol molecules Detailed investigations were carried out on the density distribution profiles of water molecules confined in Pri-g-C3N4/Ben-g-C3N4 layers along the Y-axis, which were perpendicular to the Pri-g-C3N4/Ben-gC3N4 surface. Fig. 6 shows the density profiles of water and ethanol molecules inside a confined region of 20 Å for pure water or 5 wt% 83
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repelling water molecules. Fig. 6-c indicates that hydroxyl groups of ethanol are closer to the surface of Pri-g-C3N4 than methyl groups of ethanol. This is because that hydrogen atom of the hydroxyl group of ethanol forms hydrogen bonds with nitrogen atoms in Pri-g-C3N4. This result displays the differences in the absorption of ethanol in Pri-g-C3N4 and graphene. Gao et al. [50] reported on the simulation of alcohols absorbed by the graphene layer, presents that methyl groups in ethanol are closer to graphene than hydroxyl groups in ethanol. This is due to the absence in the formation of hydrogen bonds between ethanol and graphene surface. Fig. 6-e and 6-f indicate the density distributions of water in 5 wt% ethanol solution in the confinement of Pri-g-C3N4. It could be noted from these figures that the density distributions of water molecules in the mixture are similar to that of pure water. Since the hydroxyl groups of ethanol form hydrogen bonds with the surface of Pri-g-C3N4, the methyl groups on the other part of ethanol have less influence on the density distributions of water molecules. 3.2.2. Normalized distributions of orientation angle To analyze the configuration of ethanol in confinement, angles α and β are defined as the orientation angles of CH3–CH2 and O–H bonds of ethanol, respectively to the Y-axis. Fig. 7-a and 7-b are the distributions of these angles. In 20 Å Pri-g-C3N4 confinement, both α and β in Fig. 7-a show a maximum peak at 90°. This indicates that the CH3–CH2 and O–H bonds prefer to be parallel to the surface of Pri-gC3N4 membrane. Compared to β, it could be noted that the angle α has a broader distribution, which may be due to the formation of the hydrogen bond between ethanol and membrane. This indicates that the interaction between Pri-g-C3N4 membrane and the hydroxyl group is stronger than that of the methyl group. Fig. 7-a exhibits broad distributions of the orientation angles, α and β. This indicates that ethanol molecule is affected not only by Pri-g-C3N4 but also by the surrounding water molecules. In 20 Å Ben-g-C3N4 confinement, Fig. 7-b presents that the distributions of orientation angles of ethanol molecules are to a certain extent different than that in Fig. 7-a. It could be noticed that the intensity of the maximum peak around 90° of the orientation angle β was less for Ben-g-C3N4 than that in Pri-g-C3N4 confinement. Two other intense distributions of angle β appear around 54° and 126° (Fig. 7-b). These results confirm that the distorted triazine-ring (Ben-g-C3N4) affects the configuration of ethanol molecules, as the protuberant benzene in Ben-g-C3N4 causes triazine-ring to protrude from the surface of the membrane (inset snapshots of Fig. 7-b).
Fig. 3. MD simulations of water, hydrogen, neon, etc. across Pri-g-C3N4 membrane as a function of time; the y coordinate is the number of molecules across the membrane and t = 0 corresponds to appearance of solvent molecules on the permeated side.
ethanol solution. For pure water, Fig. 6-a indicates that the density distribution of water is symmetric about the center of the confinement which signifies a distinct layered structure in Pri-g-C3N4. A peak of maximum density peak for water appears around ∼0.36 nm away from the surface of Pri-g-C3N4, indicating that water molecules form a high adsorption layer. On the other hand, Fig. 6-b shows a different distribution of water molecules on the surface of Ben-g-C3N4 (Fig. 6-b). The position of maximum density peak also appears around ∼0.36 nm, but the maximum intensities are significantly reduced than that in Prig-C3N4. This could be due to that the protuberant benzene ring destroys the adsorption of water molecules. Fig. 6-c and 6-d demonstrate the density distributions of –CH3 and –OH groups of an ethanol solution of 5 wt% (Nethanol/Nwater = 150/ 7525) in Pri-g-C3N4 confinement. A peak of maximum density peak for hydroxyl groups of ethanol appears around ∼0.27 nm away from the surface of Pri-g-C3N4, while that of methyl groups of ethanol appears around ∼0.36 nm. Compared to water molecules (∼3.6 nm), hydroxyl groups of ethanol is closer to the surface of the Pri-g-C3N4. It's because that ethanol molecules move from the reservoirs into the confinement and preferentially adsorb onto the Pri-g-C3N4 nanosheets, thereby
3.2.3. Movement of solvent molecules between nanosheet layers To further understand the fluidic behavior of water in Pri-g-C3N4 confinement, the residence time of water molecules was calculated. Compared with water molecules in the center of confinement, which
Fig. 4. Snapshots of equilibrium configurations (side views) for (a) pure water in Pri-g-C3N4, (b) 5 wt% ethanol in Pri-g-C3N4, and (c) the configuration of confined water inside Pri-g-C3N4. 84
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remains inside that region. As shown in Fig. 8, the CA(t) of water molecules in Ben-g-C3N4 decays faster than that in Pri-g-C3N4. It denotes that the residence time is shorter in Ben-g-C3N4 and the mobility of water molecules is larger. Table 2 lists the calculated diffusion coefficients (D), mean residence times of water (τwater) and mean lifetimes of hydrogen bond (τHB) for water and ethanol molecules. Based on the slope of the mean square displacement (MSD), the calculated diffusion coefficient of bulk water was 5.51 × 10−5 cm2 s−1, which agrees well with the reported literature value [53,54]. The data in Table 2 shows that in both pure water and 5 wt% ethanol systems the diffusion coefficient of water in Ben-g-C3N4 is larger than that in Pri-g-C3N4. This result is consistent with the results of residence time. Thus, water molecules in Ben-g-C3N4 are more mobile than in Pri-g-C3N4. Therefore, it could be concluded that the pure water flux of Ben-g-C3N4 system is larger than that of Prig-C3N4 system. Both the density distributions (Fig. 6-b) and orientation angle distributions (Fig. 7-a) of water and ethanol molecules indicated that hydroxyl groups of ethanol was closer to the surface of the Pri-g-C3N4 surface. Compared to hydroxyl groups of ethanol, methyl groups in ethanol can not form hydrogen bonds with water molecules, thus the methyl groups of ethanol have less influence on the movement of water molecules. Table 2 shows that the diffusion coefficient of water molecules decreased with the addition of ethanol. This may be due to that the addition of ethanol reduces the movement of water molecules in space. On the other hand, Table 2 demonstrates that the diffusion coefficients of water are significantly larger than that of ethanol. This reveals that with incorporation of Pri-g-C3N4 into dense polymeric membranes, the permeability and separation ability were improved. The separation of ethanol and water was mainly due to the interactions between water/ethanol molecules and Pri-g-C3N4 but not due to the size effect.
Table 1 Table of External Force (Fexternal), Kinetic Diameter (Dk), Initial number of molecules (N0), and Number of molecules across the membrane (Nt) for water, hydrogen, neon, hydrogen cyanide, and ethanol, etc. molecules.
H2 Ne H2Oa H2Ob H2Oc H2Od HCNe HCNf C2H5OH a b c d e f g h
Fexternal/kcal/mol/Å
Dk/Å
N0
Nt
Referencesg
0 0 0 50 0 0 0 0 0
2.89 2.79 2.80 2.80 2.80 2.80 2.92h 2.92h 4.50
4203 4203 4203 4203 4065 4067 143 4203 83
1060 77 0 0 0 0 0 0 0
[39] [42] [26] [26] [26] [26] [46] [46] [25]
Pure water. Pure water under pressure. Water in 5 wt % HCN mixture system. Water in 5 wt % ethanol mixture system. HCN in 5 wt % HCN mixture system. Pure HCN. Molecular kinetic diameter references. Section diameter of HCN.
were similar to bulk water, the high adsorption confined water layer may have different movement behaviors. Referring to Fig. 6-a and 6-b, four particular regions along Y direction were selected: (−9.0677 to −5.3240 Å) and (5.2035–8.9734 Å) for Pri-g-C3N4 system; (−9.8558 to −4.5255 Å) and (6.5740–9.7931 Å) for Ben-g-C3N4 system, respectively. The normalized residence autocorrelation function was calculated using equation (1), where A(t) was unity when water was inside a particular region at time t and A(t) was zero otherwise. Also, the angular brackets denote ensemble average. The mean residence time was calculated by the integration of the residence autocorrelation function (equation (2)) [51,52].
CA (t ) =
τ=
∫0
t
〈A (t )⋅A (0) 〉 〈A (0)⋅A (0) 〉
(1)
CA (t ) dt
(2)
3.2.4. Microstructural analysis for analyzing the capacity of molecule mobility Related to the mobility of water, the H-bond network was considered to have a significant influence, and thus the amount of H-bond between water molecules in different membrane systems was calculated. In this system, the H-bond was determined when the distance between the hydrogen atom and the acceptor was less than or equal to 2.5 Å and the angle formed by the donor, hydrogen and acceptor atoms was larger than 139° [50,55]. The length of the hydrogen bond is defined as the distance between the hydrogen atom of donor and the acceptor atom.
The time correlation functions and residence times of water molecules in the particular region are shown in Fig. 8. The CA(t) decays from 1 to 0 with an increase in the simulation time, indicating that water molecules continuously entering and moving out of the particular region. The faster CA(t) decays to zero, the shorter is the time that water
Fig. 5. Snapshots of equilibrium configurations (side views) for (a) pure water in Ben-g-C3N4, (b) 5 wt% ethanol in Ben-g-C3N4; the configuration of confined water inside (c) Protuberant benzene, (d) depressed benzene. 85
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Fig. 6. Density profiles of molecules in 20 Å confined region: (a) pure water in Pri-g-C3N4, (b) pure water in Ben-g-C3N4, (c) methyl (green line) and hydroxyl groups (blue line) in ethanol in Pri-g-C3N4, (d) methyl (green line) and hydroxyl groups (blue line) in ethanol in Ben-g-C3N4, (e) water in the mixture in Pri-g-C3N4, and (f) water in the mixture in Ben-g-C3N4. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
averages. As shown in Table 3, it could be noted that the total number of Hbonds in Ben-g-C3N4 is 150, which is larger than that of 103 in case of Pri-g-C3N4. The calculated mean numbers of H-bonds (number of hydrogen bond/number of N atoms), however are in the reverse order: 0.96 in Pri-g-C3N4 and 0.88 in Ben-g-C3N4. The hydrogen bonds formed between water molecules and membrane lead to the reduction in the mobility of water. The smaller mean number of H-bonds indicates larger mobility of water. An analysis in the hydrogen bond suggestes that water moves faster in Ben-g-C3N4 than in Pri-g-C3N4. This agrees
Then, the lifetime autocorrelation function was calculated via equation (3) and the value of the lifetime autocorrelation function was integrated to acquire mean lifetime (equation (4)) [51,55].
CHB (t ) =
τ=
∫0
t
〈H (t )⋅H (0) 〉 〈H (0)⋅H (0) 〉
CHB (t ) dt
(3) (4)
Where H(t) was unity when H-bond was formed as time t and H(t) was zero otherwise. Also, the angular brackets denote ensemble 86
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Fig. 7. Normalized orientation angle (α, β) distributions of confined ethanol molecules within the 20 Å Pri-g-C3N4 (a) and Ben-g-C3N4 (b) for 5 wt% ethanol−water mixture. The inset snapshots show the structure of α and β. Table 3 Table of total number of H-bond formed by water molecules and different membrane surfaces(NT), number of N atoms forming hydrogen bonds on the membrane(NN), mean number of H-bond formed by water molecules and different membrane surfaces(NM) and mean value of H-bond length formed by water molecules and different membrane surfaces(LM).
g-C3N4 Ben-g-C3N4
NT
NN
NM
LM
103 150
107 170
0.96 0.88
2.20 2.25
(n = 1, 2, 3, 4) represents the formation of the number of hydrogen bonds for a single water molecule to its surrounding water molecules. From Fig. 9-b, it is seen that water molecules in Ben-g-C3N4 have the largest proportion of forming one hydrogen bond. On the other hand, the confined water in Pri-g-C3N4 has the largest proportion of forming three hydrogen bonds. This result shows that a water molecule is less restricted by the surrounding water molecules in Ben-g-C3N4 than that in Pri-g-C3N4 confinement. In addition, the analysis of H-bonds length suggested that H-bonds formed by water molecules and water molecules in Pri-g-C3N4 confinement were most stable (the data in Table 4).
Fig. 8. Intermittent time correlation functions CA(t) of water molecules and corresponding residence times in particular region.
4. Conclusions
well with the trends of the calculated diffusion coefficients of water in these two systems. Fig. 9-a exhibits the calculated correlation functions and their corresponding lifetimes of H-bonds. It shows that the lifetime in Pri-g-C3N4 confinement is larger than that in Ben-g-C3N4. Both the lifetimes and mean length (the data in Table 3) of the H-bond suggests that H-bonds formed by water molecules and Pri-g-C3N4 are more stable, thereby causing water moves slower through Pri-g-C3N4 layers. As shown in Table 4, the mean number of H-bonds formed by water molecules and water molecules in Ben-g-C3N4/Pri-g-C3N4 confinement is less than that of in bulk water. To explain the reason for the decrease in the number of H-bonds, the distribution of H-bonds of water molecules in bulk phase, Pri-g-C3N4 and Ben-g-C3N4 were investigated (Fig. 9-b). Fn represents the percentage of water molecules while n
In this study, MD simulations were successfully performed to explore the diffusion mechanisms of H2O and C2H5OH molecules in sheet-type Pri-g-C3N4 and distorted-type carbon nitride (Ben-g-C3N4) free-standing membranes. For g-C3N4 free-standing membranes, although Pri-g-C3N4 has intrinsic lattice defect, which is higher than the kinetic diameter of water, the hydrogen bonds formed between water and Pri-g-C3N4 hinder the passage of water molecules across the Pri-g-C3N4 sheet. Based on the diffusion coefficient analysis of water and ethanol molecules in Pri-gC3N4 interlayers, an improvement in the permeability and selectivity with the incorporation of Pri-g-C3N4 into dense polymeric membranes is mainly attributed to the separation of interlayer diffusion. Thus, it
Table 2 Table of Diffusion Coefficient (D), Mean Residence Times (τwater), and Mean Hydrogen Bond Lifetimes (τHB) for water molecules and ethanol molecules.
pure water 5 wt % ethanol
g-C3N4 Ben-g-C3N4 g-C3N4 Ben-g-C3N4
Dwater/( × 10−5 cm2 s−1)
Detanol/( × 10−5 cm2 s−1)
τwater/ps
τHB/ps
4.46 4.87 2.24 2.41
/ / 1.04 0.68
25.05 18.23 32.30 26.92
2.57 2.39 2.90 2.72
87
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Fig. 9. (a) Intermittent time correlation function CHB(t) of H-bond and corresponding lifetime in confinement. (b) Percentage of water molecules (Fn) with n (n = 1, 2, 3, 4) hydrogen bonds.
Chemistry of Low-Dimensional Materials (JSKC17005). Also, the authors would like to express their gratitude to EditSprings (https://www. editsprings.com/) for the expert linguistic services provided.
Table 4 Table of mean number of H-bond formed by water molecules and water molecules (NW), Maximum value of H-bond length (Lmax) formed by water molecules and water molecules, Minimum value of H-bond length (Lmin) formed by water molecules and water molecules and Mean value of H-bond length (Lmean) formed by water molecules and water molecules.
g-C3N4 Ben-g-C3N4 H2O
NW
Lmax
Lmin
Lmean
1878a 1849a 2198a
1.94 1.94 1.92
1.89 1.90 1.81
1.92 1.93 1.89
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could be concluded that Pri-g-C3N4 could be selected to separate ethanol and water mainly owing to the separation of interlayer diffusion and not due to the size of inherent lattice defect via sieve effect. For different configurations of g-C3N4, pure water in sheet-typed Pri-g-C3N4 confinement flows slower than that in distorted-type g-C3N4 (Ben-g-C3N4), which is based on the diffusion coefficient as well as the residence time of water molecules. More specifically, the residence time of water molecules in confinement decays in the order of, Pri-gC3N4 > Ben-g-C3N4, whereas the diffusion coefficient of water molecules decays in the order of, Ben-g-C3N4 > Pri-g-C3N4. Also, the lifetime of H-bonds in Ben-g-C3N4 is shorter than that in Pri-g-C3N4, thereby the hydrogen bonds between water molecules in Ben-g-C3N4 confinement are unstable. Due to this, the water molecules in Ben-gC3N4 have the greatest capacity in mobility. For ethanol/water binary mixtures, Pri-g-C3N4/Ben-g-C3N4 surfaces preferentially adsorb ethanol, and the hydroxyl group of ethanol is closer to the membrane surface. More specifically, the addition of ethanol squeezes the water molecules in space, making the actual confinement size to become smaller, as hydroxyl groups in ethanol destroy the hydrogen bonding network of water molecules rarely.
Acknowledgments The authors are grateful for the financial support of the National Natural Science Foundation of China (No. 21878118, 21406082, 21476094), Natural Science Foundation of the Jiangsu Higher Education Institutions of China (18KJA530003), Natural Science Foundation of Jiangsu Province (BK20171268), Jiangsu Province Qing Lan Project, and the open project program of Jiangsu Key Lab for 88
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