Effect of adsorption alcohol layers on the behaviours of water

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Effect of adsorption alcohol layers on the behaviours of water molecules confined in graphene nanoslit: A molecular dynamics study Qingwei Gao, Yudan Zhu, Yang Ruan, Yumeng Zhang, Wei Zhu, Xiaohua Lu, and Linghong Lu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02038 • Publication Date (Web): 01 Sep 2017 Downloaded from http://pubs.acs.org on September 2, 2017

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Effect of adsorption alcohol layers on the behaviours of water molecules confined in graphene nanoslit: A molecular dynamics study Qingwei Gao, Yudan Zhu*, Yang Ruan, Yumeng Zhang, Wei Zhu, Xiaohua Lu, Linghong Lu College of Chemical Engineering, State Key Laboratory of Materials-oriented Chemical Engineering, Nanjing Tech University, Nanjing 210009, P.R. China

* Authors to whom correspondence should be addressed: Yudan Zhu, Email address: [email protected]

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Abstract With the rapid development of two-dimensional (2D) nanomaterial, the confined liquid binary mixture has attracted increasing attention, which has significant potential in membrane separation. The alcohol/water is one of the most common systems in the liquid-liquid separation. As one of the most focused systems, recent studies found that ethanol molecules were preferentially adsorbed on the inner surface of pore wall and formed an adsorption ethanol layer under 2D nanoconfinement. To evaluate the effect of alcohol adsorption layer on the mobility of water molecules, molecular simulations were performed to investigate four types of alcohol/water binary mixtures confined under a 20 Å graphene slit. Residence times of the water molecules covering the alcohol layer were in an order of methanol/water < ethanol/water < 1-propanol/water < 1-butanol/water. Detailed microstructures analysis of hydrogen bonding (H-bond) network elucidated the underlying mechanism at the molecular scale that a small average number of H-bonds between the preferentially adsorbed alcohol molecules and the surrounding water molecules could induce a small degree of damage to the H-bond network of the water molecules covering the alcohol layer, resulting in the long residence time of the water molecules.


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INTRODUCTION In recent years, two-dimensional (2D) nanoconfinement has attracted increasing attention.1, 2 With the development of nanotechnology, a large number of 2D materials with layered lamellar structure, such as graphene nanosheets, hexagonal boron nitride (hBN), transition metal dichalcogenides, phosphorenes and MXenes, were successively developed.3-5 Nowadays, 2D materials are extensively used as separation membrane materials due to their unique atomic-scale thickness and exhibition of extraordinary performance, which has significant potential to realise separation at the molecular scale.6-9 Although 2D materials are only emerging, the theoretical study of nanoconfined slit model can be traced back to decades ago. Gubbins et al.10, 11 proposed the slit model to reflect the characteristic pore of porous materials, such as activated carbon. Their pioneer work12, 13 adopted the slit model to investigate the transport diffusion of gas in porous carbon materials to distinguish the contribution of viscous and diffusive transport. Debenedetti et al.14 adopted the slit model to evaluate the pore wall hydrophilicity effect on the phase change of water molecules under confinement. The static structural variation of water molecules was found to be more insensitive to pressure for hydrophilic slit than the hydrophobic one. Most of the theoretical studies focused on the confined properties of gas mixture or single component liquids. However, theoretical studies of the liquid binary mixture under 2D confinement are relatively rare, which are essential in membrane separation applications, such as small organics extraction and water purification. The separation of the alcohol/water mixture under confinement is significant in the fields of life, medicine and chemical engineering.15, 16 This separation is also one of the most common systems in the liquid-liquid separation. Recently, the behaviours and structures of the alcohol/water binary mixture under 2D nanoconfinement were probed by advanced experimental methods, such as atomic force microscopy17 and scanning force microscopy18, 19. The observations showed that the static structural


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properties of the alcohol/water under 2D nanoconfinement were unique and quite different from their bulk counterpart. The molecular simulation investigations of Kommu et al.20 compared the dynamic properties of ethanol/water mixture confined in graphene and hBN slit pores, respectively. They found that the hydrophilicity of wall had significant effects on the adsorption behaviours of ethanol molecules, as well as the mobility of water molecules under nanoconfinement. It is well-known that the interactions between the components in liquid binary mixtures is much stronger compared to that of the gas binary mixture. For a liquid binary mixture under 2D nanoconfinement, one of the components can be affected not only by the pore wall hydrophilicity but also the other confined component. The effects derived from the other component should be relevant to the separation behaviours. In our previous studies,21-25 we performed a series of molecular dynamics (MD) simulations to investigate the effects of pore size and pore wall chemical properties on confined pure water molecules and pure ethanol molecules, respectively. Systematic studies showed that the pore wall chemical properties played more significant influences on the microstructure variation of fluid molecules than that of pore size effect. Recently, Zhao et al.26 investigated ethanol/water mixture behaviours under a 20 Å graphene slit pore by MD simulations. They found that ethanol molecules were preferentially adsorbed on the inner surface of pore wall and formed an adsorption ethanol layer. Essentially, the adsorption alcohol layer can be equivalent to a new “interface” for the water molecules covering it. Does this adsorbed alcohol layer affect the mobility of water molecules that cover the alcohol layer? We further investigated the effects of alcohol layers on the water molecules under 2D nanoconfinement in this work to answer this question. Molecular simulations were performed to investigate four types of alcohol/water binary mixtures confined under a 20 Å graphene slit.


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MODELS AND SIMULATION METHODS The front view of the graphene-based lamellar slit model employed in this study is shown in Figure 1, which comprises six 40 Å × 60 Å parallel graphene sheets with two reservoirs at each side. The interlayer space 20 Å was selected because such was proven to facilitate the formation of the preferential alcohol adsorption layer and this gap size could ensure each kind of alcohol molecule can form an adsorption layer for better comparison. Besides, it is a typical size for nanoslit in graphene-based membranes and could be used in many applications (e.g., water purification, pharmaceutical and fuel separation) that require precise separation of large molecules and small waste molecules.2,

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Four common alcohol/water mixtures, namely

methanol/water, ethanol/water, 1-propanol/water and 1-butanol/water, were chosen to investigate the effect of alcohol adsorption layer on the mobility of water molecules at the molecular scale. Each case was represented by a specific label, namely Me_W, Et_W, Pro_W and Bu_W, respectively. The mole fraction of alcohol is 5% (Nalcohol/Nwater = 200/3800). All the initial configurations are constructed by the PACKMOL software package, and all the molecules are randomly distributed in the box.

Figure 1. Front view of the simulation box showing a graphene-based lamellar slit in the centre and the two reservoirs on both sides of the slit. (The grey atoms represent the carbon atoms in graphene. The green and blue atoms represent the oxygen and carbon atoms in alcohol molecules, respectively. The pink atoms represent the oxygen atoms in water molecules.)

The SPC/E model27 was applied for describing water molecules, whereas the OPLS-AA force field28 was used for alcohol molecules. The force field parameters are


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listed in Table 1. The particle−particle particle-mesh method29 was used to calculate the long-range electrostatic interaction with a cut-off for a real space of 10 Å. The SHAKE algorithm was implemented to freeze the O−H bond of water whereas the alcohol molecules were flexible. The simulation system was periodic in all three directions. After energy minimization, all systems were first equilibrated in the NPT ensemble for 2 ns and NVT ensemble for 35 ns. Every production run was performed for 5 ns in NVT ensemble with 1 fs time step and saved every 1 ps. The temperature in the system was controlled by Nosé-Hoover thermostat30, and the simulated temperature was maintained at 300 K. The last 5 ns of trajectory were collected for further analysis. In this work, all MD simulations were performed by the LAMMPS software package.31 The visualization was generated via Open Visualization Tool (Ovito) package32. Table 1. Force field parameters used in the simulation28 ε/kcal mol−1

Site C(graphene) C(CH3OH, RCH2OH) C(CH3ROH) C(RCH2ROH) H(CH3OH) H(CH3ROH, RCH2ROH) O(OH) H(OH) OW HW

0.07 0.066 0.066 0.066 0.03 0.03 0.17 0.0 0.1554 0.000

σ/Å 3.55 3.5 3.5 3.5 2.5 2.5 3.12 0.0 0.636 0.000

q/e 0.0 0.145 −0.18 −0.12 0.04 0.06 −0.683 0.418 −0.834 0.417

RESULTS AND DISCUSSION We first investigated the spatial distributions of the molecules nanoconfined in graphene slit. Then, we calculated the residence times of the water molecules covering the adsorbed alcohol layers to evaluate the effect of one component on the mobility of the other for a nanoconfined binary mixture. Finally, the detailed network


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microstructures of H-bonds were further analysed to elucidate underlying mechanisms at the molecular scale. Spatial distributions of the molecules nanoconfined in graphene slit Figure 2 lists the average number of alcohols and water molecules inside the 2D graphene nanoconfined channel after the systems reached equilibrium. The numbers above the columns are the proximal integer after ensemble average. Although the initial alcohol concentration is as low as 5%, the nanoslit has a higher preference for alcohol molecules than water molecules. All kinds of studied alcohols concentrated into the nanoslits and formed higher concentration compared to the mixtures outside the nanoslits. The molar concentration inside the slit for Me_W, Et_W, Pro_W and Bu_W are 8.84%, 13.83%, 17.52% and 19.20%, respectively. Similarly, Tian et al.33 also observed the preferential adsorption of alcohol into the single-walled carbon nanotube (SWNT) with a pore diameter of 17.4 Å. They explained that in such energetic phenomenon, the preferential adsorption of alcohols over water inside SWNTs can be attributed to the stronger dispersion interactions of alcohol molecules with SWNTs than water molecules. Moreover, Figure 2 shows that the number of water molecules in the slit decreases with the increase of carbon chain of alcohols, whereas the number of alcohol molecules is insignificantly affected and the error bars in Figure 2 represent fluctuation of molecules in slit. The space inside the slit for accommodating water molecules is bound to be reduced because the number of alcohol molecules does not change much for the four cases. Hence, the entry of water molecules was restricted as the chain length of the alcohol molecules increases.


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Figure 2. Number of average molecules inside the graphene slits (hollow columns represent the water molecules and the filled columns represent the alcohol molecules)

The simulation snapshots of the equilibrium configuration of each case are displayed in Figure 3. For the four cases, the alcohol molecules were preferentially adsorbed onto the inner wall of the graphene slit and formed an adsorbed layer. Most water molecules were concentrated between the two adsorbed alcohol layers. These observations agreed with the results obtained in Figure 2. Experimentally, Zhang et al.34 adopted sum-frequency vibrational spectroscopy and demonstrated that alcohols, such as methanol, ethanol and 1-propanol, were always preferentially adsorbed at hydrophilic fused silica and hydrophobic octyltrichlorosilane-covered surfaces. When Dai et al.35 investigated the friction coefficient of ethanol and water molecule confined in 20 Å width graphene slits by MD simulation, similar preferential adsorption of alcohols was also observed. As the chain length of alcohol molecules increases, the layered distribution of alcohol and water molecules confined in the slits become more obvious, which might be attributed to the decreasing polarity of alcohol molecules. This finding also suggests that the absorption layer structures of different alcohols were varied from each other, which inevitably resulted in different influences on the water molecules covering the absorbed alcohol layer.


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Figure 3. Snapshots of equilibrium configurations (side views) for (a) Me_W, (b) Et_W, (c) Pro_W and (d) Bu_W in contact with the graphene slit.

We analysed the density profiles of molecules in the direction perpendicular to the graphene surface to quantitatively describe the preferred distribution of molecules in a confined slit. For each type of alcohol molecules, the density profiles of methyl and hydroxyl groups were respectively analysed. As shown in Figure 4 (a–d), two obvious peaks in both density profiles concentrated near the slit walls for all the studied cases, indicating that the four types of alcohol molecules can form a nearly discrete layer near the graphene wall. These phenomena coincided with the findings in Figure 3. The peak positions of methyl groups were closer to the graphene walls than that of hydroxyl groups. These observations can reflect the preferential orientations of the alcohol molecules, with the methyl group facing the graphene wall whereas the hydroxyl group is oriented towards the pore inside. Ren et al.36 employed MD simulation to study the dehydration behaviours of ethanol/water mixture confined in two hydrophobic plates and observed similar preferential orientations. For all types of alcohols studied, the distance between the methyl group of alcohols and the graphene wall are all 2.95 Å. The gap between the peak positions of methyl and hydroxyl groups is more obvious as the length of the alcohol chain increases. Moreover, compared with the sharp peak of methyl groups, the profiles of hydroxyl groups are broad, which might be attributed to the different configurations exhibited by the alcohol molecules on the graphene surfaces with the different directions of hydroxyl groups.35


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Because the positions of the oxygen atoms are closer to the centre of mass of the water molecules, the spatial distribution of confined water molecules is represented by the density distribution and the Y-Z planar density distribution of oxygen atoms, respectively. Although a certain amount of water molecule (see Figure 3) also exists in the alcohol layer, especially for the methanol, this phenomenon rapidly disappears to negligible with the increase of the alcohol chain. Moreover, this work aims to evaluate the influence of the preferentially adsorbed alcohol layer on the behaviour of water molecules covering it. For this reason, we focused on the water molecules that are distributed near the adsorbed alcohol layer for the further analysis. As shown in the insets of Figure 4 (a–d), the peaks of water molecules are heterogeneously but symmetrically distributed, suggesting that the alcohol molecules have a confined effect on the water molecules. It can also be observed in 2D density maps, where the brighter region represents the higher number density of water molecules. As the carbon chain of alcohol molecule increases, the first peak of water molecules is close to the slit centre and gradually farther away from the methyl groups. The distance between the peaks of methyl groups and water molecules are respectively 2.55, 3.30, 3.60 and 3.75 Å. Dixit et al.37 found that the hydroxyl group of alcohol molecules plays a significant role in alcohol/water mixture. Compared to the other three cases, no obvious peak of water molecules exists in the slit centre for Pro_W, which might be due to the specific effects of hydroxyl groups in alcohol molecules.


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Figure 4. Density profiles of methyl (blue line), hydroxyl groups (green line) in four alcohols in the 20 Å graphene slits (y = 0 indicates the slit centre). Insets show the density profiles (red line) and the Y-Z planar density distribution (background) of oxygen atom in water molecules covering the alcohol layer.

Residence time of water molecules on the preferential adsorption alcohol layer Residence time is extensively used to reflect the mobility of molecules in a particular region in equilibrium simulations.38-40 We focused on the water molecules on the preferential adsorption layer of alcohol molecules. In general, the density profiles of the water molecules confined in two adsorbed alcohol layers are axisymmetric. Therefore, we investigated the residence time of water molecules in the particular regions. The insert snapshot which is amplified in Figure 5(a) shows the interface of propanol in the confined channel. We focused on the layer of water above the adsorption alcohol layer, which is indicated by the insert snapshots in Figure 5(a). These particular regions are −4.95 Å < y < −1.35 Å, −5.25 Å < y < −1.35 Å, −4.95 Å < y < 0.0 Å and −5.25 Å < y < −0.90 Å for Me_W, Et_W, Pro_W and Bu_W,


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respectively. The values were selected according to the positions of the first two valleys on one side in the density profile of water molecules in insets of Figure 4. The residence autocorrelation function is calculated as

(1)

where Ow(t) represents if an oxygen atom of a water molecule stays in a particular layer at time t (ns). Ow(t) = 1 represents an atom that belongs to a particular layer; otherwise, for Ow(t) = 0. The angular brackets denote ensemble averages. CR(t) decays from 1.0 to 0.0 with the time evolution, indicating that the water molecules move into and out of the particular layer, as shown in Figure 5(a). For clarity, only the first 1 ns was presented. We further calculated the mean residence time to quantitatively assess the difference of the water molecules on the preferential adsorption layer of different alcohol molecules by integrating the values of the residence autocorrelation functions (see Figure 5(b)), which was calculated according to Eq. (2):

(2)

The residence autocorrelation function curve rapidly drops, which indicates that the water molecules staying in the particular layer loss the correlation with the initial state quickly. As shown in Figure 4(a), CR(t) rapidly decays in an order of Me_W > Et_W > Bu_W > Pro_W, and the mean residence time is 0.23, 0.27, 0.33 and 0.40 ns, respectively (see Figure 5(b)). Although the values of residence times calculate depend on the length of the integration interval in Eq. (2), but extending the calculation time do not change the order (see Figure S1). The result suggested that the mobility of water molecules has no dependence on the chain length of the alcohol molecules in the adsorption layer. The water molecules that cover the propanol molecules layer were most likely to stay whereas those on the methanol layer have the highest mobility. The difference in the mobility of these confined water molecules might have relevance to their microstructures affected by the alcohol molecules.


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Under 2D nanoconfinement, each type of alcohol molecule exhibited unique spatial distribution and thus provided different confinement spaces for accommodating the water molecules. The phenomena revealed that the unique microstructures of one type of component caused by the under confinement could significantly influence the behaviour of another component in a binary mixture. We further analysed the microstructures of confined binary mixtures in detail to explain the difference in mobility of water molecules for the four cases at the molecular level.

Figure 5. Water molecules on the preferential adsorption layer of different alcohol molecules: (a) The residence autocorrelation as a function of time and (b) the residence time. The insert snapshots show the layered structure of propanol molecules and water molecules near the graphene wall.

Detailed microstructural analysis Detailed microstructures were analysed to shed light on how the confined alcohol molecules influence the residence time of water molecules. The microstructure of H-bond networks is considered as an important and effective indicator for aqueous solution.37,

41

Firstly, H-bond microstructures between the

alcohol molecules adsorbed on the wall with the surrounding water molecules were analysed (see Figure 6). Then, we analysed the H-bond microstructures of water molecules themselves covering the alcohol molecule layers (see Figures 7, 8). In this work, the geometric criteria42 were adopted to determine the formation of H-bond,


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and three criteria were used for determining whether the two water molecules (one acts as a donor and another acts as an acceptor) form H-bond or not. (1) The distance between the oxygen atom of an acceptor molecule and that of a donor molecule should be less than 3.5 Å. (2) The distance between the oxygen atom of an acceptor and the hydrogen atom of a donor should be smaller than 2.5 Å. (3) The angle of H-O···O (the first two atoms H-O belong to a donor molecule and the third oxygen atom belongs to an acceptor molecule) should be less than a threshold value of 30°. An H-bond can be formed between the two molecules only if three criteria are simultaneously met. Given that the alcohol/water mixture formed the interfacial layered structures in nanoconfined graphene slits (see Figure 2), we first analysed the H-bond formed between the hydroxyl groups in the alcohol molecule layer and the water molecules surrounding it. The number of H-bonds in these confined cases is significantly reduced compared with those in bulk and has no obvious dependence on the bulk values (see Figure S2), suggesting that the order that we obtained before is mainly due to the confined microstructure of molecules. Figure 6 shows the average number of water−hydroxyl H-bonds per hydroxyl group of alcohol molecules covering on the graphene interface for different cases. As shown in Figure 6, the average number of H-bonds per hydroxyl group generally exhibited a decreasing tendency as the chain length of alcohol molecules increases. However, the value for Pro_W is unusual, and the hydroxyl groups of the propanol molecules have the least average H-bonds with the surrounding water molecules. The sequence is Me_W > Et_W > Bu_W > Pro_W, which is in agreement with the variation tendency of the residence time of covered water molecules in Figure 5b. The low H-bond formed between the hydroxyl group of the alcohol layer and the covered water layer resulted in a less constrained formation of microstructures of water molecules themselves by the alcohol layer. This finding is probably one of the main reasons for explaining the residence time variation tendency.


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Figure 6. Average number of water−hydroxyl hydrogen bonds per hydroxyl group of four alcohols adsorbed on the graphene interface

In the preceding section, we discussed the effect of different kinds of alcohol molecules in the residence time of water molecules nearby. Then, we explored the average persistence time of hydrogen bonds and the distribution of hydrogen bonding networks of the water molecules in the layer next to the alcohol layer to elucidate the effect of the alcohol component on the water component. We further analysed H-bond network microstructures of water molecules covering the alcohol layer. Given that the number of water molecules in the covered layers is inconsistent for different cases, we define the average H-bond persistence time (HBPT) to characterise the degree of water molecules binding to other water molecules and describe the formation of H-bonds per water molecule. HBPT is the persistence time of H-bond per water covering the adsorption layer of alcohols. HBPT can indirectly reflect the degree of restriction of the water molecules induced by the adsorption alcohol layer. The function expression is as follows:

(3)

(4)


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where CHB(t) indicates the intermittent H-bond correlation function43. The variable h(t) = 1 is in agreement when a particular water-water forms H-bond at time t (ns) and zero otherwise. Essentially, CHB(t) (see Figure S3) represents the H-bond life time and is an important indicator for the structural relaxation of the H-bond network. According to the method adopted by Zhang et al44, the average persistence time per H-bond can be obtained by integrating the values of CHB(t). NHB indicates the average H-bond number per water that covering the adsorption alcohol layer. Figure 7 shows the HBPT per water for the four studied cases. As shown in Figure 7, the order of HBPT is Me_W (5.85 ps) < Et_W (8.12 ps) < Bu_W (10.49 ps) < Pro_W (12.09 ps). The persistence time of H-bond per water on the propanol layer is the longest. To some extent, this order can reflect the format at the molecular level, in which the formed H-bond microstructures of water molecules is distinctive on different layers of alcohol molecules. Among all the cases, the structure of the H-bonds of water molecules on the propanol adsorption layer is the most stable. Phan et al.45 show that the structure of H-bonds could influence the mobility of water molecules. A long HBPT suggests difficulty for water molecules to move freely. This finding is probably another main reason for explaining the residence time variation tendency in Figure 5.

Figure 7. Average hydrogen bonds persistence time per water molecules covering on the different alcohol layers


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We further analysed the distribution of H-bonds per water molecule covering the alcohols layer to provide more insight into the differences of microstructures of water molecules in four cases, which is a common method for analysing the microstructure of H-bond networks.46 Distribution of different H-bond microstructures could suggest different mobility of water molecules.47, 48 Figure 8 shows the distribution of H-bonds of water molecules covering the different layers of alcohols. Fn denotes the percentage of the water molecule that forms n H-bonds with its surrounding water molecules. The insets illustrate the characteristic microstructures of H-bond networks for F1, F2, F3 and F4, respectively. The H-bond between water molecules is determined according to the geometrical criterion described above. As shown in Figure 8, the H-bond network of water molecules is mainly distributed in the forms of F1, F2, F3 and F4. The other forms with more than four H-bonds were less than 1% and were negligible. The distribution of Fn for Pro_W was obviously different from the other cases. F1 and F2 of Pro_W only account for 14.68% and 38.61%, respectively, which were less than that in other cases. This special difference will not change as the simulation time increases. Our previous studies39 suggested that H-bonds distribution of bulk water is mainly concentrated in F3(33.59%) and F4(20.23%). This indicated that the H-bond network of water molecules on the propanol adsorption layer has been less damaged. Wang et al.21 found that in nanoconfined water molecule behaviours by MD, the destruction of complete H-bond networks would allow individual water molecules to move more freely, resulting in the increase of the mobility. The proportion of water molecules on the layer of propanol forming three or four H-bonds with the surrounding water molecules was higher than that in the other three cases, which can be responsible for the longest residence time of Pro_W (see Figure 5).


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Figure 8. The percentage of water molecules (Fn) on the adsorption alcohol layers with n (n = 1, 2, 3, 4) hydrogen bonds

Based on the preceding detailed H-bond microstructure analysis, we can obtain some basic understanding regarding the structure of the water molecules on the different adsorption alcohol layers and recognise the effect of alcohol molecules on the water molecules at the molecular level.

CONCLUSION In this work, we performed a series of MD simulations to investigate the properties of different binary alcohol/water mixtures confined in the 20 Å 2D graphene slit to clarify the effect of alcohol adsorption layer on the behaviour of water molecules at the molecular scale. The simulation result shows that distinct layered structures can be formed in the slits, and the alcohol molecules are preferentially adsorbed on the graphene wall. Residence times of the water molecules covering the alcohol layer were in an order of Me_W < Et_W < Bu_W < Pro_W. Based on the detailed microstructures analysis of H-bond network, the residence times of the variation tendency of water molecules can be explained at the molecular scale. For an


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alcohol/water binary mixture under the 2D confinement, a small average number of H-bonds between the preferentially adsorbed alcohol molecules and the surrounding water molecules could induce a small degree of damage to the hydrogen bonding network of the water molecules covering the alcohol layer, resulting in the long residence time of the water molecules. The binary mixture separation under 2D nanoconfinement is usually attributed to a combination effect of multiple factors, such as pore size, pore wall chemistry and external conditions. These results demonstrated that the effect of one component on the other in the separation of binary mixture is essential. Moreover, the results also suggest that for a confined binary mixture, the confinement microstructure of one liquid component could exert a significant influence on the mobility of other liquid components. Controlling one liquid component of microstructures by altering pore properties and external conditions may be a new avenue to improve the separation performance. This method will also provide some guidance for the alcohol/water separation under 2D nanoconfinement. Investigation on the influence of slit pore size will be performed systematically in future work.


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ASSOCIATED CONTENT Supporting Information Available: This material is divided into the following sections: (1) Figure S1: The residence autocorrelation of water molecules on the preferential adsorption layer of different alcohol molecules within a longer time. (2) Figure S2: Average number of water−hydroxyl hydrogen bonds per hydroxyl group of four alcohols adsorbed on the graphene interface and those in bulk. (3) Figure S3: H-bond autocorrelation function of water molecules on the preferential adsorption layer of different alcohol molecules.

AUTHORS INFORMATION Corresponding Authors * Yudan Zhu, Email address: [email protected] Notes The authors declare no competing financial interset. ACKNOWLEDGMENTS This work was supported by the National Science Foundation of China (21576130, 21490584), National Basic Research Program of China (2015CB655301), Project of Jiangsu Natural Science Foundation of China (BK20171464), Project of Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) and Qing Lan Project. REFERENCES (1) Radha, B.; Esfandiar, A.; Wang, F. C.; Rooney, A. P.; Gopinadhan, K.; Keerthi, A.; Mishchenko, A.; Janardanan, A.; Blake, P.; Fumagalli, L.; Lozada-Hidalgo, M.; Garaj, S.; Haigh, S. J.; Grigorieva, I. V.; Wu, H. A.; Geim, A. K. Molecular Transport through Capillaries Made with Atomic-scale Precision. Nature 2016, 538, 222-225.


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Table of Contents Graphics


Effect of adsorption alcohol layers on the behaviours of water

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