Segregation Structures and Miscellaneous Diffusions for Ethanol

Article pubs.acs.org/JPCC

Segregation Structures and Miscellaneous Diffusions for Ethanol/ Water Mixtures in Graphene-Based Nanoscale Pores Mengyao Zhao and Xiaoning Yang* State Key Laboratory of Material-Orientated Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing Tech University, Nanjing 210009, China

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on September 7, 2015 | http://pubs.acs.org Publication Date (Web): September 4, 2015 | doi: 10.1021/acs.jpcc.5b03307

S Supporting Information *

ABSTRACT: Molecular dynamics simulation was conducted to study ethanol−water mixtures and the corresponding pure species, confined within slit-shaped graphene nanopores. Extensive structural and dynamical properties of the confined fluids, including hydrogen-bonding behavior, were investigated. The effects of pore width and mixture composition on the confined behavior were illustrated. It is observed that a layered structure is formed within the confined spaces and the ethanol− water mixtures show segregation at larger pores, with ethanol molecules preferentially adsorbing on graphene surfaces. This microphase demixing behavior stems from the competitive effect of the solid−fluid and fluid−fluid interactions. Moreover, miscellaneous diffusion mechanisms have been revealed for the hydrogen-bonding mixtures within the graphene pores. In the mixtures, water and ethanol generally display analogous diffusion mechanism due to ethanol−water association, converting from short-time subdiffusion to long-time Fickian diffusion in the larger nanopores. In the smaller pore (7 Å), both ethanol and water show a suppressed single-file diffusion behavior at the initial time and then display subdiffusion or single-file diffusion behavior. The complex diffusion behavior of ethanol−water mixtures can be described by the collaborating effects of pore confinement and enhanced interaction in the hydrogen-bonding mixtures. of alcohol inside the nanopores. Rabe21 experimentally found that the slit pore, composed of mica surfaces and graphene layers, allows for the phase separation of water and ethanol. Zhou et al.22 confirmed a small amount of ethanol molecules could promote dewetting of two hydrophobic plates. According to the above studies, the hydrophobic nanopores show great potential in inducing microscopic phase separation for alcohol−water mixtures. In addition, alcohol−water mixtures confined in hydrophobic channels are critical for biological areas as well. It has been found that the microscopic origin of alcoholism caused by excessive drinking is that ethanol molecules interact with hydrophobic sites and block the ion flux within the channels.23 Extensive studies suggested that the ethanol molecules have the modulation function to ion channels, which is associated with the confined behavior of ethanol in hydrophobic nanopores.9,24,25 Moreover, as an important fundamental concern, recognition on the structural and dynamical properties of water and ethanol around hydrophobic surfaces would be useful for comprehending the phenomenon of the instability about proteins in ethanol solution.26 Compared with one-dimensional nanotubes, two-dimensional graphene nanochannels with superior flexibility, chemical stability, and spacing adjustability provide a promising hydrophobic

1. INTRODUCTION The behavior of confined molecules is very important for various applications including nanofluidics,1 biological processes,2,3 membrane separation,4,5 and energy conversion.6,7 It is wellknown that when molecular fluids are confined within nanoscale pores, their structural and dynamical properties are substantially altered compared to those in bulk phase.8 Considerable studies have been made for confined fluid molecules, especially for confined water molecules. As one of the simplest amphiphilic organic molecules, ethanol is essential in biological areas and industrial processes.9−11 In the bulk phase, ethanol is soluble in water across the entire range of concentration. But previous studies suggested that, at the molecular level, ethanol and water cannot mix completely, and the local microscopic structure of water could be enhanced by the surrounding alkyl groups.12,13 In the bulk phase, because of the hydrogen-bonding effect, the structures and physical−chemical behavior of the binary ethanol−water mixture become very complicated, as compared with pure components.14−16 Recently, aqueous alcohol mixtures in heterogeneous confined systems have received extensive interests. Several studies17−19 demonstrated that phase separation could appear for ethanol− water mixtures confined in hydrophilic nanopores. Meanwhile, the properties of ethanol/water confined in hydrophobic nanopores have been studied likewise. Molecular simulation20 showed alcohol and water can be separated within SWNTs, wherein alcohol may induce drying of nanotubes and cause accumulation © XXXX American Chemical Society

Received: April 6, 2015 Revised: August 24, 2015

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DOI: 10.1021/acs.jpcc.5b03307 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C nanofluidic material. In the graphene nanochannel, it is suspected that the unique hydrophobic surfaces can have preferential attraction toward the ethanol molecules and lead to enhanced surface adsorption, possibly inducing microscopic phase separation for the mixture. Recently, the graphene-based laminate structures with two-dimensional nanocapillary have been fabricated and show high separation performance as nanofiltration membrane.5,27,28 Thus, understanding the preferential adsorption, structure, and dynamics of ethanol−water mixtures within the two-dimensional graphene pores might help design of new types of nanostructured membranes. At present, the properties of ethanol/water confined in the hydrophobic graphene pores are far from satisfactory. In particular, it is still unclear how the mixture compositions and the pore widths affect the confined structure and phase behavior. Experiments have shown that ethanol molecules could form long chain or ring structures confined in narrow graphite channel.29,30 Extensive investigations confirmed that hydrogen-bonding interaction is helpful for water fast collective motion in confined systems.31,32 So it is highly interesting to study whether the hydrogen-bonding formations in the mixture system could influence ethanol mobility in confined graphene nanopores. At present, a direct experimental measurement about the molecule-scale properties of confined molecules remains usually difficult,33 whereas molecular dynamics (MD) simulation is an effective tool for studying the confined behavior within nanoscale.8,34,35 With the above in mind, in this study, we employ MD simulations to comprehensively investigate the confined structural and dynamical properties of ethanol−water mixtures inside slitshaped graphene nanopores. For comparison, the pure ethanol and pure water are investigated too. In this work, we consider various pore sizes and mixture compositions. Comprehensive confined structure and dynamics properties are characterized throughout this work. The simulation results will provide new insights into the confined performance for complex fluids in nanopores.

Table 1. Force Field Parameters Used in the Simulation εii (kcal/mol)

σii (Å)

q (e)

0.0557 0.066 0.066 0.03 0.17 0 0.1553 0.00

3.4 3.5 3.5 2.5 3.12 0 3.166 0.00

0 −0.18 0.145 0.06 −0.683 0.418 −0.8476 +0.4238

pore width in the graphene slit model. As shown in Figure 1, two layers of static graphene slab were modeled with varying interlayer distances of 7, 10, 15, 20, and 25 Å. These typical pore widths with subnanometer scale for graphene-based nanochannels are expected to have remarkable effect on the confined behavior of ethanol−water mixtures.42 Each graphene sheet has the size of 49.12 × 97.84 Å2 (xz plane), same as the simulation box lengths in the x and z directions. The size of the simulation box in the y direction was chosen large enough to ensure negligible interactions between the confined solution and its periodic images (Ly = 100 Å).43 The graphene sheets were placed in the middle of the box. In order to simulate the ethanol−water mixtures inside the slit pores compatible with the pressure-driven flow mode, we adopted the previously reported method44,45 to construct the initial simulation configuration. In the setup of initial configuration, a series of trial simulations were performed with the graphene nanochannel connected to two bulk mixture reservoirs. An external pressure difference was imposed to push the bulk reservoir, corresponding to ethanol−water bulk-phase mixtures, to fill the graphene slit pore. We can adjust the composition of bulk phase reservoir and external pressure to control the composition and density for the confined mixture within the slit pores. After that, the two reservoirs were removed, and the computational domain consists of only the graphene nanochannel. We reinitialized the velocities of the initial configuration for EMD simulation so that the COM motion is zero. A typical snapshot and the detailed procedure of the simulation systems are given in the Supporting Information (Figure S1). Three mixture compositions and two pure species in graphene nanochannels (with molar fraction of ethanol being 1.0, 0.7, 0.5, 0.3, and 0.0, respectively) were selected in our simulation. The final numbers and the corresponding number densities of ethanol and water molecules in the simulation systems are displayed in Table S1 and Table S2 (Supporting Information), respectively. All simulations were performed in the canonical ensemble (NVT) using the LAMMPS MD package46 with a time step of 1 fs. The Nose−Hoover thermostat47,48 was used to keep the temperature of 298.15 K. The particle mesh Ewald (PME) method was applied to calculate the long-range electrostatic interaction. The cutoff for the L-J interaction was set to be 10 Å. Three dimensional periodic boundary conditions were used throughout this work to represent infinite graphene nanochannel.

2. SIMULATION METHOD In this work, ethanol molecules were described by the optimized potential for liquid simulation in the all-atom model (OPLS-AA).36 Water molecules were simulated with the SPC/E model.37 We kept carbon atoms of graphene rigid, and they were treated as the Lennard-Jones (L-J) spheres using the parameter obtained from ref 38. The L-J 12−6 potential and the Columbic potential were implemented to model the intermolecular interactions: ⎡⎛ ⎞12 ⎛ ⎞6 ⎤ qiqj σij σij U (rij) = 4εij⎢⎢⎜⎜ ⎟⎟ − ⎜⎜ ⎟⎟ ⎥⎥ + r rij ⎝ rij ⎠ ⎦ ⎣⎝ ij ⎠

site C(graphene) C(CH3) C(CH2) H(CH3, CH2) O(OH) H(OH) OW−OW HW−HW

(1)

where rij is the distance between atom i and atom j, qi is the partial charge of atom i, εij and σij stand for energy and radius parameters. Following the previous works,22,39 the geometric combining rules were used in this simulation. Table 1 lists the L-J parameters and partial charges used in the MD simulation. In this work, the main objective is to investigate the structural and dynamical properties of confined ethanol−water mixtures inside the two-dimensional graphene nanochannels, which are being expected to act as new-typed nanofiltration membranes.27,40,41 It has been experimentally demonstrated that this nanochannel membrane structure could keep stable and uniform interlayer distance;28,42 therefore, we will use the fixed

3. RESULTS AND DISCUSSION 3.1. Confined Structures. Figure 2 shows the number density profiles of various atomic groups as a function of the distance (y) across the graphene slit pores. In this figure, y = 0 corresponds to the pore center. In general, the density profiles are symmetric with respect to pore centers. For ethanol molecules, significant layered structure is observed just like pure water confined in graphene slits.49,50 As expected, the number of layers B


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The Journal of Physical Chemistry C


Figure 1. Simulation systems and typical snapshots. The cyan walls represent the graphene sheets.

Figure 2. Density profiles of methyl, methylene, hydroxyl groups in ethanol, and oxygen atom in water in the 7−20 Å graphene slits (top to bottom).

the CH3−CH2 bonds prefer to align along the graphene surfaces. In the 15 Å pore, the orientation angle of α has a broad distribution with two small peaks at 20° and 160°, representing CH3−CH2 bonds shown in certain perpendicular position in regard to the surfaces, as supported by the presence of second peak in the density profiles of methylene groups (Figure 2). Moreover, it is noted that there exist four peaks in the hydroxyl density distributions (Figure 2) for the 10 Å nanopore, and the corresponding orientational distribution of β appears as two extra peaks at around 30° and 150°, compared to the narrowest pore (H = 7 Å). This implies that some ethanol molecules might stand on the surface, with the CH2−OH bonds pointing toward the pore center to form layer−layer hydrogen bonds, just like the previous report30 about pure ethanol confined graphite pore. According to the above structural analysis, we can conclude that the configurations of ethanol molecules convert from parallel form to standing state as pore size increases. The pore-sizedependent transformation of ethanol configurations can be found in alumina nanopores as well.17,53 Furthermore, as shown in Figure 2, the layered structure of ethanol and water molecules is insensitive to solution compositions except for the 10 Å pore, where the density peaks of ethanol near the surface gradually disappear as water molecules are added, indicating more hydroxyl groups point toward the pore center. For the 15−25 Å graphene pores, the added water molecules yield an enriched zone in the center region of pore.

increases with pore size. For pure ethanol, two density layers are formed at H = 10 Å, in agreement with previous X-ray diffraction experiments29,30 and molecular simulation30 of ethanol confined inside carbon microspores. In the methyl density profiles (Figure 2), we notice a shift in the peak position of contact layer as pore width increases; for instance, the peak position is at 3.4 Å for the 7−10 Å nanopores. However, it is at ∼3.6 Å for the 15−25 Å pores, which is close to the value of ethanol confined in CNT.51 This pore size effect is probably owing to the competition between spatial effect and interaction energy. In the ethanol surface contact layer within large pores, the −OH and −CH2− groups show certain shift away from the graphene surfaces, when compared with the −CH3 groups in the density profiles. This indicates that confined ethanol molecules orient themselves with the hydrophobic methyl groups exposing to the two plates, whereas the polar hydroxyl groups point outward to maintain a hydrogen-bonding network with adjacent solvent layer. This orientation behavior is the same as previous reports.39,52 To further characterize the configurations of confined ethanol, we defined α, β, and γ as the orientational angles between the yaxis and the CH3−CH2, CH2−OH, O−H bonds of ethanol, respectively (for the illustration of definition see Figure 3). No obvious difference is found for the ethanol orientation between the pure species and the mixtures. As for the orientation angle α, the most concentrated distribution locates at 90°, indicating that C


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Figure 3. Orientation angle (α, β, γ) distributions of confined ethanol molecules in the 7, 10, and 15 Å slits: (a) pure ethanol; (b) mixture with ethanol mole fraction of 0.5.

Figure 4. Average interaction energy distributions for ethanol (a) and water (b) molecules with two graphene surfaces in the 7−25 Å nanoslits: (solid line) pure component; (dashed line) mixture with ethanol mole fraction of 0.5.

Oe−Oe RDFs are representative of ordering of ethanol molecules. However, with regard to gow‑ow(r), the first peak amplitude in the ethanol−water solutions increases with ethanol added. This suggests an enhancement of local water structure due to the presence of ethanol molecules. For H = 7 Å, the confined pure water also has oscillating RDF, showing a solidlike structure as given in the snapshots (see Figure 7b). In the Oe−Ow RDFs for the 10−15 Å pores, the first peak becomes higher with the ethanol concentration increasing, just like bulk mixtures. However, this first peak height displays small difference in the 20−25 Å pores, which may be caused by the nearly fixed concentration within the contact layer. To quantitatively evaluate the different surface interactions, we showed the energy distributions (Figure 4) of the ethanol− surface and water−surface interactions for the 50% ethanol− water mixture and the pure components. When the pore size is 7 or 10 Å, the interaction energies of ethanol show a single distribution, ranging from −40 kJ/mol to −25 kJ/mol and from −25 kJ/mol to −10 kJ/mol, respectively. The enhanced surface interaction in the 7 Å pore can be ascribed to the confined molecules feeling the interaction simultaneously from two surfaces. With a further increase in the pore size, the surface− ethanol interaction distribution shows a second peak in the range of −5 to 0 kJ/mol, which is associated with the formation of middle layers in the density profiles (Figure 2).

It is interesting to note that the ethanol concentration of contact layer remains higher and almost unchanged (see Figure S2 in the Supporting Information), representing demixing of ethanol− water mixtures with water-enriched mixtures sandwiched between two ethanol adsorption layers near the hydrophobic graphene surfaces, as shown in the snapshot of Figure 1 (bottom panel). This segregation structure behavior has been found in the previous observation.54 As to 7−10 Å graphene pores, because of spatial confinement, no segregation occurs for the confined mixture (top panel of Figure 1). We further analyzed the radial distribution functions (RDFs) between oxygen atoms, along the direction parallel to the xz plane, within the surface contact layer. Three typical RDFs were simulated: (i) goe‑oe(r), between two ethanol oxygens; (ii) goe‑ow(r), between ethanol oxygen and water oxygen; and (iii) gow‑ow(r), between two water oxygens. The RDF results are displayed in Figure S3. All the O−O RDFs have maximum peaks at around 2.8 Å for different systems, agreeing with neutron diffraction experiments12,55,56 and molecular simulation13,57 of ethanol−water mixtures in bulk phase. For the intermolecular structure of ethanol, it is observed that the first peak height of goe‑oe(r) decreases with addition of water molecules. This shows that water can push ethanol hydroxyl groups apart and reduce the interacting extent between ethanol molecules. In the 7 Å pore, a succession of neighbor peaks in the D


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Figure 5. Average HB number profiles along the y-axis for the 50% ethanol−water mixture in different slit widths: (a) ethanol; (b) water. (c) HB number differences between middle region and contact layer for ethanol and water molecules.

Figure 6. (a) The SHB(t) and the CHB(t) of C2H5OH−C2H5OH HBs for pure ethanol inside nanochannnels as well as bulk ethanol. (b) SHB(t) for the C2H5OH−C2H5OH and H2O−H2O HBs inside the 15 Å nanochannnel with different ethanol/water compositions.

Meanwhile, because of the configuration transformation of ethanol molecules, surface interaction between the ethanol molecules in contact layer and the adjacent graphene wall also becomes weak with pore size increasing (see Figure S4). Similar observation can be seen for confined water in Figure 4b. This energy comparison shows that confined ethanol molecules have larger interaction with the hydrophobic graphene surface. Such energetic preference promotes the obvious adsorption of ethanol molecules near the graphene surfaces, thus leading to the observed demixing behavior (see Figure S2). 3.2. Hydrogen-Bonding Behavior. Parts a and b of Figure 5 respectively show the profiles of total hydrogen bonds (HBs) per ethanol and per water molecule, along the y-axis in the graphene pores for the 50% mixture. Here we used the geometric criterion definition of hydrogen bonds.58,59 The HB profiles are symmetric with respect to the pore centers, and the average HB number is reduced at the interfacial region. Unsurprisingly, ethanol has less HB number than water. We further characterized the individual HB profiles (see Figure S5) for ethanol−water mixtures in order to see the individual contribution to the total HB distribution. It is obvious that the HB number between ethanol molecules decreases and that between ethanol and water increases by adding water molecules. It implies that the ethanol− ethanol HB structure is gradually broken with water composition increasing, in line with the Oe−Oe RDF results. Obviously, the HB number of water−water increases as water content becomes higher, especially in the center region. This is because water molecules possess higher probability to form the water−water hydrogen bonds in the enriched water region. Figure 5c shows the HB number differences between the middle region and interface layer for the 15−25 Å pores. We can

see that the HB number differences per ethanol and per water are 0.08−0.24 and 0.6−1.0 in the graphene nanochannels, respectively. If the average energy per HB is ∼20.4 kJ/mol,60 then when a solvent moves from center into graphene surface, water molecule needs to overcome higher HB interaction energy (>12.3 kJ/mol) than ethanol molecule (

Segregation Structures and Miscellaneous Diffusions for Ethanol

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