Li+ Separation via Biomimetic

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Molecular Dynamics Study of Mg2+/Li+ Separation via Biomimetic Graphene-based Nanopores: The Role of Dehydration in Second Shell Yang Ruan, Yudan Zhu, Yumeng Zhang, Qingwei Gao, Xiaohua Lu, and Linghong Lu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b03001 • Publication Date (Web): 18 Oct 2016 Downloaded from http://pubs.acs.org on October 20, 2016

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Molecular Dynamics Study of Mg2+/Li+ Separation via Biomimetic Graphene-based Nanopores: The Role of Dehydration in Second Shell

Yang Ruan, Yudan Zhu*, Yumeng Zhang, Qingwei Gao, 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] Xiaohua Lu, Email address: [email protected] 1

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Abstract Residual Mg2+ reduces the performance of lithium-ion batteries. However, separating Mg2+ and Li+ is difficult because of their similar ionic properties. Inspired by the high selectivity of biological Mg2+ channels, this work utilizes atomistic simulations to investigate the ability of graphene-based nanopores with diameters of 0.789, 1.024, and 1.501 nm to separate Mg2+ and Li+ under a series of transmembrane voltages. We analyzed the spatial distribution of molecules in the nanopores’ vicinity, structure properties of ionic hydration, and potential of mean force of ions traveling through the nanopores. Separation was mainly caused by the difference in dehydration between the second hydration shells of Mg2+ and Li+. When ions traveled through nanopores, Li+ had to overcome a greater energy barrier than Mg2+ because it had to shed more water molecules and break more hydrogen bonds in the second hydration shell compared with Mg2+. Moreover, the ionic Coulomb blockade of Mg2+ occurred near the pore mouth, impeding Li+ transport and increasing selectivity when the pore diameter decreased to subnanometer.

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1 INTRODUCTION The rapid development in structure revolution techniques has resulted in the successive reporting of various biological ion channels with remarkable ionic selectivities and permeabilities, such as KscA1 for K+, NavAb2 for Na+, and GluCl3 for Cl-4. Recently, magnesium (Mg2+) channels have received both academic and industrial interest.5–8 Dalmas et al.5 adopted the crystal structure of CorA to investigate its selectivity for monovalent and divalent cations and demonstrated that its multi-occupancy Mg2+-selective channels fully exclude monovalent cations. Their findings have significance for practical applications. Residual Mg2+ is inevitable in the industrial extraction of Li+ from salt lakes for lithium-ion battery production, and influences battery performance.9 Residual Mg2+ considerably affects lithium carbonate or lithium hydroxide purity, rendering them unusable for manufacturing lithium-ion batteries.10 However, separating Li+ and Mg2+ is difficult because of their nearly identical ionic radii.9, 11 Therefore, mimicking the characteristics of biological Mg2+ channels is important in designing novel biomimetic nanopores for use in Mg2+ and Li+ separation. Graphene has potential applications in the biomimetic design of nanopores for fluid molecules separation. Graphene is an advantageous two-dimensional (2D) material with preeminent electrical, mechanical, and thermal properties, and is widely used in membranes for molecular separation because of its unique one-atom-thick structure.12,13 Surwade et al.14 utilized oxygen plasma-etching to successfully synthesize graphene nanopores with tunable diameters of 0.5–1.0 nm. Numerous experimental studies have demonstrated that graphene and its derivatives are promising materials for separating gases, water molecules, and ions.12, 13 Joshi et al.15 utilized graphene-based membranes for molecular sieving and found them impassable for solutes, such as Na+, Mg2+, Cu2+, Cl-, propanol, toluene, and octanol, with hydrated radii larger than 0.45 nm. Shen et al.16 synthesized uniform GO laminates with interlayer heights of approximately 0.4 nm. The H2 permeability of the GO

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laminate increased by two to three orders of magnitude, along with a threefold enhancement in H2/CO2 selectivity, to surpass the Robeson upper bound. Atomistic simulation has been indispensable in identifying the underlying mechanisms at the molecular scale in graphene-based material applications. Many insights have been presented on ion separation.17–21 Sint et al.17 utilized graphene nanopores to investigate the ion selectivities and fluxes of Li+, Na+, K+, F-, Cl-, and Br-. Their results demonstrated that ion selectivity and flux could be optimized by selecting the sizes, shapes, and numbers of the functional ligands attached to the pore rim. Cohen-Tanugi et al.18 found that the ability of graphene nanopores to prevent salt passage depends largely on pore size, which allows water flow and ion blockage. Moreover, given their controllable pore size and easy functionalization with different groups, graphene-based nanopores are widely used as pore models in simulations mimicking biological ion channel structures, such as in K+/Na+ separation.20, 21 Replicating the selectivity filter for amino acid arrangements is a major strategy for mimicking the function of biological ion channels. Gong et al.22 adopted a modified carbon nanotube as a pore model to imitate the amino acid arrangement in the K+ channel filter structure. Their findings suggested that the prominent selectivity of the nanotube results from the difference between the Na+ and K+ hydration structures confined within the nanopores. He et al.20 reported the high K+ selectivity of graphene-based nanopores modified with (–CO) groups to mimic the KcsA (K+) channel selectivity filter structure. However, when they elaborated similar nanopores with four carboxyl (–COO-) groups to mimic NavAb (Na+), their results implied that their method of replicating the selectivity filter for amino acid arrangements is not always effective. Another trend in biomimetic design is extracting easily controllable key features, such as pore size, for feasible material preparation in practical applications.21, 23 In our previous studies, Zhou et al.24 defined the novel term ionic hydration factor to quantitatively describe the microstructure of ionic hydration. The Cl- hydration radius derived from the hydration factor explained the mechanism of various synthetic Cl4


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channels.25–27 Shao et al.28 found that slightly changing the pore size of a hydrophobic nanotube resulted in a reversed ionic selectivity for K+/Na+ via simulations. This result suggested that hydrophobic nanopores with certain pore sizes can achieve high K+ selectivity despite the hydrophilicity of biological K+ channels. Zhou et al.29 synthesized hydrophobic K+ channels with a self-assembly method. The synthesized channels had pore sizes (0.65 nm in effective diameter) in good agreement with the high K+ selectivity predicted by Shao et al.28 in their simulation. To mimic the function of Mg2+ channels, it is essential to gain an initial understanding of the selectivity filter key structure. Pfoh et al.30 demonstrated that the crystal structure of CorA (PDBID: 4EED) from Thermotoga maritime (TmCorA) provides a detailed structure for understanding Mg2+ channel sizes. Kitjaruwankul et al.31 analyzed CorA channel selectivity filter structures and found that Gly-Met-Asn (GMN) residues bind to Mg2+ in partially hydrated forms because the distance between the metal center and five carbonyl oxygen atoms of asparagine is within 0.38–0.50 nm, and 0.42–0.52 nm for the five glycine side chains. Combined with the distance between Mg2+ and GMN motifs, selectivity filter pore diameters are approximately within 0.76–1.04 nm, as illustrated in Figure S1. In this work, we utilized atomistic simulations to investigate the Mg2+/Li+ selectivity of single-layer graphene nanopores. The nanopores were bio-inspired by the characteristic sizes (0.76–1.04 nm) of selectivity filters in Mg2+ channels. We chose three pore sizes (0.789, 1.024, and 1.501 nm in diameter) to evaluate ion selectivity and flux under various electric field intensities (0.2, 0.4, 0.6, 0.8, and 1.0 V/nm). We described the simulation details in Section II. In section III, we first adopted molecular spatial distribution in the pore vicinity to reflect the preferential ionic transport pathway. Ionic hydration structure properties were then analyzed in detail to illustrate selectivity at a molecular scale. Moreover, to explain selectivity in terms of energy, we investigated the potential of mean force (PMF) for ions traveling through nanopores. We stated our conclusion in Section IV.

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2 MODELS AND SIMULATION METHODS Figure 1 shows models of graphene-based nanopores. A graphene sheet with a central pore was placed parallel to the x-y plane in a periodic simulation box (4.176 × 4.254 × 5.00 nm3). Pore diameters of 0.789, 1.024, and 1.501 nm were selected according to the characteristic sizes of the Mg2+ channel selectivity filter. Each nanopore was obtained by removing carbon atoms from the central region of graphene. Nanopore diameter was defined as the distance between the centers of two carbon atoms. For convenience, 4-4-1, 4-4-2, and 4-4-3 indicate pore diameters of 0.789, 1.024, and 1.501 nm, respectively. For each system, the simulation box was filled with 0.25 M MgCl2 and 0.25 M LiCl mixed solution. Table S1 provides the detailed system sizes and atom numbers for all the studied simulation cases.

Figure 1. (a) Selectivity filter structure of CorA Mg2+ channels viewed from the top. (b) Graphene nanopores with pore diameters of 0.789, (c) 1.024, and (d) 1.501 nm. (e) Lateral view of the simulation box showing a graphene nanopore in the center and the two reservoirs on both sides of the nanopore. The black arrow represents the direction of the applied electric field. The gray rectangle frame is the simulation box. The TIP3P model was applied to water molecules.32 To prevent out-of-plane displacement, graphene carbon atoms were fixed to their initial positions and modeled as uncharged Lennard-Jones (L-J 12-6) particles. The force field parameters are listed in Table S2. The CHARMM27 force field was used to describe graphene and ions,33 6


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and the parameters for Li+ were from a previous work.19 The Lorentz-Berthelot combination rule34 was used to describe L-J interactions between different particles. The short-range interactions were computed using a 1.0 nm cutoff. The particle-mesh Ewald (PME) method was utilized to treat the long range electrostatic interactions with a cutoff for a real space of 1.0 nm.35 Molecular dynamic (MD) simulations were conducted by GROMACS.36 The visualization was generated via Visual Molecular Dynamics (VMD) package.37 The periodic boundary condition was applied to all three directions. All systems were first equilibrated in the NPT ensemble for 5 ns after energy minimization, and every production run was performed for 65 ns in the NVT ensemble with 2 fs time step and saved every 1 ps. All simulation data of the last 60 ns were collected for further analysis. A temperature of 300 K was maintained using the V-rescale method,38 and the Parrinello-Rahman39, 40 coupling scheme was employed to control the system at 1 bar. In all production runs, an electric field (E) was applied perpendicular to the graphene nanopores to generate the effective transmembrane voltage (V).41 The corresponding voltage was calculated via = − × , where was the length of the simulation box along the z-axis. In previous works, electrical field was used extensively as the driving force to investigate ion separations.17, 20–22, 42, 43 The electric field intensities in this work were 0.2, 0.4, 0.6, 0.8, and 1.0 V/nm, and the corresponding transmembrane voltages were 1.0, 2.0, 3.0, 4.0, and 5.0 V. 3 RESULTS AND DISCUSSION 3.1 Selectivity and Flux Figure 2a shows the Mg2+/Li+ selectivities of the three nanopores. Mg2+/Li+ selectivity was defined as the ratio of the total number of Mg2+ and Li+ passing through the nanopore. The simulation showed that all the selectivities were higher than 1.0, indicating that the three nanopores selected Mg2+ over Li+. Selectivity decreased as pore diameter increased. Under all the studied voltages, separation ratios of the 4-4-1 and 4-4-2 nanopores exceeded 2.0. The 4-4-3 nanopore had a Mg2+/Li+ 7


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ratio of approximately 1.0, indicating difficulty in separating Mg2+ and Li+. The diameter of 4-4-3 was only 0.5 nm larger than that of 4-4-2, whereas the selectivity was halved, suggesting that Mg2+/Li+ selectivity was sensitive to pore size. The maximum selectivity of 4-4-1 was 3.26 under V = 3.0 V (E = 0.6 V/nm) and 2.36 for 4-4-2 under V = 4.0 V (E = 0.8 V/nm). Under the same voltage, 4-4-1 and 4-4-2 exhibited significantly higher selectivity than 4-4-3.

Figure 2. (a) Mg2+/Li+ selectivity of each graphene nanopore under different voltages. The black dashed line represents Mg2+/Li+ ratio equal 1.0. The total number of ions traveling through the nanopores under different voltages: (b) Li+ and (c) Mg2+. The 4-4-1 and 4-4-2 nanopores had diameters within the characteristic size range of biological Mg2+ channel selectivity filters (0.76–1.04 nm). The results demonstrated that nanoporous graphene with a pore diameter range of 0.76–1.04 nm could potentially extract Mg2+ from Li+. Toward practical applications, 2D-layered graphene and its derivatives possess extraordinary separation performance in precise ionic and molecular sieving.44, 45 It would be more efficient if these single-layered porous graphene were assembled into 2D lamellar nanochannels, which have a potential in lithium recovery from salt lake brine.

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As shown in Figure 2b,c, Mg2+ and Li+ fluxes increased as voltage increased because a higher voltage enables ions to overcome energy barriers and increase ions flux. However, Figure 2a shows no obvious dependence between Mg2+/Li+ selectivity and voltage. Although Mg2+ and Li+ have different valences, increasing voltage did not directly improve separation performance, implying that voltage is not the dominant cause of Mg2+/Li+ selectivity. Given that different graphene nanopores have significantly different selectivity under V = 3 V (E = 0.6 V/nm), we investigated their preferential ionic transport pathways and hydration structure properties under this voltage to further explore the underlying selectivity at the molecular scale. We present our results in the following section. 3.2 X-Y Planar Number Density Distribution of Ions and Water Molecules

Figure 3. X-Y planar number density distribution of Li+, Mg2+, and water molecules in 4-4-1, 4-4-2, and 4-4-3 graphene nanopores. To investigate the distribution characteristics of ions and water molecules, we adopted the x-y planar number density distribution18,

43, 46–48

of ions and water

molecules inside the graphene nanopore (-0.2 nm < z < 0.2 nm, where z = 0 represents the pore position), as shown in Figure 3. The density profiles were obtained in an 9


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imaginary cylinder with its axis perpendicular to the graphene sheet. The cylinder diameter was equal to the graphene pore diameter. The effect of pore size could be obtained by 2D density maps. As shown in Figure 3, Mg2+ and water molecule density distribution in the 4-4-1 nanopore exhibited a triangular shape similar to the pore geometry. This observation indicated that Mg2+ tends to occupy the pore’s central region when traveling through the nanopore. The 4-4-1 nanopore had three apparent water passage areas symmetrically arranged around the pore center. By contrast, Li+ had a scattered density profile in 4-4-1. The density distribution map was round in the hexagonal 4-4-2 and 4-4-3 pores, with the highest probability for Li+ and Mg2+ around the pore center. However, as in previous works, water molecules still tended to cross the nanopore close to the pore edge

46, 47

because of the strong affinity of carbon with

water. Therefore, we could divide the nanopore into three nesting regions according to water and ionic distributions: (I) the Mg2+-dominated central region, (II) the Li+ middle region, and (III) the water-occupied outer layer. The difference between the ionic pathways of Li+ and Mg2+ gradually reduced as pore size increased. Combined with the selectivity shown in Figure 2a, we found that selectivity was dependent on the difference between ionic pathways. One possible reason for ionic pathway differences is that the Li+ pathway might be impeded by electrostatic repulsion from Mg2+, causing Li+ to randomly cross the pore. Electrostatic repulsion was similarly observed in biological ion channels and subnanometer sized pores. Dalmas et al.5 stated that the high Mg2+ flux in CorA is attributable to electrostatic repulsion, and that Mg2+ prevented other ions, especially monovalent ions, from approaching the binding sites of the selectivity filter. Kaufman et al.49 proved that the extraordinary selectivity of Ca2+ channels resulted from ionic Coulomb blockade, a strong electrostatic interaction. Krems et al.50, utilizing MD simulations, found that ion-ion interactions in nanopores resulted in ionic Coulomb blockade. This result signified that ions inside a nanopore with a specific capacitance impeded the flow of additional ions because of Coulomb repulsion. Moreover, highly 10


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charged and heavier ions, such as Mg2+ or Ca2+, had a stronger effect resulting from increased Coulomb interaction. Feng et al.51 reported an experimental observation on single-ion transport through subnanometer MoS2 pores, and indicated that ionic Coulomb blockade and dehydration determined ion transport through the subnanometer pore.

Figure 4. Total flux and selectivity ratios of Mg2+ and Li+ passing through the 4-4-1, 4-4-2, and 4-4-3 graphene nanopores under 3.0 V. The cyan and green columns indicate the total number of Li+ and Mg2+ traveling through the nanopore, respectively, whereas the blue and red lines represent the selectivity ratio of Mg2+/Li+ in single-component and mixed solutions, respectively. The horizontal black dashed line corresponds to Mg2+/Li+ ratio of 1.0. We investigated the influence of ionic Coulomb blockade on 4-4-1, 4-4-2, and 4-4-3 nanopores under 3.0 V (E = 0.6 V/nm) in single-component LiCl and MgCl2 solutions and with the same parameters mentioned in the Method section. Figure 4 illustrates the Mg2+/Li+ selectivity ratio. In the 4-4-1 nanopore, the selectivity ratio of the mixed solution was greater than that of the single-component solution. However, 4-4-2 had the same selectivity for both solutions. The 4-4-3 nanopore had a slightly higher selectivity ratio in the single-component solution than in the mixed solution. We calculated the charge density in a cylindrical zone (diameter equal to pore size) along the z-axis. As illustrated in Figure 5, the asymmetrical net charge density (net 11


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charge = total positive + total negative) indicated the generation of ion concentration polarization layers52 and the disappearance of polarization layers as pore size increased. As pore size decreased, more Mg2+ accumulated in the nanopores (z < 0.0 nm), especially in the 4-4-1 nanopore. However, the charge density of Li+ was virtually unchanged. Based on the above analysis, we found that the ionic Coulomb blockade was highly associated with pore size, and improved the separation of Mg2+ and Li+.

Figure 5. Charge density along the z-axis (from top to bottom: Net, Mg2+, and Li+ charge densities). The gray horizontal dashed line represents a charge density of zero. Beckstein et al.53 demonstrated that ionic hydration cannot be unnoticed when ions permeated a nanopore or channel. Ionic hydration is an essential cutting-in point to understand the mechanism of ion transport under nanoconfinement.54 The difference between the ionic pathways of Li+ and Mg2+ may be closely associated with their hydration shells. In the following sections, we further analyzed confined ionic hydration.

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3.3 Ionic Hydration Structure Detailed ionic hydration microstructures were analyzed to shed light on their involvement with Mg2+/Li+ selectivity ratios. We first investigated ionic hydration number distribution as a reflection of the different ion dehydration properties. We then clarified the underlying dehydration mechanism by calculating the preferential orientations and hydrogen bond (H-bonds) networks of water molecules in ionic hydration shells. 3.3.1 Hydration Number Distribution

Figure 6. Hydration number distributions of Li+ and Mg2+ in the first and second hydration shell at the nanopore’s entrance (-0.2 nm < z < 0.2 nm) and the bulk solution (z < -0.6 nm or z > 0.6 nm). The hydration number distributions of Mg2+ and Li+ at the entrance of the nanopore (-0.2 nm < z < 0.2 nm) and the bulk solution (z < -0.6 nm or z > 0.6 nm) were obtained by counting the numbers of water molecules in ionic hydration shells and are shown in Figure 6. The hydration number calculated method was previously used on nanopore systems for water desalination.22, 47, 48, 55 The hydration radii were determined according to the radial distribution function (RDF) of the water molecules 13


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around ions for the whole system (see Figure S2). The first hydration shell radii were approximately 0.295 and 0.24 nm for Li+ and Mg2+, respectively, and the second hydration shell radii were 0.568 and 0.512 nm, respectively. The results were in good agreement with those previously reported 56, 57 for Li+ and Mg2+ in bulk solutions. Figure 6a,b show that in the bulk solution, the first hydration number profile of Li+ was broadly distributed from 2–6 with an average of 4.4, indicating dynamic changes in hydration structure. These results were consistent with previous experiments and theoretical studies.56 Rempe et al.58 demonstrated that the first hydration shell of Li+ could be divided into two shells, with four stable water molecules in the inner shell and two water molecules in the outer shell, resulting in dynamic variation. Recently, Mason et al.59 performed neutron scattering experiments to validate these theoretical calculations. Consistent with previous results,56 Mg2+ in bulk solution had a first hydration shell number of 6 with an average of 5.9, indicating that the hydration shell structure was stable. This result demonstrated that Mg2+ has considerable electrostatic interactions with water molecules in the first hydration shell. Moreover, we also found that the ionic hydration number profiles of both ions between the pore entrance and in bulk solution were similar. As shown in Figure 6a,b, the first hydration shell of Li+ and Mg2+ contained approximately 3–5 and 5–6 water molecules, respectively, adjacent to the pore entrance. These results indicated that the first hydration shells of the majority of Mg2+ and Li+ were in line with their bulk counterparts when ions travel through the nanopore; however, there was a slight dehydration in 4-4-1. The numbers of water molecules in the second ionic hydration shell were also analyzed given the strong hydration of Mg2+ and Li+, as shown in Figure 6c,d. In bulk solution, the hydration number distributions of Li+ and Mg2+ were concentrated in the ranges of 19–23 and 13–16, respectively, with average hydration numbers of 21 and 15, respectively, coinciding with the results reported by Mogami et al.57 The difference between the hydration numbers of both ions suggested that the Li+ hydration cluster had a larger volume than Mg2+. Moreover, in the pore entrance, the 14


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hydration number distributions of Li+ had a greater shift than those of Mg2+ compared with those of their bulk counterparts, as shown in Figure 6c,d. This result suggested that the dehydration of both ions mainly occurred in the second ionic hydration shell and that Li+ must be stripped off more coordinated water molecules when ions passed through the nanopore. The ionic hydration number of Mg2+ in the second shell gradually approached the bulk value as pore diameter increased. However, the second shell dehydration of Li+ occurred even in the 4-4-3 nanopore. Based on the different dehydration properties derived by analyzing the two hydration shells of Li+ and Mg2+, the difference in hydration properties between Li+ and Mg2+ in the first hydration shell could be another explanation for the different preferential pathways of both ions shown in Figure 3, in addition to the effect of ionic Coulomb blockade. Mg2+ preferred to pass through the central region of the nanopore given its more stable binding with the water shell compared with Li+. On the other hand, the differences between the dehydration of the Li+ and Mg2+ second hydration shells suggested that Li+ would encounter a higher energy barrier than Mg2+ because more water molecules in the Li+ hydration shell had to be stripped off. Richards et al.60 pointed out that transport would be strongly hindered when the pore radius was smaller than the hydrated radius and that dehydration was the main barrier. Overall, the second shell dehydration of Mg2+ and Li+ occurred in all the selected nanopores. Therefore, we can reasonably consider that Mg2+/Li+ selectivity was governed by the dehydration of the second hydration shell of ions. 3.3.2 Water Orientation in the Ionic Hydration Shell Two characteristic angles θ and β describe the preferential orientations of water molecules in ion hydration shells. θ is the angle between the cation and the dipole moment of the water molecules, and β is the angle between the OH bond of water molecules and the distance of the water oxygen to the ion, as illustrated in the Figure 7a,b insets, respectively.61–63 The dipole and OH bond orientation at the pore entrance and in the bulk solution were analyzed. 15


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Figure 7. Distributions of orientation angles θ and β of water molecules within the first and second hydration shell of ions at the nanopore entrance (-0.2 nm < z < 0.2 nm) and the bulk solution (z < -0.6 nm or z > 0.6 nm) for Li+ (a, b, e, and f) and Mg2+ (c, d, g, and h). The insets illustrate the θ and β angle. There are two obvious peaks in dipole and OH bond orientations in the Li+ first hydration shell, as shown in Figure 7a,b. The orientation of hydration water 16


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molecules of ions is one of most important indexes of ionic hydration microstructures.54 The optimal values of θ were 20°–27°, 153°–161°, and 57°–63°, 118°–124° for β. The preferential distributions of θ and β in the three pore entrances were dynamically changed, in which one peak declined as another peak rose. These phenomena indicated that the Li+ symmetrical hydration shell structure tended toward asymmetry and restricted by the graphene nanopore. These results reflected the preferential orientation of water molecules with an oxygen atom pointing toward the ion, whereas both hydrogen atoms pointed outwards in a ‘V’ shape. As shown in Figure 7c,d, the first orientation distributions of Mg2+ had similar preferential orientations with those of Li+, but were more concentrated and almost constant in the three studied cases, indicating a more stable and stronger hydration shell of Mg2+ than Li+. For Mg2+, the orientation distribution was almost symmetrical given the stronger ion-water interaction. However, in the 4-4-1 nanopore, β declined between 116°–138° because Mg2+ was dehydrated as it passed through the nanopore. These slight dehydration phenomena were observed in hydration number distribution (see Figure 6b). Compared with the first ionic hydration shell, the ionic second hydration shell shown in Figure 7e-h did not have obvious preferential orientations for both ions (except for θ in Mg2+ second hydration shell), indicating that as the ion-water distance increased, the ion-water interaction weakened and water-water molecular interaction was enhanced. We also observed the different second hydration structure responses of Li+ and Mg2+ as ions traveled from the bulk solution into the nanopore, suggesting different dehydration behaviors for both ions. The underlying mechanisms were further investigated and combined with detailed H-bond analysis. 3.3.3 Hydrogen Bond Network In this work, a hydrogen bond (H-bond) was formed when the distance between the donor O atom in one water molecule and acceptor O atom in another water molecule was less than 0.35 nm and the O-H---O angle was less than 30°.64 We investigated the average H-bond along the pore axis (z-axis) ranging from z = -1.5 nm 17


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to z = 1.5 nm. We discussed two kinds of H-bonds: the type I H-bond, which is formed between water molecules in the ionic first and second hydration shells, and the type II H-bond, which is formed within water molecules in the second ionic hydration shell. The type I H-bond is illustrated in Figure 8a,b. Our results indicated that water molecules in the first hydration shell acted as H-bond donors, forming two H-bonds with water molecules in the second hydration shell. These observations corresponded with the orientation analysis shown in Figure 7a-b. An H-bond network cage was formed around the first hydration shell. As pore size decreased, the number of type I H-bonds per water molecule of Mg2+ decreased faster than that of Li+, indicating that more type I H-bonds were broken down in Mg2+ than in Li+ as the ions traveled through the nanopore. The more easily fractured type I H-bonds in Mg2+ compared with Li+ might result in the easy stripping of water molecules from the second hydration shell, which could be attributed to the rare exchange of water molecules between the Mg2+ first and second hydration shells. The RDF profiles of the water molecules around Mg2+ (see Figure S2) also proved that the profile vanished between the first and second hydration shells.57

Figure 8. Average H-bond numbers per water molecule resulting from water molecule formation along the z-axis in the first and second hydration shells (type I; a and b) and 18


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hydrogen bonds formed in water molecules in the second hydration shell (type II; c and d). The insets illustrate hydrogen bond formation in the ionic first and second hydration shells. The blue dashed line between water molecules represents H-bonds. Figure 8c,d show that in the three nanopores, the average number of type II H-bond per water molecule in Li+ second hydration shell was higher than that of Mg2+. This observation indicated that water molecules in the Li+ second hydration shell retained a more stable structure via water-water molecular interactions compared with Mg2+, suggesting the difficulty of stripping water molecules from the second hydration shell of Li+ as the ion traveled through the nanopore. Moreover, as pore size decreased, the number of type II H-bonds per water molecule of Li+ deceased faster compared with Mg2+, indicating that more type II H-bonds were broken down for Li+ than for Mg2+ as ions traveled through the nanopore, which opposed the finding for type I H-bonds. Moreover, the above deduction for type II H-bonds can also explain the variations in orientation distribution in the second hydration shells of Li+ and Mg2+ as the ions traveled from the bulk solution into the nanopore. Given that the hydration effect of Mg2+ was stronger than that of Li+, the dipole orientation (θ) profile of Mg2+ in the second hydration shell remained symmetrical with two broad peaks. The peaks gradually approached bulk value as pore size increased, making the second hydration shell more uniform (see Figure 7g). By contrast, for Li+, θ at the nanopore entrance was broadly distributed with a maximum peak located at 130°–150°, which had an obvious shift from the bulk value of 104° (see Figure 7e). This finding suggests that water molecules around the second hydration shell of Li+ were less ordered than those of Mg2+ and tended to form H-bonds. He et al.62 reported a direct competition between H-bonds (water-water molecular interaction) and ionic hydration (ion-water molecular interaction). Combining the results derived from analyses of both H-bond types, we found differences in H-bond types that tended to be broken for Mg2+ and Li+. H-bonds between the Mg2+ first and second hydration shells (type I) were unstable and the 19


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water-water molecular interaction in second hydration shell were weaker than that of Li+, suggesting that Mg2+ required less energy barriers when traveling through the nanopore, thereby improving the selectivity of Mg2+/Li+. 4 Potential of Mean Force (PMF) Potential of mean force (PMF) is widely used to describe nanoconfined ion behaviors and energy changes of an ion traveling through a nanopore.20, 21, 43, 46, 65 Given that the size of 4-4-2 approximated the maximum diameter of biological Mg2+ channel selectivity filter, we calculated its PMF for Mg2+ and Li+ along the z-direction in the absence of an electric field. Simulation details are described in the supplementary material.

Figure 9. PMF profiles of Mg2+ and Li+ traveling through 4-4-2 graphene nanopore along the reaction coordinate. The green vertical dashed line represents nanopore position. As shown in Figure 9, PMF profiles were symmetrically distributed around the pore. The highest free energy barriers encountered by Mg2+ and Li+ passing through the nanopore were 45.01 and 15.82 kJ/mol, respectively, with peaks located 0.13 and 0.21 nm away from the nanopore, respectively. These results indicated that both ions had to overcome great energy barriers to approach the nanopore. For Li+, the local minimum was -10.47 kJ/mol and located in the nanopore position, suggesting that Li+ 20


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favorably resided in this location, and that this valley of free energy might reduce the transport mobility of Li+.46 The local minimum for Mg2+ was 28.33 kJ/mol, indicating that Mg2+ did not prefer staying at the nanopore. Moreover, we also found that the PMF curve peak-valley differences for Mg2+ and Li+ were 16.68 and 26.29 kJ/mol, respectively. These results indicated that when leaving the nanopore, Li+ had to overcome a higher energy barrier compared with Mg2+. This finding corresponded with the dehydration properties of Li+ and Mg2+ derived from the H-bond network analysis of ionic hydration shells. The free energy values shown in Figure 9 were consistent with those reported by Yang et al.,19 who calculated the PMF for Li+ and Mg2+ passing through CNT-based membranes with a diameter of 1.085 nm. Notably, in previous works,20, 21 the low free energy in PMF usually indicated that the ion could travel through the nanopore easily, especially for monovalent ions, such as K+ and Na+. This inference is contrary to the findings in this work. We found that the free energy barrier of Mg2+ transiting through the 4-4-2 graphene nanopore was higher than that of Li+ in PMF (see Figure 9), whereas the selectivity shown in Figure 2a indicated that Mg2+ traveled through the nanopore more easily than Li+. We attributed this difference to the ionic Coulomb blockade caused by electric field (see Figure 5). 5 CONCLUSION In this work, we conducted a series of atomistic simulations with three single-layer graphene-based nanopores to investigate the selectivity of Mg2+ and Li+. The graphene-based nanopores were bio-inspired by the characteristic sizes (0.76– 1.04 nm in diameter) of the selectivity filter of biological Mg2+ channels. We evaluated the effects of pore size (0.789, 1.024, and 1.501 nm in diameter) on ion selectivity and flux under different transmembrane voltages. We adopted the spatial distribution of molecules in the pore vicinity to reflect the different ionic preferential transport pathways of Li+ and Mg2+. Ionic hydration structure properties were 21


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discussed in detail to illustrate the underlying selectivity mechanisms at the molecular scale. Moreover, we investigated the PMF of ions traveling through the nanopores to explain selectivity in terms of energy. The results demonstrated that graphene nanopores exhibited potential in extracting Mg2+ from Li+ when their pore diameters were within the characteristic sizes (0.76–1.04 nm in diameter) of the selectivity filter in biological Mg2+ channels. The selectivity ratio could reach 3.26 in the pore diameter of 0.789 nm. The difference of dehydration in Mg2+ and Li+ second hydration shells was the main cause for Mg2+ and Li+ separation. By contrast with Mg2+, Li+ had to strip off more water molecules and break more hydrogen bonds in the second hydration shell. The PMF results also indicated that Li+ had to overcome a greater energy barrier than Mg2+ when leaving the nanopore. Moreover, the ionic Coulomb blockade of Mg2+ impeded Li+ transport near the pore mouth, influencing the different ionic transport pathways and thus increasing Mg2+/Li+ selectivity as the pore diameter decreased to subnanometer sizes. Overall, pore size is easier to control than decorating functional groups to the pore edge of graphene-based nanopores. These results suggested an optimum pore size range (0.76–1.04 nm in diameter) for Mg2+ and Li+ separation. Single-layer porous graphene will be more efficient in practice if assembled into 2D lamellar nanochannels, which can recover lithium from lake brine. On the other hand, given the complex structure of biological ion channels, the simple pore model adopted in this work can more evidently reflect the influences of pore size as a single factor on ionic Coulomb blockade and dehydration, thus offering some useful hints to further understand the mechanism of biological ion channels. ASSOCIATED CONTENT Supporting Information Available: This material is divided up in to the following sections: (1) Figure S1: CorA Selectivity filter structure; (2) Table S1 the details of simulation systems; (3) Table S2: LJ potential parameters; (4) Figure S2: Radial distribution function (RDF) of water molecules around Li+ and Mg2+; (5) The details 22


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of calculated potential of mean force; (6) Figure S3: Histograms of the configurations within the sampling windows and PMF curves between different sampling intervals. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHORS INFORMATION Corresponding Authors * Yudan Zhu, Email address: [email protected] * Xiaohua Lu, Email address: [email protected] Notes The authors declare no competing financial interset. ACKNOWLEDGMENTS This work was supported by the National Basic Research Program of China (2015CB655301), National Science Foundation of China (21576130, 21490584, and 21206070), and Qing Lan Project. The authors are grateful to Dr. Xubo Lin (The University of Texas Medical School at Houston), Dr. Yu Kang (College of Pharmaceutical Sciences, Zhejiang University, China) and Dr. Qing Shao (Department of Chemical and Biomolecular Engineering North Carolina State University) for their insightful suggestions. REFERENCES (1) Zhou, Y. F.; Morais-Cabral, J. H.; Kaufman, A.; MacKinnon, R. Chemistry of Ion Coordination and Hydration Revealed by a K+ Channel-Fab Complex at 2.0 Angstrom Resolution. Nature 2001, 414, 43-48. (2) Payandeh, J.; Scheuer, T.; Zheng, N.; Catterall, W. A. The Crystal Structure of a Voltage-Gated Sodium Channel. Nature 2011, 475, 353-358. (3) Hibbs, R. E.; Gouaux, E. Principles of Activation and Permeation in an Anion-Selective Cys-Loop Receptor. Nature 2011, 474, 54-60. (4) Maffeo, C.; Bhattacharya, S.; Yoo, J.; Wells, D.; Aksimentiev, A. Modeling and Simulation of Ion Channels. Chem. Rev. 2012, 112, 6250-6284.

23


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(5) Dalmas, O.; Sandtner, W.; Medovoy, D.; Frezza, L.; Bezanilla, F.; Perozo, E. A Repulsion Mechanism Explains Magnesium Permeation and Selectivity in CorA. Proc. Natl. Acad. Sci. U. S. A. 2014, 111, 3002-3007. (6) Takeda, H.; Hattori, M.; Nishizawa, T.; Yamashita, K.; Shah, S. T. A.; Caffrey, M.; Maturana, A. D.; Ishitani, R.; Nureki, O. Structural Basis for Ion Selectivity Revealed by High-Resolution Crystal Structure of Mg2+ Channel MgtE. Nat. Commun. 2014, 5, 5374. (7) Dalmas, O.; Sompornpisut, P.; Bezanilla, F.; Perozo, E. Molecular Mechanism of Mg2+-Dependent Gating in CorA. Nat. Commun. 2014, 5, 3590. (8) Matthies, D.; Dalmas, O.; Borgnia, M. J.; Dominik, P. K.; Merk, A.; Rao, P.; Reddy, B. G.; Islam, S.; Bartesaghi, A.; Perozo, E.; Subramaniam, S. Cryo-EM Structures of the Magnesium Channel CorA Reveal Symmetry Break upon Gating. Cell 2016, 164, 747-756. (9) Vikstrom, H.; Davidsson, S.; Hook, M. Lithium Availability and Future Production Outlooks. Appl. Energy 2013, 110, 252-266. (10) Xiao, C.; Xiao, L. S.; Gao, C. J.; Zeng, L. Thermodynamic Study on Removal of Magnesium from Lithium Chloride Solutions Using Phosphate Precipitation Method. Sep. Purif. Technol. 2015, 156, 582-587. (11) Chitrakar, R.; Makita, Y.; Ooi, K.; Sonoda, A. Lithium Recovery from Salt Lake Brine by H2TiO3. Dalton Trans. 2014, 43, 8933-8939. (12) Huang, L.; Zhang, M.; Li, C.; Shi, G. Q. Graphene-Based Membranes for Molecular Separation. J. Phys. Chem. Lett. 2015, 6, 2806-2815. (13) Liu, G. P.; Jin, W. Q.; Xu, N. P. Graphene-Based Membranes. Chem. Soc. Rev. 2015, 44, 5016-5030. (14) Surwade, S. P.; Smirnov, S. N.; Vlassiouk, I. V.; Unocic, R. R.; Veith, G. M.; Dai, S.; Mahurin, S. M. Water Desalination Using Nanoporous Single-Layer Graphene. Nature Nanotech 2015, 10, 459-464. (15) Joshi, R. K.; Carbone, P.; Wang, F. C.; Kravets, V. G.; Su, Y.; Grigorieva, I. V.; Wu, H. A.; Geim, A. K.; Nair, R. R. Precise and Ultrafast Molecular Sieving Through Graphene Oxide Membranes. Science 2014, 343, 752-754. (16) Shen, J.; Liu, G. P.; Huang, K.; Chu, Z. Y.; Jin, W. Q.; Xu, N. P. Subnanometer Two-Dimensional Graphene Oxide Channels for Ultrafast Gas Sieving. ACS Nano 2016, 10, 3398-3409. (17) Sint, K.; Wang, B.; Kral, P. Selective Ion Passage through Functionalized Graphene Nanopores. J. Am. Chem. Soc. 2008, 130, 16448-16449. (18) Cohen-Tanugi, D.; Grossman, J. C. Water Desalination across Nanoporous Graphene. Nano Lett. 2012, 12, 3602-3608. 24


Page 24 of 29

Page 25 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(19) Yang, D. F.; Liu, Q. Z.; Li, H. M.; Gao, C. J. Molecular Simulation of Carbon Nanotube Membrane for Li+ and Mg2+ Separation. J. Membr. Sci. 2013, 444, 327-331. (20) He, Z. J.; Zhou, J.; Lu, X. H.; Corry, B. Bioinspired Graphene Nanopores with Voltage-Tunable Ion Selectivity for Na+ and K+. ACS Nano 2013, 7, 10148-10157. (21) Kang, Y.; Zhang, Z. S.; Shi, H.; Zhang, J. Q.; Liang, L. J.; Wang, Q.; Agren, H.; Tu, Y. Q. Na+ and K+ Ion Selectivity by Size-Controlled Biomimetic Graphene Nanopores. Nanoscale 2014, 6, 10666-10672. (22) Gong, X. J.; Li, J. C.; Xu, K.; Wang, J. F.; Yang, H. A Controllable Molecular Sieve for Na+ and K+ Ions. J. Am. Chem. Soc. 2010, 132, 1873-1877. (23) Yang, Z. H. The Size and Structure of Selected Hydrated Ions and Implications for Ion Channel Selectivity. RSC Adv. 2015, 5, 1213-1219. (24) Zhou, J.; Lu, X. H.; Wang, Y. R.; Shi, J. Molecular Dynamics Study on Ionic Hydration. Fluid Phase Equilib. 2002, 194, 257-270. (25) Gokel, G. W.; Schlesinger, P. H.; Djedovic, N. K.; Ferdani, R.; Harder, E. C.; Hu, J. X.; Leevy, W. M.; Pajewska, J.; Pajewski, R.; Weber, M. E. Functional, Synthetic Organic Chemical Models of Cellular Ion Channels. Bioorg. Med. Chem. 2004, 12, 1291-1304. (26) Gokel, G. W.; Daschbach, M. M. Coordination and Transport of Alkali Metal Cations through Phospholipid Bilayer Membranes by Hydraphile Channels. Coord. Chem. Rev. 2008, 252, 886-902. (27) Yamnitz, C. R.; Negin, S.; Carasel, I. A.; Winter, R. K.; Gokel, G. W. Dianilides of Dipicolinic Acid Function as Synthetic Chloride Channels. Chem. Commun. 2010, 46, 2838-2840. (28) Shao, Q.; Zhou, J.; Lu, L. H.; Lu, X. H.; Zhu, Y. D.; Jiang, S. Y. Anomalous Hydration Shell Order of Na+ and K+ inside Carbon Nanotubes. Nano Lett. 2009, 9, 989-994. (29) Zhou, X. B.; Liu, G. D.; Yamato, K.; Shen, Y.; Cheng, R. X.; Wei, X. X.; Bai, W. L.; Gao, Y.; Li, H.; Liu, Y.; Liu, F. T.; Czajkowsky, D. M.; Wang, J. F.; Dabney, M. J.; Cai, Z. H.; Hu, J.; Bright, F. V.; He, L.; Zeng, X. C.; Shao, Z. F.; Gong, B. Self-Assembling Subnanometer Pores with Unusual Mass-Transport Properties. Nat. Commun. 2012, 3, 949. (30) Pfoh, R.; Li, A.; Chakrabarti, N.; Payandeh, J.; Pomes, R.; Pai, E. F. Structural Asymmetry in the Magnesium Channel CorA Points to Sequential Allosteric Regulation. Proc. Natl. Acad. Sci. U. S. A. 2012, 109, 18809-18814.

25


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(31) Kitjaruwankul, S.; Wapeesittipan, P.; Boonamnaj, P.; Sompompisut, P. Inner and Outer Coordination Shells of Mg2+ in CorA Selectivity Filter from Molecular Dynamics Simulations. J. Phys. Chem. B 2016, 120, 406-417. (32) Jorgensen, W. L.; Chandrasekhar, J.; Madura, J. D.; Impey, R. W.; Klein, M. L. Comparison of Simple Potential Functions for Simulating Liquid Water. J. Chem. Phys. 1983, 79, 926-935. (33) Bjelkmar, P.; Larsson, P.; Cuendet, M. A.; Hess, B.; Lindahl, E. Implementation of the CHARMM Force Field in GROMACS: Analysis of Protein Stability Effects from Correction Maps, Virtual Interaction Sites, and Water Models. J. Chem. Theory Comput. 2010, 6, 459-466. (34) Berendsen, H. J. C.; Grigera, J. R.; Straatsma, T. P. The Missing Term in Effective Pair Potentials. J. Phys. Chem. 1987, 91, 6269-6271. (35) Essmann, U.; Perera, L.; Berkowitz, M. L.; Darden, T.; Lee, H.; Pedersen, L. G. A Smooth Particle Mesh Ewald Method. J. Chem. Phys. 1995, 103, 8577-8593. (36) Van der Spoel, D.; Lindahl, E.; Hess, B.; Groenhof, G.; Mark, A. E.; Berendsen, H. J. C. GROMACS: Fast, Flexible, and Free. J. Comput. Chem. 2005, 26, 1701-1718. (37) Humphrey, W.; Dalke, A.; Schulten, K. VMD: Visual Molecular Dynamics. J. Mol. Graph. 1996, 14, 33-38. (38) Bussi, G.; Donadio, D.; Parrinello, M. Canonical Sampling through Velocity Rescaling. J. Chem. Phys. 2007, 126, 014101. (39) Parrinello, M.; Rahman, A. Polymorphic Transitions in Single-Crystals - A New Molecular-Dynamics Method. J. Appl. Phys. 1981, 52, 7182-7190. (40) Nose, S.; Klein, M. L. Constant Pressure Molecular-Dynamics for Molecular-Systems. Mol. Phys. 1983, 50, 1055-1076. (41) Aksimentiev, A. Deciphering Ionic Current Signatures of DNA Transport through a Nanopore. Nanoscale 2010, 2, 468-483. (42) Zhao, S. J.; Xue, J. M.; Kang, W. Ion Selection of Charge-Modified Large Nanopores in a Graphene Sheet. J. Chem. Phys. 2013, 139, 114702. (43) Zhu, Y. D.; Ruan, Y.; Wu, X. M.; Lu, X. H.; Zhang, Y. M.; Lu, L. H. Electric Field-Responsive Nanopores with Ion Selectivity: Controlling Based on Transport Resistance. Chem. Eng. Technol. 2016, 39, 993-997. (44) Guo, W.; Cheng, C.; Wu, Y. Z.; Jiang, Y. A.; Gao, J.; Li, D.; Jiang, L. Bio-Inspired Two-Dimensional Nanofluidic Generators Based on a Layered Graphene Hydrogel Membrane. Adv. Mater. 2013, 25, 6064-6068. (45) Mi, B. X. Graphene Oxide Membranes for Ionic and Molecular Sieving. Science 2014, 343, 740-742. 26


Page 26 of 29

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(46) Konatham, D.; Yu, J.; Ho, T. A.; Striolo, A. Simulation Insights for Graphene-Based Water Desalination Membranes. Langmuir 2013, 29, 11884-11897. (47) Zhu, C. Q.; Li, H.; Zeng, X. C.; Wang, E. G.; Meng, S. Quantized Water Transport: Ideal Desalination through Graphyne-4 Membrane. Sci Rep 2013, 3, 3163. (48) Chen, Q.; Yang, X. N. Pyridinic Nitrogen Doped Nanoporous Graphene as Desalination Membrane: Molecular Simulation Study. J. Membr. Sci. 2015, 496, 108-117. (49) Kaufman, I. K.; McClintock, P. V. E.; Eisenberg, R. S. Coulomb Blockade Model of Permeation and Selectivity in Biological Ion Channels. New J. Phys. 2015, 17, 083021. (50) Krems, M.; Di Ventra, M. Ionic Coulomb Blockade in Nanopores. J. Phys.-Condes. Matter 2013, 25, 065101. (51) Feng, J.; Liu, K.; Graf, M.; Dumcenco, D.; Kis, A.; Di Ventra, M.; Radenovic, A. Observation of Ionic Coulomb Blockade in Nanopores. Nat Mater 2016, 15, 850-855. (52) Chen, H. Y.; Ruckenstein, E. Nanomembrane Containing a Nanopore in an Electrolyte Solution: A Molecular Dynamics Approach. J. Phys. Chem. Lett. 2014, 5, 2979-2982. (53) Beckstein, O.; Tai, K.; Sansom, M. S. P. Not Ions Alone: Barriers to Ion Permeation in Nanopores and Channels. J. Am. Chem. Soc. 2004, 126, 14694-14695. (54) Zhu, Y. D.; Ruan, Y.; Zhang, Y. M.; Lu, L. H.; Lu, X. H. Nanomaterial-Oriented Molecular Simulations of Ion Behaviour in Aqueous Solution Under Nanoconfinement. Mol. Simulat. 2016, 42, 784-798. (55) Kou, J. L.; Zhou, X. Y.; Lu, H. J.; Wu, F. M.; Fan, J. T. Graphyne as the Membrane for Water Desalination. Nanoscale 2014, 6, 1865-1870. (56) Ohtaki, H.; Radnai, T. Structure and Dynamics of Hydrated Ions. Chem. Rev. 1993, 93, 1157-1204. (57) Mogami, G.; Suzuki, M.; Matubayasi, N. Spatial-Decomposition Analysis of Energetics of Ionic Hydration. J. Phys. Chem. B 2016, 120, 1813-1821. (58) Rempe, S. B.; Pratt, L. R.; Hummer, G.; Kress, J. D.; Martin, R. L.; Redondo, A. The Hydration Number of Li+ in Liquid Water. J. Am. Chem. Soc. 2000, 122, 966-967. (59) Mason, P. E.; Ansell, S.; Neilson, G. W.; Rempe, S. B. Neutron Scattering Studies of the Hydration Structure of Li+. J. Phys. Chem. B 2015, 119, 2003-2009. 27


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(60) Richards, L. A.; Schafer, A. I.; Richards, B. S.; Corry, B. The Importance of Dehydration in Determining Ion Transport in Narrow Pores. Small 2012, 8, 1701-1709. (61) Zhu, Y. D.; Guo, X. J.; Shao, Q.; Wei, M. J.; Wu, X. M.; Lu, L. H.; Lu, X. H. Molecular Simulation Study of the Effect of Inner Wall Modified Groups on Ionic Hydration Confined in Carbon Nanotube. Fluid Phase Equilib. 2010, 297, 215-220. (62) He, Z. J.; Zhou, J.; Lu, X. H.; Corry, B. Ice-like Water Structure in Carbon Nanotube (8,8) Induces Cationic Hydration Enhancement. J. Phys. Chem. C 2013, 117, 11412-11420. (63) Wu, X. M.; Lu, L. H.; Zhu, Y. D.; Zhang, Y. Y.; Cao, W.; Lu, X. H. Ionic Hydration of Na+ Inside Carbon Nanotubes, Under Electric Fields. Fluid Phase Equilib. 2013, 353, 1-6. (64) Xu, H. F.; Berne, B. J. Hydrogen-Bond Kinetics in the Solvation Shell of a Polypeptide. J. Phys. Chem. B 2001, 105, 11929-11932. (65) Samoylova, O. N.; Calixte, E. I.; Shuford, K. L. Molecular Dynamics Simulations of Ion Transport in Carbon Nanotube Channels. J. Phys. Chem. C 2015, 119, 1659-1666.

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