Letter pubs.acs.org/JPCL
Nanomembrane Containing a Nanopore in an Electrolyte Solution: A Molecular Dynamics Approach Houyang Chen*,†,‡ and Eli Ruckenstein*,‡ †
Department of Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, P. R. China Department of Chemical and Biological Engineering, State University of New York at Buffalo, Buffalo, New York 14260-4200, United States
‡
ABSTRACT: Molecular dynamics simulation is used to acquire information about the characteristics of a nanographene membrane immersed in an electrolyte solution of KCl and subjected to an electric field. The membrane possesses one nanopore. It is shown that the solution contains in addition to hydrated ions, hydrated ion pairs, and hydrated clusters with more than two ions. The fractions of hydrated ions, hydrated ion pairs and hydrated clusters as well as their hydration numbers were also calculated. It was found that the hydration numbers remain constant at low electric fields but decrease at high electric fields. Under the action of an electric field, the K+ and Cl− ions separate on the two sides of graphene, thus generating hydrated ion polarization layers, which result in negative charge density layers and positive ones on the left and right interfaces of the water/graphene. Thus, the neutral graphene becomes asymmetrically charged. SECTION: Liquids; Chemical and Dynamical Processes in Solution the z-axis, and the pore has a square shape of size 2.2 × 2.2 (nm)2 and is located at the center of the graphene sheet. The concentration of KCl in the simulations was 1.0 mol/L. The bonds between hydrogen and other atoms were treated using the SHAKE algorithm.14 The CHARMM 27 force field15,16 and the TIP3P model17,18 for water were employed. The parameters of the carbon atoms of graphene were selected to be of the “CA” type under the CHARMM27 force field with periodic boundary conditions and particle mesh Ewald electrostatic calculations.19 Snapshots of the KCl solution at E = 1.0 V/nm are presented in Figure 1a,b. These figures reveal the presence, in addition to hydrated ions, of hydrated ion clusters. A hydrated K+ layer (see Figure 1a) and a hydrated Cl− layer (see Figure 1b) on the two water−graphene interfaces are identified. The fractions of hydrated ions and hydrated ion pairs as well as hydrated ion clusters (of the total number of K+ and Cl− ions) against the electric field are presented in Figure 1c. The fractions of hydrated ions and ion pairs are around 65% and 20%, respectively, at low electric fields, and decrease at high electric fields. The fractions of hydrated clusters with three and four ions are between 4.5% and 8% and between 3% and 5%, respectively. The fraction of hydrated clusters with more than four ions increases as the electric field increases. Figure 2 presents the conformation of hydrated K+−Cl− ion pairs and of hydrated clusters (such as K+−Cl−−K+, Cl−−K+−
T
he study of a nanomembrane containing one nanopore immersed into an electrolyte solution is of interest because of its applications to desalination,1−3 water purification,4 DNA and other bio(macro)molecules translocation through its pore(s),5,6 and as (bio)molecular detectors,7,8 etc. Cohen-Tanugi and Grossman1 used a nanopore in a singlelayer graphene to separate NaCl from water, and found that the salt rejection decreases as the pressure increases. Sint et al.9 prepared two kinds of functionalized graphene nanopores, and found that the ion selectively can be controlled by introducing some functional groups in the nanopores. Employing molecular dynamics simulations, He et al.10 found that a carboxylate activated nanopore can stimulate the separation of Na+ from K+ by applying a voltage. Hu et al.11 examined the NaCl transfer through a single layer of graphene containing a nanopore. When an ion or (bio)polymer is transferred through a nanopore, numerous factors, such as the nature of the solvent, the electric field, and the pressure as well as the nature of the salt, are significant because they affect the water structure, the structure of hydrated ions, and their distribution, and further the transfer process. Our objective is to show that charged layers are formed close to the membrane and that, in addition to hydrated ions, hydrated ion pairs and hydrated clusters are present. The simulations were performed using the nanoscale molecular dynamics (NAMD) program (version 2.8),12 and the snapshots were obtained using the VMD program.13 A system of size 6.68 × 6.52 × 10.00 (nm)3 containing water, KCl, and a bilayer graphene possessing a nanopore was employed. The graphene membrane is located at the center of © 2014 American Chemical Society
Received: July 17, 2014 Accepted: August 15, 2014 Published: August 15, 2014 2979
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Figure 1. Snapshots of the first shell of hydration for K+ (a) and Cl− (b) at E = 1.0 V/nm. Cyan color: carbon atoms; red: oxygen atoms of water molecules; white: hydrogen atoms of water molecules; green: chlorine ions; blue: potassium ions. (c) Fraction f of hydrated ions and hydrated ion pairs as well as hydrated clusters as functions of electric field E. Circle: hydrated ions; square: hydrated ion pairs; triangle up: hydrated clusters with three ions; triangle down: hydrated clusters with four ions; star: hydrated clusters with more than four ions.
Cl−, K2Cl2, K2Cl3, K3Cl2, and K3Cl3 clusters). The hydrated ion pairs have a linear structure, whereas the hydrated K+−Cl−−K+ and Cl−−K+−Cl− clusters have V-shaped structures. The KCl3, K3Cl, and K2Cl2 clusters have a triangular pyramid structure. For hydrated clusters with more than four ions, the K+ and Cl− ions are located alternately. In addition to the hydrated clusters listed above, hydrated clusters with more than six ions are formed at high electric fields. However, the conformations of such clusters are more complex. The hydrated clusters possess several configurations, but we present only some of them. The hydration numbers of hydrated K+, Cl−, ion pairs, and ion clusters are presented in Table1. The hydration number of K+ is about 6.6 for weak electric fields. This value is somewhat larger than the hydration number of 6.0 determined through neutron diffraction.20 By increasing the electric field to 5.0 V/ nm, the hydration number of K+ decreases somewhat to 6.2. This occurs because the strong fields change the orientation of water molecules around K+, and disrupt the hydrogen bonding of water. The hydration number of Cl− is around 7.4, and this value does not change under strong electric fields, probably because of the hydrogen bonding between H2O and Cl−. The hydration numbers of ion pairs, ion clusters with three ions, and ion clusters with four ions are about 11.4, 15.2, and 18.6, respectively, for weak electric fields, and decrease for strong electric fields. Of course, low electric fields do not affect the orientation of water in hydrated ions, ion pairs, and clusters, whereas strong electric fields do. Strong electric fields change the orientation of hydrating water molecules and disrupt the hydrogen bonding, thus modifying the structure of hydrated ions. To obtain information regarding the hydrated ion layers close to the membrane, the number density profiles of K+ and Cl− along the z-direction were calculated for various strengths
Figure 2. Snapshots of hydrated K+−Cl− ion pairs and hydrated clusters composed of K+ and Cl− at E = 0.1 V/nm: (a) Hydrated K+− Cl− pair; (b) Hydrated Cl−−K+−Cl− cluster; (c) hydrated K+−Cl−− K+ cluster; (d) hydrated K2Cl2 cluster; (e) hydrated K3Cl cluster; (f) hydrated K3Cl2 cluster; (g) hydrated KCl3 cluster; (h) hydrated K2Cl3 cluster; (i) hydrated K3Cl3 cluster. Red: oxygen atoms of the water molecules; white: hydrogen atoms of the water molecules; green: chlorine ions; blue: potassium ions.
of the electric field. Under the action of the electric field, Cl− and K+ generate polarization layers at the left and right of the graphene membrane, respectively. For E = 0.1 V/nm, one weak layer of hydrated Cl− and a weak one of K+ are formed on the two faces of the graphene membrane. As the field strength increases to E = 1.0 V/nm (Figure 3a,b), the hydrated ions polarization layers (of both K+ and Cl−) become stronger. Two layers of hydrated K+ and two layers of hydrated Cl− were formed. By increasing the field to 5.0 V/nm, one hydrated Cl− layer at the left and one hydrated K+ layer at the right interface were generated. The location of hydrated ions becomes closer to graphene for larger electric fields. This occurs because, for weak electric fields, the location of hydrated ion layers is dominated by the repulsive interactions between the hydrated ions and graphene, while, for strong electric fields, their location is dominated by the electric field. The presence of hydrated ions (Figure 3a,b) results in asymmetric charge density layers at the water−graphene interfaces. Thus, the neutral graphene becomes asymmetrically charged. To obtain the structure of ions on the layer closer to the membrane, the fraction of hydrated ions, hydrated ion pairs and hydrated ion clusters and their hydration numbers were calculated. For E = 0.1 V/nm, there are 73% hydrated ions 2980
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Table 1. Average Hydration Number for the Entire System 0 0.1 0.5 1 2 5
Cl−
K+
E (V/nm) 6.6 6.5 6.6 6.6 6.6 6.2
± ± ± ± ± ±
0.1 0.1 0.1 0.1 0.1 0.3
7.4 7.4 7.4 7.4 7.3 7.3
± ± ± ± ± ±
0.1 0.1 0.1 0.1 0.1 0.2
ion pair 11.4 11.4 11.4 11.5 11.7 11.2
± ± ± ± ± ±
clusters with 3 ions
0.3 0.3 0.3 0.3 0.3 0.4
15.2 15.1 15.1 15.3 15.4 14.3
± ± ± ± ± ±
0.7 0.7 0.7 0.6 0.6 1.1
clusters with 4 ions 18.7 18.2 18.5 18.8 18.8 17.6
± ± ± ± ± ±
1.6 2.5 2.0 1.8 1.0 1.4
Figure 3. Number density profiles of K+ (a), Cl− (b), H (of H2O) (c), and O (of H2O) (c) along the z-direction for E = 1.0 V/nm.
hydration numbers of both K+ and Cl− ions, ion pairs and hydrated ion clusters first remain unchanged and later decrease. Under the action of an electric field, the hydrated K+ and Cl− ions separate, thus generating hydrated ion polarization layers with a negative charge on one side of the membrane and a positive one on the other side. Thus, the neutral graphene becomes asymmetrical charged graphene.
and 19% hydrated ion pairs in the hydrated ion layers close to the membrane, and the hydration numbers of K+ and Cl− ions are 6.8 and 7.7, respectively, while the hydration number of ion pairs is between 10 and 11. As the electric field increases to 1.0 V/nm, the hydrated ion layers contain more than 90% hydrated ions with hydration numbers of 6.7 for K+ and 6.6 for Cl−. After further increase of the electric field to 5.0 V/nm, the hydrated ion layers contain more than 93% hydrated ions with hydration numbers of 5.5 for K+ and 6.4 for Cl−. In summary, for weak electric fields, the layers contain mainly hydrated ions and hydrated ion pairs, whereas for strong electric fields, the layers contain mostly hydrated ions. Two water layers on both sides of graphene are formed with asymmetric density profiles at E = 1.0 V/nm (Figure 3c). This occurs because (1) the strong electric field changes the orientation of water; (2) asymmetric hydrated K+ and Cl− layers are generated at the water/graphene interfaces. The first and second peaks of H (of H2O) at the left side are more leftshifted than those of O (of H2O), whereas the first and second peaks of H (of H2O) at the right side are more right-shifted than those of O (of H2O), indicating that, in the polarized water layers, the O (of H2O) is closer to the interface. By decreasing the electric field to a weak one (e.g., E = 0.1 V/nm), about two water layers on both sides of graphene are formed, and the profiles at the two sides of graphene become almost symmetric. By increasing the electric field to E= 5.0 V/nm, four water layers are generated on each of the two sides of graphene, which are asymmetric. The peaks are much larger than those provided by the weak or moderate electric fields, indicating that high density water layers are formed. In addition, the polarization of the water layers becomes stronger. It should be mentioned that the dipole of water generated with strong electric fields would affect the charges of the membrane. In conclusion, by employing all atom molecular dynamic simulations, it is shown that in addition to hydrated ions, hydrated ion pairs and hydrated ion clusters are present, and the fraction of ions forming ion pairs and clusters increases with increasing electric field. By increasing the electric field, the
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: hchen23@buffalo.edu (H.C.). *E-mail: feaeliru@buffalo.edu (E.R.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We wish to thank the Center for Computational Research in State University of New York at Buffalo for the computer time provided.
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