Supplementary Information for
Ion Enrichment on the Hydrophobic Carbon-based Surface in Aqueous Salt Solutions due to Cation-π Interactions
Guosheng Shi1, Jian Liu1,2, Chunlei Wang1, Bo Song1, Yusong Tu3, Jun Hu1, and Haiping Fang1*
1Department of Water Science and technology and Department of Physical Biology, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China
2Graduate School of the Chinese Academy of Sciences, Beijing, 100080, China
3Institute of Systems Biology, Shanghai University, Shanghai, 200444, China
Correspondence and requests for materials should be addressed to H. F. ()
Details of Density Functional Theory (DFT) Calculations.
Part 1. Modeling Systems.
Part 2. Adsorption of Water Clusters on the Graphite Surface.
Part 3. Adsorption of Hydrated Na+ on the Graphite Surface.
Part4. Adsorption of Hydrated Cl- on the Graphite Surface.
Part5. Full citation of Gaussian-03 program.
Part 1. Modeling Systems.
A two-dimensional graphite surface of 12.275×15.658 Å2 is used, which is large enough to obtain results with a tolerable errorS1. This surface is then used for the following study, which is fixed during geometry optimizations. As shown in Fig. S1, all edge carbon atoms with dangling bonds are passivated by hydrogen atoms.
Figure S1 | Schematic description of different adsorption sites on a finite-size graphite monolayer surface (C84H24）of 12.275×15.658 Å2 (84 carbon atoms and 24 hydrogen atoms), above the hollow (H), C-C bonds (B), and carbon atom top (T) sites of the hexagonal ring. Moreover, the geometrical structure of three-dimensional graphite models AA-stacked multilayer graphite (2C84H24-Na+-a) and AB-stacked multilayer graphite (2C84H24-Na+-b) are shown. Na+ is denoted by purple spheres. The grey hexagonal rings and small white balls are the carbon atoms and hydrogen atoms of the graphite.
We first consider the adsorption of ions on a graphite surface. We define the ion adsorption energy (ΔEi) as:
， (S1)
where Eion, EG, and Eion-G are the total energies of the isolated ion, the graphite monolayer, and the ion-adsorbed graphite, respectively.
Table S1 The ion adsorption energies (ΔEi), average ion-carbon distances (Rion-C), and residuary charges of the adsorbed ions (Charge) for different sites, i.e., above the hollow (H), C-C bonds (B), and carbon atom top (T) of the hexagonal ring at the B3LYP/6-31G(d) level.
Levels / ΔEi (kcal/mol)Na+-C84H24(H) / -39.6a
Na+- C84H24(B) / -37.6a
Na+- C84H24(T) / -37.2a
Na+-2C84H24(H)-a / -43.0
Na+-2C84H24(H)-b / -43.3
a Previously reported in ref S1
The geometrical structure of three-dimensional graphite models b (AA-stacked multilayer graphite) and c (AB-stacked multilayer graphite) with size of 12.265×15.678 (C84H24) are investigated (see Fig. S1). Table S1 shows that the adsorption energies of three-dimensional graphite models AA-stacked multilayer graphite (2C84H24-Na+-a) and AB-stacked multilayer graphite (2C84H24-Na+-b) are higher 3.4 and 3.7 kcal/mol than the adsorption energy of single layer graphite, respectively. It demonstrates that Na+ is more strongly adsorbed onto the three-dimensional multilayer graphite.
Part 2. Adsorption of Water Clusters on the Graphite Surface.
The possible geometries of the (H2O)n and graphite-(H2O)n clusters for n = 6-9 (n=1-5 in previously reportedS1) are investigated, and the most stable structures therein are shown in Fig. S2. The adsorption energies (ΔEwn) of the water clusters were calculated as follows:
， (S2)
where Eclu-G and Eclu are the total energies of the water clusters adsorbed on the graphite and the water clusters. We find that the adsorption energies of the (H2O)n clusters for n=6-9 that adsorbed on graphite were -3.2, -2.1, -2.1, and -2.4 kcal/mol, respectively. These are on the order of 4 kBT for T=300K, showing that the adsorption of water molecules on a graphite surface is unstable in an environment of thermal fluctuations.
Figure S2 | The most stable optimized geometries of the (H2O)n and graphite-(H2O)n clusters for n=6-9. Water molecules are shown with oxygen in red and hydrogen in white.
Part 3. Adsorption of Hydrated Na+ on the Graphite Surface.
The possible geometries of the Na+-(H2O)n and graphite-[Na+-(H2O)n] for n = 6-9 (n=1-5 in previously reportedS1) are investigated, and the most stable structures therein are shown in Fig. S3. The adsorption energy (ΔEin) of the hydrated Na+ (or hydrated Cl-) adsorbed onto graphite is calculated as follows:
, (S3)
where Ehyd-G and Ehyd are the adsorption energies of the hydrated Na+ (or hydrated Cl-) adsorbed onto the graphite and the total energies of the hydrated Na+ (or hydrated Cl-) , respectively. For the hydrated Na+, we find that ΔEin = -22.0, -19.1, -19.0 and -17.8 kcal/mol for n = 6-9, which consists of two parts: H2O-π interaction and Na+-π interaction. The water-graphite interaction (H2O-π interaction) energies are -3.2, -1.8, -2.5, and -1.4 kcal/mol for n = 6-9, respectively. Thus, we can obtain the Na+-π interaction by ΔEin – ΔEwn, which are -18.8, -17.3, -16.5, and -16.4 kcal/mol for n = 6-9, respectively, indicating that the water-graphite interactions decrease the Na+-π interaction.
Figure S3 | The most stable structures of the Na+-(H2O)n and graphite-[Na+-(H2O)n] clusters for n=6~9. Water molecules are shown with oxygen in red and hydrogen in white. Na+ is denoted by purple spheres. The grey structures are the graphite sheet.
To elucidate the form of the model potential between the hydrated Na+ and the graphite surface, we analyze the interaction of a Na+ with nine water molecules adsorbed on the graphite surface. We calculate the adsorption energies of the hydrated Na+(H2O)9 adsorption on the graphite surface at the different adsorption distance z. The adsorption energies are 30.5, 6.5, -4.6, -15.3, -16.4, -15.8, -14.4, -12.9, -11.5, and -10.3 kcal/mol for the adsorption distance z = 3.1, 3.2, 3.4, 3.6, 3.8, 4.0, 4.2, 4.4, 4.6, and 4.8 Å, respectively. To represent the dissolution behavior of NaCl on graphene surfaces, the orbital polarizations rejection of adjacently adsorbed Na+ on graphene surfaces are considered based on quantum calculations. Structures and repulsive potential are obtained with graphene atoms fixed and Na+-Na+ distance frozen. A repulsive potential between Na+ are complemented in the classical modeling system, with a form of scaled electrostatic repulsion and the values are 1/20 of the electrostatic repulsion energies. Moreover, the zm = 3.2 Å and e = e0 = -15.6 kcal/mol are used to further test this model potential, corresponding to the case of a Na+ with five water molecules adsorbed on the graphite surface. We calculate the adsorption energies of the hydrated Na+(H2O)9 adsorption on the graphite surface at the different adsorption distance z. The adsorption energies are 23.9, 9.9, -6.5, -13.0, -14.6, -13.7, -11.9, -9.9, -7.9, and -6.1 kcal/mol for the adsorption distance z = 2.5, 2.6, 2.8, 3.0, 3.2, 3.4, 3.6, 3.8, 4.0, and 4.2 Å, respectively. Fig. S4 shows this model potential is still available.
Figure S4 | Adsorption energies of Na+ with five (blue triangle) and nine (black square) water molecules on the graphite surface on the different distance (z, the vertical dimension between the Na+ and the surface) at the B3LYP/6-31G(d) level and the fitting potential.
Part 4. Adsorption of Hydrated Cl- on the Graphite Surface.
The possible geometries of the Cl--(H2O)n and graphite-[Cl--(H2O)n] clusters for n=1,2 were investigated, and the most stable structures therein are shown in Fig. S5.
Figure S5 | The most stable structures of the Cl--(H2O)n and graphite-[Cl--(H2O)n] clusters for n=1,2. Water molecules are shown with oxygen in red and hydrogen in white. Cl- is denoted by green spheres. The grey structures are the graphite sheet.
For the hydrated Cl-, we find that ΔEin = -3.8, and -1.8 kcal/mol for n = 1,2, which is less than 1/10 of the hydrated Na+-π interaction.
Part5. Full citation of Gaussian-03 program.
Gaussian 03, Revision D.01, Frisch, M. J., Trucks, G. W., Schlegel, H. B., Scuseria, G. E., Robb, M. A., Cheeseman, J. R., Montgomery, J. A., Vreven, T., Kudin, K. N., Burant, J. C., Millam, J. M., Iyengar, S. S., Tomasi, J., Barone, V., Mennucci, B., Cossi, M., Scalmani, G., Rega, N., Petersson, G. A., Nakatsuji, H., Hada, M., Ehara, M., Toyota, K., Fukuda, R., Hasegawa, J., Ishida, M., Nakajima, T., Honda, Y., Kitao, O., Nakai, H., Klene, M., Li, X., Knox, J. E., Hratchian, H. P., Cross, J. B., Adamo, C., Jaramillo, J., Gomperts, R., Stratmann, R. E., Yazyev, O., Austin, A. J., Cammi, R., Pomelli, C., Ochterski, J. W., Ayala, P. Y., Morokuma, K., Voth, G. A., Salvador, P., Dannenberg, J. J., Zakrzewski, V. G., Dapprich, S., Daniels, A. D., Strain, M. C., Farkas, O., Malick, D. K., Rabuck, A. D., Raghavachari, K., Foresman, J. B., Ortiz, J. V., Cui, Q., Baboul, A. G., Clifford, S., Cioslowski, J., Stefanov, B. B., Liu, G., Liashenko, A., Piskorz, P., Komaromi, I., Martin, R. L., Fox, D. J., Keith, T., Al-Laham, M. A., Peng, C. Y., Nanayakkara, A., Challacombe, M., Gill, P. M. W., Johnson, B., Chen, W., Wong, M. W., Gonzalez, C., Pople, J. A. Gaussian., Inc., Wallingford CT (2004).
References
S1. Shi, G. S., Wang, Z. G., Zhao, J. J., Hu, J. & Fang, H. P. Adsorption of sodium ions and hydrated sodium ions on the hydrophobic graphite surface via cation-π interactions. Chin. Phys. B. 20, 068101 (2011).
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