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*
1
Department of Water Science and technology and Department of Physical Biology,
Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai
201800, China
2
Graduate School of the Chinese Academy of Sciences, Beijing, 100080, China
3
Institute of Systems Biology, Shanghai University, Shanghai, 200444, China
Correspondence and requests for materials should be addressed to H. F.
([email protected])
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
1
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:
ΔEi = Eion-G - Eion - EG
，
(S1)
where Eion, EG, and Eion-G are the total energies of the isolated ion, the graphite
monolayer, and the ion-adsorbed graphite, respectively.
2
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.
ΔEi (kcal/mol)
Levels
a
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
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 threedimensional multilayer graphite.
Part 2. Adsorption of Water Clusters on the Graphite Surface.
3
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:
ΔEwn = Eclu-G - Eclu - EG
，
(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.
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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:
ΔEin = Ehyd-G
- EG - Ehyd ,
(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.
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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  = 0 = -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,
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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.
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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,
8
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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