Supplementary Material. Physisorption of Hydrophobic and Hydrophilic 1-alkyl-3- methylimidazolium Ionic Liquids on the Graphite Plate Surface

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1 Supplementary Material Physisorption of Hydrophobic and Hydrophilic 1-alkyl-3- methylimidazolium Ionic Liquids on the Graphite Plate Surface Mohammad Hadi Ghatee * and Fatemeh Moosavi (Department of Chemistry, Shiraz University, Shiraz 71454, Iran) ghatee@susc.ac.ir Fax: Tel: , * Corresponding author 1.S) Molecular structures of ionic liquids The value of H 8 L A distance and C - H8 L A angle are functions of both anion and alkyl chain length. For instance, in the case of [C 1 mim]cl the chloride anion takes a top-plane position and the H 8 L Cl distance is about. Å, which is quite close to the results of X-ray diffraction (.58 Å) and CPMD (.5 Å) reported by Del Poplo et al, 1 as well as being comparable with the hydrogen bonding distance. Therefore, it can be concluded that chloride interacts closely with C - H 8 channel. The angle of C - H8L Cl is also close to the right angle, beside the fact that the C 4 -N 3 -C -Cl dihedral angel clearly signifies the top-plane position of chloride anion. In the case of [C 4 mim]cl the position of chloride alters to an in-plane position, where the anion is perfectly co-planar with the imidazolium ring. See Table 1.S for numerical data. At this position, the chloride anion interacts with three hydrogen atoms, H 8, H 11, and H 14, (see Scheme 1) though it keeps the closest distance to the H 8 atom.,3

2 The situation with [C 1 mim]pf and [C 4 mim]pf is quite different as the relative structure of anion-cation is concerned. In both ILs, as indicated by the angle values in Table 1.S, the anion takes a top-plane position such that three hydrogen atoms, H 8, H 11, and H 14, attract the triplet fluoride atoms closely. For [C 1 mim]pf, the three hydrogen atoms are in position eclipsed with respect to each other, while in [C 4 mim]pf the H 14, contrary to H 11, tends to a staggered position with respect to H 8. The position of the C - H8L P angle is almost 15º. PF anion is such that in both [C 1 mim]pf and [C 4 mim]pf The agreement between the results in this work and reported in literature for Cl,,3 and PF, 3,4 salts is quite good. For example, top-plane structures have been obtained in references 3 and 4 for [C 1 mim]pf and [C 4 mim]pf and the corresponding molecular parameter distances and angles are very much close to the present results. The alkyl chain length has a strong influence on the relative structure of monatomic Cl anion with their respective cation, while an optimal influence can be observed in the case of polyatomic anion PF. The anion type as well as the alkyl chain length, which is worth detailed considerations, determines the conformation of highly interacting ring's atoms. The chloride anion, as the case of PF, interacts mainly with - H8 C channel. However, the situation with the halide is different in that as the alkyl chain length increases, the anion changes its position with respect to the imidazolium ring.

3 [C 1 mim]cl [C 4 mim]cl [C 1 mim]pf [C 4 mim]pf Figure 1.S. The optimized structure of ILs obtained at B3LYP/-311g level of theory. Green, chlorine; dark gray, carbon; light gray, hydrogen; dark blue, nitrogen, orange, phosphorus; and cyan, fluorine. a TABLE 1.S: The comparison of anion-cation interaction energies (E inter ) and structural parameters obtained at B3LYP/-311g level of theory for ILs studied IL a E inter (kj.mol -1 ) µ (Debye) C -H 8 -A (degree) C 4 -N 3 -C -A (degree) C - H 8 (Å) C - H (Å) b H 8 L A (Å) [C 1 mim]cl [C 4 mim]cl [C 1mim]PF [C mim]pf [ E (au) E + (au) E (au)] -1 E inter / kj.mol = 5.50 AX A X b C-H refers to C4 -H 9 and C 5 -H 10 bonds as in Scheme 1. 3

4 .S) Molecular structures of methyl imidazolium ionic liquids on coronene It can be found from Table.S that BSSE values are basically small, 1.39 kj.mol -1 for [C 1 mim]cl and 1.33 kj.mol -1 for [C 1 mim]pf. The influence of BSSE for [C 1mim]PF is insignificant as compared to [C 1 mim]cl and leads to a smaller error. TABLE.S: Comparison of H ads. with and without counterpoise (CP) for adsorbed [C 1 mim]cl and [C 1 mim]pf ILs on coronene substrate obtained at B3LYP/-311g level of theory IL -E graphite+il (a. u.) µ (Debye) a H ads. (kj.mol -1 ) CP a H ads. (kj.mol -1 ) No CP [C 1 mim]cl [C 1mim]PF a / kj.mol 5.50 [ (au) (au) (au) ] -1 H E E E ads. = graphite + IL graphite IL The interaction energy, E inter, between each cation and its corresponding anion was calculated. The strongest interaction was found in [C 1 mim]cl, which can be attributed to the anion. The calculated interaction energy between [C 1 mim] + and PF is smaller than between [C 1 mim] + and Cl by kj.mol -1, indicating the difference in the anion-cation attraction in these two ILs. See Table 1.S. This will cause [C 1 mim]pf to be less compact than [C 1 mim]cl which then leads to a stronger adsorption (to be discussed in details). However, the larger the molecule the more diffused the charge, which is also consistent with the lower interaction energy. Another factor influencing the adsorption of IL on coronene is the IL configuration and orientation upon adsorption. By the present computations, [C 1 mim]cl tends to orient perpendicularly on the graphite surface in agreement with the results of molecular dynamics simulation. 5 As it can be seen in Figure.S, the imidazolium ring interacts closely with graphite from C 4 =C 5 side, while Cl interacts with H 8, H 11, and H 14 atoms and keeps the top-plane 4

5 position. Under this situation, Cl anion tends towards the edge plane carbon atoms of the coronene. Therefore, the original structure of [C 1 mim]cl is more or less preserved when adsorbed on the graphite surface. This is also true for [C 1 mim]pf adsorption. The structural orientation of the adsorbed [C 1 mim]pf can be further evaluated by placing the imidazolium ring and the anion near the coronene surface. Noticeably, the results show that the anion and the cation compete for adsorption on coronene surface such that both the cation and the anion keep the least possible distance to the surface. Figure.S. The optimized structure of [C 1 mim]cl/coronene obtained at B3LYP/-311g level of theory. 5

6 TABLE 3.S: HOMO and LUMO energies and their gaps for ILs on the coronene plate calculated by B3LYP/-311g. Circumcoronene is showen for comparison System HOMO LUMO E (a.u.) (a.u.) (ev) [C 1 mim]cl [C 1mim]PF Coronene Circumcoronene [C 1 mim]cl+coronene [C 1mim]PF +coronene C4H1 [C1mim]Cl [C1mim]Cl+C4H1 DOS E (ev) Figure 3.S. Comparison of the DOS obtained at B3LYP/-311g level of theory before and after adsorption for [C 1 mim]cl on coronene plate.

7 8 C4H1 [C1mim]PF [C1mim]PF+C4H1 DOS E (ev) Figure 4.S. Comparison of the DOS obtained at B3LYP/-311g level of theory before and after adsorption for [C 1 mim]pf on coronene plate. 3.S) Charge transfer 0.4 a) 0. atomic charge/e C54H18 [C1mim]Cl [C1mim]Cl+C54H atom label 7

8 0.4 b) 0. 0 atomic charge/e C54H18 [C4mim]Cl [C4mim]Cl+C54H atom label 8

9 C54H18 [C1mim]PF [C1mim]PF+C54H18 c) atomic charge/e atom label C54H18 [C4mim]PF [C4mim]PF+C54H18 d) atomic charge/e atom label Figure 5.S. Comparison of the atomic charges before and after adsorption, obtained at B3LYP/- 311g level of theory. a) [C 1 mim]cl and circumcoronene. See Figure b for the atomic labels. Atom labels 1 to 17 are due to the IL. b) [C 4 mim]cl and circumcoronene. See Figure 4 for atom labels. Atom labels 73 to 98 are due to IL. c) [C 1 mim]pf and circumcoronene. See Figure 3 for the atomic labels. Atom labels 73 to 95 are due to IL. d) [C 4 mim]pf and circumcoronene. See Figure 5 for the atomic labels. Atom labels 73 to 104 are due to the IL. 9

10 References (1) Del Popolo, M. G.; Lynden-Bell, R. M.; Kohanoff, J. J. Phys. Chem. B 005, 109, () (a) Koßmann, S.; Thar, J.; Kirchner, B.; Hunt P. A.; Welton, T. J. Chem. Phys. 00, 14, (b) Hunt, P. A.; Gould, I. R. J. Phys. Chem. A 00, 110, 9-8. (3) Dong, K.; Zhang, S.; Wang, D.; Yao, X. J. Phys. Chem. A 00, 110, (4) Katsyuba, S. A.; Zvereva, E. E.; Vidis, A.; Dyson, P. J. J. Phys. Chem. A 007, 111, (5) Pinilla, C.; Del Popolo, M. G.; Lynden-Bell, R. M.; Kohanoff, J. J. Phys. Chem. B 005, 109,