Ordering of Imidazolium-Based Ionic Liquids at the α-Quartz(001) Surface: A Molecular Dynamics Study

Abstract

We report a molecular dynamics study of ionic liquids (ILs) at the R-quartz(001) surface. The studied ILs are based on the 1-butyl-3-methylimidazolium (BMI + ) cation and different anions Y -(Y ) Cl, BF 4 , PF 6 , and Tf 2 N) of increasing size and hydrophobicity. Two chemically different quartz(001) model surfaces were compared: a fully hydrolyzed surface covered by silanol groups, and a more "apolar" surface, covered by silane groups. All studied ILs are found to be well-ordered at the solid/liquid interfaces, in a different manner, depending on the solid surface. Interactions with the Si(OH) 2 surface are mainly determined by the H-bonding attractions of solvent anions with silanol groups. The BMI + cations are oriented more or less parallel to the surface, depending on the nature of Y -. This contrasts with the SiH 2 surface that displays repulsive interactions with all Yanions (excepted Tf 2 N -) and is mainly solvated by BMI + cations, oriented parallel to the surface. For the [BMI][Tf 2 N] and [BMI][Cl] ILs, the comparison of dry versus humid ILs and of "real" (polar) versus all-neutral quartz surfaces reveal small perturbations of the cations orientation at the interface, indicating that their orientation is mainly determined by their interactions with the anions and, to a lesser extent, by the IL/surface Coulombic + van der Waals interactions. We finally simulated more amphiphilic ILs, one being composed of the 1-octyl-3-methylimidazolium (OMI + ) cation and the Tf 2 Nanion, the other of BMI + cations and octylsulfate (OSF -) anions. In both cases, the octyl chains are mainly parallel to the Si(OH) 2 and SiH 2 surfaces, and the orientations of the imidazolium rings are similar. In the different systems, anisotropic distribution of ions is not only observed at the contact surface, but also more deeply, and markedly differs from that observed at aqueous or "air" interfaces.

Two-Dimensional Ordering of Ionic Liquids Confined by Layered Silicate Plates via Molecular Dynamics Simulation

“…The pervasive H bond in water and ILs suggests that it's possible for water to form H bond with Br anion and the surface. 43 Similar to the dry ILs/solid interface, Br in the system F10 shows greater correlation with surface H that is almost totally lost in system R10, i.e. the appearance of a dense water layer separates the direct contact of Br with surface hydroxyl group.…”

“…23 In comparison with system A1, the weak H-bond formed between Br and hydroxyl surface in system A2 and A3 qualifies them as systems with absence of strong attractive interaction, thus the significance of Br/surface interaction, though 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 12 perceivable, is less important than the cation/anion attraction between ILs pairs, and appears only as perturbation. 43 Well-pronounced peaks in RDFs surrounding ILs hydrogen, Figure 2a, signify the strong interaction within bulk ILs in system A2 and A3, which is possible because layers of cation share the same region with Br along z direction, as opposed to the negligible correlation with the solid surface. Closer inspection of the different behaviors of ILs in system A2 and A3 gives an overall understanding of the influence of different confinement on structural arrangement of ILs.…”

Ionic Liquids in Catalysis

“…The applied methods include X-ray photoelectron spectroscopy (XPS) [106,107,, UV photoelectron spectroscopy [138,139,165,166], inverse photoelectron spectroscopy (IPES) [165,166], X-ray absorption spectroscopy (NEXAFS) [165], soft X-ray emission spectroscopy (SXES) [166], low energy ion scattering (LEIS) [141], metastable ion spectroscopy (MIES) [138,139], time-offlight secondary mass spectroscopy (TOF-SIMS) [140], Rutherford backscattering [167][168][169][170][171], high resolution electron energy loss spectroscopy (HREELS) [138], reflection absorption infrared spectroscopy (RAIRS) [172][173][174][175], direct recoil spectroscopy (DRS) [176], and scanning tunneling microscopy [177][178][179][180][181], to name only a few. In addition, an increasing number of theoretical studies and simulations have been conducted [101,[182][183][184][185][186][187][188][189][190]. The easy access to the liquid-vapour interface of ILs under UHV conditions has also opened a new era for the spectroscopic investigation of liquid surfaces in general [102].…”

Specific distributions of anions and cations of an ionic liquid through confinement between graphene sheets

“…Therefore, it is important to have a fundamental understanding about the characteristics of the confined ILs. Investigations of the interfaces of the ILs, both solid and liquid, have been carried out using both experimental and simulation studies [11][12][13][14][15][16][17][18]. Recently, structure of the electrical double layer in IL 1-butyl-3-methylimidazolium hexafluorophosphate [BMIM][PF 6 ] near a charged and uncharged graphite surface has been studied [19].…”