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 α-quartz(001) surface. The studied ILs are based on the 1-butyl-3-methylimidazolium (BMI+) cation and different anions Y− (Y = Cl, BF4, PF6, and Tf2N) 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 SiH2 surface that displays repulsive interactions with all Y− anions (excepted Tf2N−) and is mainly solvated by BMI+ cations, oriented parallel to the surface. For the [BMI][Tf2N] 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 Tf2N− anion, the other of BMI+ cations and octylsulfate (OSF−) anions. In both cases, the octyl chains are mainly parallel to the Si(OH)2 and SiH2 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.