Complementary Molecular Dynamics and X-ray Reflectivity Study of an Imidazolium-Based Ionic Liquid at a Neutral Sapphire Interface

Abstract

Understanding the molecular-level behavior of ionic liquids (ILs) at IL-solid interfaces is of fundamental importance with respect to their application in, for example, electrochemical systems and electronic devices. Using a model system, consisting of an imidazolium-based IL ([C2Mim][NTf2]) in contact with a sapphire substrate, we have approached this problem using a complementary combination of high-resolution X-ray reflectivity measurements and atomistic molecular dynamics (MD) simulations. Our strategy enabled us to compare experimental and theoretically calculated reflectivities in a direct manner, thereby critically assessing the applicability of several force-field variants. On the other hand, using the best-matching MD description, we are able to describe the nature of the model IL-solid interface in appreciable detail. More specifically, we find that characteristic interactions between the surface hydroxyl groups and donor and acceptor sites on the IL constituents have a dominant role in inducing a multidimensional layering profile of the cations and anions.

Insights from molecular dynamics simulations on structural organization and diffusive dynamics of an ionic liquid at solid and vacuum interfaces

“…Naturally, even with the excess of positive charge at the sapphire surface, there are almost equal numbers of cations and anions integrated over the first several layers. Despite differences in geometry and force field, an analogous result was also found in our previous work 65 when plotting the 2D-histogram of the centre-of-mass positions of the ions. It was proposed there that since solid substrate is uncharged, this ordering cannot be understood from a simplistic balance of ionic interactions and that it can be attributed to hydrogen bonding of both cation and anion with the fully hydroxylated sapphire surface.…”

“…62,63,64 In previous work from our group, using an archetypical model system consisting of an imidazolium-based IL ([C 2 Mim][NTf 2 ]) in contact with a fully hydroxylated sapphire substrate, we presented and compared the results of high-resolution X-ray reflectivity measurements and extensive MD simulations with IL in a solidliquid-solid (S-L-S) configuration. 65 In that work, apart from examining which ff was suitable, we also paid close attention to which mixing rule was more relevant for simulating the IL on a solid substrate. It was established, using the experimental X-ray reflectivity as a clear benchmark, that the Maginn ff (with CHelpG charges) for the cation and the CL&P ff for the anion was preferable than the approach proposed by Kodderman.…”

Molecular scale structure and dynamics at an ionic liquid/electrode interface

“…15,16 Different experimental techniques have been used to study the molecular-scale structure of ILs at solid/liquid interfaces. Interfacial layering was observed in atomic force microscopy (AFM) [17][18][19][20] and X-ray reectivity (XRR) measurements [21][22][23][24] for several ILs on various substrates. In thin IL lms, long range ordered structures have been found by helium atom scattering.…”

π+–π+ stacking of imidazolium cations enhances molecular layering of room temperature ionic liquids at their interfaces

“…Experimentally, the interfacial structures of RTILs have been investigated by X-ray spectroscopy, [6][7][8][9][10][11] neutron reflectivity, 12,13 atomic force microscopy, 14 sum-frequency generation (SFG) spectroscopy, [15][16][17][18][19] and surface force measurements. 20,21 Owing to its selection rules, SFG spectroscopy is specifically sensitive to the near-surface region in which the symmetry of the molecular arrangement is broken.…”