[Show abstract][Hide abstract] ABSTRACT: We have studied structural transitions in the electrical double layer of ionic liquids by molecular dynamics simulations. A model coarse grained room temperature ionic liquid (RTIL) with asymmetric sized ions confined between two oppositely charged walls has been used. The simulations have been performed at different temperatures and electrode charge density values. We found that for the studied charge densities the electrical double layer has a multilayered structure with multiple alternating layers of counter- and co-ions at the electrode–RTIL interface; however, at certain charge densities the alternating multilayer structure of the electrical double layer undergoes a structural transition to a surface-frozen monolayer of densely packed counter-ions (Moiré-like structure). At this point the dense ordered monolayer of counter-ions close to the electrode surface coexists with apparently non-structured RTIL further from the electrode. These findings might bring possible explanations to experimental observations of formation of Moiré-like structures in ionic liquids at electrified interfaces. Moreover, we report the formation of herring-bone interfacial structures at high surface charge densities, that appear as a result of superposition of two ordered monolayers of RTIL ions at the electrode–RTIL interface. Similar structures were observed experimentally; however, to the best of our knowledge they have not been modelled by simulations. We discuss the dependence of the electrical double layer structure in RTILs on the ion size and the surface charge density at the electrodes.
[Show abstract][Hide abstract] ABSTRACT: By means of fully atomistic molecular simulations we study basic mechanisms of carbon nanotube interactions with several different room temperature ionic liquids (RTILs) in their mixtures with acetonitrile. To understand the effects of the cation molecular geometry on the properties of the interface structure in the RTIL systems, we investigate a set of three RTILs with the same TFSI (bis (trifluoromethylsulfonyl)imide) anion but with different cations, namely, EMIm (1-ethyl-3-methylimidazolium), BMIm (1-butyl-3-methylimidazolium) and OMIm (1-octyl-3-methylimidazolium) ions. The cations have identical charged methylimidazolium 'heads' but different nonpolar alkyl 'tails' where the length of the tail increases from EMIm to OMIm. The analysis of the simulation data results in the following conclusions: There is an enrichment of all molecular components of ionic liquids under study at the CNT surface with formation of several distinct layers even at the non-charged CNT surface. Mixing RTIL with acetonitrile decreases ion-counterion correlations in the electric double layer. Increase of the length of the non-polar 'tail' of cations increases the propensity of imidazolium-based cations to lay parallel to the CNT surface. At the CNT cathode TFSI anions and molecular cations are preferentially oriented parallel to the surface. At the CNT anode the TFSI anions are oriented parallel to the surface, however the preferred orientations of cations depend on the length of non-polar tail: EMIm cations are oriented perpendicular to the surface, BMIm cations can be in both parallel as well as perpendicular orientations, OMIm cations are oriented parallel to the surface. As a result, by applying an electric potential on the CNT electrode and/or varying the structure of molecular ions it is possible to change molecular ion orientations at the surface and, consequently, the structure of the electrical double layer at the CNT-RTIL interface.