Molecular-scale insights into the mechanisms of ionic liquids interactions with carbon nanotubes

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.