6572 Chem. Commun., 2011, 47, 6572–6574 This journal is c The Royal Society of Chemistry 2011 Cite this: Chem. Commun., 2011, 47, 6572–6574 Self-assembly in the electrical double layer of ionic liquids Susan Perkin,* a Lorna Crowhurst, b Heiko Niedermeyer, b Tom Welton, b Alexander M. Smith a and Nitya Nand Gosvami a Received 7th March 2011, Accepted 20th April 2011 DOI: 10.1039/c1cc11322d We have studied the structure of two ionic liquids confined between negatively charged mica sheets. Both liquids exhibit interfacial layering, however the repeat distance is dramatically different for the two liquids. Our results suggest a transition from alternating cation–anion monolayers to tail-to-tail cation bilayers when the length of the cation hydrocarbon chain is increased. Many applications of ionic liquids (ILs) involve their use as electrolytes in electrochemical applications such as solar cells and electrical double-layer capacitors (EDLCs, also called super- capacitors). 1,2 Currently there is relatively little understanding of the interfacial behaviour of ILs at charged surfaces, although recent experimental and theoretical work have revealed that the electrical double layer in IL is dramatically different to that in dilute electrolyte solutions. Experiments using a surface force apparatus (SFA) 3,4 and AFM 5 suggest that next to a negative surface lies a monolayer of cations, (C), followed by alternating monolayers of anions (A) and cations (C) repeating for 3–8 repeat units (i.e. C(AC) n where n = 3–8) away from the surface. This alternating cation–anion layer structure was also observed at a single sapphire surface probed by X-ray reflectometry. 6 Molecular dynamics (MD) simulations of IL in the region near an electrode surface reveal oscillations in cation and anion density with distance from the surface. 7 The space-filling nature of the ions near the electrode also leads to a qualitatively different dependence of double layer capacitance on electrode potential: a local maximum (or ‘camel shape’ with two maxima for non-spherical ions) at the potential of zero charge (PZT) is predicted and observed for an ionic liquid, 8 in contrast to the minimum expected for dilute electro- lytes. 9 For applications in EDLCs, where the electrode is composed of micro- or nano-porous materials in order to maximise the surface area, 2 the effect on capacitance of confining the electrolyte to nanoscopic geometries is important. For example it has been shown that optimal capacitance can be achieved when the pore size is less than 1 nm within which only a single ion can be trapped between the electrode surfaces. 10 Here we present high resolution force measurements which reveal the interfacial structure of two ILs, 1-butyl-3-methyl- imidazolium bis(trifluoromethylsulfonyl)imide, [C 4 C 1 im][NTf 2 ], and 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)- imide, [C 6 C 1 im][NTf 2 ] (Fig. 1), confined to nanometrically thin films. A dramatically different layering structure is observed for the two ILs, despite their similar chemical structures, which differ only in the length of hydrocarbon chain on the cation. We propose that the more amphiphilic nature of the [C 6 C 1 im] cation causes self-assembly into tail-to-tail cation bilayers at the mica surface, driven by the need to sequester the hydrocarbon chains away from the ionic regions. The force, F N , between two atomically smooth mica sheets (in crossed cylinder configuration with radius R) was measured as a function of their separation distance, D, across the IL using an SFA according to procedures described previously. 11 By the Derjaguin approximation F N /R is proportional to the interaction energy per unit area between parallel plates, E ll . Ionic liquids were synthesised according to a method described elsewhere 12 and kept in a desiccator until used. P 2 O 5 powder inside the SFA chamber prevented moisture entering the IL during the experiment, as verified by the reproducibility of results over the first 5–10 h of each experiment. When water vapour was intentionally allowed to enter the SFA chamber the oscillatory forces were observed to decrease in magnitude over a similar timescale and finally to disappear entirely. The data presented represents many force runs at different contact points on the mica sheets and from three separate experiments (different mica sheets) for each IL. Measurements of F N as a function of film thickness, D, for [C 4 C 1 im][NTf 2 ] and [C 6 C 1 im][NTf 2 ] are shown in Fig. 2. Both ILs show clear oscillatory forces with alternating repulsive and attractive regions of increasing amplitude with decreasing D. Each minimum corresponds to a stable configuration of ions between the surfaces, and the maxima represent the force Fig. 1 Chemical structures and approximate dimensions in nm, calculated from the van der Waals radii, of the ions used in this study. a Department of Chemistry, University College London, 20 Gordon Street, London, WC1H 0AJ, UK. E-mail: susan.perkin@ucl.ac.uk b Department of Chemistry, Imperial College London, London, SW7 2AZ, UK. E-mail: t.welton@imperial.ac.uk ChemComm Dynamic Article Links www.rsc.org/chemcomm COMMUNICATION Downloaded by University College London on 17 March 2012 Published on 13 May 2011 on http://pubs.rsc.org | doi:10.1039/C1CC11322D View Online / Journal Homepage / Table of Contents for this issue