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2009, 113, 7049–7052 Published on Web 04/29/2009
Pronounced Structure in Confined Aprotic Room-Temperature Ionic Liquids Robert Hayes,† Sherif Zein El Abedin,‡ and Rob Atkin*,† Centre for Organic Electronics, Chemistry Building, The UniVersity of Newcastle, Callaghan, NSW 2308, Australia, and Institute of Particle Technology, Clausthal UniVersity of Technology, Robert-Koch-Str. 42, 38678 Clausthal-Zellerfeld, Germany ReceiVed: March 30, 2009; ReVised Manuscript ReceiVed: April 8, 2009
Room-temperature ionic liquids (ILs) are attracting considerable research interest as replacements for traditional molecular solvents in a diverse range of chemical applications, mostly due to their green characteristics and remarkable physical properties. Previously, we reported the liquid structure of 1-ethyl-3-methylimidazolium acetate confined between mica and an atomic force microscope (AFM) tip, and found that approximately three solvation layers form. In this manuscript, we present new data, derived from similar experiments, for three different aprotic ILs [1-butyl-3-methylimidazolium hexafluorphosphate (BMIm PF6), 1-ethyl-3methylimidazolium bis(trifluoromethanesulfonyl) imide (EMIm TSFA), and 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl) imide (BMP TSFA)] and between five and six solvation layers are identified depending on the IL species. These new results allow us to make suggestions for molecularly designing IL architectures likely to be suitable for a particular application, depending on whether near surface order is desirable or not. Where mobility of component ions and transfer of species to and from the interface is required (DSSCs, hetereogeneous catalysis, etc.), multiple sterically hindered allylic functional groups could be incorporated to minimize substrate-IL interactions and maximize compressibility of the solvation layers. Conversely, in situations where IL adsorption to the interface is desirable (e.g., lubrication or electrode surface restructuring), symmetric ions with localized charge centers are preferable. Room-temperature ionic liquids (ILs) are attracting considerable research interest as replacements for traditional molecular solvents in a diverse range of chemical applications, mostly due to their green characteristics1,2 and remarkable physical properties.3-5 While the use of ILs as solvents for chemical synthesis has been well documented,4,6 there are far fewer studies of IL physical properties, particularly relating to the structure of the solid-IL interface. Papers that have emerged have examined the solid-IL interfaces in terms of catalysis,4,7 electrodeposition,8-10 lubrication,11 and dye sensitized solar cells (DSSCs).12,13 In many cases, distinct performance advantages over conventional liquids have been elucidated, but the factors producing improved performance remain largely unexplained. As ILs are composed solely of ions, their bulk and interfacial behavior is complex, governed by Coulombic, van der Waals, dipole-dipole, hydrogen-bonding, and solvophobic forces.14 In order to explain mechanisms that improve performance, the relationships between molecular structure, intermolecular interactions, and physiochemical properties must be ascertained. Previously, we reported the liquid structure of 1-ethyl-3methylimidazolium acetate confined between mica and an AFM tip, and found that approximately three solvation layers form.15 In this manuscript, we present new data, derived from similar experiments, for three different aprotic ILs [1-butyl-3-meth* To whom correspondence should be addressed. E-mail: Rob.Atkin@ newcastle.edu.au. † The University of Newcastle. ‡ Clausthal University of Technology National Research Centre, Cairo, Egypt.
10.1021/jp902837s CCC: $40.75
TABLE 1: Name, Abbreviation, Structure, Molecular Weight (MW), Density (G), Molecular Volume (MV), and Ion Pair Diameter (Dm) of the ILsa
a Carbon atoms are shaded gray, nitrogen are blue, fluorine are yellow, sulfur are orange, and oxygen are red. Hydrogens are not shown.
ylimidazolium hexafluorphosphate (BMIm PF6), 1-ethyl-3methylimidazolium bis(trifluoromethanesulfonyl) imide (EMIm TSFA), and 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl) imide (BMP TSFA), c.f. Table 1] and between five and six solvation layers are identified depending on the IL species. These new results allow us to draw firm conclusions relating IL molecular structure to the degree of interfacial order, and make suggestions for molecularly designing IL architectures 2009 American Chemical Society
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J. Phys. Chem. B, Vol. 113, No. 20, 2009
likely to be suitable for a particular application, depending on whether near surface order is desirable or not. The molecular organization of ILs is more complex than traditional solvents, and ILs often cannot be considered as unstructured molten salts.16 Theoretical and experimental studies by a number of groups have shown ILs are nanostructured, which helps to explain their solvating power4 and some other unusual physical properties.3 X-ray diffraction studies of aprotic 1-alkyl-3-methylimidazolium cations reveal distinct alkyl and ionic clusters,17,18 and comparable conclusions were drawn for C5 imidazolium ILs based on optical heterodyne-detected Raman-induced Kerr effect spectroscopy.19 These results are essentially consistent with recent molecular dynamics simulations.20 IL nanostructure results from solvophobic21 interactions: cation alkyl groups cluster together to form apolar domains segregated from polar ionic regions in a fashion reminiscent of surfactant self-assembly in water. Nanostructure has also been elucidated for protic ILs using SANS.22 A single, broad structure peak was measured for ethylammonium nitrate (EAN) and propylammonium nitrate, and fits to the spectra suggest a disordered lamellar or sponge-like structure. For protic ILs, electrostatic contributions to solvophobicity are enhanced by the extensive hydrogen-bond network of the liquids. While near surface oscillatory forces have been measured for a range of molecular solvents,23 the same intermolecular forces that induce nanostructure in the bulk mean that IL solvation layers are particularly well-defined, and can be detected using a standard atomic force microscope (AFM). (For molecular solvents, solvation layers may only be detected using the surface force apparatus (SFA) or AFMs modified to increase sensitivity.) IL solvation layers were first detected for the EAN-mica system using SFA,24 and have since been measured for both protic and aprotic ILs confined between Si3N4 AFM tips, and mica, silica, and graphite.15,25 In all studies, the oscillation or step period was approximately equal to the size of the ion pair and force required to rupture a layer is reduced farther from the surface as the level of ordering decreases. BMIm PF6 was purchased from Iolitec (>99%) and used as supplied. Ultrapure quantities of EMIm TSFA and BMP TSFA were sourced from Merek/EMD (chloride
