The interaction between an adsorbed 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide, [BMP][TFSA], ionic liquid (IL) layer and a Ag(111) substrate, under ultrahigh-vacuum conditions, was investigated in a combined experimental and theoretical approach, by high-resolution scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and dispersion-corrected density functional theory calculations (DFT-D). Most importantly, we succeeded in unambiguously identifying cations and anions in the adlayer by comparing experimental images with submolecular resolution and simulated STM images based on DFT calculations, and these findings are in perfect agreement with the 1:1 ratio of anions and cations adsorbed on the metal derived from XPS measurements. Different adlayer phases include a mobile 2D liquid phase at room temperature and two 2D solid phases at around 100 K, i.e., a 2D glass phase with short-range order and some residual, but very limited mobility and a long-range ordered 2D crystalline phase. The mobility in the different adlayer phases, including melting of the 2D crystalline phase, was evaluated by dynamic STM imaging. The DFT-D calculations show that the interaction with the substrate is composed of mainly van der Waals and weak electrostatic (dipole-induced dipole) interactions and that upon adsorption most of the charge remains at the IL, leading to attractive electrostatic interactions between the adsorbed species.

Aiming at a comprehensive understanding of the interaction of ionic liquids (ILs) with metal surfaces we have investigated the adsorption of two closely related ILs, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [EMIM][TFSA] and 1-methyl-3-octylimidazolium bis(trifluoromethylsulfonyl)imide [OMIM][TFSA], with two noble metal surfaces, Au(111) and Ag(111), under ultrahigh vacuum (UHV) conditions using scanning tunneling microscopy (STM). At room temperature, the ILs form a 2D liquid on either of the two surfaces, while at lower temperatures they condense into two-dimensional (2D) islands which exhibit ordered structures or a short-range ordered 2D glass structure. Comparison of the adlayer structures formed in the different adsorption systems and also with those determined recently for n-butyl-n-methylpyrrolidinium [TFSA](-) adlayers on Ag(111) and Au(111) (B. Uhl et al., Beilstein J. Nanotechnol., 2013, 4, 903) gains detailed insight into the adsorption geometry of the IL ions on the surface. The close similarity of the adlayer structures indicates that (i) the structure formation is dominated by the tendency to optimize the anion adsorption geometry, and that (ii) also in the present systems the cation adsorbs with the alkyl chain pointing up from the surface.

SummaryIn order to resolve substrate effects on the adlayer structure and structure formation and on the substrate–adsorbate and adsorbate–adsorbate interactions, we investigated the adsorption of thin films of the ionic liquid (IL) 1-butyl-1-methylpyrrolidinium-bis(trifluoromethylsulfonyl)imide [BMP][TFSA] on the close-packed Ag(111) and Au(111) surfaces by scanning tunneling microscopy, under ultra high vacuum (UHV) conditions in the temperature range between about 100 K and 293 K. At room temperature, highly mobile 2D liquid adsorbate phases were observed on both surfaces. At low temperatures, around 100 K, different adsorbed IL phases were found to coexist on these surfaces, both on silver and gold: a long-range ordered (‘2D crystalline’) phase and a short-range ordered (‘2D glass’) phase. Both phases exhibit different characteristics on the two surfaces. On Au(111), the surface reconstruction plays a major role in the structure formation of the 2D crystalline phase. In combination with recent density functional theory calculations, the sub-molecularly resolved STM images allow to clearly discriminate between the [BMP]+ cation and [TFSA]− anion.

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