Abstract
We investigate the interfacial properties of the non-halogenated ionic liquid (IL), trihexyl(tetradecyl)phosphonium bis(mandelato)borate, [P6,6,6,14][BMB], in proximity to solid surfaces, by means of surface force measurement. The system consists of sharp atomic force microscopy (AFM) tips interacting with solid surfaces of mica, silica, and gold. We find that the force response has a monotonic form, from which a characteristic steric decay length can be extracted. The decay length is comparable with the size of the ions, suggesting that a layer is formed on the surface, but that it is diffuse. The long alkyl chains of the cation, the large size of the anion, as well as crowding of the cations at the surface of negatively charged mica, are all factors which are likely to oppose the interfacial stratification which has, hitherto, been considered a characteristic of ionic liquids. The variation in the decay length also reveals differences in the layer composition at different surfaces, which can be related to their surface charge. This, in turn, allows the conclusion that silica has a low surface charge in this aprotic ionic liquid. Furthermore, the effect of temperature has been investigated. Elevating the temperature to 40 °C causes negligible changes in the interaction. At 80 °C and 120 °C, we observe a layering artefact which precludes further analysis, and we present the underlying instrumental origin of this rather universal artefact.
Highlights
Ionic liquids (ILs) consist of cations and anions, and are, salts
Note that an absolute zero of separation cannot be obtained in atomic force microscopy (AFM) force measurements, and zero separation is conventionally defined in terms of constant compliance of the cantilever deflection with actuator movement [32,33]
Similar reductions in the Hamaker constants were reported for interactions between mica and gold in 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [BMIM][TFSI], and between silicon oxide and graphene in 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [EMIM][TFSI] [16,22], but this is not a characteristic of ILs, rather, the fact that the surfaces are immersed in a medium of dielectric constant larger than 1 [15]
Summary
Ionic liquids (ILs) consist of cations and anions, and are, salts. Unlike their crystalline counterparts, e.g., table salt, they are liquid below 100 ◦C [1]. In ILs, at least one of the ionic species is organic, bulky, and asymmetric. The balance of enthalpic and entropic interactions in ILs favors the liquid state, and it gives ILs specific properties, such as being a good solvent for a range of organic and inorganic materials, low vapor pressure, high viscosity, low conductivity, and a high electrochemical window [4,5]. Once used primarily as “green” solvents, e.g., for cellulose [6], ILs are finding applications in tribology and super capacitors [7,8,9,10,11,12]
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