Influence of the aliphatic chain length of imidazolium based ionic liquids on the surface structure

Abstract

The molecular surface structure of four imidazolium based ionic liquids was studied with two surface sensitive techniques. Angle resolved neutral impact collision ion scattering spectroscopy (ARNICISS) allows us to determine elemental concentration depth profiles and to obtain information about the topography of the surface. Angle resolved X-ray photoelectron spectroscopy (ARXPS) can be used to study the chemical composition of the surface. The room temperature ionic liquids (RTILs) 1-ethyl-3-methylimidazolium [EMIM], 1-butyl-3-methylimidazolium [BMIM], 1-hexyl-3-methylimidazolium [HMIM], and 3-methyl-1-octylimidazolium [OMIM] bis(trifluoromethylsulfonyl)imide [Tf₂N] were investigated at 293 K. No evidence of surface active impurities was observed. The majority of previous studies about these RTILs with ARXPS or other surface sensitive techniques dealt only with single examples of these substances or with different combination of the anion and cation. In this present study a homologous series of the four RTILs mentioned above was investigated. This means that only the number of carbon atoms in the aliphatic chain of the cation is varied. Due to this procedure it is possible to study the influence of the chain length, which is a part of the imidazolic ring, on the composition of the surface and the surface near region. In this paper we demonstrate the potential of ARNICISS as a surface sensitive technique to study the surface structure of the RTILs. Furthermore, we combine our NICISS data with ARXPS data, to get a better comprehension of the influence of the aliphatic chain length. After the presentation of the results we develop a model of the surface structure of different RTILs. We have discovered two different surface structures that depend on the number of carbon atoms inside the aliphatic chain.