Angle resolved X-ray photoelectron spectroscopy has been used to study the surface composition of various nonfunctionalized and functionalized 1,3-dialkylimidazolium ionic liquids. For [CnC1Im][Tf2N] (where n = 2-16), an enrichment of the aliphatic carbon was observed for longer chains (n > or = 4). Enrichment of the aliphatic carbon also occurs for alkyl chains attached to the anion, as observed for [C2C1Im][OcOSO3]. Oligo(ethyleneglycol)ether (PEG) functionalities in the cation lead to a surface composition close to bulk stoichiometry and thus a loss in enrichment of the chains. This effect is attributed to attractive interactions between the oxygen atoms on the cation to the hydrogen atoms on the imidazolium ring for [Et(EG)2MIm] [Tf2N] and [Me(EG)3MIm][Tf2N].

Ten [C(8)C(1)Im](+) (1-methyl-3-octylimidazolium)-based ionic liquids with anions Cl(-), Br(-), I(-), [NO(3)](-), [BF(4)](-), [TfO](-), [PF(6)](-), [Tf(2)N](-), [Pf(2)N](-), and [FAP](-) (TfO=trifluoromethylsulfonate, Tf(2)N=bis(trifluoromethylsulfonyl)imide, Pf(2)N=bis(pentafluoroethylsulfonyl)imide, FAP=tris(pentafluoroethyl)trifluorophosphate) and two [C(8)C(1)C(1)Im](+) (1,2-dimethyl-3-octylimidazolium)-based ionic liquids with anions Br(-) and [Tf(2)N](-) were investigated by using X-ray photoelectron spectroscopy (XPS), NMR spectroscopy and theoretical calculations. While (1)H NMR spectroscopy is found to probe very specifically the strongest hydrogen-bond interaction between the hydrogen attached to the C(2) position and the anion, a comparative XPS study provides first direct experimental evidence for cation-anion charge-transfer phenomena in ionic liquids as a function of the ionic liquid's anion. These charge-transfer effects are found to be surprisingly similar for [C(8)C(1)Im](+) and [C(8)C(1)C(1)Im](+) salts of the same anion, which in combination with theoretical calculations leads to the conclusion that hydrogen bonding and charge transfer occur independently from each other, but are both more pronounced for small and more strongly coordinating anions, and are greatly reduced in the case of large and weakly coordinating anions.

Angle-resolved X-ray photoelectron spectroscopy has been used to study the influence of different types of anions on the surface composition of ionic liquids (ILs). We have investigated nine ILs with the same cation, 1-octyl-3-methylimidazolium [C(8)C(1)Im](+), but very different anions. In all cases, an enrichment of the cation alkyl chains is found at the expense of the polar cation head groups and the anions in the first molecular layer. This enhancement effect decreases with increasing size of the anion, which means it is most pronounced for the smallest anions and least pronounced for the largest anions. A simple model is proposed to explain the experimental observations.

Methylation of the C2 position of 1,3-dialkylimidazolium based ionic liquids disrupts the predominant hydrogen-bonding interaction between cation and anion leading to unexpected changes of the physicochemical properties. We found the viscosity of 1-ethyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide [C(2)C(1)C(1)Im][Tf(2)N], for example, to be about three times higher than that of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [C(2)C(1)Im][Tf(2)N]. In order to explain these macroscopic changes upon methylation we investigated the vibrational as well as the magnetic resonance structure of [Tf(2)N](-) salts involving the cations 1-ethyl-3-methylimidazolium [C(2)C(1)Im](+), 1-ethyl-2,3-dimethylimidazolium [C(2)C(1)C(1)Im](+), 1-butyl-3-methylimidazolium [C(4)C(1)Im](+), and 1-butyl-2,3-dimethylimidazolium [C(4)C(1)C(1)Im](+) by means of Fourier-transform infrared (FTIR), Raman and (13)C NMR as well as (1)H NMR spectroscopy aiming a better microscopic understanding of the cation-anion interaction. To reveal the impact of methylating the C2 position and changing the alkyl side chain length of the imidazolium a detailed assignment of the individual peaks is followed by a comparative discussion of the spectral features also considering already published work. Our spectroscopic findings deduce electron density changes leading to changes in the position and strength of interionic interactions and reduced configurational variations. Both facts are represented on a macroscopic level by the viscosity and melting point. Therefore changes on a macroscopic level clearly express molecular alterations which in turn can be observed using spectroscopic methods as Raman, IR and NMR.

A systematic study of ionic liquid surfaces by angle resolved X-ray photoelectron spectroscopy (ARXPS) is presented. By reviewing recent and presenting new results for imidazolium-based ionic liquids (ILs), we discuss the impact of chemical differences on surface composition and on surface enrichment effects. (1) For the hydrophilic ethylene glycol (EG) functionalised ILs [Me(EG)MIm][Tf(2)N], [Et(EG)(2)MIm][Tf(2)N] and [Me(EG)(3)MIm][Tf(2)N], which vary in the number of ethylene glycol units (from 1 to 3), we find that the surface composition of the near-surface region is in excellent agreement with the bulk composition, which is attributed to attractive interactions between the oxygen atoms on the cation to the hydrogen atoms on the imidazolium ring. (2) For [C(n)C(1)Im][Tf(2)N] (where n = 1-16), i.e. ILs with an alkyl chain of increasing length, an enrichment of the aliphatic carbons is observed for longer chains (n > 2), at the expense of the polar cation head groups and the anions in the first molecular layer, both of which are located approximately at the same distance from the outer surface. (3) To study the influence of the anion on the surface enrichment, we investigated ten ILs [C(8)C(1)Im][X] with the same cation, but very different anions [X](-). In all cases, surface enrichment of the cation alkyl chains is found, with the degree of enrichment decreasing with increasing size of the anion, i.e., it is most pronounced for the smallest anions and least pronounced for the largest anions. (4) For the IL mixture [C(2)C(1)Im][Tf(2)N] and [C(12)C(1)Im][Tf(2)N] we find a homogeneous distribution in the outermost surface region with no specific enrichment of the [C(12)C(1)Im](+) cation.