Lehrstuhl für Physikalische Chemie II, Department Chemie und Pharmazie, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstrasse 3, 91058 Erlangen, Germany.
Phys Chem Chem Phys. 2010 Feb 28;12(8):1905-15. doi: 10.1039/b920804f. Epub 2010 Jan 9.
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.
通过角度分辨 X 射线光电子能谱（ARXPS）对离子液体表面进行了系统研究。通过回顾最近的和呈现新的结果，我们讨论了化学差异对表面组成和表面富集效应的影响。（1）对于亲水性的乙二醇（EG）功能化离子液体[Me（EG）MIm][Tf 2 N]、[Et（EG）（2）MIm][Tf 2 N]和[Me（EG）（3）MIm][Tf 2 N]，它们在乙撑二醇单元数（从 1 到 3）上有所不同，我们发现近表面区域的表面组成与体组成非常吻合，这归因于阳离子上的氧原子与咪唑环上的氢原子之间的吸引力相互作用。（2）对于[C（n）C（1）Im][Tf 2 N]（其中 n = 1-16），即烷基链长度逐渐增加的离子液体，对于较长的链（n > 2），观察到脂肪族碳的富集，以牺牲极性阳离子头基团和位于第一层分子的阴离子为代价，两者都位于离外表面相同的距离处。（3）为了研究阴离子对表面富集的影响，我们研究了十个具有相同阳离子但具有非常不同阴离子[X]的离子液体[C（8）C（1）Im][X]。在所有情况下，都发现了阳离子烷基链的表面富集，随着阴离子尺寸的增加，富集程度降低，即对于最小的阴离子最为显著，对于最大的阴离子则最为不明显。（4）对于离子液体混合物[C（2）C（1）Im][Tf 2 N]和[C（12）C（1）Im][Tf 2 N]，我们在外表面区域发现了均匀的分布，没有[C（12）C（1）Im]（+）阳离子的特定富集。
