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Updated: Jun 20, 2026

From Molecules to Materials: Engineering New Ionic Liquid Crystals Through Halogen Bonding
Published on: March 24, 2018
Influence of different substituents on the surface composition of ionic liquids studied using ARXPS
K R J Lovelock1, C Kolbeck, T Cremer
1Lehrstuhl für Physikalische Chemie II, Lehrstuhl für Chemische Reaktionstechnik, Erlangen Catalysis Resource Center (ECRC), Universität Erlangen-Nürnberg, Egerlandstrasse 3, 91058 Erlangen, Germany.
Abstract:
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].
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The acidic strength of hydrocarbons follows the order: Alkynes > Alkenes > Alkanes. The strength of an acid is commonly expressed in units of pKa — the lower the pKa, the stronger the acid. Among the hydrocarbons, terminal alkynes have lower pKa values and are, therefore, more acidic. For example, the pKa values for ethane, ethene, and acetylene are 51, 44, and 25, respectively, as shown here.