Protic Imidazolium Cation-Based Ionic Liquids Show Unexpected Interfacial Properties.

Virtually, every investigation and application of ionic liquids (ILs) involves gas-liquid, liquid-liquid, and liquid-solid interactions. Therefore, understanding the behavior of ILs at those interfaces is critical. In this work, we studied the interfacial properties of protic and aprotic ILs with N-alkylimidazolium and 1-alkyl-3-methylimidazolium as cations and bis(trifluoromethylsulfonyl)imide, methanesulfonate, and trifluoromethanesulfonate as anions. The surface tension of these ILs is measured with the pendant drop method in a temperature range of 293.15-343.15 K and at atmospheric pressure. The contact angle measurements are performed at 293.15 K on three solid substrates: polytetrafluoroethylene, glassy carbon, and platinum. Dispersive and nondispersive components of the IL surface energy are determined from the experimental data using Fowkes theory. The most interesting result is that the protic ILs have lower surface tension and smaller contact angles than the equivalent aprotic ILs, despite the presence of high charge density on the proton associated with one of the nitrogens of the cation. Higher charge density on the anion results in a higher surface tension, and decreasing surface tension and contact angles are observed for increasing alkyl chain length on the cation.