(531h) 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 interfacial behavior of ILs is critical to improving the performance of IL-based systems. In this work, we studied the interfacial properties of twenty-six protic and aprotic ILs. They are N-alkylimidazolium bis(trifluoromethylsulfonyl)imide ([C_nim][Tf₂N]) for n = 2 to 10, 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C_nmim][Tf₂N]) for n = 2 to 6, 1-alkyl-3-methylimidazolium methanesulfonate ([C_nmim][CH₃SO₃]) for n = 3, 6, 7 and 8 and 1-alkyl-3-methylimidazolium trifluoromethanesulfonate ([C_nmim][CF₃SO₃]) for n = 2 to 4 and 6 to 10. Surface tension of these ILs is measured with the pendant drop method in a temperature range of 293.15 to 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 non-dispersive components of the IL surface energy are determined from the experimental data using Fowkes theory. The most surprising result is that the protic [C_nim][Tf₂N] ILs have lower surface tension and smaller contact angles than the equivalent aprotic [C_nmim][Tf₂N] ILs, despite the presence of high charge density on the proton associated with one of the nitrogens of the cation. The lower surface tensions of the protic ILs suggests that their surfaces are depleted of the highly charged protons on the cation. Higher charge density on the anion results in higher surface tension, and decreasing surface tension and contact angles are observed for increasing alkyl chain length on the cation. Assessment of the dispersive and non-dispersive components of the surface energy for these ILs shows that the highly charged protic [C_nim][Tf₂N] ILs have the greatest dispersive component. The four classes of ILs have dispersive components of surface energy that decrease in the following order: [C_nim][Tf₂N] > [C_nmim][Tf₂N] > [C_nmim][CF₃SO₃] > [C_nmim][CH₃SO₃].