On the interfacial behavior of ionic liquids: Surface tensions and contact angles

摘要

In this work the liquid/vapour and the solid/liquid interfaces of a series of ionic liquids: 1-ethyl-3-methylpyridinium ethyl sulfate, [EMPy][EtSO_4], 1-ethyl-3-methylimidazolium ethyl sulfate, [EMIM][EtSO_4], 1-ethanol-3-methylimidazolium tetrafluoroborate, [C_2OHMIM][BF_4], 1-butyl-3-methylimidazolium tetrafluoroborate, [BMIM][BF_4], and 1-octyl-3-methylimidazolium tetrafluoroborate, [OMIM][BF_4], were investigated. The surface tension was measured in a wide temperature range, (298-453) K. The contact angles were determined on substrates of different polarities. Both on the polar (glass) and the non-polar substrates ((poly-(tetrafluoroethylene) and poly-(ethylene)), the liquids with maximum and minimum surface tensions lead, respectively, to the highest and the lowest contact angles. The dispersive, γ_L~d, and non-dispersive, γ_L~(nd), components of the liquid surface tension, γ_L, were calculated from the contact angles on the non-polar substrates using the Fowkes approach. The polarity fraction, γ_L~(nd) / γ_L, was compared with the polarity parameter, k, obtained from the fitting of the surface tension vs. temperature data to the E?tv?s equation. Good agreement was found for the extreme cases: [OMIM][BF_4] exhibits the lowest polarity and [BMIM][BF_4], the highest. When compared with the polarity fractions of standard liquids considered as "polar" liquids, the ionic liquids studied may be considered as moderately polar.