Surface tension measurements of highly conducting ionic liquids

Abstract

The capillary rise method is used to measure the room temperature surface tension of several ionic liquids, selected mainly for their high electrical conductivity. They include salts based on the cations 1-ethyl-3-methylimidazolium (EMI⁺), 1-butyl-3-methylimidazolium (BMI⁺), and 1,3-dimethylimidazolium (DMI⁺), paired with anions such as GaCl₄⁻, FeCl₄⁻, C(CN)₃⁻, N(CN)₂⁻, SCN⁻, EtSO₄⁻, BF₄⁻, CF₃SO₃⁻, (CF₃SO₃)₂N⁻ (Tf₂N⁻) and Au(CN)₂⁻. The method consumes relatively little sample (<0.1 cm³) with measurement errors of 5%. Vacuum-dried samples are placed in the measurement cell under ambient (humid) air, but the meniscus is kept dry by a small flow of dry gas. Failure to dry the active interface leads to rapid contamination in the case of hydrophilic liquids, and to anomalously high surface tension. The highest surface tension measured (61 dyn cm⁻¹) corresponds to DMI–N(CN)₂.