Surface tension measurements of imidazolium-based ionic liquids at liquid–vapor equilibrium

A series of high quality 1-alkyl-3-methylimidazolium-based ionic liquids are synthesized and used for studying their surface tension. The capillary rise method is used for measuring the surface tension of I−, Cl−, PF6−, and BF4− salts in the temperature range 298–393 K. The capillary apparatus is evacuated and sealed under vacuum. The experimental results show that surface tension of these compounds depend systematically on temperature.For ionic liquids containing a butyl alkyl chain, the contribution of the anion to the surface tension is found to be in the order of I− > Cl− > PF6− > BF4−. As the chain length of the alkyl group is increased, the surface tension decreases and approaches the surface tension of alkyl fluids of the same chain length.The surface entropy decreases smoothly as the chain length increases while the surface energy decreases rather sharply. This resembles the behavior of normal liquids, molten salts, and molten metals. Comparison with literature suggests that the complete removal of water and dissolved atmospheric gases as well as sufficient thermal equilibrium would produce a highly correlated and thus accurate surface tension of ionic liquids.