Surface tension of binary mixtures of (ionic liquid + tributyl phosphate)

Abstract In this work, surface tension was determined for binary mixtures of imidazolium based ionic liquids (ILs) and tributyl phosphate (TBP) over the whole composition range at temperatures from (293.15 to 323.15) K and atmospheric pressure. The ILs, 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim]BF 4 ), 1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim]PF 6 ) and 1-butyl-3-methyl imidazolium bis(trifluoromethylsulfonyl)imide ([Bmim]Tf 2 N) were used in this work. According to the experimental data, surface tension deviations ( δγ ) and thermodynamic properties, including surface enthalpy ( H γ ), surface entropy ( S γ ), enthalpy of vaporisation △ l g H m o and Hildebrand solubility parameter ( S H ), were also calculated. The results show that γ values of ILs are significantly higher than that of TBP. And the contribution of the anion size to the surface tension of ILs follows the order of BF 4 −  > PF 6 −  > Tf 2 N − , which indicates the low negative charge delocalization of small anion can lead to high γ values. Moreover, the strong electrostatic interactions between cation and anion in ILs result in the higher concentration of TBP at surface. As a result, δγ values for all the studied systems are all negative over the entire range of composition. Additionally, due to the high surface organization of ILs, surface entropies for ILs are much lower than that of TBP.