Phase behavior of imidazolium and phosphonium tetrafluoroborates with dihydroxy alcohols

Liquid–liquid equilibrium curves for 1-ethyl-3-methylimidazolium tetrafluoroborate -[C2MIM,BF4], 1-butyl-3-methylimidazolium tetrafluoroborate -[C4MIM,BF4] and trihexyl(tetradecyl) phosphonium tetrafluoroborate -[P6,6,6,14,BF4] with chosen dihydroxy alcohols and their deuterated analogues have been determined. All fourteen obtained phase diagrams are described by the upper critical solution temperature (UCST) type behavior. The mutual solubility of 1-ethyl-3-methylimidazolium tetrafluoroborates decreases with the increase in alkyl chain of 1,2-dihydroxy alcohols but the opposite effect was observed for the ionic liquid with phosphonium cation. The impact of the hydroxyl group position in butanediol molecule on miscibility with trihexyl(tetradecyl) phosphonium tetrafluoroborate was examined. The results lead to the conclusion that vicinal(1,2 and 2,3)-butanediols are much better miscible than more polar (1,3) and (1,4) isomers. Deuterium substitution in both hydroxyl groups of investigated diols generates downward isotope shift of UCST both for phosphonium and imidazolium based ionic liquids thus making the miscibility better in each case.