Vaporization behavior of tetraoctylphosphonium bis(2-ethylhexyl)phosphate ionic liquid

[Display omitted] •In situ Knudsen effusion mass loss measurements with a thermogravimetric analyzer.•In situ Langmuir free evaporation measurements with a thermogravimetric analyzer.•Vaporization coefficient and vapor pressures over tetraoctylphosphonium bis(2-ethylhexyl)phosphate ionic liquid. The equilibrium vapor pressures, pe, of the ionic liquid tetraoctylphosphonium bis(2-ethylhexyl)phosphate ([P8888][DEHP]) over the temperature range 409–495K were determined for the first time using mass loss Knudsen effusion. The pe versus temperature relationship compares well to 1-methyl-3-octylimidazolium bis(trifluoromethylsulfonyl)imide ([C8mim][NTf2]) but is lower than that of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C2mim][NTf2]) when measured using the same technique. The discrepancies between the pe determined in this work for [C8mim][NTf2] and [C2mim][NTf2] with previous studies is discussed. The enthalpy and entropy of vaporization for all three fluids are estimated from the Clasius-Clapeyron relation.