Pressure-volume-temperature (PVT) measurements of ionic liquids ([bmim⁺][PF₆^-], [bmim⁺][BF₄^-], [bmim⁺][OcSO₄^-]) and analysis with the Sanchez-Lacombe equation of state

Pressure-volume-temperature measurements of the pure ionic liquids 1-butyl-3-methyl-imidazolium hexafluorophosphate ([bmim⁺][PF₆^-]), 1-butyl-3-methyl-imidazolium tetrafluoroborate ([bmim⁺][BF₄^-]), 1-butyl-3-methyl-imidazolium octylsulfate ([bmim⁺][OcSO₄^-]) were performed with a metal bellows apparatus at temperatures from 313 to 473 K and at pressures up to 200 MPa. The pressure-volume-temperature data were used to determine parameters of the Sanchez-Lacombe equation of state, which was used to calculate the hard core volume and free volume of ionic liquids. It was found that the hard core volume of the ionic liquid was related with anion size and was slightly smaller than the van der Waals volume estimated with Bondi methods. The free volumes of [bmim⁺][PF₆^-] and [bmim⁺][BF₄^-] calculated with the Sanchez-Lacombe equation of state showed a similar pressure dependence and were close to each other (ca. 10-14 cm³/mol) whereas the values for [bmim⁺][OcSO₄^-] were much higher (ca. 18-25 cm³/mol) over the range of pressure.