Density of 1-Butyl-3-methylimidazolium Bis(trifluoromethanesulfonyl)amide and 1-Hexyl-3-methylimidazolium Bis(trifluoromethanesulfonyl)amide over an Extended Pressure Range up to 250 MPa

Densities of the ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([BMIM][Tf2N]) between (273 and 363) K over an extended pressure range up to 250 MPa and densities for 1-hexyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([HMIM][Tf2N]) at 288.15 K to 175 MPa and between (273 and 363) K at 0.1 MPa are reported. The high-pressure densities were measured with a metal bellows volumometer, whereas the atmospheric pressure densities were determined with a vibrating tube densimeter. The overall uncertainties are estimated to be 0.02 % at atmospheric pressure and 0.1 % at higher pressures. The standard Tait equation of state with temperature dependent parameters B(T) and C(T) was used to represent the temperature and pressure dependences of the densities, with a standard uncertainty of 0.6 kg/m(3) or 0.03 %. The isothermal compressibility, isobaric expansivity, and the internal pressure were obtained from differentiation of the equation of state, but as the calculation of the two latter properties is known to be sensitive to the form adopted for the temperature dependence of the Tait coefficients, a comparison is made with other methods. It is concluded that great care must be taken in determining expansivities from the differentiation of pVT equations of state and that the internal pressure is best obtained by direct measurement.