Determination of derived volumetric properties and heat capacities at high pressures using two density scaling based equations of state. Application to dipentaerythritol hexa(3,5,5-trimethylhexanoate)

Abstract

Reliable equations of state (EoS) together with heat capacities at atmospheric pressure make it possible to determine properties such as the isobaric thermal expansivity, compressibility, both isothermal and isentropic, high pressure isobaric heat capacities or speed of sound. In this work, we analysed the reliability of two density scaling based EoS, Power-Law Density Scaling (PLDS) and General Density Scaling (GDS), and the Tammann-Tait EoS to determine these quantities. For this aim, dipentaerythritol hexa(3,5,5-trimethylhexanoate), diPEiC₉, was chosen because it has been recently proposed as a candidate to fill the gap of reference fluids suitable for high pressure viscometer calibration or their verification. New experimental densities measured between (283.15 and 398.15) K at pressures up to 70 MPa together with isobaric heat capacities between (282.93 and 399.92) K and thermal conductivities between (283 and 333) K at 0.1 MPa of diPEiC₉ are reported. Literature relative volumes up to 400 MPa for this compound were also used. The three EoSs give rise to coherent values of the above properties. The most difficult property to describe is isobaric thermal expansivity for which the isobaric curves can present minima and/or maxima and the isotherm curves can cross at different pressures. The loci of the maxima of the isobaric thermal expansivity in p–T diagrams of the GDS and PLDS EoSs are very close.