Experimental densities and derived thermodynamic properties of pure p-cymene, α-pinene, limonene and citral under high pressure conditions

•Density measurements of various terpenes at moderate temperatures and high pressures.•Density data were correlated applying the modified Tammann-Tait equation.•Calculation of the isothermal compressibility and the isobaric thermal expansion coefficient.•Densities of the investigated terpenes decrease: citral > α-pinene > p-cymene > limonene.•Among studied terpenes, citral is most rigid upon volume change. In order to reduce negative influence of fossil fuels on environment, use of various renewable resources is highly promoted. Terpenes, naturally occurring in plants, can be added to petroleum fuel as its substitute up to a certain share. Thermodynamic properties of a fuel under high pressure and moderate temperature conditions are of significant importance for engine efficiency. This work reports density measurements of pure p-cymene, α-pinene, limonene and citral at temperatures (293.15–413.15) K and pressures (0.1–60) MPa, applying an Anton Paar DMA HP measuring cell. Density data were fitted by modified Tammann-Tait equation where the absolute average percentage deviation between measured and calculated densities was about 0.010%. The obtained parameters were used to estimate the isothermal compressibility, the isobaric thermal expansion coefficient, the internal pressure, and the difference between specific heat capacity at constant pressure and at constant volume. For examined compounds, all thermodynamic properties, except the internal pressure, decrease with pressure rise along an isotherm and increase as temperature increases at a constant pressure. The intersection point of isotherms for the isobaric thermal expansion coefficient for pure citral was registered at pressure 47 MPa, while for other analyzed terpenes the intersection point is above 60 MPa, outside the measurements pressure range.