Thermodynamic Properties of Caffeine in Ethylene Glycol at High Pressures and High Temperatures

Ethylene glycol is recognized as a heat transfer fluid. To improve its performance, caffeine is added to pure ethylene glycol. The experimental measurements of densities at pressures of up to 60 MPa and in the range of temperatures 293.15–413.15 K were performed. Obtained experimental results were fitted by the modified Tammann–Tait equation, and parameters were used to determine some valuable thermodynamic and mechanical properties, the isothermal compressibility, the isobaric thermal expansibility, the internal pressure, and the difference of specific heat capacity at constant pressure and constant volume. The same approach was applied to the equimolar mixture of ethylene glycol and water. The results confirm negligible changes in the investigated thermodynamic properties with the addition of caffeine into pure ethylene glycol and significantly smaller dependence on temperature compared to the mixture of ethylene glycol and water.