Features of isochoric heat capacity measurements near the phase transition points

Isochoric heat capacity data of n-C7H16, n-C12H26, and CO2+n-C10H22 in the immediate vicinity of the phase transition points including thermodynamically stable and metastable phases are presented. Measurements were made using a high-temperature, high-pressure, adiabatic, and nearly constant-volume calorimeter in the temperature ranges: for n-heptane—from 323 to 422 K and densities of 657.9, 612.4, and 571.1 kg m−3; for n-dodecane—from 323 to 378 K and densities of 689.2 and 726.8 kg m−3; and for CO2+n-decane mixture—x=0.822 and 0.905 mole fraction of CO2, densities of 347.1 and 287.8 kg m−3, respectively, for temperatures between 471 and 494 K. Some features of the isochoric heat capacity measurements in the immediate vicinity of phase transition temperatures are discussed. This method allowed measurements of CV in the one- and two-phase regions including coexistence curves and metastable phases. Measurements in the stable states were carried out at constant volume by the continuous-heating (dT/dτ>0) method. Measurements near the phase transition points in the stable and metastable states were carried out using continuous-cooling (reverse direction, dT/dτ<0) runs. For each measured liquid isochore during cooling runs we found penetration to the metastable region. The magnitude of the penetration depth varied from 0.03 to 0.41 K depending on measured densities. Our experimental results indicate that the character of the temperature dependence of CV does not change, when moving along an isochore, upon the intersection of the coexistence curve and transition into the metastable state. The results of CV and measurements for n-heptane and n-dodecane in the one-phase region and on the coexistence curve were compared with values calculated from different equations of state (EOS) and previous publications.