Features of isochoric heat capacity measurements near the phase transition points

Abstract

Isochoric heat capacity data of n-C 7H 16, n-C 12H 26, and CO 2+ n-C 10H 22 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 CO 2+ n-decane mixture— x=0.822 and 0.905 mole fraction of CO 2, 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 C V 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 (d T/d τ>0) method. Measurements near the phase transition points in the stable and metastable states were carried out using continuous-cooling (reverse direction, d T/d τ<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 C V does not change, when moving along an isochore, upon the intersection of the coexistence curve and transition into the metastable state. The results of C V 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.