Effect of phase behavior and pressure on the constant-volume heat capacity and intermolecular interaction of CO_2-ethanol and CO_2-n-pentane mixtures in the critical region

摘要

Study on the properties of the fluids near the critical point of mixed systems is a key for the development of supercritical (SC) technology and for the further understanding of the features of supercritical fluids (SCFs). The constant volume molar heat capacity (C_v) of a solution is directly related to the internal energy, intermolecular interaction, and the microstructure of the solution. In this work, the C_v of CO_2-n-pentane and CO_2-ethanol systems was determined at 308.15 K in different phase regions. This focuses on how the properties of the mixtures change with pressure, composition, and the structure of the components near the critical point of the mixtures. It was found that at fixed composition, a maximum in C_v versus pressure curve exists (C_v~(max)) that occurs at the pressure at which the isothermal compressiblity (K_T) is the largest. We deduced that breaking the "clusters" in the SC mixtures is an endothermic process. It is very interesting that C_v increases sharply as the pressure approaches the critical point (CP) or bubble point (BP), while C_v is nearly independent of pressure and composition at the pressures well above the CP or BP pressure, and that the C_v at CP or BP can be several times larger than that at the high pressures. It can be deduced that at fixed composition the degree of "clustering" changes significatntly with pressure near the CP or BP, and is the largest at CP and BP.