A general heat capacity model of simple liquids using phonon theory

Abstract

To develop a general theoretical model of heat capacity is a long-term goal for liquid thermophysical property studies. However, previously reported models cannot meet this requirement due to their dependence on free fitting parameters, leading to an ambiguous interpretation of liquid heat capacity. In view of this problem, a general heat capacity model for simple liquids was proposed, without free fitting parameters. The present model was established based on the solid phonon theory (quasi-harmonic Debye model), with some modifications made for considering the distinction between liquids and solids, to calculate the liquid heat capacity. The proposed model was applied to seven simple liquids (Ar, Na, Ne, N2, CH4, CHF3, CH2F2). The calculation was performed in the temperature range between 50 and 810 K and pressures from 0.1 to 800 MPa. The results show that the calculated data are in good agreement with the corresponding literature values, yielding an average absolute deviation (AAD) of 2.07%. Moreover, compared with previous liquid phonon theory model, the present model can be employed without thermal expansion coefficients. In addition, the effect of temperature and pressure on heat capacity was explained using the liquid relaxation time τ on the basis of the present model. This work provides guidance for the measurement and modeling of liquid heat capacity.