New isotherm regularity and an equation of state for gases and liquids

In this paper an isotherm regularity has been introduced for gases and liquids based on intermolecular potential energy. The experimental data has been used to demonstrate the validity of the regularity. A non-linearity relationship exists between (Z − 1)v3 and ρ for all isotherms of liquids and gases. The basis for this regularity is intermolecular potential which is a modified Lennard–Jones potential (9, 6, 3) for repulsive, dispersion, dipole–dipole and longer-ranged interactions. The isotherm regularity is equivalent to a virial-like EOS for which the parameters of the isotherm form the corresponding second, third and fourth virial-like coefficients. The equation of state is simple and ready to use. The parameters of equation of state are determined by fitting isothermal regularity to experimental data. The new equation of state provides excellent results in homogenous gas and homogeneous liquids region to very high pressures while its predictions in gas-liquid transition have more deviations. Densities of 1828 data points of 21 components have been calculated over the entire range of data with a maximum pressure of 1000 MPa. The average absolute deviation between calculated and experimental densities for gases, liquids and gas-liquid transition region are 0.06%, 0.03% and 0.90%, respectively.