Interpretation of covolume in an extension of the Redlich-Kwong equation of state for real gases

This work discusses an extension of the Redlich-Kwong (RK) equation of state for real gases that modifies its attractive part by introducing a third constant, fitted to the nearly-universal value of the compressibility factor at the critical point, Z(c) approximate to 0.27. From this, the model predicts a covolume b = (V) over bar (c)/(6.33 +/- 0.62) for 90% of all the gases, in terms of the critical molar volume (V) over bar (c). It is found that this b value is approximately the difference between the molar volume of a monodisperse fluid near its normal melting point and the interstitial space associated with the loose packing of spheres. In other words, in contrast to the covolume in the RK equation, the present b-parameter does correspond to a molecular excluded volume. Despite the intrinsic limitations of the cubic equations of state for fluids, the present model extends their phenomenological usefulness over a larger range of values for the molar volume.