Abstract
The equation of state of ethylene in the liquid and vapour-liquid critical regions is examined in terms of two theories. Both operate with a structure function represented by a fraction of vacancies in a quasi-lattice. One (SS) is derived conceptually from cell theory, and reduces in the limit of vanishing hole fraction to the results of this theory. The other (KK) employs the model of a lattice gas, and computes the excess free energy over the no-hole state. With a multiparameter computer-generated fit to the experimental P-V-T surface, quite satisfactory results ensue from both theories. As might be anticipated from the underlying physical picture, the SS theory is more powerful in the dense liquid region, whereas KK's lattice model is superior in the low-pressure gas and critical regions.
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