Statistical thermodynamics of liquid hydrocarbons Part II—Mixtures

Abstract

The corresponding states theory for liquids consisting of polysegmented molecules is extended to mixtures of paraffinic, olefinic, naphthenic and aromatic hydrocarbons; the previously derived partition function is generalized to yield expressions for the excess free energy, enthalpy and entropy. It is found that the excess enthalpy arises from the change in potential energy of the system which is due to the change in the average potential field acting on the individual segments and from the change in lattice irregularities upon mixing. The excess entropy arises due to (a) the combinatorial term (Flory-Huggins), (b) the change upon mixing of the size of the average volume in which the segments are free to move, and (c) the change in excitation of the external degrees of freedom when the average potential field acting on the segments is changed in the mixing process. Lattice irregularities appear to have a large effect on the excess functions of mixtures whose components are of different hydrocarbon type. Comparison of predicted with experimental results is favorable.