Solutions of n-alkanes: volume-related properties and the principle of congruence

Abstract

The relationship of the simple segment theory to the Orwoll-Flory theory is examined; this suggests a restatement of the principle of congruence as five postulates. These are tested in turn against the molar volume, thermal expansivity and isothermal compressibility measurements of Díaz Peña and Benítez de Soto on the system n-hexane + n-hexadecane, using the methods of Patterson and Bardin. The molar volume data are analyzed also by a method based on the principle of corresponding states, which includes direct comparison of each solution with a reference n-alkane. The Patterson-Bardin analyses accord with the analyses of the molar volumes, with reservations because of an inconsistency in the thermal expansivity results. The solutions appear to follow the same reduced equation of state as do the n-alkanes, but their equivalent substances depart significantly from the locus of reducing parameters defined by the n-alkanes. The implications of this departure are discussed.