Additivity Rule of the Partial Molal Volume. 1. Its Validity for a Series of Normal Alkane Liquids

Abstract

AbstractA critical test of the additivity rule of the partial molal or molal volume (Ṽ) at constant temperature and pressure, expressed by Ṽ = αn + β where n is the carbon number, has been made for a wide range of n‐alkanes at a variety of temperatures and pressures. In the pure liquid state the additivity rule is obeyed approximately well by the higher n‐alkanes but broken by the lower ones having the boiling point close to the temperature of interest, and in solution at infinite dilution it is obeyed much more accurately by both high and low n‐alkanes. These facts indicate that the additivity rule holds more accurately for the configurational partial molal volume than for the partial molal volume itself. The virial theorem for classical fluids is applied here to separate the overall partial molal volume into the translational and configurational components; the translational contribution is proved to be the isothermal compressibility multiplied by the gas constant and temperature. The parameter a decreases slightly and linearly and the parameter β decreases significantly and nonlinearly as temperature is lowered or pressure is increased.