First-Order Phase Transitions of Six Normal Paraffins at Elevated Pressures

Abstract

First order phase transitions were investigated for n-nonane, n-dodecane, n-tridecane, n-pentadecane, n-octadecane, and n-tetracosane, at pressures up to 10 kilobars and temperatures up to 135°C. By a modification of standard piezometric techniques, phase transition pressures, as well as the associated isothermal isobaric volume changes were determined at approximately 25°C intervals. Correlations established between the melting temperatures and the specific volume changes associated with phase transitions and the n-paraffin chain lengths show a strong dependence upon whether the n-paraffin is of odd or even species. This dependence becomes more pronounced at higher pressures. The specific volume, enthalpy, and entropy changes showed no dependence upon chain length at the same melting temperature.