Properties of mixed-crystalline organic material prepared by zone levelling IV. Melting properties and excess enthalpies of (trans-azobenzene + trans-stilbene)

Abstract

Homogeneous mixed crystals of (trans-azobenzene + trans-stilbene) were prepared. Molar heat capacities of the pure substances and the mixed crystals were determined at temperatures from 300 to 400 K. The melting temperatures and molar enthalpies of fusion were measured for trans-stilbene and trans-azobenzene. The melting temperatures are 341.06 K and 397.40 K; the molar enthapies of fusion are 22.52 kJ·mol−1 and 27.69 kJ·mol−1, respectively. From the measurements on the mixed crystals the excess molar enthalpy difference between the liquid and solid phases was obtained, and is about −200x(1 − x) J·mol−1. Finally the solidus and liquidus points were obtained from the heat-capacity curves of the levelled samples, the solidus curve was calculated, and was found to agree with the experimental solidus points (to within 1.5 K).