Thermodynamic Studies on Order-Disorder Phase Transitions of p -Terphenyl and p -Terphenyl- d 14

Abstract

The heat capacities of p-terphenyl and p-terphenyl-d14 were measured by adiabatic calorimetry between 3 and 300 K. The phase transition associated with a molecular conformation change was observed as a λ-shaped anomaly at (193.5±0.1) K and at (180.3±0.1) K. The enthalpies of transition are (304±20) J mol−1 and (288±20) J mol−1 and the entropies of transition are (1.63±0.10) J K−1 mol−1 and (1.63±0.10) J K−1 mol−1, respectively. The anisotropy of the intermolecular interaction is discussed and the anomalous portion of the heat capacities in the transition region was compared favorably with a two-dimensional Ising model.