Heat capacity measurements were performed by adiabatic calorimetry for two diacetylene compounds, 10,12-pentacosadiynoic acid and 10,12-pentacosadiyn-1-ol. While the former did not show any anomaly in heat capacity between 5 K and 300 K, a solid-solid phase transition was found at 90.4 K for the latter. The transition entropy was estimated to be 2.80 J K−1 mol−1, which is close to (1/2)R ln2 (= 2.88 J K−1 mol−1), indicating that the transition is presumably an order-disorder type. The crystal structure, which is unknown yet, is extremely interesting to understand the mechanism of the transition. Such information is also important for the adsorbed monolayer system.
|
Fig. 1 Schematic representation of 1,4–addition reaction of diacelylene compounds to show polymerization. |
|
Fig. 2 Molecular structures of 10,12–pentacosadiynoic acid (a) and 10,12–pentacosadiyn–1–ol (b). |
|
Fig. 3 Heat capacity of 10,12–pentacosadiyn–1–ol. |
|
Fig. 4 Heat capacity of 10,12–pentacosadiyn–1–ol near the phase transition (red) and 10,12–pentacosadiynoic acid (black). |