Phase Changes in Crystalline and Glassy-Crystalline Cyclohexanol

Abstract

It has been known that cyclohexanol has low-temperature form (crystal-II) and high-temperature form (crystal-I) and that the supercooled crystal-I shows in its heat capacity curve an anomalous jump resembled to that associated with glass transition. In order to clarify whether this anomaly arises from relaxation phenomenon or not, we have reinvestigated the heat capacity of this substance from 14°K to 320°K and obtained the values of 8827 J/mol, 265.31°K, 1782 J/mol and 299.09°K for heat of transition, transition point, heat of fusion and melting point respectively. Comparing the ideal gas entropy of cyclohexanol calculated from thermal data and that from statisticalmechanical data, we have estimated that the residual entropy of the crystal-II is of zero and that the potential barrier of internal rotation of OH group is 7.0 kj/mol. From these results, the residual entropy of the supercooled crystal-I amounts to 4.72 J/(mol deg K). We have discussed the glass transition of this crystal and proposed a new term glassy crystal. We have also studied the heat capacity of metastable phase (crystal-III) and obtained the results 244.8°K and 8640 J/mol for transition point and heat of transition. The residual entropy of the crystal-III has determined to be zero.