The phase diagram of cyclohexane-methanol: a challenge in chemical education

Cloud point and cooling curve studies of the cyclohexane-methanol system we re ideally suited as a laboratory exercise for a physical chemistry curricu lum. Within 3-4 h, students can obtain a sufficient number of relatively pr ecise experimental data (temperature vs. mol fraction of. methanol) to cons truct the major pan of the solid-liquid phase diagram. The experimental pro cedure is bused on three different methods. (1) cloud points were detected using an H-Ne laser: (2) temperatures of the methanol-rich part below the m onotectic were taken when the first crystals of cyclohexane become visible: and (3) cooling curves were recorded in order to determine the melting poi nt of C6H12 and the monotectic temperature. The results of the system exhib iting an upper critical solution temperature. UCST. agreed reasonably with the most reliable data presently available. Assuming a tetrameric methanol species, the experimental T-x data of the freezing-point depression and the sub-monotectic curve were fitted to a sub-regular model with three adjusta ble parameters for the liquid phase. However. only an extended Ising model agreed with the experimental curve near the critical temperature. (C) 1998 Elsevier Science B.V.