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

Abstract

Cloud point and cooling curve studies of the cyclohexane–methanol system were ideally suited as a laboratory exercise for a physical chemistry curriculum. Within 3–4 h, students can obtain a sufficient number of relatively precise experimental data (temperature vs. mol fraction of methanol) to construct the major part of the solid–liquid phase diagram. The experimental procedure is based on three different methods: (1) cloud points were detected using an H–Ne laser; (2) temperatures of the methanol-rich part below the monotectic were taken when the first crystals of cyclohexane become visible; and (3) cooling curves were recorded in order to determine the melting point of C 6H 12 and the monotectic temperature. The results of the system exhibiting 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 adjustable parameters for the liquid phase. However, only an extended Ising model agreed with the experimental curve near the critical temperature.