Melting behaviour in the n-alkanol family. Enthalpy–entropy compensation

The melting behaviour was studied in ten systems: C15OH–C16OH, C16OH–C17OH, C17OH–C18OH, C18OH–C19OH, C19OH–C20OH with Δn = 1 (difference in chain length), C15OH–C17OH, C16OH–C18OH, C17OH–C19OH, C18OH–C20OH with Δn = 2, and C16OH–C20OH with Δn = 4. The phase that melts is either the monoclinic R′IV(C2/m, Z = 4) or the hexagonal R′II(Rm, Z = 6) rotator form. One of the most important issues in the melting of these systems is that when the two original compounds of the system are isostructural, the phase diagram does not always show total miscibility. In the systems studied here, only the C15OH–C16OH, C18OH–C19OH and C19OH-C20OH systems show total miscibility. In the other systems in which the two original compounds are isostructural, miscibility is partial, as in the systems where the two original compounds are not isostructural. In this family, as in other families of mixed crystals, there is an excess enthalpy-entropy compensation. This compensation has a temperature dimension, and is called the compensation temperature (θ) of the family and/or subfamily. In the case of the R′II and R′IV rotator forms of the n-alkanols family its value is 362 K. This value is in line with the trend show by a large group of organic and inorganic mixed crystalline materials.