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

Abstract

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.