The C19H39OH–C20H41OH system: Experimental phase diagram and thermodynamic modelling

Abstract

The experimental phase diagram of the C₁₉H₃₉OH–C₂₀H₄₁OH system has been determined and, subsequently, subjected to thermodynamic modelling. The pure components of the system are polymorphic. At low temperature they have monoclinic phases that are different: a γ phase (C2/c, Z = 8) for C₂₀H₄₁OH and a β phase (P2₁/c, Z = 8) for C₁₉H₃₉OH. At high temperature, a few degrees before melting, the two components have the same monoclinic phase R^′_IV (C2/m, Z = 4). The solid–liquid equilibrium can be explained by simple isomorphism, and the solid–solid equilibria by crossed isodimorphism. The solid–liquid domain ([R^′_IV + L]) is very narrow (less than 0.2 K), and the enthalpy of fusion is high enough to consider the alloys of the system as promising candidates for thermal energy storage. Three solid–solid domains were observed ([β + R^′_IV], [γ + R^′_IV] and [β + γ]), these are related by a peritectoid invariant at ≈323 K, with compositions from 55 and 80 mol% in C₂₀H₄₁OH. One of the domains ([β + R^′_IV]) has a minimum at ≈321 K and about 40 mol% in C₂₀H₄₁OH. The calculated phase diagram, obtained by thermodynamic analysis, is in full agreement with the experimental one.