Solid–solid and solid–liquid equilibria in the n-alkanols family: C18H37OH–C20H41OH system

Abstract

C₁₈H₃₇OH–C₂₀H₄₁OH is an example of a binary system showing isopolymorphism. The two alkanols display the same polymorphic behaviour. At low temperatures, they crystallize into the same ordered form γ (C2/c, Z = 8). On heating, γ transforms into the rotationally disordered form R′_IV (C2/m, Z = 4), at a few degrees below the melting point of the latter. However, in most mixed samples of this system a β form (P2₁/c, Z = 8), metastable in the two pure components, has also been observed at low temperatures. At high temperatures, the β form transforms into the R′_II form (Rm, Z = 3). This R′_II form is also metastable in the two pure components. The β form presents conformational defects, and molecules with all-trans conformation co-exist with molecules with CO-gt-conformation, in contrast, all the molecules in the γ form present all-trans conformation. In the R′_II form the rotational disorder is more accentuated than in the R′_IV form. The disorder of composition (molecular alloys) stabilizes over wide ranges of compositions the β (disorder of conformation) and R′_II (disorder of rotation) forms. Five solid–solid domains ([γ + β], [β + R′_II], [γ + R′_II], [γ + R′_IV] and [R′_II + R′_IV]) related by two peritectoid and eutectoid invariants, and two solid–liquid domains ([R′_IV + L] and [R′_II + L]) related by a eutectic and a peritectic invariant, are present. The [β + R′_II] domain has a minimum. All these domains are observed for compositions rich in the two pure components. The experimental phase diagram data are fully supported by the thermodynamically calculated phase diagram. The R′_II + liquid domain has a width of less than 1 K; therefore, and due to the large heat effect, the system's alloys are good candidates for the storage of thermal energy.