Binary Phase Diagram with Non Isomorphous n-Alkanes:C12H26-C15H32. Implication of the Rotator Phase RI in the Melting Behaviour of Odd-Even and Even-Odd Phase Diagrams

The phase diagram of n-C12H26 + n-C15H32 established by calorimetric and X-ray diffraction analyses exhibits no less than four single phase domains [T-12], [O-15], [Ri(15)] and [L], two invariants: a eutectic at T = 258.3 K and a metatectic at T = 263.3 K, and five two-phase domains. From a thermodynamic point of view, an interpretation of the phase diagram is given and an attempt is undertaken to modelise the [R-I + L] equilibrium. The extent in composition x of this phase as well as the width in temperature of the [R-I + L] eutectic at x = 1/2, are explained in terms of degree of isomorphism epsilon(i)(m)(R-I) and excess Gibbs energy functions G(E,sol) (x). Those quantities as well as the melting temperature difference (Delta T) at x = 1/2, are compared and discussed with other binary systems.