Phase diagram of n-hexacosane and n-octacosane: experimental determinationand calculation

The phase diagram of (n-octacosane + n-hexacosane) has been determined expe rimentally, using a crystallographic and a calorimetric method, in the n-oc tacosane mole fraction range x = (0.2 to 0.7). In this composition range, t he mixtures show an intermediate solid solution at room temperature, denote d beta ", with an orthorhombic structure, identical to the one found in oth er binary, or ternary systems, and in crude oil. With increasing temperatur e, this solid solution undergoes two solid-solid transitions, leading to an other orthorhombic phase, denoted beta(Fmmm), then to a rhombohedral "Rotat or" phase, denoted alpha-RII, before melting. The phase diagram has been ca lculated by using the following assumptions: ideality of the liquid phase m ixture, ideality of the alpha-RII rotator phase mixture, and non-ideality o f the mixture in the beta, and beta " phases whose excess Gibbs energies ar e represented by Margules equations (one- and two-parameter, respectively). The enthalpies of transformation, the temperatures of transformation, and the heat capacities of the pure compounds have been either measured, or ext rapolated from the mixtures values. This allows a good representation of th e phase diagram through the use of simple expressions. (C) 1999 Academic Pr ess.