STRUCTURAL BEHAVIOR OF N-TETRACOSANE AND N-HEXACOSANE MIXTURES

The study of the n-alkane binary system n-tetracosane (n-C24H50): n-hexacosane (n-C26H54) was carried out using calorimetry and X-ray analyses. In spite of what is generally found in the literature for the two consecutive even-numbered n-alkanes mixtures, the X-ray diffraction results at 293 K show the presence of five solid solutions when the molar concentration in n-hexacosane (n-C26H54) increases:two terminal solid solutions gamma(1), and gamma(2), isostructural with n-tetracosane (n-C24H50) and n-hexacosane (n-C26H54) respectively and three orthorhombic intermediate phases, isomorphic with the odd-numbered pure alkanes (n-pentacosane n-C25H52, for example), noted beta'(1), beta'' and beta'(2) respectively. On the basis of powder X-ray patterns, the phases beta'(1) and beta'(2) are indistinguishable and they are isostructural with the intermediate solid solutions beta'(1) and beta'(2) of the n-docosane (n-C22H46):n-tetracosane (n-C24H50) system and probably with the phases beta(1) and beta(2) of the n-eicosane (n-C20H42):n-docosane (n-C22H46) diagram. The intermediate solid solution beta'' is also isostructural with the phase beta'' of the n-docosane (n-C22H46):n-tetracosane (n-C24H50) system and certainly with the phases beta of the n-eicosane (n-C20H42):n-docosane (n-C22H46) mixtures:these two phases have been compared and discussed. With increasing temperature, X-ray diffraction and differential calorimetry led to the determination of the phase transitions at their equilibrium temperatures. The terminal solid solution gamma(1) and gamma(2) transformations are similar to those observed in n-tetracosane (n-C24H50) and in n-hexacosane (n-C26H54). The transitions of the intermediate phases beta'(1), beta'' and beta'(2) are equivalent to those of the n-docosane: n-tetracosane system and the odd-numbered n-alkanes, above the transition delta. The n-pentacosane (n-C25H52) transformations versus temperature have been described with the appearance of the rotator phases beta-RI and alpha-RII just below the melting point. They are compared with those that are observed in the phase beta''. The phase denoted beta(0) in the n-C20H42:n-C22H46 diagram probably corresponds to the rotator phase beta-RI.