Structural behaviour of n-tetracosane and n-hexacosane mixtures

Abstract

The study of the n-alkane binary system n-tetracosane ( n-C 24H 50) : n-hexacosane ( n-C 26H 54) 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-C 26H 54) increases : two terminal solid solutions γ 1 and γ 2, isostructural with n-tetracosane ( n-C 24H 50) and n-hexacosane ( n-C 26H 54) respectively and three orthorhombic intermediate phases, isomorphic with the odd-numbered pure alkanes ( n-pentacosane n-C 25H 52, for example), noted β′ 1, β″ and β′ 2 respectively. On the basis of powder X-ray patterns, the phases β′ 1 and β′ 2 are indistinguishable and they are isostructural with the intermediate solid solutions β′ 1 and β′ 2 of the n-docosane ( n-C 22H 46) : n-tetracosane ( n-C 24H 50) system and probably with the phases β 1 and β 2 of the n-eicosane ( n-C 20H 42) : n-docosane ( n-C 22H 46) diagram. The intermediate solid solution β″ is also isostructural with the phase β″ of the n-docosane ( n-C 22H 46) : n-tetracosane ( n-C 24H 50) system and certainly with the phases β of the n-eicosane ( n-C 20H 42) : n-docosane ( n-C 22H 46) 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 γ 1 and γ 2 transformations are similar to those observed in n-tetracosane ( n-C 24H 50) and in n-hexacosane ( n-C 26H 54). The transitions of the intermediate phases β′ 1, β″ and β′ 2 are equivalent to those of the n-docosane : n-tetracosane system and the odd-numbered n-alkanes, above the transition δ. The n-pentacosane ( n-C 25H 52) transformations versus temperature have been described with the appearance of the rotator phases β-RI and α-RII just below the melting point. They are compared with those that are observed in the phase β″. The phase denoted β 0 in the n-C 20H 42 : n-C 22H 46 diagram probably corresponds to the rotator phase β-RI.