Structural evolution versus temperature of the beta(0) phase of the n-eicosane/n-docosane system: Rotator transitions

The orthorhombic phase beta(0)[n-C20H42/n-C22H46] (Fmmm) determined by Gerson and Nyburg is isostructural with the orthorhombic Rotator phase beta-RI (Fmmm), observed in n-tricosane (n-C23H48) and n-pentacosane (n-C25H52) and in the two binary systems: [n-C22H46/n-C24H50] and [n-C24H50/n-C26H54]. When the temperature increases, its b/a ratio changes progressively from 1.51 at 293 K to 1.73 at 309 K; it then transforms into the rhombohedral Rotator phase alpha-RII (R (3) over bar m) just below the solidus curve. These observations allow generalization of structural behaviour of two n-alkane mixtures (19 < n < 27), whose difference between carbon numbers is equal to 1 or 2.