The experimental systems considered in this paper are normal alkanes, in the range from octane to octacosane; and their binary mixtures {(1-x) mol of CnH2n+2 + x mol of Cn′H2n′+2}, with n′ > n, and ∆n ) n′ -n taking the values 1 and 2. The alkanes and their mixtures have a rich, complex polymorphic nature, for the description of which a distinction is made between a high-temperature and a low-temperature domain. The high-temperature domain is occupied by rotator phases, and the low-temperature domain by a variety of mixed crystalline phases, referred to as the "ordered" phases. Experimental phase diagram data for 19 binariessincluding 11 binaries for which original data are presentedsare used to give a uniform and coherent description of the phase relationships for the ensemble of alkane alloys.

X-ray diffraction analyses of the pure components n-tricosane and n-pentacosane and of their binary mixed samples have enabled us to characterize the crystalline phases observed at "low temperature." Contrary to what was announced in literature on the structural behavior of mixed samples in odd-odd binary systems with Dn 2, the three domains are not all orthorhombic. This work has enabled us to show that two of the domains are, in fact, monoclinic (Aa, Z 4), and the other one is orthorhombic (P ca2 1 , Z 4). The conclusions drawn in this work can easily be transposed to other binary systems of n-alkanes.

The experimental phase diagram of the binary system implying the nonisomorphous consecutive n-alkanes heneicosane (C 21 H 44 ) and docosane (C 22 H 46 ) has been determined by combining X-ray diffraction and microcalorimetric methods. The phase diagram is very complex. X-ray experiments performed on powder samples at 288 K have shown the presence of five monophasic domains at "low temperature": three orthorhombic (Oi, Odci, and Op), one monoclinic (Mdci), and one triclinic (Tp). With increasing temperature occur successively the rotator forms RV, RI, and RII. No less than ten solid-solid and two solid-liquid biphasic domains are observed, as well as three peritectoid, one eutectoid, and one peritectic invariants. Moreover, cell parameters of mixed samples corresponding to the forms Odci (space group Pnam), Mdci (Aa), and Op (Pca2 1 ) have been determined at 288 K.