Solid−Solid and Solid−Liquid Equilibria in the Heneicosane−Docosane Binary System

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 ( Pca 2 1 ) have been determined at 288 K.