Effect of Pressure on the Phase Relations of Some n-Paraffins

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Abstract
The phase diagrams of n-C21H44, n-C25H52, n-C22H46, n-C24H50 and n-C32H66 were studied to 40 kbar by means of differential thermal analysis and volumetric techniques. n-C21H44 and n-C25H52 have orthorhombic/hexagonal/liquid triple points near 3 kbar, with the triple points marked by definite inflections in the melting curves. n-C22H46 transforms to a suspected monoclinic phase at ∼2.6 kbar, 22°C. It is concluded that even nparaffins below n-C22H46 will all have similar high-pressure transformations. n-C24H50 has the triclinic/monoclinic transition at ∼44°C, 1 bar. The high-pressure melting curves of the even n-paraffins studied here all refer to the monoclinic phases, and are remarkably similar in slope and curvature, but have considerably less curvature than the orthorhombic odd n-paraffins. The monoclinic (or triclinic)/hexagonal/liquid triple points of the even n-paraffins studied are located at or below 1 kbar. The usual monoclinic phase of even n-paraffins above ∼n-C26H54 is a high-pressure structure with respect to the usual triclinic phase of even n-paraffins below ∼n-C26H54.

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