Review on the energetic and crystallographic behaviour of n-alkanes.: II.: Series from C22H46 to C27H56

A review on the energetic and crystallographic behaviour of the series of n alkanes from C22H46 to C27H56 is proposed. The polymorphism is very rich (characterization of 12 transitions, 10 solid phases). The molecules are always parallel, their end methyl groups forming parallel layers. At low temperature ordered crystalline forms are observed. Their nature depends on the parity of n the number of carbon atoms. In the odd subfamily the molecules are perpendicular or nearly perpendicular to the layer; three different forms have been orthorhombic O, O' and monoclinic M*. In the even subfamily, the molecules are no more perpendicular to the layer and crystallize either in the triclinic T or monoclinic M lattice. On the contrary, the nature of the phases before melting are not dependant on the parity of n. These phases are orientationally disordered. Five rotators phases may be successively encountered RV, RI, RII, RIII, RIV with increasing temperature. None of the n-alkanes shows more than 3 of these rotator phases. Despite the high degree of disorder the crystalline cohesion remains important, the enthalpy of melting being higher than 158 J/g.