Phase transitions of n-alkanes as rotator crystals

Abstract

Phase transitions of n -alkane homologues with n =17–24, their solid solutions, two-phase mixtures and multicomponent mixtures with n =17–37, of biological, geological and technological origin have been studied by high-temperature X-ray powder diffraction. In keeping with the rotator nature of n -alkanes, their thermal deformations and polymorphic transformations are discussed as a function of the thermal torsional motion of the molecules. We have shown that not only orthorhombic n -alkanes but also triclinic n -alkanes undergo consistent phase transitions from the crystal state (cryst) to the low-temperature (rot.1) and high-temperature (rot.2) rotator states. When molecules (atoms) of different kinds combine in a structure one more rotator state of n -alkanes (rot.1+2), intermediate between the low-temperature (rot.1) and high-temperature (rot.2) rotator states, was identified. Each of these states is characterized by a specific form of the molecular thermal oscillation motion. The existence of crystal phase V and rotator phase RV is discussed on the basis of X-ray powder diffraction and literature data. Phase transitions of the mixtures of two crystal phases were shown to depend on the molecular symmetry (parity) of the mixed components and the difference in chain length (Δ n ). The distinguishing feature of the phase transition to the rotator state of multicomponent mixtures is step-wise phase separation of the solid solution during heating.