Raman analysis of intermediate phases during melting and freezing in long-chain n-alkanes

Structural changes of the n-alkanes (n = 18, 20, 21, 22) have been investigated by Raman spectroscopy during the melting from solid phase to the intermediate transition phase to the molten phase. With increasing temperature docosane (C-22) undergoes an orthorhombic-hexagonal (rotator, R) phase follow by an alpha (alpha) phase. In addition, the alpha phase as an intermediate transition between the solid and liquid phases is observed in heneicosane (C-21). For eicosane (C-20) and octadecane (C-18) the transition from solid to liquid phase is almost instantaneous unlike docosane (C-22) and heneicosane (C-21) where the intermediate transitions are observed. During freezing from the liquid phase, the even n-alkanes show the rotator phase (R) a few degrees lower than melting and just before the crystalline solid phase. The intermediate phase transition temperature can be spectrally determined with the 1100-1350 cm(-1 )Raman region of the CH2 band. (C) 2021 Elsevier B.V. All rights reserved.