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

Abstract

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 (C22) undergoes an orthorhombic-hexagonal (rotator, R) phase follow by an alpha (α) phase. In addition, the α phase as an intermediate transition between the solid and liquid phases is observed in heneicosane (C21). For eicosane (C20) and octadecane (C18) the transition from solid to liquid phase is almost instantaneous unlike docosane (C22) and heneicosane (C21) 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.