Confined crystallization of binary n-alkane mixtures: stabilization of a new rotator phase by enhanced surface freezing and weakened intermolecular interactions.

The present work reports the confined crystallization behaviours of binary even-even normal alkane (n-alkane) mixtures of n-octadecane (n-C(18)H(38)) and n-eicosane (n-C(20)H(42)), which are microencapsulated in monodisperse microcapsules, using the combination of differential scanning calorimetry (DSC) and temperature-dependent X-ray diffraction (XRD). A new metastable rotator phase (RII) absent in the bulk state, has been detected for the n-alkane mixture in confined geometry under all the investigated compositions. Such a crossover is attributed to the lower interfacial energy due to the same in-planar hexagonal structure of the surface monolayer and RII, as well as the weakened intermolecular interaction in alkane mixtures. This is the first time that RII is found in such a binary even-even n-alkane mixture that neither of the components contains RII phase in the crystallization process. Furthermore, based on the variation of alkane molecule conformation and in-planar structure with temperature, the correlations between the phase transition temperature and composition have been discussed.