X-ray and thermal studies on the rotator phases of normal higher alcohols C17H35OH, C18H37OH, and their mixtures

Abstract

The structure of the rotator phase and the phase transitions in n-alcohols C17, C18, and their mixtures are studied by differential scanning calorimetry (DSC) and by x-ray diffraction. Both the heating and cooling phase diagrams of the binary system are determined. During cooling, a new intermediate phase α′ is observed under a temperature region of the usual rotator phase α. These α and α′ phases are found to correspond to the rotator-II phase and the rotator-I phase, respectively, of n-paraffins. The dynamical crystal structure in the α phase is revealed by investigation of the diffuse scattering from the single crystal. The characteristic diffuse scattering observed originates from molecular fluctuations due to the translation along and the rotation around the molecular axes. Both modes of the molecular motions are studied separately from the diffuse scattering on the first and on the second layer lines. The translational disorder is described by the averaged relative translation between the nearest neighbor molecules; it is found to be as much as 20% of the repeat period 2.55 A of the polymethylene chain. On the other hand, it is found that the rotational disorder of the molecules is much smaller than the translational. The molecules are found to have a tendency to make their molecular planes parallel. It is suggested that the crystal in the rotator phase consists of domains, within which the molecules are ordered with respect to the orientations of their molecular planes but are making fairly active translations. Such structural model of the rotator phase is found to be quite consistent with that obtained in our recent computer simulation.