Phase transitions among the rotator phases of the normal alkanes

Abstract

We present the first calorimetric study of the normal alkanes CH3–(CH2)n−2–CH3 (21≤n≤30) covering the temperature range of the five rotator phases (whose structures were previously identified using x-ray scattering) with sufficient resolution to observe the various rotator to rotator transitions. We find first-order hexagonal–orthorhombic distortion transitions; second-order azimuthal tilt-angle rotation transitions, and two types of second-order tilting transitions, one of which has the higher symmetry phase at low temperature. These transitions appear to be mean field in character, in that they are without significant pretransitional fluctuations. We discuss the calorimetric signatures for the transitions in terms of the order parameters obtained from x-ray scattering data. In addition to the transitions, we find strong temperature variation of the heat capacity in the rotator phases not associated with the transitions.