X-ray study of the ‘‘rotator’’ phase of the odd-numbered paraffins C17H36, C19H40, and C21H44

Abstract

We present here a study of the ’’rotator’’ phase displayed by the odd-numbered normal paraffins CnH2n+2 with n ranging from 17 to 21. A structural model of the rotator phase can be deduced from x-ray experiments performed both on single domains and on powder samples. Its structure very much looks like the crystalline phase structure, i.e., the molecules are packed within layers forming a bilayer structure with the molecular axes oriented perpendicularly to the layer planes. The comparison of the space group Ccmm with the steric dimensions of the molecules implies the appearance of an orientational disorder of the molecules around their long axes which is the main characteristic of the crystalline→rotator phase transition. Such a conclusion is in agreement with the dynamical measurements which state a uniaxial rotation of the molecules.