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.

New insights on the crystalline forms in binary systems of n-alkanes: Characterization of the solid ordered phases in the phase diagram tricosane + pentacosane

“…C (1) 0.014 0.186 0.0239 0.90 C (2) 0.092 0.314 0.0435 1.00 C (3) 0.014 0.186 0.0631 1.00 C (4) 0.092 0.314 0.0827 1.00 C (5) 0.014 0.186 0.1023 1.00 C (6) 0.092 0.314 0.1219 1.00 C (7) 0.014 0.186 0.1415 1.00 C (8) 0.092 0.314 0.1611 1.00 C (9) 0.014 0.186 0.1807 1.00 C (10) 0.092 0.314 0.2003 1.00 C (11) 0.014 0.186 0.2199 1.00 C (12) 0.092 0.314 0.2395 1.00 C (13) 0.014 0.186 0.2591 1.00 C (14) 0.092 0.314 0.2788 1.00 C (15) 0.014 0.186 0.2987 1.00 C (16) 0.092 0.314 0.314 1.00 C (17) 0.014 0.186 0.3376 1.00 C (18) 0.092 0.314 0.314 1.00 C (19) 0.014 0.186 0.3770 1.00 C (20) 0.092 0.314 0.3964 1.00 C (21) 0.014 0.186 0.4160 1.00 C (22) 0.092 0.314 0.4356 1.00 C (23) 0.014 0.186 0.4552 1.00 C (24) 0 and binary mixed samples do not change significantly with chain length), and z͞c coordinates were obtained by extrapolation along the z-axis of the chain model used by Dorset. 51 The fractional atomic occupancies (see Table I) which are applied to the chain ends have no effect on the angular position of the diffraction lines if not on the intensities and specially those of the 00l lines.…”

Influence of Pressure on the Size of Free Volumes in Some Waxes

“…The positron annihilation lifetime spectroscopy (PALS) has been successfully applied to estimate the size of free volume in hydrocarbons, which have a relatively simple structure in condensed phases (rigid crystal, rotator and liquid phase) [15]. In many applications (and in the nature) alkanes appear as a mixture (wax) composed of molecules of various chain length.…”

Positronium as a probe of small free volumes in crystals, polymers and porous media

“…for C17 the concentration of kinked molecules is about 8%, while for C29 it approaches 70% [118]. Mismatching the kink conformers and all-trans ones leads to the creation of free volumes inside the lamella.…”

“…Top -temperature dependence of the o-Ps lifetime in the range of rotator phase for several odd-numbered n-alkanes (C17, C19, C21), bottom -temperature dependence of the crystalline axis a for the n-alkanes shown above. Crystallographic data from [118].…”