Molecular disorder in even-numbered paraffins

Conformational and displacive molecular defects in even-numbered paraffins ${\mathrm{C}}_{\mathrm{n}}$ ${\mathrm{H}}_{2\mathrm{n}+2\mathrm{}}$, with 22≤n≤28, are studied as a function of temperature by means of small-angle x-ray diffraction. The amplitude of the molecular longitudinal motion in the rotator phase increases with molecular length, as it was also found to do in previous studies of odd-numbered paraffins. This amplitude is temperature independent in rotator phases of even paraffins with 22≤n≤26. The voids between molecular layers in the crystalline phase of even paraffins are narrower than those of odd paraffins. This indicates a closer packing of layers in systems in which molecules are tilted. Structures of the high-temperature hexagonal rotator phases of odd and even paraffins exhibit a minor but significant difference which has been associated with different interactions between molecular ends.