Mixtures of numerous different n-alkanes:: 1. Structural studies by X-ray diffraction at room temperature -: Correlation between the crystallographic long c parameter and the average composition of multi-alkane phases

Two quaternary equimolar alloys (C-22/C-26/C30C34 and C-24/C-28/C-32/C-36) and three commercial products, which respectively consist of 23 and 33 consecutive n-alkanes with chain lengths between 20 and 52 carbon atoms, are studied by X-ray diffraction analyses. A single orthorhombic solid solution, identical to one of the two intermediate phases seen in binary n-alkane systems, is observed in two commercial products and in their 50-50 wt% mixture. The n-alkane molar concentrations and the long crystallographic c parameters have been determined and allow us to demonstrate that the molecule layer stacking periodicity is quasi-equal to the average length of n-alkane chains of these complicated mixtures: this periodicity corresponds to that of a hypothetical orthorhombic purr n-alkane whose equivalent carbon atom number is quasi-equal to the average carbon atom number of commercial multi-n-alkane mixtures. Discontinuous distributions of chain lengths lead to the observation of two or three orthorhombic phases in the quaternary alloys and the presence of an orthorhombic solid solution and of an amorphous solid in the third commercial multi-il-alkane product. (C) 1999 Elsevier Science Ltd. All rights reserved.