Crystal Structure of Wax Lamellar InterfacessA Residual Petroleum Fraction Characterized by Electron Crystallography Melanie Rademeyer Department of Chemistry and Biochemistry, Rand Afrikaans UniVersity, P.O. Box 524, Auckland Park 2006, South Africa Douglas L. Dorset* ,† Electron Crystallography Laboratory, Hauptman-Woodward Medical Research Institute, Inc., 73 High Street, Buffalo, New York 14203-1196 ReceiVed: NoVember 17, 2000; In Final Form: March 26, 2001 After a multicomponent paraffin assembly was constructed to model a petroleum residue wax (M w /M n ) 1.009), its structure was characterized by electron crystallography. Consistent with a single lamellar spacing, two endotherms in a DSC scan are typical of paraffin chain solid solutions and represent the premelt transition to a “rotator” phase and the true melt. The average chain packing in the crystal structure is that of the paraffin n-C 32 H 66 , in space group Pca2 1 with a ) 7.42, b ) 4.96, and c ) 85.0 Å. An attempt to account for the lamellar disorder with a chain-end occupancy model based on the chemical distribution of chain lengths is only partially successful. A better fit is found when lower chain-end occupancies are used. This discrepancy could be due to conformational defects in which the chain end atoms do not lie on strict methylene subcell lattice sites. Introduction Recently, single-crystal electron diffraction data from poly- disperse assemblies of linear alkanes have effectively distin- guished the structure of true lamellar waxes from those in which well-defined layers are never formed. 1 It is now clear why the former, exemplified by relatively narrow petroleum distillate cuts, are physically deformable solids whereas the latter, exemplified by as-synthesized Fischer-Tropsch products and low-molecular-weight linear polyethylene, are brittle solids. With lamellar layering, defects due to chain length differences, including nonplanar conformations, are concentrated at a layer surface. 2-4 For the brittle solids in which lamellae are never formed, there is only an average “nematocrystalline” ordering of the chain axes, leading to an internally reinforced chain packing array, despite an often larger polydispersity value. 1 The distribution of chain disorder has often been studied in lamellar waxes. It readily became apparent that, in terms of diffraction experiments, a suitable model at layer surfaces for dissimilar chain lengths within a lamella would include a distribution of fractional methylene atomic occupancies, 5 with the greatest deviation from unity occurring at the surface. Spectroscopic measurements, on the other hand, indicate that the packing arrangement might be more complicated than just partial methylene group occupancies. A more realistic structure would include, for example, nonplanar conformational defects. 4 Recently, 2 a Gaussian model for fractional occupancy was correlated to a lamellar disorder term described by Strobl and co-workers 6 and was shown to account well for the falloff of lamellar diffraction peaks in patterns from a number of paraffin waxes. Later, the accuracy of this model was questioned 3 because, in principle, this distribution of chain methylene positions should be related to the actual chemical distribution of chain lengths within the solid. After examination of a polydisperse artificial wax with very low polydispersity (M w /M n ) 1.003) 3 based on one characterized earlier by Stokhuyzen and Pistorius, 7 it was decided that a model accounting for the actual chain length distribution was more accurate than the best Gaussian distribution of methylene occupancies. In this paper, a wax with a broader distribution is studied to evaluate this chemically based model further. Materials and Methods Model Wax and Its Crystallization. The model paraffin wax used in this study was based on a residual petroleum wax, termed “Wax J” when originally characterized by Gupta and Severin 8 using high-temperature gas chromatography and su- percritical fluid chromatography to determine the carbon chain distribution. The original residual wax comprised a distribution of chains from n-C 21 H 44 to n-C 39 H 80 , with the assayed concen- trations of individual species tabulated by these authors. 8 For this study, a closely similar chain distribution (Figure 1) was obtained by weighing pure n-paraffins into a vial. This mixture was then co-melted on a hot plate, cooled, broken up and stirred, and remelted several times to obtain a uniform solid solution. From the known chain length distribution, the calculated polydispersity 9 (M w /M n ) is 1.009. A DSC measurement of the reconstructed wax (Figure 2) yields results typical of narrow distillate cuts 8 where one can discern a premelt transition of an orthorhombic solid to a rotator phase before the true melt. The appearance of the premelt transition indicates the presence of a stable paraffin solid solution, as is also supported by diffraction measurements. The premelt temperature of 55.6° and the melt temperature of 65.7 * Author to whom correspondence should be addressed. † Current address: ExxonMobil Research and Engineering Co., 1545 Route 22 East, Annandale, NJ 08801. 5139 J. Phys. Chem. B 2001, 105, 5139-5143 10.1021/jp004233o CCC: $20.00 © 2001 American Chemical Society Published on Web 05/09/2001