The crystal structure of waxes

Abstract

Quantitative electron crystallographic studies have been carried out on epitaxially oriented multi-component waxes. Intensities from two paraffin-based samples, an artificial six-component medium wax (equimolar distribution of chain lengths) and a petroleum-based wax (Gaussian distribution of chain lengths) have been used to determine their crystal structures. As found earlier for binary paraffin solid solutions, differences in molecular volume are compensated by longitudinal molecular shifts within individual lamellae. Nevertheless, each lamellar surface must remain fiat enough, and with enough crystallographic order intact, to nucleate the next lamella, thus accounting for the observed long-range correlation in these crystals. Recrystallized beeswax also has a layer packing somewhat similar to the paraffin waxes. However, in this case, the lamellar order is 'frustrated' so that a certain amount of 'nematically' ordered material must be present, spanning the nascent lamellar interfaces. IntroductionPolydisperse combinations of linear molecules are often found in biologically and commercially significant materials, ranging from the lipids to synthetic polymers. Multicomponent paraffin solid solutions, themselves, have been significant for the study of petroleum wax crystallization, as a model for insect and plant cuticle waxes and as a model for polydisperse polyethylene lamellae.Crystallographically, the description of such polydisperse linear-chain combinations is not well advanced. Progress has been made recently in the study of binary n-paraffin solids in the form of single crystals. Low-angle X-ray diffraction measurements had often been made on such samples (Mnyukh, 1960;Fischer, 1971;Asbach, Geiger & Wilke, 1979;Craievich, Doucet & Denicolo, 1984) and, more recently, on polydisperse solids (Asbach & Kilian, 1991;Basson & Reynhardt, 1992). However, the first single-crystal characterization of an n-paraffin binary solid solution was reported only two decades ago (L0th, Nyburg, Robinson & Scott, 1974). In this qualitative description, it was proposed that the copacking of dissimilar chain lengths was compensated by longitudinal molecular disorder in individual lamellae. ©1995 International Union of Crystallography Printed in Great Britain -all rights reserved Vibrational spectroscopic (Maroncelli, Strauss & Snyder, 1985; Kim, Strauss & Snyder, 1989) as well as NMR measurements (Basson & Reynhardt, 1991) established that conformational disorder near the chain ends was also important for minimizing the average nonoverlapping volume. Recent quantitative crystal-structure analyses of such solid solutions, based on electron (Dorset, 1990a) or X-ray diffraction (Gerson & Nyburg, 1994) intensities, support the longitudinal disorder model.In an earlier electron diffraction study of binary paraffin combinations (Dorset, 1987), it was found that similar patterns could also be obtained from an oriented commercial petroleum wax. In this paper, the study of paraffin-based multicomponent waxes is extended to f...

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