Crystallography of the Polymethylene Chain: An Inquiry into by Douglas L. Dorset

By Douglas L. Dorset

By way of contemplating the forged nation packing of linear chain wax elements, this ebook goals at knowing 3 issues: to start with, which differences of molecular elements are allowed for keeping good good recommendations; secondly, what occurs whilst balance stipulations are traversed and fractionation starts and thirdly, the constitution of fractionated arrays. The co-compatibility of molecular components is taken into account when it comes to their shapes and relative sizes, following an strategy initially proposed by means of Kitaigorodskii. As tested profusely through the crystal buildings of natural part forms (e. g. alkanes, fatty alcohols, fatty acids, lengthy chain esters, cholesteryl esters) and insertion of useful teams (e. g. chain branches, unsaturation, heteroatoms), attribute molecular packing arrays supply vital geometric info for realizing the co-packing of other molecules in combinations. unmarried crystal and spectroscopic info from polydisperse arrays can then be evaluated to reach at believable constructions of stable options and fractionated platforms. the consequent buildings usually are not in simple terms suitable to the knowledge of so-called waxes but in addition contain sure periods of polymers. The ramifications of this paintings expand into any strong kingdom array of polymethylene chains, together with lipid foodstuffs.

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Additional resources for Crystallography of the Polymethylene Chain: An Inquiry into the Structure of Waxes (International Union of Crystallography Monographs on Crystallography)

Sample text

The energetic possibilities for the occurrence of such polytypes or non-180Њ rotational twins across the (001) planes were considered by Boistelle and Aquilano (1977) and compared to experimental observations of crystal growth. 5 Orthorhombic crystal structure of the even-chain n-paraffins The X-ray crystal of n-C36H74 in its orthorhombic form was determined by Teare (1959). e. two layers). 5Њ acute angle (Dawson and Vand, 1951; Anderson and Dawson, 1953). From systematic absences in the diffraction pattern, two possible space groups could be chosen: Pca21 or Pcam.

The structural basis for polymorphism associated with thermotropic transitions is also given. Crystal growth is thus justified from preferred assembly of chains in oblique and rectangular layers at various temperatures and for various chain lengths. The thermodynamically stable crystal structures of n-paraffins, n-CmH2m ϩ 2, can be reviewed as follows (Asbach and Kilian, 1970): For the odd-chain paraffins in the range 11 Յ m Յ 43, the chains pack in an orthorhombic unit cell. Following principles of organic chemical crystallography (Kitaigorodskii, 1961), the perpendicular mirror plane in the molecular point group is utilized by the space group symmetry.

While it is true that the orthorhombic polymorph appears to be expressed preferentially for the longer chain even n-paraffins—for example, single layers from n-C82H166 (Dorset, 1986), there is a subtle change in crystal morphology, most noticeable for chain lengths near n-C50H102 and larger while the chains are still extended in the lamellae. As noted in bright-field electron micrographs of lamellae nucleated from hot solvent (Fig. 12(a)), the crystals are sectorized so that striations occur along Ͻ130Ͼ in {110} sectors.

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