The Normal Paraffins Revisited

Abstract

The low temperature modifications of the normal paraffins n-CnH2n+2crystallize in three groups (Broadhurst, 1962). The structure is triclinic for n even, 6 < n < 26 (Muller and Lonsdale, 1948; Nyburg and Luth, 1972); orthorhombic for n odd, 11 < n < 39 (Smith, 1953; Teare, 1959); and monoclinic for n even, 28 < n < 36 (Shearer and Vand, 1956). In all of these structures the hydrocarbon chains are linear and in trans configuration. The chains are parallel to one another, the terminal methyl groups forming the surfaces of lamella which are more or less perpendicular to the chain axis. For n < ca.36, it is apparently the interlamellar interaction between end methyl groups which dictates the symmetry. For longer chains the structure is usually orthorhombic and comparable to the structure of highly crystalline polyethylenes. Chains do not fold (as they undoubtedly do in polyethylenes) unless n is greater than 102 (Bidd and Whiting, 1985; Ungar and Keller, 1986).The several crystal forms differ in the manner in which the nearest neighbor chains are related to one another. In the triclinic lattices the packing is such that a triclinic sublattice containing one methylene group is evident. In the other modifications the sublattice is orthorhombic and contains four methylene groups. If the overall symmetry is orthorhombic the long chain axes are perpendicular to the interlamellar surface; the x and y translations, perpendicular to the long axis, are common to both cells. If the nearest neighbor chains are displaced by two or four methylene groups along the chain axis, overall monoclinic symmetry results (Sullivan and Weeks (1970)).

Phase diagram ofn-hexacosane andn-octacosane: experimental determination and calculation

“…These observations suggest the existence of a two-phase domain in this temperature range: low temperature β phase, and β(Fmmm) orthorhombic phase. The β(Fmmm) phase has already been observed in oddnumbered pure n-alkanes (1,2,16,17,22,72) and in many n-alkanes systems. (43)(44)(45)(46)(47)(48)(49)51) When the characteristic peaks of the β phase disappear completely at T = 321.5 K, the rotator state RI of the β phase is detected by a small shift in the β phase (020) line.…”

“…According to Clydesdale and Roberts (13) and Barbillon, (42) it is triclinic. For Heyding et al (2) and Barbezat-Debreuil, (15) it is triclinic and monoclinic.…”

“…According to the literature, (2,10) C 28 has a monoclinic structure at room temperature. However, from our own observations, it can also exist in two phases, depending on the thermal history of the sample.…”

“…The evolution of the temperature and enthalpy of this transformation against the number of carbon atoms n are given by the following expressions, that we have determined by fitting the pure odd n-alkanes literature data, (2,74) (19 ≤ n ≤ 33): The heat capacities are extrapolated from the mixture values. They cannot be considered independent of the temperature.…”

Understanding the Polymorphic Behavior of Sibenadet Hydrochloride through Detailed Studies Integrating Structural and Dynamical Assessment

“…Polymorphs I and II of sibenadet HCl show diffraction behavior similar to that shown by other long chain molecules that form layer-type structures, for example n-alkanes, fatty acids, surfactants, and triglycerides. [33][34][35][36][37][38] In contrast, III does not exhibit the regularly spaced reflections below 158 2y. Although the XRPD pattern of this form of the compound exhibits reflections, thus suggesting crystallinity, the reflections are rather broad thus suggesting either a rather strained lattice structure or one with significant degrees of lattice site disorder.…”