Crystal structure of n-paraffin solid solutions - an electron diffraction study

Abstract

Single-crystal electron diffraction data from epitaxially crystallized n-paraffins are used to characterize the structures of their solid solutions. The even-even binary chain system n-C32H66/n-C36H74 is shown to be continuous, preserving the orthorhombic polymorphic packing at all compositions. Intensity data (0kl) from a ca. 1:1 solid solution are used to verify the validity of a packing model similar to one proposed by Asbach et al. The continuity of the n-C33H6g/n-C36H74 system is also demonstrated, contrary to predictions that these paraffins cannot form stable solid solutions because of dissimilar symmetry. It is shown, however, that epitaxial crystallization of the odd-chain paraffin produces a higher energy twinned monoclinic polymorphic form, which can more readily combine with the orthorhombic even-chain paraffin than can the lowest energy polymorph.

Direct structure analysis of a paraffin solid solution.

“…2 that the electrostatic potential distribution of a 1:1 solid solution of n-C32H66/n-C36H74 resembles the structure expected for n-C34H70 (i.e., the intermediate alkane structure). The lamellar thickness limit, therefore, is not determined by the length of the longest paraffin, as suggested in earlier model studies (1,2). From the distribution of chain carbon occupancies, it is apparent that some random translational displacement of the shorter chain component must take place in order to minimize the volume occupied by voids in a lamella, as stated previously by Asbach et al (10).…”

“…A model with fractional chain end atomic occupancies was also found to fit the observed data reasonably well (2). Although the indices of major Okl reflections corresponded to the crystal structure of n-C34H70, the continuous scattering from a model based on the unit cell of n-C36H74 was sampled at reciprocal lattice positions corresponding to the C34 alkane, thus incorporating some features of the longer alkane transform into the structural model.…”

“…Although, historically, numerous studies have been made of these binary combinations in an effort to understand the structural rules that determine the stable cosolubility of such molecules in the solid state [e.g., as discussed in the 1960 review by Mnyukh (1)], the recent availability of epitaxially oriented microcrystalline preparations has enabled these determinations to be made routinely in terms of a single-crystal structure (2)(3)(4)(5)(6). The molecular volume differences tolerated as a stable solid solution have been shown experimentally (6) to be dependent upon the mean molecular chain length, with a relationship similar to the prediction of Matheson and Smith (7).…”

“…My first attempt to determine the structure of a solid solution quantitatively with single-crystal diffraction data was carried out with electron diffraction intensities from a 1:1 solution of n-dotriacontane/n-hexatriacontane (2). A model with fractional chain end atomic occupancies was also found to fit the observed data reasonably well (2).…”

Modelling Phase Equilibria in Systems with Organic Solid Solutions

“…Reports on the formation of alloys, or solid solutions, of organic compounds, although still rare, have been appearing regularly in the last few years for aromatic compounds, such as substituted benzenes and naphtalenes [1][2][3], pyrene and anthracene [4][5][6], for carbohydrates [7], terpenes [8], neopentane derivatives [9][10] and fulerenes, besides solid solutions of C 60 with C 70 [11][12] solid solutions of C 60 in sulphur have been reported [13]. Nevertheless it is among the molecules with long n-alkyl chains such as n-alcohols [14], fats [15], soaps [16] and alkanes [17][18][19][20] that solid solutions appear more frequently. Although most of these alloys are still just interesting academic curiosities, these new materials seem quite promising and interesting applications for some of them have been identified.…”