Nanoscale Structures of Alkane and Alcohol Wax Thin Films Crystallised From a Melt

Session

Poster Session Two

Authors

Miss Emily Wynne (3), Dr Neil George (2), Dr Adam Keates (2), Dr Rachael Shinebaum (1), Dr Helen Blade (1), Professor Andy Brown (3), Dr Sean Collins (3)

Affiliations

1. Oral Product Development, Pharmaceutical Technology & Development, Operations, AstraZeneca
2. Syngenta
3. University of Leeds

Keywords

4D-STEM, SED, crystallography, structure determination, paraffins

Abstract text

Nanoscale structure determines key properties in many multi-component systems, including organic layered products. Plant leaves are central to crops and to crop protection for global food security.  They have wax layers that control vital processes such as transpiration. These waxes contain complex crystalline nanoscale structures modulating the movement of water and other molecules across a leaf, yet the precise mechanisms of transpiration control are still largely unknown. The intracuticular wax (IW) structure is believed to resemble a “bricks and mortar” model, with highly ordered crystalline domains interspersed in an amorphous matrix assumed to be a fast diffusing path [1] [2]. The wax layer consists mainly of very long chain aliphatics with functional group endings thought to form the disorder within the amorphous zones. Simplified, replica leaf waxes based on the composition of the Schefflera elegantissima plant can be formed from long chain hydrocarbons [3]. Previous work carried out on similar paraffin systems has used selected area electron diffraction (SAED) with limited spatial resolution to assess unit cell structure and global disorder across two different chain orientations [4]. 

We present low-dose scanning electron diffraction (SED) work on chain orientation control of carbon alkyl chains in two single-phase wax end members (C₃₁H₆₄ and C₃₀H₆₁OH) and binary mixtures prepared by two different TEM grid preparation methods. Drop-casting sample preparation confirms the [001] orientation of carbon chains, with an epitaxial orientation method confirming the [100] orientation for a Pbcm unit cell. SED at sufficiently low dose reveals crystalline domain structure and tilt between crystals by Bragg and diffuse scattering from a C₃₁H₆₄ paraffin wax. SED offers a spatial resolution of ~5 nm [5], and can be applied at doses of < 5 e^-Å^-2 to ensure compatibility with our independently measured critical fluences of 6-7 e^-Å^-2 for these wax materials. SED analysis overcomes limited spatial resolution in X-ray diffraction and photon spectroscopies to develop a simple nanoscale structure model of the crystallographic packing of the two endmember phases. Breakdown in domain structure in binary mixtures across a range of compositions of increasing alcohol content (C₃₀H₆₁OH) are identified. Further work will involve exposing these structures to small organic molecules used in agrochemical products to determine any changes in structure. Developing a mechanism for movement of these molecules across this wax barrier will then aid in the design of crop protection formulations. 

References

[1] Riederer, M. Annual Plant Rev, Volume 23, Biology of the Plant Cuticle. Wiley, 2007.

[2] V. V. Zeisler-Diehl, W. Barthlott and L. Schreiber, in Hydrocarbons, Oils and Lipids: Diversity, Origin, Chemistry and Fate, Springer International Publishing, 2018, pp. 1–16

[3] I. Basson and E. C. Reynhardt, Chem. Phys. Lett., 198, (1992) 367–372.

[4] Dorset et al., Proc. Natl. Acad. Sci. USA, 81, (1984), 1913-1917. 

[5] D.N. Johnstone et al., J. Am. Chem. Soc., 142, (2020), 13081–13089.