.....revealing that process of crystal growth is more active than was previously thought. Although crystal growth is usually seen as a grouping of individual atoms, molecules or small clusters adding to a larger block that remains in a fixed translational relationship to the rest, a team of scientists from New York University and St. Petersburg State University in Russia have developed a crystal that continually changes its shape as it grows.
The team, whose research is published in the Journal of the American Chemical Society [Shtukenberg, et al. J. Am. Chem. Soc. (2010) DOI: 10.1021/ja101491n], examined crystals from hippuric acid, discovering that when molecules are added to the end of fine crystalline needles, stresses built up at the tips of the crystals, resulting in a helical twist, similar to the double helix in DNA. However, the process was reversed when crystals thickened from the other end of the growing tip, which undid the twisted formations, due to the elasticity of the crystals decreasing as they become thicker.
Bart Kahr, a co-author of the study, said “This competition between twisting and untwisting creates needles with a rainbow of colors, which is a characteristic of tightly wound helices, as well as ribbons that have become completely untwisted. This is a very strange and new perspective on crystal growth.”
With hippuric acid, the twisting is due to the presence of a decomposition product, which causes both mismatches and strain in lattices. If there is a lack of symmetry of the systems, twists can occur, an internal stress mechanism that could easily apply to many other systems.
The concept of helical crystals that twist like a drill bit originated in a book by Ferdinand Bernauer, "Gedrillte" Krystalle, published in the 1920s. Although forgotten for decades, the book was revived when helical crystals became topical again in the 1950s with the emergence of the synthetic polymer industry and the controversy over how and why high-polymers twist. The team investigated the book and the history of crystal growth in order to understand the crystallization process.
It is hoped that this study will bring a greater understanding of high-polymers, which are used in a range of consumer products, such as clothing and liquid crystal displays. It would be useful to be able to prevent crystallization in polymers, and to understand the twisting process, or even stop deleterious crystallization processes.
The chemists now hope to find further evidence that these mechanisms are brought about by internal stresses, and intend to examine mixed crystal structures and the form of the pointed needle tips as they grow.