Crystals derived from mucic acid represent the most rigid organic crystals documented thus far. Their strength is attributed to a tightly woven network of hydrogen bonds among their molecules, which impart mechanical attributes akin to those of metals.
Crystals are frequently perceived as fragile, yet this isn’t universally accurate. Organic crystals, composed of small molecules linked by intermolecular forces, can exhibit mechanical flexibility because these interactions can adjust under pressure. Typically, these relatively weak bonds constrain the rigidity of the crystals. However, there are instances where organic crystals can be remarkably rigid – a potential that inspired Panče Naumov from New York University Abu Dhabi in the United Arab Emirates and his team to investigate the maximum bounds of this characteristic in such substances.
Numerous potential contenders for rigid organic crystals are well recognized and readily available. Although their physical properties have been explored, their mechanical properties have often been neglected. With no dependable computational approaches for predicting rigid molecular crystals, Naumov remarks, “these are exceedingly tough to locate,” adding, “currently, it is impossible to systematically search for them.”
Consequently, the team depended on their chemical insight and expertise to pinpoint a fitting candidate, seeking indicators like high density and the potential for extensive intermolecular interactions. Mucic acid, also referred to as galactaric acid, is a compound long utilized in numerous applications – and it fulfilled both criteria, pairing unusual high density with an abundance of carboxylic acid and hydroxyl groups, facilitating extensive hydrogen bonding.
Stiffness, in basic terms, refers to a material’s resistance to elastic deformation and is evaluated experimentally via Young’s modulus. To ascertain this, researchers frequently employ a technique known as nanoindentation. In a prior investigation, the team discovered that the majority of organic crystals possess a Young’s modulus ranging from 10 to 25GPa, with merely about 8% of the categorized substances exceeding 25GPa and classified as extraordinarily rigid. In their recent investigation, the team measured the Young’s modulus on the (100)/(100) planes of the mucic acid crystal at approximately 50GPa, classifying it as the stiffest organic crystal documented to date. Computational analysis corroborated this result and projected an even greater value of 68.5GPa. Other accessible planes of the crystal exhibited Young’s moduli near 30GPa – still among the highest ever recorded.
The team characterizes these mucic acid crystals as ‘ultrastiff’ due to their unmatched rigidity, which is comparable to certain metals and inorganic materials. For instance, aluminum has a Young’s modulus of around 70GPa.
Additionally, the team assessed the hardness of the crystals, or their capacity to endure pressure from a sharp object. They discovered that mucic acid crystals merge high hardness with extraordinary stiffness. When combined with their high density, these attributes create a mechanically robust yet lightweight organic crystal. Marijana Đaković, a specialist in crystallography and crystal engineering from the University of Zagreb in Croatia, states, “this favorable arrangement of properties is attained within a single crystalline system, while previously known ultrastiff crystals generally excelled in only one or two mechanical parameters.”
The mechanical characteristics of mucic acid crystals stem from both the abundance of intermolecular interactions and the manner in which the molecules assemble to create a dense network of relatively strong hydrogen bonds. Đaković emphasizes that the mucic acid crystals “underscore how strategically organized hydrogen-bonding networks can surpass molecular flexibility and bestow a crystal with remarkable stiffness and mechanical durability.”
Typically, organic crystals are lightweight and biodegradable, advantages not mirrored in their inorganic rivals. Thus, organic crystals may present appealing alternatives for a variety of applications. “The concept outlined paves the way for incorporating organic crystals into technologies that require high mechanical robustness alongside minimal weight,” adds Đaković.