Millimolar levels of water accidentally taken in from the atmosphere into a solvent were sufficient to alter the thermodynamic equilibrium between two polymorphs of a supramolecular monolayer, new findings reveal. By tracing seemingly spontaneous structural discrepancies back to the background of a solvent, the study serves as a reminder that even minor impurities can have significant impacts and demonstrates how solvent management can lead to issues in reproducibility.
Crystal polymorphism refers to the phenomenon where a compound can crystallize in multiple configurations. Researchers are eager to enhance their understanding of this structural occurrence: beyond scholarly interest, it bears important ramifications for drug safety and effectiveness.
The factors that dictate polymorphism in self-assembled interfacial monolayers remain only partially elucidated. Although solvent, solute concentration, temperature, and surface characteristics are recognized as influencers of which monolayer polymorph emerges, researchers sometimes face spontaneous polymorphism, where an unforeseen structure appears without any clear reason. Markus Lackinger and his team at the University of Munich, Germany, encountered this exact predicament while examining monolayers of trimesic acid (TMA) on graphite.
In previous studies, the group had demonstrated that the configuration taken by a single layer of TMA on graphite is reliant on the solvent utilized: short-chain fatty acids promote a dense ‘flower’ formation, while longer-chain solvents lead to a more open ‘chickenwire’ structure. Nonetheless, when they subsequently tried to conduct similar experiments, the system exhibited significantly different behavior.
“We’ve experienced occasional instances where we could not replicate our own results, [but] these are typically resolved by starting anew,” notes Lackinger. “We aimed to explore polymorphism on other graphitic surfaces like graphene and intercalated graphite and obtained a new batch of solvent [heptanoic acid], but we only observed the flower structure; this was concerning because we wanted to conduct experiments with the chickenwire structure. So, we acquired yet another bottle – the same issue persisted!”
In search of an explanation, the team employed scanning tunneling microscopy along with UV–vis absorption spectroscopy and gas chromatography–mass spectrometry to analyze how the concentration of water in heptanoic acid influenced monolayer polymorphism on graphite. They found that the ratio of the flower polymorph increased as water concentration rose and, paired with molecular dynamics simulations, concluded that the water impurities interfere with the hydrogen bonding in the trimesic acid monolayers. This interference was more significant for the chickenwire polymorph due to its lower packing density, leading to the flower polymorph prevailing at elevated water concentrations.
It became evident that the solvent was inadvertently absorbing minute quantities of water from the air, indicating that bottles from the same supplier could yield varying monolayer structures based on the batch or the duration for which they had been opened.
“Water as a trace impurity is always a prime suspect when reproducibility issues crop up in chemistry,” remarks Fernando Pablo Cometto, a nanoscience and surface chemistry researcher at the National University of Córdoba in Argentina. “What is genuinely astonishing, and notable, is the level of sensitivity involved. The fact that concentrations in the range of tens of millimoles per liter – hardly a trace of water in an organic solvent – can entirely shift the manifested polymorph is remarkable.”
“This research serves as a timely reminder that handling solvents requires the same level of scrutiny we usually apply to our reagents,” warns Cometto. “Moving forward, it is prudent to store solvents more meticulously, monitor how long bottles have been opened, and – especially for experiments sensitive to concentration – transition away from the convenience of saturated solutions toward accurately measured concentrations. These adjustments are not drastic, but they are the kind of disciplined practices that distinguish reproducible science from perplexing irreproducibility.”
Lackinger and colleagues now plan to explore how water influences the thermodynamic stability of monolayer polymorphs. “This is a somewhat unique system because the two polymorphs are very similar in terms of thermodynamic stability. It would be intriguing to see if we can detect the impact of water even if the same polymorph is obtained,” concludes Lackinger.