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Catalysis and chemistry

‘Frustrated’ water molecules drive supramolecular self-assembly

04 Jun 2018 Belle Dumé
Bert Meijer and Nathan van Zee
Bert Meijer and postdoc Nathan van Zee. Courtesy: Bart van Overbeeke

Although oils only contain trace levels of water molecules, they can direct supramolecular processes by forming new hydrogen bonds. Since many chemical processes, both industrial and non-industrial, take place in oil, this unexpected new result means that much previous research will have to be reevaluated to take into account the effect of water. The findings will also be important for designing supramolecular materials in a host of applications, including electronics and catalysis.

Oils such as alkanes contain less than 0.01% dissolved water, so its chemical role in these compounds has often been overlooked. “We have now found that this water can direct structural changes in one-dimensional supramolecular structures,” explains Bert Meijer of the Eindhoven University of Technology, who led this study. “Since supramolecular assembly is in many cases performed in hydrocarbon solvents, it is important to take this water into account.”

Thanks to spectroscopy, calorimetry and theoretical modelling, Meijer and colleagues found that water molecules form new hydrogen bonds with supramolecular structures in oils. “Water molecules are polar – that is, one side of the molecule is negatively charged and the other positively charged,” explains study first author Nathan Van Zee. “They bond via hydrogen bonding but since oil is hydrophobic it repels water. This repulsion means there is little space for the water molecules to bond with other water molecules, which leaves them isolated.

Thermodynamic driving force

“These ‘frustrated’ water molecules thus possess potential enthalpic energy in the form of unrealized hydrogen bonds,” says Van Zee. “And this energy in fact becomes a thermodynamic driving force for the molecules to hydrogen-bond with the supramolecular structures in the oils instead.”

The researchers unexpectedly obtained their result when studying the self-assembly of biphenyl tetracarboxamide. In the bulk, this compound forms helical liquid structure aggregates. It also forms 1D aggregates when diluted in solvents such as methylcyclohexane (MCH). At micromolar concentrations in MCH, however, the helical aggregates sometimes have a clockwise structure and sometimes an anticlockwise structure.

The underlying cause of this puzzling variation in chirality turned out to be directly linked to the amount of water present in the sample – even though it is only a few parts per million.

 “Non-covalent synthesis”

“We suspect that many previous reports of unexplained phenomena in oils – be they changes in structure, size or processing – are fundamentally related to interactions with water,” says Meijer. “We hope that researchers will now consider reexamining their previously (unexplained) results in light of our new findings,” he tells Physics World.

“Importantly we believe that our results take the topic of self-assembly and self-organization onto a whole new level. Indeed, we think this is the beginning of ‘non-covalent synthesis’, a completely new field in supramolecular chemistry.”

Songhi Han of the University of California Santa Barbara, who was not involved in this work, agrees: “This study illustrates ‘the wonder of water’ in mediating self-assembly, as it can metamorphose into a reactant (as shown here), or into a lubricating layer or a solvent (as revealed in other studies), and everything in between.”

The Eindhoven researchers say that they will now be looking to exploit water molecule interactions in oils and create new stimuli-responsive gels. “These interactions might also be used to modulate the structure of materials like 1D supramolecular polymers. These polymers can be designed to have a somewhat flexible hydrogen bonding network that holds them together, which makes them highly sensitive to water binding and consequential structure changes. Water is important in dictating the structure of rigid 1D supramolecular fibres too, since it appears to play a role in controlling the lateral interactions between supramolecular fibres.”

Full details of the research are detailed in Nature 10.1038/s41586-018-0169-0.

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