Will It Be a Solid or a Liquid? The Molecular Structure Has the Answer
Oppositely charged polymer chains can be “right-handed,” “left-handed,” or have no “handedness” at all, which controls whether a solid or liquid forms.
The Science
“Smart” coatings could protect chemical reactors from corrosion and help build safer batteries, but creating these coatings means controlling how the films assemble their building blocks. These blocks are charged polyelectrolytes, or molecular chains of hydrogen, carbon, oxygen, and nitrogen atoms plus various molecular groups. The scientists found that these building blocks form solid complexes when all the chains have the same “handedness” (that is, chirality). However, a liquid is formed when there is an equal mixture of right-handed and left-handed chains.
The Impact
Controlling how polyelectrolyte complexes form could lead to better smart coatings. These coatings could result in enhanced corrosion resistance for metals and new solid-state electrolytes for batteries and other energy storage applications. Understanding how these complexes form permits scientists to predict and control materials’ properties.
Summary
Polyelectrolyte complexes offer new opportunities for studying and guiding the assembly of easily deformed soft matter, including certain biological materials. The liquid forms of these complexes have useful characteristics such as very low and tunable interfacial tension with water, which makes them easy to spread on solid surfaces. However, the factors determining whether the complexes form as solids or liquids remain unclear. Oppositely charged polyelectrolyte chains were used to examine the effects of “handedness,” or chirality, on complex formation. For dilute solutions of oppositely charged chains, it was discovered that the chirality determines whether a liquid or solid complex is formed from the interactions of the molecules. The explanation for this finding is based on the combination of electrostatic and hydrogen-bonding interactions. Fluid complexes form when at least one of the chains is a mixture of both right- and left-“handedness,” which disrupts hydrogen bonding networks. In contrast, chains that have only one “handedness” form compact, fibrillar solids with a beta-sheet structure. This secondary structure was clearly revealed in detailed molecular dynamics simulations. “Handedness” was found to be an exploitable tool for manipulating material properties in polyelectrolyte complexation. With this new discovery, self-assembled polyelectrolyte materials such as smart coatings may be improved in the future.
Contact
Juan J. de Pablo
Institute for Molecular Engineering, University of Chicago
Argonne National Laboratory
[email protected]
Funding
This work was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences.
Publications
S. Perry, L. Leon, K. Hoffmann, M. Kade, D. Priftis, K. Black, D. Wong, R. Klein, C. Pierce III, K. Margossian, J. Whitmer, J. Qin, J. Pablo, and M. Tirrell, “Chirality-selected phase behaviour in ionic polypeptide complexes.” Nature Communications 6, 6052 (2015). [DOI: 10.1038/ncomms7052]
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