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Chirality-dependent friction of bulk molecular solids
1School of Engineering and Applied Sciences and ‡Departments of Chemistry and Chemical Biology and of Physics, Harvard University , Cambridge, Massachusetts 02138, United States.
Langmuir : the ACS Journal of Surfaces and Colloids
|August 8, 2014
Summary
Solid-solid friction depends on material chirality. Friction differences between same-chirality (homochiral) and different-chirality (heterochiral) solids vary with temperature and material, offering insights into molecular interactions.
Area of Science:
- Materials Science
- Physical Chemistry
- Tribology
Background:
- Chirality, a property of non-superimposable mirror images, is common in molecular solids.
- Friction in molecular solids is complex, influenced by various intermolecular forces and dissipation mechanisms.
- The role of chirality in the macroscopic property of friction remains underexplored.
Purpose of the Study:
- To investigate the influence of relative chirality on solid-solid friction in molecular materials.
- To quantify chiral asymmetries in friction coefficients.
- To correlate friction behavior with material properties like melting point and racemic form.
Main Methods:
- Experimental measurement of solid-solid friction coefficients for bulk chiral molecular solids.
- Systematic variation of temperature to observe friction changes.
- Analysis of friction differences between homochiral and heterochiral interfaces.
- Comparison of friction data with melting point data and racemic behavior (compound vs. conglomerate).
Main Results:
- Solid-solid friction is dependent on the relative chirality of interacting surfaces.
- Heterochiral friction can be lower or higher than homochiral friction, depending on the specific compound (e.g., menthol, 1-phenyl-1-butanol, ibuprofen).
- Chiral friction asymmetries are temperature-dependent and can reach up to 30%.
Conclusions:
- The sign of the friction difference between heterochiral and homochiral interactions correlates with the melting point difference between racemate and pure enantiomer.
- A stable interfacial racemic compound is not necessary for observing chirality-dependent friction.
- Chirality-dependent friction measurements offer a novel method to probe short-range intermolecular forces in molecular solids.
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