Abstract
The biogenic substance E-indigo can form supramolecular, hydrophobic structures using self-organization. These structures show a low coefficient of friction as a gliding layer against polar surfaces. The formation of primary particles with platelet morphology based on hydrogen-bonded E-indigo molecules is ideal to produce the gliding layer. Structures with excellent gliding properties can be achieved by means of directed friction and high pressure, as well as through tempering. The resulting hard, thin gliding layer of E-indigo does not easily absorb dirt and, thus, prevents a rapid increase in friction. Field tests on snow, with cross-country skis, have shown promising results in comparison to fluorinated and non-fluorinated waxes. Based on quantitative structure–activity relationship (QSAR) data for E-indigo, and its isomers and tautomers, it has been demonstrated that both the application and abrasion of the thin indigo layers are harmless to health, and are ecologically benign and, therefore, sustainable.
Highlights
Archaeological research identified indigo-dyed textiles dating back to at least 7800 years ago [1]
Indigo is well known as the chromophore of the pre-Columbian pigment Maya Blue [2]
The unusual stability of Maya Blue has inspired the use of indigo as a chromophore in other host–guest-based colorants [3]
Summary
Archaeological research identified indigo-dyed textiles dating back to at least 7800 years ago [1]. For the application of indigo as a gliding layer, a high content of E-indigo is necessary For this purpose, the usual separation methods of the isomers, i.e., by sublimation or via reduction and reoxidation, are insufficient. E-indigo is stable as a supramolecular cluster with intermolecular hydrogen bonds Solid lubricants, such as graphite or molybdenum disulfide, reduce friction due to the sheets being able to slide on each other. The formation of indigo nanoplatelets (Figure 4) is ideal to produce gliding layers The toxicology of these nanoparticles is insignificant, because once released, the hydrogen bonds are cleaved, and enzymatic degradation starts from the polar end groups [59]. Experiments with the runners of a luge have shown that the bonding of an indigo layer to a surface of iron or steel, together with its hydrophobicity, is sufficient to protect against corrosion
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