Cosmetic touch at the nanoscale: role of covalently-bound lipids on the nanotribology of the hair surface

18-Methyleicosanoic acid (18-MEA) is a branched chained fatty acid, which is covalently-bound, via a thioester linkage, to the outer surfaces of mammalian keratin fibres. The role of this unusual fatty acid is still uncertain, although the side methyl group is thought to disrupt packing of the monolayer thereby producing a more disordered surface of lower surface energy. We report the use of atomic force microscopy (AFM) and lateral force microscopy (LFM) to investigate the adhesion and frictional behaviour of the outer surfaces of human hair fibres that have been subjected to various chemical treatments. The results are discussed in terms of the adhesion and nanotribology of the 18-MEA layer on the cuticular surfaces. Friction coefficient mapping has also been used to investigate the structural heterogeneity of cuticular surfaces. Self-assembled monolayers (SAMs) of 18-MEA and other fatty acids have also been fabricated onto mica surfaces. Good agreement has been found between nanotribology and AFM adhesion studies of these coated surfaces and measurements obtained from hair fibers. In addition, contact angle goniometry studies have shown that 18-MEA SAMs possess a lower surface energy than those from fatty acids analogs with straight chain.