Distribution and Localization of Hydrophobic and Ionic Chemical Groups at the Surface of Bleached Human Hair Fibers

A chemical mapping with high lateral resolution using an atomic force microscope in the pulsed force mode with chemically modified tips, introduced as “dynamic chemical force microscopy” (dCFM), was carried out to investigate the chemical properties of the cuticle of human hair and its changes following an oxidative treatment. Chemically modified atomic force microscopy (AFM) tips, CH3- and NH2-terminated, were applied to achieve a defined chemical contrast (hydrophobic and ionic) in aqueous medium. A comparative Fourier transform infrared spectroscopy–attenuated total reflection identified the dominant chemical groups of the surface vicinity of the hair fiber resulting from the bleaching process. The combined experimental results lead to the conclusion that the hydrophobic top layer is partially removed after bleaching, resulting mostly in hydrophilic SO3 – end groups at the top of the surface of the hair, with a mean surface density “δmean” of negatively charged groups of approximately 2.2 molecules/nm2, corresponding to ∼600 μg/m2 cysteic acid. This indicates that thioester bonds are disrupted and fatty acids are removed as a result of cysteine oxidation. At the molecular level, our results indicate a clustered “self-assembled monolayer” alignment of cysteic acid with a crystal-like structuring, reminiscent of the “fluid mosaic model of cell membranes”, with a surface energy of approximately 0.04 N/m. Despite previous extensive works of AFM on human hair, this is, to our knowledge, the first time that the hydrophobic and ionic sites at the top surface of hair have been imaged at the nanoscale with dCFM.