Model surfaces for human hair: screening perfumes and surfactants

The thesis describes the development of surfaces that model the surface of human hair in order to characterize the adsorption of detergents and perfumes. Three types of model surfaces were investigated consisting of (i) a self-assembled monolayer of proteins extracted from human hair, (ii) fatty acid monolayers and (iii) a composite fatty acid-protein layer where the fatty acid is covalently attached on the underlying protein layer. Attenuated total reflection infrared spectroscopy was used to study adsorption and desorption kinetics of surfactants and perfumes (molecules present in daily hair care formulations) to the different model surfaces on solid supports. Two cationic surfactants, dimethyl dioctadecylammonium chloride (DODAC) and trimethyl octadecylammonium bromide (TODAB), and perfumes with different hydrophobicities were tested. The obtained results helped to identify several parameters that were crucial for the interaction of surfactants and perfumes with the model surfaces. For the model surface comprising only the fatty acids, we found that the fatty acid coverage on the sensor surface influenced the amounts of deposited surfactant (lower coverage higher deposition). In this context, the electrical charge density of the underlying supporting surface (Ge, silanes) played an important role. For the protein containing model surfaces, the adsorption/desorption kinetics of surfactants depended on the chemical properties of the surfactant structure. The TODAB/OD surfactant mixture showed a lower affinity towards the surface than DODAC. In addition, the adsorption/desorption kinetics could not be described with a simple Langmuir mechanism, most likely due to reorganisation processes of the primary adsorbed vesicles, which could be transformed into planar supported bilayers. The affinity of perfumes to the protein surface was influenced by the perfume hydrophobicity. The developed sensor technology allows to monitor on-line, quantitatively and simultaneously the adsorption of different compounds (detergents and perfumes) on different supported layers with yet unreached signal to noise ratio and compared resolution. In addition, the ATR-FTIR spectra deliver valuable structural information (conformational and orientational) of the adsorbed molecules. The developed technology opens the possibility to screen compounds of interest for the cosmetic industry. Beyond these potential of industrial R&D; applications, the reported approach will be of general interest for investigating molecular interactions on designed surfaces ranging from material science to biochemical and biophysical research. A comparison of three fatty acids (18-MEA, 19-MEA and EA) was carried out in order to understand why nature has chosen a branched fatty acid as major component for the hair surface. It was found that the methyl group in the anteiso position (18-MEA) induces the largest molecular area in a monolayer at the air-water interface. In turn, less molecules are needed to obtain a monomolecular layer with similar surfaces properties such as hydrophobicity than its straight chain homologue. The compressibility of the 18-MEA monolayer at the air-water interface was higher than for the 19-MEA monolayer. A supported monolayer of higher compressibility can compensate the loss of fatty acids more easily while maintaining its hydrophobic surface properties than a supported monolayer of lower compressibility. This might explain, why nature has preferred 18- MEA to 19-MEA to coat hair's surface.


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