The Role of Monoethanolamine in Hair Bleaching and Dyeing: Mechanistic Insights from Model Formulations

Primarily, the focus of this project was to investigate hair bleaching and dyeing mechanisms, in the presence of ammonia or ethanolamine (MEA), at room temperature. Firstly, the mechanism of hair bleaching by alkaline hydrogen peroxide was explored, using homogeneous solutions of Sepia melanin free acid (MFA) as a model for hair melanin. UV-vis spectroscopy was applied to study the rate of melanin bleaching under various conditions. It was established that both hydroxyl radicals and perhydroxyl anions are involved in the bleaching of melanin.
Hydrogen peroxide decomposition and Sepia melanin oxidation were then monitored using homogenous model bleaching solutions, to see if differences in hair bleaching when MEA is used instead of ammonia could be explained by a change in chemistry. Dissimilarities were found in ligand-free and etidronic acid (HEDP) systems when the base was altered, due to the presence of differing metal complexes. However, when strong chelating ligands such as ethylenediaminetetraacetic acid (EDTA) are used, no differences were apparent in homogenous model bleaching systems.
The mechanism of dye formation inside hair fibres was then investigated, due to the observation that catalase accelerates the oxidation of dye primaries in aqueous solutions. Dye formation was studied by UV-vis spectroscopy. It was shown that metal ion centres are predominantly responsible for the formation of dyes in the hair cortex. Fe(III) proved to be a more effective catalyst for dye production than Cu(II).
Finally, the effect of MEA on the rate of hair dye formation in aqueous systems was studied, using HPLC and UV-vis spectroscopy. The rate of colour formation in MEA based formulations was found to be greater than in ammonia systems, possibly due to slower degradation of the dyes in MEA systems. It was also found that nucleophilic attack of MEA on preformed dye molecules leads to the formation of different dyes, which incorporate the base into their structure. The formation of these dyes greatly changes the colour of model aqueous dye solutions.