Role of transition metal ions in oxidative hair colouring

The objective of this Ph.D project was to study the role of transition metal ions in oxidative hair colouring. Model systems corresponding to real-life hair colouring conditions were designed to examine copper(II) and iron(III) catalysed decomposition of alkaline hydrogen peroxide and hydroxyl radical formation. In a chelant-free system, copper(II) ions were more active in decomposing alkaline hydrogen peroxide compared to iron(III) ions. For copper(II) ions, the initial rate of decomposition of hydrogen peroxide and hydroxyl radical formation increased with an increase in initial concentration of copper(II) ions. Adding chelants to the reaction solution altered the catalytic activity of metal ions. EDTA and EDDS chelants with iron(III) generated more hydroxyl radical and decomposed higher amounts of hydrogen peroxide than the corresponding complexes of these chelants with copper(II) ions. Most studied chelants supressed catalytic activity of copper(II) ions except HEDP chelant which rapidly decomposed hydrogen peroxide. The results highlight that different metal-chelant systems have different level of catalytic activity in the decomposition of hydrogen peroxide. Adding large excess of calcium ions to the reaction solution influenced the binding of copper(II) ions. Unlike other chelants, only EDDS showed selective binding of copper(II) ions in the presence of calcium and suppressed the decomposition of hydrogen peroxide. Similar results were obtained for copper treated hair fibres, where EDDS again showed strong preference and selectivity for copper(II). This suggests that EDDS is the best chelant to control free radical mediated protein hair damage during oxidative hair colouring. The selectivity of EDDS chelant was explained using speciation plots. Catalytic activity of copper(II) ions was also examined in the presence of aromatic dye precursors. PPD/MAP combination suppressed decomposition of alkaline hydrogen peroxide which suggests that oxidative hair dyeing is likely to induce less protein damage to hair as compared to the bleaching systems. It is believed that some unknown intermediates are formed which chelate copper catalysts changing their chemical activity. Among the chelants examined in the current study, HEDP is an exception as its mixture with copper(II) ions led to rapid decomposition of alkaline hydrogen peroxide and showed a very unusual kinetic profile. A mechanistic study showed that the decomposition reaction proceeds via formation of an active catalyst that degrades the chelant and eventually seeds formation of catalytically-inactive basic copper phosphate/carbonate nanoparticles. The nanoparticles prevent any further catalytic reaction as freshly added Cu2+ ions quickly adsorb on their surface and do not form active catalyst. In a separate study, human hair samples were analysed to quantify the amount of calcium carbonate present in the bubble shaped structures lying over the hair shaft. SEM Images were analysed to estimate the amount of material present while quantitative gas IR analysis showed that the amount of calcium carbonate increased with increasing bubble count level. Calcium carbonate found by IR analysis was less than the values estimated by image analysis which suggested that calcium carbonate may not be the only material present in the bubble and some other unknown material may also be present.