Role of Redox Metals in Color Formation in a Hair Colorant JENNIFER M. MARSH, ROBIE L. LUCAS, CHRIS COWANS, and SHANE WHITAKER , The Procter & Gamble Company Mason, Ohio, 45040 (J.M., R.L., C.C., S.W.) Accepted for publication January 13, 2021. Synopsis The objective of this work was to identify if low levels of redox metals such as copper would accelerate color formation on hair and to understand the consequent impact on initial color formation and color fade. Kinetics of color formation with oxidative dyes in solution in the presence of varying concentrations of copper ions were assessed via imaging and color measurements. Color uptake on hair and color fade were measured with a spectrophotometer, and copper levels in hair were measured with inductively coupled plasma atomic spectroscopy after hair digestion. In this work, the role of redox metal ions such as copper and iron on accelerating rates of oxidative dye formation was demonstrated. Kinetics of dye formation were measured in solution for three dye couples—p-phenylene diamine (PPD) plus resorcinol, PPD plus 5-amino-2-methylphenol (AHT), and 4,5-diamino-1-(2-hydroxyethyl) pyrazole sulfate (HDAP) plus AHT— in a solution that also contained ammonium hydroxide and hydrogen peroxide at pH 10. Low levels of copper were added at a concentration range from 0.01 μg/g to 0.1 μg/g and the rate of color formation measured over 2 h. All th ree dye couples showed signifi cant color acceleration that increased with increasing levels of copper. A mechanism where initial oxidation of primary intermediate PPD or HDAP is accelerated is proposed. This mechanism is demonstrated to become important when trace levels of copper are in hair and a hair colorant added. Color formation is accelerated outside versus inside hair, and ultimately, color uptake is reduced after the colorant is rinsed off hair. Noticeable color fade versus the starting hair color is also increased. T his work provides evidence for the role of copper ions in color formation in hair and strategies to reduce copper levels in hair using a chelant such as histidine in a shampoo or conditioner before coloring. I N TRODUCTION H air colorants constitute a signifi cant category in the cosmetics market—it is estimated that more than 70% of women in the developed world have used hair color, and a large proportion of those do so regularly. Although each woman may have a very individual rea- son for coloring her hair, covering gray can be considered a universal key motivator. Other desired performance aspects include enhancing the existing color, wanting a different color from the on e given to oneself by nature, or achieving a more striking looking appearance. The most widely used products in the colorant category are Permanent Level 3 colorants that consist of a combination of an alkalizer (typically ammonia, but ethanolamine is also Address all correspondence to Jennifer Marsh at marsh.jm@pg.com. J. Cosmet. Sci., 72, 268–278 (May/June 2021) 268

used), hydrogen peroxide, and a range of dye precursors (1). There are tw o chemical pro- cesses that take place during the permanent dying process, both of which contribute to observed fi nal color. First i s oxidation of melanin and previously deposited dyes that lightens the underlying hair color. Second is oxidation of dye precursors that have dif- fused inside hair to form colored chromophores. The rate of color formation is important: if too fast color will form outside hair or in the bottle before application is completed, and if too slow not enough color is formed during a typical development time of 30 min. Penetration of dyes inside hair is important for color longevity as the larger size and re- duced solubility of the dyes formed mean they are trapped inside hair, making them harder to be removed during washing. The basic mechanism of dye formation is an oxida- tive reaction between hydrogen peroxide and a primary intermediate to form a reactive intermediate called a diimine that then reacts with a coupler to form diphenylamine (leuco dye) which then undergoes a rapid oxidation to form a binuclear indo dye. Depend- ing on the coupler structure, there can be subsequent reactions with additional primary intermediates to form trinuclear and larger dyes (2,3). T he role of redox metals in accelerating this chemistry has been studied. A work by Kojma and coworkers showed via isotopically labeled dye precursors, and nano-secondary ion spectrometry that formed chromophores were localized in or close to melanin gran- ules and linked this to the presence of transition metals in melanin. A previously pub- lished work has shown that consumer hair contains redox metals such as copper and iron that are adsorbed during washing in tap water (4) and that these exogenous metals are present in the outer layers of the hair cuticle (5). The objective of this work was to determine whether these exogenous redox metals are infl uencing color formation rates and color uptake. M ATERIALS AND METHODS H AIR SOURCE C hemically virgin natural white Caucasian-source hair was purchased from International Hair Importers & Pr oducts Inc. (Gl endale, NY). Individual tresses (2 gm, 6in with hot wax tab at the top), formed by evenly blending hair from multiple ponytails, were used for all experiments. Colored hair was created by treating virgin hair tresses once with an oxidative permanent commercial colorant (Nice ʹN Easy Extra Light Blonde Shade, Clairol, Stamford, CT). The mixed colorant was thoroughly massaged onto hair at a dose of 4 g of product per g of hair and then incubated for 30 min in an oven at 30°C. The product was then completely rinsed from hair. Initial copper levels in hair were 20 μg/g. DETE RMINATION OF METAL CONTENT IN HAIR The metal content of hair samples was determined by inductively coupled plasma atomic spectroscopy (ICP-OES) with an Optima 5300 DV Optical Emission Spectrometer (Perkin Elmer Life and Analytical Sciences, Shelt on, CT). Samples of 100 mg of hair were di- gested overnight with 2 mL of high-purity concentrated nitric acid. The digestive mix- ture also contained 150 μL of 100 μg/g yttrium internal standard (Inorganic Ventures, REDOX METALS IN COLOR FORMATION IN A HAIR COLORANT 269