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Aqua-Mane-ia: The Science and Art of Water on Your Hair

Part 4. The Impact of Copper






Technical Content Creator


Summer: the sun, the sea, the pool, the daquiris….and the green hair….an affliction that usually affects blondes and often occurs from swimming in a pool. The technical term for green hair is chlorotrichosis, and, contrary to urban legend, it is the interaction between hair and copper salts that cause the green color rather than between hair and chlorine. However, copper can do more than just turn hair green. This article will consider copper and its interaction with hair, why chlorotrichosis occurs, if damaged hair is more at risk from turning green, and if there is anything that can be done to avoid it or treat it. As with the other topics discussed in this mini-series, the condition of the hair when it is exposed to the copper is key. Damaged hair (e.g. through bleaching) can lead to an increase in the likelihood of metal coordination.



Where does the copper come from?


Worldwide, copper is present in hair at levels of between 10 and 200 μg/g with minimal visible impact upon the hair fibers. However, copper can play a role in hair damage, especially when hair is exposed to UV light. The reason that most people notice hair turning green after swimming in a pool is that copper sulfate (CuSO4) is sometimes added to the water as an algaecide, which leads to raised copper levels in the water that leads to increased copper coordination to hair fibers and therefore a color-change. However, it’s not only in the pool that there is a risk. Coordination of copper salts to hair can also occur if a house has freshly installed copper pipes because freshly installed pipes do not have a layer of limescale build-up on the surface, which has a protective effect and inhibits leaching of copper(II) ions. In addition, water with a low pH (i.e. is acidic) can facilitate leaching of copper into the water supply that can subsequently coordinate to hair fibers.


Copper accelerates UV damage


In 2014, workers at Procter and Gamble found that there was an increase in levels of a protein degradation biomarker upon exposure of hair to both copper and UV light rather than just UV light, then in 2015 they further elaborated on this work. They suggested that under UV light reactive oxygen species (ROS) are formed, which can both damage the protein framework leading to degradation and oxidize any lipids present. The presence of copper, a redox-active metal, further enhanced this effect by promoting formation of highly reactive hydroxyl radicals that also oxidized proteins or lipids in the hair, and then rapidly degraded these oxidized species by further radical pathways (these pathways were not fully elucidated).


They postulated that the copper entered the hair from tap water and then displaced calcium ions, becoming chemically bonded to residues in the hair. The levels of copper were highest in the endo-cuticle, where the levels of sulfur-containing residues were lowest. Removing the copper residues was possible with shampoo containing a chelating agent, either N,N’-ethylenediamine disuccinic acid (EDDS) or histidine, where histidine was most effective.


Copper and chlorotrichosis


Recent work at TRI Princeton has also investigated how metal salts can ingress into the hair cuticle. To gain baseline data, tresses were washed in a copper sulfate solution and colorimetry was used to quantify the color changes, Figure 1.


Image showing color changes of blonde hair after treatment with copper(II) solution
Figure 1: Virgin blonde hair before (L) and after (R) treatment with a 0.5 ppm copper sulfate solution for one hour. The color-change is clearly observed.

Three methods where then compared to see if the greening of hair from copper could be avoided (by coating hair with a protective layer of oil) or remedied (by either washing affected hair with regular shampoo and conditioner, or an acidified shampoo with pH 3.4). Quantification of color change using colorimetry relies on two axes: the a axis, which is the red-green color-scale, and the b axis, which is the yellow-blue color scale.


Hair that was neither pre-treated with a protective agent nor washed after exposure (the control sample) had the smallest a value (derived from the CIELAB color space), with the lower a value indicating a comparatively greener hue, Figure 2. When exposed hair was washed five times either with 15% SLES solution, shampoo with pH 3.4, or was pre-treated with oil and then washed five times with 15% SLES solution, the a value was increased compared with control sample, showing a reduction in the green-ness. However, between the three modes of treatment there were minimal differences. The largest a value was observed when, after exposure to copper sulfate solution, hair was washed using a drug-store shampoo (a value: –8.25).


Plot showing how a value of hair changes with different treatment.
Figure 2: Plot showing change in α value for the tresses under investigation. The a value runs along the red-green color-scale, with a lower a value showing a comparatively more green hue.

Other metal salts in hair


To definitively prove which metal salts are coordinating with hair, ToF-SIMS, a type of mass spectrometry, is required. ToF-SIMS is a powerful technique that can identify the precise metal ions present in the hair fiber and their location. For example, metal species such as iron (Fe), aluminum (Al), calcium (Ca), magnesium (Mg), copper (Cu), zinc (Zn) and arsenic (As) can all be identified, Figure 3. This is useful when looking for ingress of metal salts into the hair. In addition, removal of these metal salts using different techniques can also be monitored.


Image showing ToF-SIMS images of hair after soaking in different metal salt solutions.
Figure 3: Data generated using ToF-SIMS, demonstrating the distribution of specific elements in a hair cross section, after soaking in water containing varying metal salts.

The science part


In water, copper sulfate ionizes into two distinct ions: copper, present as Cu(II), and sulfate, Figure 4.


Image showing possible hydration spheres for Cu(II) and sulfate ions.
Figure 4: Possible hydration shell of copper(II) and sulfate ions upon dissolution in water. The copper(II) salt will likely adopt a octahedral structure, with six water molecules coordinating through the lone pair of electrons on the oxygen atom. Hydration of the sulfate ion is less well-defined, although is likely to be through a δ positive hydrogen atom on the water molecule. This image is to illustrate the overall effect rather than provide a definitive structure.

The copper(II) ions can bind (chelate) with any lone pairs of electrons (e.g. on nitrogen or sulfur atoms) or negative charges on the hair surface, Figure 5a. Hair that has been chemically altered, for example by bleaching or perming, contains more negative charges on the surface due to chemical changes to the structure. This means that these hair fibers are more able to chelate with metal salts, leading to an increased instance of bonding to these metal ions, and explains why chemically damaged hair is more susceptible to chlorotrichosis.


As noted earlier, copper ions can be removed using a chelating agent. Over-the-counter chelating agents include N,N’-ethylenediamine disuccinic acid (EDDS), ethylenediaminetetraacetic acid (EDTA) and histidine, all of which can be incorporated into a shampoo formulation. Penicillamine is a medical treatment for chlorotrichosis that also works through chelation, and contains both a sulfur atom and a nitrogen atom. When penicillamine comes into contact with a copper(II) species, two sulfur atoms bind strongly to one copper(II) atom bringing the nitrogen atoms into close-proximity, therefore the nitrogen atom is also able to chelate by using the lone pair of electrons leading to a stable complex that can dissolve in water and be washed away, Figure 5b. The same principle applies to EDDA, EDTA and histidine, although the exact atoms binding in each will differ.


Image showing change in chemical structures of proteins or penicillamine when exposed to Cu(II) salts.
Figure 5: (a) Chemical interaction of common amino acid residues with copper(II) salts; (b) Chelation of copper by penicillamine.

That’s nice, but what do I need to do to keep my hair blonde when swimming in a pool?


As with all of the topics discussed in this mini-series, if hair is in good condition then it is less likely to be affected by environmental factors e.g. hard water, salt, or chlorine. Ensuring that hair is healthy and has minimal damage (e.g. through chemical straightening, perming or bleaching) goes a long way towards ensuring that hair does not go green in a swimming pool, as there are fewer carboxylate and sulfonate groups (present as cysteic acid) that are formed by oxidative cleavage of peptide bonds and disulfide bridges, respectively, during bleaching or straightening treatments. In addition, protecting hair from UV light will help mitigate damage from copper due to radical reaction pathways, therefore wearing a swimming hat will help. Washing hair with a chelating agent in the shampoo, e.g. penicillamine, will help prevent chlorotrichosis as a medical intervention, but TRI has also shown that a store-bought shampoo and conditioner also goes a long way towards restoring red-tones in hair and removing a greenish tinge.


How can TRI Princeton help me?


TRI Princeton provide a suite of analytical services that can measure the impact of water or metal ions upon the physical properties of the hair. For example, ToF-SIMS can be used to show distribution of metal ions within the surface of a hair fiber, colorimetry can show how red-green and blue-yellow hues change under different conditions, SEM can be used to visualize degradation of hair fibers upon exposure to pollutants, infra-red/Raman spectroscopy and mass spectrometry can interrogate structural changes to hair fibers, and Differential Scanning Calorimetry (DSC) can probe the hair protein structure.







Director – Hair Research


Thank you to Ernesta, and the TRI Princeton hair research team, for providing supporting data.


 

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