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

Part 1. The impact of water hardness


Many people say that their hair looks and feels different when they go on holiday or move to a different part of the world. This isn’t just their imagination, the hardness of the water coming out of your tap can affect your hair when you wash it. This short article explains how water hardness affects the hair, and how cosmetic products can help.


Image showing build-up of limescale on a tap

Water hardness usually refers to the amount of magnesium (Mg) or calcium (Ca) salts dissolved in water. Hard water contains high levels of Mg(II) and Ca(II) ions that can interact with the hair fibers and mainly collect on the cuticle (the hair fiber’s surface). In terms of the impact upon hair, the extent of metal ion uptake and the impact of these metal ions have both been studied.


The extent of uptake of Mg(II) and Ca(II) ions found in hard water are directly related to the condition of the hair, with both virgin and damaged hair (either through bleaching or chemical straightening) absorbing the metal ions, but damaged hair absorbing more overall. Regardless of the condition of the hair, more Ca(II) is absorbed than Mg(II). Water’s pH also has an impact, with higher water pH (more alkaline) leading to a great uptake of metal ions.


Work performed at TRI, back in 2007 showed that the introduction of divalent Ca(II) into bleached and dialyzed hair increased the tensile modulus. It was assumed, at the time, that divalent ions, such as Ca(II) form bridges between negatively protein sidechains. However, more recent work on the effects of hard water treatment on the tensile and torsional properties by Evans et al showed only very small effects. Experiments show only slight increases in tensile or torsional modulus with hard water, and, often, no change at all. Clearly, more work needs to be done in this area.


It might be expected that the build-up of lime scale crystals on the surface of the hair, in hard water areas would cause an increase in surface friction, and some laboratories have reported this. However, work by Evans et al suggests that hard water might reduce hair tangling by increasing hair stiffness and alignment. Again, more work needs to be done in this area to figure-out what is going on.


The science part


The propensity of damaged hair to absorb more metal ions, and therefore be affected by these ions, is likely due to the chemical entities present within the hair fiber itself, Figure 1a. Damaged hair contains more carboxylate and sulfonate groups (present as cysteic acid) due to oxidative cleavage of peptide bonds and disulfide bridges, respectively, during treatment with peroxide, Figure 1b, and can readily make a strong bonds to both Mg(II) and Ca(II) ions. The preference for bonding to Ca(II) rather than Mg(II) is likely due to the energy required; Ca(II) has a larger ionic radius than Mg(II) (as it is further down Group 2 of the periodic table) so the forces between the metal ion and surrounding water molecules will be smaller, so the hydration sphere will be removed more easily allowing the carbonate and sulfonate groups to coordinate instead of water.


The impact of pH is also related to the presence of carboxylate and sulfonate groups, Figure 1c. A lower pH leads to an increase in protons (H+) in solution, meaning the carboxylic acid and sulfonic acid residues will be protonated. This reduces their potency for binding to the metal ions because they are no longer anionic (negatively charged). Washing or treating hair at higher pH increases the levels of Mg(II) and Ca(II) ions present in the hair because the carboxylate and sulfonate groups are deprotonated, so can readily bind to the metal ions.


Image showing chemical binding interactions between protein chains, chemical changes to chemical groups during bleaching and change in chemical structures due to pH
Figure 1: (a) Key chemical binding interactions between protein chains in hair; (b) Chemical changes to proteins that occur during hair bleaching; (c) changes in residual group chemical composition at low (acidic) and high (basic) pH.

That’s nice, but what do I need to do to keep my hair soft when washing in hard water?


When washing hair in hard water, the key is to protonate the anionic (negatively charged) residues through use of an acidic shampoo and/or conditioner, or the use of a chelating agent that sequesters Mg(II) or Ca(II) ions. The product should ideally have pH <6. This will ensure that any metal ions are less likely to interact with the hair, and hair should be less affected.



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Figure 2.  Data generated using Tof-SIMS, demonstrating the distribution of specific elements in a hair cross section, after soaking in hard water.
Figure 2. Data generated using Tof-SIMS, demonstrating the distribution of specific elements in a hair cross section, after soaking in hard water.

 

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