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

Part 2. The Impact of Salt

Anyone who has swum in the sea or a salty swimming pool, has probably noticed the change in the feel and appearance of their hair afterwards. Hair can become dull or lackluster, as well as feeling heavy and tangling easily. However, on the positive side, having the beach hair look can be used by some as a fashion statement, providing fun, relaxed hair styles with strong curl definition. Beach hair effects, good and bad, are created from deposits of sodium chloride (NaCl) on the hair fibers, which changes their physical properties. This short article reveals how NaCl residues affect the hair fibers, and how scientists at TRI Princeton use advanced analytical techniques to quantify these changes.

Research undertaken at TRI, Princeton

Four analytical techniques were used to probe the impact of saline water upon tresses: ease of combing; scanning electron microscopy (SEM) imaging; shine analysis, and; tensile strength testing. In all cases hair was submerged in a beaker of 3.5% (w/w) NaCl solution for 30 minutes with stirring, then hair was dried overnight at 60 ± 2% relative humidity, and 22 ± 2 °C.

Hair Friction: quantifying the degree of tangling within hair tresses

Tangled hair is a common complaint after swimming in saline water, and a combing force measurement allows for quantification of this change. Researchers at TRI Princeton showed that both medium brown virgin hair and 6% bleached hair required a significant increase in combing force after exposure to salty water, indicating increased friction between hair fibers that could lead to substantially increased risk of split-ends or fiber breakage. However, rinsing the hair with freshwater reduced the impact, while washing the tress with shampoo and conditioner restored the tress to the levels of the control sample, Figure 1.

Figure 1: Comparison of change in force required to comb hair after differing treatments for virgin hair and 6% bleached hair.

Some people believe that treating their hair with oil before swimming can prevent or reduce the impact of NaCl upon tresses, as hair feels less tangled afterwards. To test this hypothesis, tresses were pre-treated with oil and then subjected to saltwater conditions. In both cases an increase in force required for brushing was observed when compared with virgin hair, but the use of oil led to a 78% reduction in combing force required when compared with untreated hair, Figure 2.

Figure 2: Comparison of change in force required to comb hair between non-treated hair and hair pre-treated with oil.

Does hair really look duller after going in the sea?

SEM imaging is an advanced technique that allows individual crystals of NaCl to be identified and the location of these crystals on the hair shaft. Using SEM imaging, no discernible differences between the quantity or location of NaCl coating on either virgin or bleached hair fibers could be identified, Figure 3. However, hair shine analysis revealed that salt residues led to a 22% reduction in shine.

Figure 3: SEM images showing salt (NaCl) crystals on the surface of virgin and bleached hair.

Does saltwater exposure cause hair to be damaged more easily?

Thus far, NaCl has been shown to increase friction between hair fibers and reduce shine, which are both due to deposits of NaCl upon the hair fibers themselves. However, as well as these physical impacts upon hair fibers, there are also sub-microscopic impacts too, where the sodium and chloride ions change interactions between different chemical groups in the hair’s structure.

For example, exposure of bleached fibers to salt water caused the cross-sectional area of each hair fiber to increase by 3.2% and Young’s modulus to reduce by 12%. In terms of physical changes, the reduction in Young’s modulus shows that the bleached hair fibers become less stiff. In addition, the break stress (i.e. the stress required to break each hair fiber) for bleached hair changed from 214 MPa to 197 MPa, an 8% decrease. One possible explanation is that at a molecular level the NaCl is disrupting both hydrogen bonds and ionic bonds within hair fibers (by solvating any H-bond acceptors or ionic residues), reducing both the overall strength and rigidity of the hair, Figure 4.

Figure 4: (a) Key chemical binding interactions between protein chains in hair; (b) NaCl disrupting ionic bonding (blue) or hydrogen bonding (green).

That’s nice, but what do I need to do to keep my hair soft when I swim in the sea?

There are numerous take-home messages from this work, with the main one being that removing saline residues as soon as possible can prevent changes to hair structure at a chemical level and readily restores hair to its condition prior to exposure. Ideally, hair should be washed with shampoo and conditioner, but simply rinsing with fresh water can mitigate some of the physical impacts. If this is not possible, pre-treatment of hair with an oil-containing product seems to protect hair to some extent, particularly in relation to grooming and mechanical treatments.

What if I want the beach-look?

A variety of cosmetic salt-water sprays exist on the market that can simulate the effects of swimming in salty sea water. They don’t actually contain sea water, but just the sodium chloride that we have spoken about in this article. If used correctly these products do give good hair texture and curl definition and wash away very easily. However, as you would expect, the hair will feel gritty and rough.

How can TRI Princeton help me?

TRI Princeton provides a suite of analytical services that can measure the changes of a particular set of conditions upon the physical properties of the hair. For example, SEM can be used to visualize build-up of materials on the hair shaft, hair shine levels can be fully quantified and texture analysis can probe the smoothness of the hair fibers.

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