The influence of surfactants on the pH

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Surfactants are the cleaning agents that create the foam in shampoos. They act by weakening the adherence forces binding impurities and residues to hair. Surfactants dissolve dirt and oils by preventing them from attaching to the hair shaft or the scalp. Surfactants are also the ingredients creating the foam.

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The increasingly popular expression “SLS-free shampoo” refers to preparations without Sodium Lauryl/Laureth Sulfate, which has a medium pH (around 6-7) but has strong washing characteristics.

pH: Definition and importance

Hair is extremely sensitive to the pH variations of products applied to its surface.

Any product applied to hair that has a pH higher than 3.67 causes an increase in the negative charge of the hair, that is, an increase of static electricity. This causes repulsion between strands. The pH of water is 7.0; therefore, an increase in negativity occurs during rinse, the negatively charged strands repel the shampoo.

The more the hair is negatively charged, the more it will tangle, and become hard to comb, thus causing the frizz effect. Besides, in a low (above 7)  pH state, hair has an increased capacity to absorb water. Water causes hydrolysis, that is, temporarily breaks the hydrogen bonds and makes the molecule malleable. Consequently, the hair strand becomes fragile due to a decrease in its elasticity. There’s also an increase in its plasticity, which means that if the wet hair is deformed, it does not regain its original shape. When the hair is wet, the cuticle scales lift, leading to increased cuticle removal, cuticle fragmentation, and cracks to the fiber axis.

The pH of the scalp, however, is around 5.5 like the rest of the skin, which is more alkaline than the hair shaft’s pH. Although there is not a standard definition for the concept of low-pH shampoo, in scientific publications, a low-pH shampoo means with a pH of 6.5 lower.

Studies on shampoos pH

Two main studies can be found in the literature. The first has analyzed a total of 123 shampoos. About 38.21% of all analyzed shampoos had a pH of 5.5, and 61.78% had a pH > 5.5.

100% of the children’s shampoos that were analyzed had a pH > 5.5. Twenty-six anti-dandruff shampoos were also analyzed.

The dermatological shampoo group had 42.1% with a pH of 5.5, and 57.9% presented with a pH > 5.5. Among the commercial products, many had a pH of 5.5 and above.

A second study analyzed 38 samples of shampoo, 24 (63%)  had a pH within the range of 6  to 7.

This study also found that the soaps commonly used by the population at large have a pH ranging between 7 and 9. Experts generally recommend balanced acid shampoos and the study found that 81.58% of commercial shampoos had an acidic pH. When compared to soaps, shampoos have much better pH for hair.

Specialists remind us of the need to consider the pH of soaps and shampoos they so what we use on our hair is friendly.

Most of the analyzed products had a final pH higher than the hair shaft pH of 3.6 and even higher than the scalp pH of 5.5. There is no standardized pH for any specific indication of hair shampoo, whether commercial/popular or anti-dandruff products.

It is quite a different scenario with the products for professional use in hair salons, where 75% of them have a pH value within the optimal range of 5.5 or lower. According to the current literature, the usage of shampoo with a pH higher than 5.5 may increase friction and cause frizz, hair breakage, and increased hair tangling.

Soaps for hair 

Soaps have a high pH, and in contact with water, they leave an alkaline residue that is harmful to the hair because it precipitates in the form of calcium salts. These accumulate on the hair strands, leaving them opaque and tangled. Such effects do not happen with the newest surfactants such as SCI (Sodium Cocoyl Istionathe), which are smooth cleansers and gentle to the hair.

Conclusion

As the hair fiber has a pH of 3.67, a shampoo pH closer to 3.67 has less possibility of increasing the negative electric charge of hair, which in turn results in less tangling. For this reason, the ideal shampoo pH should be between 5.5 and 6.5. Using such shampoos avoids a significant increase in static electricity, which is the leading cause of frizzy hairs.

 

Sources
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4158629/
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4171909/

 

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