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Hair contains approximately 5% moisture at 15% RH, while this value rises to around 17% at 80% RH.
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Soaking hair in a 5% glycerol solution leads to increased water content at elevated humidity.
Hair soaked for 60 min in a 5% resorcinol solution showed a ~20–25% reduction in water content, relative to the untreated control.
Adsorption and desorption isotherms show a water-retarding ability; diffusion of these species into hair appears relatively slow, hence the prolonged soaking and elevated temperatures conditions used in this work.
To scientists familiar with the technical literature, the comments and demands of hair care consumers appear puzzling. Consumers worry greatly about the “drying out” of hair and subsequently demand products that provide “moisturization.” However, as shown earlier, many forms of damage actually lead to higher levels of water in hair. Therefore, to the hair scientist, the popularly used descriptor “dry-damaged hair” is actually an oxymoron. Similarly, when considering consumer demands, one observes that highest moisture content is attained when hair is allowed to equilibrate in high humidity environments, but this condition typically represents the very definition of “a bad hair day.” As such, there is a definite disconnect between “consumer language” and “scientific language,” where an individual’s self prognoses and the technical root cause are indeed different.
This point was dramatically illustrated by Davis and Stoffel, who equilibrated hair tresses at both 15% and 80% RH before presenting them to panelists (N = 50) for evaluation. The isotherm shown in Figure 1 demonstrates how hair contains approximately 5% moisture at 15% RH, while this value rises to around 17% at 80% RH. When panelists were asked to select the tress that felt most moisturized, approximately 75% opted for hair that had been equilibrated at 15% RH. Namely, three out of four panelists rated hair with the lowest water content as feeling “most moisturized.” Similarly, the tresses equilibrated at low humidity also scored equally lopsided wins in terms of smoothness, being less tangled, and feeling less damaged. These findings dramatically illustrate how higher water content in hair overwhelmingly leads to considerably diminished sensorial properties. An explanation for these findings likely returns to the ability for water to induce swelling of the hair fibers, where higher water content produces increased swelling, slight uplifting of the cuticle scales, and a rougher feel.
It appears that consumers essentially use the words “conditioning” and “moisturizing” interchangeably. In their minds, a rough, course hair feel (presumably the consequence of a degrading cuticle structure) is equated with dryness. It is likely that this belief arises due to an analogy with skin care, where a similar thought process persists. Therefore, this symptom is actually alleviated by lubrication rather than any true mitigation of moisture content. Conventional conditioner products (which are addressed in Chapter 3 of Practical Modern Hair Science) are extremely effective at masking poor feel properties and consequently, in consumer terms, the removal of this “dry” feel is described as “moisturization.”
While current conventional treatments do little to alter the water content of hair, some non-conventional approaches are able to induce significant effects. If the “moisturization” term is taken literally, attempts at claim substantiation often involve the inclusion of humectants in a formulation. Humectants are hygroscopic materials that attract moisture, and so their deposition within hair may be expected to increase the overall water content (this said, it is worth highlighting the difficulty associated with depositing and retaining such water-loving materials within hair from an aqueous product that is applied in the shower).
Figure 2 shows how soaking hair in a 5% glycerol solution leads to increased water content at elevated humidity. (It is noted that an adsorption isotherm for glycerol and water is Type III in nature, and therefore the anticipated hygroscopic nature only arises at elevated humidity). Similar, but lesser, effects have been seen with other humectant materials.
Conversely, it is very likely that reducing the moisture content of hair may be more desirable. Namely, lower water levels appear to improve feel, while also increasing tensile properties. There is precedence in the scientific literature for this occurrence. Perhaps most notably, the ability for ninhydrin to reduce the water content of wool was demonstrated in the mid 1960s. Similarly, the work of Breuer mentions how treatment of hair with phenols is able to produce similar effects. Internal studies by the author have been confirming these findings.
Figure 3 shows adsorption and desorption isotherms for hair soaked in a 5% resorcinol solution for 60 min. Results show around a 20–25% reduction in water content relative to the untreated control. Indeed similar results have been obtained for comparable treatments involving a variety of phenolic molecules. A hypothesis for this occurrence involves these molecules adsorbing within the hair with subsequent blocking of water adsorption sites. Thus, moisture uptake is precluded by steric factors.
Internal studies have also shown how many carboxylic acids are able to produce similar effects. For example, Figure 4 shows adsorption and desorption isotherms for hair soaked in 10% citric acid for 2 hr at 40°C. While this water retarding ability is intriguing, it is noted that diffusion of these species into hair appears relatively slow, hence the prolonged soaking and elevated temperatures conditions used in this work. This may appear surprising, as it may have been supposed that the ready penetration of liquid water into hair leads to the equally easy diffusion of dissolved species. However, this does not appear to be true, and an analogy may be drawn to liquid chromatography, where the mobile phase readily passes though a stationary phase while dissolved components progress at different rates depending on interaction energies.
Nonetheless, an ability to manipulate the water content provides a means for tailoring the mechanical properties of hair. It is worth pausing to highlight the novelty of this idea. Conventional daily use hair care products are based on large molecules (e.g., surfactants, polymers, oils) whose functionality lies predominantly at the hair surface. Therefore, there may be a whole next generation of products still waiting to be discovered that act to modify the internal properties of hair.
This information is an excerpt from the book Practical Modern Hair Science. To learn more about this topic or to purchase the entire book, visit www.Alluredbooks.com.