Measuring the Water Content of Hair

Feb 21, 2014 | Contact Author | By: Trefor A Evans, T.A Evans LLC
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Title: Measuring the Water Content of Hair
moisturex measurementx adsorption isothermx relative humidityx Dynamic Vapor Sorptionx
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Keywords: moisture | measurement | adsorption isotherm | relative humidity | Dynamic Vapor Sorption

Abstract: Consumers have demonstrated a clear desire for hair that isn’t “dried out” while also demonstrating a clear distaste for the effects of high humidity on hair. To find the balance in creating products, it is necessary to have an accurate means of measuring hair’s water content. This article describes equipment used to perform this task while highlighting experimental variables that can produce suspect results and lead to incorrect conclusions.

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T Evans, Measuring the Water Content of Hair, Cosm & Toil 129(2) 64 (2014)

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Continuing a previous exploration1 of measuring hair-related attributes and related concerns, another commonly heard worry is that hair will “dry out” and accordingly, consumers may avoid treatments, environmental conditions and even ingredients that are perceived to induce this state. When this condition is recognized by the consumer, products that moisturize or hydrate hair are demanded in an attempt to “restore hair’s natural moisture balance.” However, to the hair care scientist, these statements are puzzling. For a start, it is well-recognized that the water content of hair is overwhelmingly dictated by the relative humidity (RH) of the environment—but while consumers profess a desire for “maximum moisturization,” the high humidity conditions that induce this state represent the very definition of a bad hair day. Moreover, technically, as hair becomes more damaged it has a higher affinity for water. The number of water adsorption sites is increased within the fiber as hydrophobic cross-linking disulfide bonds are converted to hydrophilic cysteic acid moieties. Also, the outermost, water-repellent surface of hair is more likely to be removed (the lipid “f” layer). Therefore, theoretically, the consumer term dry, damaged hair would seem contradictory.

To unravel the mystery of these contradictions, it is necessary to have a precise and accurate means of measuring the water content of hair. This article describes the use of commercially available scientific equipment to perform this task while highlighting experimental variables and pitfalls that can produce suspect results and lead to incorrect conclusions.

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Figure 1. Water-hair adsorption isotherms

Figure 1. Water-hair adsorption isotherms

In the case of water, its abundance in the environment is represented by the RH, and so the hair-water isotherm in Figure 1 illustrates the equilibrium water content of hair, sometimes called the regain, as a function of humidity.

Figure 2. Schematic of DVS*

Figure 2. Schematic of DVS*

In today’s modern electronic era, the technique used for this measurement is commonly termed Dynamic Vapor Sorption (DVS).

Figure 3. Output from a typical DVS adsorption experiment

Output from a typical DVS adsorption experiment on hair

There are a number of parameters that contribute to the moisture adsorption rate (noted later), but for the experiment shown in Figure 3, it will generally take somewhere around 6–8 hr for hair to reach a reasonable approximation of equilibrium at each new condition; although in actuality, the weight will still be increasing slightly, even after 24 hr.

Figure 4. Reduction in water content of hair after soaking in phenol solutions

 Figure 4. Reduction in water content of hair after soaking in phenol solutions

Figure 4 shows adsorption and desorption isotherms for hair soaked in a 5% Cl-resorcinol solution, illustrating how it is possible to reduce the water content of hair by up to 40%.

Biography: Trefor Evans, PhD

Trefor Evans, PhD

Contributing author Trefor Evans, PhD, has worked in the hair care industry for more 20 years, with the majority of his time spent as a manager in the product development labs of Helene Curtis and Unilever. He also served for fi ve years as director of measurement services at TRIPrinceton before establishing his own consultancy, TA Evans LLC.

Evans holds a doctorate in physical-analytical chemistry and specializes in measurement science. He holds seven patents related to hair care, and has published numerous articles in trade magazines and scientific literature. His work has been awarded twice by the Society of Cosmetic Chemists, and he is a co-author and co-editor of the book Practical Modern Hair Science, published by Allured Business Media.

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