The Role of Water in Hair Fibers ©

The proteins of the hair have a special affinity for water. The water molecules are bound in the structure of hair and they affect the physical properties of the hair. Brown Caucasian hair has water in the range of 6.0 to 13.6% at relative humidities between 29.2% to 70.3%[i]. Untreated Caucasian hair fibers were used to determine water/moisture contents at various humidities, using Microwave Resonance Technique and results are shown in the Figure 2.8.

[i] CR Robbins. (2002). Chemical and Physical Behavior of Human Hair. 4th Ed. New York: Springer. p. 89.

Figure 2.8

 Figure 2.8: The water/moisture contents of Caucasian hair at various humidities from 30% to 80%. (Avlon Research Center Unpublished Report 10-37)

 The moisture content of Caucasian hair was in the range of 6.86% to 12.89 % in the humidity range of 30 to 70% RH[i]. These values are similar to the values reported by Robbins. The curve in Figure 2.8 has a regression coefficient (R-Square) value of 0.96, which suggest that moisture in hair is linear against % Relative Humidity. The Microwave Resonance technique requires 4 g tresses for each measurement. Since African-descent fibers are not available abundantly, this test was not conducted on African descent hair.

African descent fibers contain less water or moisture than their counterpart Caucasian hair. The fibers that equilibrated at 65%RH and heated to 200°C using thermogravimetric techniques show that African descent hair has 10.54 % moisture compared to Caucasian hair that has 11.39% moisture[i]. Brazilian hair seems to have 10.53 % moisture under similar conditions. The water molecules that are loosely bound to the surface structure of the hair evaporate quickly at around 50 - 70ºC[i]. Those water molecules that are deeply embedded in the structure of hair evaporate around 150 to 170ºC , while the crystalline microfiberillar proteins denature and driven off from the hair at around 240 to 255ºC[i].

 Another aspect of water in the hair is its retention by hair fibers upon wetting with water. Hair fiber proteins have lots of affinity for water and upon wetting, they can retain up to 30% water to their dry weight. In the case of dry hair below 5 %RH, water binds to the hydrophilic sites of the proteins which are acidic and basic amino acids. Between 5 to 75% relative humidity, water forms weaker links such as hydrogen bonds with proteins. Above 75% relative humidity, water forms clusters of water molecules and is bunched together[i]. The extent of water bonding also depends on the origin of hair fibers. For example, African-American hair has lesser water uptake than its counterparts European and Asian hair fibers [i]. The African-descent hair fiber has a smaller cortex and proteineous matrix because of its highly irregular shape and consequently it inherently attracts less moisture.

 Swelling of Hair in Water:

 

Figure 2.9

Figure 2.19 The swelling of hair fibers in water

The human keratin fibers swell in water with respect to their diameter. The diameter increases to a significant degree and the length of the fiber decreases slightly. When hair fiber is immersed in water, it takes five to six minutes for the fiber to swell to maximum degree and then the swelling tapers off, as shown in Figure 2.9. The swelling of the hair fibers increases to a great deal when the fibers are immersed in solutions of permanent waves containing ammonium thioglycolate and ammonia. Similarly, when fibers are immersed in 0.5 N sodium hydroxide solution, the swelling of hair fibers reaches to maximum of 20 % of its diameter in 20 minutes, shown in Figure 2.9. Similarly, fibers swell even more in potassium hydroxide and guanidine hydroxide solutions[i]. Figure 2.9: The swelling of hair fibers in water and in 0.50 N Sodium Hydroxide solution

When fibers swell to a great deal in alkaline solutions, they tend to lose their elasticity proportionally. In other words, the higher the swelling of the fibers, higher is the loss in fiber elasticity.The control of swelling during exposure to alkaline solutions can help reduce the loss of fiber elasticity. Some of the de-swelling agents know in the literature are sodium chloride, ammonium chloride, and potassium chloride[i].

References

[1] CR Robbins. (2002). Chemical and Physical Behavior of Human Hair. 4th Ed. New York: Springer. p. 89.

 Figure 2.8: The water/moisture contents of Caucasian hair at various humidities from 30% to 80%. (Avlon Research Center Unpublished Report 10-37)