Adsorption Properties of Hair 352 with uniform deposits to provide such a water-barrier. Secondly, it is often suggested that the primary pathway for water access into hair involves the endocuticle layer (again, the reader is referred back to Chapter 1) and so coating of the cuticle face presumably would not significantly alter adsorption and desorption rates. To date, we are not aware of any studies that convincingly support the aforementioned supposition. Consequences of Moisture Adsorption on Fiber Properties 1) Changes in mechanical properties: In 1924, Peirce20 noted a similarity between the shape of the cotton-water adsorption isotherm and the manner in which the torsional stiffness of fibers altered with the relative humidity, and subsequently in 1944, Speakman10 performed similar experiments on wool (Figure 13 in Chapter 7 shows results for our own torsion studies on hair as a function of the relative humidity). As outlined already, it is now well recognized that increased water content depletes secondary structuring by hydrogen bonds and salt bridges with a commensurate decrease in mechanical properties. This topic has been covered in Chapter 7 of this book, and there is no need to re-address it here. Instead, it is simply reemphasized that stress- strain curves for hair become flatter and longer with increased water content as the break stress, modulus and plateau stress all decrease, while the break extension increases. Similarly, as was noted in Chapter 8, the tendency for fiber breakage in fatigue experiments is hugely increased with rising relative humidity. Therefore, it is again emphasized that any dry-state mechanical testing must be performed under controlled humidity conditions. 2) Swelling: Speakman21 published single fiber swelling data for wool in 1929, while notable studies on hair have been performed by White & Stam22,23 and also Valko & Barnett.24 Not surprisingly, the shape of this swelling versus relative humidity curve closely resembles the adsorption isotherm. As such, it has been shown that totally dry, healthy hair will rise in diameter by approximately