Volume Changes During Water Binding to Hair Fibers

In 1980, researchers studied how hair fibers' volume changes when they bind with water, using an optical method to measure changes in hair diameter in response to humidity. They found that intact, bleached, and descaled hair fibers all increased in diameter by 8-9% when relative humidity rose from 0 to 93%. The study revealed that the partial molal volume of water in hair was lower than in liquid water, with chemically treated hair showing smaller values than virgin hair. This suggests that water molecules fill pre-existing voids within the hair, particularly in the more porous cortex compared to the cuticle. Bleaching was shown to alter the pore size distribution, affecting water uptake. The study also calculated the free energy changes due to water binding and concluded that water forms monomolecular layers within the keratin structure. The findings challenge Flory's polymer swelling theory, suggesting that site binding is the primary process for water absorption in hair, and that chemical treatments modify the hair's water uptake characteristics.