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J. Cosmet. Sci., 71, 303–320 (September/October 2020) 303 Quantifi cation of Human Hair Moisturization with Cosmetic Products by Dynamic Vapor Sorption Y. K. KAMATH , Kamath Consulting Inc., Princeton, NJ Accepted for publication April 24, 2020 . ABSTRACT An attempt has been made to evaluate and quantify the moisturizing effi cacy of cosmetic products by dynamic vapor sorption method. The possibility of quantifying this effect by the application of the second law of thermodynamics to sorption hysteresis has been demonstrated. The results show that the ordinary conditioner actives in rinse-off formulations show limited moisturizing capabilities. However, specially formulated products can show, quantifi ably, both moisturizing and desiccating effects. From the Brunauer, Emmett and Teller (BET) theory, total pore volume obtained from the literature by nitrogen adsorption, and the sorption data from this work, it has been possible to show that the sorbed water in hair does not distribute uniformly in the hair. This is in agreement with the X-ray diffraction measurements and the more recent work on small angle neutron scattering on D2O-saturated hair. INTRODUCTION Mechanical properties of hair such as bending and torsion moduli, which sensitively affect the softness of hair, are governed by its moisture content. An optimally moisturized hair is supposed to be shiny, smooth, and soft to touch and beautiful in appearance. How- ever, drastic changes in the humidity surrounding the hair can change the moisture con- tent with a consequent change in its softness and appearance. From a material science viewpoint, hair is a unique composite fi ber structured from spindle-shaped cells (cortical cells) held together by cell membrane complexes (CMCs) (cortex) and wrapped around by a multilayered sheath of fl at cuticle cells (1). Cortical cells themselves are composite cells consisting of rod-like intermediate fi la- ments embedded in an amorphous protein matrix. Included in the cortex are some cells which were left empty without protoplasm, forming the medulla. In thick hair, medulla can be a continuous tube, almost like a capillary. In thinner hair, the medul- lary channels could be narrow and discontinuous (1, pp. 45–48, 2). Excessive medul- lary structures can affect the shine of hair (3). These structures in hair may have a special association with water because of their volume and less-keratinous (low cys- tine) nature. Address all correspondence to yashkamath@verizon.net.