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.

JOURNAL OF COSMETIC SCIENCE 304 The importance of water to hair can be experienced by running one’s fi ngers through the hair in an environment of low humidity ( 20% RH). The hair feels coarse and rough to touch and has a “fl yaway” effect due to the development of static charges on the surface of hair. On the other hand, hair in an environment of high humidity ( 70% RH) has a limp and heavy feel because of interfi ber adhesion from the condensed moisture fi lm on the surface. In this communication, we consider the interaction of water with hair in its natural state, or in combination with cosmetic formulations and actives, and grooming processes. EXPERIMENTAL MATERIALS The hair sample used in these experiments was European dark brown hair obtained from DeMeo Brothers of New York. Sodium lauryl sulfate was obtained from Sigma Aldrich. Cosmetic products used in this study were either raw materials or confi dential experi- mental formulations from commercial sources. METHOD Because the behavior of a hair assembly depends on its moisture content, which in turn depends on the transport of water vapor in and out of hair, a device suitable for studying the sorption and desorption of water from hair is used in this work. The device is a dynamic vapor sorption (DVS) instrument supplied by Surface Measurement Systems (Allentown, PA). It is a gravimetric device using a microbalance enclosed in a temperature-controlled incubator. The sample pan of the microbalance is enclosed in an environment in which the humidity is controlled with an accuracy of ±0.4%, using dry and moisture saturated nitrogen (bubbled through distilled water) in an appropri- ate proportion with a humidity controller. In this work, all experiments were carried out at 25°C. I n a typical experiment, about 20 mg of hair is placed in the sample pan and the humidity set for 0% RH. The sample is completely dried to a constant weight and the weight is recorded. Then the humidity is increased by steps of 10% up to 95% RH, the last step being 5%. The weight of the sample is recorded at the end of each step when the equilibrium is reached. The attainment of equilibrium is automatically sensed by the instrument based on the dM/dt value set in the software (for hair this value is generally 0.001%/min). If the behavior of the sample is known from previous experiments, then the equilibrium criterion can also be set to a maximum time limit (2–3 h for hair) for each step. Equilibrium is attained in this relatively short time because of the small sample size (20 mg). A part from obtaining sorption-desorption (S-D) isotherms, the instrument can also be used to determine moisture contents of materials at a given humidity. It should be noted, however, that with materials which show sorption hysteresis, such as hair, the moisture content at a given humidity depends on whether we approach that humidity from the dry side or the wet side.