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j. Soc. Cosmet. Chem., 34, 301-316 (September/October 1983) Kinetic studies of hair reduction using a single fiber technique R. RANDALL WICKETT, The Procter & Gamble Company, Miami Valley Laboratories, P.O. Box 39175, Cincinnati, OH 45247. Received April 19, 1983. Presented at the Society of Cosmetic Chemists Annual Scientific Seminar, CindnnatL Ohio, May 1983. Synopsis A technique for investigating reduction kinetics of hair using single hair fibers is described. The method is based on stress relaxation caused by disulfide bond breakage, and can be used for fundamental studies of parameters affecting hair reduction rates. Analysis of our data in light of mathematical models indicates that under conditions where reduction is fast compared to diffusion into the hair, the reaction proceeds as a sharp front or moving boundary, and when reaction is slow compared to diffusion, the reaction front is diffuse and pseudo first-order kinetics are followed. We have used the method to investigate the effects of pH, temperature, and reactant concentration with a monothiol reducing agent, sodium thioglycolate (TG), and with two dithiol reducing agents, dithiothreitol (DTT) and sodium dihydrolipoate (6-8 dithiooctanoate). The two dithiol reducing agents displayed moving boundary kinetics under all conditions investigated, while SFTK data obtained with TG was characteristic of pseudo first-order kinetics except at pH values of 10 or greater. We have also observed that while different hairs from the same individual generally have a fairly narrow distribution of reaction rates, hair from different individuals may vary significantly in susceptibility to reductive treatment. INTRODUCTION Hair protein is highly crosslinked by cystine disulfide bonds, and hair that has been extended in water and stress relaxed to constant stress will further stress relax if its disulfide bonds are broken. Reese and Eyring (1) used this fact to study the kinetics of disulfide bond breakage by sodium bisulfite, sodium sulfide, and sodium hydroxide. In this report we present a modification of their method that is well suited to fundamental studies of disulfide bond reduction in hair or other keratin fibers. We call it the single fiber tensile kinetics (SFTK) method. SFTK data can be analyzed to obtain information about the rates and mechanisms of reactions of reducing agents with hair. By making some straightforward assumptions about the nature of the reaction and its relationship to stress decay, it is possible to derive mathematical methods of the reduction kinetics. One of these is the pseudo-first order model obtained by Reese and Eyring (1), where reaction occurs throughout the hair. We also present a new model which requires the presence of a moving boundary or sharp front of reducing agent within the hair. 301