j. Soc. Cosmet. Chem., 47, 49-58 (January/February 1996) The kinetics of disulfide bond reduction in hair by ammonium thioglycolate and dithiodiglycolic acid MELISSA A. MANUSZAK,* EDWARD T. BORISH, and R. RANDALL WICKETT, The University of Cindnnati College of Pharmacy, 3223 Eden Avenue, Cincinnati, OH 45267 (M.A.M., R.R.W. ), and L & F Products, I Philips Parkway, Montvale, NJ 07645-I575 (E.T.B.). Received July I, I994. Synopsis A study was conducted tO determine the physical behavior of human hair in the presence of ammonium thioglycolate (ATG) and the effect of dithiodiglycolic acid (DTDG) on this behavior. The method utilized in this study was a modification of the single fiber tensile kinetics (SFTK) method (1). SFTK experiments were performed using the miniature tensile tester (Dia-stron) on virgin hair from a single source (DeMeo Brothers) by monitoring stress-relaxation and stress/strain behavior (20% index). The results indicate that the addition of dithiodiglycolic acid does not affect the kinetics of stress relaxation when the fiber is held under a constant strain (1.5 %). Similarly, the addition of dithiodiglycolic acid does not diminish the extent of fiber weakening prior to neutralization. However, hair reduced in the presence of dithiodiglycolic acid is stronger after neutralization then hair reduced in the absence of dithiodiglycolic acid. INTRODUCTION Numerous studies have been undertaken to determine the effects, both physical and chemical, that reducing agents have upon the hair. It has been well established that the mercaptans used to produce a permanent wave attack the disulfide bonds in the hair, cleaving these bonds so that they may later be reformed in a new configuration. Previous studies have investigated the kinetic behavior and mechanisms of action of various mercaptans by monitoring the stress-relaxation and stress/strain behavior of hair or wool fibers (1-11). The findings of these studies indicate the dramatic effects that may result when parameters such as temperature, pH, or concentration are varied. The overall reactions involved in the waving of hair (Eqs. 1 and 2) are: Ker-S-S-Ker + 2RS-H • 2 Ker-S-H + R-S-S-R keratin mercaptan reduced keratin disulfide (cystine) (cysteine) (1) *Current addresses: Melissa Manuszak, Department of Chemistry, Louisiana State University, Baton Rouge, LA 70803 (504) 388-2985. email: mmanusz@unixl. sncc. lsu.edu Edward Borish, Zotos Corporation, 100 Tokeneke Road, Darien, CT 06820 49

50 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 2 Ker-S-H q- H202 --• Ker-S-S-Ker q- 2820 (2) reduced keratin hydrogen peroxide new keratin bond (cysteine) (cystine) Although other oxidizing agents, including atmospheric oxygen, have been utilized, hydrogen peroxide (Eq. 2) is the most commonly used oxidizing agent to date. The overall reaction (Eq. 1) proceeds through two steps. Each of these steps is a displacement reaction by the mercaptan ion, RS-, on a sulfur atom of the disulfide bond, then by the mercaptan ion on a sulfur atom of the mixed disulfide (12). The two displacement reactions (Eqs. la and lb) are: Ker-S-S-Ker q- RS-H •-• Ker-S-S-R q- keratin mixed disulfide (cystine) Ker-S-S-R + RS-H •-• R-S-S-R + mixed disulfide mercaptan disulfide Ker-S-H (la) reduced keratin (cysteine) Ker-S-H ( 1 b) reduced keratin (cysteine) The overall equilibrium constant, Ka, of the above reactions (Eqs. la and lb) is ex- pressed as: [R-S-S-R] * [Ker-SH] 2 Kam [Ker-S-S-Ker] * [R-SH] 2 (3) The equilibrium positions and extent of the reactions given by Equations 1 and 2 are governed by pH, concentration, tension applied during rolling, nature of mercaptan employed, degree of fiber swelling, time, and temperature (12, 13). Much research has been performed to describe the kinetics and mechanisms by which reducing agents function (1-11). Currently, there are two mechanisms cited in the literature: pseudo first-order kinetics and moving boundary kinetics. Pseudo first-order kinetics are exhibited by sodium thioglycolate (TGA) below pH 9 (1) and bisulfite (4). The assumptions made in applying this kinetic model are: 1) the diffusion of the reducing agent into the hair fiber occurs rapidly, 2) the reducing agent is present in large excess and remains constant during the reaction, and 3) the rate of disulfide reduction proceeds slowly and is the rate-determining step. The rate of disulfide bond cleavage is described by Equation 4: d(S-S) - - k Co(S-S) (4) dt where k is the reaction rate constant, C O is the concentration of the reducing agent, and (S-S) is the number of disulfide bonds remaining at any given time (1,4). This equation is then integrated to arrive at Reese and Eyring's relationship for describing the tensile stress (Eq. 5) (4): F• = Foex p- kCo• (5) where F• is the force at any given time, F o is the force before applying the reducing