DISULFIDE BOND REDUCTION IN HAIR 51 agent, C O is the concentration of the reducing agent, k is the rate constant, and t is the time (1,4). There are two further assumptions made when applying the pseudo first-order kinetic model to chemical stress-relaxation studies of hair. The first assumption is that the tensile stress is proportional to the number of disulfide bonds remaining at any given time. The second is that all the stress-supporting bonds are equally reactive or totally nonreactive. A plot of-ln (Ft/Fo) versus time, t, yields the rate constant, k. If the reaction is truly pseudo first-order, then a plot of the In [(Ft-Ff)/(Fo-Ff)] versus time will be linear with a slope of kCo and an intercept of zero (1,2). The rate constant, k, should remain constant regardless of the concentration of the reducing agent. Single fiber tensile kinetics (SFTK) is a method for investigating reduction kinetics of hair using single fibers based on stress relaxation caused by disulfide bond cleavage. This method is useful for basic studies of interaction of thioIs with hair (1). The data obtained from SFTK measurements are used to elucidate information about the rates and mech- anisms of reactions of reducing agents and to derive mathematical models (1). Wickett used SFTK in conjunction with strain cycling to study the effects of changing pH, temperature, and concentration on reduction of individual hairs using sodium thiogly- colate, dithiothreitol, and sodium dihydrolipoate solutions (1). Wickerr and Barman used SFTK to study the efficacy of perms affected by the reaction with disulfide bonds and the ease of penetration using solutions of dihydrolipoic acid, dithiothreitol, 1,3- dithiopropanol and its derivatives, and 1,4-dithio-2-butanol and its derivatives (2). For the purposes of this study utilizing the miniature tensile tester, the SFTK method was modified in order to study the effects of reduction by ammonium thioglycolate (ATG) at different pHs and the effects produced by the addition of dithiodiglycolic acid (DTDG) to an ATG solution. If the reducing agent in Equation la is ATG (HS-CH2- COOH), then the disulfide in Equation lb is DTDG (HOOC-CH2-S-S-CH2-COOH). The data obtained from stress-relaxation and stress/strain measurements were used to determine the reaction rate constants (k) and mechanisms of reduction. SFTK results from stress/strain measurements (20% index) showing the effects of reoxidation of hair after treatment with an ATG/DTDG solution are also discussed. MATERIALS AND EQUIPMENT Medium-brown, virgin hair from a single source was obtained from DeMeo Brothers, New York. This hair sample was used for all the studies described in this report. The hair fibers were shampooed with a 10% (w/w) solution of sodium lauryl sulfate in Millipore water, rinsed thoroughly, and allowed to dry. Stress-relaxation and stress/strain measurements were made on the miniature tensile tester (Dia-stron) interfaced to an IBM 386 personal computer that ran the Rheopc software. Temperature was controlled by a Techne © Model 1252-00 circulating water- bath. Reducing agents were obtained from Evans Chemetics, Waterloo, NJ. The re- ducing solutions used in this study contained only the mercaptan, Millipore water, and ammonium hydroxide to adjust pH. The neutralizer contained hydrogen peroxide, Millipore water, and phosphoric acid to adjust pH.

52 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS METHODOLOGY FIBER SELECTION TEST Hair fibers were preselected by straining the untreated fibers into the yield region in water (25% extension) and measuring the force necessary to extend the fiber. After the untreated hair was measured, it was allowed to re-equilibrate overnight in water to recover its tensile properties. Each 30-mm fiber segment was allowed to soak in water for 60 minutes. The fiber was extended 20% in water and held for five seconds. Only those fibers exhibiting a linear Hookean region and an acceptable yield region (turnover of 2-5 % and slope equal to approximately 0.1 times the slope in the Hookean region) were used for SFTK measurements. STRESS RELAXATION When a hair fiber is extended in water by less than 2% of its original length, stress relaxation is complete in approximately 30 minutes (1). The utilization of a strain cycling procedure will reduce the amount of time necessary for stress relaxation. The following modification of the SFTK method was used with the miniature tensile tester to stress relax the hair fibers. The fiber was extended to 2.0% strain in a buffer solution the same pH and temperature as the reducing solution. Following extension, the hair fiber was allowed to stress relax for 60 seconds. The amount of strain was then reduced to 1.50% and the fiber was allowed to stress relax until a constant level of stress was achieved (2-3 minutes). After stress relaxation, the buffer solution was replaced rapidly by the reducing solution. SFTK data were collected to monitor the stress relaxation of the disulfide bonds in the hair fiber caused by the reducing agent. The data were displayed graphically as grams of force versus time by the Rheopc software (Dia-stron). Plots of the natural logarithm of [(Ft-Ff)/Fo-Ff)] versus time were prepared, and the reaction rate constant (k) was calculated from the slope. It is known that as hair is reduced, its swelling in water increases. Therefore, it would seem possible that some portion of measured decrease in force could be attributed to increased zero strain length of the hair due to swelling. Wortman and Souren (14) have shown that length setting does not occur therefore, changes in length from swelling are unlikely. This result indicates that the measured decreases in force can be attributed solely to decreased fiber strength. REDUCTION METHOD Two types of solutions were prepared for the following studies: ammonium thioglycolate and ammonium thioglycolate + dithiodiglycolic acid. The first type of solution con- tained 1M ATG in Millipore water adjusted to either pH 8.0, 9.0, or 9.5 with ammonium hydroxide. The second type of solution was prepared by combining the appropriate volumes of a 2 M ATG solution at pH 9.0 with a 1 M DTDG solution at pH 9.0 and readjusting the pH to 9.0 in order to acquire the desired solution combi- nations. Each 30-mm hair fiber was prestretched in a phosphate buffer of the appropriate pH and reduced in 50 ml of reducing solution. Stress-relaxation data were collected for 30 minutes during reduction.