J. Soc. Cosmetic Chemists 15 643-650 (1967} ¸ 1967 Society of Cosmetic Chemists of Great Britain The behaviour of hair at low pH values M. M. BREUER* and D. M. PRICHARD*p Synopsis--The effects of exposure to acidic solutions (pH2) on hair were investigated. It was found that considerable changes occur in the titration curve, the mechanical properties, and the reactivity of the disulphide bonds as a consequence of soaking hair in solutions having pH values less than 2. The experimental results were interpreted by assuming that a dis- sociation of disulphide bonds occurs in the presence of acid. INTRODUCTION It was previously reported that human hair undergoes irreversible structural changes when exposed to solutions of pH less than 2 (1). In particular the acid titration curve, the water regain and the value of the 30ø//0 index of hair was found to be effected by exposure to acid. In the present communication we intend to report further experimental data on the behaviour of human hair at low pH values, and suggest a molecular mechanism which accounts for these phenomena. EXPERIMENTAL Materials Virgin hair was used throughout the experiments. The hair samples were first washed with hot distilled water to remove foreign matter and then Soxhlet-extracted with diethylether for 24 hr. Subsequently the hair was soaked in distilled water for 4 days, the water being changed twice *Unilever Research Laboratory, Isleworth, Middx. tWork carried out as part of DMP industrial training programme for B.Sc. of University of Bath. 643

644 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS a day. Afterwards it was dried in vacuo over phosphorus pentoxide, and finally chopped to pieces about 1 cm in length. Methods A nalar grade chemicals were used in all instances except when otherwise stated. Volucon volumetric solutions, prepared with freshly boiled distilled water, were used for the titrations. The tetrakis (hydroxy-methyl) phosphonium chloride (THPC) was supplied by Albright and Wilson as an 80% solution, and was used without further purification. The Pye Dynacap multi-range pH meter and an E.I.L. automatic titrimeter were used with standard electrode fittings for maintaining and measuring the pH. A Unicam SP500 spectrophotometer was used for optical density determination and an Instron tensometer (table model) was employed for the strain-stress measurements. All experiments were carried out at 25øC (+0.5øC) using thermostatically controlled water baths. Rate of acid uptake This was measured by the following procedure: The hair samples (2g) were introduced into a four-necked, round bottomed flask, containing 200ml 0.1M NaC1 solution. The pH was adjusted to the required value and kept constant within +0.05 units by means of an automatic titrimeter. The amount of acid required to keep the pH constant was recorded as a function of time. {Acid of strength 0.05M was used). The determinations of N-terminal amino acids were carried out using Hille's {2) method. Cystine analysis The Zahn-Traumann method (3) was employed. Reduction of hair THPC reduction was effected by treating dried hair samples with a solution of THPC (30%) for 4 hr at 35øC in a water bath (4). The thioglycolate reduction was carried out by exposing lg hair to 9 w/v ammonium thioglycolate solution (10ml) at pH 9 for 15 min. In both cases the reduced hair was washed with water, and then methylated to block all the free cystine residues using Jenkins' and Wolfram's pro- cedure (4).