THE BEHAVIOUR OF HAIR AT LOW pH VALUES 645 Measurement of the work to 20% stretch and hysteresis indices The Instron tensometer was used for these experiments, suitably set to measure the work to 20% stretch index (120) of single hair fibres im- mersed in water. First the extension curves of single hairs were measured after 2 hr soaking in water. These hairs were then allowed to relax in water for 30 hr and soaked overnight in hydrochloric acid solutions of pH 2.0, pH 1.t3 and pH 1.2 respectively. Afterwards the hairs were washed free of acid and their 120 again measured. The hysteresis index H20, was cal- culated from the difference between the stretching work and the work recovered during the relaxation of the fibre. RESULTS 2O 0.5 f/ i I I ii i/ pH 2.2 pH 2.4 0 50 I00 150 I000 TIME {rains.} pH !-4 pH 1.6 Figure Curves representing the acid uptake of hair as a function of time First we investigated the kinetics of the acid uptake using the auto- titrimeter. In a three necked flask the hair was immersed in a solution of fixed pH and the acid required for maintaining the pH was measured as a function of time. The curves (Fig. 1) revealed that, down to approximately pH 2, the uptake occurs in a single step, reaching equilibrium in about 15

646 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS min. However, when the pH is lowered below 2, a second step occurs after about 110-120 min resulting in very much higher acid uptakes. The exact time of occurrence of the second step varied between 100-130 min after the initial exposure, possibly depending on the thickness of the hair. In Fig. 2 the final acid uptake (i.e. after 7 hr) is plotted against the pH. The curve of Fig. 2 indicates clearly that in the region of pH 2 sudden 8'5 x 8.0 I• ' 0-5 o I 2 3 4 pH Figure 2 Acid uptake of virgin hair after seven hours' exposure to acid at 25øC changes occur in the hair structure, causing a very large increase in acid uptake. We checked the reproducibility of the second step very carefully (Table I), and found that this observation could not be an artefact. Table I Acid uptake of hair Run Final uptake (moles HC1/g hair) I 8.5x 10-3 2 8.8x lO -3 3 7.9x 10 -3 In a further series of experiments, hair samples were exposed to N/10 HC1 for 12 hr at room temperature, brought back to pH 7, and washed