QUATERNARY SURFACTANTS ON HAIR 143 Figure $. pH 95 o 5 I0 20 30 40 (M•NUTES) I/2 Effect of pH on sorption of DTAB. Hair, lg. Buffer at pH 3.6, citrate at 6.5, acetate at pH 9.5, carbonate Table II Equilibrium Sorption of DTAB and CTAB (24 hours, pH 9.4) Micromoles % Solution Sorbed % Sorbed Cohen. DTAB 150 69.9 0. 029 CTAB 160 80.7 0. 020 Table III Effect of Change in Amount of Hair Grams Hair • Solution pH Mg Sorbed Per G Concn. % Depiction 1 3.6 13.5 0.086 18 2 3.6 10.4 0. 067 [/2 1 6.9 35.7 0. 054 48 2 6.9 21.0 0. 035 64 Table IV Effect of pH on CTAB Sorption in the Acidic Range Mg Sorbed/G of Hair Final pH 1 Min 24 Hr CTAB Analysis at 24 Hr % Conch. % Depletion 3.7 O.46 7.6 4.7 O.86 15.0 5.7 1.38 18.5 6.4 2.28 21.0 0.078 22 0.054 44 0.042 56 0. 035 64

144 Figure 4. JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS • 2GM // õ •2 GM 5 tO 20 30 40 Effect of hair amount. Buffer at pH 3.6, citrate at 6.9, acetate tion. Results to be reported separately will show, however, that certain cationics of higher molecular weight than CTAB do penetrate more slowly, particularly at acidic pH. Effect of Change in Amount of Hair In the preceding experiments, approximately 1 g of hair was used for 60 ml of cationic solution. For experiments in the acidic range only, the use of a 2:60 ratio was investigated as a means of effecting larger concentration changes and thereby improving precision. The effect of this change on the sorption curves for CTAB at low and at neutral pH is shown in Fig. 4. On a "mg sorbed per gram of hair" basis, sorption on 2 g of hair is appreciably less than on 1 g. This fact is more evident from the data in Table III taken from the sorption curves at 24 hours. The sorption is less for the "2-g" experiments because the depletion of cationic from solution is greater and the curves tend toward lower equi- librium sorption. Effect of pH Change in the Acidic Range The pH range between 3.6 and neutral is of major interest from a formulation standpoint and it includes the isoelectric region of hair. Increasing the pH in this range causes the hair to change in volume (33, 35) and to shift from a positively-charged species to a noncharged species to a negatively-charged species relative to the liquid phase. It seemed desirable, therefore, to determine whether any abrupt change in sorption occurs in this range.