356 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS lOO 80 60 40 20 0 I pH 3.5, 60ø I 0 2o 4o 6o treatment time (minutes) -•-0.5 % '•' 1.0 % •2.0 % Percent anionic surfactant (P-6) applied 1.0% cationic surfactant (CTAB) pretreatment Figure 4. % Uptake anionic P-6. effect is only observed where the amount of anionic surfactant applied is not greater than the amount of cationic surfactant already present on the wool. Hence for desorption the complex probably has a net cationic character. In a further experiment wool was pretreated with 1% P-6 and then with various amounts of CTAB at pH 7, 60øC (see Figure 5). Desorption of anionic surfactant occurs at pH 7, but the rate of desorption is increased by low concentrations of CTAB (0.5 or 1.0%).

CATIONIC-ANIONIC INTERACTIONS 357 3O 25 2O 15 10 5 O I I I pH 7, 60 ø t=5 t--10 t=20 t=40 t=60 treatment time (minutes) 0%o CTAB • 0.5% CTAB • 1.0% CTAB 1.5% CTAB • 2.0% CTAB • 5.0%o CTAB Percent cationic surfactant treatment Figure 5. % Extraction anionic P-6 (1.0% applied). This result is consistent with the formation of an anionic-cationic complex containing excess anionic surfactant that desorbs from the fiber at pH 7. At higher CTAB con- centrations, lower levels of desorption are observed, consistent with formation of a complex of greater cationic character that retains an affinity for the fiber at pH 7. The desorption of the anionic complex during the cationic surfactant treatments at pH 7 and of the cationic complex during the anionic surfactant treatments at pH 3.5 may be simply a function of the pH of the liquors. The effect of variation of pH on the sorption/desorption has not been studied. However, the sensitivity to pH would not be anticipated from the study of desorption of anionic surfactants from wool, which was shown to occur only slowly at pH 7 (9). The sensitivity of the treatment of wool with cationic surfactants to the previous sorption of anionic surfactants suggests that the conditioning behavior of hair may depend on the sorption of anionic surfactants during shampooing. Robbins et al. (4, 12) noted the increased sorption of the cationic surfactant decyltri- methylammonium bromide by hair if the hair had been pretreated with sodium decyl- sulphate. They also observed the sorption/desorption phenomenon noted here, but only when the application solutions were diluted. The different results are probably due to the chain length and amount of surfactants applied and the pH of treatment (pH 3.6) with the cationic surfactant rather than to the differences between wool and hair. Finkelstein and Laden (3) found that sorption followed by desorption occurred for treatments of hair with 2.5 X 10-4 M solution of octadecyltrimethylammonium bro- mide but not with 2.5 x 10-3 M solution. Desorption was not observed with the dodecyltrimethyl analogue. The desorption behavior was therefore considered to be related to the much better conditioning performance found with the octadecyl deriva-