354 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS lOO 80- 6O 40 20 OJ I I pH 7, 60 ø I 0 20 40 60 treatment time (minutes) •0.2 % •0.5 % [] 1.0 % Percent cationic surfactant (CP-6) applied 0.5% anionic surfactant (SDS) pretreatment Figure 2. % Uptake cationic CP-6. presumably because it reacts rapidly with the large amount of SDS present on the wool. However, even in this case, further desorption occurs after 20 min. For wool pretreated with 0.5% SDS, sorption of CP-6 followed by desorption is observed with 0.2% and 0.5% applications of CP-6 but not with 1.0% CP-6 (Figure 2). The sorption/desorption phenomenon is observed only when the amount of cationic surfactant applied is not greater than the amount of anionic surfactant initially on the

CATIONIC-ANIONIC INTERACTIONS 3 5 5 wool. Presumably an anionic-cationic complex is formed on the wool which, at neutral pH, will be positively charged if the cationic is in excess and negatively charged if the anionic is in excess. At pH 7 wool is negatively charged (isoionic point approximately pH 4-5) and the cationic complex will be taken up in much the same way as CTAB, whereas the anionic complex will be less strongly bound and can desorb into the treatment liquor. The same sorption/desorption phenomenon is observed for wools treated with the an- ionic surfactant P-6 at pH 3.5 following treatment with CTAB (Figures 3,4). Again the lOO • 80 60 40 20 Ol I I pH 3.5, 60 i 0 2o 40 60 treatment time (minutes) -0--0.0 % [] 0.5 % ,•, 1.0 % -0-2.0 % Percent cationic surfactant (CTAB) pretreatment Figure 3. % Uptake anionic P-6 (1.0% applied).