190 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS of DMSA, a tress combing test favored the neutral pH for easier combing although the decision was not unanimous as expected from friction values. FRICTION MINIMA Friction values as determined correlate well with combing results on similarly treated tresses and both tests support the existence of friction minima for certain surfactants as concentration is changed. Accepting this, an important question is why higher solution concentrations and consequently higher fiber surface concentrations cause friction to increase rather than decrease. A difference in arrangement of surfactant molecules at the fiber surface was considered in the context of micelies absorbing at high concentrations. The critical micelle concentration (CMC) of CTAB is 0.03% in pure water and is reduced to about 0.02% in salt solutions. Above these concentrations in Figure 2, friction values appear high. Steigman, et al. (34) discuss change of CTAB CMC by salts. At 86øF and 0.03% CTAB, the CMC is exceeded in KBr solution and not yet attained in NH4Br solution of the same molarity. At 0.1% CTAB, the CMC is far exceeded with either of these salts but is not yet attained with tetraethylammonium bromide (TEAB). Friction was measured for CTAB under these conditions for the data in Table XI. Apparently, the presence or absence of micelles has no appreciable effect on friction. Table XI Tests for Miceliar Effects t F.Co % CTAB KBr NH4Br TEAB 0 .51 .55 .47 0.03 .31 .28 -- 0.04 .27 .31 -- 0.10 .29 .29 .29 •DM, BI./H, Rubber in 0.39 M salt solution at 86øF. Schwartz and Knowles (24) state that the surfactants of their experiments acted at best as boundary rather than hydrodynamic lubricants. However, an analogy of friction minima in our experiments and in other research is obvious. Roder (21) for example found that with increments of oil, friction decreases, passes through a minimum and then increases as more is applied. This appears to be a widely accepted general observation (8) and the friction increase is explained in terms of hydrodynamic lubrication. However, with cationic surfactants, as concentration increases, formation of the multilayers associated with hydrodynamic lubrication is disputed (6). With no tenable explanation for why more cationic causes higher friction, an interesting analogy to the friction minima was recollected from earlier static charge experiments (33). The method (36) employs an oscilloscope with camera for recording charge impulses from a hard rubber comb during strokes of a treated tress suspended in a low humidity cabinet. With cationic treatments the impulses can be reduced to negligible levels on the scope, i.e. static charge build-up is reduced. With additional cationic on the tress, the impulses again rise but with altered pattern. The hair fibers

EFFECTS OF SURFACTANTS ON HAIR FRICTION 191 evidently change from electronegative to neutral to electropositive, relative to the hard rubber surfaces, depending on the amount of cationic present. Friction reduction is in large part due to non-polar long chains but, as for sorptivity and static, the polar ends of sorbed molecules may also play a direct role. RINSE TIME Fibers were measured in 0.1% SAAC solution at pH 3.6 and then remeasured in water as a function of time. Results depicted in Figure 3 indicate that excess surfactant is removed with water to minimize friction and further removal is extremely slow. A 0.5 0.4 0.2 0.1 • MEASURED IN O. I ø/o SOLUTION "'Oo0 • o o o 0 I I I I I ' 55 I I I 50 I00 150 200 •0 1200 MINUTES IN I•ATER Figure 3. Single fiber measurements in 0.1% stearalkonium chloride (SAAC) at pH 3.6, followed by measurements in water. portion of surfactant must remain on the hair surface despite prolonged soaking with perhaps some replenishment by sub-surface surfactant. The experiment was repeated with similar results for DMSA at pH 3.6. IMMERSION TIME IN SOLUTION Fibers immersed in cationic solutions were measured after a few minutes and at 20 hours, for the data in Table XII. Each solution was buffered at pH 3.6 with acetate. Frictional changes for most cationics appear small indicating that immersion time is a minor factor in friction determinations. This was anticipated since friction is primarily a surface property. Sorption of surfactants reaches equilibrium in a short time at fiber