346 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 0.7' 0.6 0.5, 0.4, 0.2 0.1 0 a) nylon • Tress length First (0), seoond (D) andthird (o)-combing force 1.5 120 no % 100 • •, 90 80 ø• 1.( 70 } 6O 40 • 0.• 30 } 2O 10 0 b) •olyethylen• Tress length First (o) ,second (a) and third (V) combing c) teflon •ress length First (o),second (9) and third (V) combing 1.6 -5.0 Tress length First (O), second (•1) a•l third (V) e) alumin• . • Tress length First (O), s•3nd (f7) and third (•) Figure 3. Triboelectric charge distributions on untreated hair after multiple combings with combs made of various polymers or aluminum no discharging between combings. finding that comb material has no effect on both combing work and maximum combing force (10). Teflon, nylon, and polyethylene generate a high density of positive charges with a characteristic bimodal or trimodal distribution pattern. The polycarbonate comb, on the other hand, produced a high-intensity negative charge which probably indicates that piezoelectric potential (which we believe determines the sign and the magnitude of transferred charges in rubbing triboelectrification experiments (3)) does not influence the direction of electron transfer under low-stress conditions of comb-ker- atin contact in combing experiments. In general, the charge density in the tip-end and upper portions of hair tresses increases to a similar extent after multiple combings. This contrasts with the selective increase of the insertion peak in distributions produced by multiple combing of discharged tresses with a charged comb, as shown in Figure 2d.

TRIBOELECTRIC CHARGE DISTRIBUTIONS ON HAIR 347 The characteristic apportionment of transferred charges and the progressive increase in the charge density is probably related to the fact that multiple combings produce an increased number of contacts between uncharged sections of contacting surfaces. The gradual increase of potential after multiple combings can continue until equilibrium surface charge density is reached. Also, the electrical breakdown of the surrounding atmosphere might limit surface charge density to less than 7 ø 10 9 C/cm 2 (2), the value never reached after a few combings. THE EFFECT OF HAIR SURFACE MODIFICATION Adsorbed long-chain alkyl quaternary ammonium salts and cationic polymers signifi- cantly modify the electrochemical potential of the fiber surface and affect the process of electron transfer (2). Adsorbed long-chain alkyl quaternary ammonium salts cause a considerable decrease in the electrochemical potential of hair (2). In consequence of this, the probes characterized by the work function lower (poly(methyl methacrylate)) and close to keratin (polycarbonate) generate negative charges on quat-treated hair. Charging against teflon, which has a high work function value, produced positive charges on hair because the reduction of the electrochemical potential of the fiber sur- face was not sufficient to match that of the teflon surface. An opposite effect, increased electron-donating character of hair, is caused by adsorp- tion of the cationic polymer PDMPDAMC. Triboelectrification, in an experimental set- up similar to the one described earlier (1,2), revealed a consistent increase in the elec- trochemical potential of PDMPDAMC-modified hair surface using polycarbonate, alu- minum, and poly(methyl methacrylate) probes. Triboelectrification results from combing of hair treated with cationic substances are given in Figures 4a-d and Figures 5a-c. The polycarbonate comb generated trimodal distributions of high positive-charge density for PDMPDAMC-modified hair (Figure 5c) and high negative-charge density for fibers treated with the cationic surfactant (Figure 4d). Reduction of the electrochem- ical potential gap between the comb and quat-modified hair, accomplished by polyeth- ylene, nylon, or teflon combs results in a reduced overall density of positive charge on hair. Correspondingly, an increase in the electrochemical gap between the comb and PDMPDAMC-modified hair produced an enhancement in the overall charge density of the hair. This indicates that the electrochemical surface-potential gap between the rub- bing element or comb material and keratin is a decisive factor in determining the magnitude and sign of the generated charge. Apart from the overall magnitude of the charge density produced by combing, the quat and PDMPDAMC treatments exert an influence on the charge distribution profiles. In the case of quat-treated hair, the charge concentrates mainly in the upper portion of the tress. Selective increase of the intensity of the peak in the upper portion of the tress after consecutive combings is similar to that observed in combing of discharged tresses with a charged comb. On the other hand, the middle and tip-end peaks are considerably re- duced. Since the adsorbed stearalkonium chloride reduces the tip-end peak combing force by about 10-30% as compared to untreated fibers (compare combing force curves presented in Figures 3a and 4a), this is probably related to a combined effect of reduced electrochemical potential gap and friction (lower combining force should result in a decreased number of fiber-comb contacts and consequently produce less triboelectric charge). Triboelectric charge distributions, similar to those shown in Figures 4a-d,