200 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS complex. In this case, charge decays are very fast, of the order of 10 • sec, and t•/2 is constant with •. However, generation of higher surface charge densities results in electrical breakdowns which seem to affect the kinetics of discharge. That is probably why an increase in t•/2 was observed after the first electrical breakdown occurred (between the 7th and 8th contact, Figure 5). It is noteworthy that very fast surface charge decays are again non-exponential. This was observed in the case of the fibers modified with cationic polymers and surfactants, when a high density of ionic species was introduced on the fiber surface (Figure 8, Table II). Table II Half-Times and First-Order Rate Constants for the Charge Decays from Fibers Treated With Various Long Chain Alkyl Quaternary Ammonium Salts Surfactant T1/2 K (min) Decyltrimethyl ammonium bromide 11.7 Dodecyltrimethyl ammonium chloride 8.9 Tetradecyltrimethyl ammonium bromide 4.4 Hexadecyltrimethyl ammonium bromide 1.4 Octadecyltrimethyl ammonium iodide 1.2 (min- •) 0.059 0.078 0.157 0.495 Nonexponential EFFECT OF ADSORBED LONG CHAIN ALKYL QUATERNARY AMMONIUM SALTS Long chain alkyl quaternary ammonium salts (quats) are readily adsorbed and retained on the surface of keratin fibers due to coulombic and hydrophobic interactions (30,31). Adsorption studies of cationic detergents on keratin as well as on other anionic or amphoteric surfaces indicate that surfactants may form well-defined monolayers (ver- tically or horizontally oriented in relation to the surface, depending on the concentration of the treatment solutions) or bilayers on the surface. Some experimental data obtained for human hair suggest that the cationic surfactant can penetrate into the cortex of the fiber. As a result of surfactant adsorption, reductions of wet and dry combing forces are usually observed. Long chain alkyl quaternary ammonium salts are also known to impart antistatic properties to hair (21,32), wool, and many other natural or synthetic textiles. The mechanism of static charge elimination is not obvious. It might be con- nected with either increased surface conductivity (21) or with a reduction of the work function gap between the hydrocarbon-modified surface and hydrocarbon comb mate- rials (hard rubbers, polyethylene, polypropylene, etc.). We have found that adsorbed long chain alkyl quaternary ammonium salts cause a considerable decrease of the electrochemical potential of the fiber surface (by increasing the value of the effective work function). The diagram in Scheme 2 illustrates quali- tatively the relative changes in electrochemical potentials of contacting surfaces for hexadecyltrimethyl ammonium chloride treated fibers as compared to untreated hair. This diagram was constructed based on the following observations. Both modes of rubbing (root-to-tip and tip-to-root) of quat-treated fibers by the probes characterized by the work function lower (PMMA) and close to keratin (PC) result in a high density of negative charges on hair. In the case of PC, the directional triboelectric effect observed for untreated fibers disappears (Figure 6a). Rubbing against teflon, © which is characterized by a very high work function value (Table I), in the direction from

TRIBOELECTRIC CHARGING OF HAIR 201 Electrochemical Potential Untreated Keratin PC Root to Tip PC Tip to Root Stainless Stainless Steel Skeel Teflon Hexadecyl ¾rimethyl Ammonium Chloride Treated Fibers Teflon to Tip •T•p to Roo Scheme 2. Relative changes in electrochemical potential of hexadecyltrimethyl ammonium chloride treated fibers as compared to untreated hair. root to tip produces a high density of positive charges (Figure 6b). Apparently, the reduction of the electrochemical potential of the fiber surface is not sufficient to match the one of the teflon © surface. For rubbing in the direction from tip to root, the electrochemical potential gap between contacting surfaces is smaller and some reduction of transferred charge was observed for hexadecyl and decyl derivatives (Figure 6b). This could be the result of mass transfer caused by increased roughness of the keratin fiber surface (rubbing against the cuticles). The ability of various long chain alkyl quaternary ammonium salts to modify the electrochemical potential of the fiber surface is best demonstrated by rubbing against stainless steel (Figure 6c). Although the value of the work function for stainless steel is not known, it can be inferred from the charging data (Figure 3) that it lies somewhere between the values characteristic for teflon © and polycarbonate. The data shown in Figure 6c indicate that the keratin fibers modified with short chain alkyl quats (decyl, dodecyl) exhibit weaker electron acceptor properties than the ones with adsorbed long chain alkyl analogues (octadecyl, stearalkonium). There may be two explanations of this result: (1) Van der Waals interactions between the adsorbate and the surface increase with the number of methylene units in the hydrocarbon chain. Hence, surface density of longer chain alkyl quats might be higher, producing a larger number of the surface acceptor states and (2) if a complete monolayer or bilayer of surfactant is formed on the fiber surface, it should be thicker for longer chain alkyl quats and consequently provide a more efficient barrier against the electron transfer from keratin to the stainless steel probe. A similar effect of quat chain length was found for a homologous series of alkyldimethylpropylmethacrylamide ammonium halides which is discussed in a separate paper (33). The deposition of cationic surfactants considerably increases the fiber conductivity. Table II shows the half times and first-order rate constants for the charge decays from