SHAMPOO ANALYSIS 317 ,• ( oqtun ) ,,d. lAIJ. OrlaNOO = 6 • 6 • 6 • 6 • 6 = •,• ( ALU ) -IVlJ. N=lJ. Od VñEtZ ++ + +$+ oooo • 6 g • o • g õ o ø g ( ALU ) -IVlJ. N=lJ. Od DNII•V=IEIJ. S 6 6 6 õ • g •, ( oas/oo ) =1/VEI MO-I-I

318 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS surface. After prolonged rinsing, zeta potential increases again, reaching a value similar to that measured for the untreated hair. The changes in streaming and zeta potentials are accompanied by a relatively slow decrease in the conductivity, from a value of about 10 !xmho/cm after the treatment to about 4.5 !xmho/cm after 30 minutes rinsing with the test solution. Both streaming potential and conductivity data suggest a continuous desorption of an anionic detergent from both the bulk and the surface of hair. The rate of desorption was calculated from the conductivity curves by assuming the first-order kinetics. The calculated rate constants for several anionic surfactants are presented in Table I. They are reproducible within -10%. According to these data, sodium lauryl sulfate (SLS) is the slowest to desorb from hair, followed by ammonium lauryl sulfate and sodium laureth sulfates with one and two moles of ethoxylation. Sodium lauroyl sarcosinate, in which the ionic head is a carboxylate group, also desorbs from the fibers at a relatively high rate. The affinity of SLS to cationically modified hair was investi- gated in an experiment that included a treatment of hair with 0.5% solution of a cationic polymer, a 30-minute rinse with the test solution, a treatment with 0.5% SLS, and a final 30-minute rinse with the test solution, which allowed us to follow the rate of desorption of SLS. The calculated desorption rates of SLS from cationically modified hair are about 25% higher than those obtained for untreated hair (Table I). This may reflect the fact that a cationic polymer creates a barrier on the surface, preventing the detergent molecules from penetrating deep into the fiber (5). The obtained result may also suggest that the surfactant superficially bound to oppositely charged polymer on the fiber surface desorbs faster than that bound by the hair protein. Cationic surfactants and polymers bind to the hair surface and reverse the sign of both streaming and zeta potential from negative to positive. In the case of cationic surfac- rants, the reversal of the sign of the streaming potential is transient, and depends on the affinity of the surfactant to hair surface. For example, quats characterized by a lower water solubility (longer hydrophobic chains), which form crystalline dispersions, usually show slower rates of decline in zeta potential as a function of rinsing time and higher equilibrium values of zeta potential. High substantivity is also evident for surfactants comprising two positive charges such as Schercoquat DAS (Figure 2). The zeta and Table I Rate Coefficients of Desorption of Various Surfactants From Untreated and Cationic Polymer-Pretreated Hair Rate constants (1/min) Surfactant After first treatment After second treatment Untreated hair SLS 0.076 ALS 0.093 SLES (1 mole) 0. 104 SLES (2 mole) 0.116 Sodium lauroyl sarcosinate 0.100 Hair pretreated with a cationic polymer Merquat 100/SLS 0.097 Merquat 550/SLS 0.099 Merquat 280/SLS 0.096 0.088 0.130 0. 109 0.155 0. 127 Polymer pretreatment concentration 0.5 %, surfactant concentration 0.5%.