334 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS (a) 20 15- •' 10 • E v 5- z O- LU o uJ -10- N -2( 0 1 '0 2'0 3'0 4'0 5'0 6'0 7'0 TIME ( min ) 80 (b) 4.50 4.00- 3,50- (.3 • 3.00- v 2.50- 2.00- O 1.50- ._1 LL 1.00- 0.50- 0.00 0 [] C(1%)/A(1%) + C(1%)/A(10%) • M(1%)/A(1%) 1 ..•1-•+4'++ ++ +.4+ + ++•. 1'0 2'0 3'0 4'0 5'0 6'0 7'0 TIME ( min ) 80 (c) -- o z o o [] 0(1%0)/A(1%) + 0(1%)/A(10%) • M(1%)/A(1%) 30 20- 15- 10- 5- •!t• 0 0 1'0 2'0 3'0 4'0 5'0 6'0 7'0 80 TIME ( min ) [] C(1%)/A(1%) + C(1%)/A(10%) • M(1%)/A(1%) J Figure 12. Zeta potential (a), flow rate (b), and conductivity (c) as a function of time for the following combinations of treatments: (1) 1% Shampoo C-1% Shampoo A, (2) 1% Shampoo C-10% Shampoo A, and (3) 1% Shampoo M-l% Shampoo A.

SHAMPOO ANALYSIS 3 3 5 sorption on rinsing with the test solution. Note that the conditioning shampoo used in this experiment does not affect the permeability of the plug. CONCLUSIONS The application of streaming potential, conductivity, and permeability measurements in a dynamic mode, by using hair plugs as substrates and shampoo solutions as adsorbates, allows a detailed analysis of the sorption/desorption phenomena occurring during the process of shampooing hair. Streaming and zeta potentials were found to be sensitive to the presence of both negatively and positively charged species in the formulations, while the permeability of a fiber plug was affected by the surface deposition of conditioning layers thick enough to restrict the flow of the test solution. The conductivity data provide a measure of the rate of desorption of ionic surfactants or polymers ad(b)sorbed during the treatment stage. They were analyzed by taking into consideration two parameters: (1) the value of conductivity at the beginning of the rinsing with the test solution (one rinsing cycle after the treatment stage), and (2) the rate constant of the conductivity decrease during the rinsing stage, calculated by assuming the first-order kinetics in the range of conversions 0-80%. It should be stressed that the observed conductivity changes are related to the removal of strongly bound ionic species. Unad- sorbed or weakly bound surfactants or polymers from the treatment solution, which give rise to the plug conductivity of the order of mmho/cm, i.e., 2-3 orders of magnitude higher than the values recorded during the rinsing stage, are quickly washed off the plug (within 1-2 rinsing cycles after the treatment), unless the treatment reduces the per- meability of the plug, which may delay this process by several rinsing cycles. Nonconditioning shampoos, based on anionic surfactants, show the binding of deter- gents to hair by making it more negatively charged than untreated hair. The effect is transient, and zeta potentials return to the values characteristic for untreated hair as a result of desorption of anionic surfactant during the rinsing with the test solution. The kinetics of this process could be further monitored by the conductivity measurements, which showed relatively high values at the beginning of the rinsing cycle (8-14 }xmho/ cm for 1% treatment solutions) and rate coefficients ofdesorption in the range from 0.08 to 0.19 min- • The rates of desorption were usually higher for more concentrated shampoo solutions, and also increased with the number of consecutive treatments with the same composi- tion. The dependence of the desorption rate on concentration makes the comparisons of various commercial formulations difficult, since the quantitative content of surfactants in the investigated formulas was not known. Since it was also demonstrated that the rates of desorption of single anionic surfactants depend on their chemical structure (3), it may be further presumed that the surfactant residues of shampoos would desorb with different rates, depending on the surfactant blend used in the formulation. In addition to this, other formulation components, including nonionic or amphoteric surfactants or emulsifiers, which form mixed micelies or other colloidal structures with the primary surfactants and are largely invisible by the employed technique due to the lack of the electrical charge, may codeposit on the fiber surface and affect the process of desorption. Relationship between the shampoo composition in terms of the employed surfactant type (or other auxiliary ingredient) and the process of their deposition and desorption was not investigated in this paper.