MORPHOLOGY OF COMPLEX AGGREGATES IN SHAMPOOS 297 Icomplex, measured at varying compositions during dilution for the model shampoo solutions containing LES/LPB = 3/1 as a surfactant, CC with α = 0.38 and 0.21, and CD with α = 0.30 as the polymer. All model shampoo solutions were of until their compositions reached the highest dilution ratios in the fi gure. Their dilution produced increases in the relative intensity of the light scattered by the polymer–surfactant complex, and the formation of complex coacervates was observed when the solution composition was be- yond those at the highest dilution ratios. The increase in the relative scattered light intensity, Icomplex, means that the solubilized complexes adhered due to dilution. The Icomplex for CC before coacervation sharply increased when the cationic charge was high. The increase in the Icomplex was more abrupt for CD at α = 0.30 than for CC at α = 0.38. Table I shows the shape and size of the polymer in salt solution by SLS measurement. In fact, the shape of CC, with a rigid chain, was rod-like, and that of CD, with a bendable chain, was contracted and coil-like. In other words, CC and CD favorably assume a rod- like shape and a contracted coil-like shape in the model shampoo solution, respectively. As shown in Figure 11, in the complex adhesion by dilution, it is suggested that the coil-like complexes for CD densely gathered each other and that the rod-like complexes for CC cross-linked and essentially formed a mesh-like structure. The density of the con- tracted coil-like chain for CD caused a higher Icomplex than that of CC as well as an abrupt increase in the Icomplex, even if the cationic charge and surfactant composition were the same. On the other hand, decreasing the Icomplex in CC complexes caused the mesh-like structure of the complexes precipitated to form. CC became looser. That means that the Figure 10. Change in relative scattered light intensity of the solubilized complex with increasing dilution ratio. Surfactant: LES/LPB=3/1. Table I Shape and Radius of CC and CD in 3wt% Na2SO4 Solution α Shape Radius (nm) CC 0.38 Rod 116 0.21 Rod 122 CD 0.30 Coil 36
JOURNAL OF COSMETIC SCIENCE 298 cross-links of the rod-like complexes before coacervation were reduced by lowering the cationic charge. From these considerations, we found that the morphologies of the pre- cipitated complexes were formed by the adhesion state of the solubilized complexes before coacervation, where the membranous structure was generated from the dense complex aggregates and the mesh-like structure from the looser complex aggregates. RHEOLOGICAL PROPERTIES OF THE PRECIPITATED COMPLEXES AND THE TOUCH OF HAIR IN THE RINSING PROCESS Polymer–surfactant complexes precipitated during dilution adhere to hair and determine its texture upon rinsing. Figure 12 shows the viscoelastic parameters for the complex con- sisting of CC at α =0.38 and LES, which leads to a membranous morphology, and that consisting of CC at α = 0.21 and LES/LPB=3/1, which forms mesh-like aggregates. The former fi gure suggests the presence of a gel-like bridging structure in the precipitated com- plex because the curves for the storage modulus of elasticity, G′, and the loss modulus of elasticity, G″, crossed at around ω = 10 [rad/s] and the complex viscosity, η∗, decreased on the high frequency side. In contrast, the latter fi gure shows that η∗ was low and indepen- dent of the frequency and that G′ and G″ increased linearly, showing the fl uid-like character of the complex. It is clear that the decrease in the cross-links of the rod-like CC complex is full of fl uidity in the precipitated complexes. Figure 13 schematically shows the touch of the hair in the process of rinsing for the model shampoo solution. The precipitated complex consisting of CC at α = 0.38 and LES, which formed dense membranous aggregates, gave a sticky touch, whereas the complex consisting of CC at α = 0.21 and LES/LPB=3/1, which Figure 11. Difference between CC and CD in the coacervation process of cationic polymer and of anionic micelles in the dilution process.
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