J. Cosmet. Sci., 61, 289–301 (July/August 2010) 289 Morphological study of cationic polymer–anionic surfactant complex precipitated in solution during the dilution process M. MIYAKE and Y. KAKIZAWA, Functional Materials Research Laboratories (M.M.) and Beauty Care Research Center (Y.K.), Lion Corporation, 2-1 Hirai 7-Chome, Edogawa-ku, Tokyo 132-0035, Japan. Accepted for publication March 16, 2010. Synopsis We investigated the phase diagrams and the morphology of the complexes that were formed by cationic polymers, cationic cellulose (CC) and cationic dextran (CD), and by anionic surfactant-based sodium poly(oxyethylene) lauryl ether sulfate (LES). The anionic charge of the LES-based surfactants was changed by adding an amphoteric surfactant, lauryl amidopropyl betaine acetate (LPB), or a nonionic surfactant, poly- oxyethylene stearyl ether (C18EO25). We discuss the relationship between the complex aggregation process and the morphology of the precipitated complexes. The morphologies of CC complex aggregates, which precipitated during the dilution process in a model shampoo solution, changed from membranous forms to mesh-like forms by decreasing the charges of both the CC and the surfactant. Their touch on hair in the rins- ing process changed from sticky to smooth and velvety, corresponding to their rheological properties. In contrast, CD complex aggregates had a membranous form and a smooth touch independently of the charges on the polymer and surfactant. These results suggested that the control of the charges of both the polymer and surfactant and the choice of polymer structure are important for excellent conditioning effects upon rins- ing with shampoo. INTRODUCTION Polymers and surfactants are important materials in the fi elds of cosmetics and toiletries. The variety of properties generated from the interaction of these materials determines the functions of the fi nal products. In particular, the so-called trigger system, in which dilu- tion of the solution causes complexes formed between oppositely charged polymers and surfactants to precipitate, is the basic mechanism in conditioning shampoos these days (1). Since anionic surfactants are the basic material in cleaning agents, cationic polymers represented by cationic cellulose are utilized in shampoo (2–5). The complexes coacer- vated through dilution adhere to hair, prevent hair entanglement in the rinsing process, and promote the adhesion to hair of emollients such as silicone, thereby producing a con- ditioning effect (6,7). The effi ciency of shampoo is controlled by varying polymer species and by surfactant composition, and these factors determine the touch of hair in rinsing (8,9). Address all correspondence to M. Miyake.
JOURNAL OF COSMETIC SCIENCE 290 Complex precipitation (CP) regions have been systematically studied in terms of phase diagrams in basic research on the interaction between oppositely charged polyelectrolytes and surfactants (10–12). The binding of surfactants to polymers was the main subject of these studies at surfactant concentrations lower than those used in the precipitation re- gion (13–15), and the structure of solubilized complexes in solution was also observed (16). In contrast, the complexes were solubilized due to the adsorption of surfactant mi- celles at higher surfactant concentrations in the CP region. The interactions between polymer chains and micelles increased with the increasing anionic charge of the surfac- tant (17) and with the decreasing concentration of the surfactant (18), and the shielding effect of salts was also revealed to affect the interaction (19,20). The solubilized com- plexes near the CP region were observed to grow in size probably because of the decreas- ing surfactant and salt concentrations in the solution (18). Shampoos contain anionic surfactants at high concentrations where cationic polymers are solubilized in micelles however, the change in the CP region by mixing amphoteric and nonionic surfactants and the morphology of the surfactant–polymer complexes deposited during the dilution process of the shampoo solution have not been investigated systematically. The present study aims to develop the relationship between the mixed surfactant compo- sition and the complex precipitation during the dilution process and to observe the mor- phology of the complexes precipitated by the dilution of a model shampoo solution, which contains typical cationic polymers used in shampoo, cationic cellulose and cationic dextran, and anionic surfactants with LES as the base component. In addition, the rela- tion between the morphology and rheological properties of complex aggregates on the hair surface was elucidated, and the effects of the structure of the polymer molecules and the composition of the surfactants on the morphology are discussed. MATERIALS AND METHODS MATERIALS Three kinds of cationic cellulose (CC) were obtained from Lion Chemical Co. (Leogarde® series). The degrees of cationic substitution per unit of glucose (α) were 0.38, 0.21, and 0.10, and the average molecular weights determined by light scattering were 5.3 × 105, 5.1 × 105, and 5.5 × l05, respectively. Cationic dextran (CD) was obtained by the reaction of dextran (Meito Sangyo Co., Ltd.), with a molecular weight of about 2 × l05, with gly- cidyltrimethylammonium salt in solution in the presence of NaOH as a catalyst in a ni- trogen stream, and the product was neutralized and dried. The values of the degrees of cationic substitution of the obtained CD were 0.30 and 0.38. Figure 1 shows their chem- ical structures. Anionic sodium poly(oxyethylene) lauryl ether sulfate (LES, with a mean oxyethylene chain length of 3 mol), and amphoteric lauryl amidopropyl betaine acetate (LPB) were purchased from Taiko Oil & Fat Company and Ipposha Oil Industries Co., Ltd, respec- tively. Nonionic poly(oxyethylene) stearyl ether (C18EO25) was supplied by Nihon Emul- sion Co., Ltd. All of these surfactants were used as supplied. The salt used to adjust the ionic strength of the solution was reagent grade sodium sulfate (Na2SO4) (Tokyo Kasei Ind. Co.). All experiments were performed with distilled water.
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