Study On The Behavior Of Cationic Surfactant DDABDT Onto Human Hair Surface

The hair is an important accessory tissues of body skin. It not only has the physical functions, but also plays an vital role in beautifying the body. Quaternary ammonium surfactants are important ingredients that are frequently formulated into hair care products to modify the peoperties of hair surface. The adsorption kinetics, isotherms, association structures and wetting properties of cationic surfactants on hair surface, however, are not fully understood due to the heterogeneous nature of human hair fibers. In this work, a quaternary ammonium of surfactant, dimethylpabamidopropyl laurdimonium tosylate (DDABDT) was chosen as a probe to investigate the adsorption behavior of cationic surfactant on cuticles of scalp hair. The main results are below:1. The results reveal that the adsorption kinetics fit to a pseudo-second-order kinetic model and the adsorption isotherm fit to the Freundlich adsorption model. The adsorption isotherm can be divided into five regions. According to documentaries, the monomers of DDABDT molecules adsorb on hair surface when the surfactant concentration is below the 1/4 of its cmc value; as the concentration increases, the association structure of DDABDT molecules begins to be monolayer on the solid-liquid interface; when the equilibrium concentration of DDABDT is between 1/4cmc and 1/2cmc, it is found that the structure is mainly monolayer; when the equilibrium concentration of DDABDT is >1/2cmc, the structure of DDABDT molecules is the mixture of monolay and bi-layer; and the structure becomes bi-layer when the equilibrium concentration of DDABDT is >cmc.2. The wetting properties of hair vary the same trends with adsorption isotherm as the concentration of DDABDT increase. When the equilibrium concentration of DDABDT is below 1/4cmc, the contact angle between water and hair decrease rapidly; when the equilibrium concentration of DDABDT is between 1/4cmc and 1/2cmc, the value of contact angles become stable, but the hair is still hydrophobic; when the equilibrium concentration of DDABDT is >1/2cmc, the contact angle decrease again, and the hair become hydrophilic; when the equilibrium concentration of DDABDT is >cmc, the contact angle becomes stable again.3. The results of AFM show that there are obvious differences in topography between initial hair and hair treated with DDABDT, The thickness of surfactant film was calculated, and the results supported the presumption of bilayer structure.4. The stress of hair cross section changes a lot also, which are relative to the concentration of DDABDT. The results of AFM are the support of the presumption about association structure of DDABDT on human hair.