Effect of Adding Lecithin and Nonionic Surfactant on α-Gels Based on a Cationic Surfactant-Fatty Alcohol Mixture

α-Gels are often used as base materials for cosmetics and hair conditioners. α-Gel-based commercial products typically contain many types of additives, such as polymers, electrolytes, oily components, and other surfactants, in addition to the three basic components. However, few systematic studies have been conducted on the effect of such additives on α-gels. In this study, we chose surfactant as an example to initiate the effect of such additives on the structure and rheological properties of α-gel samples formulated using cetyl alcohol (C₁₆OH) and cetyltrimethylammonium chloride (CTAC). Optical microscopy analysis demonstrated that the size of the vesicles in the α-gel samples in this study was decreased via the addition of hydrogenated soybean lecithin (HSL) and penta(oxyethylene) cetyl ether (C₁₆EO₅), a nonionic surfactant, to them. Rheological measurements revealed that at high C₁₆OH/CTAC ratios, the viscosity and yield stress of the α-gel samples decreased owing to the addition of surfactants to them. Conversely, at low C₁₆OH/CTAC ratios, the opposite tendency was observed. Small-angle X-ray scattering analysis indicated that for the α-gel samples with high C₁₆OH/CTAC ratios, the addition of HSL or C₁₆EO₅ to them decreased the interlayer spacing of their lamellar bilayer stack, which led to the changes in the rheological properties of the α-gel samples.