# Phase Behavior and Rheological Properties of AES/CAPB/H\({}_{2}\)O Ternary System
###### Abstract
Although most surfactant mixtures exhibit viscous characteristics when applied (G\({}^{\prime}\) \(<\) G\({}^{\prime\prime}\)), the fluid will form a gel-like state and exhibit elastic characteristics rather than viscous characteristics (G\({}^{\prime}\) \(>\) G\({}^{\prime\prime}\)) when there are strong interactions between surfactant molecules similar to cross-linking. Thus, the phase behavior and rheological properties of surfactant mixtures may differ for different types of surfactants or compositions. Different from LAS (anionic surfactant) and poly(vinyl alcohol) (non-ionic surfactant), cocamidopropyl betaine (CAPB), made from dimethylaminopropylamine and coconut oil fatty acids (Figure 1), is a zwitterionic surfactant [18]. For example, TEGO\({}^{\circledR}\) BETAIN F50, a concentrated cocamidopropyl betaine (active concentration up to about 38%) produced by Evonik, is often used in conjunction with various surfactants to make personal care products such as shampoo and shower gel [19]. As one of the most important zwitterionic surfactants, TEGO\({}^{\circledR}\) BETAIN F50 has been widely used in cleaning products [20] and extensively studied in various surfactant systems [21; 22]. The AES/CAPB system is one of the research hot spots in cleaning products, because both AES and CAPB are safe for use as cleaning products (including cosmetics) in their safety assessments and undergo rapid primary and ultimate biodegradation in the natural environment [23; 24]. Compared with AES/Ethanol/H\({}_{2}\)O [10], AES/LAS/H\({}_{2}\)O [12], and AES/poly(vinyl alcohol)/H\({}_{2}\)O [13] systems, the anionic surfactant AES may interact more strongly with CAPB than with ethanol, LAS, or poly(vinyl alcohol) due to the cationic chemical structure (quaternary ammonium) in CAPB. However, as far as we know, although there have been some studies on the solubility limits of fatty acids [25] in the AES/CAPB systems, the synergistic growth of giant wormlike micelles [26] and the foam properties of the AES/CAPB systems [27], no report has been published on the phase behavior and rheological properties of the AES/CAPB/H\({}_{2}\)O quasi-ternary system (For the simplicity of presentation, it will be referred to as the AES/CAPB/H\({}_{2}\)O ternary system below). In this work, the AES/CAPB/H\({}_{2}\)O ternary system is a typical normal (oil-in-water) system because the commercial materials AES and CAPB used in this system are both aqueous solutions, and the samples were prepared according to the weight percentage of each point in the AES/CAPB/H\({}_{2}\)O ternary phase diagram. The phase of each sample was preliminarily determined using polarizing optical microscopy (POM) and then identifiedusing small angle X-ray scattering (SAXS). Finally, the identified phases were used to construct the phase diagram of the AES/CAPB/H\({}_{2}\)O system. In addition, the viscosity distribution of the AES/CAPB/H\({}_{2}\)O ternary system was investigated by a viscometer, and rheological properties were also studied with a rheometer. This work not only enriches the phase diagram of surfactant systems, but also has important guiding significance for the design and development of cleaning products.