# Synergistic Effect and Phase Behavior of SCG-CAPB-H\({}_{2}\)O
## 1 Introduction  
Surfactants are amphiphilic molecules possessing both hydrophilic and hydrophobic groups. Due to their amphiphilic nature, surfactants self-assemble into various structures in water through hydrophobic interactions, such as micelles [1; 2; 3; 4; 5] and lyotropic liquid crystals [6; 7; 8; 9; 10; 11]. Typically, phase diagrams are used to describe the different self-assembled structures in surfactant systems. Lyotropic liquid crystals, as important assemblies of amphiphilic surfactants, have attracted extensive research interest among scholars. In aqueous surfactant solutions, with increasing surfactant concentration, various lyotropic liquid crystal phases can form, including hexagonal phase (H), lamellar phase (L\(\infty\)), and cubic phase (V) [6; 12; 13; 14; 15; 16]. For surfactant mixtures, the system may exhibit more complex self-assembly behaviors [17; 18; 19; 20], and the microstructures may vary with different compositions. Understanding phase behavior is crucial for determining cleaning product formulations [21; 22; 23]. By understanding the phase behavior of different component mixtures, one can predict and identify surfactant concentrations, thereby forming stable and semi-transparent solutions containing various surfactants. Consequently, lyotropic liquid crystal phases formed by surfactant mixtures have been extensively studied.  
Sodium cocoyl glycinate (SCG) [24; 25; 26] is an amino acid-based surfactant known for its mildness, low irritation, biocompatibility, and renewable raw materials. It also exhibits a low critical micelle concentration, requiring only a small amount to significantly reduce the surface tension of water [27; 28; 29; 30]. Currently, it finds wide application in personal cleansing products such as shampoos and body washes. [PERSON] et al. [31] investigated the application of sodium cocoyl glycinate in low-rank coal flotation processes, where its excellent surfaceproperties shine. However, due to its relatively high cost and poor solubility in water, sodium cocoyl glycinate is seldom used alone in practical applications. Moreover, single surfactants often fail to meet the required performance criteria. Therefore, sodium cocoyl glycinate is typically used in conjunction with cationic surfactants or amphoteric surfactants to achieve the desired performance.  
Cocamidopropyl betaine (CAPB) [32; 33; 34] is also a commonly used amphoteric surfactant in personal care products. Due to its mildness and cost-effectiveness, it is often combined with other surfactants to achieve optimal performance in personal care formulations.  
The present study extensively investigates the physical properties of SCG, laying a theoretical foundation for subsequent studies on complex formulations. Specifically, the SCG/CAPB complex system was selected for detailed investigation, focusing on the synergistic effects within this system. Surface tension measurements were conducted to determine various synergistic parameters of the system. Additionally, the phase behavior of the SCG/CAPB/H2O ternary system was studied. Polarized optical microscopy (POM) and small-angle X-ray scattering (SAXS) were employed to identify the types of phases present. Viscosity measurements were analyzed to understand the relationship between viscosity and liquid crystal phases. Furthermore, based on the results of small-angle X-ray scattering, the phase transition mechanisms between different phases were further explored.  
Despite the extensive use of sodium cocoyl glycinate (SCG) in daily chemical and petroleum industries [25; 31; 35; 36], there has been no detailed report on the synergistic effects and phase behavior of the SCG/CAPB/H2O ternary system to date. This study holds significant implications for detergent formulation design, serving as a valuable reference for subsequent formulation designs in the commercial sector. Additionally, it enriches the understanding of phase behavior of amino acid-based surfactants, contributing to the broader research landscape in this field.