# Synergistic Effect and Phase Behavior of SCG-CAPB-H\({}_{2}\)O
## 4 Conclusions  
In this study, we extensively investigated the physical properties of sodium cocosyl glycinate (SCG), including critical micelle concentration (CMC), Krafft point, and critical packing parameter (CPP). The synergistic effect of the SCG/CAPB surfactant complex system was examined, and the synergistic parameters of the complex system were determined based on the critical micelle concentration under different blending ratios (X\({}_{1}\)) obtained using a surface tension meter. It was observed that the interaction between SCG and CAPB in mixed micelles and mixed monolayers is stronger than the interaction of individual surfactants alone. Moreover, the surface activity of the SCG/CAPB complex system was significantly superior to that of the SCG system, with the CMC and \(\gamma_{\text{CMC}}\) of the SCG/CAPB complex system being much lower than those of the SCG system. These findings collectively suggest that the combination of SCG and CAPB surfactants yields superior results compared to using a single surfactant.  
Furthermore, the phase behavior of the SCG/CAPB/H\({}_{2}\)O ternary system was investigated in detail. Solutions with different mass fractions of surfactants exhibited diverse phase behaviors, including micellar phase (L\({}_{1}\)), micelle-solid mixed phase (L\({}_{1}\)-S), hexagonal phase (H), and lamellar phase (L\({}_{\alpha}\)). Overall, when the mass fraction of SCG in the solution is relatively high, due to its poor solubility, undissolved surfactant monomers form hydrated crystals through hydration and remain free in the aqueous solution. Conversely, when the mass fraction of CAPB in the solution is higher, the samples tend to form lamellar or hexagonal phases. During the investigation of the mechanism of phase transition, it was discovered that the cationic head groups of CAPB exert a strong electrostatic attraction on the anionic head groups of SCG molecules, leading to a weakening in the hydrogen bonding interactions between SCG monomers. As a result, electrostatic forces replace the hydrogen bonding interactions, causing the rupture of intermolecular hydrogen bonds.  
This study enhances our understanding of surfactant complex systems involving acyl amino acid surfactants, enriches the research on the phase behavior of acyl surfactants, and lays a theoretical foundation for the widespread application of acyl amino acid surfactants.  
Figure 7: The SAXS results of (**a**) sample 9 and (**b**) sample 10; the polarized light microscopy characterization of (**c**) sample 9 and (**d**) sample 10.  
**Author Contributions:** Conceptualization, Z.Z.; methodology, Z.Z.; software, Z.Z.; validation, Z.Z. and J.Z.; formal analysis, Z.Z. and F.W.; investigation, Z.Z.; resources, L.D.; data curation, Z.Z. and W.F.; writing--original draft preparation, Z.Z.; writing--review and editing, Z.Z.; visualization, Z.Z.; supervision, H.W.; project administration, L.D.; funding acquisition, H.W. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research received no external funding.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** Data are contained within the article.  
**Conflicts of Interest:** The authors declare no conflicts of interest.