Self-Assembly of Hydration-Dependent Quasi-Spherical Mixed Micelles into Selective Mesoscale Complex Crystalline Structures

Over the past three decades, a variety of complex structures mimicking intermetallic compounds have been discovered in soft matter systems. However, a complete understanding of the mechanisms that govern the self-assembly of these complex structures in aqueous solution is still lacking. Herein, we investigate the formation of mesoscale complex crystal structures with micelle packing of nonionic amphiphilic molecules in aqueous solutions using small-angle X-ray scattering (SAXS). The SAXS measurements revealed both unary-micelle and binary-micelles liquid crystalline phases, including face-centered cubic (FCC), body-centered cubic (BCC), Frank-Kasper (FK) σ, and FK A15 and NaZn₁₃, FK C14, and FK C15 phases, respectively, which arise from the interplay of composition, temperature, and time. Quantitative SAXS analyses with Le Bail refinements and electron density reconstruction indicated that EO hydration, the interfacial curvature of micelles, and micellar packing play important roles in the formation of mesoscale complex crystalline structures during the self-assembly process of the nonionic ternary system. This study is the first demonstration of binary mesoscale complex crystalline structures with quasispherical close packing in nonionic amphiphilic aqueous solution, offering broader insights for the self-assembly mechanism of the complex crystalline structures on soft materials.