From Solvent-Mediated Micellization to Packing in a Face-Centered Cubic Structure of Poloxamers

Ethylammonium nitrate (EAN) is one of the most studied protic ionic liquids (ILs) offering unique properties for self-assembly of poloxamers. Understanding how solvent composition in EAN-water mixtures governs block copolymer self-assembly has yet to be investigated in full detail. This study systematically investigates the temperature-driven self-assembly of Poloxamer 407 in the full composition range of EAN-water mixtures, combining small-angle X-ray scattering (SAXS), rheology, and differential scanning calorimetry (DSC) to resolve micellization and lyotropic liquid crystal (LLC) formation, also termed gelation in the literature. This work reveals a distinct nonmonotonic dependence of both micellization and LLC transitions on EAN content in the solvent, where a minimum at 60:40 wt % EAN:water ratio is observed. We identify a universal trend in the evolution of micelle volume fraction against normalized temperature independent of solvent nature and surfactant concentration. Extrapolation of this sigmoidal trend shows a critical micelle volume fraction of approximately 0.49 required for face-centered cubic (FCC) ordering, which remains invariant across all solvent environments and agrees with the entropic bonding theory of colloidal crystallization. Moreover, we resolve two temperature-dependent micellization regimes in the transition region, characterized by a decrease in micellar number density at early stages, followed by micellization and micelle growth through unimer incorporation, resulting in an increase in number density.