Molecular Microheterogeneity: Structure Formation in Amine-Water Mixtures

Aqueous alkylamine mixtures reveal an unexpected form of molecular self-assembly analogous to more complex soft matter systems. Using X-ray scattering, we uncover exceptionally intense structural prepeaks, sometimes surpassing the first sharp diffraction peak, which indicate stable microheterogeneity over nanometer scales. Temperature- and concentration-dependent measurements across the single-liquid, lamellar, and two-liquid regimes demonstrate that alkylamine headgroups stabilize extended water domains through bilayer-like arrangements. This molecular architecture persists in a macroscopically homogeneous solution, a behavior in strong contrast with aqueous alcohol solutions. Molecular dynamics simulations corroborate the experimental findings, showing that amine headgroups saturate water interfaces and prevent demixing. The aqueous amines represent a new class of molecular emulsions, where bilayer-stabilized domains account for both the unusual scattering signatures and the remarkable miscibility. Beyond rationalizing the prepeak anomaly in X-ray diffraction, these results call for a general theoretical framework to understand how molecular-scale amphiphilicity can mimic soft-matter architectures.