Self-Assembly at Oil–Water Interfaces Driven by Solubility Differences and Polar–Hydrophobic Interactions: An Insight into a Highly Mechanical Performance Gel with Gradients

Interfacial self-assembly offers a promising route to fabricate functional materials, yet achieving robust mechanical performance remains challenging. Here, the self-assembly of nonionic surfactant polyoxyethylene monoalkyl ether (AEO-9) at oil-water interfaces was systematically investigated to form a high-strength interfacial gel. During the self-assembly process, the mechanical strength of the interfacial gel progressively increased, reaching a maximum equilibrium value of 4200 Pa after 24 h. Furthermore, significant gradients in the composition, microstructure, and micromechanical properties of the interfacial gel along the sample height were revealed by fluorescence microscopy, small-angle X-ray scattering (SAXS), and atomic force microscopy (AFM). This unique interfacial self-assembly behavior was further studied via dynamic light scattering and molecular simulations. The solubility differences and polar-hydrophobic interactions are the key factors. Directional migration of AEO-9 from the oil phase (low solubility) to the aqueous phase (high solubility), driven by solubility differences, was found to establish a transient interfacial concentration gradient. This gradient facilitated interfacial enrichment of AEO-9, which was subsequently organized into gradient lamellar liquid crystals (LC) through compositional heterogeneity and polar-hydrophobic interactions. The gradient variations in micromechanical properties were correlated with the gradient structural packing. Moreover, the effects of AEO-9 concentration, brine, temperature, and oil type on the gel's mechanical properties were examined, highlighting their roles in modulating the polar-hydrophobic balance. This study reveals the dual control of solubility-driven migration and interfacial interactions on gradient formation, establishing a framework for the design of high-performance interfacial materials.