Stabilization of Immiscible Liquids using Surfactant / Fatty Acid Mixtures that Self-Assemble into Nanostructured Interfaces

Great efforts have been made to stabilize immiscible liquid interfaces or to compartmentalize discrete domains using surfactant molecules, in most cases focusing on equilibrium morphology. Here we present work highlighting how to use surfactants to create liquid columns that are composed of an aqueous cationic surfactant, cetylpyridinium chloride (CPCl), and a fatty acid, oleic acid (OA), via the formation of a self-assembled nanostructured interfacial layer under non-equilibrium conditions. The interfacial layer or column wall exhibits particular elastic behavior such as wrinkling, buckling, or rupture. The resulting interfacial layer forms owing to rapid self-organization of CPCl and OA molecules, which is similar to established surfactant systems. We determine that the morphology of the interfacial layer formed during the flowing column experiment is lamellar, and exhibits a domain spacing on the order of 20 nm using small angle X-ray scattering (SAXS). Furthermore, we find that the viscoelastic response is one of a soft-solid with gel-like properties using interfacial rheology. The characteristics are studied as a function of CPCl solution concentration, followed by a comparison with the bulk equilibrium phases in the mapped-out ternary phase diagram (water, OA, and CPCl).