Effect of surfactant tail length and ionic strength on the interfacial properties of nanoparticle-surfactant complexes

Mixed nanoparticle-surfactant systems are effective foam stabilizing agents, but the lack of colloidal stability of the bulk dispersions makes interfacial characterization challenging. This study investigates the adsorption of C_nTAB/SiO₂ complexes at air/water interfaces through surface tension and interfacial rheology measurements. The effects of surfactant tail length, ionic strength, and interfacial processing on the surface properties are measured utilizing a bulk reservoir exchange methodology to avoid bulk destabilization. The surfactant structure controls the surface tension of the system, but has minimal impact on particle surface coverage or interfacial mechanics. Once adsorbed, nanoparticles remain pinned at the surface, while the surfactant is able to desorb upon bulk exchange with deionized water. Particle packing on the interface governs the interfacial mechanics, which can be modified by increasing the ionic strength of the bulk solution. Fully rigid interfaces can be generated at low particle coverages by controlling the ionic strength and interfacial processing. These findings contribute to the understanding of mixed particle-surfactant systems and inform formulation and process design to achieve the desired interfacial mechanical properties.