Interfacial Activity of Nonamphiphilic Particles in Fluid-Fluid Interfaces

Surfactants can adsorb in fluid-fluid interfaces and lower the interfacial tension. Like surfactants, particles with appropriate wettability can also adsorb in fluid-fluid interfaces. Despite many studies of particle adsorption at fluid interfaces, some confusion persists regarding the ability of (simple, nonamphiphilic) particles to reduce the interfacial tension. In the present work, the interfacial activity of silica nanoparticles at air-water and hexadecane-water interfaces and of ethyl cellulose particles at the interface of water with trimethylolpropane trimethacrylate was analyzed through pendant drop tensiometry. Our measurements strongly suggest that the particles do significantly affect the interfacial tension provided that they have a strong affinity to the interface by virtue of their wettability and that no energy barrier to adsorption prevents them from reaching the interface. A simplistic model that does not explicitly account for any particle-particle interactions is found to yield surprisingly good predictions for the effective interfacial tension in the presence of the adsorbed particles. We further propose that interfacial tension measurements, when combined with information about the particles' wetting properties, can provide a convenient way to estimate the packing density of particles in fluid-fluid interfaces. These results may help to understand and control the assembly of nonamphiphilic nanoparticles at fluid-fluid interfaces, which is relevant to applications ranging from surfactant-free formulations and food technology to oil recovery.