Interfacial Activity of Nonamphiphilic Particles in\nFluid–Fluid Interfaces

Abstract

Surfactants\ncan adsorb in fluid–fluid interfaces and lower\nthe interfacial tension. Like surfactants, particles with appropriate\nwettability can also adsorb in fluid–fluid interfaces. Despite\nmany studies of particle adsorption at fluid interfaces, some confusion\npersists regarding the ability of (simple, nonamphiphilic) particles\nto reduce the interfacial tension. In the present work, the interfacial\nactivity of silica nanoparticles at air–water and hexadecane–water\ninterfaces and of ethyl cellulose particles at the interface of water\nwith trimethylol­propane trimethacrylate was analyzed through\npendant drop tensiometry. Our measurements strongly suggest that the\nparticles do significantly affect the interfacial tension provided\nthat they have a strong affinity to the interface by virtue of their\nwettability and that no energy barrier to adsorption prevents them\nfrom reaching the interface. A simplistic model that does not explicitly\naccount for any particle–particle interactions is found to\nyield surprisingly good predictions for the effective interfacial\ntension in the presence of the adsorbed particles. We further propose\nthat interfacial tension measurements, when combined with information\nabout the particles’ wetting properties, can provide a convenient\nway to estimate the packing density of particles in fluid–fluid\ninterfaces. These results may help to understand and control the assembly\nof nonamphiphilic nanoparticles at fluid–fluid interfaces,\nwhich is relevant to applications ranging from surfactant-free formulations\nand food technology to oil recovery.