Giant Pickering droplets: effect of nanoparticle size and morphology on stability

The interaction between a pair of millimeter-sized nanoparticle-stabilized n-dodecane droplets was analyzed by high-speed video camera. The droplets were grown in the presence of either poly(glycerol monomethacrylate)-poly(benzyl methacrylate) (PGMA-PBzMA) diblock copolymer spheres or poly(glycerol monomethacrylate)-poly(2-hydroxypropyl methacrylate)-poly(benzyl methacrylate) (PGMA-PHPMA-PBzMA) triblock copolymer worms prepared by polymerization-induced self-assembly (PISA). The effect of nanoparticle morphology on droplet coalescence was analyzed by comparing 22 nm spheres to highly anisotropic worms with a mean worm width of 26 nm and comparable particle contact angle. Both morphologies lowered the interfacial tension, providing direct evidence for nanoparticle adsorption at the oil-water interface. At 0.03 % w/v copolymer, at least 90 seconds was required to stabilize the n-dodecane droplets in the presence of the worms, whereas no ageing was required to produce stable droplets when using the spheres, suggesting faster diffusion of the latter to the surface of the droplets. The enhanced stability of the sphere-coated droplets is consistent with the higher capillary pressure in this system as the almost planar interfaces approach. However, the more strongly adsorbing worms ultimately also confer stability. At lower copolymer concentrations (≤ 0.01% w/v) worm adsorption promoted droplet stability, whereas the spheres were unable to stabilize droplets even after longer ageing times. The effect of mean sphere diameter on droplet stability was also assessed while maintaining an approximately constant particle contact angle. Small spheres of either 22 nm or 41 nm stabilized n-dodecane droplets, whereas larger spheres of either 60 or 91 nm were unable to prevent coalescence when the two droplets were brought into contact. These observations are consistent with the greater capillary pressure stabilizing the oil-water interfaces coated with the smaller spheres. Addition of an oil-soluble polymeric diisocyanate cross-linker to either the 60 nm or the 91 nm spheres produced highly stable colloidosomes, thus confirming adsorption of these nanoparticles.