Effect of nanoparticles on spontaneous Ouzo emulsification

Particles stabilize fluid interfaces. In particular, oil/water Pickering emulsions undergo limited coalescence, yielding droplets of smaller size as the amount of particles is increased. Herein, we studied the effect of hydrophobic nanoparticles (<10 nm, alkyl-coated) on submicronic droplets (ca 100 nm) formed in an Ouzo system. We investigated thoroughly the water/tetrahydrofuran (THF)/butylated hydroxytoluene (BHT) reference diagram, in the absence and in the presence of nanoparticles, using the Nanoparticle Tracking Analysis (NTA) technique. This allowed us to characterize the size distributions in a much finer way than what is usually obtained using conventional Dynamic Light Scattering (DLS). Both a Surfactant-Free Microemulsion (SFME, thermodynamically stable) and an Ouzo (metastable spontaneous emulsion) domains were identified and the transition from one to the other could be characterized by specific features of the droplet size distributions. We found that the presence of the nanoparticles limits coalescence in the metastable domain. We also show that the alkyl-coated nanoparticles are irreversibly attached to the liquid-liquid interface.