Adsorption of Anionic Nanoparticles at Oil-water Interfaces Driven by Ion Partitioning*

Adsorption of particles at oil-water interfaces is the basis of Pickering emulsions, which are common in nature and industry. For anionic particles, however, the negative potential at the water-oil interface inhibits spontaneous adsorption, which limits the scope of useful materials. Here we address this problem by adding ions that selectively partition in the two phases, thereby changing the interfacial potential and driving anionic particle adsorption. We add oil-soluble tetrabutyl ammonium perchlorate (TBAP) to the nonpolar phase and Ludox silica nanoparticles to the aqueous phase. We find a threshold TBAP concentration, above which emulsions are stable for months. This threshold increases with particle concentration and with the oil’s dielectric constant. Adding salt to the water raises the threshold and causes spontaneous coalescence. The results are consistent with a model based on Poisson-Boltzmann theory, which predicts that TBAP anions (ClO₄^-) migrate into the water phase and leave behind a net positive charge in the oil. Our results clarify the role of interfacial electrostatics and show how a large class of inorganic anionic nanoparticles can be used to stabilize emulsions.