Adsorption of particles at oil–water
interfaces is the basis
of Pickering emulsions, which are common in nature and industry. For
hydrophilic anionic particles, electrostatic repulsion and the absence
of wetting inhibit spontaneous adsorption and limit the scope of materials
that can be used in emulsion-based applications. Here, we explore
how adding ions that selectively partition in the two fluid phases
changes the interfacial electric potential and drives particle adsorption.
We add oil-soluble tetrabutyl ammonium perchlorate (TBAP) to the nonpolar
phase and Ludox silica nanoparticles or silica microparticles to the
aqueous phase. We find a well-defined threshold TBAP concentration,
above which emulsions are stable for months. This threshold increases
with the particle concentration and with the oil’s dielectric
constant. Adding NaClO4 salt to water increases the threshold
and causes spontaneous particle desorption and droplet coalescence
even without agitation. The results are explained by a model based
on the Poisson–Boltzmann theory, which predicts that the perchlorate
anions (ClO4
–) migrate into the water
phase and leave behind a net positive charge in the oil. Our results
show how a large class of inorganic hydrophilic, anionic nanoparticles
can be used to stabilize emulsions in a reversible and stimulus-responsive
way, without surface modifications.