Pickering emulsions: what are the main parameters determining the emulsion type and interfacial properties?

We synthesized surface-active lipophilic core-hydrophilic shell latex particles, and we probed their efficiency as emulsion stabilizers. The relative weight percentage of the shell, RS/P, was varied to trigger the balance between lipophilicity and hydrophilicity of the particles. Particle wettability could concomitantly be tuned by the pH of the aqueous phase determining the surface charge. Emulsions covering a wide range of RS/P and pH values were fabricated, and their type, oil-in-water (O/W) or water-in-oil (W/O), and kinetic stability were systematically assessed. By adapting the particle gel trapping technique to pH-variable systems and by exploiting the limited coalescence process, we were able to determine the proportion of oil/water interfacial area, C, covered by the particles as well as their contact angle, θ. All of these data were gathered into a single generic diagram showing good correlation between the emulsion type and the particle contact angle (O/W for θ < 90° and W/O for θ > 90°) in agreement with the empirical Finkle rule. Interestingly, no stable emulsion could be obtained when the wettability was nearly balanced and a "bipolar"-like behavior was observed, with the particles adopting two different contact angles whose average value was close to 90°. For particles such that θ < 90°, O/W emulsions were obtained, and, depending on the pH of the continuous phase, the same type of particles and the same emulsification process led to emulsions characterized either by large drops densely covered by the particles or by small droplets that were weakly covered. The two metastable states were also accessible to emulsions stabilized by particles of variable origins and morphologies, thus proving the generality of our findings.