Can Janus particles give thermodynamically stable Pickering emulsions?

Abstract

Emulsions stabilised by solid particles with homogeneous surfaces are thought to be kinetically rather than thermodynamically stable. In the absence of effects due to negative line tension in the contact line around adsorbed particles, the free energy of forming an emulsion droplet coated with a monolayer of adsorbed spherical particles is expected always to be positive, and is dominated by the presence of a fraction of uncovered oil–water interface on the droplet between the particles. It is believed however that particles with well-defined surface areas of different wettability (Janus particles) can be adsorbed considerably more strongly than particles of homogeneous wettability. Here we explore how the greater magnitude of the adsorption free energy of Janus particles can affect the free energy of emulsion formation and show, using a simple approach, that negative free energy changes appear possible. We take into account lateral interactions between adsorbed particles at droplet surfaces arising from electrical, hydration and van der Waals forces.