Repulsive van der Waals forces enable Pickering emulsions with non-touching colloids

Abstract

Emulsions stabilized by solid particles, called Pickering emulsions, offer promising applications in drug delivery, cosmetics, food science and the manufacturing of porous materials. This potential stems from their high stability against coalescence and 'surfactant-free' nature. Generally, Pickering emulsions require that the solid particles are wetted by both phases and as a result, the adsorption free energy is often large with respect to the thermal energy (kBT). Here we provide the first experimental proof for an alternative scenario: non-touching (effectively non-wetting), charged, particles that are completely immersed in the oil phase through a balance of charge induced attractions and repulsions caused by van der Waals forces. These particles nonetheless stabilize the emulsion. The main advantage of this novel adsorption mechanism is that these particles can easily be detached from the interface simply by adding salt. This not only makes the finding fundamentally of interest, but also enables a triggered de-emulsification and particle recovery, which is useful in fields like enhanced oil recovery, heterogeneous catalysis, and emulsion polymerization.

Tuning Colloid-Interface Interactions by Salt Partitioning

“…Very recent experiments on the same system by Elbers et al [13] revealed that the colloidal particles in fact do not penetrate the water-CHB interface, but are trapped at a finite ∼ nanometer distance [13,14], completely circumventing the irreversible wetting effects de- * j.c.everts@uu.nl Figure 1. (a) Geometry of a colloidal sphere with radius a = 1 µm and a dielectric constant c = 2.6 in cylindrical coordinates (r, z) at a distance d from an oil-water interface, with the position on the particle surface indicated by the polar angle ϑ.…”

Colloid–oil–water-interface interactions in the presence of multiple salts: charge regulation and dynamics

“…In this paper, we discussed colloid-oil-water-interface interactions and ion dynamics of PMMA colloids dispersed in a non-polar oil at an oil-water interface, in a system with up to three ionic species. We have applied a formalism that includes ion partitioning, charge regulation, and multiple ionic species to recent experiments [5], to discuss (i) how the charges on the water and oil side of the oil-water interface can change upon addition of salt, (ii) how charge inversion of interfacially trapped non-touching colloidal particles upon addition of salt to the oil phase can drive particles towards the bulk over long distances, followed by reattachment for large times, (iii) that particles that cannot invert their charge stay trapped at the interface, and (iv) that colloids in bulk can be driven closer to the interface by adding salt to the water phase. We used equilibrium and dynamical calculations to show that these phenomena stem from a subtle interplay between long-distance colloid-ion forces, middistance image forces, short-distance vdW forces, and possibly out-of-equilibrium diffusiophoretic forces.…”

“…We consider two experimental systems from Ref. [5], to which we will refer as system 1 and 2. Both systems are suspensions with sterically stabilized poly(methylmethacrylate) (PMMA) colloidal particles of radius a = 1.4 µm and dielectric constant c = 2.6 [5,32].…”

“…In this example, the concentration of multiple ionic species is used to tune biological processes. However, this scenario is not limited to living systems, as it can also be important for ionic liquids [2], batteries [3], electrolytic cells [4], and colloidal systems [5][6][7], as we show in this paper.…”

Solvo-osmotic flow in electrolytic mixtures

“…The preferential wetting of one liquid at a solid surface or the presence of a liquid-liquid interface therefore also affects the electrostatics of the mixture. It leads to a modification of colloid-colloid (Law et al 1998;Bonn et al 2009;Hertlein et al 2008;Nellen et al 2011;Samin et al 2014) as well as colloid-interface interactions (Leunissen et al 2007a,b;Elbers et al 2016;Banerjee et al 2016;Everts et al 2016). The strength of preferential solvation is measured by the Gibbs transfer energy k B T g α of an ion species α between two solvents, where k B T is the thermal energy, and |g α | ∼ 1 − 10 for aqueous mixtures of relatively polar organic solvents (Kalidas et al 2000;Marcus 2007), but can be as large as 15 in less polar solvents containing antagonistic salts (Onuki et al 2016).…”