The Phenomenon of Pickering Stabilization: A Basic Introduction

STEFAN A. F. BON

1.1 A Brief Historic Perspective on Pickering Stabilization

The ability of solid particles to adhere to soft deformable interfaces, for example to the surface of emulsion droplets or bubbles, is currently the subject of renewed interest in material science. The phenomenon that solid particles can reside at the interface of droplets and bubbles, thereby providing them with resistance against coalescence or fusion, and (debatable) coarsening or Ostwald ripening, is known as Pickering stabilization and named after Spencer Umfreville Pickering. Food science and flotation technology show a steady stream of research over the 20th century using Pickering stabilization in, for example, table spread/margarine formulations, where fat crystals sit on the surface of water droplets dispersed into the oil matrix. Interestingly, the origins of Pickering stabilization in the area of (froth) flotation lie further back than the cited works by Pickering (1907) and Ramsden (1903). Patents by William Haynes (1860) and the Bessel brothers (1877) clearly reported the phenomenon, the latter patent interestingly illustrating the concept with graphite flakes attached to bubbles. In the area of polymer chemistry the idea of using solid particles as stabilizers for the fabrication of polymer beads by suspension polymerization was explored to some extent from the 1930s to the 1950s. A revival of the concept of using solid particles as stabilizers in heterogeneous polymerizations did not emerge until 50 years later with the development of Pickering mini-emulsion polymerization and Pickering emulsion polymerization. The idea of using Pickering stabilization as a way of assembling colloidal particles into intricate supracolloidal structures drew attention from the soft matter physics crowd initiated by the works of Velev et al. and Dinsmore and coworkers, the latter coining the term 'colloidosomes' for the semi-permeable hollow structures made by assembly of particles onto droplets. Not only does the fabrication of supracolloidal structures receive great attention, but also the underlying physics is studied and discussed widely, for example looking at why particles adhere to a liquid–liquid interface, how strong the interaction energy is, and what the interplay between particles at the droplet surface is.

1.2 A Basic Physical Understanding of Pickering Stabilization

This short introductory chapter does not aim to provide a thorough literature review of the underlying physics of Pickering stabilization, but merely to give the reader a basic understanding. The question of why a particle would prefer to sit at the interface of an emulsion droplet instead of being dispersed in either the water or oil phase has already been raised and discussed by, for example, Hildebrand and coworkers in 1923. They said that for solid particles to adhere to and be collected at the surface of emulsion droplets, the powder had to be wetted by both liquids. They stated that in general particles have a preference for one of the two liquids, which meant that the particles would reside for longer in that liquid. They described how the assembly of particles onto the oil–water interface will cause the interface to bend in the direction of the more poorly wetting liquid, thereby facilitating its emulsification into droplets. They concluded that the type of emulsion, i.e. oil-in-water or water-in-oil, could be predicted on the basis of this wettability, and thus on the basis of the contact angle of the interface with the solid.

To describe the behaviour of a single particle at the liquid–liquid interface we often see the following expression for the adhesion energy as a function of the contact angle:

ΔE = πR2σ12(1 [+ or -] cosθ12)2

Care must be taken to define the right contact angle and whether to use a plus or minus sign within the brackets to calculate the escape energy needed to remove the particle from the interface and place it into either phase 1 or 2. This can lead to confusion. An approach that circumvents this issue is to calculate the energy well completely, as reported by Pieranski in 1980. In his work, he studied the adhesion of polystyrene spheres at the air–water interface.

Imagine a thermodynamic type of experiment as shown in Figure 1.1. We take a perfectly smooth spherical particle that we disperse in a liquid phase, which we call phase 1. We ignore all dynamics (kinetics) and external force fields, such as gravitational, electrical, optical and magnetic. We also do not consider any surface roughness of the particle, and we ignore any ionic (Coulombic) interactions, dielectric effects and thus van der Waals interactions. The question we pose is what would happen to the free energy if we move the particle, which is dispersed in phase 1, all the way to phase 2, and thus through the interface?

To answer this question we need to take into account the interfacial energies upon placing the particle at various heights, z. These are the energy between the particle and phase 1, EP1, the energy between the particle and phase 2, EP2, and the energy between phase 1 and phase 2, E12. For this we need to know the interfacial tensions between the particle and phase 1, σP1, the particle and phase 2, σP2, and the interfacial tension between phase 1 and phase 2, σP1, and multiply these by the respective contact areas.

z0 = z/R; ST = 4πR2; AT = πR2

EP1 = σP1 ST (1 + z0)/2

EP2 = σP2 ST (1 - z)/2

E12 = -σ12 AT (1 - z02)

The sum of the three energies and its division by kBT leads to the following quadratic expression for the energy well (see also Figure 1.2):

E0 = EP1 + EP2 + E12/kBT = [πR2σ12/kBT] (z02 + 2(σP1 -...