Deformation of the water/oil interface during the adsorption of sterically stabilized particles.

Abstract

The adsorption of sterically stabilized colloids at water/oil (w/o) interfaces is studied experimentally by the formation of Pickering emulsions. Specifically, the effect of the steric stabilizer with respect to the rate of particle adsorption is investigated. Uniform, micrometer-sized poly(methyl methacrylate) (pMMA) particles, which are sterically stabilized with poly(isobutylene) (pIB), are used. The pIB concentration on the particle surface (ΓPIB) is controlled during the synthesis by adjusting the pIB/monomer ratio. Pickering emulsions are formed directly by the addition of water to the nonaqueous pMMA dispersions and subsequent emulsification. A strong dependence of the rate of particle adsorption on ΓPIB is found. The rate constant k for particle adsorption decays exponentially with ΓPIB, which suggest the use of a Boltzmann factor to model the experimentally found rate constants. The experimental results can be explained when the activation barrier for particle adsorption EA is of the same order as the average kinetic energy EK of a particle colliding with an emulsion droplet, which is equivalent to 10(5) kBT. Interestingly, this makes EA several orders of magnitude greater than the steric interaction with another particle. A possible mechanism that can lead to such a significant repulsive force is the inhibited drainage of solvent between the particle and o/w interface. Deformation of the o/w interface then occurs, when the solvent does not have time to drain, which results in a dramatic increase in the interfacial energy. This study identified the relevance of drainage in the formation of Pickering emulsions.