A Generic Single‐Step Synthesis of Monodisperse Core/Shell Colloids Based on Spontaneous Pickering Emulsification

Abstract

Monodisperse core–shell colloids are widely applied in various fields such as biomedicine, catalysis, and optics, and are also important for fundamental studies in colloid and interface science. The synthesis of particles with a well-defined core–shell morphology still comprises several separate steps such as core preparation, its subsequent functionalization with a coupling agent and the deposition of one or several shells. This multi-step approach towards composite colloids has some serious drawbacks. First, both coupling chemistry and synthetic pathways need to be specifically designed for the materials involved. Second, multi-step reactions often require intermediate purifications or separations that not only reduce the net yield but also hamper the up-scaling of composite-colloid synthesis for applications such as those referred to above. In this communication we report a novel single-step synthesis of monodisperse latex-based core–shell colloids that completely relies on self-assembly of the components involved (Fig. 1). Recently we discovered that nano-particles may induce spontaneous emulsification of oil in water to thermodynamically stable, monodisperse Pickering emulsions. This novel emulsification phenomenon, first observed for magnetite (Fe3O4) nano-particles, turns out to be quite generic since also cobalt ferrite (CoFe2O4) and commercial silica particles (Ludox) lead to stable Pickering emulsions. It was also shown that using the chemically reactive oil (methacryloxypropyltrimethoxysilane, TPM), Pickering emulsions could be polymerized by addition of a radical initiator (potassiumpersulphate, KPS) resulting in stable colloidal dispersions (Fig. 2a and b). Here we investigate the use of such particle-stabilized emulsions (hereafter TPM-emulsions) in a seeded growth polymethylmethacrylate (PMMA) polymerization, to obtain in one single and simple step, PMMA colloids with various inorganic cores. Initially we focused on TPM-emulsions merely as a source for monodisperse inorganic cores to seed a surfactant-free PMMA polymerization. However, exploring what would happen when TPM-emulsions are prepared directly in the presence of methylmethacrylate (MM) we noticed that, surprisingly, MM does not interfere at all with the TPM emulsification. In fact, during the latter emulsification, MM remains present as a separate oil phase (Fig. 2c insert). Addition of a water-soluble radical initiator, however, not only induces polymerization of the TPM-emulsion droplets to solid spheres, but simultaneously also triggers the polymerization of PMMA shells onto these spheres. Thus the floating MM phase (Fig. 2) acts as a monomer source for this PMMA shell formation and, indeed, the shell thickness continues to grow until the whole MM phase is consumed. It turns out that, in addition, polymerized TPM-emulsions are also excellent seeds dispersions for growing polystyrene shells via surfactant free emulsion polymerization, as well as silica shells via the classical Stober synthesis, showing that the spontaneous Pickering emulsification allows for the preparation of polymeric as well as inorganic composite model colloids. C O M M U N IC A TI O N