Pickering Emulsions - Particles as Surfactants

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Original entry by Hyerim Hwang, AP 225, Fall 2011.

Reference

Shin-Hyun Kim, Gi-Ra Yi, Kyu Han Kim, and Seung-Man Yang, "Photocurable Pickering Emulsion for Colloidal Particles with Structural Complexity", Langmuir 2008 24, 2365-2371

Keywords

Oil-in-water emulsions, Dispersion polymerization, Patchy particles, Drug discovery, Hard sphere, Latex particles

Introduction

Figure 1. Schematic for Fabricating Patchy Particles

This research demonstrates the preparation of anisotropic colloidal particles with well-coordinated patches using particle-stabilized oil-in-water emulsions. In particle-stabilized Pickering emulsions or bubbles, the colloidal particles are adsorbed on the liquid-liquid or liquid-gas interface to reduce the surface energy. In particular, when the particles are bound to the interface strongly, only tangential movement is allowed with a constant contact angle which determines the position of the contact line. Furthermore, in this study, structure can be fixed by UV irradiation in a few seconds because the emulsion drop is photocurable and stabilized with adsorbed particles without any consolidating additives.

Results

Here, the authors examined the structures of colloidal clusters, which were partially covered with photocurable resin of ethoxylated trimethylolpropane triacrylate (ETPTA), and their various derivatives of different morphologies. In doing this, ETPTA resin was emulsified first into an aqueous medium which contained colloidal PS particles. Colloidal particles were adsorbed spontaneously on the emulsion interface to reduce the total interfacial energy, forming so-called Pickering emulsions. Finally, the structures of the particle-stabilized Pickering emulsion droplets were captured by UV exposure. A schematic of UV-induced fabrication of composite patchy particles is illustrated in Figure 1.

Figure 2. Patchy clusters and spherical assembly of PS particles with sulfate groups

Typical SEM images of patchy clusters of colloidal PS particles are shown in Figure 2. Since the surface property of the PS particles was uniform, the contact angle was constant and close to 145˚. ETPTA phase was nonvolatile with low diffusivity, the PS microspheres formed either close- or non-close-packed composite clusters depending on the initial volume of ETPTA resin. This volume-dependent particle configuration was different from the case in which the drop phase is evaporated to induce self-assembly. Patchy clusters were doped with rhodamine B and distribution of dye molecules in the structure was analyzed with a laser scanning confocal microscope as shown in Figure 3. Surface Evolver determines the minimum energy state that corresponds to the most favorable structure of the patchy cluster for a given emulsion volume and number of particles. In Figure 4, some of the simulated patchy clusters were reproduced for N=2-8 when the ETPTA drop volume was sufficiently small that the colloidal microspheres came into close contact.

Figure 3. Laser scanning confocal microscope images for rhodamine B-doped patched clusters
Figure 4. Patched clusters with the packing configuration modeled by Surface Evolver

Discussion

The purpose of this study is to demonstrate a simple strategy for fabricating patchy colloidal clusters from a particle-stabilized Pickering emulsion. They use UV-curable oil as the emulsion phase and an aqueous colloidal suspension as the continuous phase. The structures of the particle-armored Pickering emulsions were stabilized by adsorbed particles and fixed by UV polymerization to produce patchy particles. The size of the parches depends on the volume of the emulsion drop and the surface properties such as the contact angle. Finally Surface Evolver simulation was performed to predict the structures of patchy clusters and the surface energy reduction by the particle adsorption as a function of the emulsion drop volume.