Colloid Surfactants for Emulsion Stabilization

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  • [1] Kim, J., Lee, D., Shum, H., & Weitz, D. Adv. Mater. 20, 3239-3243 (2008).


Kim, Lee, Shum, and Weitz use solid particles in the place of surfactant molecules and qualitatively compare both methods of stabilizing emulsions. Emulsions stabilized with particles are called Pickering emulsions.

Figure 1. Geometry of the fabricated particles. The particle is approximately 2 microns long. From figure 1 of [1].

The solid particles that the researchers fabricate look like two connected spheres of different radii (see figure 1). Surprisingly, these particles are not formed by connecting two pre-existing spheres. Rather, Kim et. al. heat crosslinked polystyrene spheres which have been "swollen" with styrene and a couple other chemicals. The heat causes an elastic stress on the spheres which causes the spheres to squeeze out some of the material inside them. The first sphere shrinks, and a new attached sphere grows. This process is called the "seeded monomer swelling and polymerization technique." The researchers have a method of modifying surface chemistry to make only the first sphere hydrophilic.

The reason for creating dimer particles rather than spheres is to allow control over the particle's geometry as well as chemistry. A molecular surfactant's aggregation structure (micelle, vesicle, bilayer, or inverted micelle) depends on its packing parameter <math>P_{packing}</math> which in turn depends on the geometry of the molecule. People also use the packing parameter to determine whether an emulsion will be oil in water or water in oil. Kim et. al. vary the size of the dimer spheres to study analogous packing parameter for pickering emulsions. Figure 1 shows the dimensions of the dimer particles taken as the dimensions for the packing parameter calculation, while the definitions below are for a surfactant molecule.


<math>\nu=</math> "volume of the hydrophobic tail"

<math>a_0=</math> "optimum surface area of the head groups"

<math>l_c=</math> "fully extended chain length of the tail"

By changing the chemistry during the formation of the dimer particles, the researchers vary the geometry of the particles and reach <math>P_{packing}</math> between .6 and 1. Similarly to how surfactants assemble into micelles, the dimer particles assemble into clusters in water. The dimer's hydrophobic bulbs in the center. Although, micelles have a characteristic size and the clusters of dimer particles did not.

Stabilize emulsion droplets

Soft Matter Details

Surfactants: -packing parameter -control of emulsion shapes

Experimental Techniques: -Bright field microscopy -SEM -Fluorescence Microscopy using fluorescein isothiocyanate (FITC).

Colloidal Aggregation vs. Micelle Formation: -do the colloids aggregate because of hydrophobic/hydrophilic spheres, or some other driving force between particles. I don't see that the hydrophobic spheres are in the middle of the clusters.