Colloid Surfactants for Emulsion Stabilization

From Soft-Matter
Jump to: navigation, search

Overview

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

Summary

Kim, Lee, Shum, and Weitz use solid particles in 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 single spheres is to allow control over the particles' 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 an analogous packing parameter for Pickering emulsions. Figure 1 shows how the dimensions of the dimer particles relate to the packing parameter dimensions for surfactant molecules (listed below).

<math>P_{packing}=\frac{\nu}{a_0l_c}</math>

<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 (relative sphere sizes) and reach <math>P_{packing}</math> between .6 and 1. Similarly to how surfactants assemble into micelles above a critical micelle concentration, the dimer particles assemble into clusters in water (Does this aggregation also happen only above some critical concentration?). Although micelles have a characteristic size, the clusters of dimer particles do not.

Lastly, the scientists looked at an emulsion of hexadecane in water. The stabilizing dimer particles are large enough to image directly (as opposed to surfactant molecules). A startling difference between molecularly-stabilized emulsions and dimer-stabilized emulsions is that the dimers can stabilize non-spherical droplets (figure 2)! The authors hypothesize that the dimer particles jam and are thus not able to rearrange into a relaxed spherical distribution.

Figure 2. Three differently shaped emulsion droplets. From figure 4 of [1].

Soft Matter Details

Surfactants: The packing parameter is a simple geometric factor one can control to control molecular assembly and the formation of structures. Colloidal surfactants may have a broader range of easily accessible geometries than molecular surfactants, leading to a broader range of emulsion structures.

Experimental Techniques: The authors characterize the particles they make with scanning electron microscopy. By labeling the hydrophilic spheres with fluorescein isothiocyanate (FITC), a fluorescense microscope image shows the location of the hydrophilic spheres. In addition, the authors use simple bright field microscopy.

Colloidal Aggregation vs. Micelle Formation: I am curious about the driving force behind the aggregation of the colloidal surfactants. These aggregates are a hybrid of a simple colloidal aggregate and a micelle. If the hydrophobic/hydrophilic nature of the particles drives aggregation, I would expect the hydrophobic spheres to be gathered together in the center of clusters just like micelles. The images in this article do not entirely convince me that hydrophobic/hydrophilic interactions are the only significant force in aggregation. Maybe electrostatic interactions or van der Waals interactions also play an important role.

Open Questions:

  • The authors state that the mechanism responsible for producing the dimer particles is not entirely understood (p. 3239).
  • Kim et. al. discuss qualitative observations of emulsion drop shapes, but state that the correlation between the packing parameter and the properties of a resulting emulsion had not been determined at the time of publication (p. 3242). From the images in the article, it seems that particles of one packing parameter can stabilize many different shapes and sizes of droplets. The emulsion parameters which correlate to the packing parameter are not obvious.