Particle-stabilized Foams and Emulsions with Outstanding Stability

Mixing gases and liquids of limited miscibility to form foams and emulsions has been part of everyday life since the advent of bread and dairy products in ancient Egypt. Nowadays, foams and emulsions are used in a variety of applications, ranging from food and pharmaceuticals to cosmetics, oil recovery and materials manufacturing.

The particle-stabilized foams and emulsions with outstanding stability have been prepared using short amphiphilic molecules to tailor in situ the wettability of the particle surface [1,2].

Method

Short fatty acids containing between 1 and 6 carbons in the hydrophobic tail have been used to change the wettability of oxide particles in water by adsorbing on and partially hydrophobizing initially hydrophilic oxide surfaces [3]. Due to their high solubility and high critical micelle concentration in water, these short amphiphiles can be used to modify high concentrations of submicron- and nanosized particles of large surface area.

When exposed to the droplets or gas bubbles introduced during mixing, the surface modified particles in the suspension irreversibly adsorb onto fresh gas-liquid or liquid-liquid interfaces (Fig.1). The high concentration of modified particles in the system result in a fast coverage of the droplet/bubble surface, leading to the formation of foams and emulsions throughout the entire mixture volume without any coalescence or phase separation after mixing is stopped. The as-prepared wet emulsions and foams are viscoelastic materials which can keep their shape upon deformation.

The microstructure of foams and emulsions stabilized using this method can be tailored within a wide range by varying the content of the second immiscible phase and the composition of the initial suspension, namely the amphiphile concentration, amphiphile type and length, particle size, particle concentration and ions present in the liquid phase. By changing these parameters, we have prepared stable foams and emulsions with average droplet/bubble size ranging from 10 to 500 μm and air/oil content varying between 45 and 92 vol% [4,5].

Particles of different chemical compositions can be used to stabilize foams and emulsions based on the principles outlined above. The versatility stems from the fact that the anchoring group and length of the amphiphile tail can be deliberately chosen according to the surface chemistry of the colloidal particles to be surface modified. This has been demonstrated for a series of metal oxides particles varying widely in chemical composition and surface properties [2,3]. By using alkyl amines, alkyl gallates and fatty acid amphiphilic molecules, we were able to stabilize foams and emulsions using oxide particles of various chemical nature ranging from acidic SiO₂ to alkaline Portland cement.

Applications of Foams and Emulsions Stabilized with new Method

Particle-stabilized foams and emulsions prepared with this approach can be either used in the wet state in areas such as cosmetics, food and pharmaceuticals  or can be further processed into porous and composite materials of interest for biomedical and engineering-related applications.

The properties of the foam and emulsion can be tuned according to their specific applications. Dilution enables the preparation of fluid foams and emulsions that can be used for dip coating spin coating and spraying deposition on substrates. Upon further high dilution of the mixture with the continuous liquid phase an enormous number of oil or air-filled capsules coated with particles are produced (Figure 2). This simple and inexpensive approach can be used to prepare millions of capsules in just a few seconds. These capsules have become attractive materials for the encapsulation and delivery of active agents in food processing, pharmaceutical and agricultural industries and biomedicine. The use of inorganic particles with tailored properties can lead to capsules with unique functionalities, including biocompatibility, high-temperature resistance, high strength and magnetic response.

The wet foams and emulsions exhibit viscoelastic behavior with a high yield stress and can be easily shaped.  Drying and heat treatment leads to porous materials  that can be used in many applications, as for instance thermal insulators, catalytic supports, filters, separation membranes, light weight structures, scaffolds for tissue engineering and bone replacement, porous electrodes for fuel cells, as well as sensors and actuators. In this case, the microstructure of the wet foams and emulsions has to be properly tailored in order to address the specific needs of each application. We recently showed that the methods employed for the consolidation, drying and sintering of wet emulsions and foams can be adjusted to produce macroporous materials with closed or open-cell structures, as shown in Figure 3 [5-8]. Based on the concepts described here, we have also been able to produce macroporous metallic and polymeric foams using metallic and polymeric colloidal particles as foam/emulsion stabilizers. The combination of air and oil droplets in a single mixture has also allowed us to prepare hierarchical porous structures, with pore varying more than one order of magnitude in size.

New Projects

On the basis of this new simple technique described above a few projects in different scientific directions were initiated in D-MATL:

• High-temperature insulation materials characterized with high porosity and strength.
• Metallic porous materials with tuned pores size and pore size distribution. (in cooperation with Laboratory of Metal Physics and Technology, Department of Materials, ETHZ)
• Porous materials from nonmeltprocessable polymers, such as PTFE, PEEK (in cooperation with Center of Structure Technologies, Department of Mechanical and Processing Engineering, ETHZ)
• Bone graft materials
• Functional Colloidal capsules

Commercialization

Thanks to the versatility and the simple nature of the method as well as the outstanding properties of the porous materials, several collaborations with industrial partners were established in order to make the needed steps to go from the invention towards a product on the market.

Authors

A. R. Studart, E. Tervoort, U. T. Gonzenbach, I. Akartuna and L. J. Gauckler,
Nonmetallic Inorganic Materials, Department of Materials, ETHZß

References

1. L. J. Gauckler, A. R. Studart, E. Tervoort, U. T. Gonzenbach, and I. Akartuna, Patent application: PCT/CH2005/000744 (2005).
2. U. T. Gonzenbach, A. R. Studart, E. Tervoort, and L. J. Gauckler, Angewandte Chemie-International Edition, 45 [21] 3526-3530 (2006).
3. U. T. Gonzenbach, A. R. Studart, E. Tervoort, and L. J. Gauckler, Langmuir, 22 [26] 10983-1098 (2006).
4. U. T. Gonzenbach, A. R. Studart, E. Tervoort, and L. J. Gauckler, Society, 90 [1] 16-22 (2007).
5. U. T. Gonzenbach, A. R. Studart, E. Tervoort, and L. J. Gauckler, Langmuir, 23 [3] 1025-1032 (2007).
6. U. T. Gonzenbach, A. R. Studart, D. Steinlin, E. Tervoort, and L. J. Gauckler, Journal of the American Ceramic Society, 90 [11] 3407-3414 (2007)
7. A. R. Studart, U. T. Gonzenbach, E. Tervoort, and L. J. Gauckler, Journal of American Ceramic Society, 89 [6] 1771-1789 (2006).
8. A. R. Studart, U. T. Gonzenbach, I. Akartuna, E. Tervoort, and L. J. Gauckler, Journal of Materials Chemistry, 17 [31] 3283-3289 (2007).