Ultrastable particle-stabilized foams.

Aqueous foams are important in a variety of different applications, ranging from food and cosmetics to oil recovery, blast mitigation, and fire extinguishing. Well-established and emerging applications that use foams as an intermediate structure to produce macroporous materials are also widely used in the field of engineering to fabricate thermal insulating materials and low-weight structures, as well as in medicine to produce artificial implants and scaffolds for drug delivery and tissue engineering. The thermodynamically unstable nature of liquid foams is a critical issue in all these applications. Foam instability arises from the high energy associated with the gas–liquid interface, and constitutes a driving force for decreasing the total interfacial area of the foam through coalescence and disproportionation (Ostwald ripening) of the bubbles. Such processes can be partially hindered by using long-chain surfactants or biomolecules such as lipids and proteins to adsorb at the air–bubble surface and reduce the gas–liquid interfacial energy. In addition to surfactants and biomolecules, colloidal particles have long been exploited to stabilize oil droplets in Pickering emulsions. However, it was only recently recognized that partially hydrophobic particles can also attach to gas–liquid interfaces and stabilize air bubbles in surfactant-free diluted suspensions. The attachment of particles at the gas–liquid interface requires an optimum balance between the solid– liquid, solid–gas, and liquid–gas interfacial tensions and is therefore dependent on the wetting behavior at the particle surface (Figure 1a,b). A number of approaches have been described to change the lyophobicity and wetting properties of solid particles so as to favor their attachment at gas–liquid interfaces. In the flotation industry, for example, wetting is usually controlled through the adsorption of long-chain surfactants (typically > 10 carbon atoms) on the particle surface. Hydrophobic silane species have also been deliberately grafted on to the surface of silica nanoparticles to enable model investigations in the absence of surfactants to be performed. However, in all the particle-stabilized foams reported so far, the concentration of modified particles in the liquid medium is not sufficiently high to stabilize a large gas–liquid interfacial area. Therefore, the initially aerated suspension undergoes extensive drainage and creaming before a stable floating foam is achieved on top of the liquid phase. The stabilization of a high concentration of sub-millimeter-sized air bubbles that do not undergo drainage or creaming would, however, be highly advantageous in many foam applications. We report here a simple and versatile approach to prepare ultrastable particle-stabilized foams that percolate throughout the entire liquid phase and exhibit no drainage or creaming effects. The novelty of our method is the fact that it enables the surface modification of a high concentration of colloidal particles in the liquid phase, thus allowing the stabilization of a large gas–liquid interfacial area against disproportionation, coalescence, drainage, and creaming. Herein we describe and discuss: 1) our approach to surfacemodify a large number of particles in the liquid phase, 2) the resulting attachment of lyophobized particles at a gas–liquid interface, 3) the foaming behavior after surface modification, and finally 4) the foam stability achieved. The examples described herein illustrate the universal nature of the method, which in principle can be extended to any type of oxide or non-oxide particles regardless of their initial wetting behavior. Colloidal particles of various chemical compositions (Figure 1c) were surface-lyophobized through the adsorption of short-chain amphiphilic molecules on to the particle surface. A key feature of our approach is the use of short amphiphiles (typically < 8 carbon atoms) which exhibit high solubility and high critical micelle concentrations in the aqueous phase. This is a primary requisite to enable the [*] U. T. Gonzenbach, Dr. A. R. Studart, Dr. E. Tervoort, Prof. Dr. L. J. Gauckler Department of Materials ETH Zurich Wolfgang-Pauli-Strasse 10, HCI G 539 8093 Zurich (Switzerland) Fax: (+41)44-632-1132 E-mail: [email protected]