Foaming-Antifoaming in Boiling Suspensions ² Darsh Wasan,* Alex Nikolov, and Anal Shah Department of Chemical and Environmental Engineering, Illinois Institute of Technology, 10 West 33rd Street, Chicago, Illinois 60616 Particle-stabilized aqueous foams are encountered in radioactive waste treatment and im- mobilization processes and in food, chemical, and agricultural products. The cause of foaminess in the presence of finely divided solids during boiling and in the absence of any surface-active agents is not well understood. Our research has identified at least two kinds of particles in such foaming systems, hydrophilic (i.e., water wet) colloidal particles dispersed in the aqueous phase and biphilic particles (partially wetted by water). The biphilic particles are attached to the air-water surface. In this study we used an advanced optical technique to characterize and monitor the number of nonattached (i.e., hydrophilic) and attached (i.e., biphilic) particles at the gas-liquid surface. The results clearly show that foaming increases with an increase in each of the two types of particles but to a different degree. The presence of biphilic particles causes a significantly higher degree of foaminess than the hydrophilic colloidal particles. Introduction Particle-stabilized aqueous foams are encountered in the processing of solid waste (e.g., during boiling), food, chemical, and agricultural products, froth flotation, and radioactive waste treatment and immobilization pro- cesses. 1 Uncontrollable foaming can severely impact the production rate and ultimately the cost-effectiveness of a chemical process. The solid particles in boiling sus- pensions, in the absence of any surfactants, stabilize the foam lamella and enhance the foaminess. Previously, 1 we identified at least two types of particles in such three-phase foaming systems: hydrophilic colloidal particles dispersed in the aqueous phase and biphilic particles (i.e., with some area of the particle wetted by water and the other part is not). These biphilic (or amphiphilic) particles attach to the surfaces of the foam lamella, provide a steric barrier against the coalescence of bubbles, and thereby enhance foam lamella stability and foaminess. A foam lamella is formed during the generation and interaction of bubbles during the boiling of aqueous suspensions. Hydrophilic colloidal particles get trapped inside the lamella. Subsequently, due to the confined boundaries of the film (lamella), these particles form a layered (i.e., stratified) structure inside the foam lamel- la. 2,3 Monte Carlo simulations of the film containing particles show that the concentration of the colloidal particles is higher in the film/lamella than that in the bulk. 4 Furthermore, our theoretical calculations show that, at a higher particle concentration, a better particle in-layer structure develops that increases the energy stabilization barrier, inhibiting particle diffusion from the film to the bulk meniscus. 3,5,6 The repulsive struc- ture barrier (i.e., the structural disjoining pressure) arising due to the colloid particle in-layer structure formation offers a novel stabilization mechanism for macrodispersions such as foams and emulsions. In fact, we have produced aqueous foams in surfactant-free particle suspensions using nanosized silica particles. 2,7 Our thin film experiments have clearly shown that there exists a critical lamella size below which at least one layer of particles always stays in the film. This critical lamella size is dependent upon particle size and concentration. The critical lamella size seems to increase almost exponentially with particle concentration. 5 Our observations show the phenomenon of lamella stratifi- cation (i.e., layering) is very much dependent on lamella (or bubble) size. 5,8 The classical concept of foam lamella stability is based on the disjoining pressure isotherm (a thermodynamic quantity which is independent of lamella size). Therefore, the disjoining pressure iso- therm is unable to predict the stability of the foam lamella stabilized by colloidal particles, the stability of which depends on both particle concentration and its size. An important factor that affects the colloidal particle structuring and layering phenomena in confined films, and thereby the film stability, is the polydispersity in particle size. Polydispersity has a significant effect on the structural disjoining pressure. Studies have shown that a 30% polydispersity in particle size can decrease the energy structural barrier by a factor of 3, while the effect on the depletion well is smaller. 3,4,9 This suggests that a simple way to destabilize a stable foam is to increase the polydispersity of the suspension or simply add a small amount (e.g., 1 vol %) of large particles. The large particles trapped inside the foam lamella weaken the structure of colloidal particles, decrease the struc- tural barrier, and destabilize the foam lamella, reducing the foaminess and foam stability. 3,9 The first part of our paper presents results of foaming tests during the boiling of simulated nonradioactive waste containing both hydrophilic and biphilic particles. We used an advanced optical technique to monitor these particles. The second part of our paper describes using this knowledge to develop a new antifoam to suppress severe ² During the past 8 years, our research group at IIT has collaborated with the U.S. Department of Energy Savannah River Technology Center to obtain a fundamental understand- ing of the physicochemical cause of foaming and have used this knowledge to develop novel antifoaming agents that are effective in the harsh environment of high-level radioactive waste processing. The results of this effort are summarized in this paper. * To whom correspondence should be addressed. Tel.: (312) 567-3001. Fax: (312) 567-3003. E-mail: wasan@iit.edu. 3812 Ind. Eng. Chem. Res. 2004, 43, 3812-3816 10.1021/ie0306776 CCC: $27.50 © 2004 American Chemical Society Published on Web 03/30/2004