pubs.acs.org/Langmuir © 2009 American Chemical Society
Thinning of a Vertical Free-Draining Aqueous Film Incorporating Colloidal Particles Su N. Tan,† Yujie Yang, and Roger G. Horn*,‡ †
Ian Wark Research Institute, University of South Australia, Mawson Lakes, SA 5095, Australia. Current address: CSIRO Minerals, Bayview Avenue, Clayton, Victoria 3168, Australia. ‡ Current address: Deakin University, Institute of Research Training, Burwood, Victoria 3125, Australia. Received June 12, 2009. Revised Manuscript Received September 28, 2009
The drainage under gravity of a vertical foam film formed on a wire frame has been investigated. Dual-wavelength optical interferometry was used so that unambiguous fringe order assignments could be made, enabling absolute film thicknesses to be calculated with confidence. Films were stabilized by nonionic polypropylene glycol surfactant. Halfmicrometer silica particles with varying degrees of hydrophobicity were added to the film-forming liquid to investigate their effect on film drainage rate and stability. Hydrophilic particles had little or no effect, while hydrophobic particles slowed the drainage of the film and caused a minor increase in film lifetime, from ∼10 to ∼30 s. In both the hydrophilic and hydrophobic cases the films ruptured when they reached a thickness of ∼2 particle diameters. Particles of intermediate hydrophobicity had the most significant effect, increasing film lifetime by an order of magnitude over that for hydrophilic particles. The intermediate particles allowed films to thin down to a thickness less than the particle diameter, indicating that particles bridge across the entire film. This did not occur with more hydrophobic particles even though they were embedded in each of the two film surfaces. These results correlate well with previous literature on particle-laden foams. The film thickness and drainage measurements allow drainage mechanisms for the different particles to be identified, thus providing a mechanistic explanation for the observation by several previous authors that foams formed in the presence of particles, for example during mineral processing, have the greatest stability when the particles are of intermediate hydrophobicity.
1. Introduction Foams are present in our daily life in various forms, such as food and beverages (whipped cream, cake, and beer), packaging, and personal care products (shampoo and detergents); in nature, such as foaming in rivers and the sea; and in industrial processes like enhanced oil recovery, paper recycling, and minerals processing. In many situations particles are present in the foam, forming what is sometimes referred to as a froth, and these particles may dictate the stability and the efficiency of processes such as those listed above. In flotation, mineral particles attached to bubble surfaces stabilize the foam, assisting the recovery of valuable minerals. The froth recovery is dictated by two key parameters. Water recovery from the froth phase controls particle recovery by the entrainment mechanism, and froth stability controls the recovery of attached particles. Nonionic surfactants are often used to enhance froth stability. Particle and surfactant properties are important factors that influence froth stability.1-3 This paper focuses on understanding the drainage behavior of an aqueous foam film in the presence of model particles and nonionic surfactants that are relevant to mineral processing. The ability of small particles (nanometer-micrometer) to stabilize foam in surfactant-free systems has been clearly demon*Corresponding author E-mail:
[email protected]. (1) Lekki, J.; Laskowski, J. S. In A New Concept of Frothing in Flotation Systems and General Classification of Flotation Frothers; 11th International Mineral Processing Congress; Universita de Cagliari: Cagliari, Italy, 1975; pp 427-448. (2) Jameson, G. J. In Physical Aspects of Fine Particle Flotation; The Wark Symposium, Parkville, 1983; Jones, M. H., Woodcock, J. T., Eds.; AIMM: Parkville, MO, 1983; pp 215-232. (3) Johansson, G.; Pugh, R. J. Int. J. Mineral Proc. 1992, 34, 1. (4) Basheva, E. S.; Danov, K. D.; Kralchevsky, P. A. Langmuir 1997, 13, 4342.
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strated in the literature (e.g., refs 4-13). When a film containing monodisperse colloidal particles that are only present inside the liquid film drains, at sufficiently high particle concentration the thinning of the film proceeds in a stepwise manner where each layer of particles is successively removed. Film stabilization by long-range oscillatory structural force14 has been reported for hydrophilic silica6 and latex particles4,5 in surfactant-free systems. In the case of hydrophobic particles that attach to the airliquid interface, ultrastable foam can be attained in surfactantfree systems if the particles form an armored particle shell on the bubble surface which prevents bubble coarsening. Particle shape, hydrophobicity, and concentration are important factors dictating foam stability. Highly anisotropic polymer microrods (contact angle ≈ 80° with average length of 23.5 μm and an average diameter of 0.6 μm) at low concentration (0.22.2 wt % in water) were demonstrated to be a very effective foam (5) Sethumadhavan, G. N.; Nikolov, A.; Wasan, D. T. J. Colloid Interface Sci. 2001, 240, 105. (6) Sethumadhavan, G. N.; Bindal, S.; Nikolov, A. D.; Wasan, D. T. Colloids Surfaces A 2002, 204, 51. (7) Alargova, R. G.; Warhadpande, D. S.; Paunov, V. N.; Velev, O. D. Langmuir 2004, 20, 10371. (8) Binks, B. P.; Horozov, T Angew. Chem., Int. Ed. 2005, 44, 3722. (9) Gonzenbach, U. T.; Studart, A. R.; Tervoort, E.; Gauckler, L. J. Langmuir 2006, 22, 10983. (10) Gonzenbach, U. T.; Studart, A. R.; Tervoort, E.; Gauckler, L. J. Angew. Chem., Int. Ed. 2006, 45, 3626. (11) Gonzenbach, U. T.; Studart, A. R.; Tervoort, E.; Gauckler, L. J. Langmuir 2007, 23, 1025. (12) Pugh, R. J. Langmuir 2007, 23, 7972. (13) Fujii, S.; Iddon, P. D.; Ryan, A. J.; Armes, S. P. Langmuir 2006, 22, 7512. (14) Wasan, D. T.; Nikolov, A. D.; Aimetti, F. Adv. Colloid Interface Sci. 2004, 108-109, 187.
Published on Web 11/03/2009
DOI: 10.1021/la9021118
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stabilizer.7 The foam remained stable for weeks and survived harsh conditions, such as vacuum treatment and drying. The authors attributed the high foam stability to the ability of the polymer microrods to sterically stabilize the foam, through steric forces mediated by dense thick hairy layers.7 Binks and Horozov8 demonstrated that particle hydrophobicity influences foam behavior. Spherical silica particles 30 nm in diameter with varying degrees of hydrophobicity (surface content of silanol varied from 100 to 14%) were prepared. Particles with intermediate hydrophobicity (32 and 20% surface silanol group) in water (0.86 w/v %) were found to give the most stable foam. The bubbles were nonspherical (5-50 μm) and covered with aggregates of branched particle chains. The authors suggested that aggregation of particles on the bubble surface increases the viscosity of the aqueous phase which results in slow liquid drainage in foam and the high foam stability. Denkov showed that highly hydrophobic particles (contact angle > 90°) destabilize foam while hydrophobic particles (contact angle < 90°) stabilize foam.15 Pugh12 reported that titanium dioxide (180 nm) particles with intermediate hydrophobicity generate the most stable foam. Coagulated titanium dioxide particles destabilize foam because large coagula are more difficult to attach to the bubble surface and are more likely to detach from the surface during the turbulent foaming condition than individual dispersed particles. Gozenbach and co-workers9,10 demonstrated that spherical inorganic colloidal particles (30-600 nm) at high concentration (15-45 vol %) can produce stable foam. Foam microstructure can be tailored by adjusting the initial colloidal composition.11 The foam stability was attributed to the formation of an attractive particle network at the air-liquid interface which retards liquid drainage in the foam. Foam stabilizing ability of sterically stabilized latex (100 μm) particles is also shown in the literature.13 Many particle-laden dispersions also include surfactants. The presence of surfactant in aqueous suspension can complicate the system in a number of possible ways: surfactants can adsorb at the air-liquid interface; adsorb at the particle surface and modify the particle hydrophobicity; influence the interparticle forces; and change solution viscosity. In general, adsorption/desorption is dynamic so the solution concentration, and adsorbed amounts of surfactant can vary with time (aging effects). All these factors can have an effect on the foam behavior. For instance, Alargova7 reported that addition of sodium dodecylsulfate to a microrodstabilized foam causes foam destabilization as the surfactant adsorption at the microrod surfaces renders the particles more hydrophilic and results in the detachment of the microrods from the bubble surface. Gredelj and co-workers reported low froth recovery of valuable minerals in a flotation system with methyisobutyl carbinol (MIBC) collector, because most of the MIBC was lost through adsorption on carbonaceous materials.16 Previous studies of drainage of vertical films have focused on aqueous suspensions in the presence of ionic surfactant. Only a few studies have investigated the drainage of large vertical films when the film thickness ranges from micrometers to nanometers, particularly in the presence of particles. Hudales and Stein17 studied the effect of particle size on film and foam drainage. The presence of 1-10 μm hydrophilic silica particles can retard both the film and foam drainage. The increase in the film and foam stability was attributed to dispersion of surface waves by particles protruding through the film surface. Smaller particles (15) Denkov, N. D. Langmuir 2004, 20, 9463. (16) Gredelj, S.; Zanin, M.; Grano, S. R. Miner. Eng. 2009, 22, 279. (17) Hudales, J. B. M.; Stein, H. N. J. Colloid Interface Sci. 1990, 140, 307.
64 DOI: 10.1021/la9021118
Tan et al.
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