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Langmuir 2003, 19, 3106-3108
Outstanding Stability of Particle-Stabilized Bubbles Zhiping Du,† Maria P. Bilbao-Montoya,† Bernard P. Binks,‡ Eric Dickinson,† Rammile Ettelaie,† and Brent S. Murray*,† Food Colloids Group, Procter Department of Food Science, University of Leeds, Leeds LS2 9JT, United Kingdom, and Surfactant & Colloid Group, Department of Chemistry, University of Hull, Hull, HU6 7RX, United Kingdom Received January 9, 2003 Methods have been used to generate air bubbles beneath a planar air-water interface, stabilized by partially hydrophobic quasi-spherical silica particles (primary diameter of 20 nm) in pure water. Particles tended to aggregate at the planar interface, and all the silica dispersions had low foamability. However, those bubbles that were formed (with radii of 5-200 µm) were completely stable to disproportionation for several days, in contrast to similar bubbles stabilized by the best protein foam stabilizers, which typically shrank and disappeared in 1 or 2 h.
Introduction Small particles are able to stabilize colloid dispersions of larger particles, as first observed by Pickering.1 There are many practical examples in which particles are implicated in emulsion stability, such as the presence of wax crystals in water-in-crude-oil emulsions and also triglyceride fat crystals in food emulsions.2 However, it was only recently that Binks et al.3,4 provided systematic evidence of the mechanism and demonstrated the versatility and efficacy of using solid particles in stabilizing a range of water-in-oil or oil-in-water emulsions.3-6 A model system of quasi-spherical nanoparticles of varying surface hydrophobicity was used to stabilize oil + water emulsions.3,5 The key to the stabilization mechanism is that if the surface energy (or contact angle) of the particles with the aqueous phase is in the correct range, then the adsorption energy per particle can be several thousand kT,4,7 where k is the Boltzmann constant and T is the absolute temperature. Thus, once particles are adsorbed at the interface of a droplet, it is almost impossible to force them out of the interface, either by shrinkage of the droplet or through coalescence between droplets. By dispersing the particles initially in either bulk phase, say by using high-energy homogenization, it is possible to obtain emulsion droplets coated in a close-packed layer of particles that prevents coalescence or Ostwald ripening. So far, there has been relatively little work on the ability of such particles to stabilize foams. Kam and Rossen8 recently analyzed a hypothetical case of two-dimensional “bubbles” stabilized by a rigid shell of close-packed hydrophobic solid particles, where the bubbles are of comparable size to the particles. There have also been some hints in the literature that finely dispersed particles may enhance foam stability in some circumstances, but * To whom correspondence should be addressed. Tel: 44 (0)113 3432962. Fax: 44 (0)113 3432982. E-mail: b.s.murray@ food.leeds.ac.uk. † University of Leeds. ‡ University of Hull. (1) Pickering, S. U. J. Chem. Soc. 1907, 91, 2001. (2) Walstra, P. In Food Structure and Behaviour; Blanshard, J. M. V., Lillford, P., Eds.; Academic Press: London, 1987; Chapter 6. (3) Binks, B. P.; Lumsdon, S. O. Langmuir 2000, 16, 8622. (4) Binks, B. P.; Clint, J. H. Langmuir 2002, 18, 1270. (5) Binks, B. P.; Lumsdon, S. O. Langmuir 2000, 16, 2539. (6) Ashby, N. P.; Binks, B. P. Phys. Chem. Chem. Phys. 2000, 2, 5640. (7) Aveyard, R.; Clint, J. H.; Nees, D. Colloid Polym. Sci. 2000, 278, 155. (8) Kam, S. I.; Rossen, W. R. J. Colloid Interface Sci. 1999, 213, 329.
generally not where particles are the only surface active agent in the system. Conversely, the ability of hydrophobic particles to destabilize foams is well-known.9 Our own motivation for investigating particle-stabilized foams comes from recent work on protein-stabilized foams with respect to disproportionation, that is, the diffusion of gas between bubbles of different radius, r, as a consequence of their different Laplace pressures (2γ/r, where γ is the surface tension). Although theory10 suggests that films with high mechanical rigidity, such as those formed from adsorbed protein, should be able to prevent disproportionation and bubble shrinkage, experimental measurements reveal that even some of the most highly viscoelastic protein films cannot halt disproportionation, but can only slow it down very slightly. It is therefore of interest to see if surface active particles, with their very high adsorption energy, could generate a sufficiently rigid shell to prevent bubble shrinkage due to disproportionation, as indicated by the theoretical work of Kam and Rossen.8 Disproportionation of a film of bubbles at an open air-water (A-W) interface has been measured, as this situation is more accessible to study, both experimentally and theoretically.11 Materials and Methods Fumed silica particles of nominal diameter 20 nm were used (Wacker-Chemie). Some of these had been treated with a silylating reagent such that 24, 40, or 51% of the surface Si-OH groups remained untreated.3 In this initial study, however, only those with 40% Si-OH produced sufficient numbers of bubbles for study via the various foaming methods used (see below). Double-distilled water was used throughout. The apparatus used to observe bubble shrinkage has been described in detail previously.11,12 Briefly, bubbles were injected beneath an air-water interface contained within a stainless steel cell. The planar A-W interface was enclosed by a small hole in a very thin sheet of wax-coated mica, floating at the interface, which served to prevent bubbles disappearing from the field of view. Bubbles were illuminated from below via a fiber optic light source and observed from above via a microscope and video camera. Images were recorded digitally, for subsequent analysis. (9) Garrett, P. R. In Defoaming: theory and industrial applications; Garrett, P. R., Ed.; Marcel Dekker: New York, 1993; Chapter 1. (10) Kloek, W.; van Vliet, T.; Meinders, M. J. Colloid Interface Sci. 2001, 237, 158. (11) Dickinson, E.; Ettelaie, R.; Murray, B. S.; Du, Z. J. Colloid Interface Sci. 2002, 252, 202. (12) Murray, B. S.; Campbell, I.; Dickinson, E.; Maisonneuve, K.; Nelson, P. V.; So¨derberg, I. Langmuir 2002, 18, 5007.
10.1021/la034042n CCC: $25.00 © 2003 American Chemical Society Published on Web 03/20/2003
Letters
Langmuir, Vol. 19, No. 8, 2003 3107
Figure 1. Bubble radius (r) versus time (t) for bubbles injected beneath the A-W interface of a 0.006 wt % dispersion of partially hydrophobic silica (method 1): bubble 1 (filled circle B); bubble 2 (filled triangle 2); bubble 3 (open circle O); bubble 4 (open triangle 4). Several different methods were used to try to generate bubbles for study, briefly described as follows. Method 1 involved attempting to disperse 0.006 wt % of particles in water via vigorous hand shaking in a 500 mL flask. The dispersion was then carefully poured down the inside wall of the cell, so that bubbles were not created. Bubbles were then injected beneath the interface via a syringe, as previously for protein-stabilized bubble experiments.11 Method 2 involved creating a 0.006 wt % dispersion as above and then deliberately pouring this into the bottom of the cell from a height, so as to create bubbles as the dispersion was added. Thick layers of highly aggregated particles were visible at the A-W interface, using both methods 1 and 2, which made it difficult to make clear observations on bubbles. There was little point in using a higher concentration of particles to try to increase the number of bubbles formed, as this simply exacerbated the problem of particle aggregation at the interface. Sonicating the silica suspensions with a high-power ultrasonic probe (UIP50G Ultrasonic Processor, Hielscher GmbH, Stuttgart) was also used. Visibly, this dispersed the silica better and resulted in less aggregated layers of particles at the A-W interface, but there was little change in the stability of the bubbles formed when the sonicated suspension was poured into the cell, as in method 2. Method 3 involved forming a 0.006 wt % dispersion of particles in the cell via method 1, sealing the system, and then reducing the pressure rapidly from 1 to 0.5 bar within 2 s to nucleate bubbles within the dispersion.
Results and Discussion In all cases, the particles were not good foaming agents; that is, it was not possible to produce a large number of bubbles using any of the foaming methods. There were, however, differences in the stability of the bubbles that were formed, depending upon the method used. Method 1: Injection beneath the Interface. Very few stable bubbles were obtained; most coalesced immediately with the interface. Occasionally, however, some stable bubbles were obtained, and Figure 1 illustrates the typical behavior of these. Most frequently, any stable bubbles observed tended to be in the larger size range, though severe aggregation of the silica particles at the interface may have obscured the presence of smaller bubbles. Figure 1 (bubbles 1 and 3) illustrates that the larger bubbles were usually not stable to disproportionation until their size had shrunk to a small fraction of their original size. After this shrinkage, they appeared to be indefinitely stable, even at quite low bubble radii, where disproportionation is very rapid for all other systems that we have so far studied (stabilized by proteins).11,13 Figure 1 also shows that some large bubbles were not stable to (13) Ettelaie, R.; Dickinson, E.; Du, Z.; Murray, B. S. J. Colloid Interface Sci., in press.
disproportionation (e.g., bubble 2), while very occasionally bubbles appeared to be stable right from the start of the observations (e.g., bubble 4). It is difficult to make firm conclusions about the behavior of the bubbles formed via method 1, because of the wide variability in their behavior. An obvious explanation for this variability is the poor state of dispersion of the silica, which appeared to aggregate rapidly, with many aggregates adhering to the planar A-W interface. As a result, when the bubbles were injected into the cell, there was undoubtedly a very low concentration of particles in the aqueous phase. Even the primary particles (20 nm in size) are expected to diffuse much more slowly than typical surface active molecules, and the time it took for the bubbles to rise to the planar interface was quite short (
