Langmuir 2006, 22, 1273-1280
1273
Effect of High Salt Concentrations on the Stabilization of Bubbles by Silica Particles Thomas Kostakis, Rammile Ettelaie, and Brent S. Murray* Food Colloids Group, Procter Department of Food Science, UniVersity of Leeds, Leeds LS2 9JT, United Kingdom ReceiVed August 11, 2005. In Final Form: NoVember 17, 2005 The stabilization of air bubbles by hydrophobically modified silica particles has been investigated in detail. The silica particles used had a nominal primary particle size of 20 nm and were made hydrophobic by treatment with dichlorodimethylsilane to yield particles with varying percent grafting of alkyl chains (“% SiOR”). Contact-angle (θ) measurements of pure water droplets on flats made from compressed samples of the particles showed a steep increase in θ above ca. 20% SiOR. Other measurements also showed a significant increase in θ when the salt concentration was raised to 1-3 mol dm-3. Bubbles were formed in a sonicated dispersion of particles by suddenly lowering the pressure. Maximum stability was obtained with 33% SiOR particles and 2-3 mol dm-3 NaCl. Under these conditions, θ was around 40°. Above a threshold size of around 70 µm, bubbles were extremely stable to disproportionation and coalescence and bubble stability increased significantly with an increase in the NaCl concentration from 0.5 to 3 mol dm-3. Furthermore, rheological measurements showed that at NaCl concentrations in this range weak particle gels were formed with a finite yield stress. The strength of these gels increased with an increasing NaCl concentration between 0.5 and 3 mol dm-3 and with an increasing time of aging the dispersions, implicating this as part of the mechanism leading to an increased bubble stability in these systems. Dispersions in the absence of NaCl showed little or no foamability at all. Use of CaCl2 and Al(NO)3 at similar ionic strengths showed that equivalent stability could not be obtained with these salts. Atomic force microscopy (AFM) measurements of the adhesion between a pure (0% SiOR) silica sphere and flat showed a significant increase in the adhesion between 0.5 and 3 mol dm-3 NaCl, even though in this concentration range no significant change in the electrostatic repulsion might be expected. It is concluded that the increased particle-particle adhesion, effective hydrophobicity, and bubble-stabilization properties of the particles at high NaCl concentrations are probably due to the collapse of protruding polysilicic acid chains on the surface of the silica.
Introduction Ramsden1 and Pickering2 were the first to recognize that small insoluble particles are able to stabilize emulsions. Later on, Briggs3 observed that silica particles could stabilize oil-in-water (o/w) emulsions, while carbon black stabilized water-in-oil (w/o) ones. Broad interest has recently been re-awakened in the study of solid particles as stabilizers of dispersed systems. Work from many researchers4-9 has shown that solid particles (latex, clay, silica, etc.) are effective stabilizers of o/w and w/o emulsions. Particles of intermediate hydrophobicity, obtained by coating hydrophilic particles to different extents, are irreversibly attached to interfaces and provide a rigid layer around droplets that impedes coalescence.10 Much of this work has involved nanosized silica particles. Binks and Lumdson11 demonstrated that whether modified silica stabilizes w/o or o/w emulsions depends to a large extent upon the hydrophobicity of the particles. In the last two decades, there have been many studies to elucidate the role of solid particles on foaminess and foam stability. In most of these studies, surfactants were used either to create * To whom correspondence should be addressed. Telephone: 44-(0)113-3432962. Fax: 44-(0)113-3432982. E-mail:
[email protected]. (1) Ramsden, W. Proc. R. Soc. London, Ser. A 1903, 72, 156. (2) Pickering, S. U. J. Chem. Soc. 1907, 91, 2001. (3) Briggs, T. R. J. Ind. Eng. Chem. 1921, 13, 1008. (4) Abend, S.; Bonkke, N.; Gutschner, U.; Lagaly, G. Colloid Polym. Sci. 1998, 276, 730. (5) Midmore, B. R. Colloids Surf., A 1998, 132, 257. (6) Midmore, B. R. J. Colloid Interface Sci. 1999, 213, 352. (7) Binks, B. P.; Lumsdon, S. O. Phys. Chem. Chem. Phys. 1999, 1, 3007. (8) Binks, B. P.; Lumsdon, S. O. Langmuir 2001, 17, 4540. (9) Ashby, N. P.; Binks, B. P. Phys. Chem. Chem. Phys. 2000, 2, 5640. (10) Binks, B. P.; Rodrigues, J. A. Langmuir 2003, 19, 4905. (11) Binks, B. P.; Lumsdon, S. O. Langmuir 2000, 16, 8622.
foam or to modify the surface characteristics of the particles used to either stabilize or destabilize the foam. It was found by many researchers12-15 that solid particles increase foam stability. The effectiveness of these solids in stabilizing foams depends upon factors such as particle size, interparticle interactions, and the wettability of the particles. Tang et al.15 examined a wide range of spherical monodisperse hydrophobic silica particles (20-700 nm in diameter) and reported that foam stability decreased with an increasing particle size and increased with an increasing particle concentration, but without explaining the exact role of the particles on the stability. Ip et al.16 investigated the dependence of the stability of metallic aluminum foams on silica particle size, wettability, and concentration. They concluded that, only silica particles of the correct wettability can stabilize foams and again that stability increased with an increasing particle concentration and a decreasing particle size. On the other hand, Hudales and Stein14 stated that only “large” (1-10 µm) glass particles in the presence of a surfactant (CTAB) could inhibit thin-film rupture and delay film drainage. Theoretical work from Kam and Rossen17 demonstrated that surface-active particles, with their very high adsorption energy, could generate a sufficiently rigid adsorbed film to prevent disproportionation. In addition, theoretical work from Kaptay18 postulated that solid (12) Adamson, A. W. in Physical Chemistry of Surfaces, 4th ed.; WileyInterscience: New York, 1982. (13) Ralston, J. AdV. Colloids Interface Sci. 1983, 19, 23. (14) Hudales, J. B. N.; Stein, H. N. J. Colloid Interface Sci. 1990, 140, 307. (15) Tang, F.-Q.; Xiao, Z.; Tang, J.-A.; Jiang, L. J. Colloid Interface Sci. 1989, 131, 498. (16) Ip, S. W.; Wang, Y.; Toguri, J. M. Can. Metall. Q. 1999, 38, 81. (17) Kam, S. I.; Rossen, W. R. J. Colloid Interface Sci. 1999, 213, 329. (18) Kaptay, G. Colloids Surf., A 2004, 230, 67.
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particles can stabilize bubbles only when the bubbles are not larger than 3 and 30 µm. The effectiveness of particles in stabilizing bubbles is often rationalized in terms of the attachment energy, E, i.e., the energy required to expel the particles from the interface into one of the bulk phases.19 If the contact angle between the particle and the water phase is in the correct range, the attachment energy per particle can be several thousand kT, where k is the Boltzmann constant and T is the absolute temperature. In 2001, Sethumadhavan et al.20 illustrated the possibility of stabilizing bubbles via monodisperse hydrophilic silica particles. The proposed mechanism of stability was the stepwise film thinning (stratification) of the intervening film of liquid between bubble surfaces, which was also found to be dependent upon the concentration and size of the particles. The use of polydisperse silica particles had the opposite effect, because of the weakening of particle layering.21 More recently, Du et al.22 demonstrated experimentally that hydrophobic silica particles can also stabilize bubbles against disproportionation. The particles were made hydrophobic by chemically modifying some of the surface silanol (Si-OH) groups. The optimum percentage of silanol groups remaining on the surface of the particles to give the best stability was 40%, but the particles were highly aggregated. The bubbles were formed by nucleating them within an aggregated hydrophobic particle layer, by a rapid lowering of the gas pressure, and were found to be much more stable in comparison to bubbles stabilized by proteins.23 Abend et al.4 reported that bubbles fully coated with such particles are apparently more stable because of the formation of a nanoparticle envelope around the bubbles. Recently, Alargova et al.24 found that hydrophobic polymer microrods of a diameter less than 1 µm but tens of micrometers long can stabilize foams for many days in the absence of any surfactant. The stability of these foams was attributed to the formation of a dense thick “hairy” layer around the bubbles. In further work,26 we have demonstrated that even more hydrophilic silica particles than used by Du et al.22 can give rise to the stabilization of bubbles against disproportionation, provided that sufficient background electrolyte is added to the solution. Under these conditions, a highly stable dispersion of air bubbles was formed by nucleating them in a solution supersaturated with air, by means of a sudden pressure drop. The formation and stability of the particle-stabilized bubbles was found to depend upon a delicate balance between the tendency of the hydrophobic particles to adsorb at the bubble surface and their tendency to aggregate rather than remain dispersed in water. When extensively aggregated, particles cannot adsorb fast enough to stabilize the bubbles. It was found that stabilization depended upon the background salt concentration in the dispersion. Enhanced foamability and foam stability was also obtained at higher particle concentrations and by breaking up some of the larger particle aggregates by sonicating the system prior to applying a pressure drop. Under such conditions, when stable bubbles are obtained, it also appears that a weak particle gel with a finite yield stress is formed throughout the system. From confocal microscopy, it (19) Simonovic, S.; Prestidge, C. A. Langmuir 2003, 19, 3785. (20) Sethumadhavan, G. N.; Nikolov, A. D.; Wasan, D. T. J. Colloid Interface Sci. 2001, 204, 105. (21) Sethumadhavan, G.; Bindal, S.; Nikolov, A.; Wasan, D. Colloids Surf., A 2002, 204, 51. (22) Du, Z.; Bilbao-Montoya, M. P.; Binks, B. P.; Dickinson, E.; Ettelaie, R.; Murray, B. S. Langmuir 2003, 19, 3106. (23) Dickinson, E.; Ettelaie, R.; Murray, B. S.; Du, Z. J. Colloid Interface Sci. 2002, 252, 202. (24) Alargova, R. G.; Washadpande, D. S.; Paunov, V. N.; Velev, O. D. Langmuir 2004, 20, 10371.
Kostakis et al.
appears that the aggregated particle layer around the bubbles is actually part of a continuous three-dimensional particle network, forming the weak particle gel.25 Interparticle forces play a key role in the aggregation of particles. Recent developments in atomic force microscopy (AFM) provide a very useful tool to measure forces between different kinds of surfaces. A method originally proposed by Ducker et al.26 involves particle attachment to the cantilever tip and measurement of the colloidal interaction forces between two particles or a particle and a flat surface. The method has the advantage of directly measuring forces between surfaces of fixed geometry, so that reasonably accurate theoretical analysis of the results can be made. Also, because the area of interaction is small, the method is less prone to contamination problems27 as compared to say the surface force apparatus.28 AFM can be also used to image the morphology of the interacting surfaces. In this paper, partially hydrophobic silica particles have been used to stabilize bubbles as in the previous study25 but the effect of a wider range of salt concentrations as well as the electrolyte valency on bubble stability has been explored. In addition, the relationship between the hydrophobicity of the silica particles, the contact angle θ with water, and the rheological properties of the dispersions has been investigated in detail. We also examine the direct pull-off force between silica surfaces as a function of the salt concentration using AFM and speculate on the influence of these forces on the stability of bubbles stabilized by silica particles. Materials and Methods Materials. Fumed silica particles were used, of nominal diameter 20 nm, which had been treated with a silylating reagent to different extents. The particles were specially made by Wacker-Chemie GmbH (Munich, Germany). The silylating reagent used was dichlorodimethylsilane. In this work, samples with 33% of the surface Si-OH groups treated (“33% SiOR”) were mainly used to stabilize bubbles. Advancing contact-angle measurements of a water droplet on the silica particles were also made with 0, 20, 30, 32, 33, 35, 50, 55, 60, 65, 76, and 86% SiOR particles. Water from a Milli-Q system (Millipore Ltd., Watford, U.K.), free from surface-active impurities and with a conductivity of less than 10-7 S cm-1, was used throughout. AnalR-grade NaCl, CaCl2, and Al(NO)3 were from Sigma (Poole, Dorset). Bubble Generation. In controlled stability measurements, sample bottles containing 1 wt % dispersions of the particles were sonicated for 1 h in a Grant model XB14 ultrasound bath (Grant Instruments Ltd., Shepreth, U.K.) at an operating frequency of 38 kHz and 162 W of RMS power. These dispersions were then foamed as described in detail elsewhere.25 A brief outline of the methods used is as follows. Bubbles were generated in a specially designed bubble cell apparatus. This was filled with the required dispersion up to approximately 90% of its capacity, sealed, and then left pressurized overnight (approximately 12 h) at 5 bar to ensure that the aqueous phase became saturated. (This was the usual pressurization procedure, but in one specific case, 20 bar for 2-3 h was also used instead.) Then, the excess pressure was suddenly (i.e., in less than 1 s) released back to atmospheric pressure, causing nucleation of bubbles within the aqueous dispersion. We have found that this in situ method of creating bubbles is a more reliable method in generating reproducible results. The behavior of the resulting bubbles was observed at the planar air-water interface above the suspension and also within the (25) Dickinson, E.; Ettelaie, R.; Kostakis, T.; Murray, B. S. Langmuir 2004, 20, 8517. (26) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Nature 1991, 353, 239. (27) Bowen, W. R.; Hilal, N.; Lovitt, R. W.; Wright, C. J. Colloids Surf., A 1999, 157, 117. (28) Tabor, D.; Winterton, R. H. S. Proc. R. Soc. London, Ser. A 1969, 312, 435.
Stabilization of Bubbles by Silica Particles dispersion via a microscope and digital video camera (Hitachi, model KP-MIEK/K). Image Capture and Analysis. The whole experimental sequence was recorded to videotape, and then selected frames or sequences of frames were played back to a Perception digital video recorder system and software (Digital Processing System, Inc., Farnham, U.K.) for subsequent analysis. The sizes of the individual bubbles were measured via the software Image Tool version 2.00a3 (University of Texas, TX), after appropriate contrast adjustment, thresholding, and calibration to a standard-sized object. Bubbles that were touching had to be specified as separate objects by the user of the software. When there were many bubbles and/or particle aggregates in the field of view, sizing all of the visible bubbles as a function of time became difficult and time-consuming. Therefore, as an alternative, in some instances, the bubble stability was represented in terms of the remaining bubble fraction, F. This was defined as the ratio of the number of bubbles, irrespective of their size, still visible after a certain time, to the original number of bubbles present at the start, i.e., immediately after their formation. In this case, the number of bubbles (and hence F) was determined by manual counting. Note that the bubbles did not drift in and out of the field of view but, once formed, were very static at the planar air-water interface. The field of view was typically 8 × 8 mm. Bulk Rheology. Dispersions of 1 wt % of 33% SiOR particles in 0-3 mol dm-3 NaCl were investigated by creep rheometry using a Bohlin CVO-R controlled stress rheometer (Bohlin Instruments, Cirencester, U.K.). A concentric cylinder measurement cell with 24/27 double gap geometry (outer gap of 1.25 mm diameter and inner gap of 1.1 mm diameter) was used, with a total sample volume of 10 mL. The temperature was maintained at 25 ( 0.1 °C. The dispersions were sonicated for 1 h as described above and left overnight before being gently transferred to the rheometer, to have conditions nearly identical to when the bubbles were formed via the pressure drop method. Rheology measurements commenced 30 min after the transfer to the rheometer. Creep compliance measurements were made on the dispersions at increasing values of applied stress in steps of 0.01 Pa. The stress at which a significant (>4 m2 N-1) increase in compliance suddenly occurred after a few seconds (
