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Surface and Capillary Forces Affecting Air Bubble-Particle Interactions in Aqueous Electrolyte Matthew L. Fielden, Robert A. Hayes,* and John Ralston Ian Wark Research Institute, University of South Australia, The Levels, SA 5095, Australia Received February 16, 1996. In Final Form: April 26, 1996X The interaction between hydrophilic silica particles and air bubbles in aqueous electrolyte has been studied by colloid probe atomic force microscopy. The interaction was found to be monotonically repulsive on approach. The silica surface was also hydrophobized by dehydroxylation and by treatment with octadecyltrichlorosilane (OTS). In these cases a repulsion was observed at long range with an attraction evident as the bubble-particle separation decreased. For a freshly prepared OTS-silica surface the intervening thin film rapidly collapsed, resulting in particle engulfment or establishment of a three-phase line. For an aged OTS-silica surface a stable film was formed which could be ruptured as the loading force was increased. In all cases adhesion resisted bubble-particle separation. This behavior was rationalized in terms of either attractive surface forces or capillary forces and contact angle hysteresis operating after formation of a three-phase line (TPL).
* Corresponding author. Telephone: 61 8 302 3225. Fax: 61 8 302 3683. E-mail:
[email protected]. X Abstract published in Advance ACS Abstracts, July 1, 1996.
faces forces that are neither electrical or van der Waals in origin have been identified, for example the short range hydration repulsion16 and the longer range hydrophobic attraction. The hydrophobic attraction, which is presumed to play a key role in bubble-particle interaction, is not at all well understood, despite being the focus of much scientific endeavor. Most experimental studies have focused upon the symmetric interaction between solid surfaces (e.g. mica) that have been hydrophobized, usually by silanation or Langmuir-Blodgett deposition. In these cases the range of the interaction has been shown to vary greatly in magnitude from 5 to 100 nm17-19 and to be critically dependent upon the type and mode of preparation of the hydrophobic layer. Mechanisms proposed either have involved cavitation20 or have been electrical18 in nature, the latter presumably arising from the patchwise nature of the deposited hydrophobic layer, a topic which has received recent theoretical attention.21,22 The cavitation mechanism has been supported by the detection of microbubbles, or ‘bubbstons’, in aqueous solutions.23 We note that it is extraordinarily difficult to prepare solid surfaces which are free of trapped air nuclei,24,25 which may possibly be responsible for the length variation of the hydrophobic attraction.26 The various suggested mechanisms have now been further challenged by recent observations of long range attraction between a hydrophilic surface and a hydrophobic one27 which exceeds that previously observed between a pair of hydrophobic surfaces.28 In this context it is perhaps not surprising that
(1) Sutherland, K. L.; Wark, I. W. Principles of Flotation; Australasian Institute of Mining and Metallurgy: Melbourne, 1952. (2) Sebba, F. Ion Flotation; Elsevier: New York, 1962. (3) Read, A. D.; Kitchener, J. A. J. Colloid Interface Sci. 1969, 30, 391. (4) Blake, T. D.; Kitchener, J. A. J. Chem. Soc., Faraday Trans. 1 1972, 68, 1435. (5) Schulze, H. J. Miner. Process. Extr. Metall. Rev. 1989, 5, 43. (6) Pashley, R.; Kitchener, J. A. J. Colloid Interface Sci. 1979, 71, 491. (7) Crawford, R.; Ralston, J. Int. J. Miner. Process. 1988, 23, 1. (8) Starov, V. M.; Kalinin, V. V.; Ivanov, V. I. Colloids Surf., A 1994, 91, 149. (9) Hewitt, D.; Fornasiero, D.; Ralston, J. J. Chem. Soc., Faraday Trans. 1 1995, 91, 1997. (10) Israelachvili, J. N.; Adams, G. E. J. Chem. Soc., Faraday Trans. 1 1978, 74, 975. (11) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Nature 1991, 353, 239. (12) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Langmuir 1992, 8, 1831. (13) Derjaguin, B. V.; Landau, L. Acta Physicochim. 1941, 14, 633. (14) Verwey, E. J. W.; Overbeek, J. Th. G. Theory of Stability of Lyophobic Colloids; Elsevier: Amsterdam, 1948.
(15) Hunter, R. J. Foundations of colloid science; Clarendon Press: Oxford, 1987; Vol. 1. (16) Grabbe, A.; Horn, R. G. J. Colloid Interface Sci. 1993, 157, 375. (17) Meagher, L.; Craig, V. S. J. Langmuir 1994, 10, 2736. (18) Tsao, Y.-H.; Yang, S. X.; Evans, D. F. Langmuir 1991, 7, 3154. (19) Claesson, P. M.; Herder, P. C.; Blom, C. E.; Ninham, B. W. J. Colloid Interface Sci. 1987, 118, 68. (20) Christenson, H. K.; Claesson, P. M. Science 1988, 239, 390. (21) Miklavcic, S. J. J. Chem. Phys. 1995, 103, 4794. (22) Miklavcic, S. J.; Chan, D. Y. C.; White, L. R.; Healy, T. W. J. Phys. Chem. 1994, 98, 9022. (23) Vinogradova, O. I.; Bunkin, N. F.; Churaev, N. V.; Kiseleva, O. A.; Lobeyev, A. V.; Ninham, B. W. J. Colloid Interface Sci. 1995, 173, 443. (24) Urban, M. J. Ph.D. Thesis, Imperial College, University of London, 1978. (25) Newcombe, G. N. M. App. Sci. Thesis, University of South Australia, 1989. (26) Wood, J.; Sharma, R. Langmuir 1995, 11, 4797. (27) Tsao, Y.-H.; Evans, D. F. Langmuir 1993, 9, 779. (28) Tsao, Y.-H.; Evans, D. F.; Wennerstrom, H. Science 1993, 262, 547.
Introduction The nature of the interaction between air bubbles and particles has been the focus of research for over forty years simply because this interaction is central to the froth flotation process, used extensively for mineral concentration1 and water treatment.2 Hydrodynamic aspects of bubble-particle interactions, the stability of attachment, and the nature of the surface forces which dictate the stability of the liquid film between a particle and an air bubble have been considered in detail.3-7 Since flotation is a kinetic process, many fundamental experimental investigations have focused upon the drainage behavior of the intervening liquid film between a macroscopic flat plate and an air bubble, using interferometric methods,8,9 the drainage rate being the key focus. The fundamental surface forces affecting the drainage process have received far less attention from experimentalists. In contrast the forces of interaction between smooth solid surfaces have been studied in great detail, with the aid of the surface forces apparatus10 and, more recently, the atomic force microscope (AFM).11,12 The methodologies for the study of interactions between essentially nondeformable surfaces are well established, and the results have been shown to be in reasonable agreement with theoretical DLVO predictions.13-15 However even for nondeformable sur-
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little progress has been made regarding the direct measurement of forces operating between an air bubble and a particle, as one must confront the complication of a deformable interface in addition to the asymmetric nature of the interaction. However recent theoretical progress by Grabbe´29 and Chan30 allows the accurate computation of the electrical double-layer interaction for the asymmetric case, while the mathematical complexities associated with an interaction involving a deformable surface have been addressed by Miklavcic et al.31 Concurrently the first reports of direct force measurements between particles and deformable surfaces, an air bubble32,33 and a liquid,34 have emerged. These studies were based on the technique of ‘colloid probe’ atomic force microscopy.11,35 Butt32 measured the interaction between 20 µm glass spheres, untreated (and presumed hydrophilic) and after hydrophobizing with dichlorodimethylsilane, and 4 mm bubbles. Preliminary work indicated that the range of movement required to separate the air bubble and the particle after contact was beyond the range of a normal AFM. The AFM cantilever and substrate, to which the particle was attached, was retained, but the remainder of the apparatus was modified and the cantilever deflection was measured with a separate position sensitive device. The mechanical instability of the cantilever which occurs when attractive forces exceed the spring constant of the cantilever, resulting in a ‘jump’, were followed with an oscilloscope. From the time course of the cantilever movement, and by assuming Newtonian behavior, the force-distance relationship for the interaction of the air bubble and the particle was extracted. For the untreated glass particle the interaction with the air bubble was entirely repulsive at low loads, while there was evidence of hysteresis in the approach-separation cycle and sometimes the particle was observed to snap into the bubble above a certain load threshold (20-200 nN). For hydrophobized particles, no repulsive interaction was observed and the particle always snapped into the bubble. For the hydrophilic particle these results are generally consistent with theory,13,14 which predicts an overall repulsion as both the van der Waals and electrical components are repulsive. Indeed the theory predicts that for an asymmetric interaction the electrical component can give rise to an attraction if one or both of the surfaces interact under constant potential conditions.36 For the case of a bubble and a particle this corresponds to the ‘contactless flotation’ model of Derjaguin et al.37 Alternatively the attraction at high load may be explained by the particles being slightly hydrophobic and therefore giving rise to a shorter range attraction than was the case for the particle that was deliberately hydrophobized. The latter possibility is consistent with measurements of the advancing contact angle of similar particles38 to those used by Butt which showed that the minimum value obtained was never less than 30°, despite the stringent mode of (29) The use of the symmetric algorithm is described in: Grabbe, A. Langmuir 1993, 9, 797. (30) McCormack, D.; Carnie, S. L.; Chan, D. Y. C. J. Colloid Interface Sci. 1995, 169, 177. (31) Miklavcic, S. J.; Horn, R. G.; Bachmann, D. J. J. Phys. Chem. 1995, 99, 16357. (32) Butt, H.-J. J. Colloid Interface Sci. 1994, 166, 109. (33) Ducker, W. A.; Xu, Z.; Israelachvili, J. N. Langmuir 1994, 10, 3279. (34) Mulvaney, P.; Perera, J. M.; Biggs, S.; Grieser, F.; Stevens, G. W. J. Colloid Interface Sci., submitted. (35) Butt, H. J. Biophys. J. 1991, 60, 1438. (36) Atkins, D. T.; Pashley, R. M. Langmuir 1993, 9, 2232. (37) Derjaguin, B. V.; Dukhin, S. S.; Rulyov, N. N. Surf. Colloid Sci. 1984, 13, 71. (38) Diggins, D.; Fokkink, L. G. J.; Ralston, J. Colloids Surf. 1990, 44, 299.
Fielden et al.
cleaning employed. It was concluded that the contamination (presumed organic) was not confined to the surface and was a legacy of the preparative conditions employed. For the hydrophobized particle the results of Butt32 indicate the presence of a dominant long range attractive force. The interaction between the particles and a water droplet was also investigated. Both the untreated particle and the hydrophobized particle were found to snap into the water drop. While such an observation was expected for the untreated particle, a repulsion would have been expected between the hydrophobized particle and the water droplet. It is probable that the particles are heterogeneous, the untreated particle for the reasons already outlined and the hydrophobized particle due to the nature of the silanation reagent employed. The deficiencies of such reagents in terms of the formation of stable hydrophobic monolayers have recently been reported.39,40 One of the key qualitative observations of Butt32 was that during interaction, particularly the ‘snapping-in’, the movement of the air bubble toward the particle was much more significant than the movement of the particle, monitored directly by the cantilever deflection. Ducker et al.33 used an unmodified AFM to measure the interaction between 6-10 µm diameter glass particles, untreated and hydrophobized, and an air bubble of diameter 500 µm. The interaction between a glass particle and an oxidized silicon wafer hydrophobized with octadecyltrichlorosilane (OTS) was also investigated. A long range attraction was observed between an untreated particle and an air bubble, although its strength and range was less than that measured between the bubble and a hydrophobized particle. The interaction between an untreated particle and hydrophobized silica obeyed DLVO theory at large separations, while at separations less than 75 nm there was clear evidence of a long range attraction of variable magnitude and range. The observation of a long range attraction between a hydrophilic surface and a hydrophobic one was in agreement with the results of Tsao et al.,27 while in the latter case the range of the attraction was also found to exceed that measured between two hydrophobic surfaces. Ducker et al.33 rationalized their results in terms of a cavitation mechanism. In the work reported here we have used an AFM to investigate the interaction between silica particles, before and after hydrophobizing, and an air bubble. Materials and Methods Silica Particles. Silica particles (radius, R ) 0.38 and 1.43 µm, bimodally distributed) were obtained from Geltech Inc. (Alachua, FL). They were characterized using various techniques. XPS and SEM/EDS results41 showed the particles to be pure silica. Gas adsorption (nitrogen) measurements indicated that the particles were nonporous. Particles were cleaned by exposure to a radio frequency plasma (Harrick POC-32G) in water vapor. The mounted particles were placed into the plasma chamber, which was evacuated to 10-2 Torr. The source was run for 20 s on low power (40 W). This cleaning procedure conducted on a silicon wafer produced a surface which showed the formation of Newton’s rings upon evaporation of water, indicating that the surface is clean and completely wetted by water (receding contact angle zero). Hydrophobic Particles. Silica particles were hydrophobized by reaction from solution using octadecyltrichlorosilane42 (OTS, Aldrich, 97%). The solution consisted of a 5 × 10-3 M solution of OTS in a mixture of CHCl3, CCl4, and bicyclohexyl (TCI, 99%) (39) Trau, M.; Murray, B. S.; Grant, K.; Grieser, F. J. Colloid Interface Sci. 1992, 148, 182. (40) Biggs, S.; Grieser, F. J. Colloid Interface Sci. 1994, 165, 425. (41) Fielden, M. L. Ph.D. Thesis, University of South Australia, 1996. (42) Maoz, R.; Sagiv, J. J. Colloid Interface Sci. 1984, 100, 465.
Air Bubble-Particle Interactions in Aqueous Electrolyte
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Figure 1. Schematic diagram of experimental arrangement for the measurement of forces between a particle and a bubble. The bubble and particle diameters were approximately 600 and 3 µm, respectively, and the cantilever typically had a spring constant, Ks, of 0.050 N/m. in a ratio of 2:3:10. A small quantity of particles were precleaned, using the method outlined above, and were dispersed in the solution for 20 min. The particles were then filtered from solution through a 0.4 µm nylon filter and left to equilibrate for 15 min. The particles were rinsed with 10 mL of n-heptane, 10 mL of ethanol, and 10 mL of n-heptane and then placed in a furnace and cured overnight at 150 °C. A piece of silicon wafer was subjected to the same treatment, and its emergence from the solvent mixture dry indicated that a successful surface modification had taken place. All solvents were AR grade and were obtained from May and Baker. Silica particles were also dehydroxylated to form a surface of intermediate hydrophobicity, performed by heating in an electric furnace at 1050 °C for 16 h.43 Dehydroxylated particles were cleaned in a low power (40 W) air plasma for 2 s. Air Bubbles. A hydrophilic template was prepared by drilling a 350 µm hole in a piece of mica and annealing it to a piece of clean polypropylene sheet. Bubbles were then placed onto the template using a 10 µL GC syringe (Hamilton Inc.) with a 25gauge needle. The bubble diameter varied between 500 and 700 µm. A schematic of the apparatus is shown in Figure 1. The bubble holder was cleaned by placing a drop of 4 M NaOH on it for 30 s. It was then rinsed with copious amounts of water and then ethanol, with a final water rinse. A drop of water was then placed on the holder and left for several minutes, followed by rinsing. This procedure was repeated three times. The syringe was cleaned by flushing with ethanol (10×) and water (10×), with the needle given a final water plasma clean. Aqueous Solutions. NaCl (AR grade, BDH) was baked at 600 °C overnight prior to use. Water was purified using a reverse osmosis fed Elgastat UHQ PS system. The resultant water had a surface tension of 72.8 mN/m at 20 °C and a conductivity less than 0.5 × 10-6 S/m. Prior to being dispensed from a UV sterilized tip, it was filtered at 0.05 µm. Light-scattering measurements performed at a wavelength of 436 nm on this water yielded a disymmetry ratio of 1.00. The bubble residence time was less than 3 s. Glassware was cleaned by soaking in warm 4 M NaOH solutions for 30 s, followed by thorough rinsing with high-purity water. Salt solutions (pH 5.8 ( 0.2) were then prepared to the desired concentration. Force-Distance Measurements. A Digital Instruments (DI) Nanoscope III atomic force microscope (AFM) was employed. A standard fluid cell and a scan rate of 0.2 Hz were used for all measurements, unless otherwise stated. AFM cantilevers were triangular, tipless, silicon nitride (DI). The spring constant was measured by the method of Cleveland et al.,44 which involves measuring the cantilever resonant frequency in the presence and absence of a known mass, in this case provided by a tungsten sphere. Using this method, spring constants of approximately 0.05 N/m were accurately measured for the cantilever of nominal spring constant 0.06 N/m. Colloid probes were prepared by attaching a particle to the end of a cantilever. This was achieved using an optical microscope (Olympus BH-2), with a micromanipulator attached to the stage. A heated stage was used for the gluing process, as the resin used (Shell Epikote 1004) had a melting point of 100 °C. A 20 µm glass fiber was attached to the micromanipulator and used to place about 10-15 L of resin at the (43) Vansant, E. F.; VanDerVoort, P.; Vrancken, K. C. Characterization and chemical modification of the silica surface; Elsevier: Amsterdam, 1995. (44) Cleveland, J. P.; Manne, S.; Bocek, D.; Hansma, P. K. Rev. Sci. Instrum. 1993, 64, 403.
Figure 2. Raw data for the air bubble-silica interaction in water. The interaction is monotonically repulsive on approach while on separation there is clear evidence of adhesion with Fdet ) 3 mN/m (normalized force). The loading force, Fl, prior to separation was 6 mN/m. A denotes the region of particlebubble approach where surface forces operate, and B, the ‘constant compliance’ region where the loading force is being monotonically increased. C relates to the bubble-particle adhesion which becomes evident when the bubble is moved away from the particle. end of the cantilever. A fresh fiber was then used to collect a single particle, and place it on the resin. The particle was sized after being mounted, using a CCD camera attached to the microscope’s trinocular head, a frame grabber, and an image analyzer (Galai, Cue 3, Israel). The image acquired could be magnified to allow easy and precise sizing ((5%) of the particles. Extraction of force versus separation results from AFM ‘force mode’ (i.e. XY-scan disabled) data is well established for nondeformable surfaces.12 Recent work has set the precedent for the interaction of a deformable surface with a nondeformable surface.33 In this case when a glass particle was pushed against a bubble, the cantilever and bubble were assumed to act as two springs in series:
1 1 1 ) + Km Ks Kb
(1)
where Kb is the spring constant of the bubble, Ks is that for the cantilever, and Km is the measured stiffness. Kb can be calculated by measuring both the cantilever deflection per unit sample translation against the bubble (Cb) and the deflection per unit sample translation against a solid surface (Ch), which can be related by
Kb )
(
Ks
) (
Ks -1 Km
)
Ks
)
Ch -1 Cb
(2)
Measurement of attractive forces which have a gradient exceeding the spring constant is not possible with the AFM. The result of such a force is a ‘jump into contact’. In the present case, two springs are involved, one of which is the air/water interface. This complicates the analysis of data because the rate of movement and profile of the air/water interface under the influence of an attractive force are unknown. To try and gain more information regarding attractive forces, an oscilloscope (Thurlby DSA524) was used to capture the position of the cantilever as a function of time during the jump. A maximum temporal resolution of several nanoseconds was possible.
Results and Discussion Silica. The interaction between a silica particle and an air bubble was found to be monotonically repulsive on approach (Figure 2), in agreement with earlier bubble against plate observations.3,4,9 This finding may be compared with that of Ducker et al.,33 where an attraction between a glass particle and an air bubble was observed.
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Figure 3. Normalized force (F/R) versus separation (D) for air bubble-silica interaction in aqueous electrolyte. Symbols represent experimental data measured in 10-4 M (2, 4) and 10-2 M (O) sodium chloride. The lines correspond to constant charge (s) and constant potential (- - -) fits to DLVO theory. The fitting was performed with the asymmetric DLVO program of Alexis Grabbe´. ψp was obtained from force measurements between silica particles under identical conditions,41 and was -100 and -27 mV at sodium chloride concentrations of 10-4 and 10-2 M, respectively, with ψb maintained at -34 mV.51 The analytical concentration of electrolyte was used in the fitting procedure. The Hamaker constant for the interaction52 was -1.0 × 10-20 J. In the inset the detachment force-loading force data, normalized by particle radius, Rp, measured on separation of the air bubble and particle are displayed.
In the present work a glass particle obtained from the same supplier (Polysciences) was substituted for the silica particle with no significant difference in interaction behavior being observedsthe interaction remained monotonically repulsive. However on removal, for an air bubble and silica, a small adhesion (
