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Langmuir 1999, 15, 2202-2208
On the Role of Hydrophobic Particles and Surfactants in Defoaming Geng Wang, Robert Pelton,* Andrew Hrymak, Nehad Shawafaty,† and Yew Meng Heng McMaster Centre for Pulp and Paper Research, Department of Chemical Engineering, McMaster University, Hamilton, Ontario, Canada L8S 4L7, and Dorset Industrial Chemicals, 2725 Ford Boulevard, Chateauguay, Quebec, Canada Received May 27, 1998. In Final Form: November 2, 1998 The role of hydrophobic particles in mineral oil-based defoamers was investigated by fluorescence labeling and microscopy. Defoamer emulsion droplets in water adhere to the air/water interface to become lenses that nucleate bubble coalescence. Fluorescent labels were covalently bonded to silica, and confocal laser scanning microscopy showed that the hydrophobic silica particles concentrate in the oil/water interface near the three-phase contact line. Furthermore, surfactants added to defoamers to facilitate emulsification are required for the transport of the silica to the water/oil interface. Removal of excess silicone oil from the emulsification process lowered defoamer performance, suggesting that very small amounts of free silicone oil have a role in the defoaming mechanism.
Introduction For the last 40 years there has been a small but sustained activity in the colloid literature directed at understanding the mechanisms by which defoamer chemicals function. This research has been driven by the fascinating scientific complexity of the problem, coupled to the fact that defoamers are commercially significant. This paper addresses the mechanism of oil-based filled defoamers, which are an important subclass of aqueous defoamers. These materials consist of a dispersion of small hydrophobic particles, the active foam-breaking agents, in mineral or silicone oil. Modern commercial formulations can contain mixtures of functional materials such as silicone surfactants, silicone gels, and two or more types of hydrophobic particles. The present work focuses on relatively simple formulations in an effort to clarify mechanisms. Many of the early scientific contributions were from Bikerman1 and Ross, and these, with more recent work, have been summarized in an excellent review by Garrett2 published in 1993. To this point, the role of the size and shape3 of hydrophobic particles has been established both theoretically and experimentally. Much of the focus was based on the premise that hydrophobic particles, with or without oil, were lodged in the thin lamella separating bubbles in foam. Dewetting of the hydrophobic particles in the lamella initiated bubble coalescence. Garrett makes strong arguments that the oil is present as lenses at the air/water interface and that these lenses cause bubble coalescence.4 He also states that the hydrophobic particles used in defoamers are lodged in the lens oil/water interface. His evidence for this is the work of Sinka and Lichtman,5 * To whom correspondence should be addressed. E-mail:
[email protected]. Phone: 905 525 9140. Fax: 905 528 5114. † Dorset Industrial Chemicals. (1) Bikerman, J. J. Foams; Springer-Verlag: New York, 1973. (2) Garrett, P. R. The Mode of Action of Antifoams. In Defoaming, Theory and Industrial Applications; Garrett, P. R., Ed.; Dekker: New York, 1993; pp 1-127. (3) Frye, G. C.; Berg, J. C. J. Colloid Interface Sci. 1989, 130, 54-59. (4) Garrett, P. R. J. Colloid Interface Sci. 1980, 76, 587. (5) Sinka, J.; Lichtman, I. Int. Dyer, Text. Printer, Bleacher Finish. 1976, (May), 489.
as well as unpublished work of his own based on comparing the properties of oil/water emulsions with or without hydrophobic particles. A recent series of papers from Wasan’s laboratory has expanded our understanding past the benchmark of Garrett’s book.6,7 On the basis of the direct observation of foam rupture events and single film studies, they confirmed that lenses are important. Furthermore, they emphasized that the lenses are located in the plateau borders and not in the foam lamella as implied by the previous literature. Reported in this work is the first direct measurement of the location of hydrophobic particles in defoamer lenses using fluorescing particles and confocal laser scanning microscopy. We also illustrate the role of surfactants in the transport of the hydrophobic silica to the interface. In their discussion of the defoaming action of oil lenses, Koczo et al.7 emphasized the importance of the lens size. If a lens is too small, the defoamer activity is reduced because time must be given for the foam to drain sufficiently for the lens in the plateau border to bridge neighboring bubbles. It was proposed that defoamers “wear out” with time because the oil is gradually emulsified to give droplets and thus lenses which are too small. The intriguing statement was made that a worn out defoamer could be rejuvenated by the addition of unfilled oil which collects the small defoamer emulsion droplets to give active larger lenses. We have been unable to rejuvenate our defoamers by the subsequent addition of oil. Experimental Section Materials. This work employed many surfactants, oils, dyes, and defoamers. The commercial materials are summarized in Table 1 whereas Table 2 identifies the modified silicas prepared in this work. Synthetic Methods. PDMS-Silica. To prepare hydrophobic silica, 10 g of silica was placed in a 2 L glass beaker and heated to 200 °C with a heating plate under constant stirring with a glass rod for 3 h. Silicone oil (PDMS; 2.5 g) was added dropwise (6) Koczo, K.; Koczone, J. K.; Wasan, D. T. J. Colloid Interface Sci. 1994, 166, 225-238. (7) Koczo, K.; Lobo, L.; Wasan, D. T. J. Colloid Interface Sci. 1992, 150, 492-506.
10.1021/la980618a CCC: $18.00 © 1999 American Chemical Society Published on Web 02/03/1999
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Table 1. Summary of Commercial Chemicals Used in This Study designation silica PDMS mineral oil DTAF Solvent Green 4 Oil Red O Dorset surfactant SDS TDAS DoTAB Tergitol NP-4 APTES CPTES TMED lignin Gel
description fumed silica, 6-8 µm diameter, 140 specific surface area poly(dimethylsiloxane), 350 cst aliphatic oil, 20 cst 5-([4,6-dichlorotriazin-2-yl]amino)-fluorescein surfactant used in commercial defoamer sodium dodecyl sulfate sodium tetradecanoate dodecyltrimethylammonium bromide nonylphenol poly(ethylene glycol) ether 3-aminopropylenetriethoxylsilane 3-chlorolpropylenetriethylsilane N,N,N′,N′- tetramethylethylenediamine indulin C, sodium salt of kraft pine lignin Gel/Mount, a proprietary slide mounting medium. It is a liquid which slowly gels at room temperature when it is open to air
Table 2. Modified Silicas Used in This Study material PDMS-silica FL-silica PDMS-FL-silica
supplier
m2/g
description hydrophobic silica silica bearing grafted fluorescent label 5-DTAF PDMS hydrophobed fluorescent labeled silica
with stirring, the mixture was heated a further 10 min, and the product was stored in a desiccator. FL-Silica. 3-Aminopropylenetriethoxylsilane (6 g) was added to 10 g of silica dispersed in 300 mL of a 0.95/0.05 ethanol/water solution in a Teflon beaker. The dispersion was stirred using a magnetic stirrer for 1 h at room temperature. The product was isolated by centrifugation and washed by three successive
Dorset Industrial Chemicals Dorset Dorset Sigma BASF Aldrich Dorset BDH, Omipur grade Sigma Aldrich Union Carbide Corp. Aldrich Dow Corning Aldrich Westvaco Biomeda Corp., Foster City, CA
treatments with ethanol followed by centrifugation and decantations. The product was cured for 1 h at 100 °C. Ten grams of dried silane-modified silica was dispersed in 300 mL of water in a glass beaker, and 5 mg of DTAF (see Table 1) was added to the dispersion, which was stirred for 24 h at room temperature. The product was washed three times with water followed by three times with anhydrous ethanol. The labeled silica was dried for 24 h at 120 °C. PDMS-FL-Silica. FL-silica (10 g) was added to a solution of 2.5 g of silicone oil dissolved in 50 mL of ethyl ether. After 20 min of mixing, the solvent was evaporated and the solid was dried for 24 h at 120 °C. More extreme conditions were avoided to prevent decomposition of the fluorescent label. Standard Defoamer. PDMS-silica (0.3 g) and 0.2 g of Dorset surfactant were added to 9.5 g of mineral oil, and the mixture
Figure 1. Sequence of capture video frames showing the migration of a defoamer lens into the film between two air bubbles, causing their coalescence.
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Wang et al. microscope slide (Fisher, 76 × 26 mm2, 1.4-1.6 mm thick, the well is 18 mm in diameter and 0.5 mm deep). The result was a monolayer of small air bubbles contained in the well. Standard defoamer was dyed with ∼0.01% Oil Red O dye, and 0.05 mL of the defoamer was place about 5 mm outside of the well on top of microscope slide. A clean polycystine monofilament was placed between the defoamer drop and the well to prevent premature mixing of defoamer and foam. A glass cover slip was placed over the defoamer and the well. The slide was placed on the stage of a Zeiss Axioplan Microscope (Germany) equipped with a Panasonic wv-CL320 (Japan) video camera. After focusing on the foam and starting the video recording, the experiment was initiated by carefully pulling out the monofilament.
Results
Figure 2. Differential interface contrast micrograph (top view) of Solvent Green 4-dyed defoamer droplets on the surface of Gel containing 0.1% SDS.
Figure 3. CLSM cross section of a Solvent Green 4-dyed defoamer lens sitting on top of Gel containing 0.1% SDS. The lines were drawn in to enhance claritysthe angles at the threephase contact points could not be measured and so are not quantitative. was agitated with a 1 cm magnetic bar in a 20 mL vial for 12 h at room temperature. The product was a milky white dispersion. Other defoamers based on modified silica powders were prepared in a similar manner. Defoamer Droplets on a Gelled Surfactant Solution. The shape of defoamer lenses supported on a gelled surfactant solution was measured together with the location of the silica particles within the lens. Initial attempts to observe defoamer droplets on the surface of black liquor or surfactant solutions were unsuccessful because the lenses were not stationary. This problem was solved by replacing low-viscosity surfactant solutions with high-viscosity gels. A few gel-forming materials were tried, and a commercial aqueous mounting media, called Gel herein, was used. The trade name and supplier of Gel are given in Table 1. For most of the work 2.5% lignin and 0.1% SDS were dissolved in the Gel. The solution formed a gel after exposure to air at room temperature for 2-3 h. Individual defoamer droplets were deposited on the gel/air interface with a 0.4 mm i.d. stainless steel tube delivering 1020 mg of defoamer. The drops were imaged with a Zeiss LSM10 confocal laser scanning microscope fitted with a Sony 3CCD Video Camera, running Northern Exposure 2.9 software (Empix Imaging Inc., Canada). Direct Observation of Defoamer Droplet/Air Bubble Interactions. A thin layer of surfactant-stabilized air bubbles was placed on a microscope slide, and defoamer-induced bubble rupture events were videotaped. For this, 20 mL of a solution of 1% SDS and 0.6% Na2SO4 at pH 10 was put in a 250 mL glass beaker. Foam was formed by air entrainment induced by rapid stirring with a 4 cm magnetic stirring bar. About 0.2 mL of foam was transferred with a 2 mL glass pipet into the well of a glass
Described in the following sections are results from a variety of experiments aimed at understanding defoaming mechanisms. The order in which the results are presented goes from a macroscopic view (i.e. bubble rupture) to a microscopic view (i.e. the location of the colloidal silica) and finally to a molecular view (i.e. the role of surfactant). Direct Observation of Defoaming. A single layer of foam bubbles formed from a solution of 1% SDS and 0.6% Na2SO4 at pH 10 was place on a microscope slide and exposed to dyed standard defoamer. Figure 1 shows a sequence of captured video frames showing a bubble coalescence event. The defoamer is present both as an emulsion droplet (i.e. the small sphere in Figure 1A) and as lenses on the air/water interface. In this sequence the largest lens located in the top center of Figure 1A moves down toward a second lens located near the lamella separating the large and small bubbles. By the third frame, the two lenses appear in contact. The fourth frame (Figure 1D) shows that the two defoamer lenses coalesced, as did the two air bubbles. The transformation from the image shown in the third frame to that in the fourth frame occurred within 1 frame of video (1/32 s). A similar experiment was conducted in which hydrophobic silica was not present in the dyed defoamer. Lenses formed and moved near bubble/bubble junction points as in Figure 1A; however, no bubble rupture occurred. Thus, hydrophobic silica, invisible in the images shown in Figure 1, is a crucial foam-breaking agent. Spreading of Defoamer Droplets on the Surfactant/Solution Surface. Oil-based defoamers are usually added to the surface or dispersed into an aqueous solution. In either case, it is important that the defoamer be dispersed into small droplets and spontaneously spread at the air/water interface. A common practice in the defoamer industry is to observe the spontaneous spreading of defoamer on the surface of a black liquor solution. Figure 2 shows the intermediate stage of the spreading of a droplet (∼20 mg) of dyed standard defoamer on the surface (1.6 cm2 total area) of a Gels solution containing 0.1% SDS. The image shows that the added defoamer droplet disintegrated into small oil lenses with diameters from 40 to 500 µm. The bridges between connecting neighboring lenses broke and retracted with time to give circular lenses. Similar spreading behavior was observed with the spreading of defoamer both on simulated black liquor (e.g. 2.5% lignin + 1% SDS + 0.6% Na2SO4 at pH 10) and on pure water surfaces. The 3% dispersed silica in standard defoamer may influence the rate of defoamer spreading. To test this, mineral oil containing 0.25% silicone oil and 2% Dorset surfactant was spread on surfactant and model black liquor solutions. In both cases the oil rapidly dispersed into small droplets with no evidence of the intermediate bridges shown in Figure 2. Thus, we conclude that the
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Figure 4. Top view of a standard defoamer drop on Gel containing 2.5% lignin + 0.1% SDS. The background is fluorescence emission from lignin in the Gel, and the light particles are the PDMS-FL-silica. The hydrophobic particles were preferentially located at the oil/aqueous interface.
dispersed silica particles do slow defoamer dispersion on the air/water interface. Shape of a Defoamer Lens. Confocal laser scanning microscopy (CLSM) was used to image the cross section of a lens. Figure 3 shows a dyed standard defoamer droplet sitting on the surface of a Gel containing 2.5% lignin and 0.1% SDS. The droplet thickness was about 90 µm, and the diameter of the lens was 400 µm. The shape of the defoamer lens in Figure 3 is controlled by the balance of surface tension forces and gravity. The angles between the vectors defining the three-phase contact are related by Newmann’s triangle.8 Location of the Silica Particles in Defoamer Lenses. A standard defoamer based on PDMS-FL-silica (hydrophobic fluorescent labeled silicassee Experimental Section) was prepared, and defoamer lenses on top of Gel containing 2.5% lignin + 0.1% SDS were imaged with a Zeiss LSM10 fluorescent microscope. Figure 4 shows a typical image taken from above the Gel surface. The dark background is due to the fluorescence emission of the lignin dissolved in the Gel whereas the FL-silica appears as light particles. Note that in this case the mineral oil was not labeled and so is invisible. The most striking feature of the image in Figure 4 is the absence of silica in the center of the lens. Since in this view the entire thickness of the lens is visible, the implication is that the FL-silica is located on the surfaces of the lens. Another example of a lens is shown in Figure 5, which shows 1/4 of a 1 mm defoamer lens. In this case the Gel contained only 0.1% SDS, so there was no fluorescence emission from lignin. The defoamer was also different. The Dorset defoamer in the standard recipe was replaced by 4% sodium tetradecanoate, and the silica was replaced (8) Ross, S.; Morrison, I. Colloidal Systems and Interfaces; WileyInterscience: New York, 1988; Chapter 11A, p 86.
Figure 5. Top view of 1/4 of a defoamer lens sitting on Gel containing 0.1% SDS. The dark regions represent fluorescent emissions from PDMS-FL-silica, which appears to be concentrated on the oil/aqueous interface.
with 3% PDMS-FL-silica. As in Figure 4, the hydrophobic silica seems to be concentrated at the lens surface. Scanning laser confocal microscopy was used to obtain optical cross sections of defoamer lenses containing PDMS-FL-silica. In this technique images of horizontal planes were captured and vertical cross sections were generated in software. Figure 6A shows a top view of a lens sitting on Gel containing SDS and lignin. The defoamer was based on the standard defoamer with the silica replaced by 3% PDMS-FL-silica and the Dorset
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Figure 6. Top view and CLSM cross sections of PDMS-FL-silica on Gel containing lignin and SDS. 2% SDS replaced Dorset defoamer in the standard defoamer recipe.
surfactant replaced with 2% SDS. The horizontal lines in Figure 6A denote the locations at which the vertical slices, shown in Figure 6B, were computed. The gray bands in Figure 6B were caused by lignin emissions from the Gel. Thus, the top surface of these bands corresponds to the surface of the aqueous phase. Since neither the oil nor the air phase was visible, the indentations in the gray bands define the shape of the oil/aqueous interface. The fuzzy images on top of the gray bands were caused by emissions from the labeled silica. It is clear that no silica was present in the center of the oil lens or on the lens/air interface. The largest silica agglomerates, corresponding to the most intense emissions, tended to accumulate at the oil/water interface near the bottom center. By contrast, the smaller particles were more concentrated at the oil/ aqueous interface near the air/oil/aqueous three-phase contact line. This experiment helps us understand why silica is effective at only a few percentsthe particles sit at the oil/water interface where they can interact with bubbles. Role of Surfactant in Oil-Based Defoamer. Most oil-based defoamers contain some kind of surface active agent which is added to help the oil disperse in water and to cause the oil to spread on the air/water interface as shown in Figure 2.9 We observed that the surfactant also influenced the location of hydrophobic silica in the defoamer lens. Figure 7 shows the top view of the lens sitting on Gel containing 2.5% lignin and 0.1% SDS. The defoamer only contained mineral oil and 3% PDMS-FLsilica (i.e. no surfactant). In contrast to the previous work, the image in Figure 7 shows that the silica was uniformly distributed in the oil phase. Thus, it was concluded that the surfactant eliminated from this experiment participates in the transport of silica to the oil/aqueous interface. A series of defoamers based on different hydrophobic particles and surfactants was prepared with fluorescent labeled silica. The spreading characteristics on Gel solution, containing lignin and SDS, were recorded together with the location of the silica particles in the lenses, and the results are summarized in Table 3. (9) Allen, S. L.; Allen, L. H.; Flaherty, T. H. In Defoaming Theory and Applications; Garrett, P. R., Ed.; Surfactant Science Series, Vol. 45; Dekker: New York, 1993; p 151.
Defoaming characteristics were measured in 1% SDS solutions. The defoamers labeled “bad” showed no defoaming characteristics whatsoever in a simple shake test. The foam rise measurements using an air sparger10 distinguished “fair” defoamers from “good” ones. A number of conclusions come from the results in Table 3. First, every “good” and “fair” defoamer had silica located at the oil/aqueous interface, as did one “bad” defoamer. Thus, interfacial silica is a necessary but not sufficient condition for good defoaming. Second, every type of surfactant studied caused the hydrophobic silica to migrate to the interface whereas, without surfactant, no migration occurred. Surprisingly, Dorset defoamer caused hydrophilic silica (FL-silica) to be transported to the aqueous phase, whereas SDS or DTAB caused FL-silica to remain in the oil phase and not at the interface. As expected, all formulations with hydrophilic silica were poor defoamers. Role of Free Silicone Oil. The second entry in Table 3 summarizes a result in which the PDMS-FL-silica was exhaustively washed with ether before addition to the mineral oil. This treatment did not prevent migration of the silica to the interface; however, the resulting defoamer was poor. We believe that the washing procedure removed excess silicone oil, which may have a role in defoaming. To test this hypothesis, a small amount of silicone oil was added to this defoamer, which resulted in high defoaming efficiency. Discussion This work has confirmed the supposition of Garrett and others that hydrophobic particles in filled, oil-based defoamers become lodged in the oil/water interface of defoamer lenses. The results in Table 3 show that defoaming activity requires particles to be present at the interface. On the other hand, some of the defoamers with interfacial particles were not good defoamers. Clearly, other criteria are important. An unknown factor is the extent to which the interfacial particles extend into the aqueous phase. Presumably, the farther that asperities protrude from a lens, the more quickly an approaching bubble wall will interact and coalesce. Higher magnifica(10) Pelton, R. Pulp Pap. Can. 1989, 90, T61-T68.
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Figure 7. Top view of a defoamer lens based on labeled hydrophilic silica (FL-silica) and mineral oil (i.e. no surfactant in the defoamer), sitting on Gel. The silica is located throughout the oil phase. Table 3. Spreading Characteristics and Silica Locations in Oil Lenses Spread on Gel Solution Containing Lignin and SDS particle type
defoamer surfactant
spontaneous spreading
particle location
defoamer activity
PDMS-FL-silica washed PDMS-FL-silica washed PDMS-FL-silica PDMS-FL-silica PDMS-FL-silica PDMS-FL-silica PDMS-FL-silica FL-silica FL-silica FL-silica FL-silica
Dorset surfactant Dorset surfactant Dorset surfactant + silicone oil SDS TDAS DTABa none none SDS DTABa Dorset surfactant
yes yes yes yes yes yes no no no no yes
interface interface interface interface interface interface interior interior interior interior in water
good fair good fair fair fair bad bad bad bad bad
a
Defoamer lenses containing cationic surfactant were spread on Gel containing no other additives.
tion techniques are required to probe the details of the oil/water interface. Surfactants are routinely added to oil-based defoamers to facilitate dispersion in aqueous media. We were surprised to observe that the surfactants appear to have a role in the transport of the hydrophobic silica particles to the oil/water interface. More work is required to clarify the mechanism and identify the links between particle transport mechanisms and surfactant structure. Like this work, many of the investigations of defoamer mechanisms employ silicone-treated silica as the hydrophobic particles. Discussions of these results usually focus on the size, shape, and surface energy (contact angles) of the particles. However, it is possible that silicone treatment gives specific effects. We have shown that washing silicone-treated silica, which removes ungrafted silicone oil, transforms good defoaming particles into poor ones. One possibility is that our hydrophobization methods (120 °C for 24 h) gave low degrees of grafting and poor hydrophobization. Observation by CLSM showed that, like the good defoamer, the washed silica was present at the
oil/water interface, suggesting favorable surface energies. It is possible that, in the good defoamer, excess silicone oil from the hydrophobization is present at the oil water interface. Furthermore, since silicone polymer is not soluble in mineral oil, particles bearing grafted silicone will not be compatible with mineral oil. In other words, silicone oil is not a good steric stabilizer for colloids in mineral oil. Therefore, the silica particle will try to concentrate in the excess silicone oil on the surface of the mineral oil lens. Efforts are in progress to identify the location of the excess silicone oil in the mineral oil defoamers. Conclusions Direct observations of the defoamer-induced coalescence of a monolayer of air bubbles support the conclusion of Koczo et al. that defoamer lenses in the plateau borders are the active foam-breaking agents. Hydrophobic silica particles tend to accumulate at the interface between the defoamer oil and the aqueous solution.
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Surfactants added to defoamers to help dispersion and spreading appear to be required for the transport of the hydrophobic particles to the oil/aqueous interface.
financial support from the Natural Sciences and Engineering Research Council of Canada (NSERC) and Dorset Industrial Chemicals Ltd.
Acknowledgment. We thank Professor Mitchell A. Winnik for useful suggestions. Acknowledged is the
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