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Effect of Surfactant on the Drainage of an Aqueous Film between an Oil Droplet Approaching a Hydrophobic Solid Surface D. G. Goodall,† M. L. Gee,*,† and G. W. Stevens‡ School of Chemistry and Department of Chemical Engineering, University of Melbourne, Parkville, Victoria 3010, Australia Received December 15, 2000. In Final Form: April 3, 2001 The effect of cetyltrimethylammonium bromide concentration above the critical micelle concentration on the profile of aqueous films between an approaching squalene droplet and a hydrophobic silica surface is investigated using the technique of imaging reflectometry. Drainage through the film periphery was observed to display viscous fingering effects and occurred via dendritic channels. This is explained to result from a combination of viscosity and interfacial tension effects. To our knowledge, this is the first time behavior such as this has been observed in a system involving the approach of a droplet to a solid surface.
Introduction Understanding coalescence phenomena in emulsion systems is of considerable interest to industrial areas such as food science, liquid-liquid extraction, paint manufacturing, and wastewater treatment. For example, when two droplets approach each other in an emulsion system, a thin liquid film forms between the two droplets. For a stable system, this film will thin to an equilibrium thickness at which no further thinning of the intervening film will occur. If the system is unstable, however, the film will thin to a critical thickness at which point an instability develops, rupture of the film occurs, and the two droplets coalesce to form a larger droplet. This droplet then undergoes a similar process with other droplets, and so on, until complete phase separation is achieved.1 The manner by which the film drains, the profile of the film, and the dynamics of film drainage are affected by the degree to which the droplet(s) deform on approach to an interface. Although most immiscible liquids can be made to form a short-lived emulsion, the presence of a third component such as a surfactant, polymer, or electrolyte is generally required to stabilize an emulsion system to a reasonable degree. To fully understand the coalescence process in systems containing additives such as these, knowledge of their effect on the profile and drainage of films between two approaching droplets or a droplet and an interface is imperative. To date, experimental data in this area is sparse,2-5 and as a result, our understanding of the coalescence process, and hence ability to accurately predict this phenomena for a wide range of systems, is limited. To this end, in the present study, we have applied the technique of imaging ellipsometry/reflectometry to investigate how the addition of surfactant affects the profile of an aqueous film as it drains from between an ap* To whom correspondence should be addressed. † School of Chemistry. ‡ Department of Chemical Engineering. (1) Sherman, P. Emulsion Science; Academic Press: London, 1968. (2) Fisher, L. R.; Mitchell, E. E.; Hewitt, D.; Ralston, J.; Wolfe, J. Colloids Surf. 1991, 52, 163. (3) Hewitt, D.; Fornaiero, D.; Ralston, J.; Fisher, L. R. J. Chem. Soc., Faraday Trans. 1993, 89, 817. (4) Goodall, D. G.; Stevens, G. W.; Beaglehole, D.; Gee, M. L. Langmuir 1999, 15, 4579. (5) Velikov, K. P.; Velev, O. D.; Marinova, K. G.; Constantinides, G. N. J. Chem. Soc., Faraday Trans. 1997, 93, 2069.
proaching oil droplet and a hydrophobic surface. Specifically, we have looked at how cetyltrimethylammonium bromide (CTAB) at a concentration above its critical micelle concentration (cmc) affects the drainage of an aqueous film between an approaching squalene droplet and a hydrophobic silica surface. We present here observations of some very interesting phenomena that, to our knowledge, have not been reported previously in studies on film drainage in such systems. It should be noted that a more detailed investigation is currently ongoing. Experimental Section Imaging Ellipsometry/Reflectometry. Measurements of film profiles for the system of an oil droplet approaching a hydrophobed silica surface in a continuous aqueous medium containing CTAB above the cmc were made using a modified Beaglehole Instruments imaging ellipsometer.6 The imaging ellipsometer differs from a conventional ellipsometer in that a beam area of 1 cm2 is employed instead of the usual laser light source of beam area ∼1 mm2. A condensing lens, an objective lens, and a CCD detector are also inserted in the optical lineup to provide the microscopic imaging. This combination allows simultaneous sampling over a 1 cm2 area, thereby enabling accurate determination of the film profile at any given time. Additionally, the CCD camera analyses the film thickness spatially, and hence, film thickness resolution is not compromised as the lateral resolution is defined by the pixel size of the camera. Two images, one polarized and one unpolarized, can be obtained by varying the polarization of the incident light using the imaging ellipsometer. The polarized picture gives an image of the coefficient of ellipticity over the sample surface, whereas the unpolarized picture gives an image of intensity of light reflected from the sample surface, i.e., a grayscale image. The imaging ellipsometer therefore has the adaptability to be run in either ellipticity or reflectance mode, hence giving it the flexibility to study both thick and thin films. Details of the microscopic imaging ellipsometer/reflectometer are described elsewhere.4,7 A film thickness profile can be obtained by imaging the reflected intensity or coefficient of ellipticity over a given area. For the system investigated in the present study, the imaging ellipsometer was run in reflectance mode, and film profile images captured were in a grayscale format. The variation in intensity of the reflected light from the sample surface is indicated by the variation in the grayscale, where white corresponds to very thick films and black corresponds to very thin (Newton black) films. (6) Beaglehole Instruments Ltd., Kelburn Pde, Wellington, New Zealand. (7) Beaglehole, D. Rev. Sci. Instrum. 1988, 59, 2557.
10.1021/la001759r CCC: $20.00 © 2001 American Chemical Society Published on Web 05/30/2001
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Consequently, the degree of shading observed in the experimentally obtained images gives a direct topographic profile of the sample surface. The thickness of a film from between an approaching droplet and a silica substrate can be determined from experimentally measuring the ratio of the intensity of the light reflected from the sample surface to the intensity of the incident light. This ratio is commonly known as the reflectance, Rf, and is given by8
1 Rf ) (Rp2 + Rs2) ) 2 -i2β 1 rs12 + rs23e 2 1 + rs12rs23e-i2β
((
) ( 2
+
rp12 + rp23e-i2β
))
1 + rp12rp23e-i2β
2
(1)
where Rp and Rs are the Fresnel reflection coefficients of the reflected beam, r12 and r23 are the Fresnel reflection coefficients at the 1-2 (substrate/film) and 2-3 (film/droplet) interfaces respectively, and β is given by9
(dλ)(n
β ) 2π
2 1
- n22 sin2 φ1)1/2
(2)
where d is the thickness of the film, λ is the wavelength of the incident light, n1 and n2 are the refractive indices of the ambient and film phases, respectively, and φ1 is the angle of incidence. Specific details of the experimental rig are described in an earlier paper.4 Briefly, a silica prism was placed to seal on top of a glass cylindrical cell filled with the fluid of the continuous phase (aqueous solution). Droplets (oil phase) were formed from a steel capillary tip, which was accurately positioned at a set distance from the silica/aqueous interface. The capillary was connected to a gastight syringe contained in a dispenser providing accurate and repeatable control over the droplet volume. The droplet volume employed was 0.02 mL. In a typical experiment, a droplet is rapidly expanded from the capillary tip, thus pressing up toward the silica surface. For this particular system, the droplet immediately detached from the capillary after expansion and consequently approached the interface. Reflectance images of the droplet profile were taken as film drainage occurs between the droplet and the silica surface. An image is captured approximately every second. This time resolution allows for accurate monitoring of changes in the shape of the droplet, i.e., film profile, as the droplet continually approaches the surface. Materials. The two immiscible fluids used in the experiments were squalene (droplet phase) and water (continuous phase). Squalene (C30H50), a triterpene, was obtained from Sigma Aldrich Chemical Co. Pty. Ltd. (98+%) and purified by vacuum distillation. The water used in the experiments was produced using a Milli-Q filtration system. The surfactant, cetyltrimethylammonium bromide (CH3(CH2)15N(CH3)3Br) was obtained from Ajax Chemicals (95% purity) and purified by recrystallization from an ethanol/acetone mixture.10 The background electrolyte concentration employed in the continuous phase was 10-3 M NaCl. Sodium chloride was obtained from Ajax Chemicals (99.9+%) and used without further purification. The silica surface was rendered hydrophobic using a method reported previously.11 Briefly, a hydrofluorocarbon layer is formed by physisorbing (tridecafluoro-1,1,2,2-tetrahydrooctyl)methyldichlorosilane vapor (C9H7Cl2F13Si) onto the silica surface followed by heat treatment to facilitate cross linking. The contact angle of water on the silica surface following this preparation was greater than 90°, thereby indicating the surface was hydrophobic. The (tridecafluoro-1,1,2,2-tetrahydrooctyl)methyldichlorosilane was obtained from ABCR (Germany) and used without further purification. Solution conditions were such that the aqueous phase contained 10-3 M CTAB ([CTAB]cmc ∼ 9 × 10-4 M), with a background (8) Hecht, E.; Zajac, A. Optics; Addison-Wesley Publishing Co. Inc.: Reading, MA, 1974. (9) Azzam, R. M. A.; Bashara, N. M. Ellipsometry and Polarised Light; Elsevier North-Holland, Inc.: New York, 1977. (10) Herder, P. J. Colloid Interface Sci. 1990, 134, 346. (11) Parker, J. L.; Claesson, P. M. J. Phys. Chem. 1994, 98, 8486.
salt concentration of 10-3 M NaCl. The temperature at which experiments were performed was ∼25 °C.
Results and Discussion Reflectance images obtained for the system of a squalene droplet expanded toward a hydrophobic silica surface in a continuous phase of 10-3 M CTAB are shown in Figure 1. This figure contains a series of grayscale images that show the evolution of the film profile in chronological order, commencing with (a), which is the first image, through to the final image (l). The variation in film thickness over the droplet area approaching the solid surface is indicated by the variation in the grayscale of the image. Recall that a darker area corresponds to a thinner film. Note also that an image of the droplet’s profile is captured every second during the course of an experiment, but not all images are shown here. The images presented in Figure 1 best represent the evolution of the film profile. Repeat experiments were performed on this system showing that the data obtained and phenomena observed were reproducible. The time taken for the entire film drainage process to occur (i.e., from droplet formation until equilibrium) was 260 min. This is significantly longer than the time taken for film drainage to occur in the same system containing no surfactant, where the oil droplet coalesced with the hydrophobic flurocarbon surface immediately after the droplet was expanded (∼1 s). In this system, where both the droplet and silica surfaces are hydrophobic, the presence of an intervening aqueous film is thermodynamically unfavorable. Rapid approach of the droplet to the surface therefore occurs in order to expel the aqueous film as quickly as possible. As seen in Figure 1a, the intervening film adopts a dimpled profile after expansion of the droplet to the surface. Dimpling behavior has been observed previously in several studies investigating the approach of air bubbles to solid surfaces.12-14 The dimpling that occurs at the beginning of the film drainage process is thought to be a direct result of the curvature of the spherical droplet. Thus the periphery of the film is relatively thick compared to the center of the film, which is relatively thin since it corresponds to the closest point of the droplet to the surface. Consequently, drainage from the film’s edges is opposed by only a small viscous drag compared to the viscous drag opposing drainage from the thinner, central film area. As a result of this, drainage rates differ over the film area, and drainage from the film’s periphery is initially greater than that from the thinner, central region of the film. This results in a buildup of fluid at the film’s center; i.e., the film forms a dimple. Following formation of the dimple, drainage is observed to continue at the film’s periphery. This is seen as a darkening of the grayscale image at the film periphery when comparing the first image Figure 1a, to the second image, Figure 1b. Similarly, drainage is observed to continue primarily at the film periphery until the film undergoes a localized collapse to a very thin film. This point of localized film collapse corresponds to the black region at the left-hand side of the profile in Figure 1c. This behavior is illustrated more clearly in Figure 2, which contains a cross section (i.e., film thickness versus lateral distance along the droplet) through the center of the film obtained at this time. This figure clearly shows the dimple (12) Platikanov, D. J. Phys. Chem. 1964, 68, 3619. (13) Fisher, L. R.; Mitchell, E. E.; Hewitt, D.; Ralston, J.; Wolfe, J. Colloids Surf. 1991, 52, 163. (14) Hewitt, D.; Fornaiero, D.; Ralston, J.; Fisher, L. R. J. Chem. Soc., Faraday Trans. 1993, 89, 817.
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Figure 1. Unpolarized images of a squalene droplet approaching a hydrophobic silica surface in a continuous aqueous medium of 10-3 M CTAB at a background electrolyte concentration of 10-3 M NaCl: (a) 9, (b) 35, (c) 50, (d) 55, (e) 64, (f) 70, (g) 72, (h) 85, (i) 90, (j) 141, (k) 214, (l) 260 min. The length of each the image in the horizontal direction is 5.87 mm; the length in the vertical direction is 4.94 mm.
Figure 2. Cross sectional area through the center of the aqueous film (10-3 M CTAB) between a squalene droplet and a hydrophobic silica surface obtained at 50 min. The data show the film is thinner on the left-hand side of the profile in comparison to the right-hand side.
in the droplet where the thickness of the film between the droplet and the surface is around 6000 nm. On either side of the dimple, i.e., the barrier ring, the film is much thinner and the droplet is flattened over a significant lateral distance. At the left-hand side of the dimple, i.e., the black region in Figure 1c, the film is significantly thinner than at the right-hand side. This is where the localized collapse, referred to above, has occurred. The time taken for this partial film collapse to occur from initial expansion of the droplet is 50 min. To understand the film drainage in this system, we must first consider the adsorption of surfactant at both the fluorcarbon and droplet surfaces and how this affects surface charge and so surface interactions between the
droplet and the fluorcarbon surface. Previous studies indicate that, at concentrations above and around the cmc, the surface excess of surfactant adsorbed onto a hydrophobic surface from aqueous solution maximizes at monolayer coverage.15-17 Such a monolayer-covered surface will therefore bear a net surface charge by virtue of the charged headgroups of the surfactant molecules. In the case of CTAB adsorbed onto a hydrophobic surface, it has been determined previously that a CTAB monolayer gives rise to an electrostatic surface potential of approximately +30 mV at a background electrolyte concentration of 10-3 M NaCl.18 Similarly, adsorption of surfactant at an oil/aqueous interface does not continue beyond the formation of a close-packed monolayer. In the case of CTAB, a complete monolayer is formed at the oil/ aqueous interface when surfactant concentration approaches the cmc,19,20 which for CTAB is approximately 9 × 10-4 M.21 Hence, as a result of the positively charged surfactant headgroups, the oil/aqueous interface also bears a net positive charge.20 At 10-3 M CTAB, this surface potential has been measured to be around +80 mV when there is no background salt.22 The surface potential is suggested to drop to approximately +40 mV at a back(15) Wirth, M. J.; Piasecki-Coleman, D. A.; Montgomery, M. E., Jr. Langmuir 1995, 11, 990. (16) Tiberg, F. J. Chem. Soc., Faraday Trans. 1996, 92, 531. (17) Grant, L. M.; Tiberg, F.; Ducker, W. A. J. Phys. Chem. B 1998, 102, 4288. (18) Keesom, W. H.; Zelenka, R. L.; Radke, C. J. J. Colloid Interface Sci. 1988, 125, 575. (19) Churaev, N. V.; Esipova, N. E.; Iskandarjan, G. A.; Madjarove, E. A.; Sergeeva, I. P.; Sobolev, V. D.; Svitova, T. F.; Zakharova, M. A.; Zorin, Z. M.; Poirier, J,-E. Colloids Surf. 1994, 91, 97. (20) Gu, Y.; Li, D. Colloids Surf. 1998, 139, 213. (21) Mukerjee, P.; Mysels, K. J. Critical Micelle Concentration of Aqueous Surfactant Systems; NSRSS: Washington, DC, 1970. (22) Gu, Y.; Li, D. Colloids Surf. 1998, 206, 346.
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Figure 3. Schematic diagram of the orientation of CTAB molecules in the system of a squalene droplet approaching a hydrophobic silica surface in a continuous aqueous medium of 10-3 M CTAB.
ground electrolyte concentration of 10-3 M NaCl.20 Thus, in light of these earlier studies, we expect that for our system, where the CTAB concentration is above the cmc, there is a surfactant monolayer at both the fluorocarbon/ aqueous and squalene/aqueous interfaces and that these interfaces bear net surface potentials of the order of +30 and +40 mV, respectively. This is illustrated schematically in Figure 3. As the fluorcoarbon and droplet surfaces are both net positively charged (refer to Figure 3), double-layer repulsion between the droplet and fluorcarbon surface would therefore be expected. Such repulsion would tend to give some stability to the aqueous film between the droplet and the surface and hinder drainage of the film. Our results indeed suggest that the double layer forces hinder film drainage, since the length of time taken for film drainage to occur until partial film collapse was 50 min, and total film collapse was 240 min. As mentioned previously, this is significantly longer than the time taken for drainage to occur in the system of a squalene droplet approaching a hydrophobic surface in 10-3 M NaCl with no added surfactant and no double layer interactions, where the film collapsed in 1 s. It should be noted that during the initial stages of drainage (i.e., Figure 1a and Figure 1b) when the film is thick, hydrodynamic forces govern film drainage.23 Recall that the droplet detaches from the syringe and its buoyancy allows it to rise toward the fluorcarbon surface. It is only when the film is thin enough that surface forces impact on the drainage process. As seen when looking at the progression from images c to f of Figure 1, the initial region of partial film collapse spreads over an increasingly larger area as time evolves. In addition to this, other regions of partial collapse begin to appear around the barrier ring. Such regions are seen in images e and f of Figure 1. It can also be seen from the reflectance images shown in images b-e of Figure 1 that there are “channels” extending from the dimple across the barrier ring where the reflected intensity is brighter, indicating that the aqueous film is relatively thick along these paths. These channels originating from the dimple appear almost dendritic in nature. The profiles shown in Figure 1 are stills, but when watching the film profile evolve in real time, these regions were observed to fluctuate in intensity slightly, suggesting that fluid flow through the barrier ring occurs by means of channels. A possible explanation for this behavior can be found by considering the viscosity difference between the droplet and film phases, as well as the interfacial tension of the system. The viscosities of the droplet phase (squalene)24 and the film phase (an aqueous CTAB solution)25 employed (23) Reynolds, O. Chem. News 1881, 44, 211. (24) Merck Index, 10th ed.; Merck: U.S.A. 1983.
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in this study were 12 and ∼1 cP, respectively. The interfacial tension between the droplet and aqueous phases was measured to be ∼0.5 dyn/cm using the drop weight method.26 It is suggested27,28 that when a less viscous fluid displaces a more viscous one, the fluid flow is uneven, and the less viscous phase “fingers” into the more viscous phase. The fingers are sometimes observed to split, forming a dendritic pattern. This phenomenon is commonly known as viscous fingering and has previously been observed in fluid flow through porous media27 and systems involving the injection of fluid into another phase confined between two flat plates, such as in experiments involving the HeleShaw cell.28,29 This is analogous to our system where a thin aqueous film is confined between an oil droplet and a flat plate. Here, the aqueous film fluid of low viscosity “fingers” into the more viscous droplet phase as it flows through the barrier ring, This behavior is facilitated by the low interfacial tension associated with the droplet/ aqueous interface in this system, since deformation of the droplet surface is almost effortless in this case. Consequently, film drainage via the channels formed is enabled. As seen from Figure 1g, the region of closest approach was observed to spread around the barrier ring, resulting in the presence of a cavity of fluid at the film center. Following this, a decrease in the area associated with the barrier ring was observed on one side of the film (refer to right-hand side of the images shown in images g-i of Figure 1). This suggests bulk movement of the fluid at the film center in this direction. As seen from images j and k of Figure 1 (refer to top right-hand corner of images), this movement of fluid caused one side of the barrier ring to lift up. Fluid from the film center was then expelled through this region resulting in total film collapse to a thin black film of uniform thickness. The area of the film is seen to increase significantly with this process, indicating that once the drainage process is complete, spreading of the droplet at the interface is enabled. This spreading occurs due to the readily deformable nature of the droplet surface, which is a result of the low interfacial tension of the system, brought about by the added surfactant. The presence of a black film between the droplet and the silica surface indicates the presence of a thin film of thickness e100 Å. Another interesting feature observed in images e-k of Figure 1 is that bright circular regions can be seen within the collapsed portions of the film, indicating the presence of trapped pockets of aqueous fluid. Similar phenomena have been observed in foam systems.30 As time evolves, these bright circular regions are observed to decrease in number and increase in size (refer to images i and j of Figure 1), suggesting that coalescence of the fluid pockets occurs. This indicates that they are unstable in nature. The observations described here indeed show some very interesting phenomena. To our knowledge, viscous fingering has not previously been observed in a system involving the approach of a droplet to an interface in a continuous aqueous medium. Clearly, further studies are required to fully understand drainage behavior in this system. LA001759R (25) Cappelaere, E.; Cressely, R.; Decruppe, J. P. Colloids Surf., A 1995, 104, 353. (26) Adamson, A. W. Physical Chemistry of Surfaces, 2nd ed.; Interscience Publishers Inc.: New York, 1967. (27) Homsy, G. M. Annu. Rev. Fluid Mech. 1987, 19, 271. (28) Chen, J.-D. J. Fluid. Mech. 1989, 201, 223. (29) Maher, J. V. Phys. Rev. Lett. 1985, 54, 1498. (30) Klitzing, R. V.; Espert, A.; Asnacios, A.; Hellweg, T.; Colin, A.; Langevin, D. Colloids Surf., A 1999, 149, 131.
