Particle Zips: Vertical Emulsion Films with Particle Monolayers at Their

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Particle Zips: Vertical Emulsion Films with Particle Monolayers at Their Surfaces Tommy S. Horozov,* Robert Aveyard, and John H. Clint Surfactant & Colloid Group, Department of Chemistry, University of Hull, Hull HU6 7RX, U.K.

Bernd Neumann School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1TS, U.K. Received August 10, 2004. In Final Form: December 17, 2004 Vertical emulsion films with particle monolayers at their surfaces have been studied by direct microscope observations. The effects of particle wettability and surface coverage on the structure and stability of water films in octane and octane films in water have been investigated. Monodisperse silica particles (3 µm in diameter) hydrophobized to different extents have been used. It is found that the structure and stability of emulsion films strongly depend on the film type (water-in-oil or oil-in-water), the particle contact angle, the interactions between particles from the same and the opposite monolayer, and the monolayer density. Stable films are observed only when the particle wettability fulfills the condition for stable particle bridgess in agreement with the concept that hydrophilic particles can give stable oil-in-water emulsions, whereas hydrophobic ones give water-in-oil emulsions. In the case of water films with dilute disordered monolayers at their surfaces, the hydrophilic particles are expelled from the film center toward its periphery, giving a dimple surrounded by a ring of particles bridging the film surfaces. In contrast, the thinning of octane films with dilute ordered monolayers at their surfaces finally leads to the spontaneous formation of a dense crystalline monolayer of hydrophobic particles bridging both surfaces at the center of the film. The behaviors of water and octane films with dense close-packed particle monolayers at their surfaces are very similar. In both cases, a transition from bilayer to bridging monolayer is observed at rather low capillary pressures. The implications of the above finding for particle stabilized emulsions are discussed.

1. Introduction When two emulsion droplets approach each other, usually a thin liquid film is formed between them. The film can thin further to its equilibrium thickness or eventually can break, resulting in droplet coalescence.1 In the case of particle stabilized emulsions, the approaching droplets are covered by (curved) monolayers (or multilayers) of particles.2-4 There are studies in the literature concerned with the role of solid particles in stabilizing or destabilizing thin liquid films. Most of them, however, deal with the case when particles are initially (and mainly) in the bulk (continuous or disperse) phases and eventually are adsorbed at the film surfaces during the experiment.5-8 Only very recently, during the preparation of this manuscript, Stancik et al.9 published results for emulsion films formed between particle-laden drops and planar monolayers of polystyrene particles. In general, there is a lack of systematic thin liquid film experiments in which the film surfaces have been covered with particles before the film formation. The present work concerns the latter case, which in turn is closely related to and resembles * Corresponding author. E-mail: [email protected]. (1) Thin Liquid Films; Ivanov, I. B., Ed.; Marcel Dekker: New York, 1988. (2) Binks, B. P.; Kirkland, M. Phys. Chem. Chem. Phys. 2002, 4, 3727. (3) Vignati, E.; Piazza, R.; Lockhart, T. P. Langmuir 2003, 19, 6650. (4) Tarimala, S.; Dai, L. L. Langmuir 2004, 20, 3492. (5) Kumar, K.; Nikolov, A. D.; Wasan D. T. Ind. Eng. Chem. Res. 2001, 40, 3009. (6) Bergeron V.; Cooper, P.; Fischer, C.; Giermanska-Kahn, J.; Langevin, D.; Pouchelon, A. Colloids Surf., A 1997, 122, 103. (7) Velikov, K. P.; Durst, F., Velev, O. D. Langmuir 1998, 14, 1148. (8) Denkov, N. D.; Cooper, P.; Martin, J. Y. Langmuir 1999, 15, 8514. (9) Stancik, E. J.; Kouhkan, M.; Fuller, G. G. Langmuir 2004, 20, 90.

the situation of approaching droplets in the real particle stabilized emulsions. There is experimental evidence that the wettability of solid particles reflects significantly on the type (oil-inwater (o/w) or water-in-oil (w/o)) and stability of particle stabilized emulsions.10 However, the structure and thinning of liquid films and their impact on the stability of such emulsions are far from being understood. Our recent studies with modified micron size silica particles showed that particle wettability has a very strong influence on the structure of particle monolayers at horizontal oilwater interfaces.11 Silanized monodisperse silica particles with contact angles smaller than ∼115° at the octanewater interface gave totally disordered, aggregated monolayers, whereas more hydrophobic particles, with contact angles greater than ∼129° gave well ordered monolayers of separated particles at large interparticle distances. On the basis of the pair interaction energy analysis, we have concluded that the disorder-order transition and the ordering of the monolayers at large particle separations are consistent with Coulombic repulsion acting through the octane phase as a result of charges at the particleoctane interface.11 One can expect that the observed differences in monolayer structure for particles with different hydrophobicities should affect the properties and thinning of the liquid films formed between two particle monolayers. For instance, the observed long-range electrostatic repulsion between particles through the oil should act also between particles attached at the opposite surfaces of the oil films in water, thus opposing the thinning and (10) Binks, B. P. Curr. Opin. Colloid Interface Sci. 2002, 7, 21. (11) Horozov, T. S.; Aveyard, R.; Clint, J. H.; Binks, B. P. Langmuir 2003, 19, 2822.

10.1021/la047993p CCC: $30.25 © 2005 American Chemical Society Published on Web 02/08/2005

Vertical Emulsion Films

Figure 1. General scheme of the experimental setup (a) and close-up of the frame for water films: side view cross section (b) and front view (c). The frame for oil films has the same geometry and size, but the glass ring is replaced by a PTFE ring and the steel needle is straight and mounted at the upper wall of the ring.

increasing the film stability. That is why our aim in the present work is to investigate the effect of particle hydrophobicity (wettability) and surface coverage on the structure, thinning, and stability of emulsion films formed between approaching particle monolayers. 2. Materials and Methods We used the same particles as those in our previous study of monolayer structure.11 They were synthetic amorphous silica particles with a diameter of 3.00 ( 0.05 µm and a density of 2.0 ( 0.2 g/cm3 (Tokuyama Corp., Japan). The particle wettability was adjusted by silanization of their surfaces by the procedure described previously.11 The silanizing agents used in the present work were hexamethyldisilazane (HMDS, +99%, Lancaster) or dichlorodimethylsilane (DCDMS, +99.5%, Fluka AG) both dissolved in dry cyclohexane (99.7%, Prolabo, for UV spectroscopy) at concentrations within the range 1 × 10-4 to 1 × 10-1 M. The contact angle of the modified particles at the octane-water interface was in the range 65-152° measured through the water (for details, see ref 11). The n-octane (99%, Lancaster) was passed several times through basic alumina (BDH, grade I for chromatographic analysis) in order to remove polar impurities. The aqueous phase with a pH of 5.6 ( 0.2 was deionized water, obtained from a Milli-Q purification unit (Millipore). All experiments were performed at room temperature (24 ( 1° C).

3. Experiments 3.1. Experimental Setup. The scheme of the experimental setup is shown in Figure 1. A rectangular glass cuvette with the inner dimensions 20 × 50 mm and a 40 mm height is partially filled with water and octane. The vertical emulsion films are formed by crossing the particle monolayer (formed in advance

Langmuir, Vol. 21, No. 6, 2005 2331 at the octane-water interface) with a circular frame with an inner diameter of 6.2 mm and a thickness of 2 mm. Two different frames were used, one for water films in octane (Figure 1b,c) and another for octane films in water. The frame for water films consists of a glass ring fitted inside a PTFE holder. A U-shaped steel needle protrudes into the holder and the glass ring through a small hole at the bottom of the frame, as shown in Figure 1b,c. The other end of the needle is connected to a glass syringe (Hamilton, 1 mL) by means of flexible PTFE tubing. The frame for octane films has the same dimensions and similar construction, but the glass ring is replaced by a PTFE ring and a straight steel needle protruding into the top of the frame wall is used. The other end of the needle is connected to another syringe similar to the frame for water films. The frames are mounted on two separate holders (not shown in Figure 1) attached directly to the microscope stage. They both are fitted with micrometer screws allowing vertical movement and adjustment of the frames. The syringes are mounted on a separate stage (not shown) near the microscope. Their pistons can be driven precisely by two micrometer screws. The observations are carried out by a microscope (Optiphot 2, Nikon) with infinity-corrected optics. It was modified by inserting a first surface mirror in the optical path between the objective and the tube lens, as shown in Figure 1a. In this way, observations in a horizontal direction (i.e., parallel to the microscope stage) can be made. The images of the film taken in transmitted or reflected light are captured by the CCD camera (TK-C1381, JVC), recorded by VCR, and processed with Lucia image analysis software (Nikon). 3.2. Experimental Procedure. All parts of the experimental setup which were in contact with the liquids used were cleaned thoroughly before the experiments. The PTFE tubing was flushed first with a large amount of chloroform and then with absolute ethanol. The frames were immersed in chloroform and sonicated for at least 10 min by using an ultrasonic bath. The sonication was repeated replacing the chloroform with ethanol and finally with deionized water. Then, the tubing and frames were rinsed with copious amounts of deionized water and dried before use. The cuvette and the syringes were cleaned with sulfochromic acid and rinsed with large amounts of deionized water. The experiments were performed in the following way. The respective syringes and tubing were filled entirely with water or octane. First, the glass frame (for water films) was immersed entirely in the water, while the PTFE frame (for oil films), in the oil. Then, the monolayer of particles at the octane-water interface was formed by spreading ∼6 wt % particle suspension in 70 wt % 2-propanol (for details, see ref 11). The density of the monolayer was varied from very dilute (∼2 × 105 particles/cm2) to very dense (close-packed particles) by changing the spread volume in the range 1-80 µL. Very thick vertical films (water-in-oil or oilin-water) are formed after lifting the glass frame from water into the oil phase or immersing the PTFE frame from octane into water. Both surfaces of the thick films formed were covered by particle monolayers. The film thickness was decreased by sucking out (or increased by pumping in) the liquid from the film meniscus by using the syringes. Oil films and water films were studied consecutively, but the monolayer densities at their surfaces were approximately equal, because they were formed from one and the same horizontal planar particle monolayer.

4. Results 4.1. Films with Dilute Particle Monolayers at Their Surfaces. Both water and octane films formed just after crossing the octane-water interface by the frames were very thick. The smallest thickness close to the film center was larger than 200 µm, as estimated by focusing on the opposite film surfaces. Hence, these very thick films can be considered as two noninteracting vertical monolayers. Images of the front film surface of these very thick octane films in water taken 1 h after their formation are shown in Figure 2. The particles with contact angles of 65° have sedimented with time, forming an ordered array of close-packed particles at the bottom of the frame and leaving a bare octane-water interface at the top (Figure 2a). The same was true for the particles with contact angles

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Figure 3. Images of vertical water films in octane taken in reflected (a and c) or transmitted (b and d) light in the absence (a) or in the presence of particles at the film surfaces with contact angles equal to 99° (b), 85° (c), and 65° (d). In all cases, a dimple is formed which is clearly seen in reflected light. The scale bar is equal to 50 µm.

Figure 2. Vertical silica particle monolayers at the octanewater interface 1 h after their formation in a circular frame with a diameter of 6.2 mm. The images are taken at the bottom (a and c) and top (b) of the frame. Particle contact angles measured through the water are 65° (a) and 152° (b and c). All scale bars are equal to 25 µm.

of 85 and 99°. In contrast, the vertical monolayers of most hydrophobic particles did not sediment significantly (Figure 2b,c). The interparticle distance at the bottom and at the top was practically the same and equal to ∼15 µm. The reasons for the strong particle repulsion in the latter case will be discussed later. Once formed, the very thick films were forced to thin by sucking a liquid out of the meniscus (so-called “opening” of the film). Vertical films without particles at their surfaces were studied first. The octane films were very unstable and ruptured at large film thicknesses during the opening. The water films in this case were more stable and survived for several minutes. A dimple has been spontaneously formed initially, even at a very gentle opening of the films (Figure 3a). The dimple has gradually disappeared, resulting in a relatively thick plane parallel film (very dark blue in color when observed in polychromatic reflected light) which eventually broke after several minutes. Films with dilute particle monolayers containing ∼(2-3) × 105 particles/cm2 at their surfaces were studied shortly after they were formed in order to diminish the effect of particle sedimentation. Water films with the most

hydrophobic particles (contact angle 152°) were very unstable and ruptured during the film opening. More stable water films were obtained with less hydrophobic particles, but their stability and structure depended on the speed of opening. At a slow rate of film opening (gentle suction of water out of the meniscus), particles were expelled out of the film with a dimple, as shown in Figure 3b. At rapid film opening, particles were also pushed out of the film but a ring of particles at the film periphery appeared. Such a ring was not observed when particles with a contact angle of 99° were used. Instead, the films ruptured during rapid opening. The particles with contact angles of 85 and 65° which formed the ring did not sediment. The latter observations suggest that the ring consists of particles which bridge the opposite film surfaces. This suggestion is supported by the behavior of the particles from the ring during the film closing caused by pumping water into the film meniscus (Figure 4). During closing, the film shrunk together with the particle ring. The ring diameter had decreased simultaneously with an increase of its thickness without losing any particles during shrinking (Figure 4a,b). The particles outside the ring did not follow the film shrinking, but a large circular region with bare film surfaces adjacent to the ring appeared. At these early stages, the process was fully reversible. We were able to open the film, thus increasing the ring diameter and decreasing its thickness to only one particle. Further closing of the film caused shrinking of the particle ring until a crystalline disk of ordered close-packed particles was formed. This crystalline disk is in fact a monolayer of close-packed particles which bridge (zip) the opposite film surfaces. Its domain structure with a small defect in the center is well seen in Figure 4c. Further supply of water into the film meniscus led to the detachment of particles from one of the film surfaces at the periphery of the disk. They still remained attached to the other film surface but sedimented (Figure 4d). The structure of the oil films with dilute particle monolayers at their surfaces was remarkably different


Figure 4. Consecutive images of a vertical water film in octane taken during film closing by pumping water in the meniscus in the case of silica particles with contact angles equal to 65°. The large ring of particles bridging the film periphery (a) shrinks, decreasing its diameter and increasing its “wall” thickness (b) until a 2D crystal disk of close-packed particles is formed (c). Then, peripheral particles detach from one of the film surfaces remaining attached to the other one and start sedimenting (d). This melting of the colloidal crystal continues until some critical size of the crystalline disk is reached followed by a sudden separation of the film surfaces with no bridging particles. The scale bar is equal to 50 µm.

from that of the water films. We were able to obtain stable octane films only in the case of the most hydrophobic particles with a contact angle of 152°. Although the exact film thickness was unknown, the structure of the monolayers on both film surfaces was observed and documented during the film thinning. Images of the thinnest part of the film close to the frame center are shown in Figure 5. When the octane film was relatively thick, the monolayers at both film surfaces were easily distinguishable because both surfaces cannot be in focus simultaneously due to the small depth of field of the optical system (∼1 µm). The particles at the front film surface in Figure 5a look apparently bigger than those at the back film surface. The particles at both film surfaces are well ordered in triangular lattices randomly oriented to each other. At a smaller film thickness, the structure of monolayers changes from triangular to square lattice (Figure 5b). The particles at both film surfaces are still well distinguishable. Further thinning of the film results in a triangular lattice of the monolayers but with a smaller lattice constant compared to that of the thicker film (cf. parts a and c of Figure 5). The particles from both film surfaces are simultaneously in focus; hence, their equators are situated within a layer with a thickness smaller than 1 µm (the depth of field). Up to this point, the film has been forced to thin by sucking octane out of the meniscus. This is followed by a spontaneous decrease of the lattice constant with time inside the central film region with a diameter of ∼50 µm due to particle movement toward each other (Figure 5d). A crystalline disk of particles bridging both film surfaces appears 40 ms later (Figure 5e) and spontaneously grows by sucking particles from the surroundings (Figure 5f). The crystalline disk of bridging particles grows very rapidly in the beginning. Then, the

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Figure 5. Images taken during the opening of octane film in water in the case of particles with a contact angle of 152°. The particle monolayers at the thick film surfaces are well ordered in triangular lattices randomly oriented to each other (a). Interactions between particles from the opposite film surfaces cause rearrangement in a square lattice at smaller film thickness (b). Further thinning of the film results in a triangular lattice of the monolayers but with a smaller lattice constant (c) followed by a spontaneous decrease of the lattice constant with time due to the particle movement toward each other (d). A crystalline disk of particles bridging both surfaces appears later (e) and spontaneously grows by sucking particles from the surroundings (f). The time interval between the images is 720 ms (c-d) and 40 ms (d-e and e-f). The scale bar is equal to 50 µm.

growth slows down and after several seconds finally stops when the disk diameter is ∼500 µm (Figure 6a). A closer look shows that the particles inside the disk are not in close contact but separated at ∼4.2 µm between their centers (Figure 6a, inset). The well ordered crystalline dense monolayer of bridging particles (∼6.5 × 106 particles/ cm2) coexists in equilibrium with the two ordered dilute monolayers (∼0.5 × 106 particles/cm2) at the film surfaces around the disk. This film was very stable, and it was possible to be observed for more than 1 h without any detectable changes in its structure. During the film closing by pumping octane into the meniscus, the disk initially shrinks without losing any particles until they become close-packed (Figure 6b). Further pumping a liquid into the meniscus causes a decrease of the disk diameter due to unzipping of the film surfaces at the periphery; that is, the peripheral particles are detached from one of the film surfaces and become part of the monolayer at the other surface. The unzipping process at this stage can be stopped if the pumping of octane into the meniscus is terminated. When some critical size of the disk is reached (typically ∼200-300 µm), both surfaces unzip spontaneously and the ordered monolayers at separated film surfaces are restored. The latter is illustrated in Figure 6c,d. The process of zipping the film surfaces by bridging particles and unzipping by pumping oil into the meniscus can be repeated many times (more than 10) without breaking the film.

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Figure 6. Consecutive images of vertical octane films in water taken during film closing by pumping water in the film meniscus in the case of silica particles with a contact angle equal to 152° before the beginning of the film closing (a), during closing before the beginning of crystal melting (b), just before spontaneous unzipping when the disk reached some critical diameter (c), and just after spontaneous unzipping 40 ms later (d). The separated monolayers restore their well ordered structure (d, inset). The insets in parts a and b show the well ordered structure of the central crystal of particle monolayer bridging the film surfaces. The inset width is equal to 40 µm (a and b) and 205 µm (d). All scale bars are equal to 50 µm.

4.2. Films with Dense Particle Monolayers at Their Surfaces. Films with close-packed particle monolayers at their surfaces behaved differently from those with dilute monolayers. Water films did not contain a dimple. Instead, when stable, they were forced to thin until a spot of bilayer of sticking particles from the opposite film surfaces was formed (Figure 7a, inset). Further suction of water out of the film meniscus caused a transition from bilayer to monolayer of particles bridging the opposite film surfaces in the center of the film (Figure 7a). The films in the case of less hydrophobic particles with contact angles of 65 and 85° were stable. It was very difficult to break them by further suction of water out of the meniscus. The latter caused an enormous increase in size of the bridging monolayer (occupying almost the whole area of the frame) in coexistence with a bilayer at the periphery! Stable water films with the more hydrophobic particles have not been observed. On the contrary, the octane films were stable only in the case of the most hydrophobic particles with a contact angle of 152°. They behaved similarly to the water films, showing a bilayer to monolayer transition during thinning and exhibiting a very high stability (Figure 7b). 5. Discussion 5.1. Dilute Particle Monolayers at Vertical Octane-Water Interfaces. This case, which is illustrated in Figure 2, will be analyzed in detail separately elsewhere. Here, we will give some arguments that the long-range repulsion between the most hydrophobic particles is due to Coulombic repulsion through the oil caused by charges at the particle-octane interface. The latter has been suggested by Aveyard et al.12 and supported by the results for silica particle monolayers.11 Other possible sources of long-range repulsion are dipolar interactions through the oil due to (i) charges at the particle-water interface13-15 (12) Aveyard, R.; Clint, J. H.; Nees, D.; Paunov, V. N. Langmuir 2000, 16, 1969.

Figure 7. Water in octane (a) and octane in water (b) films formed between two monolayers of close-packed particles with contact angles of 65 and 152°, respectively. The central region in part a and the left part in part b marked with B are monolayers of particles bridging the film surfaces coexisting with a bilayer (A). The inset shows the film just before bridging. The central zone (A) is a bilayer of closely sticking particles from the opposite film surfaces. The light patterns around in the meniscus (M) are due to light diffraction between ordered particle domains at the film surfaces with different mutual orientations (see ref 36). All scale bars are equal to 50 µm.

or (ii) dipoles at the particle-oil interface.15-17 According to Pieranski,13 the asymmetric distribution of the free ions around the interfacial particles in water should lead to an effective dipole moment oriented perpendicular to the fluid interface. Hurd14 has shown that the effective dipole moment, Pw, of each interfacial colloidal particle with radius R and its associated counterionic cloud in water is given by the expression Pw ) qpw/(κxw), where qpw ) Apwσpw ) 2πR2(1 + cos θ)σpw is the total charge at the particle-water interface with area Apw and surface charge density σpw, κ-1 is the Debye screening length, and w is the dielectric constant of water (see also ref 15). In our case, Apw of the most hydrophobic particles with a contact angle of 152° is about 12 times smaller than that of the (13) Pieranski, P. Phys. Rev. Lett. 1980, 45, 569. (14) Hurd, A. J. J. Phys. A: Math. Gen. 1985, 18, L1055. (15) Moncho-Jorda´, A.; Martı´nez-Lo´pez, F.; Quesada-Pe´rez, M.; Cabrerizo-Vı´lchez, M. A.; Hidalgo-A Ä lvarez, R. In Surface and Colloid Science; Matijevic´, E., Borkovec, M., Eds.; Kluwer Academic/Plenum Publishers: New York, 2004; Vol. 17, Chapter 4. (16) Robinson, D. J.; Earnshaw, J. C. Langmuir 1993, 9, 1436. (17) Martı´nez-Lo`pez, F.; Cabrerizo-Vilchez, M. A.; Hidalgo-A Ä lvarez, R. J. Colloid Interface Sci. 2000, 232, 303.


particles with a contact angle of 65°. The same is true for the net charge, qpw, and the dipole moment, Pw. Since the dipolar repulsion between two particles is ∼Pw2, one should expect it is more than 140 times smaller for the most hydrophobic particles than that for the less hydrophobic ones. The results shown in Figure 2 suggest just the opposite, that the repulsion between the particles with a contact angle of 152° is stronger. Hence, the dipolar repulsion through the oil due to charges at the particlewater interface cannot be responsible for the long-range repulsion between silica particles observed by us. Similar arguments hold for the repulsion between dipoles originating from the polar surface groups at the particle-oil interface. It has been shown that at large interparticle distances (as in Figure 2b,c) the net dipole moment of particles is P0 ) πpσR2 sin2 θ, where p is the average dipole moment of one polar group (an elementary dipole) and σ is the surface density of polar groups at the particleoil interface.11 Hence, the net dipole moment of particles with a contact angle of 152° is ∼3.7 times smaller than that at θ ) 65°. Therefore, the dipolar repulsion, which is proportional to Po2, should be more than 10 times smaller in the case of the most hydrophobic particles if compared to the less hydrophobic ones. The latter is not supported by the results in Figure 2. Hence, the repulsion due to dipoles at the particle-oil interface is inconsistent with our experimental observations. However, the Coulombic repulsion through the oil due to charges at the particleoctane interface is in general agreement with our findings. It is proportional to the square of the net charge at the particle-oil interface given by the expression qpo ) σpoApo, where σpo and Apo ) 2πR2(1 - cos θ) are the surface charge density and the area of that part of particles which is exposed to oil, respectively.11,12 Since Apo(θ ) 152°)/Apo(θ ) 65°) ≈ 3.26, the Coulombic repulsion between the most hydrophobic particles should be about 10 times greater than that between the particles with a contact angle of 65° at the same σpo. This is in accord with the results shown in Figure 2. The values for the surface charge density of silica particles in nonaqueous media (from 1 to 80 µC/m2, see ref 11) obtained by others18-20 are also in favor of Coulombic repulsion through the oil as a major reason for the long-range repulsion between adsorbed silica particles detected in our experiments. 5.2. Films with Dilute Particle Monolayers at Their Surfaces. The thinning behavior and stability of water and octane films in the absence of particles is remarkably different. Octane films are very unstable and rupture at large film thicknesses during the opening without forming a dimple, whereas water films are more stable with a spontaneously formed dimple inside them. This difference can be attributed to different hydrodynamic conditions (octane has a smaller viscosity than water) and also to the difference in the disjoining pressure. The results suggest a positive disjoining pressure opposing the thinning and stabilizing the water films. This seems reasonable bearing in mind the oil-water interface is negatively charged due to the adsorption of hydroxyl ions21 which should lead to significant repulsion between the approaching film surfaces at the very low ionic strength in our experiments. The sucking capillary pressure in the film meniscus is relatively low due to the large dimensions of the frames used. It can be estimated to be ∼20 Pa, (18) Labib, M. E.; Williams, R. J. Colloid Interface Sci. 1987, 115, 330. (19) Philipse, A. P.; Vrij, A. J. Colloid Interface Sci. 1989, 128, 121. (20) Danov, K. D.; Kralchevsky, P. A.; Boneva, M. P. Langmuir 2004, 20, 6139. (21) Marinova, K. G.; Alargova, R. G.; Denkov, N. D.; Velev, O. D.; Petsev, D. N.; Ivanov, I. B.; Borwankar, R. P. Langmuir 1996, 12, 2045.

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assuming the film surfaces are part of spheres (see the Appendix). This, coupled with the (positive) electrostatic disjoining pressure, can explain the relatively large thickness of the plane parallel water film (∼100 nm) formed at the last stages of film thinning in the absence of particles. This thickness, however, is much smaller than the particle diameter. Hence, if particles are present, they should bridge the film surfaces at the final stages of its thinning. Whether the film will remain stable after bridging or will rupture depends on the particle contact angle. Water films could be stable if the bridging particle has a contact angle smaller than 90°, but they should rupture immediately if a particle with a contact angle greater than 90° bridges the film surfaces. Our results show that bridging occurs only at rapid opening of the water films. At slow opening, the particles are dragged by the hydrodynamic flow out of the expanding film, as shown in Figure 3b. They do not enter the film with a dimple even if the opening process is stopped, although the particles which are above the film are pushed toward it by the gravity. This suggests that there is a barrier for attachment of silica particles to the octane-water interface. This barrier should be of electrostatic origin,22 since both the fluid interface and particle are negatively charged. Hence, the particles do not bridge the water film surfaces at slow opening for two possible reasons: (i) the electrostatic barrier for particle attachment to the charged film surface and (ii) the large lateral mobility of the particles in the dilute monolayers at the water film surfaces allowing the hydrodynamic flow to drag particles out of the film region very easily. The latter is supported by the fact that there is not strong long-range repulsion between the silica particles with contact angles smaller than 99°; therefore, they sediment, giving an array of close-packed particles (see Figure 2a). The faster opening of the water film does not influence the factors discussed above. In this case, however, the particles attached to one of the film surfaces at the periphery of the expanding film move very rapidly in a lateral direction close to the opposite film surface. Such fast particle movement can create deformation of the fluid interface,23 which could be large enough to cause bridging of the film surfaces by some of the peripheral particles (see Figure 8a,b). This mechanism of particle bridging at fast opening of water films is consistent with the results for foam films obtained by others7 and with our results shown in Figure 3c,d. It is seen that some of the particles with contact angles of 65 and 85° are located in the thinnest region of the water film at the periphery of the dimple. They form an incomplete ring and do not sediment with time. The latter is strong evidence that those particles are attached simultaneously to both film surfaces, forming a particle bridge. The formation of a dimple and particle ring has been observed by Stancik et al.9 in the case of water films formed between an oil drop covered with latex particles and a planar particle monolayer at the oil-water interface. In their experiments, however, particles forming the ring have been trapped inside the dimple and hence they did not bridge the film surfaces. This is consistent with the large particle contact angle (∼130°) reported by them and the fact they have observed stable oil films. The requirement for a stable water film with bridging particles (a contact angle smaller than 90°) is not fulfilled in some of our experiments as well. For that reason, we have not observed stable water films when particles with contact angles of 152 and 99° (22) Paunov, V. N.; Binks, B. P.; Ashby, N. P. Langmuir 2002, 18, 6946. (23) Danov, K. D.; Aust, R.; Durst, F.; Lange, U. Chem. Eng. Sci. 1995, 50, 263.

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Figure 9. Sketch showing consecutive stages of thinning of the octane film in water corresponding to Figure 5a-e.

Figure 8. Bridging of water film surfaces by hydrophilic particles with a contact angle smaller than 90° during the film opening (a and b) and increase of the particle ring thickness during the film closing (c, cf. Figure 4b). The block arrow points to the deformation of the film surface caused by the fast lateral movement of a particle attached at the opposite film surface (a).

were used. In the latter case, the water films do not break during slow opening (no bridging particles, Figure 3b) but rupture at fast film opening when some of the particles bridge the film surfaces. In contrast, the particles with contact angles of 65 and 85° give stable bridges opposing the film thinning, thus stabilizing the water films. This is supported by the behavior of the bridging particles during closing of the film by pumping water into the meniscus (Figure 4). The bridging particles remain at the periphery of the shrinking film, forming a complete ring of touching particles (Figure 4a) during the film closing. Further shrinking of the film does not lead to particle detachment. Instead, the bridging particles rearrange, thus increasing the ring thickness (Figure 4b). If the process of film closing is stopped at this stage, the water film remains very stable for two reasons: (i) the thinnest part of the film at its periphery is stabilized by the bridging particles from the ring, while (ii) the central bare film region is thick due to the arrested dimple and therefore stable (see Figure 8c). In addition, the spontaneous process of film flattening (dimple disappearance) observed with films without particles is considerably slowed in this case due to the significant hydrodynamic resistance of the ring surrounding the dimple. The ring can be forced to shrink further by pumping water into the film meniscus, leading to disappearance of the dimple and formation of a disk of well ordered close-packed bridging particles (Figure 4c). Then, the two-dimensional colloid crystal starts “melting” at its periphery. The peripheral particles detach from one of the film surfaces, allowing them to sediment (Figure 4d). After reaching some critical size of the disk of bridging particles, the film surfaces suddenly separate. The unbridged particles remain randomly distributed between the opposite film surfaces and start sedimenting. The implications of the above findings for the stability of oilin-water emulsions will be discussed later. Stable octane-in-water films were obtained with the most hydrophobic particles with a contact angle of 152°.

The most significant difference between these particles and less hydrophobic ones is the former strongly repel each other and can give well ordered monolayers at interparticle distances larger than 5 particle diameters (see Figure 2b,c and ref 11). This is consistent with the strong long-range Coulombic repulsion through the oil due to charges at the particle-octane interface. Hence, in the case of octane films in the presence of particles with a contact angle of 152°, one can expect (i) a decreased lateral particle mobility due to repulsion between particles in the monolayer and (ii) long-range repulsion between the particle monolayers at the opposite film surfaces through the octane. The latter should act even at large film thicknesses, slowing down the thinning, thus stabilizing the octane film. The film thickness, h (therefore the rate of thinning), was not directly accessible in our experiments, but an estimate is given in Table 2. The observed changes in the monolayer structure during film thinning are clear evidence for the repulsion between particles attached at opposite film surfaces (see Figure 5a-c and the Appendix). When the octane film is thick (h > ∼12 µm), the repulsion between particles from the opposite film surfaces is smaller than that between the adjacent particles in the monolayer. Particle monolayers at the opposite film surfaces are well ordered in triangular lattices with the same lattice constant (15.3 ( 0.4 µm) randomly oriented to each other (Figures 5a and 9a). At film thicknesses smaller than ∼12 µm, however, a change in the monolayer structure from triangular to rhombic and then to square lattice (Figure 5b) is observed, resulting from the mutual interference between the monolayers. The square lattices of both monolayers have practically the same lattice constants with the triangular ones from which they have originated. The square lattices of both monolayers are well correlated. Particles from the one film surface are located exactly below the centers of the lattice cells (squares) at the opposite surface (Figures 5b and 9b). At smaller film thicknesses (h < ∼5 µm), achieved by sucking octane out of the meniscus, a transition from square to triangular lattice occurs (Figure 5c) and the lattice constant is approximately 1.5 times smaller than that of the noninteracting monolayers at large film thicknesses. The particles in this film region are simultaneously in focus; hence, the particles from the opposite film surfaces interdigitate (Figure 9c). The observed transitions in the monolayer structure, occurring only at the thinnest part of the octane film, are in agreement with the experimental findings of other researchers for the structural transitions in planar crystallized ion plasmas24 and in confined colloidal crystals.25 This is more evidence for the existence of Coulombic repulsion between the most hydrophobic silica particles due to charges at (24) Mitchell, T. B.; Bollinger, J. J.; Dubin, D. H. E.; Huang, X.-P.; Itano, W. M.; Baughman, R. H. Science 1998, 282, 1290. (25) Van Winkle, D. H.; Murray, C. A. Phys. Rev. A 1986, 34, 562.


the particle-octane interface. Up to the point shown in Figure 5c, the film has been forced to thin by sucking octane out of the meniscus. Further changes in the structure of the monolayers occur spontaneously and are consistent with bridging (zipping) the film surfaces by the particles in the thinnest part of the film. Our understanding of this process is the following. When some of the particles in the thinnest region of the film touch the opposite film surface, they become attached to both surfaces, thus bridging them. The bridging particles adjust their position in the middle of the film, keeping their contact angle, θ, approximately equal to that before bridging. Obviously, the latter requirement can be fulfilled only if the film surfaces are deformed, as shown in Figure 9d, because the film thickness is larger than that corresponding to a plane parallel film with a bridging particle, h0 ) -2R cos θ (θ > 90°). The deformations around the bridging particles overlap which creates a capillary force of attraction bringing particles together.26 The attractive force increases with a decrease of the interparticle distance;26 hence, the movement of bridging particles toward each other should accelerate when they become closer. This is in agreement with our experimental observations. It takes 720 ms to decrease the distance between bridging particles from ∼10 to ∼8 µm (Figure 5c-d), but 40 ms later, the interparticle distance decreases to ∼4 µm inside the crystalline disk of bridging particles (Figure 5e). Such a fast and dramatic decrease of interparticle distance in the central part of the film creates a “void” inside the monolayers of the meniscus surrounding the disk. For that reason, particles from the meniscus, which are at the periphery of the void, are pushed toward the crystalline disk due to interparticle Coulombic repulsion. Their trajectories are well seen in Figure 5e,f. After entering the thinnest film region close to the bridging monolayer (the disk), those particles bridge the film surfaces. The bridging creates deformation of the film surfaces, and due to capillary attraction, the particles which come from the meniscus become part of the bridging particle disk. The disk grows, increasing its diameter by collecting more and more particles from the film meniscus. Its growth stops when the film thickness in the meniscus at the disk periphery becomes large enough to prevent bridging of the incoming particles. At this point, two dilute monolayers of ordered particles in the meniscus are in equilibrium with the ordered dense (but not close-packed) bridging monolayer in the film center. The latter makes the film very stable. At some stage during disk growing, the menisci around the bridging particles could invert from convex to concave as a result of film thinning (cf. parts d and e of Figure 9). Our findings are consistent with the results of Stancik et al.9 for the thinning of oil films in the presence of hydrophobic latex particles. Stable octane films in the case of less hydrophobic particles with a contact angle of 99° have not been observed in our experiments, although the condition for a stable particle bridge (contact angle larger than 90°) is fulfilled. This is in agreement with the theoretical analysis of Denkov et al.27 for the stability of emulsion films with bridging particles. They have shown that the threshold capillary pressure in the film meniscus needed for film rupture drastically decreases when the particle contact angle approaches 90°. In some aspects, the behavior of octane films with bridging particles during film closing is very similar to (26) Kralchevsky, P. A.; Nagayama, K. Particles at Fluid Interfaces and Membranes; Elsevier Science: Amsterdam, The Netherlands, 2001. (27) Denkov, N. D.; Ivanov, I. B.; Kralchevsky, P. A.; Wasan, D. T. J. Colloid Interface Sci. 1992, 150, 589.

Langmuir, Vol. 21, No. 6, 2005 2337

that of the water films. In the beginning, the bridging monolayer shrinks without losing any particles (Figure 6a) until a two-dimensional crystal (disk) of close-packed particles is formed (Figure 6b). Then, the crystal starts melting at its periphery until some critical disk size is reached (Figure 6c), followed by sudden detachment (unzipping) of the film surfaces. In this case, however, the unbridged particles do not sediment but become part of the ordered monolayers at the film surfaces due to longrange Coulombic repulsion between particles (Figure 6d, inset). These results suggest that unzipping (unbridging) of the film surfaces is not a single act but a process. This could be important for interpretation of the results when the particle contact angle is determined by unbridging experiments.28,29 In summary, there are some similarities but also distinct differences in the thinning behavior of water and octane films with dilute particle monolayers at their surfaces. In both cases, films with bridging particles can be formed, in which stability depends on the particle contact angle. However, particles are expelled out of the water film, forming eventually a ring of bridging particles at the film periphery, thus leaving bare film surfaces at the central region. In contrast, the particle density in the center of the octane film increases during the final stages of thinning due to spontaneous formation and growth of a crystalline disk of bridging particles. The observed difference in thinning of water and octane films can be attributed to the different hydrodynamic conditions related to the viscosity difference between the film and the bulk phase and also to the different lateral mobilities of particle monolayers. The latter depends on the viscoelastic properties of the monolayers which in turn are influenced by the particle interactions.30 The monolayer elasticity in the case of very hydrophobic particles with a contact angle of 152° should be significantly greater than that of hydrophilic particles due to long-range Coulombic repulsion through the oil which is much stronger in the former case. The surface viscosity of the particle monolayers should be also very important.7,30 It seems to be low in our experiments with hydrophilic particles but could increase with the increase of monolayer density,30 especially if a network of interconnected particles is formed (see, e.g., ref 11, Figure 2a). Both the surface elasticity and viscosity should be high in the case of monolayers of close-packed particles irrespective of particle interactions. This case is considered below. 5.3. Films with Dense Particle Monolayers at Their Surfaces. The thinning of water and octane films with close-packed particle monolayers is very similar. The lateral mobility of film surfaces is practically missing in this case resembling the thin liquid film drainage between two solid surfaces. This can explain the absence of a dimple inside the water films in the considered case. The rate of film thinning should be slowed especially in the case of octane films due to strong long-range Coulombic repulsion between particle monolayers from the opposite film surfaces. When forced to thin, the water and octane films give a spot of bilayer of sticking particles from the opposite film surfaces (Figure 7a, inset). The stability of the hexagonally close-packed particle bilayer formed between emulsion drops has been theoretically analyzed in refs 31 and 32. (28) Ashby, N. P.; Binks, B. P.; Paunov, V. N. Chem. Commun. 2004, 436. (29) Stancik, E. J.; Fuller, G. G. Langmuir 2004, 20, 4805. (30) Tambe, D. E.; Sharma, M. M. J. Colloid Interface Sci. 1994, 162, 1. (31) Nushtayeva, A. V.; Kruglyakov, P. M. Colloid J. 2003, 65, 341.

2338


Horozov et al.

phase, and Rm ) (2/x3 - 1)R is the smallest radius of the toroid pore, that is, the radius of a circle inscribed between three touching spherical particles with radius R (Figure 10a). The maximum pressure, Pmax, is reached when the three-phase contact line is located below the equatorial radius,33 that is, at R > 90° (but not at R ) 90°, as assumed in refs 31 and 32, which underestimates Pmax). Pmax is given by the expression33

Pmax )

Figure 10. Sketch of a water film in oil with hexagonally closepacked particle monolayers at its surfaces: top view (a) and its cross section through the plane k-k′ perpendicular to the film plane (b) where only “T” and “B” particles are shown for simplicity. For other details, see the text.

Due to the (negative) pressure difference between the meniscus and the film (the sucking capillary pressure, Pc), the liquid inside the bilayer (the film) is displaced by the outer liquid, which penetrates into triangular pores formed between three touching particles in the monolayers (Figure 10a). The curvature of the liquid menisci inside the pores increases with the increase of capillary pressure, passes through a maximum, and then decreases. The maximum curvature corresponds to the maximum capillary pressure, Pmax; therefore, a further infinitesimally small increase of the pressure makes the film unstable and it should rupture. The problem for obtaining the maximum (critical) pressure for rupturing a liquid film stabilized by a particle bilayer is similar to that arising in porosimetry, where the pore size is determined from the maximum pressure needed for the penetration of test liquid into the porous solid. The model of a toroid pore33 is often used in porosimetry. It has been exploited in refs 31 and 32 for analyzing the film stability by using the following expression for the capillary pressure, Pc,

2γow sin(R - θf)

2γowx1 - 3 sin2 θf/4 R(2/x3 - sin Rmax)

(2)

where Rmax ) θf + arccos(x3 sin θf/2). Since in our experiments γow ) 50.6 mN/m and R ) 1.5 µm, by using eq 2, we obtain Pmax = 230 kPa for the water films at θf ) θ ) 65° and Pmax = 392 kPa for the octane films at θf ) 180° - θ ) 28°. The inner radius of the needle used for sucking liquid out of the meniscus is Rneedle ) 200 µm, hence the maximum capillary pressure which can be achieved in our experiments (2γow/Rneedle), is ∼0.5 kPa. The latter is much smaller than the critical values of the capillary pressure suggested by eq 2. Therefore, the films at the conditions corresponding to Figure 7 should remain stable in our experiments. The central part of the film with bilayer structure (Figure 7a, inset) should grow in size during film opening until occupying the whole film. Instead, a transition from bilayer to monolayer of particles bridging opposite film surfaces occurs shortly after the monolayers become in contact at the center of the film (Figure 7a). Hence, the particle bilayers become unstable and restructure at much lower capillary pressures well before reaching the critical values predicted by eq 2. The reasons are analyzed below. If the monolayers in contact are hexagonally stacked, each particle is in contact with three others from the opposite monolayer (Figure 10a). As a result, the forces acting normal to each of the monolayers generate lateral forces in the opposite monolayer acting to increase the lateral distance between particles, and therefore to decrease the bilayer thickness. The latter will remain constant only if the lateral stress (the lateral force per unit length) acting at the periphery of the bilayer is balanced by the lateral stresses in the two separate monolayers from the meniscus. We will assume that the three-phase contact line around the particles is a circle with radius Rc ) R sin R at any capillary pressure.27 Then, the normal force exerted on each adsorbed particle from one of the film surfaces (Figure 10b) is27

F ) 2πRcγow sin(R - θf) + πRc2Pc

(3)

The lateral component of the force which one particle from the monolayer exerts on another one from the opposite monolayers at the point of contact is given by

FL ) (F/n) cos φ sin φ

(4)

cos φ ) Hb/2R

(5)

where

(1)

and n is the number of particles from the opposite monolayer which are in contact with the considered

where γow is the interfacial tension at the oil-water interface, R is the central angle shown in Figure 10b, θf is the particle contact angle measured through the film

(32) Kruglyakov, P. M.; Nushtayeva, A. V. Adv. Colloid Interface Sci. 2004, 108-109, 151. (33) Mason, G.; Morrow, N. R. J. Colloid Interface Sci. 1994, 168, 130.

Pc )

R(1 + Rm/R - sin R)


Langmuir, Vol. 21, No. 6, 2005 2339

Table 1. Stability of Water Films in Octane and Octane Films in Watera film surfaces with dilute monolayers

film surfaces with dense monolayers

contact angle (deg)

water films in oil

oil films in water

water films in oil

oil films in water

65 ( 3 85 ( 2 99 ( 2 152 ( 2

stable (ring formation) stable (ring formation) unstable unstable

unstable unstable unstable very stable (crystallization)

very stable stable unstable unstable

unstable unstable unstable very stable

a Unstable films break during their formation or several seconds later. Stable films survive for up to ∼30 min, while those denoted as “very stable” live more than 1 h.

particle (Figure 10b). In the case of hexagonally ordered particle monolayers, n can be equal to 1, 2, or 3 depending on the stacking between the monolayers which form the bilayer. For instance, n equals 3 for hexagonally stacked monolayers (Figure 10). Substitution of eq 3 into eq 4 gives

FL )

[

]

2πRγow sin2 R sin(R - θf) R + P sin φ cos φ n sin R 2γow c (6)

It is seen that FL equals zero at Pc ) 0 (R ) θf, see eq 1). At Pc > 0, the lateral force, FL, should be equal to zero at φ ) 90°, which corresponds to a monolayer of bridging particles (Hb ) 0), or in the special case when each particle from one monolayer is located exactly above one particle from the opposite monolayer, therefore φ ) 0° (Hb ) 2R, n ) 1). The latter situation is hardly realized in practice. Hence, in the general case of randomly stacked monolayers and Pc > 0, the lateral forces within the bilayer should create an additional lateral stress ∆τf(Pc) proportional to FL

∆τf(Pc) ) τf(Pc) - τf0

(7)

where τf(Pc) and τf0 are the lateral stresses in the film (the bilayer) at the capillary pressure equal to Pc and zero, respectively. If the film is in mechanical equilibrium, the lateral stress balance at the film periphery reads

τf(Pc) ) 2τm(Pc)

(8)

where τm(Pc) is the lateral stress in each of the particle monolayers from the meniscus. From eqs 7 and 8, we obtain

τm(Pc) ) τm0 + ∆τf(Pc)/2

(9)

where τm0 ) (τf0/2) at Pc ) 0. Hence, the lateral stress in the monolayers from the meniscus should grow up with an increase of the capillary pressure. Compression experiments performed with a Langmuir trough have shown that the lateral stress in particle monolayers at oil-water interfaces cannot increase indefinitely.12,34 There is some critical value, τ/m (corresponding to a close-packed particle monolayer), at which the monolayer starts bending, therefore keeping the lateral stress almost constant and practically equal to γow (the surface pressure reaches a plateau at small trough areas).12,34 Hence, there is a critical capillary pressure, P/c , corresponding to the critical lateral stress in the monolayer τm(P/c ) ) τ/m. If P/c is exceeded, the monolayers in the film meniscus should start to deform (bend) and the film (the bilayer) should expand, decreasing its thickness, Hb, until a monolayer of bridging particles is formed (Hb ) 0, FL ) 0, see eqs 5 and 6 and Figure 10b). Therefore, the stability of a particle bilayer is determined (34) Aveyard, R.; Clint, J. H.; Nees, D.; Quirke, N. Langmuir 2000, 16, 8820.

from the value of τ/m, which in turn should depend on the bending rigidity of the monolayers in the meniscus. Our experiments suggest that the critical capillary pressure, P/c , at which a transition from bilayer to bridging monolayer occurs, is much smaller than the critical pressure, Pmax, needed for rupturing the film stabilized by a particle bilayer. Hence, the overall film stability is determined by the stability of the bridging particle monolayer, which has already been analyzed theoretically in ref 27. This should be very important when the stability of particle stabilized emulsions is considered (see below). 5.4. Implications of the Results for Particle Stabilized Emulsions. The above results shed more light on the behavior and stability of thinning emulsion films with particle-laden surfaces. Such films are formed between colliding drops in particle stabilized emulsions; therefore, the present results have an impact on better understanding of the mechanisms of emulsion stabilization by solid particles. The results for the film stability are summarized in Table 1. Stable films are observed only in those cases when the particle contact angle, θ, fulfills the condition for a stable particle bridge, that is, θ < 90° for water films in oil (which correspond to oil-in-water emulsions) or θ > 90° for oil films in water (which correspond to water-in-oil emulsions). Hence, the results are in general agreement with the well accepted concept that hydrophilic particles can give stable oil-in-water emulsions, whereas hydrophobic ones, water-in-oil emulsions.10 The formation of a bridging particle monolayer seems to be a common feature of the emulsion films irrespective of the particle coverage at the surface. This should lead to a strong droplet flocculation in the emulsions stabilized by solid particles. In addition, the long-range capillary attraction between bridging particles, resulting from the curved menisci around them, brings particles together, thus increasing particle coverage in the thinnest part of the emulsion films. This phenomenon, which is more pronounced in the systems with long-range lateral repulsion between adsorbed particles, should stabilize the films formed between emulsion droplets partially covered with particles. This can explain some experimental observations of stable emulsions at low particle coverage at their droplets.3 The formation of a dense monolayer of bridging particles in the thinnest part of the films strongly depends on the film type (water film-in-oil or oil film-inwater) and the viscoelastic properties of the particle monolayers. A dense bridging monolayer (crystalline disk) is formed spontaneously during the thinning of oil films with dilute monolayers at their surfaces. This should also happen during the collision between the water droplets in w/o emulsions, thus preventing them from coalescing. In contrast, the particle density in the central part of the water films is substantially decreased during the thinning. This should make the oil droplets in o/w emulsions more vulnerable to coalescence at their collisions than the water droplets in w/o emulsions. However, if the water film does not rupture, it can become very stable during the separation of the emulsion droplets in the agitated emulsion due

2340


to the formation of a bridging particle ring with a thick arrested dimple in its interior. In general, one can expect that the most hydrophobic particles used in this study (θ ) 152°) could give stable w/o emulsions at lower particle coverage of the emulsion droplets in comparison to that in the case of o/w emulsions stabilized by the most hydrophilic particles (θ ) 65°). The latter hypothesis is supported by our very recent experimental data which will be published soon separately. Many experiments show that very stable emulsions can be formed if the emulsion droplets are covered by closepacked particle layers.2,10,35 This has been attributed to the significant reduction of the rate of film thinning between emulsion droplets and to the steric hindrance for droplet coalescence30 reflecting on the very high capillary pressure needed for the film rupture.31,32 The high stability of emulsion films with dense particle monolayers at their surfaces observed by us is in agreement with the above findings (Table 1). However, our experiments also show that a transition from bilayer to bridging monolayer can occur at not very large capillary pressures. This suggests that the stabilizing mechanism of large emulsion drops could be different from that of small ones due to the greater capillary pressure in the latter case. The large drops could be stabilized by a particle bilayer, whereas the smaller ones, by a monolayer of bridging particles. This should depend on the bending rigidity of the particle monolayers around the colliding droplets. The above hypothesis needs further verification. 6. Conclusions We have studied vertical emulsion films with particle monolayers at their surfaces. The obtained results show that the structure and stability of the emulsion films strongly depend on the film type (water film-in-oil or oil film-in-water), particle contact angle, interactions between particles from the same and the opposite monolayer, and monolayer density. Stable films are observed only when the particle wettability fulfills the condition for stable particle bridgess in agreement with the concept that hydrophilic particles can give stable oil-in-water emulsions (i.e., stable water films in oil), whereas the hydrophobic ones, water-in-oil emulsions (i.e., stable oil films in water).10 The structural changes in ordered monolayers observed during the thinning of octane films with hydrophobic particles are very similar to the structural transitions in planar crystallized ion plasmas.24 This confirms the existence of Coulombic repulsion between the most hydrophobic silica particles through the oil due to charges at the particle-octane interface. Such strong long-range particle repulsion is absent in the case of hydrophilic particles; therefore, their lateral mobility is much greater than that of very hydrophobic ones. That is why the hydrophilic particles are expelled out of the thinning water film, forming eventually a ring of bridging particles at the film periphery. The bare film surface formed at the central region of the water films should make them more vulnerable to rupture. In contrast, the particle coverage in the center of the octane film increases during the final stages of thinning, thus stabilizing the film. A dense particle disk bridging the film surfaces is spontaneously formed due to the long-range capillary attraction between bridging particles, resulting from the curved menisci (35) Tambe, D. E.; Sharma, M. M. J. Colloid Interface Sci. 1993, 157, 244. (36) Lazarov, G. S.; Denkov, N. D.; Velev, O. D.; Kralchevsky, P. A.; Nagayama, K. J. Chem. Soc., Faraday Trans. 1994, 90, 2077.

Horozov et al.

around them. This can explain some experimental observations of stable emulsions at low particle coverage at the droplets.3 The formation of a bridging particle monolayer between the film surfaces seems to be a common feature of the studied systems. It occurs spontaneously in emulsion films with dilute monolayers at their surfaces. When the film surfaces are covered with close-packed particle monolayers, the bridging monolayer is formed as a result of bilayer to monolayer transition. This transition occurs at some critical value of the capillary pressure which is much smaller than the critical pressure for rupturing the film stabilized by a particle bilayer. This can be attributed to the limited ability of the monolayers in the meniscus to sustain the additional lateral stress generated in the bilayer at an elevated capillary pressure. Hence, the overall film stability should depend mainly on the stability of the bridging particle monolayer. These findings have important implications for understanding the mechanisms of emulsion stabilization by solid particles. Acknowledgment. The authors gratefully acknowledge the provision by the EPSRC of a ROPA grant (GR/ N02778) for Dr. T. S. Horozov. Appendix Estimation of the Film Thickness and Capillary Pressure. Let us consider a liquid film formed between two spherical fluid surfaces with radii Rf (Figure 11). The periphery of the film is attached to the edge of a circular frame with radius a and thickness b. The thinnest part of the film has a thickness h0. It can be easily shown that the film thickness, h, at distance r from the center fulfills the following equation

[ (

h0 h + Rf 2 2

)]

2

+ r2 ) Rf2

(A.1)

with the following boundary conditions:

h ) h0

at r ) 0

(A.2a)

h)b

at r ) a

(A.2b)

From eqs A.1 and A.2, we obtain

h ) h0 + 2(Rf - xRf2 - r2)

(A.3)

where

Rf )

b - h0 a2 + 4 b - h0

(A.4)

Equations A.3 and A.4 allow one to calculate the film thickness (the smallest distance between the fluid film surfaces) at any distance r from the center of the film if h0 and the dimensions of the frame (a and b) are known. The capillary pressure, Pc, can also be calculated by the expression

Pc )

2γow 8γow(b - h0) ) 2 Rf 4a + (b - h )2

(A.5)

0

if the interfacial tension, γow, of the fluid interface is known. Equation A.5 can be further simplified if h0 , b to read

Pc ) 8γowb/(4a2 + b2)

(A.6)


Langmuir, Vol. 21, No. 6, 2005 2341 Table 2. Monolayer Structure, Lattice Constant, and Film Thickness Obtained from Figure 12 (See the Appendix) region type of lattice lattice constant (µm) film thickness (µm) I II III

Figure 11. Sketch of the cross section of a liquid film formed in a circular frame with radius a and thickness b.

triangular square triangular

∼10 ∼15 ∼15

2.824-4.9 4.9-11.4 >11.4

An image taken during the opening of octane film in water in the case of particles with a contact angle of θ ) 152° just before bridging of the film surfaces is shown in Figure 12a. This image is similar to that in Figure 5c but has been taken at a lower magnification; therefore, a larger area of the film is seen. Three coexisting regions with different monolayer structure can be distinguished. Region I corresponds to the thinnest part of the film. Particles from the opposite film surfaces are undistinguishable. They are organized in a triangular lattice formed from the interdigitated particle monolayers, as shown in Figure 9c. The particle monolayers at larger film thicknesses in region II are ordered in square lattices. Far from the film center at even larger thicknesses, the particle monolayers at the film surfaces consist of hexagonally ordered lattices which are randomly oriented to each other (region III). The film thickness in the center of region I is determined by the height, hoil, of that part of particles which is immersed in oil. Hence,

h0 ) hoil ) R(1 - cos θ)

Figure 12. Image taken during the opening of octane film in water in the case of particles with a contact angle of 152° just before bridging of the film surfaces (a) and the corresponding film thickness versus radial distance (b) calculated by eq A.3 with h0 ) 2.824 µm, a ) 3.1 mm, and b ) 2 mm (cf. Figure 5c). The monolayer structure, lattice constant, and film thickness corresponding to the film regions I-III are summarized in Table 2. The scale bar is equal to 50 µm.

In our experiments, a ) 3.1 mm, b ) 2 mm, and γow ) 50.6 mN/m; therefore, at h0 < 200 µm, the capillary pressure is Pc ≈ 20 Pa (eq A.6).

(A.7)

Using the values for the particle radius R ) 1.5 µm and contact angle θ ) 152° in eq A.7, we obtain h0 ) 2.824 µm. The latter value was used to calculate the film thickness by eqs A.3 and A.4, and the result is plotted versus radial distance in Figure 12b. The film thicknesses corresponding to regions I-III are summarized in Table 2. One should expect that the changes in monolayer stucture during the film thinning (Figure 5a-c) occur at film thicknesses approximately equal to those in Table 2 estimated by the above considerations. Supporting Information Available: A movie showing the thinning of octane film in water in the presence of hydrophobic silica particles with a contact angle of 152° at its surfaces (cf. Figure 5). This material is available free of charge via the Internet at http://pubs.acs.org. LA047993P

Particle Zips: Vertical Emulsion Films with Particle Monolayers at Their

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