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Aqueous foams stabilized by Laponite and CTAB
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Colloids and Surfaces A: Physicochem. Eng. Aspects 317 (2008) 406–413
Aqueous foams stabilized by Laponite and CTAB Shuiyan Zhang, Qiang Lan, Qian Liu, Jian Xu, Dejun Sun ∗ Key Laboratory for Colloid & Interface Chemistry of Education Ministry, Shandong University, Jinan, Shandong 250100, People’s Republic of China Received 6 June 2007; received in revised form 17 October 2007; accepted 7 November 2007 Available online 17 November 2007
Abstract Aqueous foams prepared by cetyltrimethylammonium bromide (CTAB) and disk-like Laponite particles dispersions are studied. The particle/CTAB dispersions have a synergistic effect on foam stability at intermediate CTAB concentrations. The foam stability increases with increasing particle concentration. At a fixed particle concentration, the foam stability first increases with increasing CTAB concentration and reaches a maximum at a CTAB concentration around 1.7 cec. After that, the foam stability decreases and then remains unchanged. The effects of hydrophobicity and electrical property of modified Laponite particles on foam stability are discussed. The hydrophobicity of Laponite particles is predominant in stability of foams compared with the electrical property of particles. The most stable foams were obtained by particles with the maximum hydrophobicity (the contact angle is still smaller than 90◦ ). Two mechanisms promote foam stability: (a) adsorption of modified particles on the bubble surface; (b) formation of an aggregate structure by the armored bubbles and particles in the bulk phase, which is confirmed by laser-induced fluorescent confocal microscopy. © 2007 Elsevier B.V. All rights reserved. Keywords: Foam stability; Laponite particles; CTAB; Hydrophobicity; Adsorption
1. Introduction Foams stabilized by colloidal particles have received considerable attention in recent years due to their wide applications, ranging from food and cosmetics to oil recovery, flotation [1,2], fire extinguishing and water-borne coatings [3]. They are also used as intermediate structures [4,5] or templates [6] to produce new materials. For particle-stabilized foams, it has been widely accepted that stable foams are difficult to form when the particles are hydrophobic, e.g. with a contact angle generally greater than 90◦ . The hydrophobic particles are believed to behave as antifoaming agents by a bridging–dewetting mechanism [7]. Only partially hydrophobic or hydrophilic particles can be used to stabilize foams. The foams are stabilized in two ways: (a) adsorption of colloidal particles at the air–water interface [4,8–12], which is a crucial factor for preparing stable foams and (b) stratification of non-adsorbing particles in the intervening thin film separating
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the dispersed phases [13–17], which improves the stability of foams against drainage. Many researchers [4,8–12] have emphasized that the strong attachment of particles at the bubble surface and a network or a dense layer of particles at the interface are responsible for foam stability. In most cases, the adsorption or assembly of colloidal particles at air–water interfaces is controlled by adjusting the surface hydrophobicity of colloidal particles. Binks and Horozov [18] and Murray and co-workers [19,20] studied foams stabilized by partly hydrophobic fumed silica particles which were modified by chemical reaction with a silane coupling agent. They also investigated the influence of particle hydrophobicity [18] and salt concentrations [20] on foam stability. Studart and co-workers [4,5,21] accomplished in-situ hydrophobization of particles through the adsorption of short-chain amphiphiles. High-volume and ultrastable foams were prepared by those particles. Fujii et al. [11,22] reported that highly stable foams were formed by micrometer-sized, sterically stabilized polystyrene (PS) latex particles. Particle wettability can also be modified by the adsorption of long-chain surfactants on the particle surfaces. Wilson [23] investigated foams prepared by addition of a cationic surfactant
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to aqueous dispersions of polystyrene latex. He found that both low charge and relatively high hydrophobicity of particles were responsible for the foam stability. Velikov et al. [24] investigated the influence of different surfactants on the dynamics of film thinning in the presence of negatively charged polystyrene latex particles. For SDS-stabilized films, all of the particles were expelled from the film due to their repulsion from the film surfaces. For cationic surfactant CTAB-stabilized films, however, the same particles became partially hydrophobic and a layer of particles was adsorbed simultaneously at the two film surfaces. Garrett et al. [3] studied the effect of latex particles on foaming and antifoaming in surfactant solutions. The particles can lead to three effects: depletion of surfactant concentration by adsorption loss, increase in viscosity and stratification stabilization of thin liquid films by the particles. Addition of surfactant to particle dispersions does not always promote foam stability. Alargova et al. [12] found that the stability of foams prepared by mixtures of hydrophobic rod-shaped particles and sodium dodecyl sulfate (SDS) was decreased compared with that by the particles alone, meaning that SDS acted as a defoamer. It is possible that the adsorption of SDS onto the particles rendered them more hydrophilic and then the particles lost their affinity for the solution/air interface. Fujii et al. [11] found that foams could not be formed using poly Nvinylpyrrolidone (PNVP)-stabilized polystyrene particles if they contained excess PNVP. Weak, short-lived foams were formed but collapsed within 10 s after two centrifugation/redispersion cycles to remove the nonadsorbed PNVP. After the third centrifugation/redispersion cycle, highly stable foams that remained stable for more than one year were obtained, but this behavior was not explained. Subramaniam et al. [25] reported the destabilization of particle-stabilized bubbles exposed to various concentrations of surfactant solutions. They proposed a microstructural mechanism, which recognized the role of interfacial jamming and stresses in particle stabilization and surfactant-mediated destabilization of armored bubbles. In this paper, disk-like Laponite particles and CTAB surfactant are used to prepare aqueous foams. Firstly, the properties of Laponite/CTAB aqueous dispersions are studied. Then the stability of foams prepared by Laponite/CTAB dispersions are described and discussed systematically in relation to particle hydrophobicity and zeta potential. The stabilization mechanisms of the foams are also discussed. 2. Experimental 2.1. Materials The water was deionized water purified by ion exchange. Laponite RD, a synthetic hectorite, was supplied by Rockwood Additives Ltd. (UK) as a white powder. It is composed of rigid disk-shaped crystals with a well-defined thickness of 1 nm and an average diameter about 30 nm. The molecular formula of Laponite is Na0.7 [(Si8 Mg5.5 Li0.4 )O20 (OH)4 ]. The surfactantmediated of the synthetic clay is about 0.75 meq/g. Each particle has a central layer made of Mg2+ cations in octahedral coordination to oxygen atoms or hydroxyl groups; this central layer is
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sandwiched between two silicate layers where the silica atoms are in tetrahedral coordination to oxygen atoms. Some of the Mg2+ sites of the central layer are substituted with Li+ cations. This creates a charge imbalance, which is compensated by Na+ counterions located at the surface of the outer layers. When the powders are dispersed in water, the Na+ ions on the particle surface are released and a strongly negative charge appears on the faces of the disks. On the other hand, the edges of the platelets carry some Mg OH, Si OH and Li OH sites which are weak bases. Because of the protonation of the hydroxyl groups by water, a weakly positive charge appears on the rim of the disks. The surfactant used was cetyltrimethylammonium bromide (CTAB) purchased from Sinopharm Chemical Reagent Co. Ltd. (China) with a purity >99%. 2.2. Methods 2.2.1. Preparation of CTAB/Laponite aqueous dispersions The Laponite stock dispersion was prepared by dispersing a known mass of Laponite into deionized water using a Multimixer (Baroid Co., USA). The Laponite dispersions were sealed and laid aside at room temperature (25 ◦ C) for 1 week before use. The Laponite/CTAB dispersions were prepared by diluting the stock dispersions with CTAB solutions. The prepared dispersions were stirred for at least 12 h to attain adsorption equilibrium. The initial CTAB concentration in the dispersions is expressed as a multiple of the clay’s CEC (0.75 mmol/g). 2.2.2. Adsorption of CTAB on Laponite particles The prepared dispersion is centrifuged for 60 min at 20,000 rpm in order to separate the clay particles from supernatant. CTA+ in supernatant is analyzed by TOC technique. The adsorption amount was calculated from the difference between initial and equilibrium concentrations of surfactant and divided by the mass of the dried solid. 2.2.3. Stability of the Laponite/CTAB aqueous dispersions The prepared dispersions were first transferred into a stoppered, graduated glass tube with internal diameter 1.6 cm and length 15 cm. Then the phase behavior of the dispersions was observed after 72 h. 2.2.4. Zeta potential measurements The zeta potentials of Laponite/CTAB dispersions were measured with a JS94H microelectrophoresis instrument (Shanghai Zhongchen Digital Technic Apparatus Co., China). The particles in the sediment phase were diluted with the upper clear liquid to make the particles visible under the microscope before measurement. 2.2.5. Three-phase contact angle of particles The three-phase contact angle of the Laponite particles was measured using the classic captive drop method [4,20,26]. Laponite/CTAB aqueous dispersions were first washed with deionized water to remove the unadsorbed CTAB molecules. Then, the samples were dried at 60 ◦ C and crushed into powders. The powders were compressed into a circular disk with
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a 769YP-15 Press at a pressure of 16 MPa. The thickness of the compressed disk was about 1 mm. A drop of water was first placed on the disk surface and then the shape of the water drop on the disk was immediately photographed. The contact angle was directly measured using a protractor. 2.2.6. Stability of foams Foams were prepared with Laponite dispersions, CTAB solutions, and Laponite/CTAB dispersions respectively using a lab homogenizer (Shanghai Forerunner M&E Co., China) operated at 8000 rpm for 5 min. After homogenization, foams were immediately transferred into the glass tubes mentioned above (25 ml foams). The time needed for 5 ml of liquid to drain from the 25 ml foams was recorded to evaluate the foam stability. The morphology of the bubbles was observed with an Axioskop 40 microscope (ZEISS, Germany). 2.2.7. Adsorption of particles at the bubble surface A laser-induced confocal microscope (Olympus Fluoview 500, Japan) was used to investigate the adsorption of Laponite particles at the bubble surface. The fluorescent probe was Rhodamine B, which is negatively charged in basic solution and has a maximum excitation wavelength at 543 nm. The CTABmodified Laponite particles were first labeled with Rhodamine B. Then the particle dispersions were washed with deionized water to remove the free Rhodamine B in the bulk. Foams stabilized by the labeled particles were prepared and fluorescent images of the foams were observed under the microscope.
Fig. 1. Adsorption isotherm of CTAB on the Laponite of 1.0 wt% at 25 ◦ C.
concentrations above 2.0 cec, the sediments at the bottom of the tube redisperse. Labbe et al. [28] indicated that addition of a surfactant over the CEC caused the aggregates to partially redisperse. Hanley et al. [29,30] reported that a complex must adsorb CTAB to an amount corresponding to at least two multiples of the clay’s cation exchange capacity to be dispersed. Here, the sediment redisperses when the CTAB concentration is above 2.0 cec, corresponding to the results of Hanley et al. [29,30]. 3.2. Zeta potentials of Laponite/CTAB aqueous dispersions
3. Results and discussion 3.1. Properties of CTAB/Laponite aqueous dispersions The pH of CTAB/Laponite aqueous dispersion was fixed at 9.0. The adsorption isotherm of CTA+ on the Laponite of 1.0 wt% at 25 ◦ C is shown in Fig. 1. And obviously, the adsorption isotherm is “S” type. It is like the four regions isotherms described by Brahimi et al. [27], corresponding to the exchange with the anionic clay sites, to the adsorption by van der Waals interactions, to the hemimicellization on the Laponite surface, and to the micellization in the bulk. The phase behavior of Laponite/CTAB aqueous dispersions is shown in Fig. 2. At CTAB concentrations below 0.3 cec, the dispersions are sols. At CTAB concentrations from 0.3 to 2.0 cec, phase separation occurs due to the flocculation of the Laponite particles. At CTAB
The change of zeta potential of the Laponite particles as CTAB concentration increases is shown in Fig. 3. The zeta potential of the particles decreases initially with increasing CTAB concentration, and changes to a positive value around 1.0 cec of CTAB. Then the zeta potential of the particles increases with increasing CTAB concentration. 3.3. Hydrophobicity of modified particles The hydrophobicity of particles is generally described by contact angle. It is an important parameter in determining the adsorption of particles at the air–water interface [8] and the foam stability [31]. Here, the variation of wettability of Laponite particles with CTAB concentration was studied (Fig. 4). In the absence of CTAB, the contact angle of Laponite is very small
Fig. 2. Appearance of the Laponite/CTAB dispersions. The CTAB concentrations are shown in the picture (cec). The particle concentration is fixed at 1.0 wt%.
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Fig. 5. Drainage time of aqueous foams stabilized with CTAB alone. Fig. 3. Zeta potential of Laponite/CTAB aqueous dispersions. The particle concentration in the dispersions is 2.0 wt%.
(less than 10◦ ), meaning the particles are very hydrophilic. In the presence of CTAB, the contact angle first increases with increasing CTAB concentration up to a maximum value at 1.7 cec. Further increases of CTAB concentration lead to a decrease of θ. When the CTAB concentration is over 3.0 cec, the CTABmodified Laponite particles become hydrophilic again (less than 20◦ ). So the hydrophobicity of particle reaches a maximum at 1.7 cec. 3.4. Properties of foams stabilized by CTAB and Laponite particles 3.4.1. Foam stability To understand the effect of the particles and the surfactant on the foam stability, three different systems were investigated, including particle dispersions, surfactant solutions and Laponite/CTAB dispersions. Foams cannot be prepared solely by Laponite dispersions, even in the presence of electrolytes such as NaCl. In Fig. 5, the stability of foams prepared by surfactant solutions first increases with increasing CTAB concentration and
Fig. 4. Three-phase contact angle of particles as a function of CTAB concentration.
then remains unchanged at CTAB concentrations above its cmc (about 0.98 mM). The stability of foams prepared by Laponite/CTAB dispersions is shown in Fig. 6. The concentrations of Laponite in the dispersions investigated were 0.5, 1.0, 1.5 and 2.0 wt%, and the CTAB concentrations varied from 0 to 5.0 cec at each particle concentration. The foam stability increases with increasing particle concentration. At each particle concentration, the most stable foams are formed at the same concentration ratio of CTAB to Laponite (about 1.7 cec). At a fixed particle concentration, four regions were defined according to the foam stability. No foams can be formed at relatively low (0–1.0 cec) CTAB concentrations (region I). The foam stability increases gradually with increasing CTAB concentration and reaches a maximum at 1.7 cec (region II), then decreases (region III). In region IV, the foam stability is low, but does not decrease further. In region I, the stability of foams prepared by Laponite/CTAB dispersions is much lower than that of foams prepared with surfactant solution (above cmc) alone. Stable foams are hardly formed. In regions II and III, the foams stabilized by Laponite/CTAB are more sta-
Fig. 6. Drainage time of foams stabilized with Laponite/CTAB system as a function of CTAB concentration at different particle concentrations indicated above (wt%).
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ble than those stabilized by CTAB alone (above cmc). In region IV, the foam stability is almost the same as that of pure CTAB solution (above cmc). It can be deduced that the particle/CTAB system has a synergistic effect on foam stability at intermediate surfactant concentrations. The adsorption feature of CTAB on Laponite particles was further understood in virtue of the foam stability. When the concentration of CTAB is lower than 1.0 cec, the adsorption results from the electrostatic interaction between particle surface and head groups of the ionic surfactant. After that, the adsorption process is no longer driven by electrostatic interaction and that hydrophobic interactions probably are involved. Patches of two-dimensional aggregates on the surface seem to be formed. The solid becomes more hydrophobic. Such an association is analogous to micelle formation in the bulk solution and results from the increase in the entropy of the system when the alkyl
chains are removed from the aqueous environment. The twodimensional aggregates are called hemimicelles by Gaudin and Fuerstenau. The hemimicelle concept was developed by them in 1955 [32] for the adsorption of ionic surfactant on hydrophilic surfaces. If the adsorbed ions associate tightly through van der Waals attraction between hydrocarbon chains, a minimum surface area of hydrocarbon chain will be in contact with water. In recent references of [33,34], Fuerstenau inferred that adsorbed surfactant aggregates can exist with their hydrophobic tails oriented towards the solution (classical hemimicelles), or with some of the adsorbed surfactant ions having their hydrophilic heads oriented towards the water bulk phase (reverse hemimicelles), or as an actual bilayer (complete monolayer coverage before reverse orientation begins to form a complete second layer in reverse orientation). When the concentration of CTAB reach about 1.7 cec, the hydrophobicity and flocculation of par-
Fig. 7. (a–d) Optical microscope images of foams stabilized by Laponite and CTAB dispersions. From (a) to (d), the CTAB concentrations in the dispersions are 1.4 cec, 1.7 cec, 2.0 cec and 2.5 cec. (e) Size distributions of these bubbles. The Laponite concentration was fixed at 1.0 wt%.
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Fig. 8. Confocal fluorescence image (a) and optical microscopy image (b) of foams stabilized by fluorescently labeled CTAB-modified Laponite particles. The particle concentration is 1.0 wt% and surfactant concentration 1.7 cec.
ticles both attain the maximum values, so the foam stability also obtains the maximum. After this point, a second surfactant layer (admicelle) with head groups oriented toward the solution, that is, the reverse hemimicelles form gradually. So, the hydrophobicity of particles begin to decrease, the flocculated particles begins to redisperse and the foam stability declines. When the adsorption attains a plateau, the free micelles are formed in dispersions. The particles cannot adsorb on the air–water interface, as a result the foams are only stabilized by CTAB molecules. It can be concluded that foam stability is related with the adsorption feature of CTAB on Laponite particles. Optical microscope images of the foams stabilized by CTAB/Laponite dispersions are shown in Fig. 7. The bubble size initially decreases, then increases with increasing CTAB concentration. The bubble size and polydispersity have been shown in Fig. 7(e). The bubble size of the most stable foam is the smallest. The bubble size distribution in the foam is wide at low particle concentration but is narrowest for the most stable foams (at 1.7 cec). The bubble size results agree well with the foam stability results. 3.4.2. Adsorption of particles on the bubble surface Adsorption of CTAB on the Laponite particles changes their hydrophobicity and then the partially hydrophobic Laponite particles are able to attach to air bubbles. Laser-induced confocal scanning microscopy experiments were performed to confirm the adsorption of Laponite at the bubble surface in regions II and III, as shown in Fig. 8. Aggregates of Laponite particles adsorb on the bubble surface. In addition, flocculation is observed clearly between the armored bubbles and the unadsorbed particles in the surrounding continuous phase. As Murray et al. [10,19] suggested, the particles adsorbed onto the bubbles are part of a particle network in the bulk aqueous solution. Photographs of dried foams were taken 12 h after preparation (Fig. 9). In regions II and III, the particles are located within the dried foam phase, which indicates that the particles take part in stabilizing the foams. In region IV, there is nothing left of the dried foam phase. It can be seen from Figs. 5 and 6 that
the foam stability is the same as that of pure CTAB solution (CCTAB > cmc). These results indicate that the particles do not adsorb on the bubble surface and the foams are stabilized only by free surfactant in the dispersion. The effect of particle concentration on the foam stability in different regions can be explained by adsorption of the particles on the bubble surface. In region I, the hydrophilic Laponite cannot stabilize the foams. The foam stability increases with increasing particle concentration in regions II and III. In these two regions, the adsorbed particles form an interfacial armor around the bubble surface that mechanically impedes bubble shrinkage and coalescence. Hence, the increase of particle concentration can improve the foam stability. Tang et al. [35] and Gonzenbach et al. [36] have reported that foam stability increases with increasing particle concentration. In region IV, the foam stability is independent of the particle concentration. This is due to the particles becoming hydrophilic again so that the foams are stabilized only by the free CTAB molecules. 3.4.3. Effect of surface charge and hydrophobicity of particles on foam stability In region I, foams cannot be produced. The surface charge of the Laponite particles decreases initially with increasing
Fig. 9. Appearance of dried foams prepared with Laponite/CTAB dispersions. The Laponite particle concentration is 1.0 wt%. The CTAB concentration in the dispersion is indicated above (cec).
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CTAB concentration and approaches zero at 1.0 cec. The particle–particle and particle–interface electrostatic repulsion decreases due to the decreased zeta potentials of particles, which will promote the particle adsorption at the interface [37]. Williams and Berg [38] and Abdel-Fattah and El-Genk [39] have also found that the reduction of particle surface charge promotes the adsorption of hydrophobic, negatively charged particles at the interface by decreasing the electrical barrier between the particles and the interface. El-Genk et al. [39,40] have proposed that adsorption of colloidal particles at the air–water interface is mainly controlled by the particle–interface and particle–particle interactions. Besides the electrostatic interactions between the particle and the interface, the hydrophobicity of the particles is also important for particle adsorption at the interface. In region I, the particle is still hydrophilic so the hydrophobic interaction between particles and the interface can be neglected. Here, the foams cannot be produced more likely due to the strong hydrophilicity of Laponite particles. Moreover, the free CTAB molecules in the dispersions are too few for stable foams to be obtained. Ashby and Binks [41] prepared the Pickering emulsion stabilized by Laponite clay particles. They found that a reduction of the particle surface charge was enough to stabilize emulsion. However, in our system of Laponite and CTAB, the reduction of the particle surface charge is not enough to stabilize foam. In region II, the foam stability increases with increasing CTAB concentration. The zeta potential of the particles is positive and slowly increases in this region, but the hydrophobic attractive interaction between the particles increases. The maximum zeta potential is about 20 mV in this region. The change of zeta potential is so small that the hydrophobic attractive interaction between particles is dominant here. The fact that no flocculated particles redisperse in this region (Fig. 2) also supports this conclusion. The increased particle hydrophobicity and the hydrophobic attractive interaction between the particles increase the density of the particle layer adsorbed on the bubble surface and thus improve the foam stability. The adsorption of the particles on the bubble surfaces, a crucial mechanism for foam stability, was confirmed by confocal microscopy. Once the particles adsorb at the interface, the adsorption is irreversible [8]. In region III, the particle zeta potential increases continuously, and the particle hydrophobicity decreases with increasing CTAB concentration. The electrostatic repulsion between the particles increases and the hydrophobic attractive interaction between the particles decreases. These two factors both weaken the foam stability. Therefore, the foam stability is reduced in this region, but particle-stabilized foams can still be prepared even though the particles surface charge is relatively high (about 40 mV), because the particles are still partially hydrophobic. In region IV, the zeta potential of particles changes little. And the particles become hydrophilic again and cannot adsorb at the air–water interface. The aqueous bubbles obtained at CTAB concentrations above 3.0 cec are stabilized only with CTAB, which is indirectly confirmed in three ways. First, the drainage time of foams prepared by these dispersions is close to that prepared by pure CTAB solutions (above cmc). The drainage time is all about 300 s. Second, there is nothing left in the dried
foam phase as seen in Fig. 9. The phenomenon is the same as the foam prepared by surfactant solution solely. If particles exist in foam phase, they will locate within the dried foam phase or the wall of the test tube. Third, foam stability is independent of the particle concentrations in region IV (Fig. 6), which also indicates that the foams are stabilized only by free CTAB. From the above analysis, it can be concluded that to achieve relatively stable particle-stabilized foams, the particle hydrophobicity is more important than the particle surface charge. The stability of foams prepared by CTAB-modified Laponite particles improves with increasing particle hydrophobicity. 4. Conclusions We have systematically studied the stability of foams prepared by CTAB and disk-like Laponite dispersions. The Laponite particles cannot stabilize foams alone. Stable foams can be obtained by combination of CTAB surfactant and Laponite particles in aqueous dispersions. The particle/CTAB system leads to a synergistic stabilization of foams at intermediate surfactant concentrations. The hydrophobicity of the particles is crucial to obtain stable foams by CTAB/Laponite system compared with the electrical property of particles. The most stable foams were obtained by particles with the maximum hydrophobicity (but the contact angle is still smaller than 90◦ ). Foam stability is promoted by the adsorption of CTAB-modified particles on the bubble surface and the formation of aggregated structures between the armored bubbles and the unadsorbed particles in the bulk phase. Acknowledgments This work was financially supported by a grant from the National Natural Science Foundation of China (no. 20373036). The authors thank Prof. Pamela Holt (Shandong University) for assistance in editing the manuscript. References [1] O. Paulson, R.J. Pugh, Langmuir 12 (1996) 4808. [2] S. Ata, P.D. Yates, Colloids Surf. A 277 (2006) 1. [3] P.R. Garrett, S.P. Wicks, E. Fowles, Colloids Surf. A 282–283 (2006) 307. [4] U.T. Gonzenbach, A.R. Studart, E. Tervoort, L.J. Gauckler, Angew. Chem. Int. Ed. 45 (2006) 3526. [5] A.R. Studart, E. Tervoort, L.J. Gauckler, J. Am. Ceram. Soc. 89 (2006) 1771. [6] S.S. Shankar, U.S. Patil, B.L.V. Prasad, M. Sastry, Langmuir 20 (2004) 8853. [7] P.R. Garrett, J. Colloid Interface Sci. 69 (1979) 107. [8] B.P. Binks, Curr. Opin. Colloid Interface Sci. 7 (2002) 21. [9] S.I. Kam, W.R. Rossen, J. Colloid Interface Sci. 213 (1999) 329. [10] Z. Du, M.P. Bilbao-Montoya, B.P. Binks, E. Dickinson, R. Ettelaie, B.S. Murray, Langmuir 19 (2003) 3106. [11] S. Fujii, A.J. Ryan, S.P. Armes, J. Am. Chem. Soc. 128 (2006) 7882. [12] R.G. Alargova, D.S. Washadpande, V.N. Paunov, O.D. Velev, Langmuir 20 (2004) 10371. [13] A.D. Nikolov, D.T. Wasan, Langmuir 8 (1992) 2985. [14] K.L. Mittal, P. Kumar, Emulsions Foams and Thin Films, Marcel Dekker, Inc., New York, 2000.
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Colloids and Surfaces A: Physicochem. Eng. Aspects 317 (2008) 406–413
Aqueous foams stabilized by Laponite and CTAB Shuiyan Zhang, Qiang Lan, Qian Liu, Jian Xu, Dejun Sun ∗ Key Laboratory for Colloid & Interface Chemistry of Education Ministry, Shandong University, Jinan, Shandong 250100, People’s Republic of China Received 6 June 2007; received in revised form 17 October 2007; accepted 7 November 2007 Available online 17 November 2007
Abstract Aqueous foams prepared by cetyltrimethylammonium bromide (CTAB) and disk-like Laponite particles dispersions are studied. The particle/CTAB dispersions have a synergistic effect on foam stability at intermediate CTAB concentrations. The foam stability increases with increasing particle concentration. At a fixed particle concentration, the foam stability first increases with increasing CTAB concentration and reaches a maximum at a CTAB concentration around 1.7 cec. After that, the foam stability decreases and then remains unchanged. The effects of hydrophobicity and electrical property of modified Laponite particles on foam stability are discussed. The hydrophobicity of Laponite particles is predominant in stability of foams compared with the electrical property of particles. The most stable foams were obtained by particles with the maximum hydrophobicity (the contact angle is still smaller than 90◦ ). Two mechanisms promote foam stability: (a) adsorption of modified particles on the bubble surface; (b) formation of an aggregate structure by the armored bubbles and particles in the bulk phase, which is confirmed by laser-induced fluorescent confocal microscopy. © 2007 Elsevier B.V. All rights reserved. Keywords: Foam stability; Laponite particles; CTAB; Hydrophobicity; Adsorption
1. Introduction Foams stabilized by colloidal particles have received considerable attention in recent years due to their wide applications, ranging from food and cosmetics to oil recovery, flotation [1,2], fire extinguishing and water-borne coatings [3]. They are also used as intermediate structures [4,5] or templates [6] to produce new materials. For particle-stabilized foams, it has been widely accepted that stable foams are difficult to form when the particles are hydrophobic, e.g. with a contact angle generally greater than 90◦ . The hydrophobic particles are believed to behave as antifoaming agents by a bridging–dewetting mechanism [7]. Only partially hydrophobic or hydrophilic particles can be used to stabilize foams. The foams are stabilized in two ways: (a) adsorption of colloidal particles at the air–water interface [4,8–12], which is a crucial factor for preparing stable foams and (b) stratification of non-adsorbing particles in the intervening thin film separating
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the dispersed phases [13–17], which improves the stability of foams against drainage. Many researchers [4,8–12] have emphasized that the strong attachment of particles at the bubble surface and a network or a dense layer of particles at the interface are responsible for foam stability. In most cases, the adsorption or assembly of colloidal particles at air–water interfaces is controlled by adjusting the surface hydrophobicity of colloidal particles. Binks and Horozov [18] and Murray and co-workers [19,20] studied foams stabilized by partly hydrophobic fumed silica particles which were modified by chemical reaction with a silane coupling agent. They also investigated the influence of particle hydrophobicity [18] and salt concentrations [20] on foam stability. Studart and co-workers [4,5,21] accomplished in-situ hydrophobization of particles through the adsorption of short-chain amphiphiles. High-volume and ultrastable foams were prepared by those particles. Fujii et al. [11,22] reported that highly stable foams were formed by micrometer-sized, sterically stabilized polystyrene (PS) latex particles. Particle wettability can also be modified by the adsorption of long-chain surfactants on the particle surfaces. Wilson [23] investigated foams prepared by addition of a cationic surfactant
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to aqueous dispersions of polystyrene latex. He found that both low charge and relatively high hydrophobicity of particles were responsible for the foam stability. Velikov et al. [24] investigated the influence of different surfactants on the dynamics of film thinning in the presence of negatively charged polystyrene latex particles. For SDS-stabilized films, all of the particles were expelled from the film due to their repulsion from the film surfaces. For cationic surfactant CTAB-stabilized films, however, the same particles became partially hydrophobic and a layer of particles was adsorbed simultaneously at the two film surfaces. Garrett et al. [3] studied the effect of latex particles on foaming and antifoaming in surfactant solutions. The particles can lead to three effects: depletion of surfactant concentration by adsorption loss, increase in viscosity and stratification stabilization of thin liquid films by the particles. Addition of surfactant to particle dispersions does not always promote foam stability. Alargova et al. [12] found that the stability of foams prepared by mixtures of hydrophobic rod-shaped particles and sodium dodecyl sulfate (SDS) was decreased compared with that by the particles alone, meaning that SDS acted as a defoamer. It is possible that the adsorption of SDS onto the particles rendered them more hydrophilic and then the particles lost their affinity for the solution/air interface. Fujii et al. [11] found that foams could not be formed using poly Nvinylpyrrolidone (PNVP)-stabilized polystyrene particles if they contained excess PNVP. Weak, short-lived foams were formed but collapsed within 10 s after two centrifugation/redispersion cycles to remove the nonadsorbed PNVP. After the third centrifugation/redispersion cycle, highly stable foams that remained stable for more than one year were obtained, but this behavior was not explained. Subramaniam et al. [25] reported the destabilization of particle-stabilized bubbles exposed to various concentrations of surfactant solutions. They proposed a microstructural mechanism, which recognized the role of interfacial jamming and stresses in particle stabilization and surfactant-mediated destabilization of armored bubbles. In this paper, disk-like Laponite particles and CTAB surfactant are used to prepare aqueous foams. Firstly, the properties of Laponite/CTAB aqueous dispersions are studied. Then the stability of foams prepared by Laponite/CTAB dispersions are described and discussed systematically in relation to particle hydrophobicity and zeta potential. The stabilization mechanisms of the foams are also discussed. 2. Experimental 2.1. Materials The water was deionized water purified by ion exchange. Laponite RD, a synthetic hectorite, was supplied by Rockwood Additives Ltd. (UK) as a white powder. It is composed of rigid disk-shaped crystals with a well-defined thickness of 1 nm and an average diameter about 30 nm. The molecular formula of Laponite is Na0.7 [(Si8 Mg5.5 Li0.4 )O20 (OH)4 ]. The surfactantmediated of the synthetic clay is about 0.75 meq/g. Each particle has a central layer made of Mg2+ cations in octahedral coordination to oxygen atoms or hydroxyl groups; this central layer is
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sandwiched between two silicate layers where the silica atoms are in tetrahedral coordination to oxygen atoms. Some of the Mg2+ sites of the central layer are substituted with Li+ cations. This creates a charge imbalance, which is compensated by Na+ counterions located at the surface of the outer layers. When the powders are dispersed in water, the Na+ ions on the particle surface are released and a strongly negative charge appears on the faces of the disks. On the other hand, the edges of the platelets carry some Mg OH, Si OH and Li OH sites which are weak bases. Because of the protonation of the hydroxyl groups by water, a weakly positive charge appears on the rim of the disks. The surfactant used was cetyltrimethylammonium bromide (CTAB) purchased from Sinopharm Chemical Reagent Co. Ltd. (China) with a purity >99%. 2.2. Methods 2.2.1. Preparation of CTAB/Laponite aqueous dispersions The Laponite stock dispersion was prepared by dispersing a known mass of Laponite into deionized water using a Multimixer (Baroid Co., USA). The Laponite dispersions were sealed and laid aside at room temperature (25 ◦ C) for 1 week before use. The Laponite/CTAB dispersions were prepared by diluting the stock dispersions with CTAB solutions. The prepared dispersions were stirred for at least 12 h to attain adsorption equilibrium. The initial CTAB concentration in the dispersions is expressed as a multiple of the clay’s CEC (0.75 mmol/g). 2.2.2. Adsorption of CTAB on Laponite particles The prepared dispersion is centrifuged for 60 min at 20,000 rpm in order to separate the clay particles from supernatant. CTA+ in supernatant is analyzed by TOC technique. The adsorption amount was calculated from the difference between initial and equilibrium concentrations of surfactant and divided by the mass of the dried solid. 2.2.3. Stability of the Laponite/CTAB aqueous dispersions The prepared dispersions were first transferred into a stoppered, graduated glass tube with internal diameter 1.6 cm and length 15 cm. Then the phase behavior of the dispersions was observed after 72 h. 2.2.4. Zeta potential measurements The zeta potentials of Laponite/CTAB dispersions were measured with a JS94H microelectrophoresis instrument (Shanghai Zhongchen Digital Technic Apparatus Co., China). The particles in the sediment phase were diluted with the upper clear liquid to make the particles visible under the microscope before measurement. 2.2.5. Three-phase contact angle of particles The three-phase contact angle of the Laponite particles was measured using the classic captive drop method [4,20,26]. Laponite/CTAB aqueous dispersions were first washed with deionized water to remove the unadsorbed CTAB molecules. Then, the samples were dried at 60 ◦ C and crushed into powders. The powders were compressed into a circular disk with
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a 769YP-15 Press at a pressure of 16 MPa. The thickness of the compressed disk was about 1 mm. A drop of water was first placed on the disk surface and then the shape of the water drop on the disk was immediately photographed. The contact angle was directly measured using a protractor. 2.2.6. Stability of foams Foams were prepared with Laponite dispersions, CTAB solutions, and Laponite/CTAB dispersions respectively using a lab homogenizer (Shanghai Forerunner M&E Co., China) operated at 8000 rpm for 5 min. After homogenization, foams were immediately transferred into the glass tubes mentioned above (25 ml foams). The time needed for 5 ml of liquid to drain from the 25 ml foams was recorded to evaluate the foam stability. The morphology of the bubbles was observed with an Axioskop 40 microscope (ZEISS, Germany). 2.2.7. Adsorption of particles at the bubble surface A laser-induced confocal microscope (Olympus Fluoview 500, Japan) was used to investigate the adsorption of Laponite particles at the bubble surface. The fluorescent probe was Rhodamine B, which is negatively charged in basic solution and has a maximum excitation wavelength at 543 nm. The CTABmodified Laponite particles were first labeled with Rhodamine B. Then the particle dispersions were washed with deionized water to remove the free Rhodamine B in the bulk. Foams stabilized by the labeled particles were prepared and fluorescent images of the foams were observed under the microscope.
Fig. 1. Adsorption isotherm of CTAB on the Laponite of 1.0 wt% at 25 ◦ C.
concentrations above 2.0 cec, the sediments at the bottom of the tube redisperse. Labbe et al. [28] indicated that addition of a surfactant over the CEC caused the aggregates to partially redisperse. Hanley et al. [29,30] reported that a complex must adsorb CTAB to an amount corresponding to at least two multiples of the clay’s cation exchange capacity to be dispersed. Here, the sediment redisperses when the CTAB concentration is above 2.0 cec, corresponding to the results of Hanley et al. [29,30]. 3.2. Zeta potentials of Laponite/CTAB aqueous dispersions
3. Results and discussion 3.1. Properties of CTAB/Laponite aqueous dispersions The pH of CTAB/Laponite aqueous dispersion was fixed at 9.0. The adsorption isotherm of CTA+ on the Laponite of 1.0 wt% at 25 ◦ C is shown in Fig. 1. And obviously, the adsorption isotherm is “S” type. It is like the four regions isotherms described by Brahimi et al. [27], corresponding to the exchange with the anionic clay sites, to the adsorption by van der Waals interactions, to the hemimicellization on the Laponite surface, and to the micellization in the bulk. The phase behavior of Laponite/CTAB aqueous dispersions is shown in Fig. 2. At CTAB concentrations below 0.3 cec, the dispersions are sols. At CTAB concentrations from 0.3 to 2.0 cec, phase separation occurs due to the flocculation of the Laponite particles. At CTAB
The change of zeta potential of the Laponite particles as CTAB concentration increases is shown in Fig. 3. The zeta potential of the particles decreases initially with increasing CTAB concentration, and changes to a positive value around 1.0 cec of CTAB. Then the zeta potential of the particles increases with increasing CTAB concentration. 3.3. Hydrophobicity of modified particles The hydrophobicity of particles is generally described by contact angle. It is an important parameter in determining the adsorption of particles at the air–water interface [8] and the foam stability [31]. Here, the variation of wettability of Laponite particles with CTAB concentration was studied (Fig. 4). In the absence of CTAB, the contact angle of Laponite is very small
Fig. 2. Appearance of the Laponite/CTAB dispersions. The CTAB concentrations are shown in the picture (cec). The particle concentration is fixed at 1.0 wt%.
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Fig. 5. Drainage time of aqueous foams stabilized with CTAB alone. Fig. 3. Zeta potential of Laponite/CTAB aqueous dispersions. The particle concentration in the dispersions is 2.0 wt%.
(less than 10◦ ), meaning the particles are very hydrophilic. In the presence of CTAB, the contact angle first increases with increasing CTAB concentration up to a maximum value at 1.7 cec. Further increases of CTAB concentration lead to a decrease of θ. When the CTAB concentration is over 3.0 cec, the CTABmodified Laponite particles become hydrophilic again (less than 20◦ ). So the hydrophobicity of particle reaches a maximum at 1.7 cec. 3.4. Properties of foams stabilized by CTAB and Laponite particles 3.4.1. Foam stability To understand the effect of the particles and the surfactant on the foam stability, three different systems were investigated, including particle dispersions, surfactant solutions and Laponite/CTAB dispersions. Foams cannot be prepared solely by Laponite dispersions, even in the presence of electrolytes such as NaCl. In Fig. 5, the stability of foams prepared by surfactant solutions first increases with increasing CTAB concentration and
Fig. 4. Three-phase contact angle of particles as a function of CTAB concentration.
then remains unchanged at CTAB concentrations above its cmc (about 0.98 mM). The stability of foams prepared by Laponite/CTAB dispersions is shown in Fig. 6. The concentrations of Laponite in the dispersions investigated were 0.5, 1.0, 1.5 and 2.0 wt%, and the CTAB concentrations varied from 0 to 5.0 cec at each particle concentration. The foam stability increases with increasing particle concentration. At each particle concentration, the most stable foams are formed at the same concentration ratio of CTAB to Laponite (about 1.7 cec). At a fixed particle concentration, four regions were defined according to the foam stability. No foams can be formed at relatively low (0–1.0 cec) CTAB concentrations (region I). The foam stability increases gradually with increasing CTAB concentration and reaches a maximum at 1.7 cec (region II), then decreases (region III). In region IV, the foam stability is low, but does not decrease further. In region I, the stability of foams prepared by Laponite/CTAB dispersions is much lower than that of foams prepared with surfactant solution (above cmc) alone. Stable foams are hardly formed. In regions II and III, the foams stabilized by Laponite/CTAB are more sta-
Fig. 6. Drainage time of foams stabilized with Laponite/CTAB system as a function of CTAB concentration at different particle concentrations indicated above (wt%).
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ble than those stabilized by CTAB alone (above cmc). In region IV, the foam stability is almost the same as that of pure CTAB solution (above cmc). It can be deduced that the particle/CTAB system has a synergistic effect on foam stability at intermediate surfactant concentrations. The adsorption feature of CTAB on Laponite particles was further understood in virtue of the foam stability. When the concentration of CTAB is lower than 1.0 cec, the adsorption results from the electrostatic interaction between particle surface and head groups of the ionic surfactant. After that, the adsorption process is no longer driven by electrostatic interaction and that hydrophobic interactions probably are involved. Patches of two-dimensional aggregates on the surface seem to be formed. The solid becomes more hydrophobic. Such an association is analogous to micelle formation in the bulk solution and results from the increase in the entropy of the system when the alkyl
chains are removed from the aqueous environment. The twodimensional aggregates are called hemimicelles by Gaudin and Fuerstenau. The hemimicelle concept was developed by them in 1955 [32] for the adsorption of ionic surfactant on hydrophilic surfaces. If the adsorbed ions associate tightly through van der Waals attraction between hydrocarbon chains, a minimum surface area of hydrocarbon chain will be in contact with water. In recent references of [33,34], Fuerstenau inferred that adsorbed surfactant aggregates can exist with their hydrophobic tails oriented towards the solution (classical hemimicelles), or with some of the adsorbed surfactant ions having their hydrophilic heads oriented towards the water bulk phase (reverse hemimicelles), or as an actual bilayer (complete monolayer coverage before reverse orientation begins to form a complete second layer in reverse orientation). When the concentration of CTAB reach about 1.7 cec, the hydrophobicity and flocculation of par-
Fig. 7. (a–d) Optical microscope images of foams stabilized by Laponite and CTAB dispersions. From (a) to (d), the CTAB concentrations in the dispersions are 1.4 cec, 1.7 cec, 2.0 cec and 2.5 cec. (e) Size distributions of these bubbles. The Laponite concentration was fixed at 1.0 wt%.
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Fig. 8. Confocal fluorescence image (a) and optical microscopy image (b) of foams stabilized by fluorescently labeled CTAB-modified Laponite particles. The particle concentration is 1.0 wt% and surfactant concentration 1.7 cec.
ticles both attain the maximum values, so the foam stability also obtains the maximum. After this point, a second surfactant layer (admicelle) with head groups oriented toward the solution, that is, the reverse hemimicelles form gradually. So, the hydrophobicity of particles begin to decrease, the flocculated particles begins to redisperse and the foam stability declines. When the adsorption attains a plateau, the free micelles are formed in dispersions. The particles cannot adsorb on the air–water interface, as a result the foams are only stabilized by CTAB molecules. It can be concluded that foam stability is related with the adsorption feature of CTAB on Laponite particles. Optical microscope images of the foams stabilized by CTAB/Laponite dispersions are shown in Fig. 7. The bubble size initially decreases, then increases with increasing CTAB concentration. The bubble size and polydispersity have been shown in Fig. 7(e). The bubble size of the most stable foam is the smallest. The bubble size distribution in the foam is wide at low particle concentration but is narrowest for the most stable foams (at 1.7 cec). The bubble size results agree well with the foam stability results. 3.4.2. Adsorption of particles on the bubble surface Adsorption of CTAB on the Laponite particles changes their hydrophobicity and then the partially hydrophobic Laponite particles are able to attach to air bubbles. Laser-induced confocal scanning microscopy experiments were performed to confirm the adsorption of Laponite at the bubble surface in regions II and III, as shown in Fig. 8. Aggregates of Laponite particles adsorb on the bubble surface. In addition, flocculation is observed clearly between the armored bubbles and the unadsorbed particles in the surrounding continuous phase. As Murray et al. [10,19] suggested, the particles adsorbed onto the bubbles are part of a particle network in the bulk aqueous solution. Photographs of dried foams were taken 12 h after preparation (Fig. 9). In regions II and III, the particles are located within the dried foam phase, which indicates that the particles take part in stabilizing the foams. In region IV, there is nothing left of the dried foam phase. It can be seen from Figs. 5 and 6 that
the foam stability is the same as that of pure CTAB solution (CCTAB > cmc). These results indicate that the particles do not adsorb on the bubble surface and the foams are stabilized only by free surfactant in the dispersion. The effect of particle concentration on the foam stability in different regions can be explained by adsorption of the particles on the bubble surface. In region I, the hydrophilic Laponite cannot stabilize the foams. The foam stability increases with increasing particle concentration in regions II and III. In these two regions, the adsorbed particles form an interfacial armor around the bubble surface that mechanically impedes bubble shrinkage and coalescence. Hence, the increase of particle concentration can improve the foam stability. Tang et al. [35] and Gonzenbach et al. [36] have reported that foam stability increases with increasing particle concentration. In region IV, the foam stability is independent of the particle concentration. This is due to the particles becoming hydrophilic again so that the foams are stabilized only by the free CTAB molecules. 3.4.3. Effect of surface charge and hydrophobicity of particles on foam stability In region I, foams cannot be produced. The surface charge of the Laponite particles decreases initially with increasing
Fig. 9. Appearance of dried foams prepared with Laponite/CTAB dispersions. The Laponite particle concentration is 1.0 wt%. The CTAB concentration in the dispersion is indicated above (cec).
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CTAB concentration and approaches zero at 1.0 cec. The particle–particle and particle–interface electrostatic repulsion decreases due to the decreased zeta potentials of particles, which will promote the particle adsorption at the interface [37]. Williams and Berg [38] and Abdel-Fattah and El-Genk [39] have also found that the reduction of particle surface charge promotes the adsorption of hydrophobic, negatively charged particles at the interface by decreasing the electrical barrier between the particles and the interface. El-Genk et al. [39,40] have proposed that adsorption of colloidal particles at the air–water interface is mainly controlled by the particle–interface and particle–particle interactions. Besides the electrostatic interactions between the particle and the interface, the hydrophobicity of the particles is also important for particle adsorption at the interface. In region I, the particle is still hydrophilic so the hydrophobic interaction between particles and the interface can be neglected. Here, the foams cannot be produced more likely due to the strong hydrophilicity of Laponite particles. Moreover, the free CTAB molecules in the dispersions are too few for stable foams to be obtained. Ashby and Binks [41] prepared the Pickering emulsion stabilized by Laponite clay particles. They found that a reduction of the particle surface charge was enough to stabilize emulsion. However, in our system of Laponite and CTAB, the reduction of the particle surface charge is not enough to stabilize foam. In region II, the foam stability increases with increasing CTAB concentration. The zeta potential of the particles is positive and slowly increases in this region, but the hydrophobic attractive interaction between the particles increases. The maximum zeta potential is about 20 mV in this region. The change of zeta potential is so small that the hydrophobic attractive interaction between particles is dominant here. The fact that no flocculated particles redisperse in this region (Fig. 2) also supports this conclusion. The increased particle hydrophobicity and the hydrophobic attractive interaction between the particles increase the density of the particle layer adsorbed on the bubble surface and thus improve the foam stability. The adsorption of the particles on the bubble surfaces, a crucial mechanism for foam stability, was confirmed by confocal microscopy. Once the particles adsorb at the interface, the adsorption is irreversible [8]. In region III, the particle zeta potential increases continuously, and the particle hydrophobicity decreases with increasing CTAB concentration. The electrostatic repulsion between the particles increases and the hydrophobic attractive interaction between the particles decreases. These two factors both weaken the foam stability. Therefore, the foam stability is reduced in this region, but particle-stabilized foams can still be prepared even though the particles surface charge is relatively high (about 40 mV), because the particles are still partially hydrophobic. In region IV, the zeta potential of particles changes little. And the particles become hydrophilic again and cannot adsorb at the air–water interface. The aqueous bubbles obtained at CTAB concentrations above 3.0 cec are stabilized only with CTAB, which is indirectly confirmed in three ways. First, the drainage time of foams prepared by these dispersions is close to that prepared by pure CTAB solutions (above cmc). The drainage time is all about 300 s. Second, there is nothing left in the dried
foam phase as seen in Fig. 9. The phenomenon is the same as the foam prepared by surfactant solution solely. If particles exist in foam phase, they will locate within the dried foam phase or the wall of the test tube. Third, foam stability is independent of the particle concentrations in region IV (Fig. 6), which also indicates that the foams are stabilized only by free CTAB. From the above analysis, it can be concluded that to achieve relatively stable particle-stabilized foams, the particle hydrophobicity is more important than the particle surface charge. The stability of foams prepared by CTAB-modified Laponite particles improves with increasing particle hydrophobicity. 4. Conclusions We have systematically studied the stability of foams prepared by CTAB and disk-like Laponite dispersions. The Laponite particles cannot stabilize foams alone. Stable foams can be obtained by combination of CTAB surfactant and Laponite particles in aqueous dispersions. The particle/CTAB system leads to a synergistic stabilization of foams at intermediate surfactant concentrations. The hydrophobicity of the particles is crucial to obtain stable foams by CTAB/Laponite system compared with the electrical property of particles. The most stable foams were obtained by particles with the maximum hydrophobicity (but the contact angle is still smaller than 90◦ ). Foam stability is promoted by the adsorption of CTAB-modified particles on the bubble surface and the formation of aggregated structures between the armored bubbles and the unadsorbed particles in the bulk phase. Acknowledgments This work was financially supported by a grant from the National Natural Science Foundation of China (no. 20373036). The authors thank Prof. Pamela Holt (Shandong University) for assistance in editing the manuscript. References [1] O. Paulson, R.J. Pugh, Langmuir 12 (1996) 4808. [2] S. Ata, P.D. Yates, Colloids Surf. A 277 (2006) 1. [3] P.R. Garrett, S.P. Wicks, E. Fowles, Colloids Surf. A 282–283 (2006) 307. [4] U.T. Gonzenbach, A.R. Studart, E. Tervoort, L.J. Gauckler, Angew. Chem. Int. Ed. 45 (2006) 3526. [5] A.R. Studart, E. Tervoort, L.J. Gauckler, J. Am. Ceram. Soc. 89 (2006) 1771. [6] S.S. Shankar, U.S. Patil, B.L.V. Prasad, M. Sastry, Langmuir 20 (2004) 8853. [7] P.R. Garrett, J. Colloid Interface Sci. 69 (1979) 107. [8] B.P. Binks, Curr. Opin. Colloid Interface Sci. 7 (2002) 21. [9] S.I. Kam, W.R. Rossen, J. Colloid Interface Sci. 213 (1999) 329. [10] Z. Du, M.P. Bilbao-Montoya, B.P. Binks, E. Dickinson, R. Ettelaie, B.S. Murray, Langmuir 19 (2003) 3106. [11] S. Fujii, A.J. Ryan, S.P. Armes, J. Am. Chem. Soc. 128 (2006) 7882. [12] R.G. Alargova, D.S. Washadpande, V.N. Paunov, O.D. Velev, Langmuir 20 (2004) 10371. [13] A.D. Nikolov, D.T. Wasan, Langmuir 8 (1992) 2985. [14] K.L. Mittal, P. Kumar, Emulsions Foams and Thin Films, Marcel Dekker, Inc., New York, 2000.
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