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Is Latex Surface Charge an Important Parameter for Foam Stabilization? Sarah L. Kettlewell, Andreas Schmid, Syuji Fujii,† Damien Dupin, and Steven P. Armes* Department of Chemistry, Dainton Building, UniVersity of Sheffield, Brook Hill, Sheffield, South Yorkshire, S3 7HF, United Kingdom ReceiVed July 13, 2007. In Final Form: September 5, 2007 We describe the facile production of highly stable foams stabilized solely by cationic polystyrene latex particles. Three model polystyrene latexes were synthesized using either a cationic 2,2′-azobis(2-diisobutyramidine) dihydrochloride (AIBA) or an anionic ammonium persulfate (APS) radical initiator: a 724 ( 81 nm charge-stabilized cationic polystyrene latex [AIBA-PS], an 800 ( 138 nm sterically stabilized cationic latex prepared using a poly(ethylene glycol) monomethacrylate macromonomer [PEGMA-AIBA-PS], and a 904 ( 131 nm charge-stabilized anionic polystyrene latex [APS-PS], respectively. The effect of particle surface charge, latex concentration, and solution pH on foam stability was studied in detail. The PEGMA-AIBA-PS latex proved to be the best foam stabilizer even at relatively low latex concentrations (3.0 wt %), with long-term foam stabilities being obtained after drying. The AIBA-PS latex also produced stable foams, albeit only at higher latex concentrations. However, the APS-PS latex proved to be an ineffective foam stabilizer. This is believed to be primarily due to the anionic surface character of this latter latex, which prevents its adsorption at the anionic air-water interface. This hypothesis is supported by the observation that the AIBA-PS latex no longer acts as an effective foam stabilizer above its isoelectric point (pH 7.04). Scanning electron microscopy studies revealed the formation of well-defined latex bilayers within dried foams, which indicates that the wet air bubbles are stabilized by latex monolayers prior to drying. However, little or no long-range ordering of the latex particles was observed on the surface of the bubbles, which is presumably related to the latex polydispersity.
Introduction Ramsden first described the stabilization of foams and bubbles using finely divided particles over a century ago.1 The past few years have witnessed renewed interest in this phenomenon, with a significant increase in studies of particle-stabilized bubbles and foams.2-7 In many cases, such foams can be prepared by destabilizing the colloidal particles in situ using various additives (e.g., added electrolyte, surfactant, cosolvent, etc.) in order to promote their interfacial adsorption.2,3,6 However, our group recently reported that a range of near-monodisperse, sterically stabilized polystyrene latexes can adsorb at the air-water interface (see Figure 1) and hence stabilize foams and bubbles in the absence of any additives.8-10 Since their colloidal stability is not compromised, the adsorbed particles remain mutually repulsive * Author to whom correspondence should be addressed (e-mail:
[email protected]). † Present Address: Department of Applied Chemistry, Osaka Institute of Technology, 5-16-1 Ohmiya, Asahi-ku, Osaka 535-8585, Japan. (1) Ramsden, W. Proc. R. Soc. 1903, 72, 156-164. (2) Wilson, J. C. A study of particulate foams. Ph.D. Thesis, University of Bristol, Bristol, UK, 1980. (3) (a) Binks, B. P. Curr. Opin. Colloid Interface Sci. 2002, 7, 21. (b) Binks, B. P.; Horozov, T. S. Angew. Chem., Int. Ed. 2005, 44, 3722. (c) Binks, B. P.; Horozov, T. S. Colloidal Particles at Liquid Interfaces; Cambridge University Press: Cambridge, UK, 2006. (4) Du, Z. P.; Bilbao-Montoya, M. P.; Binks, B. P.; Dickinson, E.; Ettelaie, R.; Murray, B. S. Langmuir 2003, 19, 3106. (5) Alargova, R. G.; Warhadpande, D. S.; Paunov, V. N.; Velev, O. D. Langmuir 2004, 20, 10371. (6) (a) Gonzenbach, U. T.; Studart, A. R.; Tervoort, E.; Gauckler, L. J. Angew. Chem., Int. Ed. 2006, 45, 3526. (b) Gonzenbach, U. T.; Studart, A. R.; Tervoort, E.; Gauckler, L. J. Langmuir 2006, 22, 10983. (c) Gonzenbach, U. T.; Studart, A. R.; Tervoort, E.; Gauckler, L. J. Langmuir 2007, 23, 1025. (d) Gonzenbach, U. T.; Studart, A. R.; Tervoort, E.; Gauckler, L. J. Langmuir 2006, 22, 10983. (e) Blute, I.; Pugh. R. J.; Van de Pas, J.; Callaghan, I. J Colloid Interface Sci. 2007, 313, 645. (7) (a) Subramaniam, A. B.; Abkarian, M.; Stone, H. A. Nat. Mater. 2005, 4, 553. (b) Subramaniam, A. B.; Mejean, C.; Abkarian, M.; Stone, H. A. Langmuir 2006, 22, 5986. (8) Fujii, S.; Ryan, A. J.; Armes, S. P. J. Am. Chem. Soc. 2006, 128, 7882. (9) Fujii, S.; Iddon, P. D.; Armes, S. P.; Ryan, A. J. Langmuir 2006, 22, 7512. (10) Dupin, D.; Armes, S. P.; Randall, D. P. J. Mater. Chem. Submitted, 2007.
Figure 1. Schematic representation of the three types of polystyrene latex investigated in this work and their interaction with an anionic air/water interface. Left: pure charge-stabilized cationic AIBA-PS latex adsorbed at the air/water interface. Center: sterically stabilized cationic PEGMA-AIBA-PS latex adsorbed at the air/water interface. Right: non-adsorbed charge-stabilized anionic APS-PS latex.
at the interface, and subsequent water drainage produces exquisitely ordered 2D latex bilayer superstructures that exhibit interesting optical effects.8-10 Wilson has previously shown2 that micrometer-sized, anionic, charge-stabilized polystyrene latex particles do not stabilize bubbles and foams (see Figure 1), unless certain additives such as oppositely charged surfactants, cosolvents, or added salt are employed. The same approach has been recently demonstrated to work well with inorganic sols such as silica or alumina.3,4,6 It is well-known that the air/water interface is hydrophobic, but more recent studies suggest that it can also exhibit anionic character.11,12 Very recently, Binks and co-workers described the use of poly(acrylic acid)-stabilized polystyrene latex particles to efficiently stabilize air bubbles.13 However, it was found that such particles were only effective foam stabilizers when flocculated at low pH, with positive zeta potentials being obtained under these conditions. The origin of this unexpected cationic charge is not yet clear. Bearing this observation in mind, we felt (11) Ciunel, K.; Arme´lin, M.; Findenegg, G. H.; von Klitzing, R. Langmuir 2005, 21, 4790. (12) Takahashi, M. J. Phys. Chem. B 2005, 109, 21858. (13) Binks¸ , B. P.; Murakami, R.; Armes¸ , S. P.; Fujii, S.; Schmid, A. Langmuir 2007, 23, 8691.
10.1021/la702099t CCC: $37.00 © 2007 American Chemical Society Published on Web 10/09/2007
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Figure 2. Scanning electron micrographs of (a) charge-stabilized cationic polystyrene latex [AIBA-PS] prepared by dispersion polymerization in methanol at 60 °C using a cationic azo initiator in the absence of any polymeric stabilizer; (b) sterically stabilized cationic polystyrene latex [PEGMA-AIBA-PS] obtained by latent grafting of a PEGMA macromonomer to a cationic PS latex; (c) charge-stabilized anionic polystyrene latex [APS-PS] prepared under identical conditions to the AIBA-PS latex but using an anionic persulfate initiator. The disk centrifuge particle size distribution curves obtained for these three latexes are shown in (d).
that it was worth examining more conventional cationic polystyrene latex particles to see if these particles were sufficiently strongly adsorbed at the air/water interface (see Figure 1) to allow effective foam stabilization. In the present work, we demonstrate that such cationic polystyrene latexes can indeed act as the sole stabilizer for bubble and foam stabilization, without recourse to any additives. Experimental Section Materials. Styrene (Aldrich) was passed through a basic alumina column to remove inhibitor and then stored at -20 °C prior to use. 2,2′-Azobis(isobutyramidine) dihydrochloride (AIBA) and ammonium persulfate (APS) purchased from Aldrich, monomethoxycapped poly(ethylene glycol) methacrylate (PEGMA) macromonomer (Mn ) 2000, Mw/Mn ) 1.10; supplied at 50 wt % in water by Cognis Performance Chemicals, Hythe, UK), and methanol (ex Fisher Scientific) were all used as received. The water used was deionized using an Elga Elgastat Option 3A water purifier and had a typical surface tension of 72.0 mN m-1 at 25 °C. Synthesis and Purification of Charge-Stabilized Cationic Polystyrene Latex [AIBA-PS]. Charge-stabilized cationic polystyrene latex was synthesized by dispersion polymerization of styrene in methanol in the absence of any polymeric stabilizer using the AIBA initiator at 60 °C. 900 mL of methanol and 100 mL of styrene
were added to a 2 L round-bottomed flask equipped with a magnetic stir bar, a condenser, and a nitrogen gas inlet. The solution was purged for 30 min with nitrogen and subsequently heated to 60 °C at a constant stirring speed of 250 rpm. 909 mg AIBA (1 wt % based on styrene) was dissolved in methanol (150 mL) and added to the styrene solution to initiate the polymerization. Stirring was continued for 24 h under a blanket of nitrogen. The resulting latex was purified by repeated centrifugation/redispersion cycles to remove soluble surface-active impurities. Initially, the latex was centrifuged at 3000 rpm for 30 min, and the supernatant was carefully decanted and replaced with methanol (first two cycles). Thereafter, the supernatant was replaced with deionized water, and a further four centrifugation/redispersion cycles were conducted at 5000 rpm for 30 min, with redispersion of the sedimented particles in pure water each time. The final surface tension of the supernatant was 70.1 mN m-1, indicating the removal of surface-active impurities. Synthesis and Purification of the PEGMA-Stabilized Cationic Polystyrene Latex [PEGMA-AIBA-PS]. The sterically stabilized cationic latex was prepared by a “latent grafting” technique. Styrene (50 mL) and methanol (490 mL) were added in turn to a 1 L threenecked round-bottomed flask equipped with a condenser, a nitrogen gas inlet, and a magnetic stir bar. After degassing by nitrogen purge for 30 min and heating to 60 °C, the AIBA initiator (0.452 g dissolved in a mixture of 10 mL methanol and 4 mL water) was injected. After polymerization for 3.75 h, a mixture of PEGMA (4.5 g; 10
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Langmuir, Vol. 23, No. 23, 2007 11383 Table 1. Summary of the Effect of Varying the Latex Concentration and Solution pH on the Height of Foam Obtained after Hand-Shaking Three Different Micrometer-Sized Aqueous Polystyrene Latexes for 30 s at 25 °C
latex typea AIBA-PS
PEGMA-AIBA-PS
Figure 3. Aqueous electrophoresis curves obtained for the two cationic polystyrene latexes prepared by dispersion polymerization in methanol at 60 °C using a cationic azo initiator in the absence of any polymeric stabilizer [AIBA-PS] and in the presence of the PEGMA stabilizer [PEGMA-AIBA-PS]. These two latexes exhibited cationic zeta potentials below approximately pH 7 or pH 8, respectively. An anionic polystyrene latex [APS-PS] prepared under identical conditions using a persulfate initiator exhibited negative zeta potentials over the whole pH range investigated. wt % based on styrene) and another charge of AIBA initiator (45 mg) was injected into the reaction vessel, and the polymerization was allowed to continue for a further 20 h. The latex was purified by three centrifugation/redispersion cycles at 6000 rpm for 20 min (with the supernatant being replaced with methanol each time), followed by a further three cycles at 7000 rpm for 30 min with redispersion in water. The final surface tension of the supernatant was 70.0 mN m-1. Synthesis and Purification of the Anionic Polystyrene Latex [APS-PS]. The anionic latex was prepared using a protocol analogous to that for the AIBA-PS latex described above, except that the cationic AIBA initiator was replaced by ammonium persulfate (APS). Styrene (25 mL) and methanol (130 mL) were added in turn to a 500 mL round-bottomed flask fitted with condenser and magnetic stir bar. Nitrogen was purged through the reaction solution for 30 min, followed by heating to 60 °C. APS initiator (227 mg) dissolved in methanol (180 mL) was injected into the reaction vessel, and the polymerizing solution was stirred at 250 rpm for 24 h under a nitrogen atmosphere at 60 °C. Three centrifugation/redispersion cycles were performed at 4000 rpm for 30 min with redispersion in methanol, followed by a further three cycles at 5000 rpm for 30 min with redispersion in water. The final surface tension of the supernatant was 70.2 mN m-1. Latex Characterization. Scanning electron microscopy (SEM) studies were conducted using a JEOL JSM 6400 scanning electron microscope. Samples were placed on adhesive carbon disks and sputter-coated with a thin layer of gold prior to examination. Aqueous Electrophoresis. Aqueous electrophoresis measurements were performed in 1 mM KCl solution using a Malvern Zetasizer NanoZS instrument. The solution pH was adjusted by adding HCl or KOH as required. Particle Size Analysis. Particle diameters were determined by disk centrifuge photosedimentometry (DCP; Brookhaven Instruments), which reports weight-average particle size distributions. Typical centrifugation rates and run times were 2000 to 3000 rpm and 30 to 60 min, respectively. The mean particle density of each polystyrene latex was taken to be 1.05 g cm-3. Foam Formation Studies and Foam Characterization. Foams were generated by vigorous hand-shaking for 30 s (Supporting Information Figure 1S shows digital photographs of typical foams produced with the PEGMA-AIBA-PS latex at 10.0 wt % and pH 3.4). Samples that gave stable foams were then tested under more controlled and reproducible conditions using a foam column previously described by Fujii and co-workers.8,9 Foam stability was
APS-PS
pH
latex concentration (wt %)
foam height after settling (mm)
3.4 3.4 3.4 3.4 7.0
10.0 7.0 5.0 3.0 10.0
10.3
10.0
5.8 5.8 5.8 5.8 7.9 11.0
10.0 7.0 5.0 3.0 10.0 10.0
5 5 4 2 aggregated latex aggregated latex
3.4 8.0
10.0 10.0
no foam formation no foam formation
7 6 5 3 flocculated latex, no foam aggregated latex, no foam
a AIBA-PS is a charge-stabilized cationic polystyrene latex with a mean diameter of 724 ( 81 nm and an isoelectric point of pH 7.04; PEGMA-AIBA-PS is a sterically stabilized cationic polystyrene latex with a mean diameter of 800 ( 138 nm and an isoelectric point of pH 7.85; APS-PS is a 904 ( 131 nm charge-stabilized anionic polystyrene latex with no isoelectric point.
assessed by measuring the foam height as a function of time. A vertical glass column (diameter 25 mm, length 600 mm) fitted with a sintered glass frit of porosity 4 was surrounded by a water-filled jacket maintained at 25 °C. A nitrogen line connected to both a flow rate meter and a copper coil heat exchanger was connected just below the sintered glass frit. The heat exchange ensured that the nitrogen was of the same temperature as the glass jacket. In a typical experiment, the apparatus was purged with nitrogen and equilibrated for 1 h. Before addition of the latex, the nitrogen flow rate was set to 50 mL min-1, and then 5.0 mL of the aqueous latex was carefully injected on top of the glass frit at the bottom of the glass column. After 4 min, the nitrogen supply was turned off, and the height of the foam column (if formed) was measured. The wet foam was allowed to dry in the foam column at 25 °C for 24 h (see Figure 1S), after which the dried foam height was remeasured and the foam was removed for further analysis.
Results and Discussion The synthesis of the cationic AIBA-PS latex described in this study is a little unusual in that it involves alcoholic dispersion polymerization in the absence of any polymeric stabilizer. Colloid stability is solely conferred by cationic amine groups arising from the azo initiator: this formulation is not successful if conducted in less polar solvents such as ethanol or 2-propanol or if non-ionic azo initiators are employed. This protocol was chosen simply because it was known14 to yield relatively large cationic charge-stabilized latex particles with reasonable control over the size distribution in a convenient single-shot synthesis. A second sterically stabilized cationic latex was prepared by copolymerizing PEGMA macromonomer with styrene using a “latent grafting” technique15 whereby the PEGMA stabilizer was only added to the polymerization after 3.75 h. As a control latex, anionic polystyrene particles were also prepared using an APS initiator. SEM images of these three latexes are shown in Figure (14) Schmid, A.; Fujii, S.; Armes, S. P. Langmuir 2006, 22, 4923. (15) (a) Cairns, D. B. Synthesis and Characterisation of Polypyrrole-Coated Latexes, Ph.D. Thesis, University of Sussex, Brighton, UK, 1999. (b) Cowell, C.; Li-In-On, R.; Vincent, B. J. Chem. Soc., Faraday Trans. 1 1978, 74, 337.
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Table 2. Summary of Foam Column Data Obtained for Three Polystyrene Latexes at 25 °C: Effect of Varying the Latex Concentration and Solution pHa entry no.
latex type
latex particle diameter (nm)
PEGMA stabilizer?
latex concentration (wt %)
solution pH
foam formation?
wet foam height (mm)
dry foam height (mm)
1 2 3 4 5 6 7 8 9 10 11 12 13 14
cationic cationic cationic cationic cationic cationic cationic cationic cationic cationic cationic cationic anionic anionic
724 724 724 724 724 724 800 800 800 800 800 800 904 904
no no no no no no yes yes yes yes yes yes no no
10.0 7.0 5.0 3.0 1.0 10.0 10.0 7.0 5.0 3.0 1.0 10.0 10.0 10.0
3.5 3.5 3.5 3.5 3.5 11.0 3.5 3.5 3.5 3.5 3.5 11.0 3.5 11.0
yes yes yes no no no yes yes yes yes yesb yes no no
535 420 135 N/A N/A N/A 530 455 365 355 190 350 N/A N/A
535 420 135 N/A N/A N/A 530 455 365 355 N/A collapsed N/A N/A
a Conditions: nitrogen sparge for 4 min at a flow rate of 50 mL min-1 using 5.0 mL aqueous latex dispersion. b Foam formed initially, but collapsed on drying.
2. In each case, near-monodisperse spherical particles were obtained. Disk centrifuge photosedimentometry studies (see Figure 2) indicated mean particle diameters of 724 ( 81 nm, 800 ( 138 nm, and 904 ( 131 nm for the AIBA-PS, PEGMAAIBA-PS, and APS-PS latexes, respectively. Thus, to a first approximation these three latexes have comparable mean diameters; hence particle size effects on foam stabilization performance are considered to be negligible. Aqueous electrophoresis studies were conducted on these three latexes (see Figure 3). The AIBA-PS latex had an IEP at pH 7.04. The PEGMA-AIBA-PS latex had a slightly higher IEP at pH 7.85. As expected, the APS-PS latex exhibited negative zeta potentials across the entire pH range due to the strong acid nature of its anionic sulfate groups. At first sight, it might seem strange that the two cationic latexes exhibit negative zeta potentials at higher pH, especially in the absence of any surface anionic groups. However, negative zeta potentials have also been reported at higher pH for various cationic copolymer latexes prepared using the same AIBA initiator.16 The two most likely explanations for such observations are either (i) surface-adsorbed hydroxide ions or (ii) hydrolysis of some of the surface amidine groups to produce anionic carboxylate groups. It is also noteworthy that the PEGMAAIBA-PS latex exhibits slightly higher positive zeta potentials below its IEP compared to the AIBA-PS latex.17 Nevertheless, these three latexes were considered suitable to investigate the influence of particle surface charge on foam stabilization performance. Initial assessment of foaming efficiency was achieved by simply “hand-shaking” the aqueous latexes. For the two cationic latexes, (16) (a) Elaı¨ssari, A.; Pichot, C.; Delair, T.; Cros, P.; Ku¨rfurst, R. Langmuir 1995, 11, 1261. (b) Voorn, D.-J.; Ming, W.; van Herk, A. M. Macromolecules 2005, 38, 3653. (c) Delair, T.; Pichot, C.; Mandrand, B. Colloid Polym. Sci. 1994, 272, 72. (d) Sauzedde, F.; Elaı¨ssari, A.; Pichot, C. Colloid Polym. Sci. 1999, 277, 846. (17) Two reviewers of this manuscript expressed some concern that the PEGMAAIBA-PS latex had slightly higher positive zeta potentials than the AIBA-PS latex at low pH. We were also surprised by this unexpected observation. However, 1H NMR analysis of the dissolved latex in CD Cl confirmed that the PEGMA 2 2 stabilizer is indeed grafted to the PEGMA-AIBA-PS latex (see Figure 3S in the Supporting Information). Moreover, the latent-grafting approach that we adopted in this synthesis should ensure that this PEGMA stabilizer is located on the outside of the latex particles. This is confirmed by our XPS studies, which revealed a C-O feature at approximately 286.5 eV that is not present in the AIBA-PS latex prepared in the absence of the PEGMA stabilizer (see Figure 4S in the Supporting Information). However, we calculate from the 1H NMR data that the PEGMA grafting density is only around 1.6 mg m-2. Also, additional cationic AIBA initiator was added to promote effective PEGMA grafting. Thus, we believe that the higher positive zeta potentials observed for the PEGMA-AIBA-PS latex are due to two factors: (i) the presence of additional cationic initiator fragments at the latex surface and (ii) the relatively low PEGMA grafting density (and hence reduced shielding efficiency).
the solution pH was adjusted such that the zeta potentials were approximately +20 mV, -20 mV, and 0 mV (i.e., at the IEP). Latexes with positive zeta potentials were predicted to have stronger foam stabilities due to the suggested electrostatic adsorption of the cationic particles at the anionic air/water interface.12,13 Support for this hypothesis was obtained during latex purification: the AIBA-PS latex produced no foam when methanol was the continuous phase, whereas redispersion in deionized water immediately produced copious long-lived foam, which increased with subsequent cycles. However, after three centrifugation/redispersion cycles in water, there was some reduction in the extent of foam formation. Similar observations were reported by Fujii and co-workers.9 The APS-PS latex exhibited no foaming behavior after purification and subsequently proved to be an ineffective foam stabilizer (see below). After hand-shaking aqueous latexes (1.0 mL) for 30 s, samples were left to stand for two weeks and their foam heights were measured. These data are summarized in Table 1. Both cationic latexes produced stable foams at low pH, with less foam being formed at lower latex concentrations. At their respective isoelectric points, the AIBA-PS and PEGMA-AIBA-PS latexes became flocculated, as indicated by optical microscopy. Nevertheless, some foam stabilization was observed under these conditions. Some particle aggregation also occurred at higher pH for these two latexes. After this initial evaluation, foam column experiments were undertaken using equipment identical to that used by Fujii and co-workers9 so as to ensure more rigorous and reproducible conditions. Nitrogen gas was preferred to air for foam generation: the nitrogen/water interface is not expected to be significantly different to an air/water interface with regard to particle adsorption and foam formation. A nitrogen sparge time of 4 min was employed (since this was the time normally required for the foam to reach the top of the foam column at the chosen flow rate of 50 mL min-1), and the foam heights were recorded. Like the initial hand-shaking experiments, the solution pH was first adjusted to either 3.5 or 11.0 and the latex concentration was varied from 10.0 wt % to 1.0 wt %. All foam column results are summarized in Table 2. Using the AIBA-PS latex at pH 3.5 led to stable foams of maximum height at latex concentrations of at least 5.0 wt %, with reduced foam heights being obtained at lower latex concentrations. These foam heights remained unchanged after drying. However, on switching the solution pH to 11.0, the same latex particles no longer possess cationic surface character, and no foam was formed. The PEGMA-AIBA-PS latex also proved to be an effective foam stabilizer at pH 3.5 at an even lower latex concentration of 3.0 wt %. Foaming also
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Langmuir, Vol. 23, No. 23, 2007 11385
Figure 4. Scanning electron micrographs obtained for dried foams obtained from foam column experiments. Low-magnification images show air bubbles stabilized with (a) AIBA-PS latex [foam generated at 7.0 wt % and pH 3.5] and (b) PEGMA-AIBA-PS latex [foam generated at 5.0 wt % and pH 3.5]. Higher-magnification images obtained by focusing on the top surface of selected bubbles [see selected areas in images a and b] show either (c) weakly ordered latex particles or (d) disordered latex particles. Images e and f of ruptured bubbles confirm latex bilayer formation within the bubble walls.
occurred at latex concentrations as low as 1.0 wt %, but such foams ruptured during drying. At pH 11.0, some foam was still formed within the column, but this foam contained visibly flocculated particles and completely collapsed on drying. It is emphasized that the APS-PS latex did not produce stable foams at either pH 3.5 or 11.0, even at latex concentrations of up to 10 wt %. One reviewer suggested that we should study the effect of added salt on foam stabilization. Accordingly, we conducted some additional hand-shaking experiments with both cationic latexes at 5.0 wt % solids in the presence of 10-3, 5 ×
10-3, 10-2, 5 × 10-2, and 10-1 M added salt (KCl). In each case, stable foams were observed for all but the highest salt concentration. Figure 4 shows SEM images obtained for dried foams prepared using the AIBA-PS and PEGMA-AIBA-PS latexes. Individual polydisperse bubbles of 200-900 µm diameter were observed at low magnification (see also Supporting Information Figure 2S). Focusing on the top surface of these dried bubbles confirmed the presence of the latex particles, although there is little evidence for the exquisite long-range ordering observed by Fujii et al.8,9
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This lack of long-range order is presumably related to the somewhat higher polydispersities obtained for the latexes used in the present work. If the dried foam is deliberately ruptured using a scalpel, the internal latex microstructure can be readily examined. In most cases, there was strong evidence for the formation of well-defined latex bilayers, although in some cases only latex monolayers were observed. The widespread formation of latex bilayers strongly suggests that most wet air bubbles were stabilized by monolayers of adsorbed latex particles. These monolayers are presumably forced together to form bilayers during water drainage from the drying foams. Similar results were reported by Fujii and co-workers8,9 and also by Dupin et al.10
foams. The same latexes proved to be ineffective foam stabilizers above their isoelectric points (where they possessed net negative surface charge). In contrast, the permanently anionic APS-PS latex did not produce stable foams under any conditions. This observation supports our original hypothesis that cationic and/or sterically stabilized particles adsorb much more strongly than anionic latexes at the air/water interface. Examination of dried foams by SEM confirmed that the two cationic latexes both produced foams comprising well-defined latex bilayers that were similar to those reported previously for sterically stabilized neutral latexes.8-10 However, long-range ordering of the latexes within the bubble surface was much less evident, presumably due to their somewhat higher polydispersity.
Conclusions
Acknowledgment. We thank the University of Sheffield for a Ph.D. studentship for A.S. and EPSRC for a postdoctoral grant for S.F. (GR/S69276). S.P.A. is the recipient of a 5-year Royal Society-Wolfson Research Merit Award.
Three types of micrometer-sized polystyrene latexes were successfully synthesized via dispersion polymerization in methanol using a cationic azo initiator, an anionic persulfate initiator, or the cationic azo initiator in combination with a non-ionic polymeric stabilizer. Stable foams were generated by both handshaking and also under more controlled conditions using a foam column. The foam stabilization performance of each latex was examined in detail. Below pH 7, two of the three latexes had cationic character: under these conditions, latex concentrations as low as 3.0 wt % were sufficient to generate highly stable
Supporting Information Available: Digital images of foams formed by hand-shaking and also by using the foam column; optical micrographs of latex-stabilized bubbles; 1H NMR spectra of PEGMAAIBA-PS and AIBA-PS latexes dissolved in CD2Cl2; X-ray photoelectron spectra recorded for the PEGMA-AIBA-PS and AIBA-PS latexes. This material is available free of charge via the Internet at http://pubs.acs.org. LA702099T
