7512
Langmuir 2006, 22, 7512-7520
Aqueous Particulate Foams Stabilized Solely with Polymer Latex Particles S. Fujii,*,† P. D. Iddon,‡ A. J. Ryan,‡ and S. P. Armes*,‡ Department of Applied Chemistry, Osaka Institute of Technology, 5-16-1 Ohmiya, Asahi-ku, Osaka 535-8585, Japan, and Department of Chemistry, Dainton Building, UniVersity of Sheffield, Brook Hill, Sheffield, S3 7HF United Kingdom ReceiVed March 27, 2006. In Final Form: June 12, 2006 In this article, a wide range of latexes are evaluated as possible foam stabilizers. These include near-monodisperse, poly(N-vinyl pyrrolidone)-stabilized polystyrene [PNVP-PS] latexes with diameters ranging from 170 nm to 1.62 µm, submicrometer-sized poly(ethylene glycol)-stabilized polystyrene [PEGMA-PS] latex particles, a PNVP-stabilized poly(4-bromostyrene) [PNVP-PBrS] latex with a mean diameter of 870 nm, two PNVP-stabilized poly(methyl methacrylate) [PNVP-PMMA] latexes with mean diameters of 730 nm and 1.20 µm, a PNVP-stabilized poly(2hydroxypropyl methacrylate) [PNVP-PHPMA] latex with a mean diameter of 630 nm, and a charge-stabilized anionic PS latex of 220 nm diameter. The effect of varying the particle size, latex concentration, and latex surface composition on foam stability were studied in detail. The larger PNVP-PS latexes, the PNVP-PBrS, and the two PNVP-PMMA latexes gave highly stable foams, whereas PEGMA-PS, PNVP-PHPMA, and the charge-stabilized PS latex produced either no foams or foams with inferior long-term stabilities. Scanning electron microscopy studies revealed hexagonally close-packed latex arrays in the walls of the dried foam, which leads to localized moire´ patterns being observed by optical microscopy. Moreover, these dried foams are highly iridescent in bright transmitted light.
Introduction Foams are used as either intermediates or as end-products in a wide range of industrial sectors, including mineral separation by ore flotation, food manufacturing, cosmetic formulations, and home and personal care products.1-4 Recently foams have also been exploited as a technology platform for the production of new porous materials.5 Aqueous foam stabilizers can be split into three main categories: (i) ionic or nonionic surfactants, (ii) polymeric stabilizers (including proteins), and (iii) finely divided solids or particles. A great deal of research concerning the first two categories has been reported in the literature.6-9 Solid particles have been used as both foam stabilizers and destabilizers in surfactant-stabilized aqueous foams for many years. Hydrophobic particles are often used in antifoam formulations: their mode of action is believed to involve a bridging-dewetting mechanism.2 Hydrophilic particles are considered to stabilize foams by accumulating in the plateau borders of a foam; this slows down film drainage and hence increases the kinetic foam stability.10 Hydrophilic particles can also increase foam stability by reducing gas diffusion between foam bubbles (otherwise such diffusion will lead to rupture of the foam bubbles and collapse of the * To whom correspondence should be addressed.
[email protected];
[email protected]. † Osaka Institute of Technology. ‡ University of Sheffield.
E-mail:
(1) Aveyard, R.; Binks, B. P.; Fletcher, P. D. I.; Peck, T. G.; Rutherford, C. E. AdV. Colloid Interface Sci. 1994, 48, 93. (2) Pugh, R. J. AdV. Colloid Interface Sci. 1996, 64, 67. (3) Sadoc, J. F., Rivier, N., Eds.; Foams and Emulsions; Kluwer Academic: Dordrecht, 1999; NATO ASI Series E, Vol. 354. (4) Kruglyakov, P. M.; Taube, P. R. Colloid J. 1972, 34, 194. (5) (a) Srinivasarao, M.; Collings, D.; Philips, A. Patel, S. Science, 2001, 292, 79. (b) Shirtcliffe, N. J., McHale, G., Newton, M. I.; Perry, C. C. Langmuir 2003, 19, 5626. (c) Zhang, H.; Cooper, A. I. Soft Matter 2005, 1, 107. (6) Kitchener, J. A.; Cooper, C. F. Q. ReV. 1959, 13, 71. (7) Kitchener, J. A. Recent Prog. Surf. Sci. 1964, 1, 51. (8) Sheludko, A. AdV. Colloid Interface Sci. 1967, 1, 391. (9) Ross, S. Chem. Eng. Prog. 1967, 63, 41. (10) Binks, B. P. Curr. Opin. Colloid Interface Sci. 2002, 7, 21.
foam).11,12 There may also be structural reinforcement of a foam film due to contact, friction, or cohesion between particles.13,14 In most of the foam studies reported to date, solid particles are usually used in combination with surfactants or colloidal stabilizers.1,2,10 The stability of such foams depends on the particle diameter, shape, concentration, and hydrophilicity, in addition to the type of surfactant or stabilizer that is used.2,3,15 Although Ramsden13 described the use of various inorganic sols to form persistent bubbles more than a century ago; there have been only a very small number of publications regarding foams stabilized solely by solid particles.11-13,16-20 These studies are briefly reviewed here. Wilson16 explored the foaming behavior of chargestabilized anionic polystyrene (PS) latex particles with diameters ranging from 1.02 to 3.89 µm. To the best of our knowledge, this PhD thesis is the only previous report of the production of stable aqueous foam using latex particles as the sole foam stabilizer. Although Wilson’s study has not been published in the primary literature, her results were summarized within a recent review article by Binks.10 Foaming could only be achieved after addition of either salt or cationic surfactant; or by making the latex highly acidic (pH < 1). Thus, the conditions required for generating stable foams either approached or exceeded those required for aggregation of the latex particles in bulk aqueous solution. Moreover, the foam quality decreased as the latex (11) Du, Z. P.; Bilbao-Montoya, M. P.; Binks, B. P.; Dickinson, E.; Ettelaie, R.; Murray, B. S. Langmuir 2003, 19, 3106. (12) Dickinson, E.; Ettelaie, R.; Kostakis, T.; Murray, B. S. Langmuir 2004, 20, 8517. (13) Ramsden, W. Proc. R. Soc. 1903, 72, 156. (14) Kruglyakov, P. M.; Taube, P. R. Colloid J.-USSR 1972, 34, 194. (15) Aveyard, R.; Binks, B. P.; Fletcher, P. D. I.; Peck, T. G.; Rutherford, C. E. AdV. Colloid Interface Sci. 1994, 48, 93. (16) Wilson, J. C., A Study of Particulate Foams, Ph.D. Thesis; University of Bristol: Bristol, U.K., 1980. (17) Sun, Y. Q.; Gao, T. Metall. Mater. Trans. A-Phys. Metall. Mater. Sci. 2002, 33, 3285. (18) Alargova, R. G.; Warhadpande, D. S.; Paunov, V. N.; Velev, O. D. Langmuir 2004, 20, 10371. (19) Binks, B. P.; Horozov, T. S. Angew. Chem. Int. Ed. 2005, 44, 3722. (20) Bindal, S. K.; Nikolov, A. D.; Wasan, D. T.; Lambert, D. P.; Koopman, D. C. EnViron. Sci. Technol. 2001, 35, 3941.
10.1021/la060812u CCC: $33.50 © 2006 American Chemical Society Published on Web 07/26/2006
Polymer Latex Particle Stabilized Foams
Langmuir, Vol. 22, No. 18, 2006 7513
Table 1. Summary of the Synthesis Conditions, Mean Particle Diameters, and Polydispersities of the Polystyrene Latex Particles Used in This Study reaction solvent composition (v/v%) sample ID
stabilizer
initiator
water
methanol
IPA
reaction temp (°C)
particle diameter (µm)
1 2 3 4 5 6 7 8 9 10
PNVP PNVP PNVP PNVP PNVP PNVP PEGMA PEGMA + SDS PEGMA SDS
AIBN AIBN AIBN AIBN AIBA APS AIBN APS AIBA APS
0 10 20 10 100 100 10 100 100 100
0 0 0 90 0 0 90 0 0 0
100 90 80 0 0 0 0 0 0 0
70 70 70 70 60 70 70 80 60 70
1.62 ( 0.13 (1.008) a 1.14 ( 0.08 (1.011) a 0.81 ( 0.08 (1.025) a 0.68 (0.17) b 0.26 (0.11) b 0.17 (0.18) b 0.25 (0.07) b 0.22 (0.09) b 0.19 (0.10) b 0.22 (0.06) b,c
a The weight-average particle diameter, D , as measured using a disk centrifuge. The number in brackets is the polydispersity. b The intensityw average particle diameter as measured by dynamic light scattering. The number in brackets is the polydispersity. c Also measured by disk centrifuge, which gave a Dw of 0.22 ( 0.02 µm.
diameter was reduced, with the minimum particle diameter required to obtain stable foams being approximately 1.50 µm; stable foams could not be prepared using 1.02 µm PS latex. Wilson’s observations appear to be closely related to the “surface coagulation” phenomenon previously studied by Heller and coworkers,21,22 in which colloidal dispersions of low charge density coagulated at an air-water interface after addition of salt. The critical salt concentration required to induce surface coagulation was just less than that required for coagulation in bulk solution.21,22 Finally, it is emphasized that Wilson only studied wet foams; it was not stated whether these latex-stabilized foams survived drying. Sun and Gao17 used either 1 µm poly(tetrafluoroethylene), 20 µm polyethylene, or 75 µm poly(vinyl chloride) particles to produce foams of reasonable stability in water/ethanol mixtures. However, no foams could be obtained in the absence of the alcohol cosolvent. Murray and co-workers11,12 recently used partially hydrophobic silylated 20 nm silica nanoparticles as the sole stabilizer for the formation of stable air bubbles. However, these silica particles were rather inefficient stabilizers. Salt was also used to fine-tune the hydrophobic character of the particles to further optimize foam formation and stabilization. Velev and co-workers18 described the synthesis of highly stable foams prepared using polydisperse bisphenol A-based epoxy resin microrods with an average length of 23.5 µm and an average diameter of 0.6 µm, in the absence of any surfactant. Due to their rigid, entangled structure and resistance to mechanical perturbation, these microrod-stabilized foams were very stable, even after drying. Binks and Horozov19 investigated foam formation using partially hydrophobic silica nanoparticles dispersed in water with the aid of ethanol (this cosolvent was subsequently removed by centrifugation/redispersion cycles prior to the foam studies). These silica nanoparticles were relatively polydisperse and prone to aggregation in aqueous media; it was suggested by Binks and Horozov that the air bubbles were covered by relatively thick layers of the silica nanoparticles. In all of the above studies, either additives (e.g., salt, alcohol, or acid) or unusual preparation methods were employed. Moreover, the stability and structure of dried foams does not seem to have been studied in any detail. Recently, we serendipitously discovered that micrometer-sized, near-monodisperse sterically stabilized PS latex particles of 1.57 µm diameter synthesized by dispersion polymerization can stabilize aqueous foams without requiring any additives. Moreover, foam formation results in exquisite long-range ordering of latex particles that is preserved after drying.23 In the present (21) Delauder, W. B.; Heller, W. J. Colloid Interface Sci. 1971, 35, 308. (22) Heller, W.; Delauder, W. B. J. Colloid Interface Sci. 1971, 35, 60.
work, we evaluate the performance of the following latexes as sole foaming agents: (i) near-monodisperse, sterically stabilized PS latexes of varying mean diameters; (ii) a submicrometersized, charge-stabilized PS latex; (iii) sterically stabilized poly(4-bromostyrene); (iv) sterically stabilized poly(methyl methacrylate); (v) sterically stabilized poly(2-hydroxypropyl methacrylate). Some of these latexes lead to highly stable foams that retain their structure after drying, with little or no change in volume. Although such foams were readily generated by simply handshaking, a foam column apparatus was constructed to assess foam formation more reproducibly. The effects of latex particle diameter, latex concentration, and particle surface composition on the resulting foam structure and stability were studied. Unusual optical effects for these unique foams are also described. Experimental Section Full experimental details describing the synthesis and characterization of the various latexes are given in the Supporting Information. Characterization data for the various PS latexes used in this study are summarized in Table 1. Representative scanning electron microscopy (SEM) images of the poly(N-vinylpyrrolidone)stabilized PS (PNVP-PS) latex particles are shown in Supporting Figure S1. Foam Generation. By Hand-Shaking. Aqueous latex dispersions were shaken for 30 s by hand. The shaking time, although arbitrary, was found to give reasonably reproducible foams. Hand-shaking was also found to be a useful indicator of the propensity of a given latex to stabilize foams: latexes that did not produce foams on hand-shaking invariably did not produce foams using the foam column method. Foam Column. Foam columns equipped with glass frits are widely used to generate and characterize foams.24 The foam column setup used in this study is shown in Figure 1. A sintered glass frit of known porosity (25 mm diameter, 3 mm depth) was connected to one end of a long vertical glass column (24 mm diameter; 600-650 mm in length). The column and frit were surrounded by a water-filled jacket. The space between the column and this water jacket was designed to be as small as possible and was packed at the top to reduce convective heat loss. The inner wall of the water jacket was constructed from thinner glass than that used for the outer wall, so as to aid conductive heat transfer. Dry nitrogen gas was connected in series to a flow rate meter, a copper coil heat-exchanger, before finally connecting to just below the sintered glass frit. The heatexchanger ensured the nitrogen gas was preheated to the same temperature as the glass jacket. All glassware was thoroughly washed with tetrahydrofuran and dried before use. (23) (a) Fujii, S.; Ryan, A. J.; Armes, S. P. J. Am. Chem. Soc. 2006, 128, 7882. (b) Fujii, S.; Armes, S. P.; Iddon, P. D.; Ryan, A. J. Particle-Stabilised Foams, GB Patent Application 524186.4, 2005. (24) (a) Oh, S.-G.; Klein, S. P.; Shah, D. O. Langmuir 1991, 7, 1316. (b) Oh, S.-G.; Klein, S. P.; Shah, D. O. AIChE J. 1992, 38, 149.
7514 Langmuir, Vol. 22, No. 18, 2006
Fujii et al. respectively. The synthesis of the pyrene-labeled polystyrene latex particles used in these CLSM studies has been described previously.23a The diluted air bubbles were placed on the glass slide and viewed directly (no cover glass was used to avoid disrupting the superstructure of the PS particles on the bubble surface). Scanning Electron Microscopy (SEM). SEM images were obtained using a Leo Stereoscan 420 instrument operating at a voltage of 20 kV and a beam current of 5-10 pA. Dried foam samples were placed on an aluminum stub and sputter-coated with gold to minimize sample-charging problems.
Results and Discussion
In a typical experiment, the apparatus was assembled and the water jacket temperature was set to 25 °C. The apparatus temperature was allowed to equilibrate for at least 1 h before the start of the experiment. During this time, nitrogen gas was passed through the frit so that the air in the column was displaced. Before addition of the latex, the nitrogen flow rate was set to 50-60 mL min-1. The aqueous latex (5.0 mL) was then carefully injected from the top of the glass column onto the frit. There was a short induction time as the nitrogen gas gradually reached sufficient pressure to be forced through the pores in the frit. As soon as bubbles were observed leaving the surface of the frit, the timer was started. After 5 min, the nitrogen supply was turned off, and the height of any stable foam formed and mean bubble diameters at the top, middle, and bottom of the foam were then estimated. At nitrogen flow rates of 50-60 mL min.-1, individual bubbles emerged from the pores in the glass frit.25 This was confirmed by bubbling nitrogen through deionized water (containing no latex particles) to aid visualization of the bubbles. In some cases, the nonadsorbed latex remaining after foam generation was collected for analysis. The foam was allowed to remain in the foam column for at least 24 h at 25 °C. The final foam height was measured and the foam then removed for further analysis. Sintered glass frits with two porosities [3 (16-40 µm pore diameter) and 4 (10-16 µm pore diameter)] were evaluated in preliminary foam column experiments. Unless stated otherwise, the frit used in all of the foam experiments described in this manuscript had a pore diameter of 10-16 µm. Characterization of Latex Foams. Digital Photography. A Nikon Coolpix 4500 digital camera was used to record digital photographs of foams generated using the foam column method. No further image processing was carried out on these images. Optical Microscopy. Optical micrographs of dry foams were obtained using a James Swift MP3502 optical microscope (Prior Scientific Instruments Ltd.) fitted with a Nikon Coolpix 4500 digital camera. Confocal Laser Scanning Microscopy (CLSM). A Zeiss LSM 510 Meta mounted on an Axiovert 200M microscope was used for the CLSM study. This instrument was equipped with argon ion UV and HeNe gas lasers,operating at wavelengths of 351 nm (for excitation of the pyrene groups in the PS latex particles) and 543 nm,
Foam Generation. When the PNVP-PS latex particles synthesized by dispersion polymerization were dispersed in their original 2-propanol (IPA) or IPA/water mixtures (entries 1-3, Table 1), stable foams were not obtained. However, as their purification via successive centrifugation/redispersion cycles progressed, the quantity and stability of the foam that could be generated by hand-shaking gradually increased. This is probably due to the removal of IPA and excess PNVP during the centrifugation/redispersion cycles. Surface tension measurements of the supernatant solutions decanted after each centrifugation step support this hypothesis. However, after 3-5 centrifugation cycles, the quantity of foam generated began to decline, although the foam remained very stable. Gravimetric studies indicated that the water content of the wet aqueous foam stabilized with 1.57 µm PS latex particles (used in our previous study23a) obtained immediately after hand-shaking was 86 wt %. Although simple hand-shaking was sufficient for the generation of foam, a more reliable and reproducible method was desired. Thus, a foam column was constructed for assessing the relative foam stabilization efficiencies of the various latexes under controlled experimental conditions. A diagram of the foam column is shown in Figure 1. The use of dry nitrogen to generate foam was considered preferable to the use of air by Wilson,16 which can contain variable amounts of water vapor. However, with regard to particle adsorption and foam formation, a nitrogen/ water interface is not expected to be significantly different to an air/water interface.26 Digital photographs of the foams generated in the foam column are shown in Figure 2S (see the Supporting Information). In all cases where stable foam was generated in sufficient quantity to cover the entire diameter of the foam column, the observed foam heights were not limiting: if the nitrogen flow was not terminated after 5 min, the foam continued to rise up the column. More detailed analysis showed that the rate of change of foam height with time was constant. This is due to the relatively rigid nature of the highly stable foams: the foam acts as a “plug” in the column and is simply pushed up by the inflowing gas. In principle, a continuous flow of nitrogen combined with a method for the delivery of fresh latex to the sintered glass frit would enable highly stable particulate foam to be generated continuously. Measuring the average bubble diameter at the top, middle, and bottom of the foam after its generation provides an indication of the foam uniformity. All foam column experiments were carried out at 25°C using 5.0 mL aqueous latex and a constant nitrogen flow rate of 50-60 mL min-1 for 5 min. This nitrogen flow rate and time correspond to a maximum theoretical foam height of approximately 550-660 mm. Foam Formation using the 1.14 µm PNVP-PS Latex. The 1.14 µm PNVP-PS latex (entry 2 in Table 1) was used as a model latex in our initial foam experiments. A 7.5 w/v % aqueous dispersion of this latex produced foam heights of 665 mm (this extrapolated value was calculated from the foam height achieved
(25) Dushkin, C. D.; Stoychev, T. L.; Horozov, T. S.; Mehreteab, A.; Broze, G. Colloid Polym. Sci. 2003, 281, 130.
(26) Lide, D. R. Ed. CRC Handbook of Chemistry and Physics, 85th ed.; CRC Press: Boca Raton, FL, 2004.
Figure 1. Schematic representation of the foam column apparatus. (not to scale).
Polymer Latex Particle Stabilized Foams
Langmuir, Vol. 22, No. 18, 2006 7515
Figure 2. Representative SEM images of fragments of highly stable foams prepared using the 1.62 µm diameter PNVP-PS latex (entry 1 in Table 1). Figure 2b is a magnified image of Figure 2a. Table 2. Effect of Varying Latex Concentration on Foam Formation Using the Model 1.14 µm PNVP-PS Particles (Entry 2 in Table 1) With the Foam Columna
frit pore diameter
latex conc (w/v%)
initial foam height (mm)
10-16 µm 10-16 µm 10-16 µm 10-16 µm 16-40 µm
1.0 2.0 4.0 7.5 7.5
250 630 635 665 615
initial approximate bubble diameter range (mm) top of middle of bottom of column column column c 2-5 2 1
