Adsorption of Submicrometer-Sized Cationic Sterically Stabilized

Feb 12, 2009 - E-mail: [email protected] (T.N.H.); [email protected] (D.D.)., † ... and V. N. Paunov , K. L. Thompson , A. Walsh , and S...
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Langmuir 2009, 25, 3440-3449

Adsorption of Submicrometer-Sized Cationic Sterically Stabilized Polystyrene Latex at the Air-Water Interface: Contact Angle Determination by Ellipsometry Timothy N. Hunter*,† and Graeme J. Jameson Department of Chemical Engineering, The UniVersity of Newcastle, Callaghan, New South Wales 2308, Australia

Erica J. Wanless School of EnVironmental and Life Sciences, The UniVersity of Newcastle, Callaghan, New South Wales 2308, Australia

Damien Dupin* and Steven P. Armes Dainton Building, Department of Chemistry, UniVersity of Sheffield, Brook Hill, Sheffield, Yorkshire S3 7HF, U.K. ReceiVed NoVember 24, 2008. ReVised Manuscript ReceiVed January 14, 2009 Near-monodisperse, sterically stabilized cationic polystyrene latexes of either 122 or 310 nm diameter were prepared by aqueous emulsion polymerization using cheap, readily available reagents. At low pH, these latexes stabilized foams prepared by either hand-shaking or by using a foam column. SEM studies confirmed that the dried foam mainly comprised well-defined bilayers, which suggests that each air bubble is stabilized with a latex monolayer. Adsorption of the same latexes at the planar air-water interface was studied using the Langmuir-Blodgett trough technique. Surface pressure isotherms confirmed particle desorption from the interface on repeated compression of the latex monolayers. For the 122 nm latex at pH 2, ellipsometric analysis enabled a contact angle of ∼43° to be calculated from a simple two-layer model, which suggests that these particles have only moderate wettability. Similar results were obtained for the 310 nm latex, but the data were much less reliable in this case due to additional background particle scattering.

Introduction The stabilization of bubbles and foams by adsorbed particles has been known for over a century.1 This phenomenon is important in the context of both mineral flotation2,3 and food science.4,5 Recent advances in Pickering emulsions6 has led to increasing interest in designing surface-active colloidal particles for the generation of highly stable foams.7 Indeed, this approach is already being exploited for the production of stable sintered metal8-11 * To whom correspondence should be addressed. E-mail: t.n.hunter@ leeds.ac.uk (T.N.H.); [email protected] (D.D.). † Current address: Institute of Particle Science and Engineering, School of Process, Environmental and Materials Engineering, University of Leeds, Leeds, Yorkshire LS2 9JT, U.K.

(1) Ramsden, W. Proc. R. Soc. 1903, 72, 156. (2) Nguyen, A. V.; Schulze, H. J. Colloidal Science of Flotation; Marcel Dekker: New York, 2003; Vol. 1. (3) Prud’homme, R. K.; Khan, S. A. Foams - Theory, Measurements, and Applications; Marcel Dekker, Inc.: New York, 1996. (4) Dickinson, E. An Introduction to Food Colloids; Oxford University Press: Leeds, 1992. (5) Murray, B. S.; Ettelaie, R. Curr. Opin. Colloid Interface Sci. 2004, 9, 314–320. (6) Aveyard, R.; Binks, B. P.; Clint, J. H. AdV. Colloid Interface Sci. 2003, 100-102, 503–546. (7) Hunter, T. N.; Pugh, R. J.; Franks, G. V.; Jameson, G. J. AdV. Colloid Interface Sci. 2008, 137, 57–81. (8) Banhart, J. AdV. Eng. Mater. 2006, 8, 781–794. (9) Vinod Kumar, G. S.; Garcia-Moreno, F.; Babcsan, N.; Brothers, A. H.; Murty, B. S.; Banhart, J. Phys. Chem. Chem. Phys. 2007, 9, 6415–6425. (10) Wuebben, T.; Odenbach, S. Colloids Surf., A 2005, 266, 207–213. (11) Haibel, A.; Rack, A.; Banhart, J. Appl. Phys. Lett. 2006, 89, 154102/ 1–154102/3.

and ceramic12,13 foams, as well as in the use of bubbles as templates for colloidosomes.14-16 However, our detailed understanding of the various factors that govern the interfacial activity of particles remains substantially incomplete. Previous studies suggest that important parameters include particle size,17-20 degree of aggregation,21-23 surface charge,17,21,24,25 and hydrophobic character.26-29 In some cases, the presence of (12) Gonzenbach, U. T.; Studart, A. R.; Steinlin, D.; Tervoort, E.; Gauckler, L. J. J. Am. Ceram. Soc. 2007, 90, 3407–3414. (13) Gonzenbach, U. T.; Studart, A. R.; Tervoort, E.; Gauckler, L. J. J. Am. Ceram. Soc. 2007, 90, 16–22. (14) Binks, B. P.; Horozov, T. S. Colloidal Particles at Liquid Interfaces; Cambridge University Press: New York, 2006. (15) Studart, A. R.; Gonzenbach, U. T.; Akartuna, I.; Tervoort, E.; Gauckler, L. J. J. Mater. Chem. 2007, 17, 3283–3289. (16) Subramaniam, A. B.; Abkarian, M.; Stone, H. A. Nat. Mater. 2005, 4, 553–556. (17) Blute, I.; Pugh, R. J.; van de Pas, J.; Callaghan, I. J. Colloid Interface Sci. 2007, 313, 645–655. (18) Fujii, S.; Iddon, P. D.; Ryan, A. J.; Armes, S. P. Langmuir 2006, 22, 7512–7520. (19) Gonzenbach, U. T.; Studart, A. R.; Tervoort, E.; Gauckler, L. J. Langmuir 2007, 23, 1025–1032. (20) Fujii, S.; Ryan, A. J.; Armes, S. P. J. Am. Chem. Soc. 2006, 128, 7882– 7886. (21) Binks, B. P.; Duncumb, B.; Murakami, R. Langmuir 2007, 23, 9143– 9146. (22) Dickinson, E.; Ettelaie, R.; Kostakis, T.; Murray, B. S. Langmuir 2004, 20, 8517–8525. (23) Kostakis, T.; Ettelaie, R.; Murray, B. S. Langmuir 2006, 22, 1273–1280. (24) Kettlewell, S. L.; Schmid, A.; Fujii, S.; Dupin, D.; Armes, S. P. Langmuir 2007, 23, 11381–11386. (25) Pugh, R. J. Langmuir 2007, 23, 7972–7980. (26) Binks, B. P.; Horozov, T. S. Angew. Chem., Int. Ed. 2005, 44, 3722–3725. (27) Binks, B. P.; Murakami, R. Nat. Mater. 2006, 5, 865–869.

10.1021/la803879p CCC: $40.75  2009 American Chemical Society Published on Web 02/12/2009

Adsorption of Submicrometer-Sized Latex

secondary components such as conventional surfactants can also be beneficial.30-32 Particles that are very hydrophobic (with contact angles of 70-90°) have relatively high interfacial detachment energies.33 This leads to much better resistance toward desorption during foam drainage, allowing highly stable bubbles and foams to be prepared that can remain intact over timescales of months. This enhanced stability is primarily because there is a strong steric barrier to bubble coalescence.7,26-28 Less hydrophobic particles can also dynamically stabilize bubbles by retarding drainage,7,29 either by physically ‘entrapping fluid’,34 altering bubble curvatures and threshold capillary pressures,11,35,36 or by stratifying in films.37,38 Indeed, even weakly hydrophobic latex particles can form stable foams by undergoing a 2D phase transition to form a crystalline surface layer with long-range periodicity.16 Surface charge also plays a very important role with weakly hydrophobic particles, since this can dramatically affect the interfacial interactions. It is well known that the air-water interface is highly hydrophobic and also has anionic character.39 Hence, reduction of the anionic particle surface charge by judicious adjustment of the dispersion pH can significantly increase the propensity of oxide particles to adsorb at the air-water interface, thus enhancing foam stability.17,21,25 Recently, Armes and co-workers designed a series of surfaceactive latexes for use as the sole particulate stabilizer of air bubbles and foams.18,20,24,40,41 For example, micrometer-sized sterically stabilized polystyrene latexes with no ionic surface groups were prepared by alcoholic dispersion polymerization using either poly(N-vinylpyrrolidone)18,20 or poly(acrylic acid)40 stabilizers. In some cases, latex size distributions were sufficiently narrow for the dried foam fragments to exhibit significant iridescence when viewed in strong transmitted light.18,20 However, submicrometer-sized latexes proved to be much less effective in producing stable foams. Charge-stabilized polystyrene latexes were also prepared by Kettlewell et al. via alcoholic dispersion polymerization using a cationic radical initiator, rather than a polymeric stabilizer.24 It was found that such latexes could also stabilize foams but only below their isoelectric point, where they acquired strong cationic surface charge. Conversely, similar latexes prepared using anionic initiators such as persulfate did not lead to stable foams under any circumstances. Finally, Dupin et al. used the combination of a poly(ethylene glycol) monomethacrylate (PEGMA) steric stabilizer and a cationic azo initiator to prepare three surface-active poly(2-vinylpyridine) latexes of 380, 640, and 820 nm diameter.41 These particles also proved to be effective foam stabilizers, with the smallest latex also giving rise to distinctly pink- or blue-colored foams when (28) Somosvari, B. M.; Babcsan, N.; Barczy, P.; Berthold, A. Colloids Surf., A 2007, 309, 240–245. (29) Hunter, T. N.; Wanless, E. J.; Jameson, G. J. Colloids Surf., A 2008, 334, 181–190. (30) Velikov, K. P.; Velev, O. D. Langmuir 1998, 14, 1148–1155. (31) Zhang, S.; Lan, Q.; Liu, Q.; Xu, J.; Sun, D. Colloids Surf., A 2008, 317, 406–413. (32) Zhang, S.; Sun, D.; Dong, X.; Li, C.; Xu, J. Colloids Surf., A 2008, 324, 1–8. (33) Binks, B. P. Curr. Opin. Colloid Interface Sci. 2002, 7, 21–41. (34) Melo, F.; Laskowski, J. S. Int. J. Miner. Process 2007, 84, 33–40. (35) Kaptay, G. Colloids Surf., A 2006, 282 + 283, 387–401. (36) Slobozhanin, L. A.; Alexander, J. I. D.; Collicott, S. H.; Gonzalez, S. R. Phys. Fluids 2006, 18, 082104/1–082104/15. (37) Sethumadhavan, G.; Bindal, S.; Nikolov, A.; Wasan, D. Colloids Surf., A 2002, 204, 51–62. (38) Wasan, D. T.; Nikolov, A. D. Aust. J. Chem. 2007, 60, 633–637. (39) Karraker, K. A.; Radke, C. J. AdV. Colloid Interface Sci. 2002, 96, 231– 64. (40) Binks, B. P.; Murakami, R.; Armes, S. P.; Fujii, S.; Schmid, A. Langmuir 2007, 23, 8691–8694. (41) Dupin, D.; Howse, J. R.; Armes, S. P.; Randall, D. P. J. Mater. Chem. 2008, 18, 545–552.

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viewed in reflectance mode. These wet foams also proved to be pH-responsive: addition of acid led to protonation of the crosslinked poly(2-vinylpyridine) chains and spontaneous desorption of the highly swollen cationic microgel particles from the air-water interface, leading to rapid bubble coalescence.41 In the present work, two relatively small polystyrene latexes were synthesized using the same combination of nonionic PEGMA stabilizer and cationic azo initiator as that reported by Dupin et al.41 These sterically stabilized latexes also proved to be surface-active, as expected. Having confirmed successful bubble/foam stabilization, the microstructure of the dried foams was examined using scanning electron microscopy. Moreover, the adsorption behavior of both latexes at the planar air-water interface was compared using a Langmuir-Blodgett (LB) trough combined with in situ ellipsometry. The compression of latex monolayers as a function of solution pH confirmed the influence of surface charge on the interfacial behavior of these latexes. Finally, the contact angle of the smaller latex at the air-water interface was determined by ellipsometry.

Materials and Methods Materials. Styrene (97%, Aldrich, UK) was treated with basic alumina in order to remove inhibitor. R,R′-Azodiisobutyramidine dihydrochloride (97%, AIBA; Aldrich, UK) was used as received. Monomethoxy-capped PEGMA macromonomer (Mn ) 2000; Mw/ Mn ) 1.10) was supplied by Cognis Performance Chemicals (Hythe, UK) as a 50 wt % aqueous solution. Deionized water using an Elga Elgastat Option 3 was used in all polymerizations and foam experiments. The typical surface tension of this deionized water was 72 mN m-1 at 20 °C. PEGMA-Stabilized Polystyrene Latex Syntheses via Emulsion Polymerization. Styrene polymerization was conducted at 5.0 wt % solids using the following protocol. PEGMA stabilizer (0.50 g) was dissolved in deionized water (42.0 g) in a 100 mL single-necked round-bottomed flask. Styrene monomer (2.50 g) was then added. The flask was sealed with a rubber septum, and the aqueous solution was degassed at ambient temperature using five vacuum/nitrogen cycles. The degassed solution was stirred at 250 rpm using a magnetic stirrer, heated at 60 °C with the aid of an oil bath and then the initiator solution (25 mg of AIBA dissolved in 5.0 g of water) was added after 20 min. The copolymerizing solution turned milkywhite within 10 min, and stirring was continued for 24 h at 60 °C. The PEGMA-stabilized polystyrene (PEGMA-PS) latex was centrifuged at 20 000 rpm for 3 h, followed by careful decantation of the supernatant, replacement with fresh water, and redispersion of the sedimented particles with the aid of an ultrasonic bath. Purification was continued until the serum surface tension was close to that of pure water (71 ( 1 mN m-1). A second, larger PEGMA-PS latex was also prepared by the same protocol at 10% solids using the following formulation: 5.0 g of styrene, 1.00 g of PEGMA, 39.0 g of water, 50 mg of AIBA (dissolved in 5.0 g of water). Dynamic Light Scattering (DLS) Studies. DLS studies were performed at 25 °C using a Malvern Zetasizer NanoZS Instrument equipped with a 4 mW He-Ne solid-state laser operating at 633 nm. All measurements were performed in triplicate on highly dilute dispersions. The intensity-average particle diameter (Dz) and polydispersity (µ2/Γ2) were calculated by cumulants analysis of the experimental correlation function using the Stokes-Einstein equation for dilute, noninteracting perfectly monodisperse spheres. Disk Centrifuge Photosedimentometry (DCP). DCP measurements were conducted using a Brookhaven BI-DCP instrument operating in the line start mode. This technique reports the weightaverage diameter (Dw) of hard spheres. Samples for DCP analysis were prepared by diluting a few drops of the aqueous latex mixture in 20 mL of a 1:3 v/v % methanol/water mixture. This solution was then immersed in an ultrasonic bath for 30 s prior to DCP analysis. The centrifugation rate was adjusted to between 12 000 and 15 000 rpm, depending on the particle diameter. Helium pycnometry measurements indicated a particle density of 1.055 g cm-3 for both

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Hunter et al.

Table 1. Synthesis Parameters and Particle Size Analyses for Near-monodisperse Polystyrene Latexes Prepared via Emulsion Polymerization at 60 °C Using a Cationic Azo Initiator and a Monomethoxy-Capped Poly(ethylene glycol) Monomethacrylate Macromonomer (10 wt % Based on Styrene Monomer) as a Reactive Steric Stabilizer

entry no. 1 2

styrene monomer/wt %

grafted PEGMA stabilizera/wt %

number-average diameterb/nm

weight-average diameterc/nm

hydrodynamic diameterd/nm (polydispersity)d

5.0 10.0

2.2 2.6

120 300

130 ( 14 310 ( 15

140 (0.06) 310 (0.03)

a Estimated by 1H NMR in CD2Cl2. b Estimated by electron microscopy (∼100 particles measured per latex). c Measured by disk centrifuge photosedimentometry at 20 °C. d Measured by DLS at 25 °C.

the PEGMA-PS latexes listed in Table 1. The effect of the solvated PEGMA stabilizer layer on the effective particle density was considered to be negligible. In practice, this is a good approximation for the larger latex and a reasonable approximation for the smaller latex.42 BET Surface Area Analysis. The specific surface area, As, of the smaller latex was determined using BET surface area measurements using a Quantachrome Nova 1000 instrument. The adsorbate was dinitrogen gas at 77 K (16.2 Å2/molecule). A five-point BET isotherm was determined and As was determined to be ∼50 m2 g-1 with an estimated experimental error of 5%. Using the equation As ) 3/FR (where F is the particle density) the mean particle radius R can be calculated. Electron Microscopy Studies. Scanning electron microscopy (SEM) images were obtained using a FEG-SEM Inspect F instrument operating at a voltage of 20 kV. Dried samples were mounted on adhesive carbon disks placed on aluminum stubs and then sputtercoated with gold to minimize sample charging. Transmission electron microscopy (TEM) samples were prepared by drying a drop of a dilute dispersion onto a carbon-coated copper grid and analyzed using a Phillips CM100 electron microscope operating at 100 kV. Aqueous Electrophoresis. Zeta potentials were calculated from electrophoretic mobilities using a Malvern Zetasizer NanoZS instrument. Mobilities were obtained as a function of pH on diluted dispersions (0.01 wt %) in 0.01 M NaCl by gradually adding HCl, starting from an initial pH of around 10. Each zeta potential represents an average of over 20 runs. The variance was typically within the size of the data points shown. X-ray Photoelectron Spectroscopy (XPS). Aqueous latexes were dried onto a silicon wafer at room temperature. Samples were then placed under vacuum overnight and analyzed using a Kratos Axis Ultra DLD X-ray photoelectron spectrometer equipped with a monochromatic Al X-ray source operating at 6.0 mA and 15 kV at a typical base pressure of 10-8 torr. The step size was 0.5 eV for the survey spectra (pass energy ) 160 eV) and 0.05 eV for the high resolution spectra (pass energy ) 80 eV). Spectra were typically acquired from at least two separate sample areas. Foam Generation. Aqueous latex dispersions at pH 2 and 6 were shaken for 2 min by hand. Although somewhat arbitrary, this simple protocol gave 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 handshaking invariably did not produce foams using the foam column method. The foam column setup has been recently described in detail by Fujii et al.,18 using a sintered (10 × 16 µm pore diameter) glass column. In a typical experiment, the apparatus was assembled and the water jacket temperature was set to 25 °C and equilibrated for at least 30 min before the start of the experiment. Nitrogen gas (flow rate ) 50-60 mL min-1) was passed through the column, and 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 before the nitrogen gas attained sufficient pressure to pass through the pores in the frit. Zero time was taken to be when bubbles were first observed leaving the surface of the frit and foaming was allowed to continue for 5 min before terminating the nitrogen supply. The characteristics of any resulting wet foam (height, mean bubble diameters, appearance, etc.) were noted. Each foam was allowed to (42) Cairns, D. B.; Armes, S. P.; Chehimi, M. M.; Perruchot, C.; Delamar, M. Langmuir 1999, 15, 8059–8066.

dry in the foam column for at least 24 h at 20 °C. The final foam height was measured and a portion of foam then removed for further analysis. A Nikon Coolpix 4500 digital camera was used to record digital photographs of foams generated using the foam column method. LB Trough Experiments. A 10 wt % solids aqueous dispersion of polystyrene latex was first diluted by a factor of 4 using ethanol. The particles were deposited dropwise using a 100 µL syringe onto the surface of an aqueous subphase contained within a 500 cm2 NIMA (U.K.) LB trough. In all tests, the trough was compressed at a rate of 50 cm2 min-1 and any resulting changes in the surface pressure were measured. For the larger polystyrene latex, the aqueous subphase pH was adjusted to pH 6.0, 3.5, 2.85, 2.0, and 1.5, respectively, and the equivalent of 4 mg of latex was deposited onto each subphase. For the smaller polystyrene latex, an equivalent mass of 1.4 mg was added to the aqueous subphase (at a constant pH of 2) and the adsorbed particle layer was compressed, expanded, and recompressed. The data obtained during compression were used to calculate the average interparticle distance, h, achieved during monolayer formation. This information was used for the in situ ellipsometric analysis. Ellipsometry Studies of Latex Monolayers. Ellipsometry experiments were conducted with a Nanofilm EP3 single-wavelength null-type imaging ellipsometer equipped with a 532 nm laser and CCD camera. This instrument was attached to the LB trough to allow the interrogation of liquid interfaces. Again, particle dispersions were deposited via syringe onto an aqueous subphase at pH 2. The adsorbed latex layer was compressed and the resulting surface pressure isotherm was monitored. Once the surface pressure reached 12 mN m-1 (indicating that the latex particles had formed an organized monolayer) compression was stopped. The monolayer was first split into three regions of interest (ROI), as imaged by the CCD camera, totalling ∼100 × 200 µm2 in area. Each ROI was analyzed by measuring the reflectance amplitude ratio (Ψ) and phase shift (∆) at various angles of incidence (AOI) between 49.5° and 58.5° (measured in 0.1° increments). Consistency between each ROI (within 95% confidence) was considered to be the minimum quality threshold for modeling the data. Three different particle monolayers were analyzed, affording nine separate data series for modeling. The raw ellipsometric data was compared to the theoretical two-layer model using an Excel-based ellipsometry program developed by Kondoh.43

Results and Discussion Very recently, Hunter et al.44 determined the contact angle made by a hydrophilic 265 nm diameter silica sol at the air-water interface using ellipsometry combined with a LB trough. One advantage of this approach is that very little sample is required, since the particles are introduced from the air phase and adsorb in situ. In the present study, this new technique was extended to include polystyrene latex. In general, this ellipsometric method requires the following three criteria to be fulfilled. (i) The particles must be relatively small compared to the laser wavelength to (43) Kondoh, E. Ellipsometry Spreadsheet Program-Multilayer Model, Department of Engineering; University of Yamanashi: Yamanashi, Japan, 2001. (44) Hunter, T. N.; Jameson, G. J.; Wanless, E. J. Aust. J. Chem. 2007, 60, 651–655.

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Figure 1. Schematic synthesis of PEGMA-PS latex by aqueous emulsion polymerization of styrene using a cationic azo initiator at 60 °C in the presence of a reactive PEGMA stabilizer. Subsequent self-assembly of these cationic sterically stabilized latexes at the surface of air bubbles.

Figure 2. Representative electron micrographs of the two near-monodisperse PEGMA-PS latexes shown in Table 1 after drying from neutral aqueous solution at ambient temperature: (a) 122 nm PEGMA-PS latex and (b) 310 nm PEGMA-PS latex. The particle size distribution curves for these two latexes obtained by disk centrifuge photosedimentometry are shown below.

avoid excessive light scattering;45 (ii) the particle size distribution should be relatively narrow, so that the mean monolayer thickness is uniform; and (iii) the particles should have high colloidal stability in the aqueous subphase so that only monolayers are formed. This latter point is important because many types of surface-active particles have partial hydrophobic character which usually causes some degree of flocculation in the aqueous phase.40 This leads to the adsorption of ill-defined multilayers of variable thickness, rather than well-defined monolayers. In contrast, the two sterically stabilized polystyrene latexes used in the present work have essentially no hydrophobic character. Instead, their cationic surface charge promotes adsorption at the anionic air-water interface. It is also likely that the nonionic PEGMA steric stabilizer chains also aid adsorption, since it is known that this water-soluble polymer is surface-active.46 (45) Van-de-Hulst, H. C. Light Scattering by Small Particles; Dover, Inc.: New York, 1957. (46) Henderson, J. A.; Richards, R. W.; Penfold, J.; Thomas, R. K.; Lu, J. R. Macromolecules 1993, 26, 4591–600.

Particle Characterization. Two sterically stabilized ‘PEGMAPS’ latexes were prepared via emulsion polymerization of styrene using a cationic azo initiator in the presence of a PEGMA macromonomer, as shown in Figure 1. The synthesis details are summarized in Table 1. Judicious selection of the styrene concentration allowed the mean latex diameter to be adjusted while retaining good control over the size distribution. This is confirmed by inspecting the TEM and SEM images of the two dried latexes in Figure 2. Also shown are the corresponding weight-average particle size distribution curves obtained using DCP. For both latexes, the mean particle diameters obtained using SEM, DCP, and DLS (shown in Table 1) lie in the expected rank order (i.e., SEM < DCP < DLS) given the individual biases of each technique. The weight-average diameters obtained by DCP are 130 ( 14 and 310 ( 15 nm for the small and large latex, respectively. DCP is a reliable sizing technique for hard spheres, but significant errors can occur for small sterically stabilized latexes if the stabilizer layer thickness is sufficiently large compared to the mean latex core diameter to reduce the effective

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latex density in aqueous solution. On the basis of the prior work by Cairns et al.47 with a ∼130 nm PEGMA-PS latex, the sizing error expected for our smaller latex is likely to be significant, whereas the error incurred for our larger latex should be negligible. In view of this problem, we also sized the smaller latex using BET surface area measurements. This technique provides a statistically reliable single diameter for the polystyrene latex core averaged over millions of particles. The surface area-average calculated from the BET data is 115 ( 6 nm. This is in good agreement with the number-average TEM diameter estimated from much fewer latex particles (∼100). According to Cosgrove and co-workers, the root-mean-square stabilizer layer thickness of chemically grafted PEGMA chains of Mn ) 2000 is ∼3.6 nm.48 Thus the solVated latex diameter of the smaller PEGMAPS latex is estimated to be 122 nm. This value is used to describe this smaller latex throughout the rest of this paper, while the larger latex is denoted by its DCP diameter of 310 nm. The PEGMA stabilizer contents were estimated to be around 2.2 and 2.6 wt % for the 122 and 310 nm latex, respectively, as judged by 1H NMR studies in CD2Cl2, which is a good solvent for both PS and PEGMA.49 Assuming that all of the grafted stabilizer chains are located at the latex surface, these PEGMA contents correspond to adsorbed amounts of 0.44 and 1.50 mg m-2, respectively. X-ray photoelectron survey spectra recorded for both PEGMAPS latexes, a charge-stabilized polystyrene latex and the PEGMA macromonomer are shown in Figure S1 (see Supporting Information). The additional signal at 530 eV observed in the two former latex spectra, which is not present in the spectrum obtained for the charge-stabilized latex, is assigned to the two oxygen atoms (CdO and C-O) of the grafted PEGMA stabilizer. This is consistent with the observation of a weak carbonyl feature at 289 eV in the C 1s core-line spectrum obtained for both PEGMA-PS latexes (data not shown). Curve-fitting analysis of the O 1s core-line spectrum also supports this assignment, since the two subpeaks due to the CdO oxygen (at 532 eV) and the C-O oxygen (at 534 eV) are of roughly equal intensity (data not shown). Given that the XPS sampling depth is typically only 2-5 nm, these observations provide good evidence that the grafted PEGMA stabilizer is present at the surface of both PEGMA-PS latexes, as expected.49,50 Aqueous electrophoresis studies were carried out as a function of pH (see Figure 3). The latexes exhibited isoelectric points (IEP) at around pH 6.5 and 5.8 for the 122 and 310 nm PEGMAPS latexes, respectively. As expected, positive zeta potentials of around +12 mV are observed at low pH for both latexes due to the cationic azo initiator used in these latex syntheses, see Figure 3. The magnitude of the measured zeta potential may be suppressed in comparison to the true surface potential, due to the PEGMA chains partially shielding the underlying cationic surface charge of the latex. It is also noteworthy that neither latex precipitated at its IEP; this is due to the steric stabilization conferred by the outer layer of nonionic chemically grafted PEGMA chains. Foam Formation Using PEGMA-PS Latexes. Simple handshaking of these aqueous latexes (5.0 wt % solids) at pH 2 leads to stable foams (see Figure S2 in Supporting Information). However, the long-term stabilities of these latex-stabilized foams were inferior to previously reported foams generated using (47) Cairns, D. B.; Armes, S. P.; Bremer, L. G. B. Langmuir 1999, 15, 8052– 8058. (48) Cosgrove, T.; Ryan, K. Langmuir 1990, 6, 136–142. (49) Dupin, D.; Fujii, S.; Armes, S. P.; Reeve, P.; Baxter, S. M. Langmuir 2006, 22, 3381–3387. (50) Alargova, J. C.; Warhadpande, D. S.; Paulnov, V. N.; Velev, O. D. Langmuir 2004, 20, 10371–10374.

Hunter et al.

Figure 3. Zeta potential vs pH curves obtained for 0.01 wt % aqueous solutions of PEGMA-PS latex in the presence of 0.01 M NaCl: 122 nm diameter (9) and 310 nm diameter ([).

somewhat larger sterically stabilized poly(2-vinylpyridine) latexes: Foam collapse was generally observed after drying for a couple of days. It is emphasized that essentially no foams were produced by hand-shaking at pH 6, indicating the importance of having sufficient cationic surface charge to ensure foam stabilization. In view of this observation, all foam column experiments were conducted at pH 2. In particular, the effect of varying the PEGMA-PS latex diameter and the total latex surface area on the resulting foam structure and stability was examined. Table 2 summarizes the observations made in these foam column experiments. In addition to the final foam height, the approximate range of bubble diameters observed for the upper, middle and lower regions of the foam column are also reported, with the bubble diameters within the middle of the foam being considered the most reproducible. A maximum foam column height of 650 mm was attained within 5 min with both PEGMA-PS latexes (see Figure S3 in Supporting Information). Relatively uniform bubbles were observed in the entire foam column.18,20 After each foam column experiment, the remaining latex dispersion was analyzed gravimetrically to assess its reduced solids concentration. On this basis it was estimated that ∼40% and 70% of the initial particles were incorporated into the foam column using the 122 and 310 nm PEGMA-PS latexes, respectively. Hence, the volume of nitrogen gas within the foam is calculated to be 99.9% and 99.8% for the 122 and 310 nm PEGMA-PS latex, respectively. Such foam stabilization efficiencies are relatively high compared to those previously reported by our group.18,20,41 Latex Foam Microstructure. The foam microstructure was examined using SEM. Figure 4 depicts representative SEM images obtained from dried foam fragments. Polydisperse spherical bubbles are observed at low magnification (images not shown), although drying causes some buckling and wrinkling. Some of the bubbles within the dried foam were carefully cut using a scalpel as described by Fujii et al.18 The edges of these ruptured foams shown in Figure 4a-d were examined at high magnification using SEM. Figure 4a and b show the typical particle multilayers that are observed at the top of the dried foam column. The first few bubbles generated during the foam column experiments collapsed as the foam front advanced up the column. Thus, the liberated latex particles lead to multilayer formation in the upper 5% of the foam. Such bubble collapse was not observed by Fujii and co-workers18,20 for micrometer-sized latexes, and it is noteworthy that only bilayers were formed for these larger particles. Thus, the present foams are not quite as robust as those previously reported. This suggests that the submicrometer-sized latexes are more weakly adsorbed at the air-water interface;29 this aspect is more thoroughly examined in our LB trough and ellipsometry studies (see later). In contrast, fragments from both 18,20,41

Adsorption of Submicrometer-Sized Latex

Langmuir, Vol. 25, No. 6, 2009 3445

Table 2. Effect of Varying the Latex Particle Diameter on Foam Formation Using the Two PEGMA-PS Latexes Listed in Table 1a initial approximate bubble diameter range (mm)

entry no.

particle diameter (nm)

total latex surface area, ALb (m2)

top

middle

bottom

1

122

12.0

650

644

99.9

1-5

1-5

1-5

2

310

4.8

650

645

99.8