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Surfactant Microstructure and Particle Aggregation Control Using Amphiphile Adsorption on Surface-Functionalized Polystyrene Spheres Ashish K. Jha and Arijit Bose* Department of Chemical Engineering; UniVersity of Rhode Island, Kingston, Rhode Island 02881 ReceiVed October 5, 2008. ReVised Manuscript ReceiVed NoVember 2, 2008 Surfactant microstructure and particle aggregation control upon adding surface-functionalized 100 nm diameter polystyrene (PS) spheres to a cetyltrimethylammonium bromide (CTAB)/dodecylbenzenesulfonic acid (HDBS) mixed surfactant system has been studied using cryogenic transmission electron microscopy (cryo-TEM). The key premise is that selective adsorption of surfactant onto the spheres, driven primarily by charge interactions, impacts surfactant concentrations in the solutions, thus driving structures to different, concentration-dependent states. A concurrent effect is the role played by adsorption on the clustering of the PS spheres. The effects of adsorption are quite pronounced when aldehyde-functionalized PS spheres are added to a small cationic vesicle/micelle suspension: only large vesicles remain. On the other hand, an anionic vesicle suspension remains unperturbed by the addition of these PS spheres. The addition of PS spheres to a 1:1 CTAB/HDBS mass ratio solution results in huge PS clusters that precipitate from the suspension. These large clusters are networks of PS spheres connected by surfactant bilayers arising from hydrophobic interactions with neutrally charged vesicles or vesicle fragments. These results indicate that solid surface adsorption provides a viable way to modify microstructures in a mixed surfactant system, with additional effects resulting from the aggregation of the PS particles. These effects can potentially be useful when surfactant composition must be changed without additional surfactant consumption, for rheology modification, in templated material synthesis, as well as in understanding situations where surfactant could potentially be adsorbed by neighboring solid boundaries, such as surfactant-mediated oil recovery from porous rocks and detergency.
1. Introduction A wide array of supramolecular structures can be created by the self-assembly of surfactants in a solution, including micelles, vesicles, and liquid crystals.1 The effects of surfactant tail length, type of solvent, surfactant concentration, temperature, salt concentration, and the presence of one or more cosurfactants2-4 on supramolecular morphologies have been well characterized. Newer ways to modify surfactant self-assembled structures are constantly being sought to diversify microstructural tuning for material synthesis and other applications. Surfactant molecules with hydrophobic tails and polar head groups tend to interact strongly with accessible solid surfaces. Adsorption of surfactants on solid surfaces such as polystyrene (PS) and silica has been previously reported.5,6 The selective adsorption of phospholipid and synthetic surfactant vesicles on PS spheres has also been studied.7-12 The ability of solid surfaces to adsorb surfactant molecules can be exploited to tailor microstructures in surfactant systems by varying the composition of the solution without * Corresponding author. Tel: 401-874-2804; e-mail:
[email protected]. (1) Imae, T. Colloids Surf., A: Physicochem. Eng. Aspects 1996, 109, 291– 304. (2) Koehler, R. D.; Raghavan, S.; Kaler, E. W. J. Phys. Chem. B 2000, 104, 11035–11044. (3) Kumar, S.; Aswal, V. K.; Goyal, P. S.; Din, K. J. Chem. Soc., Faraday Trans. 1998, 94, 761–764. (4) Freeman, K. S.; Tan, N. C.; Trevino, S. F.; Kline, S.; McGown, L. B.; Kiserow, D. J. Langmuir 2001, 17, 3912–3916. (5) Dixit, S. G.; Vanjara, A. K.; Nagarkar, J.; Nikoorazm, M.; Desai, T. Colloids Surf., A: Physicochem. Engi. Aspects 2002, 205, 39–46. (6) Rapuano, R.; Carmona-Ribeiro, A. M. J. Colloid Interface Sci. 1997, 193, 104–111. (7) Adams, D. R.; Toner, M.; Langer, R. Langmuir 2007, 23, 13013–13023. (8) Carmona-Ribeiro, A. M.; Herrington, T. M. J. Colloid Interface Sci. 1993, 156, 19–23. (9) Carmona-Ribeiro, A. M.; Lessa, M. D. M. Colloids Surf., A: Physicochem. Eng. Aspects 1999, 153, 355–361. (10) Carmona-Ribeiro, A. M.; Midmore, B. R. Langmuir 1992, 8, 801–6. (11) Zuzzi, S.; Cametti, C.; Onori, G. Langmuir 2008, 24, 6044–6049. (12) Pereira, E. M. A.; Petri, D. F. S.; Carmona-Ribeiro, A. M. J. Phys. Chem. B 2002, 106, 8762–8767.
addition of surfactant. An advantage of this technique is the easy modification of the nature and extent of adsorption by functionalizing the solid surface with an appropriate functional group. For example, carboxyl or aldehyde functinalized PS spheres preferentially adsorb cationic surfactant from a mixed surfactant system. Varying the particle number concentration and size and the nature of the surface functional groups provides a straightforward pathway for varying microstructures by controlled adsorption on to the solids. The particle surface charge density and polarity can be modified by changing the solution pH, leading to additional possibilities for microstructure control by tuned adsorption. The addition of particles can modify the suspension rheology, and that effect can also be exploited advantageously. In this paper, we provide new results for adsorption-induced microstructure changes in surfactant solutions. The ability to manipulate microstructures under isothermal conditions without surfactant addition can have interesting applications in templated nanomaterial synthesis,13 and provide a better understanding of self-assembly in solutions where surfactant can be adsorbed on confining surfaces.14 We also report on how amphiphile adsorption can dramatically modify the aggregation state of the particles in the suspension. More specifically, we examine changes in microstructures in mixed cetyltrimethylammonium bromide (CTAB)/dodecylbenzenesulfonic acid (HDBS) vesicles induced by their interaction with aldehyde-functionalized PS spheres. We use cryogenic transmission electron microscopy (cryo-TEM) for direct, essentially artifact-free visualization of these colloidal structures. In order to home in on the role of adsorption, we have varied the sequence of solution and particle addition, and shown that the sequence has a strong impact on surfactant morphologies as (13) Hentze, H.-P.; Kaler, E. W. Chem. Mater. 2003, 15, 708–713. (14) Ayirala, S. C.; Rao, D. N. Colloids Surf., A: Physicochem. Eng. Aspects 2004, 241, 313–322.
10.1021/la803267y CCC: $40.75 2009 American Chemical Society Published on Web 12/02/2008
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Figure 1. (a) Cryo-TEM micrograph of aldehyde modified PS spheres, 0.44 wt %. (b) Cryo-TEM micrograph of a 0.4 wt % 4:1 CTAB/HDBS solution (small unilamellar vesicles of size ∼15-30 nm are shown by squares; micelles are shown by arrows). (c) Cryo-TEM micrograph of a 0.4 wt % 1:4 CTAB/HDBS solution, showing vesicles. (d) Cryo-TEM micrograph of a 0.4 wt % 1:1 CTAB:/HDBS solution, showing large vesicles.
well as particle aggregation. Section 2 provides experimental details, and section 3 describes the key results of this investigation.
2. Experiments 2.1. Materials. The cationic surfactant CTAB (99% pure) was obtained from Sigma Aldrich, and the anionic surfactant, HDBS, was obtained from the Stepan Company. Aldehyde-modified 100 nm PS beads were purchased from Duke Scientific Corporation as 4 wt % solids in deionized water. The holey carbon grids for cryoTEM were bought from Ted Pella, Inc. 2.2. Sample Preparation. Stock micellar solutions of 0.4 wt % CTAB and 0.4wt % HDBS were prepared in deionized water. Three different compositions of vesicles were prepared by mixing the CTAB and HDBS stock solutions in 4:1, 1:4, and 1:1 volume ratios, respectively. 250 µL of the suspension of PS beads were added to 2 mL of each of these vesicle solutions and mixed for 12 h by gentle shaking. To deliberately amplify the role of adsorption, 1 mL of the PS sphere suspension was added to 1 mL of 4:1 CTAB/HDBS solution, in an additional experiment. In another set of experiments, the same net concentrations of PS spheres, CTAB, and HDBS were reached via different paths. A 1 mL portion of CTAB micellar solution was first mixed with 625 µL of the suspension of PS spheres and allowed to equilibrate for 12 h. Subsequently, 4 mL of HDBS micellar solution was added, bringing the net concentration back to 1:4 CTAB/ HDBS. In a third set of experiments, 4 mL of HDBS micellar solution was first added to 625 µL of the PS suspension and equilibrated for 12 h, followed by 1 mL of the CTAB micellar solution. All final samples were equilibrated at 25 °C for 24 h prior to being vitrified for cryo-TEM. 2.3. Cryo-TEM. Samples were vitrified in a FEI Vitrobot system.15 The vitrification chamber was maintained at 25 °C and 100% humidity to avoid sample evaporation artifacts. The holey carbon grid was automatically dipped into a vial containing ∼200 µL of sample placed inside the vitrification chamber. The sample was mechanically blotted to form thin films of liquid spanning the
grid holes. The sample-bearing grid was then plunged into a liquid ethane reservoir, close to its freezing point. Contact with the cryogen vitrifies the sample and preserves all of the microstructures in their native hydrated states. The grid was then transferred to a cold stage (Gatan 626 DH), and maintained at -170 °C during phase contrast imaging in the transmission electron microscope (JEOL JEM 2100).
3. Results and Discussion The equilibrium structures formed in the CTAB/HDBS system are known to be micelles and unilamellar vesicles depending on the relative composition of the individual surfactants. The critical micelle concentrations for CTAB and HDBS are 0.03 wt %16 and 0.08 wt %,17 respectively. In a mixture, CTAB-rich solutions form cationic vesicles up to a composition of ∼90 wt % CTAB, and HDBS-rich solutions form anionic vesicles up to ∼90 wt % HDBS. We first report images of the base suspensions in Figure 1. Figure 1a shows a cryo-TEM micrograph of a 0.44 wt % (this corresponds to the final concentration of PS spheres in all of our samples) aqueous suspension of the aldehyde-modified PS spheres of ∼100 nm diameter. Their surface charge keeps them stable and separated. Figure 1b,c,d shows images from CTAB/HDBS compositions of 4:1, 1:4, and 1:1, respectively. Figure 1b shows unilamellar CTAB-rich cationic vesicles of size ∼15-30 nm along with micelles. Figure 1c shows ∼75-100 nm diameter HDBS-rich anionic vesicles. Figure 1d shows large vesicles of size ∼100-200 nm. The vesicle size variation over this range of compositions has been observed previously, both by dynamic (15) Bellare, J. R.; Davis, H. T.; Scriven, L. E.; Talmon, Y. J. Electron Microsc. Tech. 1988, 10, 87–111. (16) Furst, E. M.; Pagac, E. S.; Tilton, R. D. Ind. Eng. Chem. Res. 1996, 35, 1566–1574. (17) Walker, S. A.; Zasadzinski, J. A. Langmuir 1997, 13, 5076–5081.
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Figure 2. (a) Cryo-TEM micrograph of solution when 250 µL of the PS sphere suspension is added to 2 mL of 0.4 wt % 4:1 CTAB/HDBS solution. White squares and arrows show small vesicles and micelles, respectively; black squares show bigger vesicles. The PS spheres are clustered. (b) Cryo-TEM micrograph of solution when 1 mL of the PS sphere suspension is added to 1 mL of 0.4 wt % 4:1 CTAB/HDBS solution. There are no micelles present in this case, and vesicles are of size ∼100-200 nm. (c) Cryo-TEM micrograph of sample when 250 µL of the PS sphere suspension is added to 2 mL of the 0.4 wt % 1:4 CTAB/HDBS solution. There is no clustering of spheres, or changes to the surfactant structures. (d) Cryo-TEM micrograph of sample with 4 mL of 0.4 wt % HDBS solution added to a mixture of 1 mL of 0.4 wt % CTAB and 625 µL of the PS sphere suspension. The spheres are clustered. (e) Cryo-TEM micrograph of sample with 1 mL of 0.4 wt % CTAB solution added to a mixture of 4 mL of 0.4 wt % HDBS and 625 µL of the PS sphere suspension. (f) Cryo-TEM micrograph of sample when 250 µL of PS suspension is added to 2 mL of the 0.4 wt % 1:1 CTAB/HDBS solution. Arrows show bilayer fragments connecting adjacent PS spheres, inset shows macroscopic cluster of PS spheres upon addition to vesicle solution. Adsorption of surfactant results in roughening of the PS sphere surfaces.
light scattering and cryo-TEM,18 and is a consequence of a combination of charge screening effects among surfactant head groups and changes to the effective volume of the hydrocarbon tail region because of the presence of the aromatic ring on the HDBS tail region. Figure 2a shows the result of the addition of 250 µL of the suspension of PS spheres to 2 mL of the 4:1 0.4 wt % CTAB/ HDBS solution. The PS spheres form clusters because of screening of surface charges by adsorption of CTAB-rich micelles and cationic vesicles. The interaction of charged particles with oppositely charged objects has been reported previously.11 In addition, very small vesicles and micelles can be seen in the proximity of the PS clusters. Some bigger vesicles (∼100 nm) (18) Lee, J.; Bose, A.; Tripathi, A. Langmuir 2006, 22, 11412–11419.
are also observed. The formation of bigger vesicles is attributed to the preferential adsorption of CTAB from the solution, causing the resulting composition to move closer toward a more equimolar ratio. Given the supplier specified charge equivalents/area on the beads, we estimate that unimolecular binding between one aldehyde group and one CTAB molecule would move this ratio from 4:1 to 3.8:1. The predicted overall concentration change is small, and the concomitant effect is also small, but repeatable and always observable. Figure 2b shows an image from a sample where 1 mL of the PS sphere suspension is added to 1 mL of the 4:1 0.4 wt % CTAB/HDBS solution. There is a complete absence of micelles, and an abundance of large vesicles, with diameters from 100 to 200 nm. Our adsorption calculation reveals that the expected composition changes from 4:1 CTAB/HDBS
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to 2.4:1 CTAB/HDBS for this case. As expected, a more dramatic change in the microstructures is observed. Figure 2c shows a cryo-TEM image of a sample when 250 µL of PS spheres are added to 2 mL of 1:4 volume ratio CTAB/ HDBS solution. No clustering of spheres is observed. The negative charge on the vesicles does not screen the charge on PS spheres, leaving them unagglomerated. Experiments with different compositions of HDBS-rich vesicles showed that even a composition slightly rich in HDBS in the mixture, for example, CTAB/HDBS ) 48:52, does not cause any clustering of the PS spheres, and the suspension remains visually clear. The vesicles sizes are similar to those in the base case without PS, suggesting negligible interaction and exchange of surfactant between vesicles and PS spheres. Figure 2d shows an image when 4 mL of the HDBS micellar solution is added to a mixture that has 1 mL of the CTAB micellar solution and 625 µL of the PS sphere suspension. Clustering seen in this case is a result of the addition of PS spheres to CTAB micelles, which causes adsorption of CTAB. The charges on the spheres get screened, and the suspension destabilizes. This depletion of CTAB results in a more HDBS-rich suspension (our calculations suggest that 1:1 binding of CTAB to an aldehyde group would result in a final CTAB/HDBS ratio of 1:5 in this case), which shrinks the vesicle size. Adding the CTAB micellar solution to a mixture of HDBS micelles and PS spheres (see Figure 2e) does not result in the clustering of spheres or a change in the vesicle size (∼100 nm). Given the availability of adsorption sites for CTAB on the PS spheres, these results suggest that the characteristic adsorption time of CTAB on PS is larger than the characteristic time for the formation of vesicles. Next, we probe the interaction of PS spheres with a 1:1 CTAB/ HDBS mixture. Vesicles here are net neutral in charge. Macroscopic clusters of PS beads become visible immediately (see insert in Figure 2f), a phenomenon that is qualitatively different from that at other compositions of CTAB/HDBS that we have probed. Figure 2f shows a cryo-TEM image of the resulting vesicle PS suspension. The PS spheres in the clusters are connected by bilayer fragments shown by arrows. The hydrophobic interiors of broken bilayers get adsorbed on to the
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PS surface (by hydrophobic interaction) and connect several spheres to form macroscopic clusters. Also, note the roughness of the surface of PS spheres as they are coated by bilayer fragments. Given the lack of charge on the vesicles, there is no preferential adsorption of CTAB onto the PS spheres. Thus, the size of the vesicles is similar to the case of the 1:1 CTAB/HDBS vesicles without any PS added. The results of this study indicate that functionalized PS spheres provide a viable way to modify microstructures in a mixed surfactant system without the addition of surfactant. These results could be important in understanding situations where surfactants are adsorbed by neighboring solid boundaries, such as surfactantmediated oil recovery from porous rocks. In the cases reported here, the final volume fraction of particles in each suspension is 0.004, leading to a negligible effect on the viscosity of the suspension. However, the addition of more particles will eventually affect the rheology of these suspensions, along with additional adsorption that can change compositions and microstructures more significantly. Both effects can be exploited to one’s advantage.
4. Conclusions Microstructure tuning upon addition of surface-functionalized negatively charged PS spheres to a CTAB/HDBS mixed surfactant vesicle has been demonstrated. Preferential adsorption of cationic surfactant by aldehyde-functionalized PS spheres modifies the vesicle sizes. PS spheres have been found to cluster due to charge screening in the presence of cationic surfactants, but the solution remains unperturbed when the PS spheres are added to HDBSrich vesicles. The PS spheres show macroscopic clusters when added to neutral vesicles made by mixing CTAB and HDBS micellar solutions in a 1:1 volume ratio, although the vesicle sizes are not changed. These variations in vesicle sizes as well as PS sphere clustering can be exploited for applications where concentrations must be changed without addition of surfactant. Acknowledgment. This work is supported by NSF grants CBET 0619440 and 0730392 to A.B., and by a URI Graduate Fellowship to A.J. We thank G. Bothun for discussions. LA803267Y
