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Langmuir 2007, 23, 8094-8102
Polymerized Rodlike Micelle Adsorption at the Solid-Liquid Interface Simon Biggs,*,† Marie Labarre,† Chris Hodges,† Lynn M. Walker,‡ and Grant B. Webber† Institute of Particle Science and Engineering, The School of Process, EnVironmental and Materials Engineering, The UniVersity of Leeds, Leeds LS2 9JT, United Kingdom, and Department of Chemical Engineering, Center for Complex Fluids Engineering, Carnegie Mellon UniVersity, Pittsburgh, PennsylVania 15213 ReceiVed March 11, 2007. In Final Form: May 8, 2007 The adsorption of rodlike polymer-micelle aggregates of cetyltrimethylammonium 4-vinylbenzoate (p-C16TVB) at the silica-water interface has been characterized using a combination of quartz crystal microbalance with dissipation monitoring (QCM-D) and atomic force microscopy (AFM) studies. Adsorption isotherm data, recorded by QCM-D, indicate a two-stage mechanism: an adsorbed film of free CTA+ ions is initially produced at low concentrations until the surface is charge reversed, whereupon the weakly anionic aggregates can adsorb and the adsorbed mass is seen to increase dramatically. The adsorbed rodlike micelle aggregates are seen to form a close-packed monolayer from AFM images with a high degree of order over micrometer length scales. AFM force-distance data indicate that the adsorbed aggregates retain their cylindrical structure and little or no flattening is seen. Rinsing of the film did not result in removal of the adsorbed layer, and the persistence of these nanoscale ordered films at the solid-liquid interface suggests many possible applications.
Introduction Complex interfacial surface coatings prepared from polymers and/or surfactants have been proposed as possible precursors in the design of a wide range of novel materials.1-4 The essential features of such coatings include the ability to self-assemble with nanoscale features that can be replicated over macroscopic surface areas.5 Typically, the nanoscale structure arises from an inherent feature of the constituent materials such as a micellar cross-sectional dimension or the block lengths in a copolymer. A wide range of surface coatings with a corresponding diversity of surface morphologies have now been reported, and basic rules for the preparation of these coatings have been elucidated.6,7 In the case of block copolymer films, it has been shown that fine control over the surface structures can be obtained through control over the copolymer chemistry, the solvent/nonsolvent effects, and the relative block sizes.8-10 There is now an obvious interest in these films as a result of the high degree of control over the resultant surface architectures that can be obtained. However, there remain a significant number of drawbacks to the use of these coatings in high-volume commercial applications. * To whom correspondence should be addressed. † The University of Leeds. ‡ Carnegie Mellon University. (1) Xu, T.; Misner, M. J.; Kim, S.; Sievert, J. D.; Gang, O.; Ocko, B.; Russell, T. P. New Polymeric Materials; ACS Symposium Series 916; American Chemical Society: Washington, DC, 2005; pp 158-170. (2) Cox, J. K.; Eisenberg, A.; Lennox, R. B. Curr. Opin. Colloid Interface Sci. 1999, 4 (1), 52-59. (3) Carswell, A. D. W.; O’Rear, E. A.; Grady, B. P. J. Am. Chem. Soc. 2003, 125 (48), 14793-14800. (4) Aizawa, M.; Buriak, J. M. J. Am. Chem. Soc. 2006, 128 (17), 5877-5886. (5) Glass, R.; Moller, M.; Spatz, J. P. Nanotechnology 2003, 14 (10), 11531160. (6) Chen, J. X.; Zhuang, H. Z.; Zhao, J.; Gardella, J. A. Surf. Interface Anal. 2001, 31 (8), 713-720. (7) Atkin, R.; Craig, V. S. J.; Wanless, E. J.; Biggs, S. AdV. Colloid Interface Sci. 2003, 103 (3), 219-304. (8) Spatz, J. P.; Eibeck, P.; Mossmer, S.; Moller, M.; Kramarenko, E. Y.; Khalatur, P. G.; Potemkin, II; Khokhlov, A. R.; Winkler, R. G.; Reineker, P. Macromolecules 2000, 33 (1), 150-157. (9) Potemkin, I. I.; Kramarenko, E. Y.; Khokhlov, A. R.; Winkler, R. G.; Reineker, P.; Eibeck, P.; Spatz, J. P.; Moller, M. Langmuir 1999, 15 (21), 72907298. (10) Kramarenko, E. Y.; Potemkin, I. I.; Khokhlov, A. R.; Winkler, R. G.; Reineker, P. Macromolecules 1999, 32 (10), 3495-3501.
For example, they are usually applied onto ultrasmooth surfaces using spin-coating technology from selective organic solvents. Environmental pressures suggest that finding aqueous-based alternatives may be highly beneficial while the ability to coat particulates will also improve the general usefulness of this technology. These are significant challenges that will need to be overcome if the use of block copolymers to form nanoscale ordered coatings is to achieve widespread acceptance. From aqueous media, the most commonly reported method for producing complex surface coatings involves the use of simple surfactants, whether ionic or nonionic.7,11-16 It has been unequivocally demonstrated over the past decade that such surfactants are capable of producing a wide range of aggregate morphologies at the solid-liquid interface; included here are spheres, rods of varying lengths, tubules, half-tubules, and bilayers. Indeed, the variety and complexity of the structures observed rival those seen in the bulk. As yet, a complete understanding of the relationships between surface and bulk structures as a function of surfactant concentration are not fully developed, but it seems clear that the two are strongly related. One complication when considering surface structures is the influence of the surface characteristics on the observed aggregates; for example, the role of surface charge when considering ionic surfactants has been shown to be very important.17 The specific details of the influence of different surface types on the structures formed from a wide range of surfactants are too broad to be presented here; interested readers are directed to a number of excellent reviews.7,15 Such surface coatings have wide appeal since they form spontaneously, they produce well-ordered layers over large areas, and they can be easily applied to particles as well as flat surfaces. It has also been demonstrated that they can be used as template layers for the synthesis of secondary structures (11) Manne, S.; Cleveland, J. P.; Gaub, H. E.; Stucky, G. D.; Hansma, P. K. Langmuir 1994, 10 (12), 4409-4413. (12) Manne, S.; Gaub, H. E. Science 1995, 270 (5241), 1480-1482. (13) Wanless, E. J.; Ducker, W. A. Langmuir 1997, 13 (6), 1463-1474. (14) Ducker, W. A.; Wanless, E. J. Langmuir 1999, 15 (1), 160-168. (15) Warr, G. G. Curr. Opin. Colloid Interface Sci. 2000, 5 (1-2), 88-94. (16) Patrick, H. N.; Warr, G. G. Colloids Surf., A: Physicochem. Eng. Aspects 2000, 162 (1-3), 149-157. (17) Liu, J. F.; Ducker, W. A. J. Phys. Chem. B 1999, 103 (40), 8558-8567.
10.1021/la700708g CCC: $37.00 © 2007 American Chemical Society Published on Web 06/20/2007
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following subsequent chemical modification.3,18,19 However, the use of such surfactant layers in complex synthetic processes or multistage manufacturing is complicated since a high bulk concentration of the surfactant is needed to maintain the integrity of the surface film. Furthermore, these films are strongly affected by changes in the environmental conditions such as pH, electrolyte concentration, use of cosolvents, or dehydration. As a result, their widespread use in the manufacture of new nanomaterials remains unlikely. Consequently, we have been investigating novel polymerized analogues of these small-molecule surfactant systems as a possible route to produce well-ordered nanoscale surface coatings.20,21 We have previously reported a method for the preparation of polymerized rodlike polymer-micelle aggregates in aqueous solution using ionic surfactants in which the counterion is a polymerizable compound.22,23 Detailed analyses of the surfactants in solution both before and after polymerization indicate that they form rodlike micelles prior to polymerization and the diameter and semiflexible nature of the micelles are maintained in the polymer-micelle aggregates formed after polymerization.23-25 The interest in these materials is that several features of the micelle template are “locked-in”, and these polymeric aggregates maintain their structure at concentrations below the cmc of the original micelles.24,25 As a result, they may provide a useful alternative for the production of complex surface coatings that are much more insensitive to changes in the environment and provide opportunities for more complex processing and manufacturing techniques. Through our earlier work, we know that in bulk solution the polymerized aggregate structure is insensitive to changes in the salt concentration, temperature, and pH and that it can be freeze-dried and resuspended without loss of the structure.24 We also know that surfactant can be utilized to tune the length of these aggregates in solution and that they can be swollen with organic species,25 both properties of use in controlling the bulk structure and, hence, the film properties. We have also reported results from initial investigations of the adsorption of these aggregates at the oxide-water interface using in situ soft-contact atomic force microscopy (AFM).20,21 This work showed that these polymer-micelle aggregates did adsorb at the interface and they produced self-organized 2-dimensional films of close-packed aggregates with a high degree of order across micrometer-sized surface areas. Adsorption at both mica and silica substrates showed that the degree of surface charge and the underlying roughness also had an effect on the quality of the ordering, with the layers on the mica appearing to show higher order. In addition, the layers on the mica were apparently more robust to washing, an effect that was attributed to the higher charge on that surface and therefore a higher adhesion of the weakly charged aggregates. These layers are appealing since they can be produced from an aqueous medium, they self-assemble, and since they are polymeric, they are kinetically trapped and will not desorb rapidly if the background solution is exchanged or the system is washed. (18) Ding, H.; Zhu, C.; Zhou, Z.; Wan, M.; Wei, Y. Macromol. Chem. Phys. 2006, 207 (13), 1159-1165. (19) Dong, J. P.; Mao, G. Z. Colloid Polym. Sci. 2005, 284 (3), 340-345. (20) Biggs, S.; Kline, S. R.; Walker, L. M. Langmuir 2004, 20 (4), 10851094. (21) Biggs, S.; Walker, L. M.; Kline, S. R. Nano Lett. 2002, 2 (12), 14091412. (22) Kline, S. R. Langmuir 1999, 15 (8), 2726-2732. (23) Gerber, M. J.; Walker, L. M. Langmuir 2006, 22 (3), 941-948. (24) Gerber, M. J.; Kline, S. R.; Walker, L. M. Langmuir 2004, 20 (20), 8510-8516. (25) Gerber, M. J. The Characterization of Polymerized Worm-like Surfactant Micelles. Ph.D. Thesis, Carnegie Mellon University, Pittsburgh, PA, 2006.
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Our initial work did not, however, include any quantitative analysis of the adsorption. In the current paper, we report a detailed study of the adsorption of rodlike polymer-micelle aggregates at the silica-water interface using a combination of AFM imaging and quartz crystal microbalance with dissipation monitoring (QCM-D) studies. The main aims of this work were to quantify the adsorption isotherm for these polymer materials and to probe the effects of rinsing the surfaces with pure water. Using AFM and QCM-D measurements, we hoped to investigate whether adsorption at an interface has any impact on this apparent stability. Experimental Section Samples and Substrates. All water used here was purified using a Millipore Milli-Q Elix-RiOs Synthesis A10 system (resistivity >18.2 MΩ cm, Millipore, U.K.). Substrates suitable for use in both the quartz crystal microbalance and atomic force microscope (see below for details) were cleaned according to a standard protocol whereby the surface is initially placed into a UV/ozone cleaner for 15 min (Scientific and Medical Products Ltd., Gatley, Cheshire, U.K.) and then ultrasonicated in a Decon 90 (2 wt %, Sigma-Aldrich, U.K.) solution for 10 min, then UV/ozone (15 min), and then Decon 90 (10 min), rinsing with copious amounts of water between each step. These surfaces were used in both AFM and QCM-D measurements and were always freshly cleaned before each experiment. The polymer-micelle aggregates were prepared according to the previously reported synthetic protocol.22 The samples were prepared and purified in-house following procedures published previously.22-24 Two samples were prepared for this study; both the samples were based on the polymerizable counterion 4-vinylbenzoate (VB-) and an associated cationic n-alkyltrimethylammonium surfactant where the alkyl chain lengths were C-16 and C-18. Free radical polymerization of the counterion was performed at 60 °C using 2.5 mol % initiator (VA-44) relative to the surfactant concentration. These conditions result in pVB- chains of molecular mass of approximately 210 000 g/mol. This chain is solubilized in a polymer-surfactant aggregate with an overall length of 140 nm, as measured by smallangle light scattering.24 The diameters of the aggregates depend on the surfactant and are measured by small-angle neutron scattering to be 4.0 nm for pC16TVB and 4.8 nm for pC18TVB.23 The aggregates are stable in solution, and the structure of each aggregate arises from a single pVB- chain and the associated surfactant molecules that act as the counterion to the anionic polyelectrolyte. Since the structure of the aggregates arises from both the polyelectrolyte (pVB-) and the associated surfactant, we refer to these structures as polymermicelle aggregates, or pCXTVB, where X ) 16 or 18 and denotes the tail length of the associated surfactant. Atomic Force Microscopy. All work reported here was undertaken on a Multimode AFM instrument with a Nanoscope IV controller (Veeco Ltd., Cambridge, U.K.) equipped with a Picoforce system. All data reported here were collected using a standard E-scanner (Veeco) which has a maximum scan size of 14 × 14 µm2. The aggregates were imaged in situ at the solid-liquid interface using the so-called “soft-contact” approach described previously.20 Imaging in this mode relies upon having a short-range repulsive force between the tip and the substrate against which the tip can be held at a finite force and distance; this repulsion can originate either from electrical double-layer forces or steric forces or a combination of the two. If the repulsive forces are weak, or absent, collection of high-resolution images may be difficult or impossible. The cantilever probes used here were NP-S series probes (Veeco Probes, U.K.). These probes are very sharp and have a nominal tip radius of 5 mg/m2. Thus, we conclude that the initial adsorption seen here results from the electrostatically and hydrophobically driven adsorption of CTA+ ions onto the silica substrate. However, when the surface charge is reversed, the adsorption of the polymer aggregates can occur and the adsorbed mass increases significantly. A closer inspection of the adsorption isotherm data for the pC16TVB sample given in Figure 2 reveals a number of interesting features. For example, the adsorbed mass appears to rise to a
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Figure 5. Soft-contact atomic AFM images (500 × 500 nm) of bare silica and pC16TVB adsorbed onto silica from solutions of 0.03, 0.1, and 0.5 mg/mL. Also shown are force-distance data recorded under equivalent conditions (concentration increases from top to bottom).
pseudoplateau in the region 0.2-0.4 mg/mL before then rising once again at higher concentrations. The value recorded at 0.1 mg/mL is clearly much greater than that seen at 0.075 mg/mL; it is not, however, quite at the level of the pseudoplateau. Further insight of these isotherm data can be obtained from a consideration of the kinetic data from the QCM-D in each region of the isotherm as well as from direct in situ images and force curve data collected using an AFM instrument. Data for the adsorbed mass (Sauerbrey) and dissipation as a function of time are shown in Figure 4 for concentrations of 0.03, 0.1, and 0.5 mg/mL of the aqueous pC16TVB sample. The corresponding in situ AFM images and associated force-distance
data are given in Figure 5, with an image and force curve taken on the clean bare silica. At 0.03 mg/mL, which is well below the apparent polymer adsorption concentration for this system, the data suggest a small initial adsorbed mass in the wash-on stage which is completely removed during the rinse off. This is consistent with wash-off data seen for CTAB on silica experiments recorded using the QCM-D and once again suggests that the initial region results from the adsorption of CTA+ ions only. The dissipation data are invariant throughout the experiment; this is also consistent with a submonolayer of adsorbed CTA+ ions. Comparison of the image recorded in pure water with that collected at 0.03 mg/mL shows little difference, highlighting the
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Figure 6. Comparison of the changes in the Sauerbrey mass as a function of time for the adsorption of pC18TVB onto silica from solutions of 0.03, 0.1, and 0.5 mg/mL. Also shown are the changes in dissipation as a function of time for the respective solutions (concentration increases from top to bottom).
absence of any significant adsorption and no visible surface aggregate morphologies. Interestingly, there is a difference in the force-distance data; at the low aggregate concentration the data show a long-range attraction and a large adhesion. These differences are consistent with a more “hydrophobic” surface which could result from the adsorption of CTA+ ions at the negative surface sites of the silica. Such an adsorption is consistent with the four-step adsorption isotherm proposed for CTAB or CTAC on silica.7 At 0.1 mg/mL imaging of the aggregate film was difficult as the surface proved to be somewhat unstable. The image obtained appears to show an amorphous thin overlayer on the silica substrate. These observations are consistent with this concentration being in the transition regime, somewhere between the CTA+ film and the fully developed adsorbed aggregate layer. At the higher concentration of 0.5 mg/mL the AFM image reveals clearly a stable adsorbed film of polymer-micelle aggregates. The corresponding force-distance data indicate the presence of a short-range steric barrier and a small push-through during approach. The retraction data show only a small adhesion between the tip and surface. Such force-distance data are consistent with the observed surface aggregates. Analyses of a number of forcedistance data sets reveal a mean value for the push-through of 4.1 ( 0.5 nm. This is consistent with the data reported in our earlier work.20 Interestingly, the expected value for a perfect cylinder of CTA+ ions is approximately 4.5 nm, i.e., twice the CTA+ chain length. This suggests that the adsorbed structures
seen here are essentially undeformed cylinders. It is worth noting of course that we do not know whether the adsorbed polymermicelle aggregates are sitting on the initially adsorbed CTA+ ion layer or the alkyl chains of these initially adsorbed CTA+ ions have penetrated into the aggregate structure. The latter scenario would appear likely, on the basis of these force-distance data. A number of images were collected at each concentration and analyzed using a fast Fourier transform (FFT) routine to reveal details of the periodic spacing seen in the images. Examples of such analyses are given in Figure 5 with each AFM image. For the 0.5 mg/mL case, a peak was found corresponding to a spacing of 8-11 nm. Analysis of a significant number of images gave a mean value of 8.40 ( 0.5 nm for the peak-to-peak spacing of the aggregates. Once again, this value is consistent with our earlier work.20 Although slightly larger than the dimensions of a single cylinder diameter, this value is consistent with a densely packed film of polymer-micelle aggregates. Some increase in the center-to-center distance may be expected as a result of local electrostatic repulsion between neighboring aggregates. Further analyses of the isotherm data for the pC18TVB sample were also undertaken using the QCM-D and AFM data, as for the pC16TVB sample above. The adsorbed mass (Sauerbrey) and dissipation data for the pC18TVB sample at 0.03, 0.05, and 0.5 mg/mL are shown in Figure 6, and an example of the corresponding AFM images and force data are shown in Figure 7. The main features of the QCM-D data are consistent with the
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Figure 7. Soft-contact atomic AFM images (500 × 500 nm) of bare silica and pC18TVB adsorbed onto silica from solutions of 0.03, 0.1, and 0.5 mg/mL. Also shown are force-distance data recorded under equivalent conditions (concentration increases from top to bottom).
results seen above for the pC16TVB sample, except that the dissipation data do not decrease quite so dramatically with rinsing. This is related to the fact that the absolute values of dissipation are much larger with the pC18TVB sample, probably indicating that a more extended layer is present on the silica surface, which is likely to contain a high percentage of water. Thus, more rinses will be required to reduce the dissipation with the pC18TVB sample than for the pC16TVB sample. Examination of the AFM images shows clear evidence of an ordered layer at the solid-liquid interface for each of the concentrations examined here. This is not surprising at the two
higher concentrations, both of which are on the plateau of the adsorption isotherm for the wash-on data. It is noteworthy, however, that at 0.03 mg/mL, which is inferred from the isotherm to be just above the transition concentration to polymer-micelle aggregate adsorption but well below the adsorption maximum, that we also see an adsorbed film with significant morphology. Analysis of the force-distance data and FFT analyses of the images suggest that on the adsorption plateau the characteristic push-through distance of these adsorbed micellar coatings is 4.0 ( 0.5 nm, which is again comparable to that from earlier work and the pC16TVB data (above). The typical peak-to-peak spacing
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is 7.5 ( 0.5 nm, which is slightly less than was reported above for the pC16TVB system but is still consistent with a closepacked layer of undeformed cylinders. Interestingly, at the lower concentration of 0.03 mg/mL where adsorption was unexpectedly seen for the pC18TVB system, we record a push-through of 3.0 ( 0.5 nm and a peak-to-peak spacing of 13 ( 1 nm. These data suggest a somewhat more flattened conformation for the adsorbed polymers. It is also worth noting that, at these lower concentrations, the Debye length is expected to be larger as a result of the reduced electrolyte concentrations, and this would contribute to a lower adsorption density. Comparison of the adsorbed mass recorded after rinsing of the surfaces with water indicates that the adsorbed polymer-micelle aggregate films produced are highly resilient. Similar experiments using CTAB as the adsorbate showed virtually complete removal of the adsorbed film after a similar rinse profile at all concentrations tested. Furthermore, rinsing below the critical adsorption concentration, where we assume aggregates first adsorb, also showed virtually complete removal of the adsorbate. This may indicate that the initially adsorbed CTA+ ions are indeed absorbed into the polymer aggregate structures after adsorption. If the polymer-micelle aggregates simply sat on the initially adsorbed CTA+ layer, then we might expect this to also be easily rinsed off. In contrast, it is well-known that high molecular weight polymers are not easily rinsed from a substrate and the desorption kinetics are sufficiently slow that adsorption is often considered irreversible. The limited desorption seen here supports the concept that the aggregates are firmly attached to the interface, and such an attachment seems most probable through
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incorporation of the initially adsorbed CTA+ ions into the polymer aggregate structure.
Conclusions The adsorption of polymerized rodlike polymer-micelle aggregates at the silica-water interface has been investigated using QCM-D and AFM measurements. The data indicate that adsorption proceeds via a two-stage mechanism. Initially, at low concentrations, free CTA+ ions adsorb as a result of electrostatic attraction to the anionic silica surface. Further CTA+ adsorption, driven by the hydrophobic attraction between the alkyl chains of the CTA+, results in charge reversal of the substrate. This cationic surface is then able to act as a substrate for adsorption of the anionic polymer-micelle aggregates, and a sharp rise in the adsorbed mass is seen. In situ AFM images of the adsorbed films show that they consist of a close-packed monolayer of the aggregates. These close-packed polymer aggregate films are seen to be highly resilient to rinsing with pure water. The closepacked nature of these polymer-micelle aggregate layers and their resilience to rinsing suggest their utility as possible template structures for multistep processing or for other applications where surface modification with nanoscale ordered layers is seen as beneficial. Acknowledgment. We thank Daniel Kuntz and Michael Gerber for preparing and characterizing the polymer-micelle aggregate samples used in this work. LA700708G
