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Nonequilibrium Mesoscale Surface Structures: The Adsorption of Polymer-Surfactant Mixtures at the Solid/ Liquid Interface Barry D. Fleming, Erica J. Wanless,* and Simon Biggs Department of Chemistry, University of Newcastle, Callaghan, N.S.W. 2308, Australia Received May 17, 1999. In Final Form: September 22, 1999 Polymer-surfactant mixtures have broad application in modern society; however, their interfacial behavior is not well understood. Soft-contact atomic force microscopy has been used to visualize the adsorbed layer at the graphite/solution interface in mixed aqueous solutions of sodium dodecyl sulfate (SDS) and poly(vinylpyrrolidone) (PVP). The nonequilibrium adsorbed layer is remarkable, consisting of micrometersized domains of surfactant-rich, ordered aggregates interspersed by disordered regions of ill-defined polymer-rich adsorbate. The ordered domains grow to cover the entire substrate after many hours. The nonuniform surface coverage and the long but finite equilibration times have important implications for colloid stability. These first observations of the heterogeneous nature of mixed polymer-surfactant adsorption highlight the importance of applying direct visualization to further our understanding of these ubiquitous formulations.
Introduction Polymer-surfactant mixtures are used extensively in modern society and are of considerable technological importance.1 Surfactants are employed in a variety of applications including detergency, mineral separation, and pharmaceuticals for their ability to adsorb at interfaces and self-assemble in bulk solution.2 Polymers are also often employed in these applications to give rheological control, and in addition they can play an important role in product appearance and, hence, commercial desirability.3 The surfactant and polymer components, although added to control single specific properties, may interact together, causing complex behavior either in solution or at an interface. The polymer-surfactant combination is frequently more effective than either component in its own right. Despite their acknowledged role, there is currently not a good understanding of the molecular level interaction of polymers and surfactants at interfaces, the very region in which these mixtures are active. In recent years, polymer-surfactant interactions in aqueous solution have been the subject of many investigations, as reviewed by Goddard1 and Hansson and Lindman.3 Measurements of these interactions have been performed using techniques such as rheology, fluorescence, IR, NMR, X-ray scattering, and neutron scattering and reflectivity. Polymers have been found to interact with surfactants in a variety of ways that are often dramatically reflected in macroscopic solution properties such as viscosity. An important finding is that polymers can induce micellization at a far lower concentration (the critical association concentration, cac) than occurs in the absence of polymer (at the critical micelle concentration, cmc). This highly cooperative association is attributed to a combination of the favorable hydrophobic interaction between the polymer backbone and the surfactant chains, and screening of the electrostatic interaction between the surfactant headgroups, thereby reducing the electrostatic penalty * Corresponding author. E-mail:
[email protected]. edu.au. (1) Goddard, E. D. Colloid Surf. 1986, 19, 255-300. (2) Hunter. R. J. Foundations of Colloid Science, Vol.1; Oxford University Press: Oxford, 1989. (3) Hansson, P.; Lindman, B. Curr. Opin. Colloid Interface Sci. 1996, 1, 604-613.
for micellization. This association between polymer and surfactant is particularly marked in the case of a polyelectrolyte and an oppositely charged surfactant. On the molecular scale there are a number of models supporting polymer-surfactant association. One of these is the visually appealing pearl necklace model of Shirahama et al.,4 in which surfactant micelle-like structures are strung along the polymer chain. Alternatively, a polyelectrolyte can be modeled as the favorable counterion to an ionic micelle, by folding around the micelle surface.5 In this model, provided that the polymer is sufficiently flexible, and particularly if it has alternating hydrophilic and hydrophobic sections, it can effectively screen the headgroup repulsions and minimize the exposure of the surfactant chains to water. This leads to a lowering of the cac, increased dissociation of ionic surfactants, and an altered environment for the carbon atoms nearest the surfactant headgroup, as confirmed by 13C NMR.1 One mixture that has been widely studied in aqueous solution is that of the common anionic surfactant sodium dodecyl sulfate (SDS) and the nonionic polymer poly(vinylpyrrolidone) (PVP).1 Although ostensibly nonionic, PVP is widely accepted to interact strongly with anionic surfactants due to the polarity of the amide moiety, leading to a polyelectrolyte-like complex.1,6 The cac of SDS and PVP is a weak function of polymer concentration, occurring in the range 2-4 mM, as detected using a range of techniques, including conductivity6 and surface tension.7 13C NMR measurements suggest that the polymersurfactant complex consists of micelle-like aggregates shrouded with polymer.7 This polyelectrolyte-type structure is confirmed by light scattering.8 The aggregation number of these micellar aggregates is significantly lower than that of pure SDS micelles.9 In any colloidal dispersion, the adsorption of solution additives at the solid/liquid interface is critical in deter(4) Shirahama, K.; Tsujii, K.; Takagi, T. J. Biochem. 1974, 75, 309. (5) Wallin, T.; Linse, P. Langmuir 1996, 12, 305-314. (6) Fadnavis, N.; Engberts, J. B. F. N. J. Am. Chem. Soc. 1984, 106, 2636-2640. (7) Chari, K.; Lenhart, W. C. J. Colloid Interface Sci. 1990, 137, 204-216. (8) Norwood, D. N.; Minatti, E.; Reed, W. F. Macromolecules 1998, 31, 2957-2965. (9) Wan-Badhi, W. A.; Wan-Yunus, W. M. Z.; Bloor, D. M.; Hall, D. G.; Wyn-Jones, E. J. Chem. Soc., Faraday Trans. 1993, 89, 2737-2742.
10.1021/la990588r CCC: $18.00 © 1999 American Chemical Society Published on Web 11/17/1999
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mining stability and rheological characteristics. Therefore, in multicomponent formulations, it is important to understand the polymer-surfactant interactions at the solid/liquid interface. While the understanding of singlecomponent adsorption mechanisms from solution is well advanced for many systems, detailed investigations of adsorption from polymer-surfactant mixtures have only recently begun.10-15 In this study we have investigated the mixed adsorption of SDS and PVP on graphite using atomic force microscopy (AFM). In situ images of the adsorbed layer have been obtained following the soft-imaging method of Manne et al.16 The key to this method is fine control of the imaging force in the repulsive regime of the tip-sample interaction, enabling the adsorbed layer to be imaged without damage. Adsorbed surfactant aggregates are generally only visible over a narrow range of applied force (∼1 nN), with the substrate imaged at higher forces. The adsorption of SDS to graphite has previously been characterized as hemicylindrical surface aggregates using soft-contact AFM.17 These aggregates cover the entire surface within minutes of exposure to the surfactant solution and are reversibly adsorbed, being easily removed by rinsing with water to re-expose the substrate. Certain additives have been investigated, including electrolyte18 and cosurfactant;19 however, there have been no reports of polymer addition to this system. Polymer adsorption has traditionally been studied quantitatively by classical solution depletion methods.20 More recently, the interface itself has been targeted directly using reflectance techniques such as neutron reflectivity,15 optical reflectometry,21 and evanescent wave spectroscopy.22 Together with adsorbed quantities, these techniques can be used to probe the kinetics of adsorption, which can involve long-lived nonequilibrium structures due to optimization of the polymer conformation. Direct force measurements between polymer-coated surfaces have been made using both the surface forces apparatus12 and AFM23 to give information about the adsorbed layer conformation away from the interface as a function of solution conditions and time. NMR and ESR measurements also yield conformational information.20,24 Adsorbed layer thicknesses can be obtained in a number of ways, including light scattering, neutron scattering, ellipsom(10) Somasundaran, P.; Krishnakumar, S. Colloid Surf. 1997, 123124, 491-513. (11) Sharma, R., Ed. Surfactant Adsorption and Surface Solubilisation; ACS Symposium Series 615; American Chemical Society: Washington, DC, 1995. (12) Claesson, P. M.; Dedinaite, A.; Blomberg, E.; Sergeyev, V. G. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 1008-1013. (13) Neivandt, D. J.; Gee, M. L.; Tripp, C. P.; Hair, M. L. Langmuir 1997, 13, 2519-2526. (14) Bremmell, K. E.; Jameson, G. J.; Biggs, S. Colloid Surf. 1998, 139, 199-211. (15) Purcell, I. P.; Lu, J. R.; Thomas, R. K.; Howe, A. M.; Penfold, J. Langmuir 1998, 14, 1637-1645. (16) Manne, S.; Cleveland, J. P.; Gaub, H. E.; Stucky, G. D.; Hansma, P. K. Langmuir 1994, 10, 4409-4413. (17) Wanless, E. J.; Ducker, W. A. J. Phys. Chem. 1996, 100, 32073214. (18) Wanless, E. J.; Ducker, W. A. Langmuir 1997, 13, 1463-1474. (19) Wanless, E. J.; Davey, T. W.; Ducker, W. A. Langmuir 1997, 13, 4223-4228. (20) Fleer, G. J.; Cohen Stuart, M. A.; Scheutjens, J. M. H. M.; Cosgrove, T.; Vincent, B. Polymers at Interfaces; Chapman & Hall: London, 1993. (21) Furst, E. M.; Pagac, E. S.; Tilton, R. D. Ind. Eng. Chem. Res. 1996, 35, 1566-1574. (22) Trau, M.; Grieser, F.; Healy, T. W.; White, L. R. Langmuir 1992, 8, 2349. (23) Biggs, S.; Proud, A. D. Langmuir 1997, 13, 7202. (24) Otsuka, H.; Esumi, K. J. Colloid Interface Sci. 1995, 170, 113119.
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etry, and direct force measurements.20 These adsorbed layer characteristics are important factors in the stabilization and flocculation of dispersions. Despite the increasing interest in the arrangement of polymers normal to the interface, little direct information is available about their molecular level arrangement parallel to the surface. In a preliminary investigation, Stipp has made a study of the spatial arrangement of adsorbed poly(acrylic acid) on mica at pH 5 using AFM.25 The adsorbed layer was a coherent film of polymer globules (e200 nm) with a roughness not more than a few nanometers. The adsorption of PVP on graphite has not previously been investigated with AFM, although strong adsorption has been reported.24 The effects of surfactants on polymer adsorption are complex, as surfactants can alter the type of bonding (i.e. hydrophobic, van der Waals, hydrogen, electrostatic) between the surface and the polymer. Also, the chronology of addition (sequential versus simultaneous) has been found to significantly affect the resultant adsorbed layer due to competitive adsorption and modification of the substrate hydrophobicity and charge during adsorption.21,26-27 Both PVP and SDS adsorb to hydrophobic surfaces and have been shown to interact strongly with each other at the air/water interface using neutron reflectivity.15 There have also been several studies of adsorption from mixed solutions onto hydrophobic carbon substrates. Ma and Li used solution depletion methods to show that the mixed adsorption on carbon black is competitive, with a constant total adsorbed amount, indicating adsorption by replacement.28 More recently, Otsuka and Esumi investigated adsorption of lithium dodecyl sulfate (LiDS) and PVP mixtures to graphite using ESR spectroscopy and ζ-potential measurements.24 They reported an initial increase and then a decrease in PVP adsorption with increasing LiDS concentration. The increase was attributed to coadsorption of surfactant and polymer, resulting in the formation of a surface polymer-surfactant complex. The ESR spectra yield information on the mobility of the adsorbed polymer chains; in the absence of surfactant, the polymer adsorbs in flattened “trains” (many bound segments) until surface crowding promotes the formation of “loops” and “tails” (fewer bound segments per chain). In contrast, above the cac, the polymer conformation changed from “loops” and “tails” to “trains”, reflecting the formation of solution polymer-surfactant complexes and the accompanying decrease in the amount of adsorbed polymer. That is, above the cac, the energy for complexation of surfactant and polymer is lower in solution than at the carbon/solution interface. In both studies, the reduced adsorption of PVP at higher surfactant concentration was explained on the basis of the conformation of the adsorbed surfactant; with a change from a monolayer in which the chains are parallel to the surface to one in which they are perpendicular. The latter was claimed to be an unfavorable environment for PVP. The traditional view of monolayer surfactant adsorption29 must be reevaluated in light of the AFM evidence that SDS adsorbs on graphite as hemicylindrical aggregates above 2.8 mM (25) Stipp, S. L. S. Langmuir 1996, 12, 1884. (26) Sastry, N. V.; Se´quaris, J.-M.; Schwuger, M. J. J. Colloid Interface Sci. 1995, 171, 224-233. (27) Shimabayashi, S.; Uno, T.; Nakagaki, M. Colloid Surf. 1997, 123-124, 283-295. (28) Ma, C.; Li, C. J. Colloid Interface Sci. 1989, 131, 485-492. (29) (a) Greenwood. F. G.; Parfitt, G. D.; Picton, N. H.; Wharton, D. G. In Adsorption from Aqueous Solution; Weber, W. J., Matijevic, E., Eds.; American Chemical Society: Washington, DC, 1968; pp 135144. (b) Zettlemoyer, A. C. J. Colloid Interface Sci. 1968, 28, 343-369.
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Figure 1. AFM height images of PVP adsorbed to the graphite/solution interface. (a) 50 ppm PVP solution after 5 h. The height scale of the image is 2.5 nm from black (low) to white (high). This coverage is typical of the range 10-200 ppm. (b) 1 ppm PVP solution after 21 h. A step in the graphite substrate is evident on the left side of the image.
rather than as a perpendicular monolayer.17 The structure below this concentration may indeed be a parallel monolayer.16 Given their association in bulk solution, it is not surprising that polymers and surfactants form complexes in adsorbed films. To date, however, there is little direct information on the structure of such films and it is not easy to predict the behavior of mixed systems at interfaces. By using AFM to directly monitor the adsorption of SDS and PVP mixtures, we have been able to further characterize the properties of these films. Experimental Section Sample Preparation. Water was prepared by passage through a Milli-Q Ultrapure filtration system. The purified water had a conductivity of 7 µS cm-1. Sodium dodecyl sulfate (BDH Chemicals, 98%) was used as received, and a constant concentration of 6 mM was maintained throughout. Poly(N-vinyl-2pyrrolidone) (Aldrich, 1.3 × 106 Mw) was used as received. Concentrations in the range 1-200 ppm PVP were investigated. Adhesive tape was used to cleave a fresh sample of graphite for each experiment from a pyrolytic graphite monochromator (grade ZYH, Advanced Ceramics, Cleveland, OH). All solutions were prepared and left overnight to equilibrate at room temperature (∼22 °C) before use. Microscopy. Images were captured using a Nanoscope III atomic force microscope (Digital Instruments, Santa Barbara, CA). Imaging was performed in soft-contact mode16 on the repulsive region of the tip-substrate interaction, at generally less than 1 nN above zero force. Two types of silicon nitride cantilevers were used: sharpened microlevers (Olympus, Japan, nominal spring constants 0.02 and 0.09 N m-1) were used throughout, except for Figure 4c, in which an unsharpened NanoProbe (Digital Instruments, nominal spring constant 0.12 N m-1) was used. The cantilevers were irradiated for 45 min with a mercury UV lamp (∼9 mW cm-2 at 253.7 nm) in a laminar flow cabinet before use. Distances in lateral dimensions were calibrated by imaging a standard 1 µm grid, and the distance normal to the surface was calibrated by measuring the etch pits of a standard grid (180 nm deep). The images presented have been flattened. Procedure. Three modes of experiment were performed: preequilibration with SDS, pre-equilibration with PVP, and simultaneous addition of SDS and PVP. Each experiment was monitored for at least 12 h after exposure of the substrate to both
polymer and surfactant components in order to follow the equilibration of the adsorbed structures.
Results and Discussion The adsorption of PVP to graphite was investigated in the concentration range 1-200 ppm, as shown in Figure 1. Figure 1a shows the adsorbed polymer layer observed in the range 10-200 ppm. The substrate was completely coated within minutes of exposure to the polymer solution, although the position of steps in the underlying substrate was always clearly evident. There was no subsequent gross change in appearance of the adsorbed layer over many hours. The adsorbed polymer layer was far more robust than the adsorbed surfactant layer,17 being visible over a wide range of applied force from 0.1 to 10 nN. Above an applied load of 1 nN, the layer could be compressed sufficiently to leave an imprint of the scanned area (up to 2.4 nm deep) but could not be completely removed from the substrate. The polymer layer was not visibly altered by flushing the AFM cell with water, providing further evidence for strong binding to the graphite. At 1 ppm, the adsorption was quite different, as shown in Figure 1b. The polymer adsorbs in an expanded chain conformation that becomes rougher and more globular over several hours to give the equilibrated structures shown. The concentration above which SDS aggregates on graphite (1.6-2.8 mM)17 and that at which it associates with PVP in solution (cac ) 2-4 mM)6,7 are similar. The adsorption of SDS and PVP mixtures was studied above this cac to permit comparison with the well-defined surface aggregates present in the pure SDS regime. The layer adsorbed from the mixture onto graphite was interesting on both micrometer and nanometer length scales. Of primary interest is the heterogeneous adsorbed layer shown in Figure 2. This layer, consisting of mesoscale domains, is a stark contrast to the uniform coverage of the substrate that occurs in the isolated adsorption of either component. The domains frequently border steps on the substrate, with the remaining unconstrained edges often exhibiting regular geometric features with frequent angles of 60, 90, and 120°. Similar coverage was observed in the range 10-200 ppm PVP, although there was a large
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Figure 2. Deflection images showing nonuniform coverage of the graphite/solution interface adsorbed from mixtures of 6 mM SDS and 50 ppm PVP. The substrate was initially exposed to a 50 ppm PVP solution for at least 12 h. The two images are from two different experiments: (a) 10 min after the addition of the mixture and (b) 25 min. As each experiment proceeded, the rough domain coverage increased to complete coverage after 12 h.
Figure 3. Force normal to the surface in the same experiment as Figure 2a. In PVP alone after 23 h in the solution, the interaction (circles) consists of a steric repulsion on approach of the tip to the surface extending ∼30 nm from zero separation. Zero separation is not necessarily true tip-sample separation but rather is defined as the point at which the compliance of the cantilever spring and that of the surface are equal. There is likely to still be a layer of compressed polymer between the tip and the graphite at this point. In polymer alone, there is a reduction in the tip-surface adhesion with exposure time. With the addition of 6 mM SDS, heterogeneous adsorption results in electrosteric interactions on both domains as shown. An instability is present in the rough domain interaction (filled triangles with line) at 1.8 nm from zero separation, but none is observed on the smooth domains (open triangles).
degree of variation in the form of the domains. This variation may be related to the step density on the substrate. Even on this scale the two regions are distinctly different, one appearing rough and the other smooth. The rough domains either consisted of an even coverage as in Figure 2a or contained fissures as in Figure 2b. There was an apparent height difference from the rough domains to the smooth regions, measured to be 1.2 ( 0.4 nm for a range of images. This difference was not found to be a function of PVP concentration. On the micrometer scale, the mixed adsorbed layer in 1 ppm PVP did not always
consist of domains, with the surface sometimes covered by small lumps of polymer. This difference reflects the differences observed for the pure polymer adsorbed layers. Importantly, the adsorbed layers shown in Figure 2 are nonequilibrium structures, as there was an increase in the surface coverage by the rough domains on the order of hours. After 8-16 h the layer was more uniform, generally covering the entire substrate. These surface changes consisted of the expansion of existing domains and, often at high polymer concentration, were accompanied by an increased roughness that was attributed to additional polymer adsorption. Additionally, the domain shape was altered by continual scanning with the AFM tip, which sometimes facilitated expansion of the rough domains. The force of interaction normal to the mixed adsorbed layer at 50 ppm PVP is given in Figure 3 for both domain types together with the pure PVP interaction for comparison. In polymer only, the interaction consists of a steric repulsion30 as the polymer-covered surface and tip approach within the scale of the radius of gyration of the polymer (calculated to be 33.5 nm).20 This repulsion extends about 30 nm from the surface, indicating polymer extension of this magnitude. The extended polymer conformation highlights the difficulties associated with measuring the thickness of the adsorbed layer. During imaging, the adsorbed polymer will be compressed by the scanning tip, thus precluding any measurement of the natural thickness. Also the tip cannot penetrate through the layer to the underlying substrate, so the compressed thickness is also unattainable. Therefore, any AFM “thickness” measurement of adsorbed layers containing polymer should strictly be qualified. With the addition of surfactant, an additional repulsion is observed within 10 nm of zero separation, resulting in an overall electrosteric interaction. The short-ranged electrostatic component is present on both rough and smooth domains and is attributed to adsorption of surfactant and possibly polymer now as a polyelectrolyte(30) Israelachvili, J. N. Intermolecular and Surface Forces, 2nd ed.; Academic Press: London, 1995.
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like complex. The major difference between the two interfacial regions is seen as the tip approaches within 2 nm of the substrate. On the rough domain, the tip reaches an instability at 2.2 ( 0.9 nm (1.8 nm in Figure 3) and moves rapidly into zero separation. There was substantial variation in both the size and magnitude of the instability; however, this was not linked to the PVP concentration. A similar attractive instability is present in pure SDS solution.17 On the smooth domains, no instability was observed and the tip moved gradually toward the substrate with increasing force. This behavior suggests that it is polymer not surfactant that is in contact with the substrate in these regions. The pull-off force showed a decrease in adhesion with the age of the adsorbed layer, but the approaching interaction was independent of layer age. Sequential adsorption and coadsorption can readily be distinguished in AFM experiments, since the observations are made in real time and the solution can easily be replaced. Preadsorbed surfactant exposed to polymer was investigated as well as preadsorbed polymer exposed to surfactant, together with simultaneous adsorption. The heterogeneous surface coverage was observed in all three cases with no significant differences between them. This is especially poignant in the case of preadsorbed polymer, where the surfactant thoroughly removes adsorbed polymer globules from some areas. Further evidence of detergent action was seen when water replaced the solution mixture. Unlike the case for strongly bound PVP, rinsing with water removed some of the mixed adsorbed layer. The results of rinsing varied widely, being less successful at higher polymer concentrations. The interaction force after rinsing was attractive, indicating a hydrophobic surface with little, if any, adsorbed surfactant. It is best to relate the higher resolution appearance of the mixed adsorbed layer to that observed in SDS alone. In SDS, the hemicylindrical aggregates are aligned with the substrate crystallography, occurring in up to three orientations at 60° to each other.17 The boundaries between these “grains” are not necessarily attributable to particular surface features, although they will often be present at a substrate step. Some representative close-up images of the rough domains adsorbed from mixtures are shown in Figure 4. In each case, hemicylindrical structures are observed, although each example differs in some way from those seen in SDS alone. In Figure 4a, small globules of polymer coexist with a large number of striped grains, as commonly observed at low PVP concentration. Three different orientations of stripes are visible. Figure 4b and c shows the common features observed from 10 to 200 ppm PVP on the rough domains. In Figure 4b the stripes appear unperturbed but the grain boundary has a definite width unlike those seen in SDS alone, in which the aggregates from each grain directly abut each other. The boundary is lower than the surrounding areas, having a depth of up to 0.5 nm, although this measurement is naturally restricted by the size of the tip. In Figure 4c, no boundary is shown; instead a general disruption of the stripes is apparent, indicating polymer influence throughout the adsorbed layer. This form was observed frequently at the higher polymer concentrations. Finally, the edge between domains is shown in Figure 4d. The edge shown is a free edge, not tethered by any substrate step. On a few occasions it took >0.5 h before any organized structures were observed in the mixed system. This was not attributed to the order of addition of the solution components and instead is likely due to the presence of disordered polymer adsorbed on, or confined between, the tip and the substrate.
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No definitive identification of the composition of the adsorbed layer is possible with AFM, although its appearance does provide several clues. The ever-present striped features on the rough domains indicate adsorbed dodecyl sulfate that is in contact with and organized by the underlying substrate despite the presence of adsorbed polymer. Similarly, Chari has predicted the formation of surface micelles in this system at the hydrophobic air/ water interface.31 The measured force curve instability distance is also consistent with adsorbed surfactant. Likewise, a layer “thickness” of 1.8 ( 0.3 nm has been measured for the adsorbed PVP-SDS layer at the air/ water interface by specular neutron reflection and not found to be a strong function of the PVP concentration.15 A similar thickness has been reported for LiDS and PVP on hydrophobic silica.32 The spacing of the hemicylindrical aggregates is identical to that reported previously; that is, there is little, if any swelling due to solubilization of polymer within the aggregates. Some polymer is likely to be present in these structures, but they are certainly surfactant-rich. Figure 4 shows that the adsorbed polymer is present in a variety of conformations. In some instances the globular form observed in the pure polymer is preserved, as seen in Figure 4a, at 1 ppm. In this case, the effect of the polymer is that a grain boundary occurs at each polymer adsorption site. This effect has been reported previously and attributed to unknown contamination.17 An alternative grain boundary influence of the polymer is shown in Figure 4b, where the boundary has a measurable width. The interaction force indicates the presence of polymer, and since this type of meandering boundary is not observed in pure SDS, it must be attributed to adsorbed PVP. Since the extended length of the polymer is several microns, it is possible that this type of boundary indicates adsorbed polymer molecules in flattened, extended conformations. Such a polymer conformation would still be expected to have associated surfactant molecules along the chain. In analogy to the case for the bulk solution, we believe this structure to be a surface-polyelectrolyte complex.8 The adsorbed PVP does not always result in an increase in the number of grain boundaries, as shown in Figure 4c, where the polymer disrupts the order, yielding roughened stripes. The presence of the grain boundaries (not shown) again suggests that it is surfactant not polymer that is in contact with the graphite in these areas. This concept is supported by the relative ease with which the mixed adsorbed layer can be rinsed off compared with the polymer layer. The lower clarity of the stripes in this instance suggests that disordered polymer is present on top or intermingled with the surfactant. There is also some evidence for this in Figure 4d, in which a haze is present on top of the stripes: visible as a variation in clarity of the stripes across the domain. This may be due to force variations during imaging; however, the fact that the intensity of the haze varies along the scan lines (horizontal) suggests that the haze may be due to overlying polymer. Given the attraction of PVP for both the surfactant tail and the headgroup,1,6 there should be polymer present on top of any adsorbed surfactant, regardless of its conformation. Our conclusion is that the ordered domains consist of weakly adsorbed polymer shrouding a surfactant-rich base with the former removed by scanning. The smooth domains also merit discussion. That they are facilitated by surfactant is obvious given their existence (31) Chari, K. J. Phys. II 1995, October, 1421-1426. (32) Otsuka, H.; Esumi, K.; Ring, T. A.; Li, J.-T.; Caldwell, K. D. Colloids Surf. 1996, 116, 161-171.
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Figure 4. Higher resolution deflection images of rough domain surfaces showing the variety of morphologies observed. (a) Small globules of polymer sit at each hemicylinder grain boundary, pre-equilibrated in 1 ppm PVP (Figure 1b) and then left overnight in the mixture. (b) Meandering fissures run through relatively unaffected hemicylinders, coadsorbed from 10 ppm PVP and left overnight. (c) Disruption of hemicylinders by adsorbed polymer, pre-equilibrated in SDS and then left for 15 min in a 200 ppm PVP mixture. (d) Domain edge, pre-equilibrated in 50 ppm PVP and then left for 35 min in the mixture.
even in the case of preadsorbed polymer. This result is in concordance with the solution depletion measurements,24,28 which indicated a decrease in polymer adsorption on addition of surfactant. The interaction forces shown in Figure 3 give a few clues as to the nature of the adsorption in these regions. The interaction force is identical normal to both domains at tip-substrate separations greater than 3 nm (see Figure 3 inset). At smaller separations the interaction is more repulsive on the ordered domains and exhibits an instability characteristic of surfactant adsorption. The lower force on the smooth domains at these separations suggests that less surfactant is adsorbed and/ or that the polymer present is in a less extended conformation. Images of these regions were featureless. This cannot be caused by ill-defined tip adsorbates lowering resolution, since the edge regions (Figure 4d) can be adequately resolved. Therefore, there are certainly
no organized structures in these regions. Whatever is adsorbed here is only loosely bound and prevents the rapid expansion of the ordered adsorbate, probably due to rearrangement of adsorbed polymer in both regions. That these regions appear lower than the ordered domains may simply reflect greater compliance than for the ordered adsorbate. There are two previous reports of direct observations of a mosaic of adsorbed domains: for quaternary ammonium surfactants adsorbed to mica33 and for SDSdodecanol mixtures on graphite.19 In the former, the mosaic was observed in the adsorption of a second bilayer, many hours after the introduction of the surfactant solution, and consisted of regions of both bilayer and (33) Lamont, R. E.; Ducker, W. A. J. Am. Chem. Soc. 1998, 120, 7602-7607.
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cylinder domains. Similarly, the domain structure in the latter reflects regions of different aggregate curvature. The key difference when comparing these observations with the current system is that here the interaction force is different on the two regions, indicating quite different structures rather than a slow change in curvature of similar structures. The geometric nature of the domain peripheries is intriguing. Edges at 60 and 120° are compatible with the substrate crystallography that templates the hemicylindrical structures.16 A variety of grain boundaries consistent with this symmetry have been reported previously.17 The repetition of these angles at domain edges indicates the importance of the substrate relative to the adjacent domain in defining these boundaries. The less common 90° edges, as shown in Figure 2b and more closely in Figure 4d, are rather unexpected. Their presence highlights the unfavorable nature of an isolated hemicylinder terminus. The perfect alignment of the hemicylindrical aggregates has been puzzling, since they were first observed because of the entropy penalty associated with such a high degree of order. The 90° edge is further evidence for strong interhemicylinder interactions. The combination of hemicylinder binary symmetry with substrate ternary symmetry necessarily involves hemicylinder terminii at domain edges. The actual edge shape must be determined by local energy considerations, and the observed variation suggests a delicate balance between substrate, hemicylinder, and adjacent domain contributions. The very features of AFM that have permitted the heterogeneous character of the mixed adsorbed layer to be observed, namely gentle, real time imaging with high lateral resolution, also highlight certain limitations of this technique. No information can be gathered on the adsorbed amounts of either the surfactant or the polymer. This information would be of invaluable assistance in interpreting the observed structures. If combined with shorter time frame observations (within the first minute), this would pinpoint the greatest differences between the three modes of adsorbed layer construction. Kinetic differences are expected between the modes of addition due to the competitive nature of the adsorption. The larger polymer molecules will diffuse to the surface more slowly than the surfactant molecules, although both are highly surface-active at the graphite/solution interface. Naturally then, except in the case of polymer preconditioning, there will be adsorbed surfactant present that will mediate the polymer adsorption. We have some inconclusive evidence that, in the latter case, the subsequent mixed adsorbed layer takes longer to equilibrate. This is consistent with other reports that sample history tends to be important when polymer adsorption is involved.32 Some comparison can be made with adsorbed quantities reported in the literature. Ma and Li reported 2.2 PVP monomers/nm2 and 1.2 DS/nm2 adsorbed on carbon black from 6 mM SDS and 1000 ppm PVP (4 × 104 Mw).28 This PVP concentration is the mole equivalent to about 30 ppm with the current sample. Otsuka and Esumi recorded the higher adsorbed quantities of 18 PVP monomers/nm2 and 3.7 DS/nm2 on graphite from 6 mM LiDS and 1.5 g L-1 PVP (1.6 × 104 Mw).24 The amount of surfactant adsorbed is of similar magnitude to that reported by Greenwood et al. for SDS alone adsorbed on graphon (∼1.8 DS/nm2).29 (34) Iksay, I. A.; Trau, M.; Manne, S.; Honma, I.; Yao, N.; Zhou, L.; Fenter, P.; Eisenberger, P. M.; Gruner, S. M. Science 1996, 273, 892898.
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This surface density of surfactant implies far more than monolayer coverage, as discussed previously.17 The reported variation for the mixture is substantial but perhaps expected, since each study used different molecular weight polymer and different hydrophobic substrates. In any case, the adsorbed mass of polymer and that of surfactant are roughly equivalent. The observed structures, however, suggest a greater amount of adsorbed surfactant. The discrepancy could well lie in an unknown quantity of weakly adsorbed polymer above the surfactant-rich base. Two important parameters for the stabilization and flocculation of dispersions are the adsorbed layer thickness and the adsorbed amount, both of which are functions of the polymer conformation at the solid/solution interface.32 The thickness of adsorbed layers can be obtained from techniques such as ellipsometry. Adsorbed amounts can be obtained using solution depletion methods, although these suffer from the comparison of high-surface-area particulate substrates with the low-surface-area planar substrate used here, where the interfaces may differ substantially. Optical reflectometry is better suited to make a direct comparison with a planar substrate. Supplementary information on adsorbed amounts, layer thickness, polymer conformation and adsorption kinetics is the subject of ongoing research and will add greatly to the interpretation and understanding of these observations. Conclusion The adsorption of PVP-SDS mixtures at the graphite/ solution interface has been studied directly using AFM in the concentration regime between the cac and the mixed cmc. The results show that adsorbed polymer still plays a significant role above the cac, despite the presence of favorable solution complexes. The first images of adsorbed polymer-surfactant layers show that the adsorbed layer is heterogeneous on the micrometer scale, consisting of two distinct types of coverage. One adsorbate region consists of ordered surfactant-rich hemicylindrical structures disrupted to varying extents by additional adsorbed polymer and is attractive to the scanning tip at an approach distance of about 2 nm. The other regions are apparently disordered and repulsive to the scanning tip at all applied loads. The heterogeneous adsorption observed across the range of PVP concentrations studied is unexpected, is of great interest, and has not previously been reported. The significance of solid/liquid interfaces bearing mesoscale regions of different surface-chemical behavior cannot be underestimated. The plethora of particulate systems reliant on interparticle surface forces to control flocculation and dispersion is one example of the type of behavior that could be vastly modified by such nonuniform adsorption. The significant lifetime of these nonequilibrium structures is also pertinent to these systems during both the mixing and settling phases, since the mesoscale structures may ultimately determine flocculation behavior. Thus, the equilibrium measurements generally present in the literature may be inappropriate when discussing the forces that govern flocculation. The heterogeneous adsorption could also be important to materials scientists if control can be gained over the shape and lifetime of the adsorbed domains. These could subsequently be used to template mesoscale structures in much the same way that surfactant aggregates have been used to assemble inorganic thin films.34 LA990588R
