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Templating of Ethyl(hydroxyethyl)cellulose on Graphite by Surfactant-Polymer Interactions Jostein Djuve,†,‡ Lachlan M. Grant,§,| Johan Sjo¨blom,†,⊥ Tanya P.Goloub,# and Robert J. Pugh*,§ Institute for Surface Chemistry, Box 5607, S-11486, Stockholm, Sweden, and Department of Chemistry, University of Bergen, Alle´ gaten 41, 5007 Bergen, Norway Received August 29, 2001. In Final Form: November 18, 2001 Ordered, templated, thin films of ethyl(hydroxyethyl)cellulose (EHEC) were produced on a freshly cleaved crystalline graphite surface by exploiting the nature of sodium dodecyl sulfate adsorption (SDS) and its interaction with EHEC. The films were prepared by exposing the graphite surface to a dilute aqueous solution containing EHEC-SDS mixtures, followed by rinsing to remove the SDS and drying. The structure of the remaining EHEC film at the graphite-air interface was investigated by atomic force microscopy and compared with images of SDS and EHEC polymer on adsorbed graphite obtained in separate experiments. The results showed that polymer networks were formed from the EHEC-SDS mixture (after rinsing out the SDS), which were predominately aligned in one of three preferential directions separated by 60°, presumably as a result of templating by coadsorbed SDS. In addition, the alignment of the EHEC polymer and spacing between the polymer strands was found to be a function of surfactant concentration. Films formed from a solution containing EHEC (10 ppm) and SDS solution (5 mM) after removal of the SDS by washing formed “tight”, highly aligned networks. At the same EHEC concentration and 20 mM SDS, a more “open” structure was formed. The results provide valuable insights into the interaction and organization of SDS and EHEC at a crystalline hydrophobic surface.
Introduction As separate entities, both low molecular weight surfactants and macromolecules can self-assemble at interfaces and form structures that are entirely different from those in solution. This topic of research has attracted much attention in recent years, particularly since the control of self-assemblies and nanostructures on well-defined interfaces forms an important basis for the rational development of fabrication processes. In the case of macromolecules, it has been demonstrated that molecularly defined nanostructures can be self-assembled on surfaces and interfaces by making use of a combination of interfacial and intra- and intermolecules forces.1 Frequently, this occurs through epitaxial crystallization, which can be used to control both the molecular chain orientation and microdomain structures on crystalline organic substrate. Surface interactions due to crystallographic matching between a polymer block and a substrate have been shown to induce a highly oriented crystalline polymer block and well-ordered parallel lamellar microphase-separated structures.2,3 * Author for correspondence. E-mail: bob.pugh@ surfchem.kth.se. † University of Bergen. ‡ Visiting research student at the Institute for Surface Chemistry. Present address: TotalFinaElf, PO Box 168, No-4001 Stravanger, Norway. § Institute for Surface Chemistry. | Present address: Biacore AB, Rapsgatan 7, SE-754 50 Uppsala, Sweden. ⊥ Present address: Statoil Research Centre, Arkitekt Ebbelsv 10, NO-7005 Trondheim, Norway. # Visiting research scientist at the Institute for Surface Chemistry. Present address: Department of Colloid Chemistry, University of St. Petersburg, St. Petersburg, Russia. (1) Rabe, J. P. Curr. Opin. Colloid Interface Sci. 1998, 3 (1), 27-31. (2) Cartier, L.; Okihara, T.; Ikada, Y.; Tsuji, H.; Puiggali, Lotz, B. Polymer 2000, 41 (25), 8909-8919. (3) De Rosa, C.; Park, C.; Lotz, B.; Wittmann, J. C.; Fetters, L. J.; Thomas, E. L. Macromolecules 2000, 33 (13), 4871-4876.
In the solution phase, it has been well established that polymers and surfactants frequently aggregate together and several extensive reviews are available on this subject.4-7 From this work, a fairly consistent picture has emerged; initial addition of polymer to a surfactant solution causes aggregation at a concentration (critical aggregation concentration, cac) well below that of the polymer-free surfactant solution (critical micelle concentration, cmc). For nonionic polymers and anionic surfactants, a detailed model for the interaction has been proposed which comprises of micellar-like aggregates (clusters) forming on the more hydrophobic portions of the polymer at the cac. The aggregation number of the clusters increases when the surfactant or polymer concentration is increased to form complexes which can adsorb at interfaces, but to date there are few studies reported on the nature of these adsorbed aggregates. Ethyl(hydroxyethyl)cellulose (EHEC) consists of a cellulose backbone modified with ethyl and nonionic oligo(ethylene oxide) units, primarily to increase water solubility. EHEC may be classified as a nonionic, amphiphilic, water-soluble polymer that exhibits reversed temperaturedependent phase behavior and a lower consolute boundary.8 EHEC is known to self-associate in aqueous solution and has been shown to interact strongly with ionic surfactants.9-11 Unmodified and hydrophobically modified (4) Kwak, J. C. T. Polymer-Surfactant Systems; Marcel Dekker: New York, 1998; Vol. 77. (5) Goddard, E. D. Colloid Surf. 1986, 19, 255-300. (6) Goddard, E. D. In Interactions of surfactants with polymers and proteins; Goddard, E. D., Ananthapadmanabhan, K. P., Eds.; CRC Press: Boca Raton, FL, 1993; pp 203-276. (7) Lindman, B.; Thalberg, K. In Interactions of surfactants with polymers and proteins; Goddard, E. D., Ananthapadmanabhan, K. P., Eds.; CRC Press: Boca Raton, FL, 1993; pp 203-276. (8) Karlstrom, G.; Carlsson, A.; Lindman, B. J. Phys. Chem. 1990, 94, 5005-5015. (9) Lindman, B.; Carlsson, A.; Karlstrom, G.; Malmsten, M. Adv. Colloid Interface Sci. 1990, 32, 183-203. (10) Piculell, L.; Lindman, B. Adv. Colloid Interface Sci. 1992, 41, 149-178.
10.1021/la011369k CCC: $22.00 © 2002 American Chemical Society Published on Web 03/09/2002
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nonionic cellulose ethers, EHEC and HM-EHEC, and their interaction with sodium dodecyl sulfate have been extensively studied in solution.12-19 In contrast to EHEC-sodium dodecyl sulfate (SDS) interactions in solution, very little is known about the organization of EHEC-SDS complexes at the solid interface. In this study, we use atomic force microscopy to determine the structure of polymer films formed from EHEC-SDS mixtures on the surface of graphite, which was subsequently rinsed with water and dried. As a control, we have also imaged dried EHEC films produced by adsorption of polymer from solution in the absence of SDS. Aggregates of pure SDS were imaged in situ, following the method of Manne and Gaub20 producing images very similar to those previously published by Wanless and Ducker21 for the SDS/graphite system. Surprisingly, imaging of the adsorbed polymer films (formed in the presence of surfactant) indicate that the final structure of the polymer film is strongly affected by the concentration of the surfactant prior to its removal from solution. Materials and Film Preparation Sodium dodecyl sulfate (purity 99.9%) (Merck, Darmstadt, Germany) was used without further purification. The graphite (pyrolytic, monochromator grade ZYH) was manufactured and supplied by Advanced Ceramics, Cleveland, OH. A fresh surface was prepared for each experiment by cleavage using doublesided tape. The EHEC polymer (Mw 100 000) was manufactured and supplied by Berol Nobel AB, Stenungsund, Sweden. Two parameters are routinely used to quantify the amount of substitution of ethylene oxide groups of EHEC polymers, MSEO, the “molar substitution” which refers to the average number of ethylene oxide groups/anhydroglucose unit, and DSethyl, the “degree of substitution” which gives the average number of hydroxyl groups that have been modified/anhydroglucose unit (and thus may have a value between 0 and 3). Values given by the manufacturer were DSethyl ) 0.6-0.7 and MSEO ) 1.8. The cloud point of the polymers was 65 °C. See Figure 1 for the structure of a section of the polymer chain. Considerable care was taken to prepare the aqueous polymer solutions by a reproducible method. Each polymer solution was dissolved in water by stirring overnight. Polymer concentrations are expressed in ppm, and surfactant concentrations, in terms of mmol/L. Milli-Q water, used for solution preparation and rinsing, was prepared by distillation and then passage through a Milli-Q RG system consisting of charcoal filters, ion-exchange media, and a 0.2 µm filter.
Atomic Force Microscopy Measurements All measurements were performed using a Nanoscope III (Digital Instruments, CA). For investigation of pure SDS aggregates imaging was carried out in situ, using the deflection (11) Lindell, K.; Cabane, B. Langmuir 1998, 14, 6361-6370. (12) Evertsson, H.; Nilsson, S.; Holmberg, C.; Sundelof, L. O. Langmuir 1996, 12, 5781-5789. (13) Medeiros, G. M. M.; Costa, S. M. B. Colloid. Surf., A 1996, 119, 141-148. (14) Thuresson, K.; Soderman, O.; Hansson, P.; Wang, G. J. Phys. Chem. 1996, 100, 4909-4918. (15) Kamenka, N.; Burgaud, I.; Zana, R.; Lindman, B. J. Phys. Chem. 1994, 98, 6785-6789. (16) Bloor, D. M.; Wanyunus, W. M. Z.; Wanbadhi, W. A.; Li, Y.; Holzwarth, J. F.; Wynjones, E. Langmuir 1995, 11, 3395-3400. (17) Singh, S. K.; Nilsson, S. J. Colloid Interface Sci. 1999, 213, 133151. (18) Nilsson, S.; Holmberg, C.; Sundelof, L. O. Colloid Polym. Sci. 1995, 273, 83-95. (19) Evertsson, H.; Nilsson, S. Macromolecules 1997, 30, 2377-2385. (20) Manne, S.; Cleveland, J. P.; Gaub, H. E.; Stucky, G. D.; Hansma, P. K. Langmuir 1994, 10, 4409-4413. (21) Wanless, E. J.; Ducker, W. A. J. Phys. Chem. 1996, 100, 32073214.
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Figure 1. Typical section of the ethyl(hydroxyethyl)cellulose (EHEC) polymer chain. mode and a liquid cell (Digital Instruments, CA). The images were obtained after the solution was left to equilibrate for 20 min. Analysis of the force curves indicated a separation of 1.9 nm before jump into contact. However, for the polymer and polymer surfactant systems, in situ imaging proved to be difficult and yielded irreproducible results. In fact, due to the erratic behavior, a quantative estimates of the layer thickness from the force curves could not be achieved. The inability to carry out in situ measurements in the EHEC and EHEC-SDS systems can be explained by the fact that the polymer chains extend into the solution and were disrupted by the approaching imaging probe. From recent NMR studies on dilute EHEC-SDS solutions, it has been reported that the EHEC is situated just inside the micellar surface with is no polymer within the core of the SDS micelle.25 Under these circumstances, it is likely that the more hydrophilic chains extend into the solution. This could explain the poor resolution of the EHEC-SDS aggregate images. To obtain reproducible measurements, we found it necessary to remove the sample from the solution and image, in tapping mode, after drying under ambient conditions. The probes used for the tapping mode measurements were 125 µm long silicon tips (Nanosensors, Gmbh). These cantilevers had a resonant frequency and spring constant of ca. 320 kHz and ca. 30-45 nm-1, respectively. All measurements were performed in the temperature range 23 ( 2 °C. The following method was used to prepare all films containing the EHEC polymer: A freshly cleaved graphite surface was inserted in 25 mL of the chosen polymer or polymer/surfactant solution and left overnight. The solution was then diluted with a large volume of distilled water (10 L) under gravity feed. The surface was then removed slowly and dried in a desiccator for a further 24 h.
Results and Discussion Imaging of SDS on Graphite. Figure 2 shows an AFM image of SDS aggregates adsorbed to the solid-liquid interface. This image clearly reveals the SDS “hemicylinders” aggregates as well-defined elongated shaped stripes with a peak-to-peak spacing of 6.2 nm. At different areas of the surface it was observed that these stripes are aligned in three directions separated by 60˚. This structure is in keeping with the results of previous studies of SDS adsorbed to graphite within the same concentration range as reported by Wanless and Ducker.21 In fact, the formation of SDS hemicylinders on graphite has been extensively documented throughout the surface chemical literature and these structures have been revealed mostly by using AFM techniques. In the earlier experiments, the relationship between coverage and the degree of order and solution (22) Nilsson, S. Macromolecules 1995, 28, 7837-7844. (23) Singh, S. K.; Nilsson, S. J. Colloid Interface Sci. 1999, 213, 152159. (24) Piculell, L. Nilsson, S.; Sjostrom, J.; Thuresson, K. ACS Symp. Ser. 2000, No. 765, 317-335 (Associated Polymers in Aqueous Solution). (25) Evertsson, H.; Nilsson, S.; Welsh, J.; Sundelof, L. O. Langmuir 1998, 14, 6403-6408.
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Figure 2. AFM deflection image of SDS aggregates adsorbed to the graphite-solution interface (bulk solution concentration 5 mM). Peak-to-peak spacing 6.2 nm.
conditions was studied in considerable detail and it was found that the shapes of the structures are not greatly affected by surfactant concentration. However, a general trend was reported in that the interaggregate spacing decreases with increasing concentration and/or increasing ionic strength. Hemicyclindrical surface aggregates have also been observed with a wide range of different types of surfactants, and the results indicate the nature of the headgroups plays only a minor role in determining the structure of the surface aggregates. It appears that the graphite lattice dictates the orientation and form of the aggregates. A hypothesis that accounts for this behavior is that this arrangement is due to a fortuitous match in the distance between alternate methylene groups in the alkane chain and the centers of the hexagonal graphite lattice. Dilution with a large volume of pure water led to only trace amounts of material being detected on the surface by the AFM imaging, indicating near complete desorption of the surfactant. This is expected on the basis of the previous ellipsometry studies that show that SDS desorbs from a hydrophobic surface when the solution is diluted with excess water. Imaging of EHEC on Graphite. The configuration of the polymer in the water is dependent on the balance between the interactions of the segments with the water and the interactions of the segments with each other. For the EHEC polymer, although the backbone of the EHEC polymer is relatively stiff, the hydrophilic chains would be flexible and coiling and entanglement could be important. It is known that EHEC strongly adsorbs on hydrophobic surfaces such as graphite due to the hydrophobic interaction between the backbone of the polymer and the hydrophobic sites on the surface but also van der Waals interactions are probably important. In Figure 3, images of the macromolecular EHEC films at the graphite surface are shown. At a concentration from 0.2 to 1 ppm, the dried imaged films showed structures resembling polymer strands and followed a network pattern, possibly caused by some entanglement of the molecules in the bulk solution and uncoiling and rearranged on attachment to the surface. With increasing concentration (1-10 ppm), more polymer is adsorbed eventually leading to a complex networks. With a further increase in concentration (from 10 to 25 ppm), the gaps within the networks were reduced producing holes during the drying process, which eventu-
Figure 3. (a) Image of EHEC on graphite. The concentration of the bulk solution was 1 ppm with coverage 20%. (b) Image of EHEC on graphite. The concentration of the bulk solution was 10 ppm with coverage 32%. (c) Image of EHEC on graphite. The concentration of the bulk solution was 25 ppm with coverage 67%.
ally closed leading to a complete polymer layer. The fractional coverages of EHEC were calculated from the
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Figure 5. Image of 10 ppm EHEC-20 mM SDS (3 µm scale) with coverage 34% and strand thickness 30-60 nm (valleyto-valley).
Figure 4. (a) Image of a 10 ppm EHEC-5 mM SDS (3 µm scale) with coverage 77% and strand thickness 40-70 nm (valley-to-valley). (b) As in Figure 4a but with 1 µm scale.
AFM pictures, and the results indicated that the fractional coverages of the polymer increased from 20% (1 ppm) to 32% (10 ppm) to 67% (25 ppm). Dilution of the adsorbed polymer/graphite system followed by rinsing and drying indicated that the EHEC was irreversibly absorbed on the graphite. Images of EHEC/SDS Mixtures on Graphite. It has been well established that SDS is adsorbed on the EHEC polymers chains in the form of charged clusters and this causes an appreciable change in configuration of the complex depending on the SDS/polymer ratio.9,10,11 The onset of adsorption and the increase in cooperate adsorption increases to a saturation maximum, and important rheological behavior (high viscosity) has been reported to occur within a fairly limited concentration range beginning at the onset of adsorption but ending long before adsorption saturation is reached.24 In this study, images were produced from polymersurfactant samples containing 10 ppm EHEC-5 mM SDS and 10 ppm EHEC-20 mM SDS (after flushing out the SDS) and are shown in Figures 4 and 5, respectively. The fractional coverages were calculated from the AFM pictures, and these values are shown in the subtext in the figures. Clearly, the results show that the structures of the EHEC polymer, produced from these EHEC/SDS
Figure 6. Schematic representation of the templating process of ethyl(hydroxyethyl)cellulose on graphite by surfactantpolymer interactions: (a) solution aggregates; (b) adsorbed aggregates on the graphite surface; (c) polymer networks produced on graphite after rinsing out the SDS surfactant. The structures are predominantly aligned in one of three preferential directions. (It has been shown from NMR studies that the EHEC is situated just inside the micellar surface and there is no EHEC within the core of the micelle.25)
complexes, were completely different from the structures from the purely adsorbed EHEC. What was surprising was that the EHEC, which remained adsorbed on the graphite after rinsing out the SDS from the polymersurfactant aggregates, showed chains aligned in similar structures as observed for the pure SDS. In fact, the images show highly ordered structures at the graphite surface. We also observe that the EHEC image produced from the
Templating of Ethyl(hydroxyethyl)cellulose
mixture 10 ppm EHEC-5 mM SDS has a different organized structure from the 10 ppm EHEC-20 mM SDS. At 10 ppm EHEC-5 mM SDS, a highly networked aggregates with minimal separation between the individual polymer strands and the formation of a more complete film by the larger networks can be seen. Yet generally, in both cases, the alignment of the polymer is similar to that of the hemicylidrical SDS structures, which are formed along the symmetrical axis of the graphite. But it is also of interest to note that the spacing of the aligned polymer strands were observed to be greater than that of the adsorbed SDS aggregates and we have used section analysis to determine the EHEC strand thickness. In Figure 4a (10 ppm EHEC-5mM SDS; 3 µm scale) an average thickness value of about 40-70 nm was found, whereas in Figure 5 (10 ppm EHEC-20 mM SDS; 3 µm scale) about 30-60 nm was found. The structuring of the adsorbed polymer was also evident in the images of the ruptured film, where equilateral triangles are visible. These experiments were repeated several times and appeared to be quite reproducible. This alignment of the polymer chains can be explained by the confinement of the EHEC by the micellar structure, and when the SDS was washed away, the aggregated polymer molecules appeared to be replicate the micellar arrangement of the SDS. The images show the adsorbed polymer resembles structured strands with angles of 60 and 120˚ relative to each other. These are the same angles as observed for pure SDS adsorbed as hemicylindrical aggregates on the graphite. In Figure 6a-c, schematic representations of the polymer-surfactant aggregates in bulk solution, the adsorption step, and the adsorbed polymer structures after rinsing out the SDS are shown. It would also appear from these results that the concentration of SDS plays an important role in defining the structure. At the higher surfactant concentration (20 mM SDS), one would anticipate a increase in clustering
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of SDS along individual polymer chains and an increase in SDS micellar species in bulk solution. Under these circumstances, as the polymer becomes more saturated with SDS, the hydrophobic nature of the polymer backbone will be altered. The change in configuration of the adsorbed polymer-surfactant complex could to some extent account for the differences in EHEC structures produced on the graphite after rinsing out the SDS and drying. A further insight into the difference in EHEC structures on graphite can be obtained from data of binding isotherms of SDS on EHEC as reported by Singh and Nilsson.23 In this study, the images produced for EHEC/SDS sample containing 5 mM SDS correspond to the region of binding isotherm where binding increases steeply due to an increase in the number of self-assembly of bound surfactant, whereas the images produced at 20 mM SDS correspond to the saturation region beyond the cmc for pure SDS system (8 mM). As the SDS concentration increases, the bound surfactant causes the EHEC molecules to be stretched with adsorption of hemicylindricallike aggregates occurring on the surface. Also, in the higher SDS concentration regions, the concentration of micelles in bulk solution may increase. As an alternative mechanism, it may be suggested that, at 20 mM SDS, competitive adsorption of both the SDS/EHEC aggregates and the regular SDS micelles could occur which may also account for the increase in spacing between EHEC strands on the graphite. Acknowledgment. This research was partly financed by a Swedish Technical Foundation (TFR) and the Norwegian Research Academy (Norfa), which provided Ph.D. funding for J.D. We also thank the Royal Swedish Academy of Sciences for providing a research grant to Dr. Tanya P. Goloub. LA011369K
