Controlling the Adsorption of Single Poly(styrenesulfonate) Sodium on

Publication Date (Web): September 13, 2001 ... Abstract. Densely packed amino-terminated monolayers of high degree of order were formed by self-assemb...
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Controlling the Adsorption of Single Poly(styrenesulfonate) Sodium on NH3+-Modified Gold Surfaces on a Molecular Scale M. Zhu, M. Schneider, G. Papastavrou, S. Akari,* and H. Mo¨hwald Max-Planck Institute of Colloids and Interfaces, 14424 Potsdam, Germany Received November 17, 2000. In Final Form: March 8, 2001 Densely packed amino-terminated monolayers of high degree of order were formed by self-assembly of amino-terminated alkanethiols. Negatively charged poly(styrenesulfonate) (PSS) molecules were electrostatically adsorbed on the NH3+ surface. The density of single PSS molecules on the surface was regulated by carefully controlling its concentration as well as adjusting the time in the dilute solution. Dynamic force microscopy in “tapping mode” was applied to map PSS polymers with molecular resolution. The lateral dimensions (15-25 nm) and heights (0.5-1.0 nm) of the adsorbed PSS macromolecules are in fair agreement with the theoretical diameters and the experimental estimations. The average area, volume, and charge density of single PSS patches absorbed on a positively charged surface is 378 nm2, 122 nm3, and 0.90e-/ nm2, respectively. The structure of adsorbed PSS polymers was discussed down to the molecular scale. The observations support the patch charge model of flocculants.

Introduction The adsorption of polyelectrolytes for surface modification is of scientific and practical interest.1-3 It allows the combination of electrostatic interactions and macromolecular dynamics on solid-liquid interfaces. The structure and amount of adsorbed polyelectrolyte chains play crucial roles concerning their utilization as flocculation inducers, colloidal stabilizer, and viscosifiers. A range of experimental technologies, such as optical reflection and scattering techniques,4-6 electrochemical and quartz crystal microbalance (QCM),7-9 neutron reflectivity,10 X-ray reflectivity,11 hydrodynamic techniques,12-14 and electron microscopy15,16 have been used to characterize the adsorption of polyelectrolytes. On a molecular scale, the scanning force microscopy (SFM) images with single polymers present a direct microscopic insight into the adsorption process. The technique gives individual results, not only an average result.17 (1) Kawaguchi, M. Adv. Colloid Interface Sci 1990, 32, 1. (2) Kawaguchi, M.; Takahashi, A. Adv. Colloid Interface Sci. 1992, 37, 219. (3) Joanny, J. F.; Castelnovo, M.; Netz, R. J. Phys.: Condens. Matter 2000, 12 (8A), A1. (4) Kawaguchi, M.; Saito, W.; Kato, T. Macromolecules 1994, 27, 5882. (5) Ladam, G.; Schaad, P.; Voegel, J. C.; Schaaf, P.; Decher, G.; Cuisinier, F. Langmuir 2000, 16, 1249. (6) Caruso, F.; Lichtenfeld, H.; Donath, E.; Mo¨hwald, H. Macromolecules 1999, 32, 2317. (7) Serizawa, T.; Kamimura, S.; Akashi, M. Colloid Surf., A 2000, 164, 237. (8) Harris, J. J.; Bruening, M. L. Langmuir 2000, 16, 2006. (9) Serizawa, T.; Takeshita, H.; Akashi, M. Langmuir 1998, 14, 4088. (10) Losche, M.; Schmitt, J.; Decher, G.; Bouwman, W. G.; Kjaer, K. Macromolecules 1998, 31, 8893. (11) Belyaev, V. V.; Tolstikhina, A. L.; Stepina, N. D.; Kayushina, R. L. Cryst. Rep. 1998, 43, 124. (12) Balastre, M.; Persello, J.; Foissy, A.; Argillier, J. F. J. Colloid Interface Sci. 1999, 219, 155. (13) Kawaguchi, M.; Ryo, Y.; Hada, T. Langmuir, 1991, 7, 1340. (14) Kawaguchi, M.; Yamauchi, T.; Ohkubo, A.; Kato, T. Langmuir 1997, 13, 4770. (15) He, J. A.; Valluzzi, R.; Yang, K.; Dolukhanyan, T.; Sung, C. M.; Kumar, J.; Tripathy, S. K.; Samuelson, L.; Balogh, L.; Tomalia, D. A. Chem. Mater. 1999, 11, 3268. (16) Lvov, Y.; Ariga, K.; Onda, M.; Ichinose, I.; Kunitake, T. Colloids Surf., A 1999, 146 (1-3), 337.

For the observation of the absorption of single polyelectrolytes on the surface with the SFM, it is necessary to prepare a molecularly flat, as well as highly charged surface to prevent aggregation. In previous work, imaging of single poly(ethylene imine) (PEI) molecules absorbed on mica or negatively charged polystyrene latexes was carried out using conventional SFM and chemical force microscopy.18-23 The present study deals extensively with the absorption behavior of negatively charged single polyelectrolytes, poly(styrenesulfonate) (PSS). For this purpose, molecularly flat and positively charged surfaces are prepared for the adsorption of single PSS polymers via self-assembly. Self-assembly provides the basis of the procedure to organize molecules in order to form densely packed monolayers of a high degree of order. Long-chain nalkanethiols with rich ω-terminated functional groups at solid surfaces allow the fabrication of different interfaces with well-defined composition.24-26 NH2-terminated alkanethiols are important due to their surface properties, such as wettability and ionization behavior. Especially their pH dependence can be used as a parameter for the preparation of surfaces of optimized properties. In addition, NH2-terminated alkanethiols can be employed to immobilize biological molecules, such as DNA and proteins onto the monolayer surface by covalent attachment.27-33 (17) Grim, P. C. M. Direct view of thin polymer films with scanning force microscopy, 87-100. (18) Akari, S.; Schrepp, W.; Horn, D. Langmuir 1996, 12, 857. (19) Akari, S.; Schrepp, W.; Horn, D. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 1014. (20) Pfau, A.; Schrepp, W.; Horn, D. Langmuir 1999, 15, 3219. (21) Regenbrecht, M.; Akari, S.; Forster, S.; Netz, R. R.; Mo¨hwald, H. Nanotechnology 1999, 10, 434. (22) Regenbrecht, M.; Akari, S.; Forster, S.; Mo¨hwald, H. J. Phys. Chem., B 1999, 103, 6669. (23) Regenbrecht, M.; Akari, S.; Forster, S.; Mo¨hwald, H. Surf. Interface Anal. 1999, 27, 418. (24) Prime, K. L.; Whitesides, G. M. Science 1991, 252, 1164. (25) Troughton, E. B.; Bain, C. D.; Whitesides, G. M.; Nuzzo, R. G.; Allara, D. L.; Porter, M. D. Langmuir 1988, 4, 365. (26) Ha¨usling, L.; Ringsdorf, H.; Schmitt, F. J.; Knoll, W. Langmuir 1992, 8, 1247. (27) Sprik, M.; Delamarche, E.; Michel, B.; Ro¨thlisberger, U.; Klein, M. L.; Wolf, H.; Ringsdorf, H. Langmuir 1994, 10, 4116.

10.1021/la001605b CCC: $20.00 © 2001 American Chemical Society Published on Web 09/13/2001

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In this work, we report on the synthesis of NH2terminated alkanethiols of long chains and the detailed investigations of the structure of the corresponding SAMs on gold surfaces. Contact angle and reflection absorption infrared spectroscopy (RAIRS) methods were employed to characterize the SAMs. By variation of the pH, NH3+modified and molecularly flat surfaces were obtained. PSS molecules were absorbed on this surface by carefully controlling the concentration and absorption times from its dilute solution. Single PSS polymers were observed with the dynamic mode of scanning force microscopy. Experiment Chemicals. Milli-Q water was used for all experiments. Poly(styrenesulfonate) sodium salt (PSS, Mw ≈ 70 000) was obtained from Aldrich Chemical Co. 11-Bromoundecanate methyl ester and was used as received from Aldrich. Synthesis and Characteristic of Alkanethiols. The alkanethiols were prepared via the nucleophilic displacement of bromide with an excess of thiourea and hydrolyzed with NaOH.25,34-37 Amides were generated from the reaction of an acyl chloride (prepare by the thionyl chloride) with an excess of ammonia.37 After a milder reduction of amide with NaBH4, respective amines were obtained. Synthesis of HS(CH2)11CONH2: Preparation of HS(CH2)11COOH. A 1.1 g (4.0 mmol/L) portion of Br(CH2)11COOCH3 was mixed with thiourea (0.35 g, 8 mmol/L) in 90% ethanol/water, under reflux for 4 h. After addition of 0.40 g (10 mmol/L) of NaOH and reflux for 2 h, ethanol was removed. The crude product was acidified with 6 N sulforic acid and then extracted from the aqueous phase with diethyl ether. The resulting product was recrystallized from ethanol. The resulting yield is 65%. 1H NMR (400 MHz, CDCl3): δ (ppm) 2.60 (t, 2H, CH2S), 2.30 (m, 2H, CH2COOH), 1.61 (m, 4H, CH2CH2SH, CH2CH2COOH), 1.201.40 (m, 14H, CH2). Anal. Calcd (found) for C12H24O2S (232.39 g/mol): C, 62.02 (61.11); H, 10.41(10.62); S, 13.80(14.11). HS(CH2)11CONH2. A 0.78 mL (10.9 mol) portion of redistilled thionyl chloride was added dropwise to 1.0 g (4.8 mmol) of HS(CH2)11COOH slowly. The solution was stirred for 2 h at reflux. The obtained HS(CH2)11COCl was added dropwise to a vigorously stirred, ice-cooled solution of concentrated ammonia (20 mL). The amide was recrystallized from ethanol. The resulting yield is 90%. The product was purified with paper chromatography using silica gel (60 mesh), with the developing solvent of 2% methanol in chloroform. 1H NMR (400 MHz, CDCl3): δ (ppm) 5.43-5.37 (s,2H, NH2), 2.68 (t, 2H, CH2SH), 2.22 (t, 2H, CH2CONH2), 1.67 (m, 2H, CH2CH2CONH2), 1.62 (m, 2H, CH2CH2SH), 1.20-1.40 (m, 14H, CH2). Anal. Calcd (found) for C12H25NOS (231.40 g/mol): C, 62.29 (61.60); H, 10.89 (11.10); N, 6.06 (5.25); S, 13.85 (14.27). Synthesis of HS(CH2)12NH2. NaBH4 (1.89 g, 50 mmol) was mixed with HS(CH2)11CONH2 (2.31 g, 10 mmol/L) in dioxane (20 mL). Acetic acid (3.0 g, 50 mmol/L) in dioxane (10 mL) was added slowly to the mixture over a period of 10 min at 10 °C. After reflux for 5 h, the reaction mixture was concentrated to dryness. The residue was recrystallized from ethanol to give the product. The resulting yield is 70%. The product was purified by paper chromatography using silica gel (60 mesh), with the developing solvent of 20:80 ethyl acetate/chloroform. 1H NMR (400 MHz, (28) Okahata, Y.; Matsuura, K.; Ito, K.; Ebara, Y. Langmuir 1996, 12, 1023. (29) Bain, C. D.; Evall, J.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7155. (30) Zhang, H. L.; Zhang, H.; Zhang, J. Liu, Z. F.; Li, H. J. Colloid Interface Sci 1999, 214, 46. (31) Sayre, C. N.; Collard, D. M. Langmuir 1997, 13, 714. (32) Kind, H.; Bittner, A. M.; Cavalleri, O.; Kern, K.; Greber, T. J. Phys. Chem. B 1998, 102, 7582. (33) Brockman, J. M.; Frutos, A. G.; Corn, R. M. J. Am. Chem. Soc. 1999, 121, 8044. (34) Okahata, Y.; Matsuura, K.; Ito, K.; Ebara, Y. Langmuir 1996, 12, 1023. (35) Tien, J.; Terfort, A.; Whitesides, G. M. Langmuir 1997, 13, 5349. (36) Miller, C.; Cuendet, P.; Gra¨tzel, M. J. Phys. Chem. 1991, 95, 887. (37) Dete, M. K. J. Appl. Phys. 1984, 55, 3354.

Zhu et al. CDCl3): δ (ppm) 2.81 (m, 2H, CH2NH2), 2.68 (t,2H,CH2SH),1.66 (m,2H, CH2CH2SH),1.61 (m, 2H, CH2CH2NH2),1.26-1.31 (m,16H, CH2). Anal. Calcd (found) for C12H27NS (217.42 g/mol): C, 66.36 (66.49); H, 12.44(12.71); N, 6.45 (6.23); S, 14.75(15.47). Preparation of Gold Substrates. Gold substrates were prepared by thermal evaporation in a vacuum of 1.0 × 10-6 mbar. Approximately 100 nm of gold was evaporated onto freshly cleaved mica for SFM measurements. Gold was also evaporated onto silicon wafers for contact angle measurement and onto glass slides for FTIR measurements. The wafer and glass slide were cleaned with 1:1:5 ammonium/ hydrogen peroxide/water at 7580 °C for 10 min. Then slides were rinsed with distilled water and dried by N2 gas before use. The substrate must be used in contact angle measurement immediately after coating with gold. Preparation of Charged Surfaces and Samples for SFM. Self-assembly monolayers were formed by immersing gold substrates into a 1.0 mM solution of HS(CH2)12NH2 in ethanol. The concentrations of thiol varied from 0.2 to 1.0 mmol/L for kinetic measurement. Derivatization with HS(CH2)12NH3+Clyielded positively charged surfaces. PSS polymers were adsorbed onto the surfaces under diffusion control from aqueous solution. NH3+-modified substrate was incubated in the PSS solution for 5 s, rinsed with water, and finally dried in air at room temperature. The concentration of PSS in solutions varied from 5.0 × 10-7 to 1.0 × 10-4mol/L (expressed in terms of monometric units). In some cases, sodium chloride was added to adjust ionic strength and buffer solution to adjust pH values. Contact Angle Measurements. The advancing contact angle, θa, and receding contact angles, θr, were measured with Millipore water as a probe liquid on a contact angle microscope (Kruess, Euromex Arnhem, Holland). A drop of 5 µL was placed on the monolayer. Measurements were performed immediately after monolayer preparation, since the surface with NH2 group is sensitive to contaminants in air. The contact angle measurements were repeated 5 times on different locations. These values were then averaged. SFM Measurements. SFM imaging of single PSS molecules was achieved by a commercially available scanning force microscope (Nanoscope Multimode IIIa, Digital Instruments, Santa Barbara). Measurements were carried out in TappingMode using silicon cantilevers (42 N/m, Olympus optical Co. Ltd., Japan) under ambient conditions. The resonance frequencies of tips are in the range of 250-280 kHz.

Results and Discussion Formation of Self-Assembly Monolayers of Alkanethiol on Gold Surfaces and Preparation of a Molecularly Flat and Positively Charged Substrate. For the SFM observations of single polymers, the roughness of the substrate must be less than the size of the structure under investigation. Mica is usually used because of its atomically flat surface. Negatively charged mica is a natural substrate for the adsorption of positively charged polyelectrolytes, such as PEI. However, it is hard to attach negatively charged polyelectrolytes, for example PSS, on a negatively charged surface because of electrostatic repulsive forces. Hence, the substrate should be chemically modified to be oppositely charged and still be smooth. In this work, a gold film was evaporated on mica followed by the self-assembly of amine-terminated alkanethiols, as indicated in Figure 1. Alkanethiol HS(CH2)12NH2 was synthesized with the purpose of formation of SAMs on the surface. The monolayer formed was characterized with reflectionabsorption infrared spectroscopy (RAIRS) and contact angle measurements. RAIRS were measured by reflection of p-polarized beam at a near-glancing angle of incidence of 86° (Figure 2). The major absorption peaks roughly match those observed for the pure sulfides. Inspection of the data in the high-frequency region revealed that d+ and d- absorptions are sharp and centered at 2850 cm-1 (CH2 symmetric stretching mode) and 2920 cm-1 (CH2 asymmetric stretching mode) in spectra of both deriva-

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Figure 1. Schematic illustration of the experimental configuration.

Figure 3. Contact angle titration curves of buffer solution on SAM of HS(CH2)12NH2 (b) and [HS(CH2)11COOH]2 (9).

Figure 2. IR spectra of C-H stretching modes in the highfrequency region of HS(CH2)12NH2: (a) transmission spectrum of bulk materials in KBr pellets; (b) RAIRS of SAM on Au.

tives. In addition, the spectra show variations in peak intensities, shapes, and positions. The values of the adsorption are slightly lower than those of pure crystalline compounds. This suggests that the monolayer is in a relatively ordered state. At the same time, individual molecules are more loosely packed in the monolayers than in the pure solid phase. The assignments for the observed adsorptions are shown in Figure 2. The twist and tilt angles of the alkane chains can be calculated from RAIRS. The intensity of the νa(CH2) peak is sensitive to the orientation of the alkane chains. In well-packed monolayers alkane chains are of all-trans conformation and perpendicular to the surface. Hence the νa(CH2) peak is very small. However in disordered and/or titled monolayers it can be quite intense. In this work, the orientation angles were estimated from the νa(CH2) peak with a slight correction of Dete38 and Zhang’s30 method. Oriented and densely packed monolayers were formed with the twist angles of 53° and the tilt angles of 30° for HS(CH2)12NH2. The monolayers of alkanethiols with polar end groups on gold could be made hydrophilic.29,33,39 The wettability (38) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558.

was examined by contact angle measurements. The exposed polar surface groups were susceptible to adsorption of contaminant species from the ambient environment. The advancing contact angle is 40 ( 1° and receding angle 30 ( 2° for HS(CH2)12NH2. The values are smaller than those of alkyl sulfides, indicating that at most few hydrophobic groups are located at the outer surface. Therefore very tightly packed monolayers of alkyl chains could be formed on gold. The monolayers in the study are quite hydrophilic and uniformly wettable due to its terminal amine. It is expected that the wettability of monolayers with NH2 and COOH groups depend strongly on pH. This fact emphasizes the potential application of pH-sensitive surfaces. Charged interfaces could be produced from the end group (NH3+ or COO-) of monolayers in acidic or basic solution. However, the structure and orientation of monolayers could also be destroyed due to strong repulsive forces between charged groups. Then the wettability of the interface would clearly change. The contact angles with drops of different pH were examined by plotting the advancing contact angle, θa, of buffered water as a function of pH, as shown in Figure 3. A transition from a more hydrophobic state at low pH to a more hydrophilic state at high pH was observed. This indicates a conversion of noncharged COOH groups to negative charged COOgroups as the drop becomes more basic. This fact is consistent with the result in the literature.25,41 In the case of HS(CH2)12NH2, the surface is more hydrophilic at low pH as expected.41-43 It is more hydrophobic at high pH, which suggests a transition of positively charged NH3+ group to neural NH2 group. We thus have a wellconstructed positively charged and molecularly flat surface. The kinetics of the monolayer formation was studied carefully in this work via time-dependent contact angle measurements (Figure 4). The equilibrium times of monolayer formation depend on the concentrations of the solutions. The wettability becomes constant after 60 min (39) Bain, C. D.; Whitesides, G. M. Angew. Chem., Int. Ed. Engl. 1989, 28, 506. (40) Laibinis, P. E.; Whitesides, G. M. J. Am. Chem. Soc. 1992, 114, 1990. (41) Whitesides, G. M.; Laibinis, P. E. Langmuir 1990, 6, 87. (42) Lee, T. R.; Care, R. I.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 741. (43) Yu, H. Z.; Zhao, J. W.; Wang, Y. Q.; Cai, S. M.; Liu, Z. F. J. Electroanal. Chem. 1997, 438, 221.

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be in an expanded configuration in a diluted salt solution, and the radius of gyration is

Rg ) RRgθ ) (KMwa/KθMw1/2)1/2.43Rgθ With the data for K and a at various ionic strengths in the handbook,50 Rg for PSS with Mw ) 70 000 is 12.2 nm in 0.01 M NaCl solution and 13.5 nm in 0.005 M solution, respectively. A patch area, Ap, could thus be estimated to be 467- 572 nm2 for a spherical patch, assuming that the patch is consisting of only one absorbed PSS molecule projected on the surface. As an alternative, one might assume a model where a globular polymer is adsorbing on the surface and spreading there to form a pancake. Assuming the 3D density to remain constant on adsorption and an adsorbate thickness of 1 nm, a molecular diameter, one may estimate the projected area A2D

Figure 4. Kinetics of monolayer formation from 0.2 mmol/L (1) or 1.0 mmol/L (2) HS(CH2)12NH2 ethanol solution by contact angle measurements.

in a concentration of 0.2 mmol/L and 30 min in that of 1.0 mmol/L. The monolayer seems to be formed mostly within 30 and 5 min, respectively. This is in agreement with the results in the literature.25,29,44,45 But it does not contradict results indicating further changes over days. Further rearrangements of the adsorbed thiols could be explored by SFM imaging.45-48 The SFM measurements on HS(CH2)12NH2 SAMs were performed after incubating the gold substrate in the thiol solutions for 10 min, 1 h, 2 h, 2.5 h, and 3 h. The theoretical film thickness of HS(CH2)12NH2 monolayers is 1.8 nm. It was observed that only 20% of the areas were covered by the monolayer homogeneously after 10 min of inversion with domains of a lateral dimension of about 50 × 50 nm2. It was increasing to 500 × 500 nm2 after 1 h to cover 50% of the surface. The result in Figure 5a indicates that over 90% of the gold surface is covered with the condensed phase after immersion for 2.5 h. The domains are formed in the first 10 min, and coalesce after 2.5 h. A continuous phase with height corrugations of less than 0.25 nm is obtained. This indicates the successful preparation of a flat surface modified with NH2 groups for the further adsorption of PSS polymers. Estimations of Single Polymer Size Using the Freeze and Drying Method. Theoretical Estimations of PSS Molecular Size The size of the PSS molecules in solution can be calculated to provide an indication of the patch size formed as it is adsorbed on gold.49 The unperturbed radius of gyration, Rgθ, of a linear PSS molecule in aqueous 4.17 M NaCl solutions is

Rgθ ) 0.0173Mw1/2 For PSS of Mw ) 70 000, Rgθ is 4.6 nm. The molecule will (44) Xu, S.; Cruchon-Dupeyrat, S. J. N.; Gano, J. C.; Liu, G. Y.; Jennings, G. K.; Yong, T. H.; Laibinis, P. E. J. Chem. Phys. 1998, 108, 5002. (45) Scho¨nherr, H.; Ringsdorf, H. Langmuir 1996, 12, 3891. (46) Tamada, K.; Hara, M.; Sasabe, H.; Knoll, W. Langmuir 1997, 13, 1558. (47) Tamada, K.; Nagasawa, J.; Nakanishi, F.; Abe, K.; Hara, M.; Knoll, W.; Ishida, T.; Fukushima, H.; Miyashita, S.; Usui, T.; Koini, T.; Lee, T. R. Thin Solid Films 1998, 327-329, 150. (48) Tamada, K.; Nagasawa, J.; Nakanishi, F.; Abe, K. Langmuir 1998, 14, 3264. (49) Leong, Y. K. Colloid Polym. Sci. 1999, 277, 299.

4π θ 3 (R ) ) A2D(1 nm) 3 g With this, one estimates projected areas of 7560 and 10 300 nm2, respectively. Freeze and Drying Method. An experimental estimation of PSS size was performed as follows. The surface was prepared by dropping 5 µL of extremely dilute PSS solution (3.4 × 10-8 mol/L) onto 0.5 × 0.5 cm2 of bare mica and immediately treating with rapid freeze and drying technology described by Brant et al.51 The concentration of PSS molecules ensures that only few molecules are left on the surface of mica within the scanning range (normally 500 × 500 nm). This procedure avoids the formation of clusters. Section analyses of over 50 spots show spots with widths ranging from 15 to 20 nm and height from 0.5 to 0.8 nm. The average area of the single patch is 265 nm2. The value is reasonable as compared with the theoretical unperturbed radius in the salt solution. This indicates that the polymer does not spread after adsorption. However, it collapses on its own projected area as indicated by the low height. We cannot determine the “absolute” size of single PSS polymers only by the SFM method. A tip is not sharp enough for scanning particles of this order of size. On the other hand, inaccurate force setting will cause distortion of the height of the spots.52 Even now, the spot dimensions measured with optimum range of parameter are within an order of the actual values. The Adsorption of Single PSS Polymers on the NH3+-Modified Surface. The amine group modified monolayers can be served as surfaces for the adsorption of PSS polymers. Figure 5a shows an image of the HS(CH2)12NH2-modified surface of gold-coated mica. A height profile along the a-b axis shows a top-valley depth of the terrace of less than 0.25 nm and width of less than 7 nm. Surfaces with single PSS polymers were obtained from salt-free solution to avoid a possible salt crystallization. Panels b-f of Figure 5 show one sequence of SFM images after exposure of the NH3+-modified surface to PSS solutions. The concentration was varied from 1.4 × 10-7 to 1.4 × 10-5 mol/L, and the time varied from 5 s to 1 min. It is shown that PSS chains could be adsorbed dispersedly on the surface. Cross section analyses of images collected for all concentrations and immersing times of 5-20 s show (50) Kurata, M.; Tsunashima, Y.; Iwama, M.; Kamada, K. In Polymer handbook, 2nd ed.; Brandrup, J., Immergut, E. H., Eds.; Wiley Press: New York, 1989; p VII-16. (51) McIntire, T. M.; Brant, D. A. Int. J. Biol. Macromol. 1999, 26, 303. (52) Stipp, S. L. S. Langmuir 1996, 12, 1884.

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Figure 5. TM height pictures (500 nm × 500 nm for a-f, 1 µm × 1 µm for g) of PSS polymers absorbed on HS(CH2)12NH2-modified gold from aqueous solution. The Z range in the height image is 1 nm (a-d) and 3 nm (e-g), respectively. The concentrations and incubation times of PSS solutions are indicated in the figures.

a fairly narrow range of heights and widths of these spots (Figure 5b,c). The width ranges from 15 to 25 nm and the heights from 0.5 to 1.0 nm. No spots are less than 15 nm in width and 0.5 nm in height. It is consistent with the theoretical and experimental size of PSS chains as discussed above. It is also consistent with the results from electrophoretic measurements and light-scattering measurement.53-54 Therefore, it appears that single PSS chains are observed in Figure 5b and Figure 5c. Two or more individual PSS molecules could aggregate partially (Figure 5d-f) or completely (Figure 5g) leading to a fast increase of the size of the patch. Aggregation is observed when the concentration of PSS molecules is increased up to 1.4 × 10-5 mol/L or at immersing times from 20 s to 1 min. The aggregation might take place when incubating for a long time although in dilute solution (Figure 5d). Therefore we could conclude that PSS patches adsorb on positively charged surfaces if the concentrations and incubation times of individual polymers are critically controlled. Adsorbate Structure and Implication for the Flocculation Model. Normally PSS chains disperse in dilute solution, as shown in Figure 6a. Observations on poly(acrylic acid) (PAA) indicate that the molecules prefer either to be adsorbed onto the surface or to remain in solution in a coiled configuration.52 No obvious unfolding of polymers on the surface could be observed. It is known that coiled macromolecules are anchored at only few sites (Figure 6b) for the first seconds of attachment when they are absorbed on the solid-liquid surface from solution (Figure 6a).55 Once contact with the surface is established, (53) Cottet, H.; Gareil, P. J. Chromatogr., A 1997, 772, 369. (54) Pflau, A.; Schuch, W. To be published.

Figure 6. Schematic illustration of the adsorption process of PSS polymers on the charged surface from its aqueous solution: (a) coiled chains disperse in the solution separately; (b) coiled chains anchor on the surface during the first seconds of attachment; (c) coiled chains spread and rearrange on the charged surface, most of their segments were adsorbed on the surface by electrostatic forces to form the conformation of the pancake; (d) multiple layers are formed in the presence of excess sodium ions.

the macromolecules spread and rearrange on the positively charged surface while most of their segments are absorbed. (55) Stuart, C.; Klein, M. W.; Krausch, G.; Cosgrovet, T.; Everett, D. H.; Ottewill, R. H.; Shull, K. R.; Jones, R. A. L.; Zachmann, H. G.; Ryan, A. J. Faraday Discuss. 1994 (98), 231.

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The molecular size of PSS polymers in aqueous solution is compable with its size obtained by the freeze and drying method. In view of swelling of PSS polymers in the presence of water, we can assume that PSS adopts a flat and planar adsorption structure without loops and tails extended from the surface (Figure 6c). The adsorption of polymers on substrates has been investigated extensively.18-20 Usually, two models (a patch charge model and a bridging model) are used to describe the adsorption process. All of the facts above strongly support the patchlike adsorption structure on the positively charged surface. A layer-layer absorption (Figure 6d) was observed (not shown in this paper) when absorbed from 0.01 M NaCl solutions. We also determined the area and volume of single patches, Apatch and Vpatch, by bearing analysis and particle analysis with the standard software of our instrument. Apatch is in the range of 252-640 nm2, with the average of 378 nm2 after analyzing over 20 particles in the SFM height pictures. The value of Apatch here is a little larger than the results obtained from freeze and drying molecules and in reasonable accordance with the theoretical results. Vpatch is in the range of 85-165 nm3, with the average of 122 nm3. This is considerably less than expected in solution (4/3)π(Rgθ)3 ≈ 80 000 nm3, again supporting the collapse after adsorption. One of the important parameters for the flocculation process is the charge density. The charge density is calculated by taking the total charge of one PSS molecule if dissociated, that is 340 (340 monomers per molecule and one charge per monomer) and dividing it by Apatch. Therefore the charge density is about 0.90 e-/nm2 for full protonation and 0.63 e-/cm2 for 70% protonation. The charge density of the NH3+-modified surface can be calculated using Sprik et al.’s model and STM data of SAM of mercaptoundecylamine.27 A value of 5.9 e/nm2 was obtained for full protonation of the NH2 group. As compared with the charge density of PSS polymers, we

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can conclude that most of the sulfuric groups are supposed to be localized at the surface, whereas few of them form loops or tails. This also contributes to its pancake conformation of relatively low monomer density. Conclusion An oriented, densely packed and ordered monolayer could be formed with amino-terminated functionalized thiols on gold surfaces. This is verified from the data of contact angle and RAIRS measurements. The surfaces of SAMs are more hydrophilic with polar amino groups on the end of monolayer. The contact angle titration curves show that the HS(CH2)12NH2 layer presents a fairly good wettability at low pH when the amino-terminated alkanethiols are positively charged. This provides a molecularly flat and positively charged surface for SFM imaging of single PSS polymers. PSS can be absorbed firmly on the HS(CH2)12NH3+modified surface. We visualized directly the adsorption of single PSS polymers. One can carefully control molecular dispersions by changing the concentration and adsorption times from dilute solution to avoid aggregation. Lateral dimensions (15-25 nm) and heights (0.5-1.0 nm) of single PSS macromolecules are obtained from the SFM images. The molecular size is confirmed by theoretical calculations and experimental estimations. Our results support the patch charge model of flocculants after a careful analysis of the absorption structure and the charge density of PSS molecules. Acknowledgment. We thank the “Bundesministerium fu¨r Bildung und Forschung” for the financial support of the project 03CO291B12. We thank Frau Anne Heilege for SFM measurements, Henning Krass for FT-IR measurement, and Dr. Dirk Kurth for many useful and informative discussion. LA001605B