Polymer Bilayer Formation Due to Specific Interactions between β

Oct 22, 2004 - polymer associations can be classified in three types: (a) hydrophobic .... function of the surface separation, D, is related to the fr...
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Langmuir 2004, 20, 10449-10454

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Polymer Bilayer Formation Due to Specific Interactions between β-Cyclodextrin and Adamantane: A Surface Force Study Eva Blomberg,*,† Atte Kumpulainen,† Christelle David,‡ and Catherine Amiel‡ Department of Chemistry, Surface Chemistry, Royal Institute of Technology, Drottning Kristinas va¨ g 51, SE-100 44 Stockholm, Sweden, Institute of Surface Chemistry, Stockholm, Sweden, and Laboratoire de Recherche sur les Polyme` res, UMR C7581, CNRS, 2-8 rue H. Dunant, 94320 Thiais, France Received July 1, 2004. In Final Form: August 19, 2004 The purposes of this study are to utilize the interactions between an adamantane end-capped poly(ethylene oxide) (PEO) and a cationic polymer of β-cyclodextrin to build polymer bilayers on negatively charged surfaces, and to investigate the interactions between such layers. The association of this system in solution has been studied by rheology, light scattering, and fluorescence measurements. It was found that the adamantane-terminated PEO (PEO-Ad) mixed with the β-cyclodextrin polymer gives complexes where the interpolymer links are formed by specific inclusion of the adamantane groups in the β-cyclodextrin cavities. This results in a higher viscosity of the solution and growth of intermolecular clusters. The interactions between surfaces coated with a cationized β-cyclodextrin polymer across a water solution containing PEO-Ad polymers were studied by employing the interferometric surface force apparatus (SFA). In the first step, the interaction between mica surfaces coated with the cationized β-cyclodextrin polymer in pure water was investigated. It was found that the β-cyclodextrin polymer adsorbs onto mica and almost neutralizes the surface charge. The adsorbed layers of the β-cyclodextrin polymer are rather compact, with a layer thickness of about 60 Å (30 Å per surface). Upon separation, a very weak attractive force is observed. The β-cyclodextrin solution was then diluted by pure water by a factor of 3000 and a PEO-Ad polymer was introduced into the solution. Two different architectures of the PEO-Ad polymer were investigated: a four-arm structure and a linear structure. After the adsorption of the PEO polymer onto the β-cyclodextrin layer reached equilibrium, the forces were measured again. It was found that the weak repulsive longrange force had disappeared and an attractive force caused the surfaces to jump into contact, and that the compressed layer thickness had increased. The attractive force is interpreted as being due to a specific recognition between the hydrophobic adamantane groups on the PEO-Ad polymer and the hydrophobic cavity in the β-cyclodextrin molecules. Furthermore, the attractive force observed on separation has increased significantly, which is a further indication of a specific interaction between the β-cyclodextrin polymer and the adamantane groups.

Introduction Supramolecular architectures involving polymers constitute an area of current interest and development due to their practical applications in solutions (associative thickeners,1 stimuli responsive gels,2 nanoparticles,3 gene carriers4) or at interfaces (biosensing devices5,6). The driving interaction mechanisms leading to polymerpolymer associations can be classified in three types: (a) hydrophobic interactions occurring in aqueous systems of amphiphilic copolymers;1-3 (b) electrostatic interactions between polymers of opposite charges, of prime importance for the buildup of well-defined polyelectrolyte multilayers at interfaces;7,8 and (c) interactions involving a molecular * Corresponding author. E-mail: [email protected]. † Royal Institute of Technology and Institute of Surface Chemistry. ‡ UMR C7581, CNRS. (1) Polymers as rheology modifiers; Glass, J. E., Ed.; American Chemical Society: Washington, DC, 1991; Vol. 462. (2) Porcar, I.; Sergeot, P.; Tribet, C. In Stimuli responsive water soluble and amphiphilic polymers; McCormick, C., Ed.; ACS Symposium Series 780; American Chemical Society: Washington, DC, 2000; p 82. (3) Akiyashi, K.; Deguchi, N.; Moriguchi, N.; Yamaguchi, S.; Sunamoto, J. Macromolecules 1993, 26, 3062. (4) Pollard, H.; Remy, J.-S.; Loussouarn, G.; Demolombe, S.; Behr, J.-P.; Escande, D. J. Biol. Chem. 1998, 273, 7507. (5) Armstrong, F. A.; Wilson, G. S. Electrochim. Acta 2000, 45, 2623. (6) Fragoso, A.; Caballero, J.; Almirall, E.; Villalonga, R.; Cao, R. Langmuir 2002, 18, 5051.

recognition process such as hydrogen bond interactions in proteins9,10 or inclusion complexes with β-cyclodextrin compounds.11-17 Cyclodextrins are cyclic oligomers of anhydroglucose with the shape of a truncated cone that has a hydrophilic exterior and a less polar cavity in the center. R-, β-, and γ-cyclodextrins consist respectively of six, seven, or eight glucose units. In aqueous solutions, cyclodextrins form inclusion complexes with substances containing lipophilic groups, provided that the shape of the hydrophobic group fits in the cavity.18,19 For instance, the adamantane group precisely fits into the β-cyclodextrin cavity.20 (7) Decher, G. In Comprehensive Supramolecular Chemistry: Templating, Self-Assembly and Self-Organisation; Sauvage, J.-P., Hosseini, M. W., Eds.; Pergamon Press: Oxford, 1996; Vol. 9, p 507. (8) Decher, G. Science 1997, 277, 1232. (9) Myers, J. K.; Pace, C. N. Biophys. J. 1996, 71, 2033. (10) Tareste, D.; Pincet, F.; Perez, E.; Rickling, S.; Miokowski, C.; Lebeau, L. Biophys. J. 2002, 83, 3675. (11) Okumura, H.; Okada, M.; Kawaguchi, Y.; Harada, A. Macromolecules 2000, 33, 4297. (12) Lu, J.; Mirau, P. A.; Shin, D. I.; Nojima, S.; Tonelli, A. E. Macromol. Chem. Phys. 2002, 203, 71. (13) Rusa, C. C.; Luca, C.; Tonelli, A. E. Macromolecules 2001, 34, 1318. (14) Sandier, A.; Brown, W.; Mays, H.; Amiel, C. Langmuir 2000, 16, 1634. (15) Amiel, C.; Se´bille, B. Adv. Colloid Interface Sci. 1999, 79, 105. (16) Auze´ly-Velty, R. M. Macromolecules 2002, 35, 7955. (17) Islam, M. F.; Jenkins, R. D.; Bassett, D. R.; Lau, W.; Ou-Yang, H. D. Macromolecules 2000, 33, 2480.

10.1021/la048370e CCC: $27.50 © 2004 American Chemical Society Published on Web 10/22/2004

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Figure 1. Schematic structure of poly-β-CDN+.

Mixing a guest polymer containing several β-cyclodextrin units (β-cyclodextrin polymer) and a host polymer containing several lipophilic groups such as adamantane (e.g., an end-capped PEO-adamantane copolymer) in aqueous media leads to macromolecular assemblies whose structural and dynamic solution properties have been studied in several cases.14-16,21-24 In particular, neutral amphiphilic copolymers of different natures (hydrophobically modified dextran, PEO) and architectures (telechelic or comblike) have been used, and it was shown that these systems are controlled by a main key parameter, related to the strength of the interaction between the host and the guest polymer. When the number of hydrophobic groups per amphiphilic copolymer chain is higher than a critical value, which is close to 3, the polymer-polymer interactions are strong enough to lead to associative phase separation. In the monophasic regions of the ternary phase diagrams, the structure and size of the soluble polymerpolymer complexes may vary as a function of the stoichiometry of the mixtures and total concentration. In this paper, well-defined architectures of the amphiphilic copolymers have been chosen: linear or four-arm starshaped poly(ethylene oxide) terminated by adamantyl groups. Phase diagrams, rheology, and light scattering experiments give evidence of the polymer-polymer associations in solution. The affinity of the cyclodextrin cavities for the adamantane moieties has also been studied by solution fluorescence measurements. (18) Saenger, W. In Inclusion Compounds; Atwood, J., Davies, J., MacNicol, D., Eds.; Academic Press: London, 1984; Vol. 2, Chapter 8. (19) Cyclodextrin Technology; Szejtli, J., Davied, J. E. D., Eds.; Kluwer Academic Publishers: Dordrecht, 1988. (20) Eftink, M. R.; Andy, M. L.; Bystrom, K.; Perlmutter, H. D.; Kristol, D. S. J. Am. Chem. Soc. 1989, 111, 6765. (21) Amiel, C.; Sebille, B. J. Inclusion Phenom. Mol. Recognit. Chem. 1996, 25, 61. (22) Amiel, C.; Moine, L.; Sandier, A.; Brown, W.; David, C. In Stimuli responsive water soluble and amphiphilic polymers; McCormick, C., Ed.; ACS Symposium Series 780; American Chemical Society: Washington, DC, 2000; p 58. (23) Weickenmeir, M.; Wenz, G.; Huff, J. Macromol. Rapid. Commun. 1997, 18, 117. (24) Renard, E.; Barnathan, G.; Deratani, A.; Se´bille, B. Macromol. Symp. 1997, 122, 229.

Molecular recognition due to specific interactions between a receptor and a ligand were measured for the first time with surface force apparatus (SFA) in the beginning of the 1990s.25 Thereafter there have been several studies of the specific receptor-ligand interactions, mainly the avidin-biotin system.26 However, only a few studies of the specific interaction between cyclodextrin and adamantane have been performed.27 The surface force apparatus has also been shown to be a powerful tool in the investigation of receptor-ligand interactions as well as in the elucidation of the interactions involved in the buildup of polyelectrolyte multilayers.28,29 In addition, one also obtains information on the structure of multilayers on the solid surface, as well as the stability and compressibility of the layers with respect to high compressive loads. Furthermore, with the SFA information about extension of loop and tails is also obtained.28,29 This is different compared to optical techniques (e.g., ellipsometry and surface plasmon resonance spectroscopy), which determine the thickness on the basis of the assumption that the layer is homogeneous and has constant optical properties throughout its thickness. Finally, the SFA has also been used to study how the adhesion between carboxylic layers is influenced by the divalent ions.30 In this study it was found that the adhesion between arachidic acid headgroups was increased about 2-fold in the presence of Cd2+. The purpose of the present study is to use the interferometric surface force apparatus to probe bilayer (25) Leckband, D. E.; Israelachvili, J. N.; Schmitt, F.-J.; Knoll, W. Science 1992, 255, 1419. (26) Zlatanova, J.; Lindsay, S. M.; Leuba, S. H. Prog. Biophys. Mol. Biol. 2000, 74, 37. (27) Auletta, T.; de Jong, M. R.; Mulder, A.; van Veggel, F. C. J. M.; Huskens, J.; Reinhoudt, D. N.; Zou, S.; Zapotoczny, S.; Scho¨nherr, H.; Vancso, G. J.; Kuipers, L. J. Am. Chem. Soc. 2004, 126, 1577. (28) Blomberg, E.; Poptoshev, E.; Claesson, P. M.; Caruso, F. Langmuir 2004, 20, 5432. (29) Kulcsa´r, A Ä .; Lavalle, P.; Voegel, J.-C.; Schaaf, P.; Ke´kicheff, P. Langmuir 2004, 20, 282. (30) Berg, J. M.; Claesson, P. M. Thin Solid Films 1989, 178, 261.

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formation based on specific interaction, and to elucidate the interactions between polymer bilayer coated surfaces. Experimental Section Materials. Synthesis of β-Cyclodextrin Polymers. Neutral β-cyclodextrin polymers (poly-β-CD) were prepared by polycondensation of β-cyclodextrin with epichlorohydrin. The synthesis and characterization have been described previously.24 The sample used in this study has a molecular weight 3.3 × 104 g/mol as determined by size exclusion chromatography in equivalent pullulan. Its β-cyclodextrin content is 65% w/w. Cationized β-cyclodextrin polymer was prepared by reacting poly-β-CD with 2,3-epoxypropyltrimethylammonium (Fluka). First, 8 g of polyβ-CD was added to an alkaline solution containing 0.8 g of sodium hydroxyl in 40 mL of water and mixed for 24 h at room temperature. An aqueous solution of the reactant (3.6 g of 2,3epoxypropyltrimethylammonium dissolved in 16 mL of water) was then added, and the reaction bath was heated at 50 °C for 24 h. The solution was then neutralized by addition of 6 N HCl until a pH value of 6 was reached. The polymer was purified by ultrafiltration on regenerated cellulose membrane, molecular weight cutoff 5000 Da. After freeze-drying, 7.95 g of poly-β-CDN+ was obtained. The ammonium group content on the copolymer was determined by potentiometric back-titration of the Clcounterions. It corresponds to an average of 0.6 positive charge per β-cyclodextrin unit. The structure of the poly-β-CDN+ is schematized in Figure 1. Modification of Poly(ethylene oxide)s. Linear poly(ethylene oxide), molecular weight Mw ) 5200, was purchased from Aldrich. Four-arm star poly(ethylene oxide), molecular weight Mw ) 21 600, was purchased from Shearwater Co. The hydrophobically modified PEO was obtained by reacting the terminal -OH groups with 1-adamantyl chloride (Aldrich, Saint Quentin Fallavier, France). The 1-adamantyl chloride excess was 14 for the linear polymer (PEO-2Ad) and 25 for the branched one (PEO-4Ad). The precursor polymer was previously dried by heating under vacuum at 40 °C overnight. The reactants were dissolved in dried 1,2dichloroethane. Dimethylaminopyridine (Aldrich) and pyridine (Aldrich) were also added. The reaction mixture was heated at 70 °C for 3 h. The polymer was then precipitated in anhydrous ether. To eliminate the pyridinium salts, the polymer was dissolved in hot anhydrous ethanol and precipitated at 4 °C. The yield of the modifications was about 90% w/w. The end group modification was close to 100%, as checked by 1H NMR spectroscopy. Methods. Interactions between β-cyclodextrin polymer coated surfaces were investigated in the absence and presence of PEOAd using the Mark-IV surface force apparatus. With this interferometric surface force technique the total force acting between two macroscopic molecularly smooth surfaces in a crossed cylinder configuration is measured as a function of surface separation.31,32 The resolution in distance determination is about 2 Å, while the detection limit of the force is about 10-7 N, corresponding to a normalized force of about 10 µN/m. The interaction force F between crossed cylinders normalized by the undeformed geometric mean radius of the surfaces, R, as a function of the surface separation, D, is related to the free energy of interaction per unit area, Gf, between two flat surfaces via the Derjaguin approximation:33

F(D) ) 2πGf(D) R

(1)

This equation is valid provided that R . D, which is the case in these experiments. The mica sheets were silvered on one side, and thereafter glued on to silica disks with the silvered side down. Adsorption and desorption of cationized β-cyclodextrin polymer were studied by adsorbing the positively charged polymer onto the negatively charged mica surfaces from a drop containing 20 ppm β-cyclo(31) Israelachvili, J. N.; Adams, G. E. J. Chem. Soc., Faraday Trans. 1 1978, 74, 975. (32) Parker, J. L.; Christenson, H. K.; Ninham, B. W. Rev. Sci. Instrum. 1989, 60, 3135. (33) Derjaguin, B. Kolloid-Z. 1934, 69, 155.

Figure 2. Specific viscosities of four-arm star PEO: (O) precursor PEO-4; (b) adamantane-terminated PEO-4Ad. dextrin polymer in pure water. The adsorption was allowed to proceed for about 1 h. The forces were then measured in the drop to obtain information on the interaction between mica surfaces across the β-cyclodextrin polymer solution and to verify the validity of the system after the dilution. In the next step, the measuring chamber was filled with pure water, which resulted in a dilution of the polymer solution by a factor of 3000, and the force was measured again.

Result and Discussion Interactions in Bulk Solution. PEO-2Ad and PEO4Ad. The adamantyl end groups of the modified PEO could be responsible for associative properties in solutions by forming hydrophobic microdomains. This behavior is usually observed with alkyl-terminated PEO.34-36 Such behavior could not be evidenced for the PEO-Ad as no hydrophobic microdomains were detected by pyrene fluorescence.14 Moreover, the viscosities of the modified polymers are always comparable to the viscosities of the precursors, as illustrated in Figure 2 for PEO-4Ad. Apparently the bulky and quite rigid structure of the adamantane groups prevents them from making microdomains with large aggregation numbers. Mixtures with Poly-β-CD. Mixing adamantyl-terminated polymer with poly-β-CD gives polymer complexes due to the affinity of the adamantyl groups for the β-cyclodextrin cavities. The complexation constants, determined by competitive fluorescence measurements or by a dialysis method, are in the range 5000-2000 L‚mol-1.15 This ensures that the majority of the possible links are established in solution between the two polymers when the host or the guest polymer concentration is larger than 2 g/L. One should keep in mind that the cooperative effects due to the multiple interactions between one poly-β-CD containing several cyclodextrin units and an adamantylterminated PEO containing two or four adamantyl groups per chain are not taken into account when these affinity constants are evaluated. Changing from linear polymer containing two adamantanes per chain to four-arm star poly(ethylene oxide) containing four adamantanes per chain leads to dramatic changes in the solution properties: the complexes are soluble in a large concentration range in the first case, whereas a biphasic domain appears at low concentration (total concentration lower than 7.5% w/w) in the second case. In the case of the soluble mixtures, the aggregate structures depend strongly on the stoichiometry. For instance, the aggregates are constituted of (34) Alami, E.; Rawiso, M.; Isel, F.; Beinert, W.; Binana-Limbele, W.; Francois, J. In Hydrophilic Polymers; Advances in Chemistry Series 248; American Chemical Society: Washington, DC.; p 343. (35) Francois, J. Prog. Org. Coat. 1994, 24, 67. (36) Benkhira, A.; Franta, E.; Rawiso, M.; Francois, J. Macromolecules 1994, 27, 3963.

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Figure 4. Normalized force as a function of surface separation between mica surfaces across water containing 20 ppm polyβ-CDN+ (circles). Squares correspond to the force measured between poly-β-CDN+-coated surfaces after dilution with water (by a factor of 3000). Filled symbols represent the force measured on approach, and unfilled symbols represent the force measured on separation.

Figure 3. (a) Viscosities of the aqueous mixtures PEO-4Ad + poly-β-CD at constant PEO-4Ad concentration (70 g/L) (O). The viscosities of the corresponding mixtures with unmodified PEO-4 are shown for comparison (b). (b) Relative viscosities of mixtures containing 70 g/L adamantane-terminated PEO and 35 g/L poly-β-CD compared to viscosities of the corresponding mixtures with unmodified PEO. Two linear PEO-2Ad polymers, molecular weights 6000 and 20 000, are reported in comparison with four-arm star PEO-4Ad polymer, molecular weight 20 000.

individual poly-β-CD chains decorated with PEO-2Ad chains when the adamantyl groups are in large excess compared to the cyclodextrin units. The stoichiometry of 1 adamantane for 1 cyclodextrin maximizes the number of possible links between the host and guest polymers, and it corresponds to the maximum viscosity for a given total polymer concentration. Increasing the total concentration leads to important thickening effects for PEO4Ad (Figure 3a), due to the increased connectivity of the system. Mixtures with linear polymers show much lower thickening properties (Figure 3b) with viscosity enhancements less than 10 times lower than in the previous case. This phenomenon is attributed to an increase of the percentage of effective links compared to the percentage of intramolecular links (not leading to increasing size of the aggregates) when the functionality of the chains is increased. Interactions at the Solid-Liquid Interface. Muscovite mica is a layered aluminosilicate mineral, where each sheet carries a negative charge due to isomorphous substitution of silicon for aluminum. This charge is in the crystal compensated by positively charged ions, mostly potassium, located between the sheets. The number of such ions present on a freshly cleaved mica surface is 2.1 × 1014 cm-2.37 Then such a surface is immersed in aqueous electrolyte solutions, and the initially surface bound ions are readily dissolved and partially replaced at the surface (37) Gaines, G. L.; Tabor, D. Nature 1956, 178, 1304.

by other ions present in solution. As a result of the dissolution of the surface ions, the mica acquires a net negative surface charge. The interactions between mica surfaces in dilute aqueous electrolyte solutions are well described by classical DLVO theory; i.e., the interaction is at large separation dominated by a strong repulsive double-layer force, whereas an attractive van der Waals force dominates at separations below about 4 nm.31,38,39 The height of the force barrier created by the interplay of double-layer and van der Waals forces in dilute aqueous electrolyte solution is typically on the order of 5000 µN/m, with the exact magnitude depending on ionic strength, pH, and type of ions present. The interaction between mica surfaces coated with a cationized β-cyclodextrin polymer, poly-β-CDN+ (Mw ) 33 000 g/mol), in pure water is illustrated in Figure 4. The long-range force is more than an order of magnitude lower than that observed in aqueous electrolyte solutions in the absence of polyelectrolyte.31,39 Hence, it is clear that under these conditions the poly-β-CDN+ polymer adsorbs onto the negatively charged mica surface, which results in a very weak repulsive long-range double-layer force. After dilution of the polymer solution about 3000 times, no repulsive electrostatic double-layer force was observed. This change in interaction as a result of dilution is due to very limited desorption, and we thus infer that prior to dilution the poly-β-CDN+ polymer very weakly overcompensated the mica surface charge. This is in line with theoretical predictions40 and experimental studies using a range of other linear highly charged polyelectrolytes.41-44 The adsorbed layers of poly-β-CDN+ are rather compact, with a rather small layer thickness of about 60 Å (30 Å per surface), but larger than that observed for more flexible highly charged cationic polyelectrolytes such as MAP(38) Israelachvili, J. N. Faraday Discuss. Chem. Soc. 1978, 65, 20. (39) Pashley, R. M. J. Colloid Interface Sci. 1981, 80, 153. (40) Fleer, G. J.; Cohen Stuart, M. A.; Scheutjens, J. M. H. M.; Cosgrove, T.; Vincent, B. Polymers at Interfaces; Chapman & Hall: London, 1993. (41) Dahlgren, M. A. G.; Waltermo, Å.; Blomberg, E.; Claesson, P. M.; Sjo¨stro¨m, L.; Åkesson, T.; Jo¨nsson, B. J. Phys. Chem. 1993, 97, 11769. (42) Dahlgren, M. A. G.; Claesson, P. M.; Audebert, R. J. Colloid Interface Sci. 1994, 166, 343. (43) Claesson, P. M.; Ninham, B. Langmuir 1992, 8, 1406. (44) De´dinaite´, A.; Ernstsson, M. J. Phys. Chem. B 2003, 107, 8181.

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Figure 5. Normalized force as a function of surface separation between mica surfaces coated with poly-β-CDN+ across a 10 ppm PEO-4Ad polymer solution (in water). Filled and unfilled symbols represent the force measured on approach and separation, respectively.

Figure 6. Normalized force as a function of surface separation between mica surfaces coated with poly-β-CDN+ across a 10 ppm PEO-2Ad polymer solution (in water). Filled and unfilled symbols represent the force measured on approach and separation, respectively.

TAC,41 PCMA,42 and chitosan.43,44 However, for branched polyelectrolytes it has been found that the layer becomes thicker when adsorbed onto an oppositely charged surface from low ionic strength solutions.45 Upon separation a very weak attractive force is observed. This is strikingly different compared to the strong adhesion forces observed between mica surfaces coated with other highly charged polyelectrolytes.41,42 The reason for this difference is the comparatively thick layer formed in the present case and the rather stiff structure of the polymer chain; both of these features counteract formation of polymer bridges when the surfaces are in close contact. The weak adhesion that is observed can well be due to van der Waals interactions. In the next step a four-arm adamantane end-capped PEO (Mw ) 20 000 g/mol) was introduced into the solution to a concentration of 10 ppm. After the adsorption of the PEO-4Ad polymer onto the poly-β-CDN+ layer reached equilibrium, the forces were measured again (Figure 5). No long-range electrostatic double-layer force was observed, whereas an attractive force caused the surfaces to jump into contact from about 280 to 140 Å, and the compressed layer thickness had increased to about 130 Å (65 Å per surface). Clearly the PEO-4Ad had adsorbed to the poly-β-CDN+ coated surfaces, resulting in the formation of a polymer bilayer. The attractive force observed upon approach is thus due to interactions between the outer layers of PEO-4Ad. Since PEO chains repel each other at room temperature, the attraction can be due to interactions between the adamantane groups or between adamantane and cyclodextrin. In light of the bulk solution studies, the most plausible interpretation is that the attraction is due to a specific recognition between the hydrophobic adamantane groups on the PEO polymer and the hydrophobic cavity in the β-cyclodextrin molecules. Furthermore, the attractive force observed on separation has increased significantly to about 1400 µN/m, which thus is interpreted as being due to specific interaction between the β-cyclodextrin and the adamantane groups. The surface separation from where the surfaces are detached (about 200 Å) is further out compared to the distance after the surfaces jump into contact (about 140 Å). We therefore note that the polymer layers are stretched

out before the force minimum is reached. This is probably due to stretching of the PEO chains. Measurements of specific interactions between immobilized oriented proteins, utilizing the surface force apparatus, was first reported by Leckband et al.46 In their study the specific interaction between streptavidin and biotin was measured, and it was observed that between two biotin surfaces coated with streptavidin an attractive minima was observed close to the hard wall contact between the surfaces. Upon separation an adhesive force of about 3500 µN/m was obtained. This was attributed to attractive forces between interdigitated streptavidin monolayers. The attraction observed was thus of the same order of magnitude as in our system, where we also have a symmetric system: in this case each surface is coated with a bilayer of polymers with the poly-β-CDN+ layer adsorbed on the surface and the PEO-4Ad polymer facing the solution. One would expect that the adhesion in an asymmetric configuration, i.e., the outer PEO-4Ad layer present only on one surface, would increase the adhesive force considerably as was found for the streptavidin-biotin system. The force needed to separate an adamantane from a β-cyclodextrin cavity has been determined by Auletta et al. by employing the atomic force microscope. They found a value of 102 ( 15 pN.27 Using this value and the adhesion force that was obtained in the present study under the assumption that the force is only due to interactions between β-cyclodextrin and adamantane, one can estimate that the measured adhesion corresponds to about 3 × 105 contacts between β-cyclodextrin cavities and adamantane groups. Auletta et al. also measured the free energy of the complexation of β-cyclodextrin and adamantane in solution and at surfaces.27 The free energy was determined to be about 11 kT both in solution and at surfaces, which was interpreted as meaning that the adamantane group did not feel the presence of the alkyl chain on the β-cyclodextrin heptathiol that was used to attach the cyclodextrin to the surface. The complexation constant between the β-cyclodextrin polymer and PEO-4Ad polymer was determined in solution to be 2300 L‚mol-1, which corresponds to about 8 kT per adamantane group provided all adamantane

(45) Claesson, P. M.; Paulson, O. E. H.; Blomberg, E.; Burns, N. L. Colloids Surf. A: Physicochem. Eng. Aspects 1997, 123-124, 341.

(46) Leckband, D.; Israelachvili, J. N. Enzyme Microb. Technol. 1993, 15, 450.

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groups participate in the complex formation. This is very close to the free energy that was found in the study of Auletta et al.27 One possible explanation for the somewhat lower free energy in our study may be that the PEO polymer folds around the adamantane groups and thus lowers the free energy of the complexation of adamantane and the β-cyclodextrin cavity. The interactions between mica coated with poly-β-CDN+ in the presence of 10 ppm PEO-2Ad were also investigated (Figure 6). The measured force is similar to that found when the PEO-4Ad polymer is used. On approach the surfaces jump into a hard wall from 250 Å. However, the compressed layer thickness is 110 Å, which is slightly smaller compared to that found in the presence of the four-arm polymer. Furthermore, on separation an adhesive force of about 400 µN/m is measured, which is larger than the adhesion measured between pure poly-β-CDN+ layers, but lower than in the presence of the PEO-4Ad polymer. Thus, we suggest that specific interactions are present also in this case but that fewer β-cyclodextrinadamantane bonds are formed. By using the value obtained by Auletta et al.27 for the attraction between one adamantane group and one β-cyclodextrin cavity, we can estimate that in this case about 8 × 104 interlayer contacts are formed when the layers come in contact, and are subsequently broken during separation. Conclusion Cationized β-cyclodextrin polymers adsorb readily to negatively charged mica surfaces. After adsorption the

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surfaces are close to uncharged. The interactions between these polyelectrolyte-coated surfaces differ in two important aspects compared to the interactions between mica surfaces coated with other highly charged cationic polyelectrolytes. The adsorbed layer thickness is larger and the adhesion force is very much smaller for the poly-βCDN+ coated surfaces. This is due to the bulky nature of the cyclodextrin unit and the stiffness of the polymer chain that counteracts the formation of polyelectrolyte bridges. A polymer bilayer can be built by first adsorbing polyβ-CDN+, rinsing, and finally adding end-capped PEO-Ad. Adsorption of PEO-4Ad results in a slightly thicker layer than adsorption of the two-arm analogue. On separation a significant adhesion force is observed. Bulk studies indicate that the attraction between the adamantane groups is not strong enough to give rise to a significant viscosity increase, whereas the interaction between adamantane and β-cyclodextrin is. Thus, specific interactions between adamantane and the cyclodextrin cavity are the most likely explanation for the attraction observed. Apparently significantly more interlayer contacts are formed, and subsequently broken during separation, when the outer layer consists of PEO-4Ad, as compared to when the outer layer consists of PEO-2Ad. Acknowledgment. This project was supported by the Swedish Research Council (VR) and the SSF nanochemistry program. Professor Per Claesson is greatly acknowledged for comments on the manuscript. LA048370E