NANO LETTERS
Adsorption of Polyelectrolyte Molecules to a Nanostructured Monolayer of Amphiphiles
2006 Vol. 6, No. 5 1018-1022
Nikolai Severin,*,† Ivan M. Okhapkin,‡,§ Alexei R. Khokhlov,§,| and Ju1 rgen P. Rabe† Department of Physics, Humboldt UniVersity Berlin, Newtonstr. 15, D-12489 Berlin, Germany, A. N. NesmeyanoV Institute of Organoelement Compounds, VaViloV str. 28, 119991 Moscow, Russia, Department of Polymer Science, UniVersity of Ulm, Albert-Einstein-Allee 11, D-89069 Ulm, Germany, and Physics Department, Moscow State UniVersity, 119992 Moscow, Russia. Received February 10, 2006; Revised Manuscript Received March 28, 2006
ABSTRACT We investigate the conformation of atactic poly(sodium 4-styrene sulfonate) (PSS) molecules both in water−ethanol solutions and adsorbed to a template provided by a nanostructured monolayer of flat-lying octadecylamines on graphite. The conformations of the PSS molecules in these solutions undergo two conformational transitions, a re-entry transition at 40 vol % and a coil−globule conformational transition at 80 vol % ethanol. On the surface the conformations are characterized by two different length scales: on a short length scale they are defined by the template, whereas the large-scale structures reflect the conformations in solution. We conclude that the conformational transitions are discontinuous on the level of single molecules and appear continuous in the ensemble.
Self-assembly is an efficient way to create materials designed down to molecular scales. Typically it requires a specific design of the self-assembling objects. For example, singlestranded DNA molecules can self-assemble, based on molecular recognition, into complex structures.1 The assembly of common synthetic polymers with precise control on position and conformation of single molecules is even more challenging.2 Template-directed self-assembly provides an improvement. In general, templates provide a potential ripple, which can be used to immobilize assembling objects at certain locations. They were applied successfully to assemble shape-persistent nano-objects.3,4 Template-directed selfassembly of flexible polymer molecules is particularly challenging because it requires control of conformations and positions simultaneously. Recently the assembly of polyelectrolyte molecules on a template of a self-assembled monolayer of amphiphiles was reported.5 Deposited from aqueous solution, polyelectrolyte molecules exhibit straight segments with sharp turns between the segments. The orientation of the polymer molecules was attributed to the potential ripple provided by an amphiphile monolayer on the basal plane of graphite with the alkyl * Corresponding author. E-mail:
[email protected]. † Humboldt University Berlin. ‡ A. N. Nesmeyanov Institute of Organoelement Compounds. § University of Ulm. | Chair of Physics of Polymers and Crystals, Physics Department, Moscow State University. 10.1021/nl060317i CCC: $33.50 Published on Web 04/15/2006
© 2006 American Chemical Society
chains oriented along the substrate axes parallel to each other, while the headgroups micro-phase separate into straight lamellae.6 The rows of headgroups that can be positively or negatively charged are separated by stripes of hydrophobic alkyl chains, providing a surface potential ripple. Two different scenarios of the polyelectrolyte molecules adsorption on the template of amphiphiles can be envisioned. In the first scenario the surface potential ripple is sufficiently deep to effectively prevent surface diffusion of the polyelectrolyte molecules. In this case their conformations on the surface should be regarded as projections of their conformations in solution7,8 locally straightened by the surface potential ripple. In the second scenario the surface potential ripple does not immobilize the polyeletrolyte molecules upon adsorption. Instead, the polyelectrolyte molecules diffuse at the solid-liquid interface and equilibrate their conformations with respect to the surface potential ripple, while only the removal of the solvent immobilizes the molecules. In the first scenario, the conformational transition of polyelectrolyte molecules in solution should be reflected in the conformations of the molecules on the surface. Polyelectrolyte molecules can undergo coil-globule transitions when the solvent quality is varied, for example, by solvent composition, temperature, or salt concentration.9-11 We chose here the theoretically and experimentally simple case of a binary mixture of good and poor
solvents. Atactic poly(sodium 4-styrensulfonate) (PSS) (Aldrich, MW 1 000 000) is a common polyelectrolyte with a negatively charged backbone, soluble in water but not in water-ethanol mixtures with ethanol fractions in excess of 80 vol %;12 therefore, one can expect PSS to undergo a coilglobule transition at ethanol fractions close to 80 vol %. Conformations of PSS molecules in water-ethanol mixtures were analyzed with dynamic light scattering (DLS) and compared to the conformations of the polymer molecules deposited on the graphite surface coated with a monolayer of C18H37NH2 (octadecylamine). PSS molecules on the surface were imaged with scanning force microscopy (SFM). Water-ethanol mixtures were prepared from fresh Milli-Q water and ultrapure ethanol (Aldrich) and mixed carefully with a glass pipet. PSS water stock solution was added to the water-ethanol mixture, resulting in the final concentration of 8 mg/L of PSS in water-ethanol solutions for SFM studies and 20 mg/L for DLS study. The concentrations were not identical because of the specific restrictions of the experimental setups. The upper limit of PSS concentration in solution for the SFM experiments was 8 mg/L because higher concentrations resulted in overcrowding of molecules on the surface and hence a difficulty of recognition of single molecules in SFM images. The lowest limit of accessible concentrations of PSS for DLS experiments was 20 mg/L. Polymer solutions for DLS measurements were clarified by filtration through Schleicher & Schuell 0.45 mkm filters with a PTFE membrane. The resulting solutions were equilibrated for 1 day at 20-22 °C. A chloroform solution of octadecylamine at a concentration of 0.1 mg/mL was spin-coated onto freshly cleaved highly oriented pyrolytic graphite (HOPG), (ZYH grade, Advanced Ceramics Corp.) at 40 rounds per second. The samples were dried for at least a half an hour at 20-22 °C. The resulting amphiphile layer was equilibrated in water: a water droplet of Milli-Q water was deposited on the surface for 15 s and removed by spinning it off. PSS water-ethanol solution was applied thereafter for 5 s and removed by spinning it off. Imaging was performed by SFM in tapping mode using a multimode head (Digital Instruments Inc., Santa Barbara, CA) and Olympus microcantilevers with a typical resonance frequency of 70 kHz and a spring constant of 2 N/m. In DLS experiments, time-intensity correlation functions were measured in pseudo cross mode with an ALV/CGS8F goniometer and ALV-5000 multi-τ digital correlator. The measurements were performed at scattering angles from 30° to 150°. The correlation functions were fitted by mono- or biexponential functions to obtain diffusion coefficients (D). If a dependence of D measured at different angles was observed, then the true values (D0) were obtained by extrapolation of the angular dependence to zero angles. Hydrodynamic radii (Rh) of PSS molecules were calculated via the Einstein-Smoluchovskii equation: Rh ) Nano Lett., Vol. 6, No. 5, 2006
kT 6πη0 D0
(1)
Figure 1. SFM images of PSS molecules deposited on a monolayer of octadecylamine on HOPG from (a) aqueous solution, (b) 40 vol % ethanol fraction in water, (c) 60 vol % ethanol fraction in water, and (d) 90 vol % ethanol fraction in water. In the height images the color palette is inverted to show PSS molecules in black color.
Spin-coating of a chloroform solution of octadecylamine covers the surface of HOPG by an octadecylamine layer. SFM imaging reveals that octadecylamine does not cover the surface homogeneously but instead forms domains with a lamellar structure. The size and density of the uniformly oriented lamella domains of octadecylamine can vary along the sample. After application of PSS, domains of lamellae can be found again, indicating remaining octadecylamine domains. In addition to the lamellar domains, linear structures can be found on top of the domains, which we attribute to PSS molecules (Figure 1a). Because the work is concentrated on adsorption of polyelectrolyte molecules on a template of the lamellar structures, only the PSS molecules that were located above domains where the lamellar structure could be clearly resolved were analyzed further, as indicated in Figures 1 and 2. The apparent height of the PSS molecules depends on SFM scanning conditions and can vary between 0.5 and 0.8 nm. PSS molecules deposited from aqueous solution exhibit straight segments about 100 nm long and occasional abrupt jumps between the segments. The straight segments appear to have a uniform width, which depends on the SFM tip sharpness. The typical width of the molecules (Figure 2) is approximately 5 nm, which is consistent with the single stretched polymer molecule broadened by the tip convolution effect. Thus, also taking into account that the PSS backbone is fully stretched in the PSS-amphiphile complex adsorbed on the graphite surface,13 we conclude that the PSS molecules are fully stretched within the straight segments. Although most of the straight segments follow the orientation of the underlying lamella, some of the segments cross the lamellae. High-resolution SFM images do not show any reorientation of the lamellae in the area surrounding the tilted segments (Figure 1b); however, it is still possible that 1019
Figure 3. Long axis size of polymer molecules on the surface (squares) and number of folded molecules (circles) as a function of the ethanol fraction in water. The dashed lines are guides for the eye.
Figure 2. SFM height image of PSS molecules deposited from 40 vol % ethanol on a monolayer of octadecylamine on graphite. The octadecylamine lamellae are the periodic stripes in the images background; the brighter linear structures on top of the lamellae are PSS molecules. The molecules marked as 1 and 2 are in the folded conformations, whereas molecule 3 is in the extended conformation. The small arrows mark four molecular ends protruding from one folded conformation. The double-sided arrow indicates the long axis size, L, of the molecule.
amphiphile molecules reorient in the close vicinity of the polymer backbone and define the tilt angle. Defects in the amphiphile monolayer may facilitate the mobility of the amphiphile molecules. Upon increasing the ethanol fraction, some molecules exhibit many kinks, folds, and self-crossings. The selfcrossings make it difficult to trace the backbone of single molecules in folded conformations; still, the length of the straight segments is visibly shorter in folded conformations than in extended ones. The presence of more than two molecular ends protruding out of some folded conformation indicates that these folded conformations can be aggregates of a few molecules. The molecules in folded conformations coexist with molecules in extended conformations (Figures 1b and 2).With further increase of ethanol fraction, the number of molecules in folded conformations, and also their compactness, vary. At high ethanol fractions the conformations become highly compact (Figure 1d). To characterize the conformations of the PSS molecules, the average long axis size of a 2D coil and the ratio of folded molecules were calculated (Figure 2). Molecular conformations were evaluated by visual examination of the SFM images. Conformations with a large number of self-crossings of polymer backbones were assigned to “folded”. We defined molecules that had more than two self-crossings as folded. Short molecules of which conformations adopted single straight segments were not taken into account for the calculation of the number of folded molecules. Both the number of folded molecules and the size of the molecules 1020
do not depend monotonically on the ethanol fraction (Figures 1 and 3). The average long axis size of the molecules decreases with the increase of ethanol fraction and reaches the first minimum at 40 vol % fraction of ethanol. With further increase of ethanol fraction, the size of the molecules increases insignificantly up to 60 vol % of ethanol fraction and decreases drastically for ethanol fractions higher than 80 vol %. The number of folded molecules reaches pronounced maxima at 40 vol % fraction of ethanol, decreases with further increase of ethanol fraction to 60 vol %, and increases again at higher fractions of ethanol. Figure 4a displays typical time-intensity correlation functions of PSS macromolecules at different ethanol contents in solution, recorded at 40° scattering angle. In pure water they are unimodal at studied PSS concentration. With addition of ethanol they become bimodal; that is, a short time-scale mode appears. The bimodal correlation functions were approximated with a biexponential model. The contribution of the short time-scale mode exhibits a maximum at 40 vol % of ethanol. The short time-scale mode is also present in the correlation functions for solutions at 20 and 60 vol % (data not presented here), where its contribution is less than that for 40 vol %. At an ethanol fraction of 90 vol %, the correlation function becomes unimodal again (Figure 4a). Figure 4b displays the dependence of the hydrodynamic radii of PSS molecules on the ethanol fraction. The size of PSS molecules is maximal in pure water. Upon an increase of ethanol content up to 40 vol %, their average size decreases. With further increase of the ethanol fraction up to 75 vol %, the calculated hydrodynamic radius increases slightly. At high ethanol content (90 vol %), it decreases drastically from 45 to 12 nm. The calculated diffusion coefficient of PSS molecules becomes independent of scattering angle at 90 vol % of ethanol, which is indicative of the formation of dense spherical particles. The concentration of PSS used in the present study falls into the region of freely diffusing molecules (dilute regime), according to the literature14 for PSS with similar molecular weight. For such solutions, the correlation functions are Nano Lett., Vol. 6, No. 5, 2006
Figure 4. (a) Intensity correlation functions for light scattering off PSS in water-ethanol solutions at a scattering angle of 40°. Two modes are typical at an ethanol fraction of 40 vol %. The correlation functions were smoothed by an adjacent averaging procedure. (b) Effective hydrodynamic radius as calculated from long time-scale modes.
unimodal and their single mode is identified with the motion of separate polymer chains.14-16 However, at concentrations exceeding overlap concentration of the solute (c*) where a translational diffusion of the individual macromolecules may no longer be observed, salt-free polyelectrolyte solutions give bimodal correlation functions. The fast modes of such correlation functions were ascribed to cooperative diffusion of entangled segments of the polymer network,14-16 whereas the nature of slow modes is still controversial. Several interpretations were proposed so far including diffusion of polymer associations,16 formation of temporal domains15 or aggregates17 in a polymer network, and reptation.14 In the present study, dilute polymer solutions are used; therefore, one can expect single-mode correlation functions characteristic of translational diffusion. This is indeed the case for ethanol-free solutions. Unexpectedly, upon addition of ethanol, a second mode appears. We call this mode a “short time scale” one to distinguish from the “fast” mode, which appears above c*. Slow modes of correlation functions for solutions above c* are characterized by significantly longer relaxation times than the single modes of correlation functions recorded in the dilute regime. In the present study, the relaxation time of the mode on a longer time scale is almost the same as that of the single modes for ethanol-free solution (Figure 4a). Therefore, the bimodal correlation functions observed in our experiment probably have a nature different from those of polymer solutions above c*, and the mode on a longer time scale can be used for calculation of diffusion coefficients of separate PSS molecules. The hydrodynamic radii of PSS molecules were calculated from the diffusion coefficients using the Einstein-Smoluchovskii equation (eq 1), which is valid for hard spherical particles. This approximation is not fully adequate for polyelectrolyte molecules in the absence of low molecular weight salt because they are much extended because of repulsive electrostatic interactions between the charges of the chains. Equation 1 is used in this study because no model was proposed so far that could satisfactorily describe the shape of polyelectrolyte molecules dissolved in salt-free solvents of variable composition. Thus, the calculated Rh values cannot be treated as absolute values Nano Lett., Vol. 6, No. 5, 2006
and are significant only to show the variation of the effective size of PSS molecules as a function of solvent composition. The hydrodynamic radius of the PSS molecules in solution and their long axis size on the surface both exhibit a similar dependence on the ethanol fraction (Figures 3 and 4b). As was expected, the size of the molecules decreases dramatically for ethanol fractions above 80 vol %, indicating collapse of the molecules due to the decrease of the solvent quality. Conformations of PSS molecules on the amphiphile monolayer deposited from higher ethanol fractions are highly compact with small protrusions, which could be explained by the templating effect of the underlying amphiphile layer. In addition to the conformational transition at 80 vol %, both DLS and SFM measurements reveal an additional re-entry conformational transition for PSS molecules at around 40 vol % of ethanol. Re-entry conformational transitions were found for polyelectrolyte gels18,19 in binary solution mixtures. The re-entry phenomenon was explained by the nonmonotonic dependence of free energy of contact between polymer segments on solvent composition.19 The enthalpy of the water-ethanol mixture is a nonlinear function of the ethanol fraction and is minimal at about 40 vol % of ethanol at room temperature. The increased attraction between solvent molecules results in effective repulsion of polymer segments by the solvent, causing compact molecular conformations. However, the resulting conformations are not as dense as the conformations in poor solvent at higher ethanol concentrations; thus, the average size of the molecules at 40 vol % is larger than at high ethanol fractions. Minor aggregation can also influence the average long axis size at 40 vol % of ethanol. Noteworthy, the maximal contribution of the fast mode to bimodal correlation functions is also observed at 40 vol % of ethanol (Figure 4a). The origin of the short time-scale mode appearing at the intermediate concentrations of ethanol is not yet clear, but it is presumably connected with the peculiar behavior of PSS molecules in the media of structured20 ethanol-water mixtures. Maximum structuring at 40 vol % corresponds to maximum manifestation of the short time-scale mode. 1021
A good agreement of the hydrodynamic radii and long axes dependences on the ethanol fraction indicates that the molecular conformations of PSS molecules on the surface reflect the polymer conformations in solution. While on a short length scale, the conformation of a polymer molecule is dictated by the potential ripple provided by the layer of amphiphile molecules; on a larger scale it can be regarded as a projection on the surface of conformation of the polymer molecule in solution. Dependence of the length of the straight segments on the conformation of the polymer molecule in solution indicates the influence of the adsorption dynamics. While DLS averages information, analysis of the SFM images provides more insight on the conformational transitions on the level of single molecules. In particular, two different conformations can be clearly distinguished on the SFM images: folded and extended conformations. The coexistence of the two different conformations indicates the discontinuity of the conformational transition of PSS molecules in water-ethanol mixtures. Initially, the coil-globule transition of a macromolecule was predicted to be continuous (a second-order phase transition21). However, as was shown later, the order of transition depends strongly on the chain stiffness.22 In flexible chains, the transition is continuous, whereas in stiff chains, it is discontinuous, indicating a firstorder transition. For polyelectrolytes, theoretical studies and experiments on single polyelectrolyte molecules show the coil-globule transition to be of the first order.11,22,23 In this work, the analysis of the SFM images indicates that the conformational transition of PSS molecules is of the first order on the level of single molecules. Thus, the first order of coil-globule transition of strongly charged polyelectrolytes appears to be in good correlation with the theoretical predictions. In conclusion, we find that PSS molecules undergo two conformational transitions in water-ethanol mixture: a reentry conformational transition at 40 vol % and a coilglobule transition at 80 vol % of ethanol fractions. The conformations of the polyelectrolyte molecules adsorbed to nanostructured amphiphile layers are characterized by two different length scales. On a short length scale they are defined by the template, whereas the large-scale structures reflect the conformations in solution. We conclude that the
1022
conformational transitions are discontinuous on the level of single molecules and appear continuous in the ensemble. Acknowledgment. Financial support from SFB 569 and 448 is gratefully acknowledged. We also thank Stefan Kirstein and Igor M. Sokolov (Humboldt University Berlin) for helpful discussions. References (1) Seeman, N. C. Nature 2003, 421, 427-431. (2) Decher, G. Science 1997 277, 1232-1237. (3) Theobald, J. A.; Oxtoby, N. S.; Phillips, M. A.; Champness, N. R.; Beton, P. H. Nature 2003, 424, 1029-1031. (4) Redl, F. X.; Cho, K.-S.; Murray, C. B.; O’Brien, S. Nature 2003, 423, 968-971. (5) Severin, N.; Barner, J.; Kalachev, A. A.; Rabe, J. P. Nano Lett. 2004, 4, 577-579. (6) Rabe, J. P.; Buchholz, S. Science 1991, 253, 424-427. (7) Rivetti, C.; Guthold, M.; Bustamante, C. J. Mol. Biol. 1996, 264, 919-932. (8) Kiriy, A.; Gorodyska, G.; Minko, S.; Jaeger, W.; Stepanek, P.; Stamm, M. J. Am. Chem. Soc. 2002, 124, 13454-13462. (9) Takahashi, M.; Yoshikawa, K.; Vasilevskaya, V. V.; Khokhlov, A. R. J. Phys. Chem. B 1997, 101, 9396-9401. (10) Aseyev, V. O.; Klenin, S. I.; Tenhu, H.; Grillo, I.; Geissler, E. Macromolecules 2001, 34, 3706-3709. (11) Katsumoto, Y.; Tanaka, T.; Sato, H.; Ozaki, Y. J. Phys. Chem. A 2002, 106, 3429-3435. (12) PSS solubility was tested macroscopically by dissolution of PSS in ethanol-water mixtures. The PSS solubility decreases dramatically for ethanol fractions above 80 vol %. (13) Severin, N.; Rabe, J. P.; Kurth D. G. J. Am. Chem. Soc. 2004, 126, 3696-3697. (14) Koene, R. S.; Mandel, M. Macromolecules 2005, 16, 973-978. (15) Buhler, E.; Rinaudo, M.; Macromolecules 2000, 33, 2098-2106. (16) Fo¨rster, S.; Schmidt, M.; Antonietti, M. Polymer 1990, 31, 781792. (17) Schmitz, K. S.; Ramsay, D. J. Macromolecules 1985, 18, 933-938. (18) Wang, G.; Kuroda, K.; Enoki, T.; Grosberg, A.; Masamune, S.; Oya, T.; Takeoka, Y.; Tanaka, T. Proc. Natl. Acad. Sci. U.S.A. 2000, 97, 9861-9864. (19) Katayama, S.; Hirokawa, Y.; Tanaka, T. Macromolecules 1984, 17, 2641-2643. (20) Dixit, S.; Crain, J.; Poon, W. C. K.; Finney, J. L.; Soper, A. K. Nature 2002, 416, 829-832. (21) de Gennes, P. G. Scaling Concepts in Polymer Physics; Cornell University Press: New York, 1979. (22) Khokhlov, A. R.; Grosberg, A. Yu. Statistical Physics of Macromolecules; American Institute of Physics, New York, 1994. (23) Vasilevskaya, V. V.; Khokhlov, A. R.; Matsuzawa, Y.; Yoshikawa, K. J. Chem. Phys. 1995, 102, 6595-6602.
NL060317I
Nano Lett., Vol. 6, No. 5, 2006
