4036
J. Phys. Chem. B 2007, 111, 4036-4042
Ion Distribution in Polyelectrolyte Multilayers with Standing-Wave X-ray Fluorescence Hauke Schollmeyer,*,† Patrick Guenoun, and Jean Daillant SerVice de Chimie Mole´ culaire, LIONS, Baˆ timent 125, C.E.A. Saclay, F-91191 Gif-sur-YVette Cedex, France
Dmitri V. Novikov Hamburger Synchrotronstrahlungslabor HASYLAB at Deutsches Elektronensynchrotron DESY, Notkestrasse 85, D-22607 Hamburg, Germany
Regine v. Klitzing Stranski-Laboratorium fu¨r Physikalische und Theoretische Chemie, Institut fu¨r Chemie, Strasse des 17. Juni 124, 10623 Berlin, Germany ReceiVed: December 19, 2006; In Final Form: February 8, 2007
Absolute ion concentration and its profile across polyelectrolyte multilayer films were studied. The films were prepared by alternating adsorption of polyanions and polycations from aqueous solution. Standingwave X-ray fluorescence was used to map the ion profile. The well-studied multilayer system PSS/PAH was investigated, and bromide ions were used as probe entities. The results show that the sign of the charge of the outermost layer and the washing procedure after finishing the preparation have a decisive effect on the ion concentration and the ion profile. Multilayers with PSS as the outermost layer contain fewer bromide ions than the PAH-terminated multilayers. Exposure to water washes the ions out, but even after 6 h of washing, not all of the bromide ions had been removed.
Introduction Because of the miniaturization of devices, ultrathin coatings have become increasingly important. One challenge in the design of such films is that they serve to modify surface properties in an easy way. Suitable films of defined charge and various permeabilities can be coated by the layer-by-layer (LbL) method wherein polyanions and polycations are alternately adsorbed from aqueous solutions.1 This preparation method offers many advantages that are of high importance in materials science. One of the main features is that the thickness can be easily tuned with angstrom precision using the number of deposited layers or the ionic strength. Additional macroscopic characteristics such as optical or conductive properties can be controlled by the types of polyelectrolyte used for the preparation. Futhermore, there are no limits with repect to the shape of the template. In addition to planar interfaces, particles or even objects with highly irregular shapes can also be coated. During the adsorption of each layer, complexes between oppositely charged polyelectrolytes are formed with the previously adsorbed polyelectrolyte layer2 through the exchange of the counterion. This is consistent with the very low counterion concentration measured by TIRF3 and concentrations even below the detection limit in radiochemical and electrochemical studies.4 This means that most of the charges within the polyelectrolyte multilayer are compensated by the opposite polymer charges (“intrinsic” charge compensation) and not by small counterions (“extrinsic” charge compensation). In connection with this intrinsic charge compensation, the strong interdigitation between adjacent layers found by neutron reflectometry5-7 might occur. * Corresponding author. E-mail:
[email protected]. † Present address: Georg-August Universita ¨ t Go¨ttingen, Insitute for X-ray Physics, Friedrich Hund Platz 1, 37077 Go¨ttingen, Germany.
The rms roughness of the internal interfaces can be on the order of the thickness of a single layer. One of the questions regarding this process is the driving force for multilayer formation. For a long time, electrostatic attraction between the polylelectrolyte in solution and the oppositely charged surface was discussed as the driving force for the formation of multilayers.8 Indeed, it has been shown that a minimum polymer charge density is required for the formation of multilayers.9-12 On the other hand, multilayers can be formed at high ionic strengths, where the electrostatic attraction between the polyelectrolyte in solution and the oppositely charged surface might be completely screened.11,13 Therefore, it is assumed that multilayer formation is entropically driven through the release of small counterions, as mentioned above. This again supports the findings in which counterions could hardly be detected within the films. However, recent studies have shown a strong effect of the type of salt on the multilayer structure,14-16 which might lead to the assumption that there are still some counterions within the multilayer. Aside from electrostatic interactions and the related gain in entropy, nonionic interactions such as hydrogen bonding and hydrophobic interactions might also play a role in the development of these films. For instance, during layer assembly, the zeta potential does not necessarily change sign.17 In this article, we demonstrate how an X-ray standing wave, generated by total external reflection, can be used to directly measure the ion concentration and distribution profile in a polyelectrolyte multilayer through the detection of counterion fluorescence. Experimental Methods Materials and Film Preparation. The polycations poly(ethylenimine) (PEI, 50 wt % solution in water) and poly-
10.1021/jp068715x CCC: $37.00 © 2007 American Chemical Society Published on Web 04/03/2007
Ion Distribution in Polyelectrolyte Multilayers
Figure 1. Schematic of the X-ray standing-wave field produced by interference of the incident and reflected waves above a mirror substrate.
(allylamine hydrochloride) (PAH, Mw ) 65000 g/mol) were purchased from Sigma-Aldrich (Steinheim, Germany). As the polyanion, poly(styrene sulfonate) sodium salt (PSS, Mw ) 70000 g/mol) from Sigma-Aldrich was used. The solutions were
J. Phys. Chem. B, Vol. 111, No. 16, 2007 4037 prepared in doubly-distilled Millipore-Q water (pH 5.5, specific resistance 18.2 MΩ cm-1). The concentration of polyelectrolyte solutions was kept constant at 10-2 mol/L of the equivalent monomer concentration. Sodium bromide was obtained from Merck (Darmstadt, Germany). The PEI solution was prepared without additional salt. The NaBr concentration in the other polyelectrolyte solutions was 1 mol/L. The silicon wafers were cleaned by treatment with a 1:1 H2O2/ H2SO4 mixture (30 min) and then rinsed with water. The wafers were coated by adsorption of polyelectrolytes using the layerby-layer technique introduced by Decher and co-workers.1 The wafers were exposed to the respective polyelectrolyte solutions for 20 min. After each adsorption step, they where rinsed three times for 1 min each with fresh Milli-Q water (without additional salt). They were dried only with a gentle air stream (filtered by active carbon filters) after the last rinsing step. Either the samples
Figure 2. (A) Map of the intensity of the E field above a model mirror surface of reflectivity 1 as a function of incidence angle normalized to the critical angle. (B) E field profile normal to the reflecting surface for normalized incidence angles of 0.2, 0.25, 0.55, and 5.00. The latter was used for calibration.
4038 J. Phys. Chem. B, Vol. 111, No. 16, 2007
Schollmeyet et al.
Figure 3. (A) Map of the intensity of the E field in a 30-nm-thick polyelectrolyte multilayer for different normalized incidence angles. (B) E field intensity in a polyelectrolyte multilayer as a function of distance to the surface for different normalized incidence angles of ϑi/ϑc ) 0.29, 0.75, and 0.98. In the present experiment, the critical angle was ϑc ) 1.73 mrad (λ ) 0.688 Å-1).
were used for the measurements as prepared, or they were kept in Milli-Q water for an additional 6 h. Standing-Wave X-ray Fluorescence. When an X-ray beam impinges on a flat interface separating two materials that have different indices of refraction, part of the energy in the travelling wave is reflected, and the remainder is transmitted. X-rays undergo total reflection for incidence angles ϑi less than the critical angle ϑc, because the index of refraction is less than unity for X-rays. The index of refraction is expressed as n ) 1 - δ - iβ, with refraction δ ) Nereλ2/2π and absorption β ) µλ/4π. Using Snell’s law, the critical angle depends on the wavelength λ and the effective electron density Ne with ϑc ) (2δ)1/2 (re is the classical electron radius, and µ is the linear absorption coefficient). As illustated in Figure 1, the incident and reflected X-ray beams interfere and generate an X-ray standing wave above the reflecting surface.18-20 Because of these waves, the E field above the surface is not homogeneous: It is constant parallel to the surface, but varies normal to the surface. By changing the incidence angle of the X-ray beam, the E field
above the surface can be changed. The photoelectric-effect cross section at the center of an atom (in the dipole approximation) is proportional to the E field intensity. This value can be used to monitor an ion distribution above the mirror surface: At a given incidence angle, the fluorescence signal is a result of the E field distribution above the surface and the density distribution of the fluorescent atoms. Variation of the incidence angle results in a change of the E field distribution above the surface. By monitoring the fluorescence signal as a function of the incidence angle, the ion distribution above the mirror surface can be determined. The E field intensity in terms of the distance z to a mirror surface in a vacuum is expressed as21
I(ϑi,z) ) |E0|2[1 + R + 2xR cos(V - 2πQz)] where Q ) 2sinϑi/λ is the magnitude of the wavevector transfer Q ) kr - ki (kr and ki initial and final momenta, respectively, of the photon) and ν is the phase. As an example, the map of the E field intensity above a surface with reflectivity R ) 1 in
Ion Distribution in Polyelectrolyte Multilayers a vacuum is shown in Figure 2A. For incidence angles above the critical angle, the signal maintains a level of |E0|2. For lower angles, one can see the variation of the E field normal to the surface for given incidence angles. As the incidence angle increases, the period of the standing wave decreases, and more oscillations of the electric field are generated in front of the surface. Figure 2B shows cuts of the electric field at four different angles of incidence. The cuts are indicated as vertical lines in Figure 2A. The critical angle of total reflection for silicon against a vacuum is 1.73 mrad for 18 keV. The example shows clearly how the distribution of the electric field changes as a function of angle of incidence. For ϑi/ϑc) 0.2, the maximum is at a distance of 600 nm. A small change of the normalized angle to ϑi/ϑc)0.25 shifts the maximum to 300 nm. A further shift to ϑi/ϑc)0.55 leads to a shift of the maximum to 120 nm, with a second occurring in the region of interest at about 480 nm. For ϑi/ϑc) 5.0, the E field is nearly constant. If there are fluorescent molecules or ions in front of the surface, mainly those within the maximal electric field are excited and contribute mostly to the integral fluorescence signal. This allows the fluorescent molecules to be mapped by changing the angle of incidence. In the present experimental case, the fluorescence signal of bromide ions in an absorbed polyelectrolyte multilayer above a silicon surface was mapped as a function of incidence angle. Of course, the multilayer adsorption led to a change in the refractive index and, therefore, in the electric field distribution. Because of the strong interdigitation and similar electron density of each layer, we considered the polyelectrolyte multilayer as a homogeneous film that could be described with one refractive index. Refractive index and thickness were then used in the matrix method of Vindal and Vincent to calculate the E field at any position z within the layer.22 Figure 3A shows a simulation of the E field of such a 30-nm-thick polyelectrolyte multilayer for different incidence angles. Because of the low thickness of the film, the E field has fewer oscillations than the example mentioned above. In Figure 3B, vertical cuts of Figure 3A show the E field intensity as a function of distance to the silicon surface for fixed incidence angles. Figure 3B shows that, for ϑi/ϑc) 0.29, the maximum of the E field is outside the multilayer, and at the respective angle of incidence, the measured fluorescence intensity is quite low, almost irrespective of the location of the fluorescent ions. At ϑi/ϑc ) 0.75, the fluorescence signal comes mainly from the intermediate part of the film, whereas for the highest ϑi/ϑc values, ions close to the substrate surface and near the outer film/continuum interface are excited. This means that, even for thin films, it is possible to distinguish between fluorescent molecules in the inner (close to the substrate), middle, and outer regions and the spatial resolution is on the order of 5-10 nm. To fit the experimental data, we assumed lateral slabs of different ion concentrations. Within a slab, the ion concentration was assumed to be homogeneous. We fitted thickness and refractive index data from three different sets of measurements, and we used 1-5 slabs to estimate the ion concentration profile. Experiments were done at HASYLAB (Hamburg) at beam line C1. The beam was tuned to 18 keV (λ ) 0.688 Å) and 20-µm size. The fluorescence signal was measured above the sample with a silicon drift detector (Roentec, Berlin, Germany). The detector was calibrated with RbBr. Figure 4 shows the sum of all fluorescence signals collected for 0 nm-1< q < 0.09 nm-1 for PEI + (PSS + PAH) after preparation. The collected spectra were fitted with a Gaussian curve at 13.47 keV, and the area of the fit was used as the intensity. All measurements were
J. Phys. Chem. B, Vol. 111, No. 16, 2007 4039
Figure 4. Sum of fluorescence for all q as a function of E (keV) for PEI + (PSS + PAH) after preparation.
Figure 5. (A) Fluorescence data for PEI + (PSS + PAH)6 as a function of ki, fit with five slabs and the contribution of each slab. (B) Ion profiles for 1-5-slab fits.
performed in a cell with Kapton windows with the option to measure both in air and in liquid. Measurements were done in air, and calibration was done in liquid: The fluorescence intensity was calibrated by injecting a 10-3 mol/L solution of RbBr in water and measuring the intensity for high q. Taking advantage of the fact that the E field is |E0|2 for high q, the fluorescence intensity was assumed to be influenced only by the amount of ions in the cross section. For calibration, the thickness of the layer was used as the cross section for the polyelectrolyte multilayer and the beam size in the case of the calibration sample. Results Figures 5-7 show experimental data, fits with five slabs and the contribution of each slab, and ion profiles for PEI + (PSS
4040 J. Phys. Chem. B, Vol. 111, No. 16, 2007
Schollmeyet et al.
Figure 6. (A) Fluorescence data for PEI + (PSS + PAH)6 after washing as a function of ki, fit with five slabs and the contribution of each slab. (B) Ion profiles for 1-5-slab fits.
+ PAH)6 and PEI + (PSS + PAH)6 + PSS without additional washing after preparation was completed. In addition, the PEI + (PSS + PAH)6 sample was measured after exposure to a water bath (Milli-Q) for 6 h. The presence of a fluorescence bromide peak, as shown in Figures 5A-7A, indicates that bromide ions (counterions) remain in the film even after extensive washing for 6 h. Depending on the preparation, the amount can be very low. As seen from the simulation of the E field in Figure 3, the sensitivity for the profiles in the z direction is on the order of 5-10 nm. This will not give very detailed profiles, but the aim of this work is to give tendencies as to whether the ions are in the inner, middle, or outer part of the film. For this purpose, even a three-slab fit would be sufficient. To check the consistency of the fits, we compared fits with one, two, three, four, and five slabs with each other. The mean of the average bromide ion concentration of all fits together is 0.4307 ( 0.008 nm-3 for the PEI + (PSS + PAH)6 film, 0.0055 ( 0.0006 nm-3 for the washed PEI + (PSS + PAH)6 film, and 0.0108 ( 0.0008 nm-3 for the PEI + (PSS + PAH)6 + PSS film. The variation of the average ion concentration is very small, showing that all fits are consistent at least with respect to the ion concentration. Assuming 5 nm-3 polymer segments in the film22 means that 8.6% of them are occupied by an ion in the PEI + (PSS + PAH)6 film, and 0.1% are occupied after washing. In the PEI + (PSS + PAH)6 + PSS film, only 0.2% of the polymer segments have a counterion. Because of the weak signals for the washed and PSS-terminated samples, it was especially difficult to evaluate concentration profiles (Figures 6A and 7A). Nevertheless, in the following, the profiles will be discussed
Figure 7. (A) Fluorescence data for PEI + (PSS + PAH)6 + PSS as a function of ki, fit with five slabs and the contribution of each slab. (B) Ion profiles for 1-5-slab fits.
more in detail. In Figure 5B, all fits using more than one slab show a depletion zone in the outer part of the PEI + (PSS + PAH)6 film. Most ions were found to be in the intermediate and inner parts of the film. This can be extracted from the three-, four-, and five-slab fits. To confirm this result, Figure 8A shows the means of the three-, four-, and five-slab fits. Error bars showing the standard deviations give an estimate of how well the three fits coincide. The smaller the error bar, the better the match of the fits. The outer lower and inner higher ion concentrations seem to be equal for all fits. The errors are higher for the intermediate part of the film. Therefore, exact profiles cannot be derived in this part. Because we know that the average ion concentration is equal for all fits, the concentration for the intermediate part of the film needs to be higher than that for the outer part. The fluorescence signals for the washed PAH-terminated and unwashed PSS-terminated samples are very low and scattered. Therefore, exact fits cannot be expected. The fits express the main features of the signal. As the signal strength indicates, the ion concentration is much lower than in the case of PEI + (PSS + PAH)6 after preparation. Figure 8B,C shows the means of the three-, four-, and five-slab fits of the washed and PSSterminated sample. The ion distribution within the PSSterminated film is nearly constant. In the case of the washed sample, the ion concentration is lower for the outer 20 nm of the film than for the inner part of the film.
Ion Distribution in Polyelectrolyte Multilayers
J. Phys. Chem. B, Vol. 111, No. 16, 2007 4041
Figure 8. Average of three-, four-, and five-slab profiles and average ion concentration in the film as a result of a five-slab fit. Error bars are the standard deviations and show how well the three fit models coincide.
Discussion All samples were prepared with bromide ions. The results show that the samples contain bromide ions irrespective of the outermost layer. Even intensive washing for 6 h cannot remove all of the bromide ions. However, the total amount of bromide ions differs greatly depending on the outermost layer and the washing procedure. First, the effect of the outermost layer will be discussed. The multilayer with PAH as the topmost layer contains more than 40 times more bromide ions than that with PSS on the outside. Obviously, the charge of the outermost layer plays an important role. The cationic PAH attracts more bromide ions than the negatively charged PSS. However, the large difference cannot be explained by an excess of bromide ions only within the outermost PAH layer. The concentration profiles support the assumption that the adsorption of PAH leads to the transport of bromide ions into the film, thus distributing them over the whole film, as the concentration is higher over the whole film (compared to that of the PSS-terminated film) and not only in the outermost layer. The outer PSS layer seems to present a kind of barrier that prevents the transport of bromide ions into the multilayer. This process of “inward” and “outward” diffusion has to take place between two sequential adsorption steps with a rinsing step with pure water between them. For a better understanding of this process, one should consider the effects of washing. After exposure of the PAH-terminated multilayer to water for 6 h, the bromide ion concentration is reduced by about 2 orders of magnitude. This means that most of the bromide ions can be washed out, and a typical rinsing step during preparation of 3 min total is too short to remove
them all. However, a much lower concentration in the outer layer after a short rinsing indicates that the ions are already partially washed out after a few minutes, and even after 6 h of washing, the concentration is higher close to the substrate than in the intermediate and outer layers, which supports the assumption that the ions of the outer part of the film are washed out first. This creates a concentration gradient within the films that shifts toward the substrate with increasing washing time. In the case of PSS-terminated multilayers, this effect does not arise, because the ion concentration is low, even before rinsing. It is worth noting that, during the short rinsing process during the adsorption of PSS (20 min), most of the bromide ions that had been in the PAH-terminated multilayer were washed out and that this process is much more efficient than washing with pure water for 6 h. If the effect was restricted to the outer layers, most of the bromide ions would remain in the intermediate and inner parts during the adsorption of PSS, which is not the case (see the homogeneous ion distribution in Figure 7). This effect could be explained by the fact that the zeta potential, which changes after each adsorption step, is extended not only toward the outer solution but also toward the inner part of the film. The negative potential that is extended toward the substrate leads to a repulsion of bromide ions from the multilayer. After each adsorption step, the chemical potential changes. The expulsion of Br ions from the multilayer might be due to a change in chemical potential after the adsorption of PSS. To estimate the range of the electrostatic potential, one has to analyze the ion concentration within the multilayer. For PAH outside, the concentration is about 0.4 nm-3 (measured in the
4042 J. Phys. Chem. B, Vol. 111, No. 16, 2007 dry state). Under the assumption that the film contains about 50% of water,23 the bromide ion concentration under the respective swollen conditions during preparation is about 0.3 mol/L. Previous studies have shown that typical concentrations of “free” ions within the multilayer are in the range of 10-410-3 mol/L3,24 or even below detection limits.4 This means that most of the bromide ions detected in the present study are “condensed” at the polyelectrolyte chains, and only less than 1% is free. The permittivity of PAH/PSS multilayers is between 20 and 50, depending on the salt concentration during preparation and the method used for the experiments.23,24 Only once have much higher values for the permittivity, up to several hundreds, been reported.26 Assuming a permittivity of 20-50 and a concentration of free ions between 10-4 and 10-3 mol/L, the screening length would be about between 10 and 20 nm. This means that the surface potential decays more or less over the whole film thickness. This might explain why bromide ions close to the substrate (i.e., far from the multilayer/solution surface) are also repelled from the film. Under the assumption that the stoichiometric ratio of PSS to PAH segments is 1:1 and that most of the bromide ions are condensed, about 17% of the PAH monomers (8.6% of the whole number of segments) are neutralized by bromide ions. This is an indication of mostly intrinsic charge compensation, even though the film was prepared at a high ionic strength of 1 mol/L. Conclusion The experiments described in the present work allow for the determination of the absolute values of the concentration and its concentration profile. The effects of the sign of the charge of the outermost layer and the step of washing with water after preparation were studied. The study shows that the adsorption of PSS reduces the ion concentration by a factor of 40, which is explained by a change in sign of the “effective” surface charge from positive to negative. This leads to a repulsion of the negatively charged bromide ions from the region near the interface. The reduction of the bromide ion concentration across the total film over a distance of 30 nm is explained by a decay of the potential over the whole film thickness. This process is much more efficient than washing for 6 h with water. In relation to former studies, it is concluded that most of the detected ions are condensed and that they neutralize about 17% of the PAH monomers. This means that the charge of most of the polymer segments is intrinsically compensated. The study shows the results of negatively charged ions. A complementary study of the concentration profile of cations is the goal of future studies.
Schollmeyet et al. Acknowledgment. H.S., R.K., P.G., and J.D. thank CNRS, CEA, DFG, and MPG: “French-German Network complex fluids: from 3d to 2d” for financial support. The authors are indebted to Christian Blot and Daniel Luzet for technical assistance. Supporting Information Available: Table containing the fit parameters for all of the fits shown. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Decher, G. Science 1997, 277, 1232. (2) Farhat, T.; Yassin, G.; Dubas, S. T.; Schlenoff, J. B. Langmuir 1999, 15, 6621. (3) Klitzing, R. v.; Mo¨hwald, H. Langmuir 1995, 11, 3554. (4) Schlenoff, J. B.; Ly, H.; Li, M. J. Am. Chem. Soc. 1998, 120, 7626. (5) Schmitt, J.; Gru¨newald, T.; Decher, G.; Pershan, P. S.; Kjaer, K.; Lo¨sche, M. Macromolecules 1993, 26, 7058. (6) Tarabia, M.; Hong, H.; Davidov, D.; Kirstein, S.; Steitz, R.; Neumann, R.; Avny, Y. J. Appl. Phys. 1998, 83, 725. (7) Lo¨sche, M.; Schmitt, J.; Decher, G.; Bouwman. W. G.; Kjaer, K. Macromolecules 1998, 31, 8893. (8) Bertrand, P.; Jonas, A.; Laschewsky, A.; Legras, R. Macromol. Rapid Commun. 2000, 21, 319, and references therein. (9) Steitz, R.; Jaeger, W.; Klitzing, R. v. Langmuir 2001, 17, 4471. (10) Glinel, K.; Moussa, A.; Jonas, A. M.; Laschewsky, A. Langmuir 2002, 18, 1408. (11) Schoeler, B.; Kumaraswamy, G.; Caruso, F. Macromolecules 2002, 35, 889. (12) Voigt, U.; Jaeger, W.; Findenegg, G. H.; Klitzing, R. v. J. Phys. Chem. B 2003, 107, 5273. (13) Klitzing, R. v.; Leiner, V.; Steitz, R. In Proceedings of the ILL Millenium Symposium and European User Meeting; Institut LaueLangevin: Grenoble, France, 2001; p 73. (14) Klitzing, R. v.; Wong, J. E.; Jaeger, W.; Steitz, R. Curr. Opin. Colloid Interface Sci. 2004, 9, 158. (15) Saloma¨ki, M.; Tervasma¨ki, P.; Areva, S.; Kankare, J. Langmuir 2004, 20, 3679. (16) Saloma¨ki, M.; Laiho, T.; Kankare, J. Macromolecules 2004, 37, 9585. (17) Neff, P. A.; Wunderlich, B. K.; Klitzing, R. v.; Bausch, A. R. Langmuir 2007, 23, 4048. (18) Bedzyk, M. J.; Bilderback, D. H.; Bommarito, G. M.; Cafrey, M.; Schildkraut, J. S Science 1988, 241, 1788. (19) Fenter, P.; Cheng, L.; Rihs, S.; Machesky, M.; Bedzyk, M. J.; Sturchio, N. C. J. Colloid Interface Sci. 2000, 225, 154. (20) Bendzyk, M. J. Synchrotron Radiat. News 1990, 3, 25. (21) Bedzyk, M. J; Bommarito, G. M.; Caffrey, M.; Penner, T. L. Science 1990, 248, 52. (22) Daillant, J.; Gibaud, A. X-Ray and Neutron ReflectiVity; Springer: Berlin, 1999. (23) Steiz, R.; Leiner, V.; Siebrecht, R.; Klitzing, R. v. Colloids Surf. A. 2000, 163, 63. (24) Neff, P. A.; Naji, A.; Ecker, C.; Nickel, B.; Klitzing, R. v.; Bausch, A. R. Langmuir 2006, 39, 463. (25) Tedeschi, C.; Mo¨hwald, H.; Kirstein, S. J. Am. Chem. Soc. 2001, 123, 954. (26) Durstock, M. F.; Rubner, M. F. Langmuir 2001, 17, 7865.
