Langmuir 1995,11, 3554-3559
3554
Proton Concentration Profile in Ultrathin Polyelectrolyte Films Regine v. Klitzing"??and Helmuth Mohwald$ Institut fur Physikalische Chemie, Universitat Mainz, Welder Weg 11, 0-55099Mainz, Germany, and MPI fur Kolloid-und Grenzflachenforschung, Rudower Chaussee 5, 0-12489Berlin, Germany Received February 22, 1995. I n Final Form: June 27, 1995@ Embedding a pH-sensitive fluorescent dye at defined distances from the surface in a polyelectrolyte film, the pH dependence of fluorescence emission as a function of film thickness, surface charge, and ion concentration is measured by total internal reflection fluorescence (TIRF). From the analysis of the data, we demonstrate that the films are permeable for protons and derive the potential profile within the polyelectrolyte film. The potential distribution within the polyelectrolyte film near the charged outer surface is described by the Guy-Chapman-Stem theory. The Debye lengths are a factor of 3 smaller than those in water at similar ionic conditions which may be ascribed to a lower dielectric constant or to a higher concentration of mobile ions in the film. The local pH inside the film can be changed in a predictible way via the nature of the outer polyelectrolyte layer and the ionic milieu of the adjacent solution.
I. Introduction Ultrathin films are a new kind of material and promise applications in different fields like integrated optics and biosensors. The films used in the measurement shown in this paper are polyelectrolyte multilayers. They are built up by alternating adsorption of anionic and cationic po1yelectrolytes.l Measurements with SAXS and Wvis spectroscopy provide first proofs for the layer structure of such self-assembly films. The total film thickness can be controlled with A precision.2 A special feature of molecular films is that the macroscopic properties can be controlled by the microscopic structure. Therefore, it is interesting to get more information about the internal structure of the films: neutron reflectivity measurements showed that the polyelectrolytes are deposited in layers with an interdigitation between the chains of about 15A.3 This is smaller but close to a layer thickness and indicates that there is no sharp alternation between polyelectrolytes of opposite charges. The driving force for the formation of such multilayers seems to be the electrostatic attraction between the oppositely charged polyelectrolytes. In this paper, measurements of the electrostatic properties via the proton distribution in the film are presented. For this purpose, a pH-sensitive dye was deposited in the film at different distances to the film surface but with the dye always experiencing the same polyelectrolyteenvironment. The dye fluorescence was measured after excitation in a TIRF apparatus dependent on the external pH value. The measured shift of the titration curve for different positions of the dye in the polyelectrolyte film yielded information about the proton concentration in the dye layer. The results can be described by the Gouy-ChapmanStern theory assuming that this theory also described the ion distribution in the polyelectrolyte film. The surface potential can be controlled via both the surface charge and the ionic e n ~ i r o n m e n t .In ~ order to check whether
* To whom correspondence should be addressed.
Universitaat Mainz. MPI fur Kolloid-und Grenzfllchenforschung. Abstract published in Advance ACS Abstracts, September 1, 1995. 8
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(1)Decher. G.:, Honc. Solid 1992.210f ~ ~ J. D.: ~ , Schmitt. ~~~~~~~~~. ~ o , J. .,.Thin ~.~ ~ Films ..... . ~ . ~. -~ ----, 211,831. (2) Decher, G.; Schmitt, J. Prog. Colloid Polym. Sci. 1992,89,160. (3)Schmitt, J.;Griinewald, T.; Decher, G.;Pershan, P. S.;yjaer, K.; Losche, M. Macromolecules 1993,26,7058.
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Figure 1. Polyelectrolytes: the polycation poly(ally1amine hydrochloride) (PAH, n = 500-600) and the polyanion poly(styrenesulfonate)sodiumsalt (PSS,m = 400). PEI is not shown here because it is not relevant for the message of this paper.
the application of the model on these polyelectrolyte films is justified, experiments were carried out with reduced surface charge and increased external salt concentration.
11. Experimental Section 11.1. Materials. Poly(ethy1enimine) (PEI), poly(styrene-
sulfonate)sodium salt (PSS),and poly(aly1aminehydrochloride) (PAH)were obtained from Aldrich (Steinheim, Germany). PSS was dialyzed against Milli-Q water and freeze-dried. PEI and PAH were used without further purification. The structural formulas for PAH and PSS are shown in Figure 1. The dye used was fluorescein isothiocyanate (FITC,Sigma)whose fluorescence shows a strong dependence on the pH value of the environment. In solution, the fluorescence intensity of fluorescein increases with increasing pH value because more and more dye molecules switch from the colorless to the fluorescent form (Figure 2).5 PAH was labeled with FITC accordine to the same standard method used for protein labeling.6 In &e following, it is called FITC-PAH. In order to vary the pH value of the external environment, a M aqueous citrate buffer solution characterized by wide buffer capacitywas used. The different pH values were obtained by adding NaCl and HC1, respectively. Glass'substrates (46 x 13 x 1.3 mm)were purchased from Hellma (Jena, Germany)and cleaned by toluene reflux and plasma cleaner. The substrate served as the top ofthe flow cell produced by Hellma (Miihlheim, Germany). It consists of a quartz block with two flow channels on the short sides of the cell.
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(4) McLaughlin, S. Current Topics Membrane Transport 1977,9,71. ( 5 ) Beyer, H.; Walter, W. Lehrbuch der organischen Chemie, 19th
Ed.; S. Hirzel Verlag Stuttgart: Mainz, 1981;p 573. (6)Nargessi, R. D.;Smith, D. S. Methods Enzymol. 1986,122,67.
0743-746319512411-3554$09.00/00 1995 American Chemical Society
Langmuir, Vol. 11, No. 9, 1995 3555
Proton Concentration Profile in Ultrathin Polyelectrolyte Films
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Figure 3. Preparation of the polyelectrolyte films by consecutive adsorption of polycation (PAH) and polyanion (PSS)from aqueous solutions. Only the structure of the decisive film coveringthe dye layer (FITC-PAH) is given in detail. According to neutron reflectivity data, the interfaces between consecutive layers are smeared by about 15 A. Because of clarity, the polyelectrolyte layers are presented without any interdigitation.
11.2. Film Preparation. The polyelectrolyte films were deposited by immersion for 20 min in aqueous solutions containing mmol of the respective polyelectrolyte and by dipping in Milli-Q-water for 3 x 1 min after each deposition step. Polyanion and polycation were deposited consecutively via this self-assembly technique. In order to obtain an increased adsorption, a precursor film with a total thickness of about 140 A consisting of layers of PEI, PSS, and PAH was deposited on the cleaned substrate. After the last PSS layer, the wafer was coated with FITC-PAH (about 6 A). Now, different numbers of layers of PSS and PAH were deposited on different substrates (Figure 3). The polyelectrolyte solutions contained 0.5 mol of NaCl. The thickness of one PSS layer was about 20 A and of one PAH layer about 10 A. The thinnest film has no further PSS or PAH layer on the FITC-PAH layer. The thickest film has eight bilayers (PSSPAH) and one further PSS layer on the FITC-PAH layer. The dye is always attached to PAH; hence, we compare dye features in identical polyelectrolyte environments. Because of clarity, the polyelectrolyte layers are presented without any interdigitation in Figure 3. The repsective film thicknesses were measured by small-angle X-ray scattering (SAXS)via analysis of the so-called Kiessig fringe^.^ 11.3. Apparatus. The TIRF experimental system used is
Figure 5. Example for a complete measurement cycle: first increasing the pH value (0)and then decreasing the pH value (0) of the external buffer solution. The data points are integrated fluorescence intensities (515-550 nm) which were normalized with respect to the result at pH 3.0. The error bars which were calculated from three different measurements correspond to the symbol size. The solid lines correspond to the fit using the Henderson-Hasselbach equation. The dashed curve is a guide to the eye. The horizontal dashed line corresponds to the relative fluorescence intensity at the respective pKa value. shown in Figure 4. An argon ion laser beam at 488 nm is coupled into the glass substrate via glass prisms obtained from Spindler & Hoyer (Gattingen, Germany). The coupling angle was fixed far away from the critical angle (about 62") at about 70". The evanescent field at the substrate surface excited the dye in the polyelectrolyte film on the bottom of the substrate. The fluorescent light was collected through a lense and filter system and an optical fiber bundle both placed perpendiculary to the substrate at the upper side and was measured by the OSMA (optical simultaneous multichannel analysis). Figure 4 shows a spectrum of the FITC-PAH which was measured in this way. The substrate with the polyelectrolyte film on its lower side served as the top of the flow cell where different aqueous buffer solutions were flushed in by a pump. 11.4. Measurement Procedure. The measurements were carried out for both increasing pH value and decreasing pH value between pH 3.0 and pH 11.0. In Figure 5, the complete result of such a measurement is shown. The displayed values are integrated fluorescence intensities (515-550 nm) which were normalized with respect to the result at pH 3.0. One realizes for this case a hysteresis of about 0.3 pH units. (Wehave encountered cases where hysteresis could be up to 1pH unit. Therefore, for a qualitative comparison, we considered only changes of the system on increasing the pH. For the quantitative analysis, we (7)Kiessig, H.Ann. Phys. 1931,10,769.
Klitzing and Mohwald
3556 Langmuir, Vol. 11, No. 9, 1995
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Figure 6. Integrated fluorescencenormalized to display equal intensities at pH 3.0 depending on the pHvalue ofthe external buffer solution. The parameter is the thickness of the polyelectrolyte film on the FITC-PAH layer. (a) Film with PAH surface: no film (01,PSSPAH (01,(PSSPAH)2(O), and (PSS/ PAH)3(+) on FITC-PAH. (b)Filmwith PSS surface: PSS (O), PSSPAWPSS (O), (PSSPAH)dPSS(O), (PSS/PAH)flSS (+), and (PSSPAH)$pSS (A) on the FITC-PAH layer. cycled the system and took the average measured on increasing and decreasing the pH.) The effects are larger than this hysteresis. Still we prefer to use the average to have comparable
values, the absolute values being less relevant. The influence of three parameters on the fluorescence of FITC-PAH was considered: (1)thickness of the polyelectrolyte film on the FITC-PAH; (2) charge of the film surafce; (3) ion concentration of the buffer solution. The variation of the signal following a pH change occurred faster than the time scale of the experiment (seconds). 111. Results
111.1. Effect of Film Thickness. Figure 6 shows the pH dependence of the fluorescence intensity of polyelectrolyte films with different thicknesses on top ofthe FITCPAH. Like in solution, the curve of the FITC fluorescence intensity vs pH shows a sigmoidal shape. The influence of the pH value of the M buffer solution decreases with increasing film thickness. To facilitate the comparison of pH dependence for different film thicknesses, the curves were normalized with respect to the result for pH 3.0. The intensity at pH 3.0stays nearly constant and varies at higher pH values. This fact justifies a normalization with respect to the value for the lowest pH. For a film thickness above loo& the intensity does not depend on film thickness. The system reaches saturation. This behavior is independent of whether there are polycations (PAH, Figure 6a) or polyanions (PSS,Figure 6b) on the film surface. To facilitate the qualitative comparison, all measured curves shown are normalized such that the intensities both at the lowest pH and the highest pH, respectively, are equal. This was achieved by multiplying the measured intensities by a proper factor and subtracting a constant value. This procedure is only relevant for the
display of data, not for the quantative analysis. For the latter, we deduce merely the pKa value, the midpoint of the titration curve, as a function of coverage. All curves for films with PAH on their surface have the same qualitative shapes with almost the same positions (they are not shown here). In contrast to that, the curves for films with PSS at their surface show qualitatively different shapes (Figure 7). With increasing film thickness, a shift of data points toward lower pH values appears. Such a data curve as shown here corresponds to a titration curve to which a pKavalue can be assigned. The pH dependence of the FITC fluorescence intensity could be described by a single proton dissociation. Therefore, the HendersonHasselbach equation is used:
For pH = P K , , ~ , ~ I(pH 11)- I(pKa) I(pKa) - I(pH 3)
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These equations were based on the assumption that the pH has no influence on the fluorescence intensity outside the range 3.0-11.0. The solid lines in Figure 5 show the fits of the respective titration curves. One realizes that eq 1 describes the data well for the upper half of the titration curve. For lower pH, the titration curve is stretched, indicating a fuzzier interface. This suggests that varying the pH also causes structural changes of the film surface at lower pH values. This effect, however, is less important for the quantitative analysis where we only consider the midpoints of the titration curve. All curves for films with PAH at the surface (Figure 6a) have almost the same pKa value of about 5.9. In contrast to this, the changes of the pKa value are obvious for films with PSS on the surface. The PK, value decreases from 7.5 for the thinnest film (PSS, 0 )to 5.8 for the thickest A). film investigated ((Pss/pAH)$pss, The apparent pKa value of FITC-PAH in solution is 4.6. This difference in the pKa values on changing the experiment is also found in the work of Soucaille et al.9 at the waterflipid interface. 111.2. Effect of Film Surface Charge. The charge of the negative PSS surface can be reduced by divalent cations that replace the monovalent Na+ ions before starting the measurement. Therfore, the film was rinsed with 1M aqueous solutions of MgClz and CaC12, respec(8)Geisow, M. J. Exp. Cell Res. 1984,150, 29. (9) Soucaille, P.;fiats, M.; Tocanne,J.F.; Teissie,J.Biochim. Biophys. Acta 1988, 939, 289.
Proton Concentration Profile in Ultrathin Polyelectrolyte Films
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Langmuir, Vol. 11,No. 9, 1995 3557
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Figure 9. Integrated fluorescence depending on the number of polyelectrolyte layers (0* no film, 1* PSS, 2 * PSSDAH, etc.) on the FITC-PAH layer. The parameter is the NaCl M concentration of the buffer solution with fixed pH 6.0: (O), 10-l M (+), and 1M (0);the data points are normalized M buffer solution with respect to the measurements with without any additional salt. tively, and Milli-Q water in both cases before starting the usual measurement procedure. Figure 8 shows that, for intermediate pH values, the data of measurements after rinsing with divalent cations (0, 0 ) are shifted toward lower pH values compared to the curve without rinsing (0)before the measurement. The results for the highest and lowest pH values are similar for all three conditions. 111.3. Effect of Ion Concentrationof Buffer Solution. The ion concentration was varied by adding NaCl to the buffer solution at a fixed pH value of 6.0. The measurement started with pure M buffer solution and was continued using buffer solutions, with increasing salt concentration from to 1M. Figure 9 shows the results normalized with respect to the values ofpure buffer solution with a dependence on the number of polyelectrolyte layers on the FITC-PAH. Films with a PSS surface (odd number of polyelectrolyte layers on FITCPAH) show an increase in intensity with increasing salt concentration. In contrast to this, measurements with PAH surface films (even number of polyelectrolyte layers on FITC-PAH) result in an intensity decay with increasing salt concentration. The influence of ion concentration both decreases with increasing thickness ofthe film above, the dye and is greater for films for PSS than with a PAH surface. Investigations into divalent ions (MgS04) show similar results. Since we measure an influence of the outer charge on the dye, although the immediate environment is not changed, we can conclude that the influence of a possible Donnan potential at the interface is not
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Figure 10. Schematic representation of the titration curves for different ion concentrations in the external buffer solution of a fixed pH value of 6.0. The arrows indicate the direction of increasing salt concentration for films with PAH surface (dashed lines) and for films with PSS surface (solid lines).
effective inside the film. Probably this potential is too small because of a small concentration of excess fixed charges. A remarkable observation was the fact that the results were not reversible for films with a PAH surface. A possible explanation could be a nonreversible change of structure by using a higher salt concentration. This would raise the question why that is not the case with films with a PSS surface. This is still unexplained.
IV. Discussion and Conclusions IV.l. Qualitative Aspects. One of the interesting aspects considering polyelectrolyte films as probable electrical devices concerns their ion permeability. We have shown above that a pH change in the outside medium is sensed by a dye inside the film. This proves that the films are permeable for protons. (They are probably also permeable to other ions.) The position of the titration curve in addition depends on the thickness of the polyelectrolytemultilayer deposited on top of the dye-containing layer. This shows that there is a pH profile along the film normal. In addition, we can reject a hypothesis that after preparation, the dyecontaining polymer is uniformly distributed. In fact, the sharp profiles which we will derive below indicate that any smearing of the dye distribution due to interdiffusion cannot exceed 10 A. The pH profile and absolute pH values inside the film drastically depend on the charge and, hence, the potential at the interface between the top layer and water. This has been clearly proved by the following results: (1) The sign of the shift of the apparent pKa value compared to the titration of the dye in solution depends on the sign of the charge of the outer polyelectrolyte film. The finding that the shifts are more pronounced for the film with PSS outside compared to PAH then suggests that the latter exhibits a lower charge density. This has in the meantime been confirmed by 5 potential measurements,1° showing potentials on the order of -100 mV for PSS and merely +10 mV for PAH as the outer layer. (2) The effects can be drastically reduced by binding divalent ions to the negatively charged surface. (3)Increasing the ionic strength and thus decreasing the surface potential also reduces the shifts. Even for thin films covering the dye-containing layer, the titration curves are shifted more and more toward the positions in the film bulk with increasing salt concentration of the external buffer (Figure 10). Therefore, at fixed pH, the fluorescence intensity in Figure 9 increases €or the films (10)Donath, E.;Lowack, K.; Helm, C.A.Unpublished 1995.
Klitzing and Mohwald
3558 Langmuir, Vol. 11, No. 9, 1995
with the PSS surface and decreases for the films with the PAH surface. Before entering a qualitative analysis of some data, we should comment on the fact that the fluorescence intensities measured at high pH value and especially for film thicknesses below 100hi depend on the thickness, although the titration curve appears to exhibit a plateau. A n explanation could be that the fluorescein dye “sees” different local environments depending on its position within the polyelectrolyte film. We assume that there is a fluorescence decrease when the environment of the dye becomes more solid. When the first PSS layer is deposited on FITC-PAH, this could not be covered completely. Chains of the labeled polyelectrolyte extend partly into the buffer solution. By depositing more polyelectrolyte layers, the dye is more and more covered, and therefore, the local environment becomes more and more solid. In this way, the fluorescence decreases step by step. After depositing a film of three bilayers (PSSPAH)and a further PSS layer, the bulk steady-state condition is reached. Even by depositing more polyelectrolytelayers, the fluorescence does not change because the environment stays likewise solid. Adsorption measurements of FITC-PAH confirm this hypothesis. Fluorescence intensity vs adsorption time shows for certain polyelectrolyte concentrations an “overshoot” with its maximum after 5-10 min.’l One could infer that when the solution is first flushed into the cell, a lot of dye approaches the substrate surface where it is excited via the evanescent field. In this way, the fluorescence intensity increases. After several minutes, the “reaction” between FITC-PAH and substrate (bulk) sets in, and the fluorescence decreases. The measurements of Soucaille et al. appear to contradict these results. The fluorescence increased during compression of the lipid-FITC film. On the other hand, it is possible that the increase of fluorescence indicates merely that the amount of fluorescent material in the investigated spot increases. IV.2. Quantitative Aspects. For a quantitative analysis, we consider the system where all environmental influences are most pronounced: films with negative charges (PSS) at the outer surface at low ionic strength MI, varying the distance of the pH-sensitive dye from the fildsolution interface. Still we should stress that findings with other surface charges and ionic conditions are in agreement with the model we apply to describe this system. As the parameter to describe the findings, we use the apparent pKa value. This value can be changed systematically by up to 2 units. To describe the ion distribution in the polyelectrolyte film, we use the Guy-Chapman-Stern mode112-14in analogy to the procedure usually applied for liquid electrolyte solutions. There the surface exhibits an excess charge which consists of the charge in the polyelectrolyte minus those of firmly bound counterions. To allow for electroneutrality, the solution near the interface contains the same excess amount of counterions. In the present case, these counterions can be in the adjacent aqueous solution and in the polyelectrolyte film. However, we are mainly interested in the ion and potential distribution within this film. For this film, we assume a dielectric constant ( E A and a charge density ([H+I(-)) at large distances from the interface and a surface charge density that is the fraction of polyelectrolyte charges minus those (11)Hitzing, R.v.; Mahwald, H. Unpublished, 1995. (12)Gouy, G. J . Phys. Radium 1910,9, 457. (13)Chapman, D. L. Philos. Mag. 1913,25,475. (14)Stern, 0. 2.Elektrochen. 1924,30,508.
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Figure 11. Potential profile in a polyelectrolyte film with a PSS surface, calculated by using the pKa values determinated by the data curves shown in Figure 7. The solid line corresponds to the fit using eq 3 and presents the upper envelope of a weak potential oscillation inside the film. counterbalanced by bound ions. The surface potentials (Y(z))along the film normal and at the surface (Y(0))are related by
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The proton concentrations far away ([H+l(-)) and near the interface ([H+l(z))are given by the Boltzmannequation
(4) Y(z)depends on all ions in the medium. The Debye length (6) is given by
I is the ionic strength of the film bulk. Therefore, the pH value is (6) and hence, the apparent pKa value is shifted accordingly:
(7) From eq 7,we derive
Equations 7 and 8 are independent on the model of a potential distribution (Y(z))we have described above by the solution of the Poisson-Boltzmann equation. In the experiment, we obtained for the thickest film pK, (z -1 = 5.8. With the experimentalvalues, we can now calculate Y(z)from eq 8. The result is given in Figure 11. The line through the data points corresponds to the expected exponential dependence with 6 30 hi and Y(0) -200 mV. The latter value is rather imprecise since it is difficult to determine the pKavalue exactly enough. Further, this potential value depends on the exact location of the surface (z = 0). One problem is the definition of the surface since the polyelectrolyte chains extend partly into the buffer solution. Because of the interdigitation, it is difficult to localize the layer exactly, as well. The other inaccuracy is the film thickness determination by SAXS. The films were investigated after drying, and it is possible that the films surrounded by buffer solution could swell. It is important to point out that the potential (Y(0))does not
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Proton Concentration Profile in Ultrathin Polyelectrolyte Films
Langmuir, Vol. 11, No.9, 1995 3559
correspond to the usual surface potential measured at the external side over a macroscopic part of the surface. On the other hand, it is interesting to compare the Debye length in the polyelectrolyte film with that calculated for aqueous solutions under the same conditions. For a M monovalent ion concentration, the Debye length amounts to 100 A. According to eq 5, the lower value observed here may either be due to a smaller dielectric constant or to a higher concentration ofdiffuse counterions. Our data suggest a dielectric constant near 10 for the polyelectrolyte film. The dielectric constant of the water/ membrane interface, i.e., the diffuse double layer, is of the order of 20-30.15 We cannot yet argue against a counterion concentration of a factor of 10 larger. On the other hand, the fact that the Debye length is well above 10 A indicates that the concentration of the mobile counterions is not above 10+ M. Indeed, in our preliminary ESCA studies on dry films, we could detect neither C1- nor Na+supporting the above limit.16 The large Debye length also requires that the charges on the polyelectrolyte be immobile. One might argue against the model presented that there is a strong oscillation potential distribution due to the buildup of the film by polyelectrolytes with alternating charges. However, neutron reflectivity
measurements have shown that adjacent layers interdigitate, thus reducing the possible potential oscillations. Still, there may be oscillations in the potential due to a residual layering of fmed charges. These would be superimposed on our measured potential. Since the dye is embedded always in the same environment with positively charged polyelectrolyte, the measured potential would correspond to a maximum of this oscillation and the solid line in Figure 11 corresponds to the upper envelope of a weak oscillation. In conclusion, our experiments yield the picture on the potential distribution inside a polyelectrolyte film. As a function of distance, the potential decays exponentially with a decay length differing from the outside of the film. The absolute value as well as the decay length can be varied via the surface charge and the total ionic concentration. It will be an interesting task for future work to determine the frequency dependence ofthe ion distribution in view of the different ion mobility in both phases."
(15) Gileadi, E.; Kirowa-Eisner, E.; Penciner, J. Interfacial Electrochemistry; Addison-Wesley: Reading, MA,1975; p 15. (16) Meisel, W. Unpublished, 1995.
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Acknowledgment. We thank Frank Essler for labeling the polyelectrolyte and Christiane Helm and Gero Decher for helpful discussions. This work was supported by the German-Israeli Foundation (GIF).
(17)Grahame, D. G. Chem. Rev. 1947,41,441.
