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Polyelectrolytes. 2. Intrinsic or Extrinsic Charge Compensation? Quantitative Charge Analysis of PAH/PSS Multilayers H. Riegler*,† and F. Essler‡ MPI-KGF, Am Mu¨ hlenberg, 14476 Golm/Potsdam, Germany, and Novosom AG, Weinbergweg 22, 06120 Halle, Germany Received February 1, 2002. In Final Form: May 13, 2002 Floating multilayers of polyallylamine/polystyrenesulfonate (PAH/PSS) adsorbed to a dimethyldioctadecylammonium bromide (DODAB) monolayer at the air/water interface (at pH 5.5) are analyzed quantitatively with respect to the internal charge stoichiometry of the polyelectrolytes. To this end the adsorbed amount of polymer is translated into charge per area. It is postulated that the strong acid PSS is fully charged (fully dissociated). The weak polyelectrolyte PAH is also assumed to be fully dissociated. This is vindicated by theoretical considerations and deduced from pH-dependent studies of the isotherms of Langmuir monolayers of negatively charged dimyristoylphosphatidic acid (DMPA) with PAH adsorbed from the subphase. With both polyelectrolytes fully charged the PAH charges significantly overcompensate the PSS charges. The polyelectrolyte charge stoichiometry is not 1:1, and the multilayer must contain some small counterions (Cl-) for charge compensation. It is suggested that this “extrinsic compensation” occurs when the charge density of the polyelectrolytes exceeds a certain threshold. If the distance between the charges is less than the Bjerrum length, the strong binding of the counterions (“Manning condensation”) prevents their release upon multilayer formation. This explains why multilayers with PAH with its unusually high charge density contain counterions whereas other polyelectrolyte films are ion-free. Literature data support this hypothesis.
1. Introduction The formation of multilayers from polyelectrolytes (“selfassembly”) is based on the consecutive adsorption of polyions with alternating charge.1-6 In a typical cross section through a polyelectrolyte multilayer film, a solid substrate carries some surface charges on which the first polyelectrolyte layer of opposite charge is adsorbed. This is followed by a layer of opposite charge which again serves as template for the next adsorbing layer, etc. It is assumed that consecutive layers within the multilayer are heavily interpenetrating and that they electrically neutralize each other (“charge compensation”), except for the outermost layer. The charge of this outer layer “overcompensates” the underlying multilayer charges. Its extra charges enable the adsorption of the next layer. Small counterions render the total system, i.e., the substrate, the polyelectrolyte multilayer, and the adjacent bulk solution, electrically neutral. It is well accepted that the electrostatic interaction between the (opposite) charges of adjacent polyelectrolyte layers is the driving force for the multilayer buildup.7-37 This interaction depends on the charge † ‡
MPI-KGF. Novosom AG.
(1) Decher, G. Science 1997, 277, 1232. (2) Decher, G.; Hong, J.-D.; Schmitt, J. Thin Solid Films 1992, 210/ 211, 831. (3) Decher, G. In Templating, Self-Assembly and Self-Organization Sauvage, J.-P., Hosseini, M. W., Eds.; Comprehensive Supramolecular Chemistry 9; Pergamon Press: Oxford, 1996; pp 507-528. (4) Bertrand, P.; Jonas, A.; Laschewsky, A.; Legras, R. Macromol. Rapid Commun. 2000, 21 (7), 319. (5) Hammond, P. Curr. Opin. Colloid Interface Sci. 2000, 4, 430. (6) Donath, E.; Sukhorukov, G. B.; Caruso, F.; Davis, S. A.; Mo¨hwald, H. Angew. Chem., Int. Ed. 1998, 37 (16), 2201. (7) Blaakmeer, J.; Bo¨hmer, M. R.; Cohen Stuart, C. A.; Fleer, G. J. Macromolecules 1990, 23, 2301. (8) Bo¨hmer, M. R.; Evers, O. A.; Scheutjens, J. M. H. M. Macromolecules 1990, 23, 2288. (9) Borukhov, I.; Andelman, D.; Orland, H. Europhys. Lett. 1995, 23, 499.
densities of the polyelectrolytes and on the concentration of the (counter)ions. The counterions indirectly (but strongly) influence the multilayer formation by adjusting the conformation of the polyelectrolyte molecules in the bulk (through shielding effects). Much, if not all, of the (10) Linse, P. Macromolecules 1996, 29, 326. (11) Chatellier, X.; Joanny, J.-F. J. Phys. II Fr. 1996, 6, 1669. (12) Borukhov, I.; Andelman, D. Macromolecules 1998, 31, 1665. (13) Netz, R. R.; Joanny, J.-F. Macromolecules 1998, 31, 5123. (14) Netz, R. R.; Joanny, J.-F. Macromolecules 1999, 32, 9013. (15) Joanny, J.-F. Eur. Phys. J. B 1999, 9, 117. (16) Castelnovo, M.; Joanny, J.-F. Langmuir 2000, 16, 7524. (17) Andelman, D.; Joanny, J.-F. C. R. Acad. Sci. Paris, t. 1. Ser. IV 2000, 1153. (18) Dubas, S.; Schlenoff, J. B. Macromolecules 1999, 32, 8153. (19) Farhat, T.; Yassin, G.; Dubas, S. T.; Schlenoff, J. B. Langmuir 1999, 15, 6621. (20) Ruths, J.; Essler, F.; Decher, G.; Riegler, H. Langmuir 2000, 16, 8871. (21) Steitz, R.; Jaeger, W.; Klitzing, R. v. Langmuir 2001, 17, 4471. (22) Schoeler, B.; Kumaraswamy, G.; Caruso, F. Macromolecules 2002, 35, 889. (23) Yoo, D.; Shiratori, S. S.; Rubner, M. F. Macromoecules 1998, 31, 4309. (24) Kolarik, L.; Furlong, D. N.; Joy, H.; Struijk, C.; Rowe, R. Langmuir 1999, 15, 8265. (25) Shiratori, S. S.; Rubner, M. F. Macromolecules 2000, 33, 4213. (26) Park, S. Y.; Barrett, C. J.; Rubner, M.; Mayes, A. M. Macromolecules 2001, 34, 3384. (27) Laurent, D.; Schlenoff, J. B. Langmuir 1997, 13, 1552. (28) Schlenoff, J. B.; Ly, H.; Li, M. J. Am. Chem. Soc. 1998, 120, 7626. (29) Farhat, T. R.; Schlenoff, J. B. Langmuir 2001, 17, 1184. (30) Schlenoff, J. B.; Dubas, S. T. Macromolecules 2001, 34, 592. (31) Dubas, S. T.; Schlenoff, J. B. Macromolecules 2001, 34, 3736. (32) Schmitt, J.; Gru¨newald, T.; Decher, G.; Pershan, P. S.; Kjaer, K.; Lo¨sche, M. Macromolecules 1993, 26, 7058. (33) Kellog, G. J.; Mayes, A. M.; Stokton, W. B.; Ferreira, M.; Rubner, M. F.; Satija, S. K. Langmuir 1996, 12, 5109. (34) Hoogeveen, N. G.; Cohen Stuart, M. A.; Fleer, G. J. Langmuir 1996, 12, 3675. (35) Lo¨sche, M.; Schmitt, J.; Decher, G.; Bouwman, W. G.; Kjaer, K. Macromolecules 1998, 31, 8893. (36) Caruso, F.; Lichtenfeld, H.; Donath, E.; Mo¨hwald, H. Macromolecules 1999, 32, 2317.
10.1021/la020108n CCC: $22.00 © 2002 American Chemical Society Published on Web 07/20/2002
Charge Analysis of PAH/PSS Multilayers
counterions will be released from the polyelectrolyte molecules in the wake of the charge complexation between the adsorbing and the already adsorbed polyelectrolytes as soon as the polyelectrolyte molecules adsorb to the multilayer film. Thus the charge compensation within the multilayer can be due to polylectrolyte complexation (“intrinsic compensation”) plus additional neutralization by counterions within the film (“extrinsic compensation”).28 For a reasonable understanding of the mechanisms of polylectrolyte multilayer formation, details on the location and number of both the polyelectrolyte charges and those of the small counterions have to be known. Although there have been several experimental and theoretical studies focusing on this topic,27-37 it is not yet fully understood. Especially the concentration of small ions within polyelectrolyte multilayers is still heavily debated. In an earlier report we have presented detailed quantitive data on the total amount of adsorbed polyelectrolyte within a multilayer floating at the air/water interface.20 In the following we will extract from these data the quantitative distribution of polyelectrolyte charges within the film. Thus we obtain information on the charge stoichiometry of the polyelectrolytes and draw conclusions on the amount of small counterions in the film. Because polyallylamine hydrochloride (PAH) is a weak polyelectrolyte, the translation from the amount of polymer to charges is not straightforward. It is necessary to determinate the degree of dissociation of PAH at the interface (which may differ from its bulk solution value23,24). This task is accomplished by a theoretical analysis of the effective pK at interfaces38 and corroborated by the analysis of isotherms of monolayers of lipid/PAH complexes39,40 as a function of the subphase pH. Full dissociation of PAH is concluded, which leads us to believe that the multilayers contain a substantial amount of small counterions. This agrees,32-37 but also conflicts27-31 with other experimental findings on different polyelectrolyte multilayer systems. We reconcile these findings by proposing a strong counterion binding (“Manning condensation”)41-44 for polyelectrolytes with high charge density. 2. Materials and Methods The experimental conditions, material, and especially the preparation procedure for the multilayers are described extensively in an earlier report,20 so a shorter description will be sufficient here. The polystyrenesulfonate sodium salt (PSS; MW ≈ 80 000-100 000) and the polyallylamine hydrochloride (PAH; MW ≈ 50 000-60 000) were purchased from Aldrich (Figure 1). The PSS was purified by dialysis (pore size 14 000) against pure water and then freeze-dried. The PAH was used as received. The lipids dimethyldioctadecylammonium bromide (DODAB) and dimyristoylphosphatidic acid (DMPA) were obtained from Sigma (both with a purity >99%) and used without further treatment. The monolayers were spread from chloroform solutions (1 mg/ mL) on water from a Millipore system (Milli-RO 35 and Milli-Q). The floating polyelectrolyte multilayers were prepared at 20 °C in a small Langmuir trough with a surface area of about 50 (37) Ladam, G.; Schaad, P.; Voegel, J. C.; Schaaf, P.; Decher, G.; Cuisinier, F. Langmuir 2000, 16, 1249. (These authors find also, similar to our results, that the incremental adsorbed amouts of PAH and PSS are roughly equal upon multilayer growth, which leads to the same conclusions if full PAH charge is assumed.) (38) Helm, C. A.; Laxhuber, L.; Lo¨sche, M.; Mo¨hwald, H. Colloid Polym. Sci. 1986, 264, 46. (39) de Meijere, K.; Brezesinski, G.; Pfohl, T.; Mo¨hwald, H. J. Phys. Chem. B 1999, 103, 8888. (40) Ahrens, H.; Baltes, H.; Schmitt, J.; Mo¨hwald, H.; Helm, C. Macromolecules 2001, 34, 4504. (41) Manning, G. J. Chem. Phys. 1969, 51, 934. (42) Manning, G. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 909. (43) Desorno, M.; Holm, C.; May, S. Macromolecules 2000, 33, 199. (44) Sens, P.; Joanny, J.-F. Phys. Rev. Lett. 2000, 84, 4862.
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Figure 1. Chemical structure of DODAB, DMPA, PSS, and PAH. cm2, which allows the exchange of the subphase (Riegler & Kirstein GmbH, Berlin). For the preparation the subphase under the Langmuir monolayer consisted of a sequence of polyelectrolyte solutions of alternating charge with flushing buffer solutions in between. Thus the controlled buildup of floating polyelectrolyte multilayers is accomplished. For all polyelectrolyte multilayers investigated here, the template for the adsorption of the first polyelectrolyte was a positively charged DODAB monolayer (surface pressure 35 mN/m). Thus the polyelectrolyte multilayers always started with a negatively charged PSS layer. The composition of the multilayers (individual layer thicknesses and polyelectrolyte densities) was monitored by ellipsometry.45 The degree of dissociation of PAH was investigated by monitoring the isotherm of DMPA with a PAH layer adsorbed underneath. The ion concentrations as indicated in the figures reflect the values in the subphase before adding the polyelectrolytes. The polyelectrolyte powders which were added to the subphase contained counterions (they were electrically neutral). Thus, the ion concentrations of the subphases after adding the polyelectrolytes were slightly higher than indicated in the figures. This means even the subphase of nominally pure water (“H2O”) contains minor amounts of ions (which explains the charge oscillations (Figure 4)).
3. Experimental Results 3.1. Degree of Dissociation of PAH at Interfaces. PAH is a weak polyelectrolyte whose allyl hydrochloride group can be charged positively at low and medium pH values. A high pH leads to the dissociation of H+ ions and the formation of an uncharged amine group. The pK of PAH in bulk solution is around pH 10,23,24 similar to the pK of an isolated amine group.46 To determine its degree of dissociation at an interface, which may be significantly different from its interfacial value,38 a layer of PAH was adsorbed onto a floating Langmuir monolayer of DMPA. The isotherms of this DMPA/PAH layer were measured (45) Paudler, M.; et al. Makromol. Chem. Macromol. Symp. 1991, 46, 401. (46) CRC Handbook of Chemistry and Physics; Lide, D. R., Frederikse, H. P. R., Eds.; CRC Press: Boca Raton, FL, 1995.
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Figure 2. Isotherms of a DMPA monolayer (a) and a DMPA monolayer with PAH (b) adsorbed from the subphase as function of the subphase pH. The PAH concentration was 10-2 M.
as a function of the subphase pH and compared to isotherms of a pure DMPA monolayer. From this comparison the degree of dissociation of the polymer can be estimated because the polymer charge will greatly influence the isotherm.39 Figure 2a presents isotherms of DMPA monolayers without adsorbed PAH as a function of the subphase pH. The headgroup of DMPA will change from uncharged at very low pH to double charged at very high pH. This is reflected in the isotherm. At pH 2.0, upon decreasing the area per molecule, the surface pressure remains constantly very low until the area reaches the cross section of the aliphatic chain. Then the pressure rises steeply. The slight pressure increase below ≈60 Å2/molecule indicates already a slight headgroup charge at this pH. When the DMPA headgroups start to become charged significantly at medium pH, repulsive interactions induce a pronounced plateau of coexistence of the liquid-expanded and liquidcondensed phases (pH 5.5 and 8.0). When the headgroup becomes doubly charged, this plateau becomes even more pronounced (pH 10.0).47 With PAH adsorbed to the DMPA the isotherm changes drastically (Figure 2b). At low and neutral pH the plateau is gone, whereas at high pH it is even more pronounced than without adsorbed PAH. At low and medium pH the positively charged PAH neutralizes the repulsive interactions between the negatively charged DMPA headgroups. At pH 8.0 the adsorbed PAH barely affects the isotherm, and at pH 10.0 the PAH adsorption even leads to an (47) It has been suggested (Caruso, F.; Grieser, F.; Thistlethwaite, P.; Furlong, N. Macromolecules 1994, 27, 77) that the headgroup dissociation is essentially complete in the region where the pressure increases (LE phase ) range of increasing pressure below the plateau pressure), whereas the dissociation is incomplete in the plateau range.
Figure 3. Monomolecular amounts per unit area of PAH and PSS in each individual layer (adsorption increments) as a function of layer number for various polyelectrolyte concentrations (a) and salt concentrations (b). The inserts show the original data on the multilayer growth20 from which the monomolecular growth increments are deduced.
increased plateau. From this, one can conclude that PAH is only partially charged at pH 10.0 and/or its charge is not sufficient to compensate the strong repulsive forces between the doubly charged DMPA headgroups. Nonelectrostatic contributions from the PAH even lead to a substantial increase of the plateau height and width. Therefore, if the adsorbed PAH significantly reduces the plateau pressure or even completely annihilates the plateau it has to be strongly charged. This is the case at pH 5.5 and 2.0. 3.2. Adsorbed Amounts of PSS and PAH Monomoles within Multilayers. Figure 3 shows the adsorbed incremental amounts (i.e., amounts per layer) of PAH and PSS in monomols per unit area as they are measured ellipsometrically in the course of the multilayer buildup for various polyelectrolyte (Figure 3a) and ion (Figure 3b) concentrations. The data are deduced from the results presented earlier20 (see inserts) by translating the layer thicknesses into monomols/m2. It is observed that (except for the first PSS layer at very low polyelectrolyte concentration (Figure 3a) all PAH layers contain substantially more monomols per unit area than their neighboring PSS layers! Typically the amounts of PAH monomols exceed those of PSS by more than 50%, in some cases by even ≈100% at high ion concentrations (Figure 3b)!
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Figure 4. Charge stoichiometry (excess beyond neutralization of the sum of all positive and negative charges in the entire multilayer) as a function of layer number for various polyelectrolyte concentrations (a) and salt concentrations (b) under the assumption that both PAH and PSS are fully charged. (c) and (d) show corresponding data with the assumption of PSS being fully charged whereas PAH is only charged by 2/3.
3.3. Sum of the Polyelectrolyte Charges within PSS/PAH Multilayers. Figure 4 shows the sum of the calculated charges of the PSS/PAH multilayers, ∑ charge, as a function of the layer numbers for different polyelectrolyte conentrations, ion concentrations, and degrees of charging. ∑ charge is calculated from the adsorbed amounts (Figure 3) of PSS and PAH via
∑ charge ) ∑j RPAHΓPAH,j - ∑i RPSSΓPSS,i
(1)
where ΓPSS,i ) adsorbed amount of PSS monomols/m2 in layer i, ΓPAH,j ) adsorbed amount of PAH monomols/m2 in layer j, i ) PSS layer number index, j ) PAH layer number index, RPSS ) degree of charging of PSS (fully charged: RPSS ) 1), and RPAH ) degree of charging of PAH (fully charged: RPAH ) 1). ∑ charge does not include the charges of small counterions and of the DODAB monolayer which served as charged template for the first PSS layer. ∑ charge reflects only the charge stoichiometry of the polyelectrolytes. In Figure 4a,b a degree of charging of 1 is assumed for both polyelectrolyte species (RPSS ) RPAH ) 1). Due to the higher amount of PAH monomers compared to PSS, ∑ charge becomes more and more positive for all polyelectrolyte and ion concentrations. If it is assumed that the degree of charging of PAH, RPAH, is 2/3 (with the PSS still assumed fully charged (RPSS ) 1)), then the PSS and PAH charges
roughly compensate each other (Figure 4c,d). In this case the total charge stoichiometry oscillates around a constant value near neutrality. 4. Discussion PAH is a weak polyelectrolyte and the degree of dissociation/charging of its amine group depends on the pH. An isolated amine group at the end of a saturated hydrocarbon chain has a pK ≈ 10.6 46 (methylamine 10.63, decylamine 10.64, octadecylamine 10.60). Because of the similarity of the chemical bonds in the vicinity of the amine group, this is also approximately the pK of an isolated amine group of PAH. In reality, the amine group is not isolated. The pK of PAH will be lower because of the mutual electrostatic interaction between neighboring amine groups, which is unfavorable for deprotonation. The lowering of the pK can be estimated by assuming that the electrostatic conditions for PAH layers within a polyelectrolyte multilayer are similar to a Langmuir monolayer with headgroups which can dissociate and thus be charged depending on the ion strength and the pH. A simplified, continuum theoretical approach, which neglects the discreteness of surface charges and counterions, reveals38 that the surface potential Ψ ≈ 300 mV in the absence of counterions and for an area per chargeable group of A ) 40 Å2. Counterions lower the surface potential and thus favor, i.e., increase, the dissociation because they partially shield and neutralize the electrostatic interactions. If the ionic strengths of monovalent counterions (NaCl) increase from cNaCl ) 10-4 M to cNaCl ) 1 M, Ψ approximately linearly decreases from ≈280 to ≈100 mV
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for A ) 40 Å2. For A ) 120 Å2 Ψ decreases from ≈240 to ≈40 mV. This means that the pK is effectively shifted by roughly 5 to 1 unit for cNaCl increasing from 10-4 to 1 M, respectively. As a result, the effective pK of a plane of amine groups with an area of A ) 40 Å2 per group is 5.6 (cNaCl ) 10-4 M), 7.6 (cNaCl ) 10-2 M), or 9.6 (cNaCl ) 1 M), respectively, instead of 10.6 for the isolated amine group. Typically, the degree of dissociation varies between 10% and 90% within 4 pH units.38 That means when the counterion strength is above 10-2 M, more than 90% of the amine groups of PAH are protonated/charged at pH 5.5. If the (neutralizing) charges of the adjacent PSS within the multilayers are additionally taken into account, then the effective pK, i.e., the degree of dissociation, will be even higher. As has been described already under Experimental Results, the isotherms from the DMPA/PAH monolayers support this theoretically suggested high degree of charging of PAH for neutral pH. Direct measurements on the PSS/PAH multilayer formation as a function of pH also agree with this assumption. At pH 3 and 5 the film formation is very similar, whereas at pH 8 and pH 10 it is different, suggesting a different degree of dissociation at the high pH values.24 If we assume that PAH is fully charged at pH 5.5, Figure 4c,d suggests that about 2/3 of the PAH charges are neutralized by complexation with the neighboring PSS; 1/3 must be neutralized extrinsically by small counterions (Cl-). Several experiments from different research groups also indicate extrinsic compensation within polyelectrolyte multilayers by small counterions,32-37 whereras others indicate that the films do not contain small ions.27-31 It is quite possible that both are correct because the investigated polyelectrolyte multilayers were prepared differently, and because different polyelectrolytes were used. It has already been proposed by Hoogeveen et al.34 that the charge stoichiometry depends on the type of polymer. A quick examination of the literature data shows that extrinsic compensation has been observed only for multilayers which contained PAH,32-34 with one exception (PVP+ 34). Whenever the multilayers were declared ionfree, other cationic polyelectrolytes were involved.27-31 A glimpse at the chemical structure reveals that PAH and PVP+ have an exceptionally high charge density. The average distance between their charged sites is very small, in the range or below the Bjerrum length lB ) e2/4πkBT ( is the dielectric constant of the aqueous medium; kBT is the thermal energy17). For all the other polyelectrolytes used for multilayer formation, the distance between the charged sites is larger than lB. Manning41-43 has shown that the interaction between counterions and charged rods changes fundamentally when the average distance between the charged sides decreases below the Bjerrum length. If the distance is less than the Bjerrum length, a fraction of the counterions is immobilized (“Manning
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condensation”) on the charged rod, such that these condensed ions neutralize the charges until the rod effectively appears to have charges with an average distance like the Bjerrum length. The residual rod charges that are not neutralized by the condensed counterions are then compensated by the usual mobile counterions. DNA, for instance, seems to have a charge density sufficiently high for Manning condensation, and it has been suggested that due to this condensation not all of the counterions are released upon adsorption of DNA onto a plane of opposite charge.44 The situation may be similar in the case of polyelectrolyte adsorption and multilayer formation: If the charge density of a polyelectrolyte is higher than a certain threshold, i.e., the average charge distance is less than the Bjerrum length, then some of the counterions remain with the polyelectrolyte even after incorporation into the multilayer film. There is a certain amount of extrinsic compensation. For polyelectrolytes with lower charge densities the charge compensation is only intrinsic; i.e., all the counterions are released upon polyelectrolyte complexation in the course of their multilayer incorporation. 5. Summary and Conclusion Based on quantitative measurements of the adsorbed amounts of PAH and PSS, we have estimated the polyelectrolyte charge stoichiometry within the polyelectrolyte multilayers. This necessitated the determination of the degree of dissociation of the weak polyelectrolyte PAH within the film. Theoretical estimations and experimental results from the influence of PAH adsorbed onto a Langmuir monolayer of negatively charged DMPA indicate that PAH is approximately fully charged at neutral pH. If this is the case, the charge stoichiometry of the polyelectrolytes is not 1:1. The PAH charges overcompensate the PSS charges. This means that the multilayers contain additional small counterions for complete charge compensation (a prerequisite for multilayer formation). This agrees with reports which also claim that polyelectrolyte multilayers contain residual conterions. Seemingly in contradiction, other studies prove that polyelectrolyte multilayers are ion-free. We suggest that the ion content depends on the charge density of the polyelectrolyte. If the average distance between the polyelectrolyte charges is less than the Bjerrum length, the multilayers contain small counterions which are immobilized (“Manning condensation”) “on” the polyelectrolyte. If the charge density is less than this threshold value, the multilayers are free of counterions because all ions are released upon film formation. Acknowledgment. We thank Frank Caruso, Gero Decher, Dirk Kurth, and Helmuth Mo¨hwald for valuable comments and suggestions. LA020108N
