Counterions and Water in Polyelectrolyte Multilayers: A Tale of Two

Between alternating exposures to the polyelectrolytes, there were three rinses in ..... This is likely one reason PAH/PSS is stiffer than PDADMA/PSS w...
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Langmuir 2007, 23, 896-901

Counterions and Water in Polyelectrolyte Multilayers: A Tale of Two Polycations Jad A. Jaber and Joseph B. Schlenoff* Department of Chemistry and Biochemistry, and Center for Materials Research and Technology (MARTECH), The Florida State UniVersity, Tallahassee, Florida 32306 ReceiVed June 26, 2006. In Final Form: October 3, 2006 Attenuated total internal reflectance Fourier transform infrared, ATR-FTIR, spectroscopy was used to compare the water uptake and doping within polyelectrolyte multilayers made from poly(styrene sulfonate), PSS, and a polycation, either poly(allylamine hydrochloride), PAH, or poly(diallyldimethylammonium chloride), PDADMAC. Unlike PDADMA/PSS multilayers, whose water content depended on the solution ionic strength, PAH/PSS multilayers were resistant to doping by NaCl to a concentration of 1.2 M. Using (infrared active) perchlorate salt, the fraction of residual counterions in PDADMA/PSS and PAH/PSS was determined to be 3% and 6%, respectively. The free energy of association between the polymer segments, in the presence of NaClO4, was about 5 kJ mol-1 and -10 kJ mol-1, respectively, for PDADMA/PSS and PAH/PSS, indicating the relatively strong association between the polymer segments in the latter relative to the former. Varying the pH of the solution in contact with the PAH/PSS multilayer revealed a transition to a highly swollen state, interpreted to signal protonation of PAH under much more basic conditions than the pKa of the solution polymer. The increase in the multilayer pKa suggested an interaction energy for PAH/PSS in NaCl of ca. 16 kJ mol-1.

Introduction Polyelectrolyte multilayers, PEMUs, made from combinations of charged polymers, are essentially ultrathin films of polyelectrolyte complex.1 While the slate of available synthetic polycations or polyanions is very broad, relatively few are commercially available and fewer yet are widely used as “representative” systems. Poly(styrene sulfonate), PSS, is the most popular strongly charged negative polyelectrolyte, and poly(allylamine hydrochloride), PAH, and poly(diallyldimethylammonium chloride), PDADMAC, are typical polycations.1 While PAH is pH sensitive, PDADMAC, a poly(quaternary ammonium), is charged at all pH values. When incorporated into multilayers, these two polycations differ strongly in other ways. For example, PDADMA/PSS is much more easily swollen than PAH/PSS,2 whereas PAH/PSS has a much higher modulus3 and presents a much greater barrier to ion transport.4 In our experience, the * Author to whom correspondence should be addressed. E-mail: [email protected]. (1) Multilayer thin films: Sequential assembly of nanocomposite materials; Decher, G., Schlenoff, J. B., Eds.; Wiley-VCH: Weinheim, Germany, 2003. (2) Dubas, S. T.; Schlenoff, J. B. Langmuir 2001, 17, 7725. (3) Nolte, A. J.; Rubner, M.; Cohen, R. E. Macromolecules 2005, 38, 5367. (4) Harris, J. J.; Bruening, M. L. Langmuir 2000, 16, 2006. (5) Farhat, T. R.; Schlenoff, J. B. J. Am. Chem. Soc. 2003, 125, 4627. (6) Jaber, J. A.; Schlenoff, J. B. Macromolecules 2005, 38, 1300. (7) Hiller, J. A.; Mendelsohn, J. D.; Rubner, M. F. Nat. Mater. 2002, 1, 59. (8) Wang, T. C.; Cohen, R. E.; Rubner, M. F. AdV. Mater. 2002, 14, 1534. (9) Wong, J. E.; Rehfeldt, F.; Haenni, P.; Tanaka, M.; Klitzing, R. v. Macromolecules 2004, 37, 7285. (10) Lebedeva, O. V.; Kim, B.-S.; Vasilev, K.; Vinogradova, O. I. J. Colloid Interface Sci. 2005, 284, 455. (11) Lulevich, V. V.; Vinogradova, O. I. Langmuir 2004, 20, 2874. (12) Salloum, D. S.; Olenych, S. G.; Keller, T. C. S.; Schlenoff, J. B. Biomacromolecules 2005, 6, 161. (13) Engler, A. J.; Richert, L.; Wong, J. Y.; Picart, C.; Discher, D. E. Surf. Sci. 2004, 570, 142. (14) Yang, S. Y.; Mendelsohn, J. D.; Rubner, M. F. Biomacromolecules 2003, 4, 987. (15) Richert, L.; Boulmedais, F.; Lavalle, P.; Mutterer, J.; Ferreux, E.; Decher, G.; Schaaf, P.; Voegel, J.-C.; Picart, C. Biomacromolecules 2004, 5, 284. (16) Engler, A.; Bacakova, L.; Newman, C.; Hategan, A.; Griffin, M.; Discher, D. Biophys. J. 2004, 86, 617. (17) Lo, C. M.; Wang, H. B.; Dembo, M.; Wang, Y. L. Biophys. J. 2000, 79, 144.

differences have been so great that neither PDADMA nor PAH is representative of all polycations in multilayers, although PAH does appear to be at the extreme in terms of the aforementioned properties. The purpose of this paper is to compare and contrast, quantitatively using ATR-FTIR spectroscopy techniques, two fundamental parameters in PSS multilayers made with PAH or PDADMA: ion pairing and water content, key factors influencing the permeability of PEMUs5,6 as well as their optical7-9 and mechanical properties.10,11 The latter is especially important when using PEMUs as active biomaterials,12-15 for example, to control cell attachment and spreading.16-19 A polyelectrolyte thin film can be switched from being cytophilic to cytophobic by tuning the assembly pH, which determines PEMU swelling and water content.20 Also, it has been shown that the dynamics of adhesion and differentiation of cells, anchored to a polymeric surface, will change according to the stiffness of the underlying interface.21 Doping by salt counterions is a thermodynamically controlled mechanism that allows for a reversible,22 fast, and effective manipulation of the degree of cross-linking3,10,23-25 and water content of PEMUs.26 The strength of the ion pairing (“electrostatic” interaction) in PEMUs is determined by the ability of salt, such as NaCl, counterions to challenge these ion pairs, as described by the following transformation:

Pol+Polm- + Naaq+ + Claq- f Pol+Clm- + Pol-Nam+ Pol+,

(1)

Pol-

where are, respectively, positive and negative polyelectrolyte repeat units, and the subscript m refers to components in the multilayer phase. As the ions enter the (18) Pelham, R. J., Jr.; Wang, Y.-L. Proc. Natl. Acad. Sci. U.S.A. 1997, 94, 13661. (19) Gaudet, C.; Marganski, W. A.; Kim, S.; Brown, C. T.; Gunderia, V.; Dembo, M.; Wong, J. Y. Biophys. J. 2003, 85, 3329. (20) Mendelsohn, J. D.; Yang, S. Y.; Hiller, J. A.; Hochbaum, A. I.; Rubner, M. F. Biomacromolecules 2003, 4, 96. (21) Discher, D. E.; Janmey, P.; Wang, Y.-L. Science 2005, 310, 1139. (22) Salloum, D. S.; Schlenoff, J. B. Electrochem. Solid-State Lett. 2004, 7, E45.

10.1021/la061839g CCC: $37.00 © 2007 American Chemical Society Published on Web 12/13/2006

Counterions and Water in Multilayers

multilayer phase, it has always been assumed (including by our group) that they bring in additional water molecules, since the counterions are small and well hydrated. Therefore, the expansion of the PEMU, expected from the doping reaction in eq 1, was assumed to include water.2 We will show that this is not necessarily the case. Because of our inability to dope PAH/PSS multilayers with NaCl, we resorted to using ClO4- ion, which is more hydrophobic27 and a stronger dopant5 than Cl-. Perchlorate is infrared active, which permits precise determination of doping levels using ATR-FTIR.5,28-32 The ATR-FTIR methods were also used to track water content in multilayers made from both polycations. Since PAH is a weak polyelectrolyte, we were also interested in the degree of hydration as a function of solution pH. Experimental Section Materials and Reagents. Sodium chloride and sodium perchlorate (Fisher) and 2-morpholinoethanesulfonic acid (MES, Sigma) were used as received. PSS, (Mw ) 6.5 × 104 g mol-1, Mw/Mn ) 1.4), PAH, (Mw ) 7 × 104 g mol-1), PDADMAC (Mw ) 3.7 × 105 g mol-1, Mw/Mn ) 2.09), and poly(ethylene glycol) (Mw ) 8 × 103 g mol-1) were from Aldrich. Deionized water (Barnstead, E-pure, Milli-Q) was used to prepare all aqueous solutions. The concentrations of polymer solutions are quoted with respect to the repeat unit. ATR-FTIR. The water and counterion content of multilayers were determined by ATR spectroscopy. ATR measurements were performed with a Nicolet Nexus 470 FTIR fitted with a 0.5-mL flow-through ATR assembly housing a 70 × 10 × 6 mm germanium crystal (Specac Benchmark). The crystal was cleaned using 50:50 v/v ethanol/H2O in saturated salt solution. Multilayer buildup on Ge was as follows: (PAH/PSS)[email protected] M NaCl,33 “PAH/PSS”, was deposited from 10 mM polymer solutions in 0.2 M NaCl and 25 mM MES buffer (pH ) 6.5) on the ATR crystal using a robotic platform and then was fitted into the ATR. Between alternating exposures to the polyelectrolytes, there were three rinses in 10 mM MES buffer with no NaCl added. Rinse and polymer solutions were approximately 50 mL each. The deposition time for each layer was 5 min, and each rinse was done for 30 s. (PDADMA/PSS)[email protected] M, “PDADMA/ PSS”, was deposited on the ATR crystal while it was loaded in the flow cell by passing polyelectrolytes (10 mM in 1.0 M NaCl) and rinse solutions (1.0 M NaCl), in an alternating manner, through the ATR. The multilayer was then annealed in 1.0 M NaCl for 48 h. All spectra were recorded with 32 scans at 4.0 cm-1 resolution. The mole ratio of water to sulfonate at different salt concentrations (sodium salts of chloride and perchlorate) was determined as follows: first, an ATR spectrum of 18 wt % PSS solution was recorded on a bare crystal, and the peak area ratios of H2O/SO3were used as a standard (i.e., corresponded to 47:1 mole ratio of H2O/SO3-). The multilayer coated crystal was then soaked in a solution of a specific salt concentration for 15 min, after which the areas under the two sulfonate peaks at 1035 and 1008 cm-1 were measured. For PAH/PSS, the salt solution contained 25 mM MES (23) Yano, O.; Wada, Y. J. Appl. Polym. Sci. 1980, 25, 1723. (24) Kovacevic, D.; van der Burgh, S.; de Keizer, A.; Cohen Stuart, M. A. Langmuir 2002, 18, 5607. (25) Kim, B.-S.; Vinogradova, O. I. J. Phys. Chem. B 2004, 108, 8161. (26) Rmaile, A.; Bucur, C. B.; Schlenoff, J. B. Manuscript in preparation. (27) Schlenoff, J. B. In Multilayer thin films: Sequential assembly of nanocomposite materials; Decher, G., Schlenoff, J. B., Eds.; Wiley-VCH: Weinheim, Germany, 2003; Chapter 4, p 99. (28) Sukhishvili, S. A.; Granick, S. Macromolecules 2002, 35, 301. (29) Kozlovskaya, V.; Ok, S.; Sousa, A.; Libera, M.; Sukhishvili, S. A. Macromolecules 2003, 36, 8590. (30) Harrick, N. J. Internal Reflection Spectroscopy; John Wiley & Sons: New York, 1967. (31) Urban, M. W. Attenuated Total Reflectance Spectroscopy of Polymers; American Chemical Society: Washington, DC, 1996. (32) Mu¨ller, M.; Rieser, T.; Lunkwitz, K.; Berwald, S.; Meier-Haack, J.; Jehnichen, D. Macromol. Rapid Commun. 1998, 19, 333. (33) Polyelectrolyte nomenclature: (A/B)x@yMA@z, where A and B correspond to the first and second polyelectrolytes in a layer pair, starting with A. x is the number of layer pairs. y and z are the ionic strength of salt MA and pH of the buildup solution, respectively.

Langmuir, Vol. 23, No. 2, 2007 897 with a pH of 6.5. The peak area ratio at different salt concentrations was then translated to a mole ratio using the standard 47:1 mole ratio of H2O/SO3-. To determine the counterion content of the multilayer, a standard solution (10 wt % NaClO4 mixed with 18 wt % PSS solution) was passed through the cell using a bare crystal. The peak area ratios of SO3- at 1035 and 1008, and ClO4- at 1100 cm-1 were used as standards. The multilayer coated crystal was soaked in a solution of a specific sodium perchlorate concentration for 15 min, after which the areas under the ClO4- peak and the two sulfonate peaks were measured. The area ratio at different salt concentrations was then translated to a mole ratio using the standard. Ellipsometry and Profilometry. For the post-buildup effect of pH on the stability of the PAH/PSS multilayer, a 20-layer-thick PEMU was deposited on a silicon wafer (Si〈100〉, 0.5-mm thick, 1-in. diameter, undoped, polished on one side, Topsil Inc.). The buildup procedure and the polyelectrolyte solution concentrations were the same as described above. The wafer was cleaned in 70% H2SO4(conc)/30% H2O2(aq) (“piranha”; caution: piranha is a strong oxidizer and should not be stored in closed containers) and then was blown dry with a stream of N2. Solution pHs covering the range of pH 6.5-11.5 were produced with either 1.0 M HCl or 1.0 M NaOH and always contained 0.2 M NaCl. Multilayer thickness on the Ge crystal was measured with a profilometer (Tencor instruments) at step edges (scratches). Multilayer thicknesses on silicon wafer were obtained using a Gaertner Scientific L116S autogain ellipsometer with 632.8-nm radiation at 70° incident angle and multilayer refractive index of 1.54.

Results and Discussion The change in the counterion and water contents of the multilayers was monitored in situ while the ionic strength of the soaking solution was increased. The PAH/PSS or PDADMA/ PSS multilayers deposited on the surface of the Ge crystal used for ATR-FTIR study had dry thicknesses of 0.9 µm and 0.75 µm, respectively. The wet thickness of both multilayers, assuming they contain about 40 vol % water,34 would be well over a micrometer, which should be thick enough (>0.65 µm) to entirely contain the evanescent IR radiation30,31 from the ATR crystal. In this situation, only water or ClO4- in the bulk of the multilayer, and not the adjoining electrolyte, will contribute to the IR signal. The technique assumes that the doping level and the water content are uniform within the film once the more hydrated surface is beyond the range of the evanescent wave. The assumption that the evanescent wave was completely contained within the film was verified experimentally by passing a 1.0 wt % poly(ethylene glycol), PEG, (in 20 mM NaCl) solution over the multilayer coated crystal (Figure 1). The neutral polymer did not permeate the multilayer, since its radius of gyration was larger than the average distance between two polymer chains in the PEMU.35,36 The absence of an absorption band at 1100 cm-1, assigned to the stretching vibration mode of the C-O groups of PEG, indicates that the evanescent wave did not reach the adjoining liquid medium. In the following sections, the doping and hydration of PAH/ PSS and PDADMA/PSS multilayers are compared. In doing so, we address two controversies that have presented themselves in the multilayer literature. The major controversy has been over whether residual counterions are left in PEMUs after they are made. According to eq 1, when salt is removed from solution, as occurs when multilayers are given their final rinse in pure (34) Lo¨sche, M.; Schmitt, J.; Decher, G.; Bouwman, W. G.; Kjaer, K. Macromolecules 1998, 31, 8893. (35) Leonard, M.; Hong, H.; Easwar, N.; Strey, H. H. Polymer 2001, 42, 5823. (36) Strey, H. H.; Parsegian, V. A.; Podgornik, R. Phys. ReV. Lett. 1997, 78, 895.

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Figure 1. a, b, and c are ATR-FTIR spectra of (PDADMA/ PSS)[email protected] M, (PAH/PSS)[email protected] M NaCl@pH 6.5, and 1.0 wt % PEG solution, respectively. The water contribution was eliminated from these spectra by subtracting the spectrum of 0.2 M NaCl solution using the dry uncoated crystal as a background. The arrow indicates the absence of the C-O stretching vibration mode from PEG for the PDADMA/PSS and PAH/PSS multilayers. For clarity, the spectra are offset by 0.1 AU along the y-axis.

water, all counterions should be expelled if the positive and negative polymer repeat units are in strict 1:1 stoichiometry. If they are not, or if there are other kinetic or thermodynamic factors, the multilayer may contain residual salt. In one study,37 it was estimated that a minimum of 10-30% of the cationic sites of PAH and PDADMA were not utilized in ion-pair formation with adjacent PSS layers in PAH/PSS and PDADMA/PSS thin films. Riegler and Essler38 suggested that about 66% of the PAH charges were neutralized by complexation with PSS (i.e., the remaining 34% were neutralized by Cl-). Others estimated that thin PAH/ PSS multilayers incorporated 0.5-0.8 counterions per monomer unit.39,40 On the other hand, we proclaimed that there are no counterions in PEMUs.41 The second controversy concerns the swelling behavior of PAH/PSS multilayers. Whereas our in-situ atomic force microscopy (AFM) measurements showed no swelling of PAH/ PSS in different NaCl concentrations,2 a PAH/PSS PEMU prepared onto a Langmuir monolayer shrunk by 60% when NaCl was added.42 In yet further contrast, a PAH/PSS system swelled on adding NaCl up to 0.1 M43 but not in more concentrated salt. More recently, a PAH/PSS multilayer thickness swelled by 15% when the NaCl concentration reached 2.5 M.10 Doping of PEMUs by Sodium Perchlorate. When a PEMU is doped, both the cation and anion of the added salt contribute equally to the transition from an intrinsic to an extrinsic charge compensation.2 In the case of sodium perchlorate, the infraredactive perchlorate anion indicates the total sodium perchlorate entering the PEMU. Figure 2 illustrates the substantial doping capabilities of ClO4which has a strong stretch at 1100 cm-1. From the slope of the two lines in Figure 2, it is clear that PSS shows stronger ionpairing interactions with PAH than with PDADMA, since more (37) Caruso, F.; Lichtenfeld, H.; Donath, E.; Mo¨hwald, H. Macromolecules 1999, 32, 2317. (38) Riegler, H.; Essler, F. Langmuir 2002, 18, 6694. (39) Schmitt, J.; Gru¨newald, T.; Decher, G.; Pershan, P. S.; Kjae, K.; Lo¨sche, M. Macromolecules 1993, 26, 7058. (40) Lourenco, J. M. C.; Ribeiro, P. A.; Botelho do Rego, A. M.; Fernandes, F. M. B.; Moutinho, A. M. C.; Raposo, M. Langmuir 2004, 20, 8103. (41) Schlenoff, J. B.; Li, M.; Ly, H. J. Am. Chem. Soc. 1995, 117, 12528. (42) Ruths, J.; Essler, F.; Decher, G.; Riegler, H. Langmuir 2000, 16, 8871. (43) Sukhorukov, G. B.; Schmitt, J.; Decher, G. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 948.

Jaber and Schlenoff

Figure 2. Doping level, y, for perchlorate ion as a function of solution NaClO4 concentration, measured in situ with ATR-FTIR for polyelectrolyte multilayers on germanium. Squares and triangles show the ClO4- content of (PAH/PSS)[email protected] M NaCl@pH 6.5 and (PDADMA/PSS)[email protected] M NaCl multilayers, respectively. The equations of the straight lines used to fit the experimental data are y ) 0.15x + 0.06 for PAH/PSS and y ) 2.37x + 0.03 for PDADMA/PSS.

sodium perchlorate ions enter PDADMA/PSS than PAH/PSS (eq 1) at the same solution salt concentration. In prior experiments,2 we did not observe any response of the PAH/PSS system to [NaCl]. In contrast to PAH/PSS, which is stable up to [NaClO4]aq ) 2.0 M, PDADMA/PSS decomposes at [NaClO4]aq ) 0.75 M (or an activity of 0.48).5 For a more quantitative view of the strength of interaction between the polymer segments in PDADMA/PSS and PAH/ PSS, the free energies of association (the reverse of eq 1) were determined. These measure the affinity of the polyelectrolyte segment pair under standard conditions (i.e., 1.0 M sodium perchlorate, 25 °C) given by27

∆Gassoc,NaClO40 ) - RT ln Kassoc

(2)

where

Kassoc )

(1 - y)[NaClO4]aq2 y2

≈

(

)

[NaClO4]aq2 y2

yf0

y is the fraction of the multilayer in the extrinsic form, (doping level), and 1 - y is the intrinsic fraction, Pol+Pol-. The respective ∆Gassoc,NaClO4 values are -9.5 and 4.5 kJ mol-1 for PAH/PSS and PDADMA/PSS. Extrapolating to [NaClO4]aq ) 0, the amount of trapped residual persistent extrinsic sites, yrpe, was determined to be 0.06 and 0.03 for PAH/PSS and PDADMA/PSS, respectively.5 This amount of residual salt in the multilayers is unambiguous. It is much less than the amount determined in previous studies37-40 but is not zero.41 The residual ion content in PDADMA/PSS films can be decreased by annealing in salt,5 but we found no evidence of a similar annealing effect in PAH/PSS. The PDADMA system is labile (i.e., the polyelectrolytes can interdiffuse) under these annealing conditions,44 whereas the PAH system appears to remain kinetically frozen. The counterion content is directly measured in our experiments. Conclusions of prior studies on a much higher residual salt content are based on indirect estimates of salt needed to balance unequal amounts of polyelectrolyte38 or as needed to fit certain structural (44) Jomaa, H. W.; Schlenoff, J. B. Macromolecules 2005, 38, 8473.

Counterions and Water in Multilayers

Langmuir, Vol. 23, No. 2, 2007 899 Table 1. Hydration Numbers of the Salt Ions Used in This Study Relative to the Polyelectrolyte Ion Pairs in PDADMA/PSS and PAH/PSS Multilayers ion pair

hydration number

NaCl NaClO4 PAH/PSS PDADMA/PSS

1545 926 6 7

Figure 3. Mole ratio of water to sulfonate at different NaClO4 concentrations. Squares and triangles correspond to the water content of (PAH/PSS)250PAH and (PDADMA/PSS)[email protected] M, respectively. For PAH/PSS, salt solutions were prepared in MES buffer (0.025 M and pH ) 6.5).

models.37,39,40 In addition, thinner films may contain a counterion contribution from the surface of the PEMU, which is known to be much richer in extrinsic charge.27 Water Uptake of PAH/PSS and PDADMA/PSS. In the present work, the term “water uptake” corresponds to the additional water entering the PEMU on doping. Counterions and their hydration shells may contribute to the expansion or swelling of the PEMU, but we did not measure dimensional changes. The water content of PDADMA/PSS or PAH/PSS multilayers exposed to sodium perchlorate is depicted in Figure 3. While the water content of PDADMA/PSS increased slightly over the range of ionic strength used, no appreciable change in the water content of PAH/PSS was observed. As seen in Figure 2, counterions are, in fact, entering the PAH/PSS PEMU. Hydration of each component in the PEMU/salt system may be represented as follows:

Pol+Pol-‚aH2Om + Na+‚bH2Oaq + A-‚cH2Oaq f Pol+A-‚dH2Om + Pol- Na+ ‚eH2Om + fH2Oaq (3) where A- is the salt anion, a the number of water molecules hydrating the intrinsic ion pair, b and c the water hydrating salt counterions in solution, and d and e the hydration numbers of the extrinsic ion pairs. f represents the balance of water molecules (and can be negative or positive). Swelling occurs when the amount of water introduced by the salt counterions is more than that originally hydrating the intrinsic ion pair, that is, a < d + e. However, if a less hydrated anion such as ClO4- is used to dope the thin film, it is possible that the amount of water hydrating the multilayer remains constant, that is, a ) d + e and f ) b + c. In such a case, doping is not accompanied by an increase in the water content (“hydration neutral”). In contrast to sodium chloride, which has a hydration number of 15 (Table 1),45 sodium perchlorate is a poor sweller with a hydration number of 9.26 Thus, it happens that the number of water molecules introduced by sodium perchlorate into the doped multilayer is not too different from that already hydrating the Pol+Pol- ion pairs. These results clearly demonstrate that additional hydration does not necessarily accompany doping. (45) Rejou-Michel, A.; Henry, F.; de Villardi, M.; Delmotte, M. Phys. Med. Biol. 1985, 30, 831.

Figure 4. Mole ratio of water to sulfonate at different salt concentrations. Squares and triangles correspond to the water content of (PAH/PSS)[email protected] M and (PDADMA/PSS)[email protected] M, respectively. For PAH/PSS, salt solutions were prepared in MES buffer (0.025 M and pH ) 6.5). The strong sulfonate stretches of PSS were used as an internal standard to obtain accurate and precise measurements of the water content at different salt concentrations. The equations of the straight lines used to fit the experimental data are y ) 0.03x + 6.3 for PAH/PSS and y ) 7.73x + 7 for PDADMA/ PSS.

Water Uptake of PEMUs in Sodium Chloride. As the concentration of NaCl in the external solution was increased, the PDADMA/PSS multilayer showed both polyelectrolyte and antipolyelectrolyte behavior,46 whereas the water content of the PAH/ PSS multilayer was unchanged (Figure 4). Extrapolating the linear portion to [NaCl]aq ) 0, the number of water molecules per ion pair, nH2O, was found to be 6 and 7 for PAH/PSS and PDADMA/PSS, respectively. Assuming the respective densities of PAH/PSS and PDADMA/PSS to be 1.2 and 1.1 g cm-3, water hydrating the Pol+Pol- ion pair occupies 35% and 33% of the volume within fully hydrated PAH/PSS and PDADMA/PSS, respectively. The apparent dehydration and contraction of the PDADMA/ PSS multilayer with higher salt concentration follows the normal behavior for free polyelectrolyte chains in solution (the “polyelectrolyte” effect). This behavior is ascribed to the residual extrinsic charge in the complex, as opposed to charge created by doping. We have previously seen multilayer shrinkage in swelling measurements2 and have attributed this to water removal by the osmotic pressure of the salt solution. At sufficiently high [NaCl], the doping reaction (eq 1, “antipolyelectrolyte” regime) brings more water back into the PEMU. There are two peculiarities in the water versus salt concentration data. First, the polyelectrolyte trend seen with PDADMA/PSS in NaCl (Figure 4) is not seen when the multilayer is immersed in NaClO4 (Figure 3). Our tentative explanation for this is that NaClO4 does not exert as much osmotic pressure as NaCl. Second, (46) Stimuli ResponsiVe Water Soluble and Amphiphilic Polymers; McCormick, C. L., Ed.; ACS Symposium Series 780; American Chemical Society: Washington, DC, 2001.

900 Langmuir, Vol. 23, No. 2, 2007

PAH/PSS does not show a polyelectrolyte regime at low [NaCl], even though it has twice as much residual extrinsic charge as PDADMA/PSS. The insensitivity of the PAH/PSS multilayer toward NaCl in the low saline concentration regime may be due to the higher cross-link density within the PAH multilayer. The osmotic pressure difference between the multilayer and the solution is counteracted by the stiffness of the multilayer chains (incompressible system). It is known that the small mesh size observed in highly cross-linked systems suppresses the uptake of the swelling agents.47 PAH/PSS and PDADMA/PSS multilayers immersed in pure water have an elastic modulus, E, of 590 ( 90 and 17 ( 2 Mpa, respectively.3,48 The discrepancies in the literature regarding swelling of PAH/ PSS are, in part, due to differences with the systems. Riegler and Essler38 formed thin PEMUs (eight layers) at the air/water interface and found shrinking with added salt (polyelectrolyte effect). This is understood by the fact that with such thin layers the extrinsic charge at the surface of the PEMU would represent a significant fraction of overall charge. Wong et al.9 studied the water content of thin PAH/PSS multilayers in saturated water vapor and showed that, for thinner films, the outermost layer, whether PAH or PSS, determined the swelling behavior of the whole film.49,50 Our multilayers are thick enough such that the surface has little impact on what occurs in the bulk. Lebedeva et al.10 used much higher NaCl concentration (2.5 M) before they observed swelling. This high concentration induces softening of PAH/PSS11,51 and probably occurs as the doping (eq 1) is enough to break some Pol+Pol- ion pairs. The only account of PAH/PSS swelling we are unable to explain is the earliest one provided by Sukhorukov et al.,43 who saw swelling at low salt concentrations only. Post-Buildup Effects of pH on PAH/PSS. Thin films of PAH/ PSS were shown to possess interesting swelling behavior52,53 when assembled under pH conditions close to the solution pKa of PAH (8.5-8.9).63-65 The degree of ionization, (R), of PAH is a function of the solution pH, although R was shown to increase considerably from the solution value when PAH was incorporated in a multilayer.54-56 For example, a multilayer assembled at pH of 9.3 had around 30%52 (instead of the calculated 60%) of the amine groups in the neutral form, that is, not involved in ionpairing formation with PSS. Itano et al.52 have shown that if this multilayer is brought in contact with a solution of pH 2.5, the free amine groups will be protonated and the thin film will be highly swollen. In our case, the PAH/PSS multilayer was prepared from a solution of pH 6.5. At this point, PAH is essentially fully ionized, and a highly interpenetrating structure insensitive to swelling by (47) Kurdikar, D. L.; Peppas, N. A. Polymer 1995, 36, 2249. (48) Jaber, J. A.; Schlenoff, J. B. J. Am. Chem. Soc. 2006, 128, 2940. (49) Carriere, D.; Krastev, R.; Scho¨nhoff, M. Langmuir 2004, 20, 11465. (50) Schwarz, B.; Scho¨nhoff, M. Langmuir 2002, 18, 2964. (51) Heuvingh, J.; Zappa, M.; Fery, A. Langmuir 2005, 21, 3165. (52) Itano, K.; Choi, J.; Rubner, M. F. Macromolecules 2005, 38, 3450. (53) Hiller, J. A.; Rubner, M. F. Macromolecules 2003, 36, 4078. (54) Mendelsohn, J. D.; Barrett, C. J.; Chan, V. V.; Pal, A. J.; Mayes, A. M.; Rubner, M. F. Langmuir 2000, 16, 5017. (55) Shiratori, S. S.; Rubner, M. F. Macromolecules 2000, 33, 4213. (56) Burke, S. E.; Barrett, C. J. Langmuir 2003, 19, 3297. (57) Rmaile, H. H.; Schlenoff, J. B. Langmuir 2002, 18, 8263. (58) Smith, A. L. Applied Infrared Spectroscopy; John Wiley &Sons: New York, 1979. (59) Socrates, J. Applied Infrared and Raman Characteristics Group Frequencies; John Wiley & Sons: New York, 2001. (60) Kim, D. K.; Han, S. W.; Kim, C. H.; Hong, J. D.; Kim, K. Thin Solid Films 1999, 350, 153. (61) Petrov, A. I.; Antipov, A. A.; Sukhorukov, G. B. Macromolecules 2003, 36, 10079. (62) Garza, J. M.; Schaaf, P.; Mu¨ller, S.; Ball, V.; Stoltz, J.-F.; Voegel, J.-C.; Lavalle, P. Langmuir 2004, 20, 7298.

Jaber and Schlenoff

Figure 5. From top to bottom, ATR-FTIR spectra of PAH/PSS multilayer in 0.2 M NaCl and pH 6.5, 7.5, 9.5, and 10.5, respectively. The water contribution was eliminated from these spectra by subtracting the spectrum of 0.2 M NaCl solution using the dry uncoated crystal as a background. The arrow indicates the absence of the -NH2 amine group absorption band. For clarity, the spectra are offset by 0.05 AU along the y-axis.

pH is expected.53 We were interested in seeing how resistant the multilayer PAH would be to deprotonation, breaking energetically favorable Pol+Pol- pairs:

PolH+Polm- + Naaq+ H Polm0 + Pol-Nam+ + Haq+ (4) We assumed that when highly hydrated Na+ ions entered the multilayer the water content would increase. The equilibrium constant may be written as57

Ka(m) )

[Pol0]m[Pol-Na+]m[H +]aq [PolH+Pol-]m[Na+]aq

(5)

A series of 0.2 M NaCl solutions covering the pH range of 6.5-11.5 were successively passed through the ATR-FTIR flow cell, housing the PAH/PSS coated Ge crystal (Figure 5). IR spectra were recorded 10 and 50 min following each pH change to ensure that the swelling response of the multilayer (if any) had reached steady state. As depicted in Figure 5, the IR spectrum of PAH/PSS multilayer shows two peaks in the range of 1500-1650 cm-1. Bands at 1530 and 1600 cm-1 are assigned to the asymmetric and symmetric bending vibrations of the protonated amine groups (-NH3+).58-60 The stepwise increase of pH from 6.5 to 10.5 did not produce a broad absorption band, usually associated with the neutral amine groups, between the NH3+ group frequencies (Figure 5).52 The stability of the Pol+Pol- bond was further demonstrated by the absence of any detectable swelling up to a pH of 10.5 (Figure 6). At a pH of 11.5, a marked increase in the water content of the PAH/PSS multilayer was observed. Here, the ATRFTIR spectrum was collected after keeping the multilayer coated crystal for 10 min in the 0.2 M NaCl solution at pH 11.5. In a separate experiment, ellipsometery measurements on a dry 20-layer-thick PAH/PSS thin film, assembled under the same conditions, revealed that the structural stability of the multilayer is compromised at this point (Figure 6). Interestingly, these experiments indicate that the PAH/PSS multilayer only starts to experience a rapid dissociation of the Pol+Pol- bonds at a pH where PAH in solution would already have completely deprotonated.

Counterions and Water in Multilayers

Langmuir, Vol. 23, No. 2, 2007 901

The ion-pair formation within the PAH/PSS multilayer is represented by the following equilibrium

PolH+Claq- + Pol-Nam+ h PolH+Polm- + Claq- + Naaq+ (8) The formation constant may be written as

Kassoc,NaCl )

[PolH+Pol-]m[Cl-]aq[Na+]aq +

-

-

+

[PolH Cl ]aq[Pol Na ]aq

)

Ka(sol) Ka(m)

(9)

The free-energy change due to ion pairing, or the “driving force” of multilayer formation, is given by eq 2. Combining eq 2 and eq 9, Figure 6. Water content of PAH/PSS as a function of solution pH in 0.2 M NaCl. Circles show the number of water molecules per repeat unit in the ATR multilayer (500-layers thick, prepared at pH 6.5), and squares correspond to the thickness of another 20-layerthick PAH/PSS thin film prepared on a silicon wafer under the same conditions. pKa(m) is assumed to be indicated by the sudden swelling as Na+ enters the film on PAH deprotonation (at ca. pH ) 11.4). Solid lines are guides to the eye.

The ellipsometry experiment also showed that the multilayer started to dissociate after 10 min of soaking in the salt solution of pH 10.5,4 with an 11% decrease in the film thickness. Leaving the multilayer in the same solution for another 40 min did not reduce the thickness any further. This erosion would bring down the wet thickness of the multilayer on the ATR crystal to 0.9 µm which is still enough to contain the evanescent wave. The stability of the multilayer structure and the absence of the amine-stretching vibrations at a pH of 9.5 indicate that PAH maintains a fully ionized form at that pH, and the effective pKa of PAH in the multilayer is greater than its pKa in solution.61 Dissociation of PAH in solution is represented by

PolH+Claq- h Polaq0 + Claq- + Haq+

(6)

The equilibrium constant may be written as

Ka(sol) )

[Pol0]aq[Cl-]aq[H+]aq [PolH+Cl-]aq

(7)

∆Gassoc,NaCl0 ) - 2.3RT(pKa(m) - pKa(sol))

(10)

For the conditions in Figure 6 (pKa(m) - pKa(sol) ) 11.4 - 8.5 ) 2.9), and ∆Gassoc0 ) - 15.6.kJ mol-1. Therefore, compared to the protonated form in solution, the complexed multilayer state of PAH is favored by ca. 16 kJ mol-1.

Conclusion Using the ATR-FTIR technique, it was possible to probe the bulk of the PAH/PSS and PDADMA/PSS multilayers, thus gaining valuable information about their water and counterion content. While PAH/PSS is a popular multilayer system, it may not be the best candidate for an active biointerface with tunable elasticity since it requires inordinately high concentrations of NaCl to produce swelling and other changes (e.g., softening).10,11 The high association energy of PAH/PSS relative to PDADMA/ PSS multilayer clearly indicates that it is a very stable system, but that it may also suffer from nonequilibrium structures because of the inability of PAH/PSS to reorganize under conditions of modest ionic strength. This is likely one reason PAH/PSS is stiffer than PDADMA/PSS with elastic moduli of 590 and 17 Mpa, respectively.3,48 On the other hand, this kinetically “frozen” property is useful when it comes to producing barriers between more labile polyelectrolyte pairs.62 Acknowledgment. This work was supported by a grant from the National Science Foundation (DMR 0309441). LA061839G