Formation of Weak Polyelectrolyte Multilayers Studied by Spin

The two polyelectrolytes have been labeled independently by a nitroxide free radical. Its electron paramagnetic resonance spectrum is mainly sensitive...
0 downloads 0 Views 119KB Size
Download PDF
Langmuir 2004, 20, 3173-3179

3173

Formation of Weak Polyelectrolyte Multilayers Studied by Spin Labeling R. Schach, H. Hommel,* and H. Van Damme Laboratoire de Physico Chimie Structurale et Macromoleculaire, UMR 7615 CNRS, Ecole Supe´ rieure de Physique et Chimie Industrielles, 10 rue Vauquelin, 75231 Paris Cedex 05, France

P. Dejardin European Membrane Institute, UMR 5635 (CNRS UM2 ENSCM), CC 047, 2 Place Euge` ne Bataillon, 34095 Montpellier Cedex 05, France

C. Amsterdamsky Laboratoire de Chimie Organique, ESA 7084 CNRS, Ecole Supe´ rieure de Physique et Chimie Industrielles, 10 rue Vauquelin, 75231 Paris Cedex 05, France Received May 12, 2003. In Final Form: January 23, 2004 Multilayers of alternately adsorbing poly(allylamine) (PAH) and poly(acrylic acid) (PAA) of opposite charges on silica have been studied by the spin labeling technique, as a function of pH. The two polyelectrolytes have been labeled independently by a nitroxide free radical. Its electron paramagnetic resonance spectrum is mainly sensitive to the local Brownian motion and shows lines typical of two different environments, namely, loops protruding in solution with a fast motion and trains adsorbed on the solid with a hindered motion. These two parts have been evaluated for each of the polymer layers separately, and the thickness of the coatings has been described more precisely by characterizing the four contributions existing, for example, for a bilayer. Complexation is demonstrated by the loss of loops and tails belonging to the first polyelectrolyte. The overall picture emerging from the data is explained in terms of compensation of charges and entropy of confinement.

Introduction The buildup of multilayers of alternately adsorbing polyelectrolytes of opposite charge (the layer-by-layer technique) is a versatile method enabling the construction of ultrathin films with well-defined thickness, composition, and chemical functionalities.1,2 Various materials can be combined, including conjugated polymers, dyes, proteins, inorganic particles, etc. For instance, it was even found that it is possible to grow alternating multilayers of poly(ethylene oxide) and Laponite, a clay mineral.3 In the simplest picture, overcompensation by adsorbing polyelectrolytes leads to reversal of the surface charge, thus allowing the subsequent adsorption of oppositely charged polyelectrolytes to form the next layer. The technique is applicable not only to flat surfaces but also to rough and curved support like colloidal particles4 or here silica. This offers the possibility for investigations requiring a large internal surface area such as magnetic resonance methods.5 The linear charge density of the polyelectrolytes is an important parameter to control, since electrostatic interactions plays a key role in this adsorption. Here the growth of bilayers based on weak polylectrolytes such as * To whom correspondence should be addressed: e-mail [email protected]. (1) Decher, G. Science 1997, 277, 1232. (2) Decher, G.; Eckle, M.; Schmitt, J.; Struth, B. Curr. Opin. Colloid Interface Sci. 1998, 3, 32. (3) Glinel, K.; Moussa, A.; Jonas, A. M.; Laschewsky, A. Langmuir 2002, 18, 1408. (4) Gao, M.; Richter, B.; Kirstein, S.; Mo¨hwald, H. J. Phys. Chem. B 1998, 102 (21), 4096. (5) Schwarz, B.; Scho¨nhoff, M. Colloids Surf. A: Physicochem. Eng. Aspects 2002, 198-200, 293.

poly(acrylic acid) (PAA) and poly(allylamine) (PAH) is monitored by making use of the fact that the ionization degrees are pH-dependent.6 Additionally, for weakly acidic substrates such as silica, the surface charge is also pHdetermined. To investigate at a molecular level the conformations of the adsorbed chains, they are alternately labeled by a nitroxide free radical. The electron paramagnetic resonance spectrum is mainly sensitive to the local Brownian motion and shows lines typical of two different environments, namely, loops and tails protruding in solution with a fast motion and trains adsorbed on the surface with a hindered motion.7 Experimental Section Materials: Silica. The silica was pyrogenic Aerosil 200 from Degussa (Degussa, Frankfurt a.M., Germany). Its specific surface area measured by nitrogen adsorption is 200 m2/g. It is not porous at a molecular scale but consists of small aggregates of particles of about 10 nm diameter. At the surface there are mainly relatively unreactive siloxane -Si-O-Si- and about 3.3 hydroxyl groups Si-OH per square nanometer that are distributed more or less randomly and can react with the polymers. Indeed, depending on the pH, they can transform into Si-O-, which are charged groups.8,9 Polymers. The polymers were the weak polyelectrolytes poly(acrylic acid) (PAA) and poly(allylamine hydrochloride) (PAH). The poly(acrylic acid) has been purchased from Aldrich (Sigma(6) Shiratori, S. S.; Rubner, M. F. Macromolecules 2000, 33, 4213. (7) Hommel, H. Adv. Colloid Interface Sci. 1995, 54, 209. (8) Legrand, A. P., Ed. The Surface Properties of Silicas; John Wiley & Sons: Chichester, U.K., 1998; 470 pp. (9) Papirer, E., Ed. Adsorption on Silica Surfaces; Marcel Dekker: New York, 2000; 753 pp.

10.1021/la030202j CCC: $27.50 © 2004 American Chemical Society Published on Web 03/09/2004

3174

Langmuir, Vol. 20, No. 8, 2004

Schach et al.

Scheme 1. General Procedure of Labeling for the Polymersa

a Acid 1 reacts with the coupling agent CDI 2 to give the intermediate compound 3. This intermediate has a good starting group on the carbonyl and reacts with the amine 4 by nucleophile substitution to give the amide 5.

Chart 1. Chemical Formulas of the Two Weak Polyelectrolytes Used, Poly(acrylic Acid) (PAA) and Poly(allylamine Hydrochloride) (PAH)

Chart 2. Chemical Formulas of the Two Spin Labels Used, the 3-Carboxyproxyl and 4-Amino-TEMPO Nitroxide Free Radicals

determined by monitoring the disappearance of CDI by TLC (thinlayer chromatography, on silica, eluent CH2Cl2/MeOH/AcOEt 60/20/20, Rf ) 0.44). 4-Amino-TEMPO (85 mg, 0.15 equiv) is then added and the solution is stirred overnight. The solution is then put in a dialysis roll and the polymer is again dissolved in water (12 × 5 L of Milli-Q water during 1 week). The end of the dialysis is marked by the disappearance of the EPR signal of the water bath (EPR is sensitive to amounts as low as a part per billion). The aqueous solution is dosed by recording the signal of a known amount of the dry component, and a concentration of 0.15 ( 0.01 mg/L is obtained. The concentration of labels is measured by EPR and is 5%, which corresponds to a reaction yield of 100%. The label is an amide, which will protonate and deprotonate with pH in the opposite way as the carboxylic functions of PAA. However, this can be neglected here: the polymer could not form intramolecular complexes. Indeed, there exists a method for the calculation of pKas for N-protonation of amides:12

pKa ) -18.6 + 1.04pKa>NH

Aldrich, Saint Quentin Fallavier, France) and has a molecular weight of 450 000 g/mol. The poly(allylamine hydrochloride) has been purchased from Polysciences (Polysciences Europe, Eppelheim, Germany) and has a molecular weight of 60 000 g/mol. Their chemical formulas are given in Chart 1. The polymers were used as received without further purification. Spin Labels and Labeling Reactions. The spin labels were 3-carboxyproxyl and 4-amino-TEMPO nitroxide free radicals from Aldrich (Sigma-Aldrich, Saint Quentin Fallavier, France). Their chemical formulas are given in Chart 2. Electron paramagnetic resonance (EPR) spectroscopy is relatively sensitive and a sufficiently low amount of labels is needed to obtain an acceptable signal without altering to much the system. To bond them to the polymers, a coupling agent, namely, N,N′-carbonyldiimidazole (CDI) from Aldrich (Aldrich, Saint Quentin Fallavier, France) has been used. Its reactivity has been extensively described.10,11 The reaction in Scheme 1 describes the general procedure adopted. More precisely, to label the PAA, the reaction was performed in dimethylformamide (DMF), which is a solvent of the polymer. PAA (200 mg, 1 equiv) is introduced into a flask with 80 mL of anhydrous DMF. CDI (15.9 mg, 0.05 equiv) is added to the solution, which is stirred for 5 h. The end of the reaction is (10) Staab, H. A.; Lu¨cking, M.; Du¨rr, F. H. Chem. Ber. 1962, 95, 1275. (11) Staab, H. A. Angew. Chem., Int. Ed. Engl. 1962, 74 (12), 407.

and here pKa>NH ∼ 10 and pKa ∼ -10 is negligible in our conditions. PAH is only soluble in water. Now water is not the best choice to use with CDI. Indeed, some authors use water to neutralize an excess of CDI.13 However, it has already been shown that it can react at least enough for a labeling in water.14 In fact, there is a competition between the reactants and the solvent, decreasing considerably the yield of the reaction. Here it is possible to synthesize first the intermediate compound in DMF, before performing the labeling of the polymer in an aqueous solution (DMF is miscible in all proportions in water). The competition is then between the expected reaction and the destruction of the intermediate. 3-Carboxyproxyl (20.1 mg, 0.05 equiv) is placed in a flask with 50 mL of anhydrous DMF. CDI (40.2 mg, 0.13 equiv) is added to the solution, which is stirred for 5 h. A polymer solution (229.25 mg, 1 equiv, in 100 mL of Milli-Q water) is then added in the flask, and the mixture is stirred for 2 days. The content of the flask is then put in a dialysis roll and the polymer is put again in an aqueous solution (12 × 5 L of Milli-Q water during 1 week). The end of the dialysis is also monitored by detecting the disappeance of the EPR signal. The aqueous solution is dosed by recording the signal of a known weight of the dry component, and a concentration of 0.30 ( 0.03 mg/L is obtained. The concentration of labels is measured by EPR and is about 1%, that is, a yield of the reaction of 20%, much less than previously but enough to obtain a good signal. (12) Fersht, A. R. J. Am. Chem. Soc. 1971, 93 (14), 3504. (13) Paul, R.; Anderson, G. W. J. Org. Chem. 1962, 27, 2094. (14) Touhami, A.; Hommel, H.; Legrand, A. P.; Serres, A.; Muller, D.; Josefonvicz, J. Colloids Surf. B: Biointerfaces 1993, 1, 189.

Formation of Weak Polyelectrolyte Multilayers Preparation of the Multilayers. The preparation of the multilayers on porous silica has been adapted from the procedure used by Rubner and co-workers on plane silicon wafers.6,18 A priori, the adsorption kinetics should be much slower here. No attempt has been made to quantify exactly this fact, and the times allowed for equilibration were simply taken much larger. To gain well-detailed information on the layers at a molecular scale, each of the polymers, PAH or PAA, must be labeled independently. Two systems of all samples were therefore prepared. In a small glass, 100 mg of silica and 25 mL of a solution of 10-2 M PAH (in monomers), with a ionic strength of 0.1 M (NaCl 10-1 M) and a pH adjusted at a value comprised between 2 and 9, were mixed and stirred for 3 h. The powder was allowed to settle for 24 h in a tube. The EPR spectrum of the supernatant was recorded and the samples were rinsed several times with an aqueous solution of NaCl 10-1 M, until the disappearance of the signal. For the second layer these same samples are recovered and a similar procedure with PAA, also in a NaCl 10-1 M solution and with the same pH, is used. In that case the signal of the supernatant also defines the end of the rinsing. Typically 3-5 rinsings were needed. EPR Spectroscopy: Samples and Apparatus. The EPR spectra were recorded on a Varian (Palo Alto, CA) E-4 spectrometer operating at X-band at 9.15 GHz. The temperature was regulated within (1 °C by a Bruker (Karlsruhe, Germany) BV2000 temperature controller, and each time a period of 10 min was allowed for thermal equilibrium of the samples. The best signals, with good sensitivity without being too far from the ambient temperature, were obtained at 50 °C. For these samples and water as a solvent, with a high dielectric constant, which induces high energy losses, flat cells (Wildmad Glass, Buena, NJ) were used. Analysis of the Spectra. The shape of the EPR spectra of nitroxide free radicals is very sensitive to the Brownian motion of the label.15 The absorption lines come from transitions between the energy levels of the spin Hamiltonian:

H ) SgB + IAS where S ) 1/2 is the spin of the unpaired electron, I ) 1 is the spin of the nitrogen nucleus, g is the gyromagnetic tensor, and A is the hyperfine tensor. If the motion is very fast, the spectrum appears as three narrow, well-resolved Lorentzian lines, which can be explained by the Kivelson theory16 and simulated on a PC computer. The rotational correlation time falls in the range (3 × 10-11)-(3 × 10-9) s. The rotational correlation time is simply derived from a comparison between experimental and calculated spectra. In the slow tumbling region, the shape is influenced by the anisotropic part of the spin Hamiltonian and particularly extra low and high field peaks appear, which can be explained by the more comprehensive Freed theory.17 It is one of the great advantages of the spin labeling method that differences in mobility appear not just as line broadenings but also as line shifts. Experimentally, when polymers are studied at interfaces, the spectra appear very often as a superposition of these two kinds of spectra. Loops and tails protruding into solution experience a fast motion, whereas trains adsorbed on the solid have a motion hindered by this contact. These two populations of segments can therefore relatively easily be detected, distinguished, and evaluated by a computer simulation. The estimated precision on these values is about 3%. In this way statistical information on the conformations of the chains, related to the concentration profile, can be gained from the EPR spectroscopy.7

Results Characterization of the Charges of the Materials. In contact with a solution, only the singly coordinated Si-OH groups can adsorb or desorb a proton. The Bro¨nsted acid/base properties of the silica surface can be simply (15) Berliner, L. J., Ed. Spin Labeling, theory and applications; Academic Press: New York, 1976; 592 pp. (16) Kivelson, D. J. Chem. Phys. 1960, 33, 1094. (17) Schneider, D. J.; Freed, J. H. In Biological Magnetic Resonance; Berliner, L. J., Reuben, J., Eds.; Plenum Publishing: New York, 1989; Vol. 8, p 1.

Langmuir, Vol. 20, No. 8, 2004 3175

represented by the two reactions involving singly hydroxylated silicon:

-Si-O- + H+s T -Si-OH K11 -Si-OH + H+s T -Si-OH2+ K12 where K11 and K12 correspond respectively to the first protonation and second protonation constants for singly coordinated Si-OH groups. Theoretical and experimental values for log K11 (11.9) and log K12 (-1.9) are consistent with the observation that -Si-OH is quite stable and that -Si-OH2+ forms only in very acidic media. Because of this difference, we may not expect two kinds of ionized sites simultaneously at a given pH. -Si-OH2+ are stable in the acidic domain (pH < 1) and the density of -Si-O- becomes significant at about pH > 6. The pH of zero charge (about 3) corresponds also to a fully undissociated silanol surface. The main difference between silica and other oxides is that there is a flat part between pH 3 and 6, where the surface charge density is very low. A strong increase is shown above pH 8.8,9 The elemental analysis of the labeled polymers showed that they had retained their purity. The same procedure was used for the titration of both polyelectrolytes. Twenty milliliters of PAA (4.4 × 10-3 M) or of PAH (3.2 × 10-3 M) are neutralized with 5.23 × 10-3 M NaOH. The starting pH allows an evaluation of the charge of the chains, as the only protons present are due to the dissociation of the groups on the chains themselves. When NaOH is added, a part of the ions neutralize the groups on the chains, modifying its charge, while the other part changes the pH. By knowing the pH and the amount of NaOH added, it is possible to record the rate of charge. The shape is not really sigmoidal like for small ions, nor linear, but evolves rather smoothly in the whole range of pH explored. For PAA near 0% of the sites are charged at pH 4, 50% at pH 7, and 100% at pH 9. For PAH 85% of the sites are charged at pH 5, 50% at pH 7, and 0% at pH 9.5. Moreover, some viscosimetry measurements of the PAH show that the crossover between the isolated coils regime and the semidiluted regime takes place for a weight fraction of 30%. First Monolayer of PAH. First it must be noted that there is no adsorption for a pH below 5, because the surface charge of silica becomes too weak, or even positive, to allow significant interactions with PAH. It has been checked that a variation of temperature has no effect on the shape of the spectra and therefore on the conformations of the chains. Indeed if the interactions between the polymer and the solid are of the order of kBT, like a van de Waals or an hydrogen interaction, it is usual that the chains are partially desorbed by the thermal agitation.7 Here the electrostatic interactions are much stronger and cannot be balanced by a relatively small shift in temperature. However, the absorption of the electromagnetic wave at a frequency of 9.15 GHz is greatly reduced at 50 °C, the signal-to-noise ratio is much better, and all spectra have been recorded at this temperature. In Figure 1 the spectra of the monolayer of labeled PAH adsorbed on silica is shown for different values of pH. Generally they present a composite shape. The labels pertaining to loops and tails, mobile in solution, give the typical spectrum composed of three narrow Lorentzian lines. On the contrary, the labels attached to trains have a motion hindered by the contact with the silica and give much broader lines. The distinction between these two environments, and the evaluation of both components,

3176

Langmuir, Vol. 20, No. 8, 2004

Figure 1. EPR spectra of the first layer of labeled PAH adsorbed on silica as a function of pH: (A) pH 5; (B) pH 6; (C) pH 7; (D) pH 8; (E) pH 9.

Figure 2. Amounts of adsorbed PAH, measured by EPR on the dry residue as a function of pH (arbitrary units; the relative intensities are significant).

Figure 3. Evolution of the fraction of labels, attached to the first layer of PAH, with a fast motion (pertaining to loops and tails) as a function of pH. The estimated precision on these values is about 3%.

gives a description more precise than simply a thickness, but rather a two-step concentration profile. In Figure 2 the total amount of adsorbed polymer measured by recording the intensity of the EPR signal of the dry residue is given. This quantity increases with pH: the charge of the silica surface increases whereas the linear charge of the polymer decreases; therefore, to compensate the charge of the solid, more polymers are needed. At pH 9 it seems that the polymer is so weakly charged that the affinity with the surface becomes finally lower. In Figure 3 the fast population of labels is plotted as a function of pH. At pH 5 the polymer is strongly charged but the silica only weakly, and the polymer can form large loops and tails protruding in solution between adsorption sites, and the fast fraction of labels is high. When the pH is increased, the fast fraction of labels decreases, indicating a more flat conformation. The charge of the silica increases probably

Schach et al.

Figure 4. Two-step concentration profile of the first layer of PAH adsorbed on silica as a function of pH. The fraction of loops and tails (white) is distinguished from that of trains (black).

Figure 5. Effect of the addition of a PAA layer on the first coating of PAH as seen on the EPR spectra (dotted lines, adsorbed labeled PAH chains alone; solid line, adsorbed labeled PAH chains covered by the PAA macromolecules). pH values of adsorption: (A) pH ) 5; (B) pH ) 6; (C) pH ) 7; (D) pH ) 8; (E) pH ) 9.

enough to accommodate ever more of the slowly decreasing charges of the polymer. At pH 8 the polymer is completely immobilized in close contact with the surface. At pH 9 a fast fraction appears again. The charge of the polymer is now so low that whatever that of the silica, it is only weakly bonded to the solid and can even form loops and tails. It must be stressed here that the fraction of trains is evaluated starting from mobility measurements. Contrary, for example, to IR results, it is not the number of actual contacts that is counted, but neighbor segments whose tumbling is strongly reduced due to the constaint of connectivity of the chains are also part of the trains. In Figure 4 the two-step concentration profile of the first layer of PAH adsorbed on silica is given as a function of pH. Taking as reference the spectra of polymers in solution at different concentrations and setting the concentration of segments in the loops and tails domain at 35% and that in the trains domain at 80%, a detailed description of the first layer of PAH, giving the extent of each of the domains deduced from the fraction of fast and slow populations, is therefore obtained. Effect of the Second Layer of PAA. When a second layer of PAA is adsorbed on the first layer of PAH, an important confinement of the first layer is observed, detected mainly by the disappearing of the fast fraction of labeled segments of PAH (Figure 5). The disappearance of the three narrow peaks corresponding to the fraction of labels with fast motion confirms first that it is indeed a bilayer which is built. The mobility may be affected by two factors: on one hand the solvent may be expelled from the outer part of the adsorbed PAH, while on the

Formation of Weak Polyelectrolyte Multilayers

Figure 6. Amount of adsorbed labeled PAA in the second layer as a function of pH measured by integrating and measuring the total intensity of the EPR spectra.

Figure 7. Evolution of the fraction of labels, attached to the second layer of PAA, with a fast motion (pertaining to loops and tails) as a function of pH. The estimated precision on these values is about 3%.

other hand the oppositely charged chains may form neutral complexes hindering the motion by steric hindrances. At this point it is not possible to conclude if the layers remain definitely distinct or if there exists some interpenetration. In Figure 6 the amount of adsorbed PAA polymer, measured indirectly through the intensity of the EPR spectra, is shown. It increases continuously from pH 5 to 9. The present evolution results from a balance between the varying charges of the three-component system with pH, namely, silica, PAH, and PAA. It is not completely obvious considering the previously observed trends for the adsorbed PAH chains alone. More precisely, the evolution of the EPR spectra of the labeled PAA chains in the second layer has been recorded as a function of pH. The shapes of the spectra again scarcely vary with temperature. Qualitatively, the relative intensities of the three peaks corresponding to the fast fraction of labels decrease rather with the pH. In Figure 7 the quantitative values of the fast population measured by integration of the peaks are shown. Starting with the high value of 100% fast population at pH 5, it decreases monotonically until pH 8 and increases again slightly at pH 9. It must be noted that a fraction of trains of 1% of the segments is not detectable even if the chain as a whole remains adsorbed as it can be inferred from the overall mobility. At pH 5 the PAA chains are only weakly charged, as well as the silica surface, leading to a moderate adsorption of PAH and probably a modest charge inversion. At this point the polymers have long neutral parts between charges, allowing the formation of large loops and tails and an important fast fraction of labeled PAA. As the pH increases, the charge of the silica increases too,

Langmuir, Vol. 20, No. 8, 2004 3177

Figure 8. Two-step thickness profiles of each of the two layers of PAH and PAA adsorbed on silica as a function of pH, evaluated by the amount of polymer in each. The fraction of loops and tails is distinguished from that of trains by different nuances of black and white. From the silica surface at the bottom to the solution at the top appear successively (a) the slow population of PAH, (b) the interpenetrated layer of PAH with PAA, (c) the slow fraction of PAA, and (d) the fast fraction of PAA.

and more PAH is adsorbed, giving rise to a more pronounced charge inversion and a stronger adsorption of PAA in a more flat conformation. This process reaches its limit at pH 9, where the PAH polymer is no longer sufficiently charged to carry on this trend. In Figure 8 all the data of the previous analyses are summarized. It shows, for each of the two layers, the adsorbed amounts and the distinction between the two environments, labels with a fast or with a slow motion, as a function of pH. Clearly the conformations of the adsorbed polymers are strongly dependent on pH. The evolution is not at all simple and results from a difficult to predict a priori balance between the charges of the three components of the systemssilica, PAH, and PAAswith pH. The thickness of the different parts, evaluated as the amount of segments included in it, is indeed accessible to the measurements by the spin labeling technique, giving a rather detailed picture of the coated solid. Discussion Expected Phenomena. The combination of polymer physics with electrostatics provides a number of interesting theoretical questions and is associated with many experimental effects and applications. In fact, the precise description of the chains’ properties and the thermodynamics in polyelectrolyte solutions are formidable problems that have not been fully solved yet.19,20 So since the introduction of polyelectrolyte multilayers, there have been a number of experiments devoted to the understanding of the formation mechanisms.1-6,21-25 For example, the effect of molecular structure, substrate, charge density, ionic strength, deposition pH, and rinsing conditions on multilayer stability were studied. Generally, multilayer stability is largely governed by polymer charge (18) Yoo, D.; Shiratori, S. S.; Rubner, M. F. Macromolecules 1998, 31, 4309. (19) Fo¨rster, S.; Schmidt, M. Adv. Polym. Sci. 1995, 120, 51. (20) Barrat, J. L.; Joanny, J. F. Adv. Chem. Phys. 1996, 94, 1. (21) Hoogeveen, N. G.; Cohen Stuart, M. A.; Fleer, G. J.; Bo¨hmer, M. R. Langmuir 1996, 12, 3675. (22) Steitz, R.; Jaeger, W.; Klitzing, R. v. Langmuir 2001, 17, 4471. (23) Serizawa, T.; Kawanishi, N.; Akashi, M. Macromolecules 2003, 36, 1967. (24) (a) Schlenoff, J. B.; Dubas, S. T. Macromolecules 2001, 34, 592. (b) Dubas, S. T.; Schlenoff, J. B. Macromolecules 2001, 34, 3736. (25) Sukharukov, G. B.; Schmitt, J.; Decher, G. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 948.

3178

Langmuir, Vol. 20, No. 8, 2004

density and solution ionic strength, with high charge densities and low ionic strengths favoring multilayer formation. Within stable multilayers, the mobility of polyelectrolyte molecules will be strongly limited, since they are strongly bound to both lower and higher layers. Immersion of consecutively adsorbed polyanion/polycation multilayer films in solutions of different ionic strength causes reversible thickness changes. Full multilayer deconstruction and desorption over a relatively narrow range of salt concentration is also possible. Polyelectrolyte multilayers of weakly charged polymers have a unique and ultimately useful feature in that their architecture depends on the polyelectrolyte degree of ionization. Key properties of layers based on PAH and PAA are specifically nanoscale control over the internal architecture (e.g., degree of interpenetration), control over surface composition (e.g., carboxylic acid content), easy and environmentally friendly processing, conformal coating over a wide variety of surfaces independent of size, and easy patternability.26 Two fully charged chains of this type in the absence of added salt will strive to form a cooperatively stitched 1:1 polyelectrolyte complex with extended sections of polycation/polyanion double-strand-like units.6 When a weak polyelectrolyte adsorbs onto a weakly acidic surface, the pH in the interphase may differ from that in the bulk solution, such that the polymer and substrate charge densities are altered, and the local apparent dissociation constants deviate from ideal behavior. Despite this complexity, coverage generally exhibits a maximum as pH is varied, a feature parallel to the influence of quaternized amine density in studies of strong polyelectrolyte adsorption.27 The effect of charge density on polyelectrolyte multilayer growth has been examined by Rubner and co-workers6,18,26 by using the pH of the adsorption solution to tune charge densities. A comprehensive investigation of adsorption conditions (via pH) revealed a complicated but controllable dependence of layer thickness on pH and thus on charge density. Very thick layers were obtained when a fully charged chain was combined with a nearly fully charged chain. The trends observed with change in pH were explained by invoking different models for the adsorption behavior at different pH: the increase in adsorbed amounts at lower charge densities were explained by charge compensation arguments, while a thermodynamic model that accounts for the loss in entropy of the polyelectrolyte upon adsorption was used at higher charge densities. The discrete nature of the charges on polyelectrolyte chains is explicit in the sticker energy and implicit in the loop entropy. In agreement with other works, layer growth proceeds linearly for the fully charged polyelectrolytes with layer number after about four layers. The number of layers required before linear growth is established depends on the nature of the surface, the polyelectrolyte type, and the adsorption conditions. Here clearly these last requirements are not met and the conformations of the chains are not expected to be decoupled from the state of the solid surface. The behavior of weak polyelectrolytes, of pH-tunable charge density is also probably different from that of copolymers of strong polyelectrolytes and hydrophilic monomers of varying compositions.28 For example, the influence of salt and of polymer charge density on the multilayer formation of strong polyelectrolytes gives the (26) Wang, T. C.; Chen, B.; Rubner, M. F.; Cohen, R. E. Langmuir 2001, 17, 6610. (27) Shin, Y.; Roberts, J. E.; Santore, M. M. J. Colloid Interface Sci. 2002, 247, 220. (28) Schoeler, B.; Kumaraswamy, G.; Caruso, F. Macromolecules 2002, 35, 889.

Schach et al.

opportunity to distinguish between the change in charge of the adsorbing polyelectrolyte chain and the surface charge.22 Here the discussion is limited to the system of weak polyelectrolytes experimentally studied. Some theories have been proposed to explain these results.29,30 Observations. Our observations fall broadly in the proposed scheme. The adsorption of the first layer of PAH on the silica surface can be simply explained by the effect of charge compensation. In fact it is a titration of the surface of the solid accessible to the polymer that is performed. At the low pH 5, the silica surface is only weakly charged and a small amount of polymer is enough to neutralize the surface sites; the immobilized charges of the chains are well separated and the conformation is rather extended with large loops. As the pH increases, the silica presents ever more charged Si-O- sites and more PAH polymers are needed to compensate them. Simultaneously, as more segments are attached to the surface, the conformation becomes more flat and even completely immobilized at pH 8. This behavior reaches its limit at pH 9 where the polymer itself is no longer charged enough to succeed in doing so. The second layer of PAA, which is then added, behaves apparently not so obviously. Complexation is clearly experimentally demonstrated by the loss of loops and tails belonging to the first polyelectrolyte, upon adsorption of the second polyelectrolyte. In fact the observed trend results probably from a balance between the charges of the three components, silica, PAH, and PAA, which are all varying in this pH range. Delicate changes in charge densities are difficult to manipulate, as the density of weak polyelectrolytes is nonlinearly changed by pH. Taken separately, each of these variations has its own shape and it cannot be expected to match exactly one another. Moreover, in the complex system where all the components are put together, the different evolution of the charges are not completely independent and even the effective pH and pK inside the polymer layer can be different from that in the bulk. Under near-neutral pH conditions, when the ionization fractions of the polyanion and the polycation approach unity, the positive and negative sites at the interface are well matched and a transition to molecularly thin bilayers is observed. Rather than pay the high cost in monomer-monomer repulsion, the chains opt to lose conformations and gain adsorption energy through complete adsorption at the interface. A transition on both sides of this pH regime of molecularly thin layers to very thick, brushlike layers appears when one component is near full ionization while the second exhibits a charge fraction that is smaller. Because one component loses charges on either side of this pH window, brushlike layers are observed at both more acidic and more basic pHs.30 Conclusion The adsorption of weak polyelectrolytes, namely, PAH and PAA, from aqueous solution onto the weakly oppositely charged silica surface has been recorded as a function of pH by studying the EPR spectra of the alternately labeled polymers. Two environments can be distinguished by this technique, namely, loops protruding in solution and trains immobilized on the solid. The number of trains and loops or tails in the first and second adsorbed layers is quantified by the spin labeling technique. The overall picture gained by EPR spectroscopy gives the thickness of all parts of the (29) (a) Joanny, J. F. Eur. Phys. J. B 1999, 9, 117. (b) Castelnovo, M.; Joanny, J. F. Langmuir 2000, 16, 7524. (30) Park, S. Y.; Rubner, M. F.; Mayes, A. M. Langmuir 2002, 18, 9600.

Formation of Weak Polyelectrolyte Multilayers

adsorbed layers: the immobilized trains of PAH in contact with silica, the interpenetrated layer of complexes between PAH and PAA, the layer of immobilized PAA, and the loops of PAA. The evolution of these components with the

Langmuir, Vol. 20, No. 8, 2004 3179

pH has been explained by considering charge compensation and the entropy of confinement. LA030202J