5188
Langmuir 2003, 19, 5188-5191
Ionization-Controlled Stability of Polyelectrolyte Multilayers in Salt Solutions Vladimir Izumrudov† and Svetlana A. Sukhishvili* Department of Chemistry and Chemical Biology, Stevens Institute of Technology, Hoboken, New Jersey 07030 Received March 1, 2003. In Final Form: April 16, 2003 In situ FTIR measurements of functional group ionization are reported for polyelectrolyte multilayer thin films, composed of a weak polycarboxylic acid and one of several polycations each having different charge densities. The stability of the multilayers in salt solutions increased dramatically when the pH of the solution was lowered below a critical point. This critical pH was shown to depend on both the charge density of the polycation and the nature of the polyacid. Stabilization of the multilayer was controlled exclusively by polyacid ionization within the film and occurred when the same fraction of carboxylic groups was ionized regardless of the nature of the polyacid or the polycation charge density.
Layered ultrathin polymer films are novel materials obtained by sequential adsorption of polymers on a solid substrate.1-9 The technique to create them is versatile and can be used to deposit a variety of synthetic and natural molecules onto solid surfaces.3 Although in the majority of cases layered polymer films are formed by the ionic pairing of oppositely charged polymers to produce polyelectrolyte multilayers,1-9 self-assembly of uncharged molecules driven by hydrogen bonding interactions is also possible.10-12 Weak polyelectrolytes have been also used to build polyelectrolyte multilayers.13-18 In the latter case, the apparent ionization of a weak polyelectrolyte incorporated into a polyelectrolyte multilayer is determined by both the external pH and the ionic strength as well as by the local microenvironment including the charge density of an oppositely charged self-assembled polyelectrolyte and the charge of a polymer included into the top layer.19,20 When a weak polyacid is incorporated into a * To whom correspondence should be addressed. † Permanent address: Department of Polymer Chemistry, Moscow State University, 119992 Moscow, Russia. (1) Decher, G.; Hong, J.-D. Macromol. Chem. Macromol Symp. 1991, 46, 321. (2) Decher, G. Science 1997, 277, 1232. (3) Bertrand, P.; Jonas, A.; Laschewsky, A.; Legras, R. Macromol. Rapid Commun. 2000, 21, 319. (4) Schlenoff, J. B.; Dubas, S. T. Macromolecules 2001, 34, 592. (5) Graul, W. G.; Li, M.; Schlenoff, J. B.J. Phys. Chem. B 1999, 103, 2718. (6) Dubas, S. T.; Schlenoff, J. B. Macromolecules 1999, 32, 8153. (7) Sukhorukov, G. B.; Mo¨hwald, H.; Decher, G.; Lvov, Y. M. Thin Solid Films 1996, 284-285, 220. (8) Kotov, N. A.; Magonov, S.; Tropsha, E. Chem. Mater. 1998, 10, 886. (9) Fisher, P.; Laschewsky, A.; Wischerhoff, E.; Arys, X.; Jonas, A.; Legras, R. Macromol. Symp. 1999, 137, 1. (10) Stockton, W. B.; Rubner, M. F. Macromolecules 1997, 30, 2717. (11) Wang, L. Y.; Fu, Y.; Wang, Z. Q.; Fan, Y.; Zhang, X. Langmuir 1999, 15, 1360. (12) Sukhishvili, S. A.; Granick, S. J. Am. Chem. Soc. 2000, 122, 9550. (13) Shiratori, S. S.; Rubner, M. F. Macromolecules 2000, 33, 4213. (14) Mu¨ller, M.; Rieser, T.; Lunkwitz, K.; Berwald, S.; Meier-Haack, J.; Jehnichen, D. Macromol. Rapid Commun. 1998, 19, 333. (15) Yoo, D.; Shiratori, S. S.; Rubner, M. F. Macromolecules 1998, 31, 4309. (16) Chen, K. M.; Jiang, X. P.; Kimerling, L. C.; Hammond, P. T. Langmuir 2000, 16, 7825. (17) Dubas, S. T.; Farhat, T. R.; Schlenoff, J. B. J. Am. Chem. Soc. 2001, 123, 5368. (18) Cao, T.; Chen, J.; Yang, Ch.; Cao, W. Macromol. Rapid Commun. 2001, 22, 181. (19) Xie, A. F.; Granick, S. J. Am. Chem. Soc. 2001, 123, 3175.
polyelectrolyte multilayer, an increase in the polyacid acidity has been observed by several groups including ours.13,20,21 We have also recently reported that the ionization of a weak polyacid within a multilayer increased with the charge density of a co-self-assembled polycation.20 Though both electrostatic and hydrogen-bonded multilayers have been previously studied, the regulated switching between electrostatic and hydrogen bonding and its effect on the salt stability of multilayers have not yet been reported. In the present work, we investigate the in situ ionization of a polyacid within a multilayer and find a sharp (in the pH scale) stabilization of multilayers at high salt concentration when the ionization of a polyacid decreases. We suggest that the stabilization is due to the hydrogen bonding between protonated carboxylic groups. We find that the transition correlates with the apparent ionization of a self-assembled polyacid and occurred at different external pH values when polycations of different charge density were used. The latter, as well as the fact that the stabilization occurred in the range of neutral or slightly acidic pH, may allow one to create enzyme-containing multilayers with controllable salt stability for use in various medicinal and biotechnology applications. Multilayers were prepared on the surface of an oxidized ATR silicon crystal by the sequential deposition of a polybase and a polyacid from 0.5 mg/mL solutions. To avoid overlap of infrared spectra with the strong water band in the region of interest to us (1500-1750 cm-1), measurements were made in D2O, rather than H2O. However the discussion below refers to pH and “hydrogen bonding” rather than pD and “deuterium bonding”, since the former terminology is much more common. The silicon crystal was installed within a flow-through stainless steel cell. The amounts of polymers adsorbed and the degree of ionization of the carboxylic groups were quantified by in situ FTIR-ATR (Fourier transform infrared spectroscopy in attenuated total reflection) from the calibrated intensities of the vibrational bands of -COO-, -COOH, and the pyridine and pyridinium rings using experimental protocols and calibration methods described elsewhere.20 Polycations of general abbreviation QPVP were obtained by ethyl bromide alkylation to varying degrees of a poly(20) Kharlampieva, E.; Sukhishvili, S. A. Langmuir 2003, 19, 1235. (21) Mendelsohn, J. D.; Barrett, C. J.; Chan, V.; Pal, A. J.; Mayes, A. M.; Rubner, M. F. Langmuir 2000, 16, 5017.
10.1021/la034360m CCC: $25.00 © 2003 American Chemical Society Published on Web 05/21/2003
Letters
Figure 1. Comparison of salt-induced disintegration of PMAA/ Q84 multilayers for various solution pH: 5.3 (open squares); 5.7 (filled squares); 6.0 (filled circles); 6.4 (open circles); 6.7 (open diamonds). Panel A shows the relative mass adsorbed as a function of salt concentration. Panel B shows the relative mass adsorbed plotted against the average ionization of PMAA within the multilayer films. The arrows show the direction in which salt concentration was increased. The experiment was done in D2O, which contained 0.01 M phosphate buffer. The multilayer films contained 10 alternating layers of PMAA and Q84, with Q84 included in the top layer.
4-vinylpyridine sample with a weight-average degree of polymerization of 900. The polycations are referenced below as Q50, Q68, and Q84 for polymers with 50%, 68%, and 84% of quaternized units, respectively. The polyacids were poly(methacrylic acid) (PMAA), with Mw 150000, or poly(acrylic acid) (PAA), with Mw 450000sweak polyacids with significantly different values of solution pKa. For low ionic strength conditions, a pKa of about 6-7 is usually reported for PMAA, while a lower pKa ranging from 5-6 is usually found for PAA. Prior to multilayer deposition, the surface of a Si crystal was pretreated with 0.1 mg/mL solution of Q84 at pH ) 9.2. This first layer was deposited to increase the adhesion of the multilayer to the surface and was always taken as a background. Ten polymeric layers were then deposited from solutions at a constant pH ranging from 5.3 to 7.2 containing 0.01 M phosphate buffer. Multilayer growth at each pH was linear, with constant amounts of polymers deposited at each step.22 All deposited films contained a positively charged polymer as the topmost layer. In subsequent studies of multilayer stability, the samples were exposed to salt solutions at the same pH at which the multilayers were created. The top panel of Figure 1 shows the stability of Q84/ PMAA multilayers in salt solutions at various pH values. Over a narrow pH range from 5.7 to 6.0 there is a dramatic
Langmuir, Vol. 19, No. 13, 2003 5189
change in the multilayer stability. Above pH ) 6, multilayers are unstable in salt solutions with NaCl concentration higher than 1 M. The latter is consistent with the electrostatic nature of self-assembled Q84/PMAA films and reflects the competition of salt ions and polyelectrolyte charged units in producing ion pairs.23 A surprisingly sharp (in the pH scale) stabilization of the film is evident when the pH is lowered to 5.7. The bottom panel of Figure 1 points to a correlation between the fractional ionization of the PMAA carboxylic groups and the multilayer stability. Note that in this and other experiments presented in this Letter, protonation of the pyridine rings did not occur because of the low pK values (ca. 3.0-3.6) of the pyridine groups in the partly alkylated QPVP. Arrows in the figure show the direction in which concentration of salt was increased. For multilayers stable at high salt concentrations, the fraction of ionized carboxylic groups was small. Note that the decrease of ionization plotted in the bottom panel of Figure 1 spanned a wide range as a result of a significant reduction of the fractional ionization of carboxylic groups as the concentration of NaCl increased. The latter is consistent with recent reports by Schlenoff et al. on the decrease of the acidity of a self-assembled weak polyacid at high concentrations of salts.23 We defined two parameters that describe the multilayer stability. First, we define [NaCl]crit as the lowest concentration of salt at which a multilayer is practically destroyed, with the fraction remained not exceeding 5%. For the data shown in Figure 1, [NaCl]crit assumed finite values from 0.7 to 1.5 M at pH g 6 but approaches an infinitely large value at pH < 5.7. For pH < 5.7 we defined a second parameter ∆ which is the fraction of a multilayer remained at the surface at [NaCl] ) 4 M (this is the highest [NaCl] we studied) after a waiting time of 20 min. Note that this time was enough to reach the steady state. The decomposition was fast, with 90% of the multilayer being destroyed during the first 2 min. Figure 2 shows the dependence of both of these parameters on the fractional ionization of the polyacid within the multilayer. Different symbols in the graph correspond to various alkylation degrees of the self-assembled QPVP. It is remarkable that regardless of quaternization degree of QPVP, all data collapsed on the same master curve. It is also important that the data obtained with a different polycarboxylic acid of higher acidity (PAA) followed the same dependence (open squares in Figure 2). This is despite the fact that significant conformational differences for PMAA and PAA molecules in solution are known,24 with PMAA undergoing pronounced conformational changes when its ionization degree is decreased. The results shown in Figure 2 suggest that the internal ionization of a polycarboxylic acid, regardless the charge density of a co-self-assembled polycation and nature of a polyacid, was an important parameter that controlled the multilayer stability. The fact that multilayer stabilization occurred in different polymeric systems at the same fractional ionization of polycarboxylic acid and over a narrow range of ionization points to a possible reason. Polycarboxylic acids are known to form strong hydrogen bondsscyclic dimers and oligomeric associated structuressin blends.25,26 Fur(22) Since PMAA dissociation is pH-dependent, larger amounts of PMAA and QPVP were deposited at lower pH, in accordance with the charge compensation mechanism of adsorption. In particular, a layer contained 4 mg‚m-2 of PMAA and 5.4 mg‚m-2 of Q84 at pH 6.7, whereas a layer contained 6.8 mg‚m-2 of PMAA and 8 mg‚m-2 of Q84 at pH 5.7. (23) Rmaile, H. H.; Schlenoff, J. B. Langmuir 2002, 18, 8263. (24) Morawetz, H. Macromolecules 1996, 29, 2689.
5190
Langmuir, Vol. 19, No. 13, 2003
Figure 2. The critical concentration of sodium chloride, [NaCl]crit, required to decompose the multilayers plotted against the average polyacid ionization within multilayers of PMAA/ Q84 (filled squares), PMAA/Q68 (filled triangles), PMAA/Q50 (filled circles), and PAA/Q84 (open squares). The critical ionization degree of 0.48, below which multilayers did not decompose completely in 4 M NaCl solutions, is marked by arrows. The inset shows relative mass ∆ of a multilayer that remains adsorbed at the surface in 4 M NaCl as a function of the average ionization of a self-assembled polyacid.
thermore, hydrogen bonding between protonated carboxylic groups persists even in aqueous solution27 where carboxylic groups exist in the hydrated form and are surrounded by hydrogen bonded water molecules. We suggest that extensive hydrogen bonding could occur within the multilayer and the formation of a hydrogenbonded network is highly possible as the concentration of protonated carboxylic groups increases. This would lead to the creation of a multilayer that could not be destroyed in solutions of high ionic strength. The formation of a hydrogen-bonded network should be facilitated both by the extremely high local concentration of functional groups in the adsorbed state and by the imperfect stratification of the polymer layers within the multilayer.2 The difference between the stable and unstable multilayer (in which a polyacid is more ionized) is that the smaller number of hydrogen bonds in the latter case does not cross-link the whole multilayer film, making it possible for macromolecules to leave the surface as ionic strength destroys the interpolymer ionic association. An alternative interpretation might argue that the multilayer stabilization at acidic pH occurred due to increased hydrophobic interactions. This possibility can be ruled out because the stability curves shown in Figure 2 overlapped for PMAA and PAA, two polyacids with noticeably different hydrophobicity. Note that self-association of carboxylic groups, probably caused by hydrogen bonding, has been seen in our experiments as an asymmetric broadening of the 1698 cm-1 carbonyl stretch band at the lower wavenumbers. However, because of significant spectral overlap of the carbonyl stretching vibration band at 1701 cm-1 with the 1643 cm-1 band associated with in-ring skeletal vibrations of pyridinium ring, quantifying the fraction of hydrogen-bonded carboxylic groups from our data was difficult. In the scale of the external pH, the multilayer stability varied significantly with the quaternization degree of the polycation and the acidity of a weak polyacid. Figure 3 (25) Lee, J. Y.; Painter, P. C.; Coleman, M. M. Macromolecules 1988, 21, 346. (26) Dong, J.; Ozaki, Y.; Nakashima, K. Macromolecules 1997, 30, 1111.
Letters
Figure 3. The critical concentration of sodium chloride, [NaCl]crit, as a function of the external pH in the PMAA/Q84 (filled squares), PMAA/Q68 (filled triangles), PMAA/Q50 (filled circles), and PAA/Q84 (open squares) systems. The arrows indicate a critical pH, below which multilayers did not decompose completely even in 4 M NaCl solutions. The inset shows the average fraction of protonated carboxylic groups in a self-assembled polyacid as a function of pH measured at a constant salt concentration of 0.4 M NaCl.
illustrates that the pH values for multilayer stabilization decreased systematically when the charge density of a self-assembled polycation increased. The inset in Figure 3 points to the underlying reason. The multilayer stabilization correlated with the fraction of protonated carboxylic groups in a polycarboxylic acid. At the same external pH, the latter was regulated by the charge density of the self-assembled polycation. In particular, a polycation with the higher charge density was more efficient in inducing ionization of a polyacid, and the critical concentration of protonated carboxylic groups was reached at a lower external pH. This is consistent with our earlier finding that ionization of PMAA within multilayers increases with the degree of alkylation of QPVP20 due to the fact that QPVP with higher linear charge density creates stronger local electric charge and, consequently, more efficiently ionizes the adjacent PMAA molecules. Figure 3 also shows that the stability could be also controlled by a polyacid acidity. For example, the stability region for PAA/QPVP multilayers was significantly shifted to lower pH, an observation which is consistent with the higher acidity of PAA. The requirement for multilayer stabilization to occur is the presence of a weak polycarboxylic acid within the multilayer. Polycarboxylic acids other than PMAA or PAA, such as polyitaconic acid, show qualitatively similar effects when self-assembled within multilayer films. In addition, cationic species other than QPVPssuch as the highly charged polycation poly(dimethyldiallylammonium) chloride or even proteins at the pH lower than their isoelectric pointsshow similar effects when self-assembled with a weak polyacid. In our recent experiments with proteins, the role of the interlayer spacing and the conformation of the self-assembled species became evident. Proteincontaining multilayers showed stabilization effects at a significantly lower fractional ionization of a polycarboxylic acid (data not shown). This could be rationalized when one considers that proteins are not molecularly thin but are folded into globular conformations. It is reasonable to suggest that when self-assembled within the multilayer, proteins strongly hinder the interassociation of the protonated carboxylic groups from the adjacent polymer (27) Tanaka, N.; Kitano, H.; Ise, N. Macromolecules 1991, 24, 3017.
Letters
layers. So far we have proven that this concept is applicable to several proteins, including lysozyme and ribonuclease, whose isoelectric points are 11.0 and 9.45, respectively. The experiments to quantify the effect as applied to proteins are underway. Practical ramification of the latter studies might include areas of biotechnology and biomaterials.
Langmuir, Vol. 19, No. 13, 2003 5191
Acknowledgment. We thank Professor Matthew Libera (Stevens Institute of Technology) for many discussions. This work was supported by the National Science Foundation through an CEEP international supplement to the award DMR-0209439. LA034360M
