Release of a Dye from Hydrogen-Bonded and Electrostatically

We found that with the PMAA/PEO films, dye release did not depend on the concentration of polymer in solution, but was largely controlled by the amoun...
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Langmuir 2004, 20, 9677-9685

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Release of a Dye from Hydrogen-Bonded and Electrostatically Assembled Polymer Films Triggered by Adsorption of a Polyelectrolyte Eugenia Kharlampieva and Svetlana A. Sukhishvili* Department of Chemistry and Chemical Biology, Stevens Institute of Technology, Hoboken, New Jersey 07030 Received May 18, 2004. In Final Form: August 20, 2004 The absorption of dyes within hydrogen-bonded and electrostatically assembled multilayers and subsequent release of the dyes from the films were studied in situ using FTIR-ATR. Multilayers were composed of poly(methacrylic acid), PMAA, and poly(ethylene oxide), PEO (hydrogen-bonded multilayers), or of PMAA and 22% quarternized copolymer of N-ethyl-4-vinylpyridium bromide and 4-vinylpyridine, Q22 (electrostatically stabilized multilayers). After multilayer deposition, the solution pH was changed to produce excess charge within the films. Dyes with charge opposite to the excess charge of the film (Rhodamine 6G for hydrogen-bonded films or Bromophenol Blue for electrostatically assembled multilayers) were then allowed to absorb within multilayers. In both systems, the dyes were uniformly included within the films. The top layers largely affected the loading capacity of the multilayers, suggesting weaker binding of the dyes with the top layers. Dye release into a 0.01 M phosphate buffer was significantly smaller as compared to release in the presence of 0.05-0.5 mg/mL solutions of adsorbing polymers whose charge was the same as the excess charge within the films. We found that with the PMAA/PEO films, dye release did not depend on the concentration of polymer in solution, but was largely controlled by the amount of charge accumulated within the adsorbing polymer layer on the top of the film. For electrostatically stabilized PMAA/Q22 systems, dye release increased with increasing concentration of Q22 in solution, suggesting a significant contribution of the competition of solution species in the release mechanism. Our findings contribute to the understanding of interactions of small molecules with polymer multilayers and might have ramifications for novel applications of multilayer films as new materials for the controlled delivery of chemicals.

Introduction Organic thin films that contain low molecular weight organic molecules and dyes have been extensively investigated due to potential optical, electrooptical, and sensor applications. Among many techniques to produce such films on a solid substrate, such as covalent attachment,1 or noncovalent deposition using thermoevaporation,2 spin coating,3 and the Langmuir-Blodgett technique,4 layerby-layer sequential adsorption of polymers from solutions has recently received much attention as a simple and promising technique to produce nanoscopically structured materials.5-10 In addition to polymers, organic molecules of opposite charge (such as porphyrins) were also deposited at surfaces using this technique.11 Self-assembly of dyes with polyelectrolytes whose chains afford many charged sites provides an attractive means to produce dyecontaining polymer films.12,13 However, when dyes and (1) Katz, H. E.; Scheller, G; Putvinstki, T. M.; Schilling, M. L.; Wilson, W. L.; Chidsey, C. E. D. Science 1991, 3, 699. (2) Yanagi, H.; Kouzeki, T.; Ashida, M. J. Appl. Phys. 1993, 73, 3812. (3) Levinson, W. A.; Arnold, A.; Dehodgins, O. Polym. Eng. Sci. 1993, 33, 980. (4) Fuchs, H.; Ohst, H.; Prass, W. Adv. Mater. 1991, 3, 10. (5) Decher, G.; Hong J.-D. Macromol. Chem., Macromol. Symp. 1991, 46, 321. (6) Decher, G. Science 1997, 277, 1232. (7) Clark, S. L.; Hammond, P. L. Langmuir 2000, 16, 10206. (8) Cheung, J. H.; Stockton, W. B.; Rubner, M. F. Macromolecules 1997, 30, 2712. (9) Schlenoff, J. B.; Ly, H.; Li, M. J. Am. Chem. Soc. 1998, 120, 7626. (10) Bertrand, P.; Jonas, A.; Laschewsky, A.; Legras, R. Macromol. Rapid Commun. 2000, 21, 319. (11) Araki, K.; Wagner, M. J.; Wrighton, M S. Langmuir 1996, 12, 5393. (12) Cooper, T. M.; Campbell, A. L.; Crane, R. L. Langmuir 1995, 11, 2713.

polyelectrolytes are sequentially self-assembled at a surface, the extraction of dyes from the film into polymer solutions occurs during multilayer buildup,13-15 impeding the formation of dye-containing films with a controlled amount of the dye. Alternatively, dyes were added to preformed polyelectrolyte multilayers (PEMs), mostly as probes to investigate the local polarity of polymer multilayer films,16 the permeability properties of the multilayer films, and to determine the number of polyelectrolyte functional groups that are not involved in the formation of polymer-polymer ion pairs.15,17-19 In preassembled PEMs, the excess of charge that creates the binding sites for dye absorption is located within the outermost region of the film.9 In good agreement, charged dyes were found to be absorbed within a finite depth of 15-20 nm from the film surface.15,18 The use of weak polyelectrolytes as film constituents20-23 represents a simple way to tune the amount of charged (13) Ariga, K.; Lvov, Y.; Kutinake, T. J. Am. Chem. Soc. 1997, 119, 2224. (14) Linford, M. R.; Auch, M.; Mo¨hwald, H. J. Am. Chem. Soc. 1998, 120, 178. (15) Tedeschi, C.; Caruso, F.; Mo¨hwald, H.; Kirstein, S. J. Am. Chem. Soc. 2000, 122, 5841. (16) Tedeschi, C.; Mo¨hwald, H.; Kirstein, S. J. Am. Chem. Soc. 2001, 123, 954. (17) Caruso, F.; Donath, E.; Mo¨hwald, H. J. Phys. Chem. B 1998, 102, 2011. (18) Caruso, F.; Lichtenfeld, H.; Donath, E.; Mo¨hwald, H. Macromolecules 1999, 32, 2317. (19) Yoo, D.; Shiratori, S. S.; Rubner, M. F. Macromolecules 1998, 31, 4309. (20) Shiratori, S. S.; Rubner, M. F. Macromolecules 2000, 33, 4213. (21) Harris, J. J.; Bruening, M. L. Langmuir 2000, 16, 2006. (22) Mendelsohn, J. D.; Barrett, C. J.; Chan, V. V.; Pal, A. J.; Mayes, A. M.; Rubner, M. F. Langmuir 2000, 16, 5017. (23) Hiller, J. A.; Rubner, M. F. Macromolecules 2003, 36, 4078.

10.1021/la048763d CCC: $27.50 © 2004 American Chemical Society Published on Web 09/30/2004

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groups within a film after the film is self-assembled. In particular, when the environmental pH changes in the region close to the apparent pKa of a weak polyacid or a polybase, a significant amount of charge of one sign is created within PEMs. This charge can be then used, for example, to fabricate novel metal-containing inorganic nanocomposite materials based on weak polyelectrolyte PEMs,24,25 as well as to bind and release low molecular compounds such as dyes.26,27 Apart from electrostatic self-assembly, multilayer growth based on hydrogen-bonding interactions28-31 or the combination of hydrogen-bonding and electrostatic interactions,32,33 has been reported. As one of us previously reported,33 dye molecules can be incorporated within hydrogen-bonded multilayered films during multilayer deposition, and then released as needed at a preselected pH as the hydrogen-bonded film disintegrates. In this paper, we study the interaction of a dye with hydrogen-bonded multilayers and contrast our findings with the data obtained for electrostatically self-assembled films. In both cases, we investigate the loading of dye into preassembled multilayers in which the excess charge of one sign is created by varying the pH after the film is constructed. A dye whose charge is opposite to that of the excess charge within the film is efficiently and homogeneously absorbed within the film, regardless of the charge and nature of the top polymer layer. We find that binding of the dye with oppositely charged polymer units embedded within the film is stronger than its binding at the film surface. The embedded dye was released from the film in response to adsorption of a polymer of the same sign of electrical charge on the top of the film, suggesting the contribution of long-ranged electrostatic interactions within the film into the release mechanism. Finally, with two polymer systems studied, stabilized by hydrogenbonding or electrostatic interactions, we find significant differences in the dye release routes. While in the case of hydrogen-bonded multilayers, dye release was mostly triggered by polymer adsorption, in electrostatically assembled multilayers, the extraction of dye from the film due to binding with polymer species in solution was also significant. These findings might have ramifications for novel applications of multilayer films as new materials for the controlled delivery of chemicals.

Kharlampieva and Sukhishvili

Figure 1. Representative infrared spectrum of a 3-bilayer 60nm thick PMAA/PEO film that was deposited at pH 2 (dashed line), then exposed to R6G solution at pH 4.2 and rinsed with a buffer solution at pH 4.2 (solid line). PEO was included within the top layer of the films. Measurements were done in D2O solution containing 0.01 M phosphate buffer, which was adjusted to the low pH using hydrochloric acid. The inset shows the molecular structure of R6G whose strongest vibrational band is centered at 1610 cm-1.

Figure 2. Loading of R6G within PMAA/PEO films of different thicknesses, from 30 to 175 nm (2-5 bilayers), at pH 3.8 (b) and 4.2 (O). PEO is included within the top layer of the films. The inset shows the ratio of mass of R6G absorbed to the amount of self-assembled PMAA at pH 3.8 (9) and pH 4.2 (0). The solvent was 0.01 M phosphate buffer whose pH was adjusted to a lower value using hydrochloric acid.

Experimental Section Materials. Rhodamine 6G (R6G) and Bromophenol Blue (BPB) were purchased from the Aldrich Chemical Co. and were used without further purification. The chemical structures of these dyes are shown in the insets of Figures 1 and 7 of this manuscipt. The polyacids were poly(methacrylic acid), PMAA with Mw 150 000, or poly(acrylic acid), PAA with Mw 450 000. For low ionic strength conditions, a pKa of about 6-7 is usually reported for PMAA, while a lower pKa ranging from 5 to 6 is usually found for PAA. Both polyacids, as well as poly(ethylene oxide) (PEO) with Mw 200 000, were purchased from Scientific Polymer (24) Wang, T. C.; Rubner, M. F.; Cohen, R. E. Langmuir 2002, 18, 3370. (25) Hammond, P. Curr. Opin. Colloid Interface Sci. 2000, 4, 430. (26) Chung, A. J.; Rubner, M. F. Langmuir 2002, 18, 1176. (27) Kharlampieva, E.; Sukhishvili, S. A. Polym. Prepr. 2003, 44, 671. (28) Stockton, W. B.; Rubner, M. F. Macromolecules 1997, 30, 2717. (29) Wang, L. Y.; Fu, Y.; Wang, Z. Q.; Fan, Y.; Zhang, X. Langmuir 1999, 15, 1360. (30) Sukhishvili, S. A.; Granick, S. J. Am. Chem. Soc. 2000, 122, 9550. (31) Sukhishvili, S. A.; Granick, S. Macromolecules 2002, 35, 301. (32) Schoeler, B.; Poptoshev, E.; Caruso, F. Macromolecules 2003, 36, 5258. (33) Kharlampieva, E.; Sukhishvili, S. A. Macromolecules 2003, 36, 9950.

Figure 3. Absorption of R6G within PMAA/PEO films as a function of the amount of self-assembled PMAA for films of different thicknesses that contained PEO (curve 1) or PMAA (curve 2) as an outermost layer. Film thicknesses ranged from 30 to 175 nm (2-5 bilayers). Conditions for film self-assembly and R6G absorption are the same as those in Figure 1. Products, Inc. To eliminate possible traces of residual monomer, the PMAA samples were dialyzed against deionized water using tubing with a molar mass cutoff of 3000 Da. The dialysis was continued for 3 days (with the water bath frequently changed to purified water during that time), after which all samples were freeze-dried. Poly(4-vinylpyridine) (PVP) with Mw 200 000 was

Release of a Dye from Polymer Films

Figure 4. Top panel: R6G release from a 3-bilayer 60 nmthick PMAA/PEO film in buffered solutions of 0.1 mg/mL PAA (O), 0.1 mg/mL PMAA (4), and 0.01 M pure buffer (0) at pH 4.2. Bottom panel: adsorption of PAA (b) and PMAA (2). PEO was included within the top layer. The inset shows infrared spectra of R6G-containing PEO/PMAA film before and after adsorption of PMAA on top of the film. Adsorption of PMAA does not result in an increase in the 1700 cm-1 band intensity because of a corresponding decrease in the intensity of overlapping R6G peaks at 1650 and 1713 cm-1 due to dye release. Conditions for film self-assembly and R6G absorption are the same as those in Figure 1.

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Figure 6. Time evaluation of PMAA amounts adsorbed (top panel) and R6G amounts retained (bottom panel) during adsorption of PMAA on 5-bilayer 175-nm thick dye-containing PMAA/PEO films. The concentration of PMAA solutions was 0.1 mg/mL (triangles) and 0.5 mg/mL (circles).

Figure 7. Representative infrared spectrum of a 3.5-bilayer 40 nm-thick PMAA/Q22 film deposited at pH 7 (solid line, A) and then exposed to BPB solution at pH 4.6 (dashed line, B). The inset shows the molecular structure of BPB whose vibrational bands are centered at 1545 and 1589 cm-1. PMAA was an outermost layer.

purchased from Scientific Polymer Products, Inc. This sample was quaternized with ethyl bromide in ethanol solution using well-established methods34,35 to obtain polymers with 22% and 90% of pyridinium units (Q22 and Q90, respectively) as determined by infrared spectroscopy. Q90 was used in the preparation of the precursor layer, and Q22 was used in the self-assembly of PMAA/Q22 multilayers. In these experiments, the solvent was D2O rather than water. This eliminated overlap of the IR spectra of polymers in the 1700-1500 cm-1 region with the strong water band. D2O with 99.9% isotope content was purchased from Cambridge Isotope Laboratories and was used as received. Although the acidity was therefore determined by deuterium rather than hydrogen ions, we nonetheless refer to it as pH rather than pD. One should note, however, that hydrogen-to-deuterium exchange has an effect on the dissociation constants of acid groups, usually resulting in

a relatively small (0.2-0.6) increase of pKa’s of acidic groups in deuterium oxide as compared to their dissociation in water.36 To control pH and ionic strength, concentrated HCl and the inorganic salts Na2HPO4 and NaH2PO4 (General Storage, pure grade) were used as received. The H2O used for glassware cleaning was deionized and further purified by passage through a Milli-Q System (Millipore). Pretreatment of the ATR Crystal. Multilayer films were deposited on a hydrophilic Si crystal which was oxidized using the procedure described elsewhere.37 To enhance the adhesion of polymers to the substrate, the surface was first modified by a precursor layer Q90. For this, Q90 was allowed to adsorb from a 0.1 mg/mL solution at pH 9.2. Adsorption occurred, probably due to electrostatic attraction at this high pH. After waiting 20 min, the amount adsorbed reached a saturated value of ∼1.5 mg/m2. The polymer solution was replaced by a pure buffer (0.01 M borate buffer at pH 9.2). The liquid cell was then filled with 0.1 mg/mL solution of PMAA at pH 9.2 and after waiting 20 min was replaced by a buffer solution at the same pH. After that, the buffer solution at pH 9.2 was replaced with a buffer solution at pH 2 or pH 7 for PMAA/PEO or PMAA/Q22 deposition, respectively.

(34) Fuoss, R. M.; Strauss, U. P. J. Polym. Sci. 1948, 3, 246. (35) Margolin, A. L.; Izumrudov, V. A.; ?vedas, S V. K.; Zezin, A. B.; Kabanov, V. A.; Berezin, I. V. Biochim. Biophys. Acta 1981, 660, 359.

(36) Salomaa, P.; Schaleger, L. L.; Long, F. A. J. Am. Chem. Soc. 1964, 86, 1. (37) Frantz, P.; Granick, S. Macromolecules 1995, 28, 6915.

Figure 5. The amount of R6G released as a function of the amount of PMAA adsorbing on the top of a 3-bilayer dyecontaining PMAA/PEO film. The conditions are the same as those in Figure 4.

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Multilayer Deposition and Dye Loading. A polymer film was produced by sequential adsorption of buffered D2O solutions of polymers onto a modified Si crystal. The protocol for in-situ multilayer deposition within the flow-through ATR-FTIR liquid cell was slightly different from the technique of alternate dipping that is commonly used for multilayer deposition. Briefly, 0.1 mg/ mL of polymer solution was allowed to adsorb onto the surface of the modified Si crystal for 20 min, after which the polymer solution was replaced by a pure buffer. Another polymer was then deposited in a similar way, and the deposition cycle was repeated. All deposition solutions, including rinsing solutions, were kept at a constant pH during multilayer growth cycles. The PMAA/PEO and PMAA/Q22 layers were sequentially deposited at pH 2 and pH 7, respectively. The process was driven by the formation of hydrogen bonds or by electrostatic interactions between functional groups in the adjacent layers of the PMAA/ PEO and PMAA/Q22 systems. In a typical experiment, 2-5 polymer bilayers were deposited, and the film thickness ranged from 30 to 175 nm for PMAA/PEO films, and from 8 to 50 nm for PMAA/Q22 films. After multilayers were built, the solution pH was changed by exposing the surface of the film to 0.01 M buffer solution at a different pH. In the PMAA/PEO system, the pH of the external solution was changed from 2 to 3.8 or to 4.2. Positively charged R6G was then allowed to absorb within the film from 0.5 mg/mL solution at pH 3.8 or 4.2 for 1 h. In the PMAA/Q22 system, the pH of the external solution was changed from 7 to 4.6, and negatively charged BPB was then loaded into the film at pH 4.6. In both systems, the dye solutions were then replaced by a pure buffer at the same pH values as the dye solutions. Methods. FTIR-ATR Measurements. 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 calibrated intensities of the vibration bands of -COO-, -COOH bands, as well as the bands associated with R6G and BPB vibrations. Infrared spectra were collected using a Bruker Equinox-55 FTIR spectrometer equipped with a narrow-band mercury cadmium telluride detector. The ATR optics and the thermostated custom-built adsorption cell (Harrick Sci.) were placed in a dry-air purged compartment within the internal compartment of the FTIR spectrometer. The ATR surface was a rectangular Si crystal of dimension 50 mm × 20 mm × 2 mm (Harrick Scientific) whose beam entrance and exit surfaces were cut at 45°. Interferograms were collected at 4 cm-1 resolution, and the number of averaged scans was 120. To obtain the absorbance spectra, each interferogram was divided by the corresponding background, measured for the same ATR cell with the same D2O buffer solution. Background spectra were obtained with the ATR crystal covered with the precursor film (1-layer Q90). In all experiments, the penetration depth of the evanescent wave was large relative to the thickness of the multilayers. For example, the penetration depth of the evanescent wave was 0.47 µm at 1643 cm-1 for the Si crystal that we used, and the largest thickness of a dry film we were working with was about 175 nm. This value is based on the adsorbed amount measured with the ATR-FTIR technique of about 175 mg/m2 (assuming a density of a dry film of 1 g/cm3). The adsorbed amount of polymers and the degree of ionization of PMAA molecules within the multilayer were calculated from calibration constants that were independently determined by measuring the infrared absorbance of polymer solutions of known concentrations as they are brought into contact with a nonadsorbing surface. The detailed procedures include integration of intensity of the absorbing species in the evanescent wave as a function of distance from the crystal surface and have been described previously.38-40 The calibration constants for Q22, Q90, and PMAA were taken from our earlier studies.33,41 The ratio of extinction coefficients for the charged and uncharged groups in (38) Harrick, N. J. J. Opt. Soc. Am. 1965, 55, 851. (39) Azzopardi, M. J.; Arribart, H. J. Adhes. 1994, 46, 103. (40) Frantz, P.; Granick, S. Langmuir 1992, 8, 1176. (41) Kharlampieva, E.; Sukhishvili, S. A. Langmuir 2003, 19, 1235.

Kharlampieva and Sukhishvili PMAA was reported earlier. In particular, equal values were found for the extinction coefficients of the 1701 and 1552 cm-1 PMAA absorption bands.33,43 The amount of PEO was calculated using a mass ratio of PMAA to PEO of 1.4 in the PMAA/PEO film. The amounts of the dyes were calculated using the same procedures described for the calibration of polymers. The calibration constants for R6G and BPB blue determined in this paper were 0.1 and 0.0063 abs.units*m2 mg-1 using 1610 and 1545 cm-1 vibrational bands, respectively.

Results and Discussion 1. Hydrogen-Bonded Multilayers. Incorporation and Release of R6G. Representative Spectra of PMAA/ PEO Films Containing R6G. Figure 1 shows a representative infrared spectrum of a 3-bilayer PMAA/PEO film deposited at pH 2 and then exposed to R6G solution at pH 4.2. The spectrum shows three major peaks: an absorption band centered at 1701 cm-1 associated with stretch vibrations of uncharged carboxylic groups (ν, CdO), and two narrow bands at 1650 and 1610 cm-1 associated with R6G vibrations. The intensities of the peaks shown in Figure 1 were integrated in two spectral regions: 18001630 cm-1 (region 1) and 1630-1575 cm-1 (region 2) using Galactic Grams/32 software. The peak integration in region 2 (band centered at 1610 cm-1) was used to determine the amount of R6G adsorbed. Note that this band was also used in a separate experiment involving solution species to determine the calibration constant for the amount of R6G absorbed within the multilayer. Spectral integration in region 1 was used to determine the amount of PMAA adsorbed. Note that in this region, R6G also has two medium-intensity vibrational bands that are centered at 1650 and 1713 cm-1 and comprise 67% of the peak intensity at 1610 cm-1. These bands overlap with PMAA vibrations and were subtracted from the total integrated intensity in this region. Also seen in the spectra (but not shown here) was a band at 1552 cm-1 (νa, COO-) corresponding to asymmetric stretching vibrations of carboxylate groups. At pH 4.2, the intensity of this band amounted to 4% of the intensity of the -COOH band and was added to the corrected intensity of the -COOH peak to calculate the amount of PMAA adsorbed. PMAA/PEO Multilayer Self-Assembly and Absorption of R6G within the Film. The growth of PMAA/PEO multilayers onto a precursor-modified Si substrate at pH 2 was in a good agreement with our earlier observations.33 Specifically, film growth was not regular, and different amounts of polymer were deposited per cycle, reflecting a low (∼0.1) fraction of polymer segments involved in intermolecular hydrogen bonding. The incremental amount of PMAA deposited per layer usually increased from 8 to 30 mg/m2 for layer numbers 3-9. Also, we observed that the reproducibility of film thickness was within 30%, which is also rooted in weak adhesion between PEO and PMAA chains. The deposited PMAA/PEO films at pH 2 did not contain excess charge because both PEO and PMAA molecules are uncharged at this pH. This was confirmed by the absence of the 1552 cm-1 COO- band in the multilayer spectrum. Multilayer growth at this pH occurred because of hydrogen-bonding interactions between adjacent layers. Excess charge was then introduced within the films by varying pH in the external solutions. The pH was raised from 2 to 3.8 and 4.2, where ∼3% and ∼4% of PMAA groups acquired charges, respectively. The fact that in these conditions PMAA/PEO multilayers carried (42) Klitzing, R.; Mo¨hwald, H. Langmuir 1995, 11, 3554. (43) Xie, A. F.; Granick, S. Macromolecules 2002, 35, 1805.

Release of a Dye from Polymer Films

charge of only one sign significantly distinguishes this hydrogen-bonded system from electrostatically assembled multilayers. Created negative charges can be utilized as binding sites for subsequent absorption of positively charged dyes. Figure 2 shows the loading capacity of PMAA/PEO multilayers as a function of the amount of self-assembled PMAA. The linear dependence of the amount of dye loaded onto the film thickness suggests homogeneous absorption of dye throughout the film. It also suggests that the PMAA/PEO multilayer is permeable to relatively small molecules of R6G (Mw 479), in good agreement with earlier observations done with PEMs.28 The fact that the data in Figure 2 can be fit with a straight line that can be extrapolated to zero PMAA and R6G concentrations also indicates that there is no preferential binding of the dye with the precursor surface. The larger loading of R6G at pH 4.2, indicated by the larger slope in Figure 2, reflects a higher proportion of ionized groups in self-assembled PMAA. From the data in Figure 2 and the known ionization of self-assembled PMAA within the multilayer, ∼3% and 4% at pH 3.8 and 4.2, respectively, we have calculated that the dye was absorbed in a 1:1 molar stoichiometric ratio with the amount of ionized groups in the film. These corresponded to the ratio of the mass of R6G absorbed to a film weight of about 0.2 and 0.3 for pH 3.8 and 4.2, respectively. Note that data shown in Figure 2 refer to multilayers containing PEO as the outermost layer. However, the amount of dye absorbed within the film was different if films were capped with PMAA. This is illustrated in Figure 3. At first glance, the fact that smaller amounts of the dye were loaded into the film when PEO was included as the top layer (curve 1) is surprising. Intuitive considerations as well as our previous data on the measurements of the degree of hydrogen bonding within PMAA/PEO multilayers33 suggest that for the top PMAA layer, hydrogen bonding of PMAA is weaker and consequently the proportion of ionized groups should be larger. However, data in Figure 3 show the opposite trend: when PMAA is included as the top layer (curve 2), R6G binding with the film is less efficient. These data suggest stronger interaction of the dye with the binding sites embedded within the film, as compared to those at the film surface. This is probably due to the different dielectric environment between the film surface and within the film interior. It is reasonable to assume that at the film surface, the local dielectric constant is higher than that within the film and is closer to the dielectric constant of a solvent. This should result in a weaker binding of the dye with the surface adsorption sites. Still another effect is also likely to contribute to the decrease in the amount of dye absorbed within PMAAcapped films. This second possibility is that the charges that are embedded within the film can also adjust to the surface charge. The latter effect has been reported earlier by several groups including ours.42,43 In the system discussed here, ionization of embedded PMAA could be reduced due to long-range interactions with the surface charges of PMAA adsorbed on the top. In this paper, however, it was impossible to distinguish between ionization at the film surface and one within the film, because of the identical chemical contributions of surface-adsorbed and embedded PMAA to the IR spectrum. The existence of this charge coupling effect and its contribution to dye release will be discussed later in this paper. Release of Rhodamine 6G from PMAA/PEO Multilayers. All multilayers used in release experiments contained PEO as the top layer, which was typically 12-16 nm thick. Because neutral PEO molecules do not have affinity to R6G, this assured that after dye absorption, R6G molecules

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were bound within the bulk of the film and no dye was adsorbed at the film surface. This also afforded binding of PMAA on the top of the film that was driven by hydrogen-bonding interactions between PMAA and PEO. Figure 4, top panel shows that 20% of the dye desorbed from the film into a 0.01 M phosphate buffer at pH 4.2 after the second rinse. Note that further repeated replacements with buffer did not result in R6G desorption from the film even when multilayers were exposed to a pure buffer overnight. At the same time, after the addition of 0.1 mg/mL PMAA solution at pH 4.2, large amounts of the dye were released during the first 15 min as seen in the top panel of Figure 4. Data shown in the bottom panel suggest that release correlated with PMAA adsorption on top of the film. Note that the amount of PMAA adsorbed per single layer at pH 4.2 was smaller than that deposited per single PMAA layer at pH 2 for a 3-bilayer film (pH, at which the film was constructed). The difference is probably due to partial ionization of PMAA at pH 4.2 (ionization of PMAA is ∼8% in solution, and ∼4% when included within PMAA/PEO multilayers). Charged segments of PMAA do not participate in hydrogen bonding with PEO, but are included in PMAA loops. This introduces electrostatic repulsions between PMAA segments and consequently results in smaller amounts of PMAA deposited. Although data in Figure 4 suggest a significant role of polymer adsorption into the dye release, a commonly considered mechanism is the extraction of the dye to solution due to competitive complexation with polymer species in solution.13-15 This type of release was observed in many research groups, particularly at high polymer concentrations.14 To find out how much complexation with polymer solution species contributes to dye release in our case, we performed a control experiment, in which a solution of polycarboxylic acid, PAA, was added to a PEOcapped dye-containing multilayer. As a polyacid of stronger acidity relative to PMAA,44-46 PAA contains a larger proportion of ionized carboxylic groups at pH 4.2 and has low affinity to PEO molecules. In our previous work, we showed that PAA/PEO multilayers were unstable at pH greater than ∼3.6. In our experiment, however, we found that a small amount, ∼1.3 mg/m2 of PAA, could still be bound to the surface of PMAA/PEO layers at pH 4.2 (bottom panel in Figure 4). This difference is probably due to a smaller total negative charge of PMAA/PEO films as compared to PAA/PEO films at a given pH. In good agreement with weaker adsorption of PAA, only a small amount of dye was released from the film after the addition of PAA solution, despite the fact that PAA solution contained a larger number of negative charges that could serve as binding sites to promote dye extraction. Also, as in the case of PMAA solution, kinetics of dye release was not a linear function of time, as could be expected if the release were controlled by dye binding with the excess solution species. As shown in the bottom panel of Figure 4, the release correlated with adsorption of PMAA at the film surface. The fact that the release of R6G was triggered by PMAA adsorption was also confirmed by the following results. The release rate of the dye was found to be limited by the rate of PMAA adsorption. The latter is illustrated in Figure 5, which shows that the amount of the R6G released from the PMAA/PEO film was proportional to the amount of PMAA adsorbed on top. This graph is plotted on the basis (44) Jager, J.; Engberts, J. B. F. N. Recl. Trav. Chim. Pays-Bas 1986, 105, 347. (45) Ikawa, T.; Abe, K.; Honda, K.; Tsuchida, E. J. Polym. Sci.: Polym. Chem. Ed. 1975, 13, 1505. (46) Spencer, H. G. J. Polym. Sci. 1962, 56, S25.

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of the results shown in Figure 4. The fact that with increased PMAA coverage the charges in the top layer remained equally efficient in triggering dye release probably reflected a relatively lower charge density within PMAA layers. On the basis of a previously described procedure,47 we have estimated the average chargecharge spacing between ionized carboxylic groups in the multilayer at pH 4.2. The found spacing of ∼22 Å is in good agreement with previously reported results by one of us, for the PMAA/PEO system.33 Note that these estimates refer to average ionization of PMAA within the multilayer, including the top layer and layers embedded within the film, because in these experiments it was impossible to distinguish between the charges on the film surface and within the film. Ionization of surface PMAA could be higher than that of embedded molecules, due to a higher dielectric constant at the film surface and weaker hydrogen bonding of the top PMAA layer with PEO.33 However, linear dependence of the amount of dye released on the amount of PMAA adsorbed indicates that the ionization of PMAA included within the top layer does not exceed a critical charge-charge distance of 7 Å, below which the ionization within a PMAA layer would be reduced due to strong unfavorable electrostatic repulsions.50 Experiments shown in Figures 4 and 5 were done with 0.1 mg/mL PMAA solution. We then intended to check whether with higher concentrations of PMAA solution the extraction due to dye binding with solution species would comprise a larger contribution of the dye release. Specifically, we compared the kinetics of PMAA adsorption and the dye release for various concentrations of PMAA in solution. In these experiments, PMAA was allowed to adsorb onto the surface of 5-bilayer dye-containing PMAA/ PEO films that were self-assembled at pH 2 and contained the same amount of PEO, 15 mg/m2, as an outermost layer. The top panel in Figure 6 shows kinetics of PMAA adsorption at the surface of these multilayers for polyacid concentrations of 0.1 and 0.5 mg/mL. One can see that larger PMAA amounts were bound at the film surface when adsorption occurred from the higher PMAA concentration. Taking into account that PMAA binding with PEO-covered surfaces is strongly irreversible because PMAA could not be desorbed in solution by a large amount of solvent, it was surprising not to find evidence for a high-affinity type adsorption isotherm. We believe that increased amounts of PMAA adsorbed are due to enhanced self-association of PMAA molecules at high PMAA concentrations. The formation of dimers between protonated carboxylic acids is well known; the dimerization also persists in aqueous environments.48 We suggest that increased intramolecular and possibly intermolecular hydrogen bonding resulted in larger amounts of PMAA bound from higher PMAA concentrations. As seen in the top panel of Figure 6, however, in both cases the amount of PMAA adsorbed saturated with time. The data on the release rate are shown in the bottom panel in Figure 6. Qualitatively similar to the polymer adsorption kinetics, the amount of dye released reached a limiting value at longer times. It was meaningful to compare the rates of dye release with those for PMAA adsorption. The amount of dye released, normalized to the amount of adsorbing PMAA, was constant with time and assumed values of (47) Sukhishvili, S. A.; Granick, S. Langmuir 2003, 19, 1980. (48) Tanaka, N.; Kitano, H.; Ise, N. Macromolecules 1991, 24, 3017. (49) Kirsh, Yu. E.; Pavlova, N. R.; Kabanov, V. A. Eur. Polym. J. 1975, 11, 47. (50) Glinel, K.; Moussa, A.; Jonas, A. M.; Laschewsky, A. Langmuir 2002, 18, 1408.

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0.85 and 0.75 for studied PMAA concentrations of 0.1 and 0.5 mg/mL (data not shown). The fact that normalized dye release was not enhanced at a higher concentration of PMAA suggests that, in the range of PMAA concentrations up to 0.5 mg/mL, the competitive binding of R6G with PMAA solution species was negligibly small on the experimental time scale of at least 600 min. An important question is the depth from which the dye is released from the film upon adsorption of PMAA. From the data collected with multilayers with different thicknesses of a top PEO layer, a decay length of about 20-30 nm was estimated. This result is in agreement with the smaller dye loading capacity of PMAA-capped multilayers, earlier shown in Figure 3. The range of charge correlation found for PMAA/PEO films is much higher than that in solution, where the Debye length is ∼3 nm for 0.01 M buffer solution. This value is also larger than values reported for multilayers composed of oppositely charged polyelectrolytes. Specifically, a decay length of ∼10 nm was reported in our earlier paper for a weak polyelectrolyte multilayer,43 and a smaller value of ∼6 nm was found by Granick et al. for layered films composed of polyelectrolytes with permanent charges.45 The higher penetration depth of the electric field in the PMAA/PEO system is probably rooted in and suggests that concentrations of free small ions that could participate in electrostatic screening interactions are small and that dye molecules are effectively immobilized around the deposited PMAA chains. Note also that in contrast to electrostatically assembled films, one of the two polymer layers deposited (PEO) does not contain charged groups and consequently does not generate counterions. Effect of Ionic Strength on Loading and Release of R6G from PMAA/PEO Multilayers. As the ionic strength of a buffer that is brought into contact with self-assembled PMAA/PEO films was increased, the ionization of carboxylic groups was enhanced. The increase was large, from ∼4% in 0.01 M phosphate buffer to ∼11% in a solution of the same buffer that additionally contained 0.15 M NaCl. This large increase of ionization reflected the fact that the film was permeable for small ions and that the negative charge created within the film by ionized carboxylic groups was effectively screened by small ions. The increased ionization did not cause film disintegration because the ratio of the electrostatic Debye screening lengths in 0.010.15 M NaCl solutions was 3.9, that is, larger than the decrease in linear charge-to-charge distance at these ion conditions which was equal to a factor of 1.4. The large increase in polyacid ionization was not accompanied, however, by a significant increase in R6G binding capacity of the film: the R6G amount adsorbed from 0.15 M NaCl solution was only 10-15% higher than that adsorbed from low-ionic strength buffer. This reflected the effective screening of negative charges within the film by salt ions. However, the dye retention within the film remained relatively high even at this high salt concentration: although 25% of the originally loaded dye could be removed from the film after two rinsing cycles, further multiple rinsing with buffer containing 0.15 M NaCl did not result in further extraction of dye. The release of R6G from the film triggered by the addition of 0.1 mg/mL PMAA solution was also not significantly affected by 0.15 M ionic strength. 2. Electrostatically Assembled Multilayers. Incorporation and Release of BPB. Representative Spectra of PMAA/Q22 Multilayer Containing BPB. In PMAA/Q22 systems, multilayer deposition involved formation of ionic pairs between quaternized units of Q22 chains and ionized units of PMAA. Multilayers were first assembled at high pH values and then exposed to lower

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Figure 8. Illustration of a curve-fitting procedure in region 2. The conditions are the same as those in Figure 7. Dashed peaks centered at 1545 and 1589 cm-1 are associated with BPB vibrations. Details of the curve-fitting are described in the text.

pH conditions to introduce excess positive charge within the film. A negatively charged dye, BPB, was then allowed to absorb with the film. Figure 7 shows a representative infrared spectra of a 3.5-bilayer PMAA/Q22 film deposited at pH 7 (solid line, A) and then exposed to BPB solution at pH 4.6 (dashed line, B). The solid spectrum shows four major peaks: an absorption band at 1701 cm-1 associated with stretch vibrations of uncharged carboxylic groups (ν, CdO), two bands associated with in-ring skeletal vibrations of pyridinium and pyridine rings centered at 1643 and 1601 cm-1, respectively, and a band at 1552 cm-1 originating from asymmetric stretching vibrations of carboxylate groups (νa, COO-). The spectrum marked with the dashed line refers to the film containing absorbed BPB. In this spectrum, the peak originally centered at 1552 cm-1 for the film before dye loading increases in intensity and changes its shape and peak frequency. These changes reflect the contribution of BPB absorbance at 1545 cm-1. Note that the integrated intensity of the 1545 cm-1 peak associated with BPB vibrations was used in a separate experiment involving high concentrations of solution species to determine the calibration constant for the calculation of the amount of BPB incorporated within the film. Integration of the spectra was performed in two regions: 1760-1630 cm-1 (region 1) and 1630-1505 cm-1 (region 2). Peak integration in region (1) was performed with peak centers fixed at 1701, 1671, and 1640, and the peak widths of 31, 49, and 14, cm-1, respectively. A peak centered at 1671 cm-1 was introduced in curve-fitting to account for the asymmetric shape of the 1701 cm-1 peak, as was discussed earlier.33,43 The intensity of this peak amounted to about 20% of the total intensity of the 1701 cm-1 peak. The curve-fitting procedure in region (2) is shown in Figure 8. The following peaks were introduced in the curve-fitting: (a) peaks at 1572 and 1518.5 cm-1 with widths fixed at 15 and 13 cm-1, respectively, associated with pyridinium rings; (b) peaks at 1604 and 1558 cm-1 with widths fixed at 12.5 and 15 cm-1, respectively, associated with pyridine rings, as was discussed earlier;43 (c) carboxylate ion bands centered at 1554 and 1534 cm-1 with widths fixed at 27 and 15 cm-1, respectively (a peak centered at 1534 cm-1 was introduced in curve-fitting to avoid an overlap of a broad carboxylate band at 1554 cm-1 with other vibration bands which are centered at 1572, 1558, and 1545 cm-1); and (d) peaks at 1545 and 1589 cm-1 with widths fixed at 13 and 17 cm-1, respectively, associated with BPB vibrations. The integrated intensity ratio of peaks associated with pyridinium and pyridine vibrations was reported in our earlier studies.43 The ratio of BPB 1545 to 1589 cm-1 was determined in this work from the solution spectrum of

Figure 9. Total fraction of Q22 charged units incorporated within a 4.5-bilayer 50-nm thick PMAA/Q22 film as a function of external pH (panel A) and correlation between the BPB inclusion and fractional amount of positive charges in selfassembled Q22 (panel B). The inset in panel A shows the fraction of positive charges in Q22 as a logarithmic function of pH in bulk solution. Except for the pH values, the conditions are the same as those in Figures 7 and 8.

BPB and was equal to 2.3. These known ratios were used to check the consistency of the curve-fitting procedure. The integrated intensities of all contributing peaks obtained as a result of curve-fitting for multilayers were in good agreement with those independently determined ratios of the peak intensities of individual multilayer components. Absorption of BPB within PMAA/Q22 Multilayers. Similar to the PMAA/PEO system, incorporation of BPB within PMAA/Q22 films occurred in conditions when large excess charge was created within the film as a result of post-self-assembly variation in pH. Figure 9 shows the effect of pH on the protonation of pyridine rings and the amount of dye incorporated within the multilayer. Panel A in Figure 9 shows a fraction of pyridinium rings (both permanently charged and protonated pyridine units, N+R + N+H) as a function of pH. At pH 7, at which the multilayer was self-assembled, this fraction was 22% and consistent with 23% of permanent charges introduced to PVP chains through alkylation reactions. No additional protonation of pyridine rings occurred at this pH, in good agreement with a pKb of 3.6 for pyridine groups reported for QPVP with low alkylation degrees.49 When the pH of a buffer solution was lowered to 4.6, an additional 6% of pyridine became charged, creating excess positive charge within PMAA/Q22 multilayer. The inset in Figure 9 shows that the percent of positive charges was a logarithmic function of pH, as is expected from simple acid-base equilibrium considerations. Panel B in Figure 9 shows that the amount of BPB loaded increased linearly with the fraction of positive charges in Q22 chains and twice the amount of BPB could be loaded at pH 4.6 as compared to that loaded at pH 7. Note that significant amounts of BPB could be incorporated within the PMAA/Q22 film even when the external pH was not varied after the selfassembly step. This agrees well with data reported by

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Figure 10. Loading of BPB into QPVP/PMAA films at pH 4.6 as a function of film thickness for PMAA/Q22 multilayers containing PMAA (curve 1) or Q22 (curve 2) as an outermost layer. The conditions for film self-assembly and inclusion of BPB are the same as those in Figure 7.

other groups15,18 on the incorporation of dyes within electrostatically assembled multilayers. In those studies, dye incorporation occurred within the top section of the film, with a penetration depth of 15-20 nm, and might reflect the binding of dye with polymer charges not directly utilized in multilayer assembly.15,18 In this paper, we did not study the thickness dependence of dye loading before external pH was varied. The dye loading at pH 4.6, though, shows that the dye absorption is proportional to the film thickness at this pH, indicating homogeneous distribution of the dye throughout the film thickness. This seeming discrepancy could be probably explained by the extension, due to introduced electrostatic stress, of the top zone of the film, after additional charges are created within the film through the pH variation. In experiments on the dye binding conducted at the same pH at which the multilayers were constructed, Caruso et al.20 found that dye binding occurred only when the outermost layer of the film was oppositely charged to the dye. Figure 10, however, shows that dye binding occurred regardless of the polymer charge of the top layer. The difference is due to excess charge opposite to the charge of the dye that was created within the film due to pH variation resulting in removal of an electrostatic barrier for dye binding. Also seen in Figure 10 is that dye loading is lower when the multilayer contained Q22 as a top layer. This observation is similar to that also found with the PMAA/PEO system and is probably explained by the weaker binding of the dye with the film surface, and probable exclusion of the dye from the film interior due to suppression of positive charges within the film as a result of Q22 adsorption at the film surface. Release of BPB from PMAA/Q22 Multilayers. As in the case of PMAA/PEO multilayers, release of BPB from the PMAA/Q22 in pure 0.01 M phosphate buffer was very slow. Specifically, desorption of ∼5% of dye from a 3.5bilayer film occurred at pH 4.6 after the second rinse with buffer. However, in the same buffer but in the presence of 0.1 mg/mL Q22 solution, 50% of incorporated dye was released from the film within an hour. The initial amount of BPB loaded within the film at pH 4.6 was 60 mg/m2; PMAA was an outermost layer (data of the experiment are not shown). In the PMAA/Q22 system, we did not further study the decay length of electrostatic field within the film. Instead, we focused our attention on the relative importance of the adsorption-triggered dye release mechanism versus dye release through competitive binding with Q22 chains in solution. Adsorption of Q22 onto a dye-containing film of the same thickness (and the same amount of polymer layers) for different concentrations of

Figure 11. Correlation between Q22 adsorption on top of a 3.5-bilayer 40 nm-thick Q25/PMAA film (top panel) and release of BPB from the film (bottom panel). The concentrations of Q22 were 0.05 mg/mL (open symbols) or 0.5 mg/mL (filled symbols). The inset shows the evolution of the average fraction of Q22 charged units for the above concentrations of Q22 in solution. The conditions for film self-assembly, inclusion of BPB, and BPB release are the same as those in Figure 7.

Q22 in solution, 0.05 and 0.5 mg/mL, is contrasted in Figure 11. One can see that, at these two different polymer concentrations, about the same amounts of Q22 were bound at the film surface (Figure 11, top panel), but the amount of dye released was drastically different (Figure 11, bottom panel), with about 2 times larger amounts of BPB released at high polymer concentration. This is different from what was observed for the PMAA/PEO system and shown in Figure 6, where approximately the same relative amounts of R6G dye were released into polymer solutions with high concentrations. We believe that strong enhancement of dye release from PMAA/Q22 films into solutions containing Q22 reflects the contribution of competitive extraction of dye from the film due to interaction of BPB with Q22 chains in solution. Also shown (Figure 11, inset) is the effect of the adsorbing Q22 chains on the average ionization of pyridine rings incorporated within the film. It is seen that Q22 adsorption resulted in deprotonation of pyridine rings and a decrease of the average number of positively charged units on Q22 chains. The time evolution of the average fraction of positively charged Q22 units followed the adsorption of Q22 on top of the PMAA/Q22 film, suggesting strong correlation between accumulation of positive charges on the film surface and deprotonation of self-assembled pyridine rings. In the same way as this occurs in the PMAA/PEO system, charge suppression reduced the number of adsorption sites for the dye and resulted in the release of dye molecules from the film. It is also important to consider that, as seen in Figure 10, dye binding with the top Q22 layer is weaker than its binding with the embedded Q22 chains. 3. Comparison of PMAA/Q22 and PMAA/PEO Systems. The results shown are drastically different from those obtained with hydrogen-bonded mutlilayers where no significant extraction of dye into solution occurred. Weaker complexation of a dye with solution species in the case of hydrogen-bonded multilayers may be due to several factors. First, the complexation of dyes with polyelectrolyte chains is known to be system-specific. It is feasible that

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the affinity of carboxylate ions to charged R6G molecules is weaker than the binding of the pyridinium ring with BPB molecules, at least in solution. The fact that pyridine units are not soluble in water unless quaternized points to the possible contribution of hydrophobic interactions in BPB binding. The difference in affinity to dye molecules between Q22 and PMAA chains may not necessarily exist within the film, where the high density of polymer chains significantly changes the microenvironment for the charge-charge interaction. A second factor is probably rooted in the fundamental differences in the nature of intermolecular adhesion for films stabilized by electrostatic or hydrogen-bonding interactions. For electrostatically stabilized mutlilayers, there exists a critical charge density below which the multilayers are not stable. This critical charge density is dependent on ionic strength and varies from 0.036 charge per angstrom of contour length at low ionic strength50 to 0.072 charge per angstrom at the ionic strength of 0.01 M.51 The latter corresponds to ∼20% content of charged units in the case of polymers in the vinyl family. If high ionic strength is used in the multilayer construction, the critical content of charged units is even higher.52 This impedes the use of polyelectrolytes with low charge density in multilayer build-up. More importantly, after a polyelectrolyte is included within an electrostatically assembled film, charged groups are largely utilized in the formation of polymer-polymer ionic pairs, and the charged groups responsible for dye binding are largely those which were introduced into the film as a result of pH variations. These additionally introduced charges comprised ∼6% of Q22 functional groups. However, for a polyelectrolyte chain in solution, all charges in the Q22 chains at pH 4.6 (about 26%, assuming the same pKb for Q22 pyridine groups in solution and within the multilayer), are available for binding with dye molecules. This creates a strong asymmetry of charges in favor of solution, resulting in a larger concentration of binding centers for the dye in solution and facilitating dye extraction. In the case of hydrogen-bonded systems, macromolecules with very low or zero charge density are usually used. Specifically, in the PMAA/PEO system, the adsorption of PMAA chains is driven by the hydrogen bonding of uncharged units, while charged units are not utilized in polymer-polymer adhesion and charge neutralization. The only source for the charge misbalance between selfassembled PMAA molecules and PMAA chains in solution is the known suppression of ionization of self-assembled PMAA due to hydrogen bonding with PEO chains.33 However, because of weak binding between PEO and PMAA molecules, this factor is small for the PMAA/PEO system, and, at pH 4.2, ionization of self-assembled PMAA chains (3-4%) is only ∼1% smaller than ionization of PMAA in solution (∼5%). As in the PMAA/PEO system, ionic strength had a strong effect on the average ionization of functional groups within PMAA/Q22 multilayers. The protonation of pyridine rings increased, due to a decrease in the Debye length at high (51) Hoogeveen, N. G.; Cohen Stuart, M. A.; Fleer, G. J.; Bo¨hmer, M. R. Langmuir 1996, 12, 3675. (52) Steitz, R.; Jaeger, W.; Klitzing, R. Langmuir 2001, 17, 4471.

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salt concentration, and the total fraction of permanently charged and protonated Q22 units increased from 28% to 31% and 38% as the concentration of NaCl in 0.01 M buffer increased from 0 to 0.15 and 0.5 M, respectively. Also, similar to the hydrogen-bonded system, this increase did not result in changes in the dye loading capacity of the film. However, as compared to PMAA/PEO systems, smaller amounts of BPB were retained within the PMAA/ Q22 film in buffer solutions with high ionic strength. For a 25 nm PMAA/Q22 film, ∼10-15 mg/m2 of BPB (30% of the amount loaded at low ionic strength) was extracted in 0.15 M NaCl buffer solutions after the first rinse. This illustrates worse retention of BPB within PMAA/Q22 films at 0.15 NaCl as compared to the R6G/PMAA/PEO system, where ∼10% of the loaded dye could be removed by a single rinse with salt-containing buffer. Note that the reverse order of dye retention strengths was observed at low ionic strength solution (0.01 M phosphate buffer), where dye removal into a pure buffer after triple rinsing comprised only ∼5% in the BPB/PMAA/Q22 system, but was about 20% in the R6G/PMAA/PEO system. We believe that this switch in dye-retaining properties of multilayers as a function of concentration of small ions in external solution might originate from fundamentally different effects of ionic strength on the multilayer environment in the case of electrostatically and hydrogen-bonded self-assembled films. While in the PMAA/Q22 system high concentrations of salts result in dissociation of electrostatic polymerpolymer pairs and promote a more polar microenvironment within the film, in the PMAA/PEO system small ions are not able to disrupt polymer-polymer hydrogen-bonded adhesion points between the PMAA/PEO chains. This might lead to a lower local dielectric constant and stronger retention of dyes within hydrogen-bonded multilayers in solutions containing a high concentration of small ions. Conclusions We studied the incorporation and subsequent release of dye molecules from hydrogen-bonded and electrostatically assembled polymer films built from weak polyelectrolytes. In the case of hydrogen-bonded multilayers, dye release is largely controlled by adsorption of a polyelectrolyte on top of the film and involves the adjustment of ionization of self-assembled weak polyelectrolytes to the charges accumulated within the top adsorbing layer that occurred through long-range electrostatic interactions within the film. A second mechanism, an extraction of a dye through the film by dissolved polymer species, had little contribution in the case of hydrogen-bonded multilayers, but significantly contributed to dye release from electrostatically assembled films. In addition, we found significant differences of the local dielectric environment between the top and the embedded layers of the film, which resulted in weaker binding of the dye with the film surface than that with the film interior. This study might have practical ramifications for controlled delivery of chemicals. Acknowledgment. We thank Tom Cattabiani (Stevens Institute of Technology) for helpful comments on the manuscript. This work was supported by the National Science Foundation under Award DMR-0209439. LA048763D