Determination of Degree of Ionization of Poly(allylamine hydrochloride

Article pubs.acs.org/Langmuir

Determination of Degree of Ionization of Poly(allylamine hydrochloride) (PAH) and Poly[1-[4-(3-carboxy‑4 hydroxyphenylazo)benzene sulfonamido]-1,2-ethanediyl, sodium salt] (PAZO) in Layer-by-Layer Films using Vacuum Photoabsorption Spectroscopy Q. Ferreira,†,‡ P. J. Gomes,† P. A. Ribeiro,† N. C. Jones,§ S. V. Hoffmann,§ N. J. Mason,∥ O. N. Oliveira, Jr.,⊥ and M. Raposo*,† †

CEFITEC, Departamento de Física, Faculdade de Ciências e Tecnologia, UNL, Campus de Caparica, 2829-516, Caparica, Portugal Instituto de Telecomunicações, Instituto Superior Técnico, Av. Rovisco Pais, P-1049-001 Lisboa, Portugal § Institute for Storage Ring Facilities (ISA), Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, DK − 8000 Aarhus C, Denmark ∥ Centre of Molecular and Optical Sciences, Department of Physics and Astronomy, The Open University, Walton Hall, Milton Keynes MK7 6AA, United Kingdom ⊥ Instituto de Física de São Carlos, Universidade de São Paulo, CP 369, 13560-970, São Carlos, São Paulo, Brazil ‡

ABSTRACT: Electrostatic and hydrophobic interactions govern most of the properties of supramolecular systems, which is the reason determining the degree of ionization of macromolecules has become crucial for many applications. In this paper, we show that highresolution ultraviolet spectroscopy (VUV) can be used to determine the degree of ionization and its effect on the electronic excitation energies of layer-by-layer (LbL) films of poly(allylamine hydrochloride) (PAH) and poly[1-[4-(3-carboxy-4 hydroxyphenylazo)benzene sulfonamido]-1,2-ethanediyl, sodium salt] (PAZO). A full assignment of the VUV peaks of these polyelectrolytes in solution and in cast or LbL films could be made, with their pH dependence allowing us to determine the pKa using the Henderson-Hasselbach equation. The pKa for PAZO increased from ca. 6 in solution to ca. 7.3 in LbL films owing to the charge transfer from PAH. Significantly, even using solutions at a fixed pH for PAH, the amount adsorbed on the LbL films still varied with the pH of the PAZO solutions due to these molecular-level interactions. Therefore, the procedure based on a comparison of VUV spectra from solutions and films obtained under distinct conditions is useful to determine the degree of dissociation of macromolecules, in addition to permitting interrogation of interface effects in multilayer films.

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tures in general depend strongly on their surface charge.30 Indeed, positively charged nanoparticles have been found to be more toxic to cells owing to the stronger electrostatic interaction with the predominantly negatively charged cell membranes.31 Another example is the high bactericide activity of chitosans, which has been attributed to their positive charge.32 Comparative experiments using chitosans and a positively charged polyelectrolyte have shown, however, that the chitosan activity does not depend solely on the electrostatic interactions.33 Hydrophobic interactions were also found to be relevant, and this is a general feature for most systems comprising charged macromolecules, particularly because these

INTRODUCTION

Designing nanomaterials is a rational way to construct functional devices1 for several applications, including in electronics,2−5 photonics,6−13 sensors,14 and biomedicine.15 Nanoarchitectures can be produced with a limited number of methods, and the layer-by-layer (LbL) technique16−19 is probably the most versatile. The LbL method, including some of its extensions as the spin-assisted LbL assembly and spray-assisted LbL assembly20 that allow for fabrication of micrometer-thick films, is essentially based on the adsorption of oppositely charged materials, but other interactions can also be exploited.21−27 Therefore, in order to identify sophisticated, optimized applications for molecules, such as natural and synthetic polymers, it is crucial to probe electrostatic and hydrophobic interactions.28,29 For example, the toxicity and therapeutic properties of metal nanoparticles and nanostruc© 2012 American Chemical Society

Received: October 11, 2012 Revised: December 2, 2012 Published: December 6, 2012 448

dx.doi.org/10.1021/la304036h | Langmuir 2013, 29, 448−455

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The thickness of the films was not measured, but measurements done in other similar films indicate a value of about 7 Å for the thickness per bilayer. Here, we emphasize that the adsorbed amount in each layer depends on the adsorption time, and consequently, the buildup of layer-by-layer films may also be dependent on this adsorption time as demonstrated in ref 49. For example, LbL films could be built with small molecules using short adsorption times.50 This means that it is not necessary for the layer to be complete to grow LbL films. The VUV photoabsorption spectra were recorded at the ultraviolet beamline (UV1) at the synchrotron radiation facility ASTRID at Aarhus University, Denmark.51 The experimental setup consists of a vacuum chamber with up to three sample disks and a reference disk mounted on a MDC SBLM-266−4 push−pull linear motion positioner. UV beam light passed through the disks and the transmitted intensity, It, was measured using a photomultiplier detector (Electron Tubes Ltd., UK). Both the transmitted light intensity and the synchrotron beam ring current were monitored at 1 nm intervals. Also recorded was the radiation transmitted through the reference disk, I0, thus allowing the absorbance to be calculated. The minimum wavelength achieved is determined by the CaF2 substrates, so that the lowest wavelength at which reliable data could be collected was ∼125 nm. In order to avoid absorption from molecular oxygen in air at wavelengths below 170 nm, the small gap between the sample chamber exit window and the photomultiplier detector was flushed with He. Films on calcium fluoride substrates were studied for wavelengths between 125 and 350 nm, while the liquid samples were analyzed in 0.1 mm optical path quartz cuvettes within the 185 to 350 nm range. The LbL films were also characterized using Fourier transform infrared (FTIR) spectroscopy with a Mattson spectrophotometer.

interactions also vary with changes in the environment which alter the charge of the macromolecules. For these reasons, the interplay between hydrophobic and electrostatic interactions, and therefore determining the degree of dissociation of macromolecules, has become of the utmost relevance. The determination of the degree of dissociation, however, is far from straightforward. The methods most commonly used are titration curves or through infrared spectra (see, for example, refs 34−39). In this paper, we show that high resolution ultraviolet spectroscopy (VUV) can also be used to determine the degree of ionization of polyelectrolytes, including in the film form. The polyelectrolytes chosen for the proof-ofprinciple experiments are poly(allylamine hydrochloride) (PAH) and poly[1-[4-(3-carboxy-4 hydroxyphenylazo)benzene sulfonamido]-1,2-ethanediyl, sodium salt] (PAZO). PAH is one of the most studied polyelectrolytes, whose results can be compared with the literature. PAZO was chosen for two reasons: First, the electronic states of ionized groups in azopolymers have barely been discussed,40,41 in spite of the detailed information available on their photoisomerization properties.27 Second, PAZO was investigated in previous works in our group,42−44 including the fabrication of layer-by-layer (LbL) films.45,46 With the VUV data, we could also estimate the degree of dissociation and the pKa of the polyelectrolytes using the Henderson-Hasselbach equation.47,48

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MATERIALS AND METHODS

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The LbL films were prepared from poly[1-[4-(3-carboxy-4 hydroxyphenylazo)benzene sulfonamido]-1,2-ethanediyl, sodium salt] (PAZO) and poly(allylamine hydrochloride) (PAH) (average Mw = 50 000−65 000g/mol), both purchased from Aldrich. Their chemical structures are shown in Figure 1. PAZO was dissolved in pure water

RESULTS AND DISCUSSION VUV Characterization of PAZO and PAH in Solution. Figure 2 shows the ultraviolet absorption spectrum of a 3 mM PAZO aqueous solution which is consistent with the computed absorption spectra of imines such as N-benzylideneaniline (NBA) in the trans and cis forms.52 The spectra at various concentrations can be deconvoluted into four Gaussian components at 353, 270, 224, and 186 nm, with the absorption intensity increasing with the concentration, as shown in the inset of Figure 2a for the last three peaks. The absorption coefficients, determined using the Beer−Lambert law, were 9.6 ± 0.7, 4.2 ± 0.3, and 3.7 ± 0.3 m2/g, for the bands at 186, 224, and 270 nm, respectively. Table 1 summarizes the band features for PAZO aqueous solutions, i.e., peak position, energy values and molar absorption coefficients. To avoid effects from the Rayleigh absorption, all the bands were obtained by subtracting the baseline defined as the straight line that passes through two minima in the spectrum. The concentration dependence for the spectra of PAZO aqueous solutions was verified by normalizing the spectra with different concentrations to the 270 nm peaks that exhibited small error bars. The normalized spectra in Figure 2b reveal hyperchromic effect for the peaks at 186 and 224 nm, while the 353 nm peak was blue-shifted (hypsochromic shift). Therefore, concentration affected the spectral shape, probably due to aggregation of azo groups. The spectra were also dependent on pH, as shown in Figure 2c. The latter spectra were normalized by analyzing the parameters used in fitting the spectra with Gaussian components, shown in Table 2. The changes appear rather small, but were found to be significant. The peak at 270 nm was the only one for which the band position was not affected by pH. The band intensity at 186 nm decreased with increasing pH, suggesting that PAZO aggregation occurs at lower pHs, in accordance with the hyperchromic effect of the

Figure 1. Chemical structures of (a) poly(allylamine hydrochloride) (PAH) and (b) poly[1-[4-(3-carboxy-4 hydroxyphenylazo)benzene sulfonamido]-1,2-ethanediyl, sodium salt] (PAZO).

supplied by a Milli-Q system from Millipore (resistivity of 18 MΩ cm) to a monomeric concentration of 10−2 M with pH varying from 5 to 9. The PAH aqueous solutions had 10−2 M monomeric concentration and pH = 4, adjusted by adding droplets of NaOH solution. The PAZO/PAH LbL films were adsorbed onto calcium fluoride and quartz substrates that had been previously hydrophilized for 30 min in a Piranha solution, H2SO4/H2O2 (1:1) bath, and then washed exhaustively with pure water. Deposition comprised the following steps: (i) immersion of the substrate in PAH solution for 30 s; (ii) rinsing the substrate+PAH layer with pure water; (iii) immersion of the substrate+PAH layer into the PAZO solution for 30 s; (iv) rinsing the substrate+PAH/PAZO bilayer in pure water. By repeating steps (i) through (iv), a large number of bilayers could be deposited. PAZO cast films were produced from 10−2 M aqueous solutions with different pHs, adjusted with HCl or NaOH solutions. 449


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the π−π* transition in the azo group, from 343 to 353 nm as the pH increased from 5 to 9. A similar shift of 12 nm was observed in the UV−vis spectra for PAZO cast films prepared from aqueous solutions with pH between 5 and 9.42 The peaks in the VUV spectra can now be assigned based on the literature. The peak centered at 186 nm (6.67 eV) with a fwhm of 27 nm, i.e., absorption in the energy interval between 6.21 and 7.19 eV, is assigned to the π−π* transitions associated with the benzene B1u and E1u symmetries and to π−σ* transitions for benzene E1g, A2u, E2u, and A1u symmetries (ref 53 and references therein). Ab initio calculations of the electronic spectra for trans and cis azobenzene transitions54−56 suggest absorption due to n−π* transitions with the 11Bg symmetry and σ−σ* transitions due to the 1Au symmetry. The peak at 224 nm (5.54 eV) may be assigned to 31Bu in π−π* transitions of azobenzene molecules,56 to π−π* transitions due to 31B2u and 31Bu symmetries of benzene molecules, or to n−π* transitions in the carboxylic group in the benzene substituent.57 Therefore, this peak should be definitely assigned to the benzene rings of azobenzene groups. There are several possible contributions to account for the peak at 270 nm (4.59 eV). It can be attributed to π−π * vibronic transitions within the benzene ring or to π−π * transitions parallel to the minor axis of the azo group.58−61 There may also be a contribution from the OH substituent in the benzene, which absorbs around 210 and 270 nm, or from the carboxylic acid group in this substituent as its π−π* transition occurs at 270 nm.57 The band at 354 nm is characteristic of n → π* transitions associated with the azo group. According to ab initio TD-DFT calculations, absorption should take place between 352 and 374 nm for trans and cis azoalkanes with solvent water molecules.62 Quantum chemical calculations in azobenzene and two bisazo derivatives led to a vertical excitation energy of 3.92 eV (316 nm), which decreased with the number of azo groups.63 The increase in the number of azo groups in the molecule leads to larger charge delocalization, as seen in the frontier orbitals calculated in ref 17. These results are consistent with the changes in electronic delocalization and vertical excitation energies induced by the presence of a carboxylate group (COO−) or a carboxylic group (COOH) in the azomolecule (chromophore group in PAZO molecule). As a consequence, the peak shifted from 343 to 353 nm as the pH of PAZO solution increased from 5 to 10. This red shift should be attributed to (a) higher percentage of ionized carboxylate groups with increasing pH; (b) the trans or cis conformation can also induce changes in the vertical excitation energies as observed by Fliegl et al.;56 and (c) chromophore aggregation.10,11,64 Thin films from PAH and PAZO were produced via adsorption of these molecules from solution onto solid calcium fluoride substrates by immersing the substrate during 45 min onto 10−2 M monomeric concentration polyelectrolyte solutions at pH = 4 for PAH and pH = 9 for PAZO. The

Figure 2. (a) Measured photoabsorption spectrum of a 3 mM PAZO aqueous solution, where the green line corresponds to the fitting with Gaussian functions. In the inset is shown the absorbance at distinct peaks as a function of PAZO concentration. (b) Normalized spectra in relation to the 270 nm peak for distinct monomeric concentrations. (c) Normalized VUV absorbance spectra in relation to the 270 nm peak of aqueous PAZO solutions with a monomeric concentration of 5 mM at various pHs.

186 nm peak in Figure 2b due to the increase in concentration. This is also consistent with the shift of the band associated with

Table 1. Parameters from the Fitting with Gaussian Curves of the VUV Absorption Spectra for PAZO Solutions, Namely, Wavelengths at Maximum Peak (λ), Full Width at Half-Maximum (fwhm), Molar Absorption Coefficient (ε), and Assignments λ (nm) 186 224 270 353

± ± ± ±

1 2 1 1

peak position (eV) 6.74 5.8 4.59 3.44

± ± ± ±

0.01 0.1 0.01 0.01

fwhm (nm)

relative peak area

± ± ± ±

0.24 0.21 0.06 0.49

26.7 61 25.0 75

0.7 8 0.6 5

450

ε (m2/g)

assignment

± ± ± ±

benzene π−π* benzene π−π* π−π* transition parallel to the minor axis π−π*

11 4.80 4.00 4.30

1 0.04 0.04 0.04


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Table 2. Fitting Parameters for Gaussian Curves of the VUV Spectra of 2 mM PAZO Solutions with Different pHs absorption band peak position fwhm (nm) peak position fwhm (nm) peak position fwhm (nm) peak position fwhm (nm)

(nm) (nm) (nm) (nm)

pH = 5 180 31 219 62 270 22 343 54

± ± ± ± ± ± ± ±

1 2 2 10 1 1 1 3

pH = 6 182 31 224 36 270 25 348 70

± ± ± ± ± ± ± ±

pH = 7

1 1 2 4 1 2 1 4

186 27 218 58 270 26 353 72

± ± ± ± ± ± ± ±

1 1 2 8 1 2 1 4

pH = 8 186 31 224 49 270 25 354 69

± ± ± ± ± ± ± ±

1 1 2 6 1 1 1 4

pH = 9 186 31 224 53 270 25 353 68

± ± ± ± ± ± ± ±

1 1 2 10 1 1 1 5

spectrum for the PAH film could be fitted with Gaussian components, as shown in the inset of Figure 3. The parameters

Figure 4. VUV absorbance spectra of (PAH/PAZO)10 LbL films prepared from PAH solution at pH = 4 and PAZO solutions with different pHs. In the inset, normalized VUV absorption spectra of a (PAH/PAZO)10 LbL film prepared from PAH solutions at pH = 4 and PAZO solutions at different pH is shown.

Figure 3. VUV absorbance spectra of PAH and PAZO cast films onto calcium fluoride substrates obtained from adsorption during 45 min from polyelectrolyte aqueous solutions. The inset shows the spectrum for the PAH film fitted with Gaussian curves.

spectra. Six Gaussian components, whose parameters are given in Table 3, were found under these conditions to fit all the spectra for different pHs. The peaks were also associated with the film components, on the basis of the assignments for PAZO and PAH above. Therefore, the peak at 156 nm can only be associated with PAH, while the peaks at 187, 270, and 355−367 nm are assigned to PAZO. The peaks near 118 and 212 nm can be attributed to either of the polyelectrolytes, due to benzene68 and nitrogen67 groups. The band at 156 nm can be assigned to electronic transitions in the PAH NH3+ groups, which is inferred from the spectrum for ammonia.69,70 Degree of Ionization of PAH and PAZO on LbL Films. The analysis of parameters in the Gaussian curves from fitting the absorption spectra of (PAH/PAZO)10 LbL films indicates that the adsorption of PAH was affected by the pH of the PAZO solutions used. Indeed, the area under the peak assigned to the PAH NH3+ group increased with the PAZO solution pH, which means an increased amount of PAH adsorbed. The wavelength for the absorption maximum assigned to the PAH NH3+ group decreased with pH. This blue shift (hypsochromic shift) points to a decreasing degree of ionization for PAH, according to the pH dependence for the degree of ionization of PAH reported by Choi and Rubner35 and reproduced in Figure 5. In summary, the increase in pH of the PAZO solution causes a decrease in the positive charges of PAH, thus leading to adsorption of a larger amount of PAH, as seen in the inset of Figure 5. The degree of ionization of PAZO molecules in LbL films was determined from the FTIR spectra, by comparing the peak areas assigned to carboxylate (COO−) and carboxylic groups (COOH)35,42 of (PAH/PAZO)30 LbL films prepared with PAZO solutions of various pHs, and keeping pH 4 for PAH

for the fitting indicate an intense peak at 172.9 ± 0.1 nm (7.172 ± 0.004 eV) with a fwhm of 22.9 ± 0.2 nm, assigned to electronic transitions associated with the protonated amine NH3+.65,66 This peak is superimposed to another at 203.8 ± 0.7 nm (6.08 ± 0.05 eV) with an fwhm of 18 ± 2, according to the spectrum in the inset of Figure 3. We have not been able to find reports on transitions of the NH3+ group in the condensed phase, but studies in the gas phase have shown Rydberg-type transitions with corresponding energies between 6 and 8 eV.66 Excitation energies close to 8 eV have been found for ammonia, namely, the nN → 3pe transition with excitation energy of 8.22 eV. Although the Rydberg states can only be identified in the gas phase, one may assume that the transition at 174 nm is associated with a transition in the PAH amino group. In addition, absorbance due to the amine group is expected for wavelengths smaller than 135 nm.67 The PAH and PAZO films were irradiated for several hours with the 140 nm light and no changes were observed. VUV Characterization of PAH/PAZO LbL Films. The VUV spectra of 10-bilayer PAH/PAZO LbL films ((PAH/ PAZO)10) prepared from pH 4 PAH aqueous solutions and PAZO solutions with different pHs are displayed in Figure 4. They contain contributions from both PAH and PAZO absorption bands, in a proportion that should depend on the adsorbed amounts of each polymer. The fitting with Gaussian functions is shown in the inset of Figure 4 for the spectrum of a (PAH/PAZO)10 LbL film prepared with PAZO solution at pH = 5. The spectra were fitted with the minimum number of Gaussian components that gave the best fitting, in addition to imposing that the same number of peaks should apply to all 451


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Table 3. Parameters for the Gaussian Curves Obtained from Fitting the VUV Spectra of PAH/PAZO LbL Films Prepared from PAZO Solutions with Different pHs absorption peak peak position fwhm (nm) peak position fwhm (nm) peak position fwhm (nm) peak position fwhm (nm) peak position fwhm (nm) peak position fwhm (nm)

(nm) (nm) (nm) (nm) (nm) (nm)

pH = 5 118 38 156 20 187 26 216 27 270 29 355 55

± ± ± ± ± ± ± ± ± ± ± ±

1 4 1 1 1 3 1 3 1 3 1 5

pH = 6 119 37 156 20 186 25 213 28 270 30 357 54

± ± ± ± ± ± ± ± ± ± ± ±

1 4 1 1 1 2 1 3 1 4 1 3

pH = 7 117 41 154 28 187 24 213 31 270 30 361 54

± ± ± ± ± ± ± ± ± ± ± ±

1 5 1 2 1 1 1 3 1 4 1 3

pH = 8 115 41 154 15 187 26 216 25 270 29 362 54

± ± ± ± ± ± ± ± ± ± ± ±

1 4 1 1 1 2 1 3 1 3 1 3

pH = 9 112 44 153 30 190 26 224 22 270 29 367 56

± ± ± ± ± ± ± ± ± ± ± ±

1 5 1 2 1 1 1 3 1 3 1 4

pH = 10 (eV) 118 34 150 33 190 26 212 27 270 29 367 55

± ± ± ± ± ± ± ± ± ± ± ±

1 3 1 2 1 2 1 3 1 3 1 5

assignment

(10.51 ± 0.02)

PAZO, PAH

(8.267 ± 0.02)

PAH

(6.53 ± 0.02)

PAZO

(5.85 ± 0.02)

PAZO, PAH

(4.59 ± 0.02)

PAZO

(3.38 ± 0.02)

PAZO

Figure 5. Position of the peak due to the transition associated with the NH3+ group of PAH and degree of ionization obtained from Choi and Rubner35 as a function of pH. The area of the peak associated with the PAH amine group is shown in the inset as a function of pH of the PAZO solution.

solutions. Figure 6a shows both the PAZO degree of ionization in LbL films and the position of the peak associated with the π−π* transition of the azo group, ∼360 nm, as a function of pH of PAZO solution. The degree of ionization for PAZO increased from 0.5 at pH = 4 to ca. 0.7 at pH = 8, which follows the behavior of the position of the azo peak. This means that this peak is associated with the degree of ionization of the PAZO molecule, the same applying for the peak at 186 nm, assigned to π−π* transitions of benzene groups. Note that the intensity of the latter peak also varies with pH in the same fashion as the position of the azo peak in Figure 6b). The pH dependence for the degree of ionization of PAZO agrees remarkably well with the change in position of the peak assigned to the π−π* transition of the azo group, as illustrated in Figure 6a). In addition, the ca. 12 nm redshift from pH = 5 to pH = 9 can be attributed to the interaction of azo conjugated group with polar species, which are known to affect the π−π* transitions. Another possible explanation for the red shift is the presence of J-type aggregates,12,71 which are associated with electrical charges, as azochromophores have facile cooperation in LbL films when the opposite polyelectrolyte has a small degree of ionization. In other words, the ionic azo groups surrounded by counterions are amenable to aggregation.44 Therefore, the change in the degree of ionization of PAZO is responsible for both aggregation and changes in the molecule electronic structure, thus yielding the red shift and changes in peak intensity.

Figure 6. (a) Degree of ionization of PAZO molecules (open squares) and wavelength of the peak (blue circles) corresponding to the π−π* transition of the azo group in LbL films as functions of pH of PAZO solutions. The pH of the PAH solutions was kept at pH = 4. (b) Position of the azo peak in LbL films (open squares) and the intensity of the peak at 186 nm (red circles) obtained from the VUV spectra of PAZO aqueous solutions with different pHs.

Dependence of Effective PAZO pK a with the Surrounding Medium. The shifts in the peaks assigned to the NH3+ group (PAH) and azo group (PAZO) point to important interface effects related to electrical charges. The interpretation of the VUV spectra may then also provide information on the interactions between polyelectrolytes which are strongly pH dependent. In PAH/PAZO LbL films, each bilayer interface is a spatial region where acid−base reactions between the carboxylic groups and amino groups take place. These reactions result in proton transfer from PAH to PAZO, with the acidity of the amino group increasing as the carboxylic group becomes more ionized. This can be seen by comparing 452


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the wavelength shift of the π−π* peak in the VUV and UV spectra for PAZO solutions, cast films,42 and LbL films plotted in Figure 7 as a function of pH for PAZO solutions. The PAZO

determining the pKa may be difficult owing to the low intensity of the absorbance spectra generated by the small adsorbed amounts in the films.77 The other commonly used method is the electrophoretic fingerprinting technique, which is limited because it requires an estimate of the apparent charge density to determine the pKa.78 Nevertheless, it has to be admitted that the VUV method provides information about the whole macromolecules, instead of concentration profiles for the ions as in other methods (see for example ref 38).

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CONCLUSIONS We reported on a simple experimental approach that provides information of molecular-level interactions between polyelectrolytes in layer-by-layer (LbL) films. It consisted of measuring the VUV spectra of the polyelectrolytes in solution, to be compared with those in cast and LbL films. The polyelectrolytes used were PAH and PAZO, whose electronic transitions in the VUV spectra could be assigned based on the literature. The amount of PAH adsorbed in LbL films varied with the pH of the PAZO solution, even using a fixed pH 4 for PAH solutions. This was attributed to charge transfer from PAH to PAZO, with the less charged PAH adopting a more coiled conformation with an ensuing adsorption of larger amounts. Most significantly, the change in the degree of ionization of PAH and PAZO could be obtained quantitatively from the VUV and FTIR data, then also allowing us to obtain their pKa, including the film form, using the HendersonHasselbach equation. PAZO was found to have its basic character increased in the LbL films as compared to in solution40−44 and in cast films, owing to the interaction with PAH, thus confirming that weak polyelectrolytes have distinct dissociation constants. The most important implication of the findings above is associated with the establishment of a relatively simple procedure to investigate molecular-level interactions in LbL films. For the shifts and changes in intensity in the VUV, data for solutions of polyelectrolytes and their LbL films can be related to the degree of dissociation. The knowledge of the charging state of macromolecules in different environments is essential for a multitude of applications, since the interplay between electrostatic and hydrophobic interactions governs many of the supramolecular properties.

Figure 7. Wavelength of the position of maximum absorbance peak between the protonated form (pH = 4) and the deprotonated form (pH = 9) as a function of PAZO pH for PAZO in solution, cast film,42 and LbL film forms. The solid lines correspond to the fitting with the Henderson-Hasselbach equation.

molecules are practically all ionized at pH = 9 and all protonated at pH = 4. The degree of ionization, α, for PAZO is proportional to the peak shift, Δλ, since both vary in similar ways; the curves in Figure 7 can be fitted with equations for the degree of ionization as a function of pH. The degree of ionization α and Δλ can be related using the HendersonHasselbach equation as follows: α pH = pK a + log (1) 1−α For the acid/base balance for polyelectrolyte chains, eq 1 can be written as72 α pH = pK a(ap) + nH log (2) 1−α where pKa(ap) is the apparent acid/base dissociation constant of the polyelectrolyte chains and the parameter nH is related to the polymer electrical charge density. The data in Figure 7 can be fitted with eq 2, from which the PAZO dissociation constant can be determined. The values, in Table 4, reveal that PAZO

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Table 4. Parameters Obtained from Fitting HendersonHasselbach Equation to the Data in Figure 7 PAZO

Δλ

pKa(app)

nH

solution cast film LbL Film

10.0 ± 0.2 10.0 ± 0.2 12.0 ± 0.2

6.02 ± 0.06 6.12 ± 0.05 7.25 ± 0.06

0.26 ± 0.08 0.44 ± 0.10 0.72 ± 0.30

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: 00351 212948576. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the European Commission for access to the ASTRID facility at the University of Aarhus, Denmark, and for their support through the Access to Research Infrastructure action of the Improving Human Potential programme. This work was supported also by the ″Plurianual″ financial contribution of ″Fundação para a Ciên cia e Tecnologia″ (Portugal) and by FAPESP and CNPq (Brazil). It resulted from taking part in the COST Action CM0601, Electron Controlled Chemical Lithography (ECCL), and in bilateral collaboration projects under the programs CAPESBrazil/Grices-Portugal.

molecules increase their basic character when in the film form, because electrically charged PAH promotes the release of PAZO counterions, favoring the electrostatic binding between PAH and PAZO in the film. This behavior is consistent with the literature,34,73−76 where the dissociation constant of weak polyelectrolytes in LbL films differs from the value found in solution. We take the view that the VUV method is more convenient than others in the literature for determining the association/ dissociation charge parameters in LbL films. For example, Fourier transformed infrared spectroscopy does allow one to obtain the degree of ionization curves in LbL films, but 453


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Determination of Degree of Ionization of Poly(allylamine hydrochloride

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