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Complexation of Polyacrylates by Ca2+ Ions. Time-Resolved Studies Using Attenuated Total Reflectance Fourier Transform Infrared Dialysis Spectroscopy Fabiana Fantinel, Jens Rieger,* Ferenc Molnar, and Patrick Hu¨bler BASF Aktiengesellschaft, Polymer Physics, 67056 Ludwigshafen, Germany Received September 9, 2003. In Final Form: January 29, 2004 The attenuated total reflectance Fourier transform infrared dialysis technique is introduced for the time-resolved investigation of the binding processes of Ca2+ to polyacrylates dissolved in water. We observed transient formation of intermediates in water with various types of coordination of the carboxylate group to Ca2+ throughout the complexation steps. Time-resolved changes in the spectra were analyzed with principal component analysis, from which the spectral species were obtained as well as their formation kinetics. We propose a model for the mechanisms of Ca2+ coordination to polyacrylates. The polymer chain length plays an important role in Ca2+ binding.
Introduction Complexation of polyelectrolytes by metal ions in aqueous phase has been widely investigated.1,2 It has been found that the interaction of some metal ions does not obey the general picture of counterion binding1-5 but is rather specific, hence the term ”site binding”.1-3 Concerning polyacrylates, the phenomenon of site binding has been observed for divalent and trivalent cations (M2/3+) and it has been related to precipitation that occurs above a critical concentration of ions.6-10 At present, several models2-5,9,11,12 have been proposed to explain the Ca2+binding process, but concerning the understanding on a molecular level not much work has been published. Although Ca2+ chelating complexes have been proposed for polyacrylates in cement pastes13,14 as well as in proteins15 the mechanisms of the local binding of Ca2+ have not yet been clarified. From a molecular point of view, three types of coordination of the carboxylate group (COO-) to metal ions have been suggested: unidentate, bidentate, and bridging.15-21 In the unidentate only one oxygen is bound to the metal * To whom correspondence may be addressed: BASF Aktiengesellschaft, Polymer Physics, Carl Bosch Str. 38, 67056 Ludwigshafen, Germany. Phone: +49 621 6073731. Fax: +49 621 606673731. E-mail:
[email protected]. (1) Manning, G. S. J. Phys. Chem. 1981, 85, 870. (2) Oosawa, F. Polyelectrolytes; Marcel Dekker: New York, 1971. (3) Manning, G. S. Q. Rev. Biophys. II 1978, 2, 179. (4) Satoh, M.; Kawatahima, T.; Komiyama, J. Polymer 1991, 32, 892. (5) Liao, Q.; Dobrynin, A. V.; Rubinstein, M. Macromolecules 2003, 36, 3399. (6) Sabbagh, I.; Delsanti, M.; Lesieur, P. Eur. Phys. J., B 1999, 12, 253. (7) Sabbagh, I.; Delsanti, M. Eur. Phys. J., A 2000, 1, 75. (8) Pochard, I.; Foissy, A.; Couchot, P. Colloid Polym. Sci. 1999, 277, 818. (9) Wittmer, J.; Johner, A.; Joanny, J. F. J. Phys. II 1995, 5, 6355. (10) Pochard, I.; Couchot, P.; Foissy, A. Colloid Polym. Sci. 1998, 276, 1088. (11) Iida, S. Biophys. Chem. 1996, 57, 133. (12) Hara, M. Polyelectrolytes; Dekker: New York, 1993. (13) Crisp, S.; Prosser, H. J.; Wilson, A. D. J. Mater. Sci. 1976, 11, 36. (14) Nicholson, J. W. J. Mater. Sci.: Mater. Med. 1998, 9, 273. (15) Dudev, T.; Lim, C. Chem Rev. 2003, 103, 773. (16) Nakamoto, K.; Fujita, J.; Tanaka, S.; Kabajashi, M. J. Am. Chem. Soc. 1957, 79, 4905. (17) Alkock, N. W.; Tracy, V. M.; Waddington, T. C. J. Chem. Soc., Dalton Trans. 1976, 2243. (18) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds, 4th ed.; Wiley: New York, 1986. (19) Deacon, G. B.; Phillips, R. J. Coord. Chem. Rev. 1980, 33, 227.
ion; in the bidentate both oxygens of the COO- are coordinating to the metal in a chelating structure; in the bridging type the two oxygens of the COO- are coordinated to two different ions. IR techniques are well suited to characterize the structure of complexes because of the sensitivity of the carbonyl stretching vibrations to the geometry of the COO- group and its environment.13,14,18,22-28 In a COO- group where the double bond is delocalized, the CO stretching vibration splits into an asymmetric (higher frequency) and a symmetric (lower frequency) stretching mode. Several intermediate symmetries are possible under different coordination conditions. For simplicity, we always denote the low-frequency CO stretching vibration as the symmetric COO- stretching, and the high-frequency one as the asymmetric COO-stretching, even when this is not rigorously correct once the symmetry is broken. Considering the ionic Na+ coordinate as a “zero reference”, a reduction of the frequency difference between asymmetric and symmetric stretching frequencies from Na+ to a bidentate (M2/3+) coordinate has been predicted and observed while an increase of this difference has been reported for unidentate (M2/3+) coordinates.15,16,18-20 For a detailed analysis, the asymmetric C-O stretching frequency of the COO- has been shown to be the most sensitive to the coordination type. The relative shifts of this frequency have been shown to provide insight into structural changes of the molecules.28 The specific Ca2+ binding was discussed by Nara: 28 going from the ionic coordination of Na+ to the site specific chelating coordination of Ca2+, a 10-20 cm-1 downshift of the asymmetric stretching was calculated (due to the decrease of the OCO angle and the symmetrization of the COO-). The symmetric stretching frequency correspondingly exhibits an upshift, but the (20) Kirwan, L. J.; Fawell, P. D.; van Bronswijk, W. Langmuir 2003, 19, 5802. (21) Katz, A. K.; Glusker, J. P.; Beebe, S. A.; Bock, C. W. J. Am. Chem. Soc. 1996, 118, 5752. (22) Ito, K.; Bernstein, H. J. Can. J. Chem 1956, 34, 170. (23) Van der Berghe, E. V.; Van der Kelen, G. P.; Albrecht, J. Inorg. Chim. Acta 1968, 2, 89. (24) Brozki, B. A.; Coleman, M. M.; Painter, P. C. Macromolecules 1984, 17, 230. (25) Wu, H.-S.; Jone, H.-C.; Hwang, J.-W. J. Appl. Polym. Sci. 1997, 63, 89. (26) Rha, C. Y.; Seong, J. W.; Kim, C. E.; Lee, S. K.; Kim, W. K. J. Mater. Sci. 1999, 34, 4653. (27) Nicholson, J. W. J. Appl. Polym. Sci. 1999, 78, 1680. (28) Nara, M.; Torii, H.; Tasumi, M. J. Phys. Chem. 1996, 100, 19812.
10.1021/la030354e CCC: $27.50 © 2004 American Chemical Society Published on Web 03/05/2004
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Figure 1. Schematic view of the dialysis ATR cell.
increase of the coordination number at the metal may counteract this effect. Therefore the symmetric stretching alone cannot be diagnostic for the structural analysis of complexes. On the other hand turning from a chelating to a unidentate coordination, the COO- becomes more asymmetric, leading to an upshift of about 30-50 cm-1 of the asymmetric COO- stretching relative to the one observed for ionic coordination of Na+. Additionally, some structures closely related to the unidentate may be observed. If a water molecule is participating in the coordination, a pseudobridge is formed, in which an H2O is bound simultaneously to the COO- and to the Ca2+; here a downshift of the asymmetric COO- stretching of about 20 cm-1 with respect to the unidentate state is expected. Also a transient pseudobridge with Na+ and Ca2+ may occur, the asymmetric COO- stretching of which has been predicted to be around 40 cm-1, shifted to lower frequencies with respect to the unidentate.28 In the present study a new attenuated total reflection Fourier transformed infrared (ATR FTIR) dialysis technique combined with principal component analysis (PCA) has been used to study the Ca2+-binding behavior of polyacrylates. The method here proposed is carried out in situ, i.e., with time resolution, and it yields information about the structure of the final complexes, which precipitate from the solution, and allows to identify the intermediates. It is based on the separation of reagents by a semipermeable membrane, so that the reaction rate is limited by the diffusion of the Ca2+ ions into the polymer solution. The detection region of this technique is located at the interface between the polymer solution and the ATR plate so that both the complexes in solution and the precipitated products can be investigated. Polyacrylates of different molecular weight interacting with Ca2+ ions have been studied. The analysis of fine details of spectra combined with the kinetics information allows a conclusion on how the binding mechanism varies with molecular mass. Experimental Section Materials. Samples analyzed were Na+ salts of poly(acrylic acid)s supplied by BASF Aktiengesellschaft, PA1.2K, PA8K, and PA30K with molar mass 1200, 8000, and 30000 Da, respectively; polymer (10 g/L) was dissolved in water at pH ) 10. With CaCl2‚ 2H2O for analysis (J.T. Baker) water solutions of 200 mM at pH ) 10 were prepared. Methods. IR spectra were collected by a FTIR spectrometer Bruker IFS 86 coupled to a differential amplifier Lecroy DA1822. A dialysis ATR-FTIR cell, Bruker BioATR A737, with two compartments and with a ZnSe crystal as internal reflection element was employed (see Figure 1). The dialysis membrane was a dialysis tubing Servapor of regenerated modified cellulose, from Serva Electrophoresis GmbH, Germany. The reaction chamber was purged with an N2 flow of 10 cm3/s. To be able to subtract completely the contribution of water in the difference spectra, the sample chamber was thermostated at 25 °C by a Thermostat Ministat Huber. The membrane separating the compartments allows the Ca2+ ions to diffuse from the upper to the lower compartment, which is filled with the polymer solution. The lower compartment is the reaction chamber in which complexation occurs. A schematic view of the cell is shown in
Figure 2. Spectra of the initial Na+ complexes (a), intermediate Ca2+ complexes (region asymmetric COO- stretching) (b), and final Ca2+ complexes (c): PA1.2K (s); PA8K (‚ ‚ ‚); PA30K (- - -). Spectra b and c are obtained from PCA loadings (see text). Table 1. Summary of COO- Stretching Frequencies frequency (cm-1)a sample MW
initial
PA1.2K 1.2K 1406 [ss] 1554 [as] PA8K 8K 1406 [ss] 1556 [as] PA30K 30K 1406 [ss] 1556 [as]
product intermediate 1 intermediate 2 1410 [ss] 1574, 1628 [as] 1547 [as] 1408 [ss] 1587, 1624 [as] 1543 [as] 1408 [ss] 1560 [as] 1541 [as]
1560 [as]
a Key: [ss], symmetric stretching; [as], asymmetric stretching. Intermediate 1 is the earlier, 2 the later formed.
Figure 1. Difference spectra against the starting solution of poly(acrylic acid) (sodium salt) were collected as an average over 264 scans, with 4 cm-1 resolution at intervals of 45 s. Data analysis was performed by means of principal component analysis (PCA), implemented in the program Unscrambler 8.0 (Camo). The PCA29,30 is based on the analysis of the covariance matrix of the data sets. A diagonalization of the covariance matrix is performed, so that the data are segregated along the direction of the maximum variance, i.e., the eigenvectors of the data matrix (called loadings) and the magnitude of each eigenvector (called the scores) is determined.31 The error matrix is reduced with an iterative procedure until a threshold of 0.001% is reached. The resulting error on the principal component’s loadings was lower than the experimental error (0.01%).
Results and Discussion Normalized IR spectra for initial Na+ salts, intermediate, and final Ca2+ complexes are shown in Figure 2. Spectra of intermediates and final species are obtained from the loadings of the first and second principal components, PC1 and PC2. The spectral details of symmetric and asymmetric stretching bands are summarized in Table 1. In Figure 3 the time evolution of the relative concentration of intermediates and final products is shown. The kinetic curves have been obtained by calculating the scores using PCA. From the time evolution of the spectra of the different species, it is possible to identify them as products or intermediates. The analysis is semiquantitative for two (29) Stordrange, L.; Christy, A. A.; Kvalein, O. M.; Shen, H.; Liang, Y.-Z. J. Phys. Chem. 2002, 106, 8543. (30) Steen, W. A.; Jeerage, K. M.; Schwartz, D. T. Appl. Spectrosc. 2002, 56, 1021. (31) Shrager, R. I. Chem. Int. Lab. Sys. 1986, 1, 59.
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Figure 3. Time evolution of the final Ca2+ complex (a) and of the Ca2+ intermediate complex (b), from PCA scores (see text): PA1.2K (2); PA8K (b); PA30K (1).
reasons. First, the Ca2+ concentration in the lower compartment is not precisely known because of the slow diffusion through the membrane. Second, the spectrum is dominated by the precipitate once the precipitation reaction has come to completion because the measured volume encompasses only a few micrometers of the solution (or precipitate) above the reflection plane. But from the evolution of the turbidity of the solution and from comparison with measurements at different pH (data not shown), it is concluded that the intermediates remain in solution whereas the final complex precipitates. Therefore we expect the species to have a gradient of concentration from the membrane to the ATR plate. The solution is likely to be homogeneous until the maximum of the second intermediate occurs. Then the precipitate dominates and the second intermediate may no longer be detected though still being present. Thus, it is concluded that the time evolution of the data shown above gives qualitative information about the complexation state of the polymers though quantitative interpretation is only possible up to the moment where a significant portion of the polymeric precipitate covers the ATR plate. From Figure 2 and Table 1 it is evident that the COOstretching frequencies of the Na+ salts do not depend on the chain length of the polymers. The Ca2+ complexes in their final form show at all chain lengths a decrease in the asymmetric COO- stretching frequency, which becomes more pronounced at higher molar masses. On the other hand, the symmetric COO- stretching frequency does not vary appreciably. Concerning the intermediates, they all exhibit an upshift of the asymmetric COO- stretching frequency, but there are many subtle differences among them: The polymer with the lowest molecular weight forms two different types of intermediates, with asymmetric COO- stretching at higher frequencies than in the case of the Na+ salts. The polymer with medium molar mass exhibits two high-frequency intermediates like the polymer with lower molar mass, but with very low concentration. Then a second intermediate with slightly lower asymmetric stretching frequency occurs. The poly(acrylic acid) with the highest molar mass in this series gives rise to one intermediate that is identical to the one formed later in the intermediate molar mass polymer. From the detailed assignment of the spectral intermediates, a general mechanism for the binding of Ca2+ ions to poly(acrylic acid)s can be proposed; cf. Figure 4. In this model, the ionic chelating complex (a) evolves either to a
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unidentate (b1), which can form a pseudobridge by coordinating a water molecule (b2) or a pseudobridge with Na+ (b3). The process leads to a chelating bidentate product (c); the yield of this step may be not complete. Within this general scheme some differences are discernible depending on the degree of polymerization. On one hand, the observed spectral details show that the final compound is a chelating type for all molar masses of polyacrylates and that its asymmetric COO- stretching frequency is reduced with increasing molar mass. This is due to a decrease of the chelating angles. Thus the Ca2+ binding is stronger in longer chains. Actually two COOgroups are likely to be involved in the Ca2+ chelation, but in order for this to occur a torsion of the chain is required, which is easier in longer chain polymers, while in the shorter chains the Ca2+ ions may be chelated by three oxygens because the chain torsion may not be complete. The increased torsion and the bichelating mechanism would thus result in a stronger binding explaining thus the decrease of the asymmetric COO- stretching frequency with MW. But further investigations are needed in order to confirm this hypothesis. On the other hand, the transient intermediates show also a regular trend with molar mass. The polymer with lowest molar mass shows two kinds of structures, i.e., a unidentate (b1) and a pseudobridge with water (b2) coexisting in solution. For the medium molar mass the same intermediates as in the low molar mass case are first formed. After a certain time lapse a different spectral intermediate is detected. The assignment of this species is not straightforward: either a mixed pseudobridge bridge with Na+ (b3) or a pseudobridge with water (b2) with tighter interaction with Ca2+ and COO- can explain the data. The latter intermediate type is directly formed without going through a unidentate for the highest molar mass sample. In fact, on inspection of the kinetic curves, in the medium molar mass sample the later intermediates seem to be produced independently from the earlier ones. In the medium molar mass case the mechanisms typical of both high and low molar mass systems seem to occur simultaneously. Furthermore, it is important to note that the formation of the intermediate that is typical of the higher molar mass sample is connected to faster reaction kinetics and to a stronger binding in the final complex. The mixed Na+-Ca2+ pseudobridge (b3) could explain this effect because this structure arises from coordination of Ca2+ without former exchange of Na+. Formation of such a mixed pseudobridge is supported by the incorporation of a small fraction of Na+ ions into the Ca precipitate observed at the precipitation threshold in saline solutions with a content as low as about 0.01 M NaCl.8 However, also a pseudobridge structure in which Ca2+ approaches more closely the COO-, thus leading more easily to the product, cannot be ruled out. In both cases positive ions are increasingly able to interact with the COO- in higher molar mass polymers, probably because a larger number of intramolecular conformational degrees of freedom is available. The above hypothesis is supported by the observation that, when Ca2+ is strongly bound to polyacrylates in sufficient concentration along the polymer, conformational changes take place.11,32,33 Concerning the precipitation equilibrium we can infer that the unidentate and pseudobridge complexes are still soluble while the bidentate precipitates. This can be well explained by the molecular model proposed here since in a monodentate or a pseudobridge complex the water belongs to the inner (32) Ikeda, Y.; Beer, M.; Schmidt; M.; Huber, K. Macromolecules 1998, 31, 728. (33) Schweins, R.; Huber, K. Eur. Phys. J., E 2001, 5, 117.
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Figure 4. Reaction scheme: initial Na+ complex ionic bidentate (a); intermediate Ca2+ complexes, unidentate (b1), pseudobridge with H2O (b2), pseudobridge with Na+ (b3); final Ca2+ complex chelating bidentate (c).
COO- coordination and participate to the whole COOstretching vibration, while in a monochelating and even more in a bichelating structure, the water molecules are far apart, thus decoupling their vibrations from those of the polymer groups. Summary The characterization of Ca2+-polyacrylate complexes and the intermediates leading to their formation have
been achieved by the dialysis ATR FTIR experimental technique, which is sensitive to very low concentrations of the transient species in water. Through the analysis of the COO- asymmetric stretching vibrations the molecular structure of intermediates and final complexes have been assigned and a mechanism has been proposed to explain the Ca2+ binding to polyacrylates. LA030354E
