Structure of Binary and Ternary Complexes Formed by Sodium Poly (2

Alexander T. Dembo, Kirill A. Dembo, Vladimir V. Volkov, Alexander I. Kokorin, Alexei A. Lyubimov, Eleonora V. Shtykova, Serguei G. Starodoubtsev, and...
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Langmuir 2003, 19, 7845-7851

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Articles Structure of Binary and Ternary Complexes Formed by Sodium Poly(2-acrylamide-2-methyl-1-propanesulfonate) Gel in the Presence of Copper(II) Nitrate and Cetylpyridinium Chloride Alexander T. Dembo,§ Kirill A. Dembo,§ Vladimir V. Volkov,§ Alexander I. Kokorin,‡ Alexei A. Lyubimov,† Eleonora V. Shtykova,§ Serguei G. Starodoubtsev,† and Alexei R. Khokhlov*,† Physics Department, Moscow State University, Leninsky Gory, Moscow 119992, Russia, Institute of Chemical Physics, Russian Academy of Sciences, Kosygina 4, Moscow 119991 Russia, and Institute of Crystallography, Russian Academy of Sciences, Leninsky Pr. 59, Moscow 117333, Russia Received September 30, 2002. In Final Form: June 30, 2003 Binary and ternary interactions of copper(II) nitrate and cetylpyridinium chloride with polyelectrolyte gel of sodium poly(2-acrylamide-2-methyl-1-propanesulfonate) (PAMPS) were studied. In contrast with sodium and calcium chlorides, the addition of copper(II) nitrate to the solution leads to additional shrinking of the gel, while in the presence of the surfactant the gel collapses. In the region of low concentration the composition of the PAMPS-copper(II) complexes is close to stoichiometric. In the highly concentrated solutions of copper(II), the salt concentration becomes the same as in the surrounding solution. ESR study demonstrates two states of the copper(II) ions in the gel phase, namely the ions that are bound to the sulfonate groups of the polymer network, and the free hydrated ions. The competition interaction of cationic surfactant cetylpyridinium chloride with the anionic network results in significant decrease of the fraction of the free copper(II) ions in comparison with bound ions. SAXS study shows that, starting from some critical concentration of cetylpyridinium chloride (ca. 0.005 M) in the solution, an interaction of anionic gel and the cationic surfactant yields a nanocomplex with a complicated structure. Addition of copper nitrate to the complex leads to competition substitution of the surfactant by copper ions from the gel. At the same time the ions of low molecular weight salt can enter the structure of the polyelectrolyte gelsurfactant complex.

Introduction Negatively charged linear polyelectrolytes and polyelectrolyte gels are known to interact strongly with biand multivalent cations. In aqueous solutions interaction with metal ions of valence 2 and higher often causes precipitation of linear polyelectrolyte or the collapse of charged gel. These phenomena are of great practical importance, and they were intensively studied both experimentally1-11 and theoretically.10-16 The attraction * To whom correspondence should be addressed. † Moscow State University. ‡ Institute of Chemical Physics, Russian Academy of Sciences. § Institute of Crystallography, Russian Academy of Sciences. (1) Michaeli, I. J. Polym. Sci. 1960, 48, 291. (2) Ikegami, A.; Imai, N. J. Polym. Sci. 1962, 56, 133. (3) Hansson, Z. J. Phys. Chem. B 2002, 106, 9777. (4) Kabanov, N. M.; Kokorin, A. I.; Rogacheva, V. B.; Zezin, A. B. Polym. Sci. U.S.S.R. 1979, 21, 230. (5) Ricka, J.; Tanaka, T. Macromolecules 1985, 18, 83. (6) Narh, K. A.; Keller, A. J. Polym. Sci., Part B: Polym. Phys. 1993, 31, 231. (7) Delsanti, M.; Dalbiez, J. P.; Spalla, O.; Belloni, L.; Drifford, M. In Macro-Ion Characterization from Dilute Solutions to Complex Fluids; Schmitz, K. S., Ed.; ACS Symposium Series 548; American Chemical Society: Washington, DC, 1994; p 195. (8) Bloomfield, V. A. Biopolymers 1991, 31, 1471. (9) Pelta, J.; Livolant, F.; Sikorav, J.-L. J. Biol. Chem. 1996, 271, 5656. (10) Olvera de la Cruz, M.; Belloni, M. L.; Delsanti, M.; Dalbiez, J. P.; Spalla, O.; Drifford, M. J. Chem. Phys. 1995, 103, 5781.

between charged chains can originate from the crosslinking of polymer molecules due to complex formation between the ions of a metal and the functional groups of a macro-ion.3,4 The short-range attraction between polyelectrolyte molecules can be a result of concentration fluctuations of condensed counterions.13,14 A model of “ion bridging” was developed by Olvera de la Cruz et al.10 This model is based on the assumption of random alternation of negative and positive charges along the polyelectrolyte chain. In another model the surface of the counterion shell is composed of a periodic alternation of positive and negative charges.15 A more recent study was based on a new theory of screening of a macro-ion by multivalent cations. In this theory the attraction between the charged rodlike chains arises from strong cation correlation at its surface.16 The investigation of the interaction between the charged networks and the ions of a metal is of great practical (11) Raspaud, E.; Olvera de la Cruz, M.; Sikorav, J.-L.; Livolant, F.Biophys. J. 1998, 74, 381. (12) Ray, J.; Manning, G. S. Langmuir 1996, 10, 2450. (13) Oosawa, F. Polyelectrolytes; Marcel Dekker: New York, 1971; p 123. (14) Barrat, J.-L.; Joanny, J.-F. In Advances in Chemical Physics; Prigogine, I., Rice, S. A., Eds.; John Wiley and Sons: New York, 1996; Vol. XCIV, p 16. (15) Rouzina, I.; Bloomfield, V. A. J. Phys. Chem. 1996, 100, 9977. (16) Nguyen, T. T.; Rouzina, I.; Shklovskii, B. I. J. Chem. Phys. 2000, 112, 2562.

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importance. The specific features of the distribution of metal ions in the charged gel should play an important role in the processes of metal nanoparticle formation in the volume of the gel.17-22 The interaction between polyelectrolytes and metal ions has a complex nature. The degree of dissociation of polyelectrolytes with weak acidic groups, such as carboxylic groups, is strongly affected by the change in pH, ionic strength, etc. Moreover, polymers with such groups are known to be effective polymer ligands and form strong polymer-metal complexes with the ions of transition metals, for instance with Cu(II) ions.3,4 On the other hand, charged polymers that are composed of strong poly(acid) salts, such as alkaline salts of poly(sulfonic) or poly(sulfo) acids, are less sensitive to the external conditions and the polymer-metal complexes in such systems are weaker. Our knowledge about the interaction between polyelectrolyte ligands and metal cations is far from being complete. In most cases the detailed structure of the complexes formed by metal ions with polymer ligand groups and molecules of the solvent is unknown. Such information can be obtained by use of paramagnetic metal ions together with electron spin resonance (ESR) technique. In this paper we have investigated the interaction of polyelectrolyte gels of sodium poly(2-acrylamide-2methyl-1-propanesulfonate) (PAMPS) with copper(II) nitrate. The formula of PAMPS is

Gels based on PAMPS and copolymers of the monomer AMPS with acrylamide and its derivatives are rather new polyelectrolyte gels, which are successfully used in many studies. Using the electron spin resonance (ESR) technique, we shall demonstrate that in aqueous media sulfonate groups of the polymer form complexes with copper(II) ions. The essential feature of the complexes is that the metal ion coordinates only with one polymer ligand group. Due to this, one group coordination does not lead to pronounced collapse of the gel. The other goal of this work was to study more complicated ternary systems, which additionally include the cationic surfactant. Polyelectrolyte gels are known to form stable complexes with oppositely charged surfactants.23,24 Due to aggregation of long hydrocarbon residues, these (17) Lopez, D.; Cendoya, I.; Mijangos, C.; Julia, A.; Ziolo, R.; Tejada, J. Macromol. Symp. 2001, 166, 173. (18) Milan, A.; Palacio, F. Appl. Organomet. Chem. 2001, 15, 396. (19) Bronstein, L. M.; Platonova, O. A.; Yakunin, A. N.; Yanovskaya, I. M.; Valetsky, P. M.; Dembo, A. T.; Makhaeva, E. E.; Mironov, A. V.; Khokhlov, A. R. Langmuir 1998, 14, 252. (20) Svergun, D. I.; Shtykova, E. V.; Dembo, A. T.; Bronstein, L. M.; Platonova, O. A.; Yakunin, A. N.; Valetsky, P. M.; Khokhlov, A. R. J. Chem. Phys. 1998, 109, 11109. (21) Svergun, D. I.; Shtykova, E. V.; Kozin, M. B.; Volkov, V. V.; Dembo, A. T.; Bronstein, L. M.; Platonova, O. A.; Yakunin, A. N.; Valetsky, P. M.; Khokhlov, A. R. J. Phys. Chem. B 2000, 104, 5242. (22) Svergun, D. I.; Kozin, M. B.; Konarev, P. V.; Shtykova, E. V.; Volkov, V. V.; Chernyshov, D. M.; Valetsky, P. M.; Bronstein, L. M. Chem. Mater. 2000, 12, 3552. (23) Khokhlov, A. R.; Kramarenko, E. Yu.; Makhaeva, E. E.; Starodubtzev, S. G. Macromolecules 1992, 25, 4779. (24) Khokhlov, A. R.; Starodubtzev, S. G.; Vasilevskaya, V. V. Adv. Polym. Sci. 1993, 109, 123.

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complexes usually have highly ordered structure with different symmetry.25-28 The highly ordered polyelectrolyte gel-surfactant complexes (PSC) were successfully used as templates for synthesis of metal nanoparticles.19 The addition of inorganic salt is known to lower the stability of PSC.23,24 On the other hand, the ions of low molecular weight salt together with surfactant ions can enter the structure of PSC. The surfactant ion-counterion pairs can fit the structure of the complexes and promote the formation of more ordered structures. The additional ordering of the structure of PSC in the presence of inorganic salts was observed earlier only for the case when the ions of the salt were univalent.29,30 We will investigate the state of copper(II) ions in the gel and the structure of the PSCs. In this paper, small-angle X-ray scattering (SAXS) is used to study the structure of self-organized gel complex with a cationic surfactant, cetylpyridinium chloride (CPC), in the presence of copper(II) nitrate. SAXS is a nondestructive method that requires no special sample preparation and provides structural information for polymer systems in aqueous environment. The scattering technique has already been successfully employed to investigate the internal structure and metal nanoparticle formation in several gel-surfactant systems.19-22,31-33 Experimental Section Sample Preparation. N,N′-Methylene(bis)acrylamide (BAA), ammonium persulfate (PS), N,N,N′,N′-tetramethylethylenediamine (TEMED), and cetylpyridinium chloride (CPC) were purchased from Fluka Chemika-Biochemika Co., AMPS was obtained from Lancaster Synthesis Inc., UK. Copper(II) nitrate trihydrate was of “chemical pure” grade (99.8 wt %). Water of Millipore quality was used for the preparation of the solutions. The gels were prepared by free-radical copolymerization in the solutions of monomers. The total concentration of AMPS and BAA in the solution was 30 wt %; the fraction of BAA was 0.5 mol %. Before copolymerization the AMPS monomer solution first of all was neutralized by sodium bicarbonate up to neutral pH. To 10 mL of the solution of monomers, 5 µL of TEMED and 50 µL of 10 wt % PS were added. Copolymerization was carried out in glass tubes 3.8 mm in diameter at 37 °C for 24 h. The gel of sodium poly(acrylate) (SPA) was synthesized as described in ref 34. The prepared gels were removed from the tubes swollen in water, cut into disks of ca. 2 mm thickness, and washed with a large excess of distilled water for 2 weeks. The water was changed every 2-3 days. After that the gels were dried in an air thermostat at 37 °C for 3 days. The dried samples were kept in closed vials. The gels were dried at 90 °C up to constant weight before use. Samples were prepared by immersing the dried disks of copolymers of known weight (3-5 mg) in closed weighted vials with calculated amounts of stock solutions of copper(II) salt. The (25) Khandurina, Yu. V.; Rogacheva, V. B.; Zezin, A. B.; Kabanov, V. A. Polym. Sci. 1994, 36A, 184. (26) Okuzaki, H.; Osada, Y. Macromolecules 1994, 27, 502. (27) Khandurina, Yu. V.; Alexeev, V. L.; Evmenenko, G. A.; Dembo, A. T.; Rogacheva, V. B.; Zezin, A. B. J. Phys. II 1995, 5, 337. (28) Chu, B.; Yeh, F.; Sokolov, E. L.; Starodoubtsev, S. G.; Khokhlov, A. R. Macromolecules 1995, 28, 8447. (29) Mironov, A. V.; Starodoubtsev, S. G.; Khokhlov, A. R.; Dembo, A. T.; Yakunin, A. T. Macromolecules 1998, 31, 7698. (30) Mironov, A. V.; Starodoubtsev, S. G.; Khokhlov, A. R.; Dembo, A. T.; Yakunin, A. T. Colloids Surf., A: Physicochem. Eng. Asp. 1999, 147, 213. (31) Shtykova, E. V.; Dembo, A. T.; Makhaeva, E. E.; Khokhlov, A. R.; Evmenenko, G. A.; Reynaers, H. Langmuir 2000, 16, 5284. (32) Evmenenko, G. A.; Theunissen, E.; Reynaers, H. J. Polym. Sci., Part B: Polym. Phys. 2000, 38, 2851. (33) Bronstein, L. M.; Platonova, O. A.; Yakunin, A. N.; Yanovskaya, I. M.; Valetsky, P. M.; Dembo, A. T.; Obolonkova, E. S.; Makhaeva, E. E.; Mironov, A. V.; Khokhlov, A. R. Colloids Surf., Part A 1999, A147, 221. (34) Novoskol’tseva, O. A.; Krupenina, T. V.; Sul’yanov, S. N.; Bel’chenko, N. N.; Rogacheva, V. B.; Zezin, A. B.; Kabanov, V. A. Polym. Sci. 1997, A39, 760.

Interaction of PAMPS Gels and Cu(NO3)2 final volume of the solution was 400 L per 1 mol of the charged groups of the network. Thus, the average concentration of nitrate anions in the solution was 0.005 M. Before measurements the samples were equilibrated at 25 °C for 2 weeks. The degree of swelling of the gel samples was characterized by the ratio F ) meq/md, where meq is the weight of the gel after reaching equilibrium and md is the weight of the dry gel. The composition of the complexes of the gels with Cu(II) ions was characterized by the charge ratio Q between the total number of charges of the ions of the metal Qm and the number of the oppositely charged ions of the network Qn. Here Qn is the number of monomer units of the gel; Qm is 2 times the number of Cu(II) ions in the gel. The amount of Cu(II) ions absorbed by the gel was determined by Cu extraction in 0.1 M solution of EDTA. The amount of solution was ca. 100-200-fold higher than the weight of the gel. The concentration of Cu(II) ions in the extraction solution was determined as the concentration of the complexes of Cu(II) with EDTA using spectrophotometry at 752 nm. Optical density was measured by a Hewlett-Packard 8452 spectrophotometer. Water content of the gels was determined gravimetrically after drying the swollen samples up to constant weight at 90 °C. ESR Study. ESR spectroscopy allows identification of the composition and the structural particularities of the coordination sphere of copper(II) complexes as well as characterization of their spatial distribution and measurement of their local concentration. This information can usually be obtained from the analyses of the anisotropic ESR spectra of vitreous solutions frozen at 77 K. The ESR spectra were recorded with a Varian E-3 X-band spectrometer in quartz tubes of 4 and 1 mm diameter at 77 and 298 K. The magnetic field was graduated by the spectra of Mn2+ in the MgO matrix and by the DPPH signal with g0 ) 2.0036. The spin-Hamiltonian parameters g|, g⊥, and g0 and hyperfine constant A| have been calculated from the ESR spectra according to recommendations of ref 35. The amount of the ESR active adsorbed Cu(II) ions has been estimated by double integration of ESR spectra of the samples and its comparison with the reference spectra (single crystal of CuSO4‚5H2O with a known number of spins). Subtraction of the experimental ESR spectra and their calculations has been carried out using the original software developed by Prof. A. Vorob’ev (Chemistry Department, Moscow State University).36 X-ray Scattering. SAXS patterns provide structural information about nanoscale inhomogeneities (particles or clusters) and about the internal ordering in the sample.37 The size and sometimes the shape of the nanoparticles can be determined from the initial part of the scattering pattern close to the primary beam (central scattering). The internal order leads to the appearance of characteristic Bragg peaks, which allow us to determine the periodicity in the investigated system. Small-angle X-ray scattering measurements were performed with the use of the diffractometer “AMUR-K”38 at the wavelength λ ) 0.1542 nm. Kratky-type (infinitely long slit) geometry was used with a sample-to-detector distance of 673 mm and with an entrance slit width of 0.2 mm to cover the range of momentum transfer 0.12 < q < 5.5 nm-1 (q ) 4π sin θ/λ, where 2θ is the scattering angle). Windows of the sample holder have poly(ethylene terephthalate) foils of 0.01 mm thickness. The thickness of the samples (about 1 mm) varied along the length of the sample holder (10 mm), so no absolute calibration was done. The data were normalized to the intensity of the incident beam, corrected for the detector response and for background scattering and desmeared following standard procedures.37 The characteristics of the ordered regions in the gels were calculated from the Bragg peaks in the scattering patterns by fitting the interactively selected peaks by Gaussian profiles using (35) Zidomirov, G. M.; Lebedev, Ya. S.; Dobryakov, S. N.; Shteinshneider, N. Ya.; Chirkov, A. K.; Gubanov, V. A. Interpretation of Complex ESR Spectra; Nauka: Moscow, 1975 (in Russian). (36) Vorob’ev, A. Kh. Kinetics of Some Chemical Reactions in Solids. D.Sc. Thesis, Chemistry Department, M. Lomonosov Moscow State University, Moscow, 1995 (in Russian). (37) Feigin, L. A.; Svergun, D. I. Structure Analysis by Small-Angle X-ray and Neutron Scattering; Plenum Press: New York, 1987. (38) Mogilevsky, L. Yu.; Dembo, A. T.; Svergun, D. I.; Feigin, L. A. Kristallografia 1984, 29, 587.

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Figure 1. Dependence of swelling ratio, F, of PAMPS (1-3) and SPA (4) gels on concentration, c, of NaCl (1, 2), CaCl2 (2, b), and Cu(NO3)2 (3, 9; 4, 1) in water. the program PEAK (Konarev and Svergun, unpublished). The mean long-range order dimension L in the gel-surfactant complexes was estimated from the Bragg peaks on the SAXS patterns by the Scherrer formula:39

L)

λ βs cos θ

(1)

where βs is the full width at half-maximum intensity of the peak (in radians) observed at a mean scattering angle of 2θ. The radius of interaction rm and the degree of disorder in the system ∆/d h were calculated as39

rm )

2

(2.5π ) βλ

(2)

x

(3)

s

∆/d h)

1 π

βsd h λ

where d h ) 2π/qmax is the characteristic size in the ordered regions in the gel-surfactant complex and ∆ is the mean-square deviation of distances between the neighboring regularly packed molecules.

Results Composition and Swelling Behavior of the Complexes. The swelling of the charged network depends on the osmotic pressure of counterions. The osmotic pressure of counterions is most pronounced in the absence of salt, while at a high ionic strength the degree of swelling of the network approaches that of the neutral gel.24 Curve 1 in Figure 1 shows the plot of the swelling ratio F vs the concentration of sodium chloride c for PAMPS gel. The value of F decreases strongly with the increase of the concentration of sodium chloride. At salt concentration higher than 1.0 M the changes of F tend to stop. In the case of calcium(II) chloride the decrease of the swelling ratio is more pronounced in comparison with sodium chloride (Figure 1, curve 2). As will be discussed below, this effect is explained by a stronger decrease of the osmotic pressure in the charged gel in the presence of oppositely charged divalent ions.40,41 The ions of copper(II) are known to form complexes with different inorganic and organic ligands. A comparison of F vs c plots for calcium(II) and copper(II) salts shows that in the wide range of the concentration the values of the swelling ratio of the gel in the solution of copper(II) salt are lower (Figure 1, curve 3). Moreover, the dependence (39) Vainshtein, B. K. Diffraction of X-rays by chain molecules; Elsevier Publishing Company: Amsterdam-London-New York, 1966. (40) Ricka, J.; Tanaka, T. Macromolecules 1984, 17, 2916. (41) Ohmine, I.; Tanaka, T. Macromolecules 1985, 18, 83.

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Figure 2. Dependence of PAMPS-Cu complex composition, Q, on concentration, c, of copper(II) nitrate in water-methanol solution with 30 vol % methanol.

of the swelling ratio, F, on the concentration of copper(II) salt passes through a minimum. The dependence of the composition Q of the PAMPSCu(II) complexes was studied in water and in 30 vol % methanol. Methanol was added in the system to prevent the crystallization of water under freezing at 77 K. Otherwise, spatial distribution of Cu(II) complexes could become different compared with the original one. Nevertheless, in both media the obtained results were very similar. Figure 2 shows the dependence of the charge composition Q of the PAMPS-Cu(II) complexes on the concentration of copper(II) nitrate in water-methanol solution containing 30 vol % methanol. It can be seen that in the wide range of concentration the composition of the complex is close to stoichiometric by charge. At a very high salt concentration the content of copper(II) ions in the gel phase increases and the concentrations of Cu(II) ions in the gel phase and in the solution equalize. It should be interesting to compare the properties of PAMPS-Cu(II) complexes with the properties of the wellknown complexes formed by Cu(II) ions with sodium poly(acrylate) (SPA).3,4 These complexes usually include two carboxylate anions in the first coordination sphere of Cu(II) ion. Curve 4 in Figure 1 shows that the values of F for the complexes of Cu(II) with SPA are much lower in comparison with those obtained for PAMPS-Cu(II) complexes. Moreover, the gel of SPA-Cu(II) complex is very rigid while the other gels obtained in the study demonstrate elastic properties. ESR Study. Typical ESR spectra of Cu(II) ions in the samples of the swollen PAMPS gels at different copper(II) salt concentration c in the solution are shown in Figure 3. At low concentrations, i.e., c e 0.01 M, all Cu(II) ions are randomly distributed in the sample, and their spectrum with a well-resolved hyperfine structure (hfs) (type A; Figure 3, curve 1) is described by the effective spin-Hamiltonian parameters A| ) 119.5 ( 2 G, g| ) 2.415 ( 0.004, and g⊥ ) 2.090 ( 0.003. These values are typical for copper(II) complexes with ligands containing oxygen atoms, and with the structure of a slightly elongated octahedron.42 Our parameters are close to that ones measured in refs 43 and 44 in the investigation of Cu(II) absorption by the cation-exchange resins KU-1 and KU-2 containing sulfo groups. (42) Al’tshuller, S. A.; Kozyrev, B. M. ESR of Compounds of the Elements of the Intermediate Groups; Khimiya: Moscow, 1972 (in Russian). (43) Vishnevskaja, G. P.; Saphin, R. Sh.; Molochnikov, L. S. Mol. Phys. 1977, 34, 1329. (44) Vishnevskaja, G. P.; Molochnikov, L. S.; Saphin, R. Sh. ESR in Ionites; Nauka: Moscow, 1992 (in Russian).

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Figure 3. ESR spectra for PAMPS-Cu complexes prepared at 0.01 M (1), 0.3 M (2), and 1.0 M (3) copper(II) nitrate in water-methanol solution with 30 vol % methanol and comparison of multiplet parameters (5) for curves 1 and 3. Subtracted spectrum of the “aggregated” copper(II) ions (4).

Comparing the data of the gel swelling and the ESR data, we can make some assumptions about the properties of Cu(II) coordination in the gel. One fraction of copper(II) ions coordinates with sulfonate groups of the network. This coordination leads to some additional gel shrinking in comparison with Ca(II) ions. However, the shrinking of the gel is only slightly pronounced in comparison with the case of SPA gel. Thus, knowing that sulfonic groups are weak ligands for Cu(II) ions, we can assume that coordination with the sulfonic group occurs only in one coordination site, namely on one of the tops of the octahedron. At c > 0.03 M (Figure 3, curves 2 and 3), the ESR spectra exhibit a superimposition of such a “multiplet” spectrum (type A) and a broad (∆H ) 245 ( 10 G) asymmetric single line (type B; geff ) 2.16 ( 0.01). The contribution of the latter in the spectrum arises with increasing Cu(II) content in the sample. A type B spectrum is usual for Cu(II) complexes with strong dipole-dipole and spin-exchange interactions between them, i.e., for a case of very high local concentration of copper(II) ions (“aggregated” complexes).44-46 Drying of the samples to the “air-dried” condition evidently demonstrates this for a sample with c equal to 0.01 M. It can be also seen (inset 5 in Figure 3) that line width and shape of individual components (for parallel hfs) of the type A spectrum remains practically unchanged with the increase of c. The latter result allows us to conclude that such complexes are separated in the sample and there is no aggregation among them. Figure 4 shows the dependences of the total amount of paramagnetic Cu(II) complexes [Cu]ESR in the PAMPS gel, as well as the dependences of the isolated (type A) and aggregated (type B) Cu(II) ions on the copper(II) nitrate concentration c in the solution. One can see that the dependence of the total number of Cu(II) ions on c is similar to the analogous plot presented in Figure 2. Thus, practically all copper(II) ions exist in PAMPS gel in the state of Cu(II) ions and are detected by the ESR technique. Special computer analysis has been done: a spectrum of the “aggregated” copper (Figure 3, curve 4) has been subtracted with the appropriate coefficient from the experimental ESR spectra. Then, we compared the double integrals calculated for both fractions (a single line and a subtracted type A multiplet) with that one, calculated for the experimental ESR spectrum. The obtained results show (Figure 4) that in a wide range of copper concentra(45) Molin, Yu. N.; Salixov, K. M.; Zamaraev, K. I. Spin Exchange; Springer: Berlin, 1980. (46) Kokorin, A. I.; Shubin, A. A. Chem. Phys. 1991, 7, 1770.

Interaction of PAMPS Gels and Cu(NO3)2

Figure 4. Concentration of “bound” (2) and “free” (3) copper(II) ions and total (1) concentration of copper(II) ions in the gel versus concentration of copper(II) nitrate in water-methanol solution with 30 vol % methanol.

Figure 5. ESR spectra for PAMPS-Cu complexes, prepared in 0.2 M aqueous solutions of copper(II) nitrate (1-3), in 0 M (1), 0.0025 M (2), and 0.0114 M (3) aqueous solution of CPC. Curve 4 is equal to curve 3 minus k × curve 1, where k is a normalizing coefficient.

tion, 0.01 < c < 1.0 M, the content of Cu(II) type A complexes remains constant while the amount of the “aggregated” (type B) complexes noticeably increases with the increase of c. All the results presented above allowed us to assume that the type A signal corresponds to Cu(II) ions, bound to the sulfonate groups of PAMPS gel. The amount of these groups does not depend on the total copper content in the volume of the gel. On the other hand, type B signal relates to “free” Cu(II) ions. Coincidence of the ESR spectra at 298 K for samples with c > 0.1 M with the spectra of Cu(NO3)2 solutions in the same solvent and with the same copper concentration confirms our interpretation. The observed poorly resolved ESR spectrum of Cu(II) ions at used values of c can be explained by the fact that a solvent CH3OH:H2O ) 3:7 is not a glassy one, and after its freezing the partial exclusion of the Cu(II) aqua complexes occurs into a separated microphase with high local concentration. Analogous behavior has been observed for the vanadyl VO2+ aqua complexes in ref 47. Competition Complex Formation of PAMPS with Cu(II) and CPC Cations. ESR spectra of the samples of PAMPS gel containing Cu(II) ions after the addition of a certain amount of CPC are shown in Figure 5. The efficiency of the spin-exchange interaction between Cu(II) ions decreases with increasing CPC concentration. This effect is manifested by the broadening of the initial single line (spectrum 1 in Figure 5) and by the appearance of the (47) Lebedev, Ya. S.; Muromtsov, V. I. ESR and Relaxation of the Stabilized Radicals; Khimiya: Moscow, 1971 (in Russian).

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Figure 6. Dependence of “free” (1) and “bound” (2) copper(II) ion quantity on concentration of CPC in 0.2 M solutions of copper nitrate.

Figure 7. X-ray intensity profiles from ternary PAMPS-CPCCu(II) (1-4) complexes and PAMPS-CPC (5) complexes in 0 M (1), 0.0025 M (2), 0.005 M (3), and 0.01 M (4, 5) aqueous solution of CPC in the presence (1-4) and absence (5) of 0.2 M solution of copper nitrate during preparation.

spectrum type A components (spectrum 4 in Figure 5). Spectrum 4 was obtained by the PC subtraction of spectrum 1 from spectrum 3 in Figure 5. Relative double integrals of both parts of the superimposed experimental spectra are plotted in Figure 6. It can be seen that part of the Cu(II) complexes coordinated with PAMPS practically did not change with the addition of CPC, while the amount of the Cu(II) aqua complexes in the PAMPS gel noticeably decreased. These results demonstrate that the ions of CPC replace mainly “free” Cu(II) ions from the gel, while the bound ions are keeping by the charged network. SAXS Study. Figure 7 presents experimental scattering data from the PAMPS gel-surfactant systems (for better visualization, the successive curves are artificially vertically shifted). Interaction of the polyelectrolyte gel with oppositely charged copper(II) ions in 0.2 M copper(II) salt solution does not lead to a collapse. The corresponding scattering curve (Figure 7, curve 1) has no significant central scattering and no characteristic peaks. On the other hand, interaction of an anionic PAMPS gel with a cationic surfactant CPC at 0.01 M concentration yields a nanocomplex with a complicated structure, as is manifested by some central scattering and central peak positioning at q ) 1.36 nm-1. The peak has two shoulders at ca. 1.21 and 1.50 nm-1. The analogous SAXS profiles were recently observed for the complexes formed between PAA and cetyltrimethylammonium chloride.48 They manifest the formation of cubic structure. Structural parameters characterizing the internal order in the PAMPS-CPC systems can be calculated from

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Dembo et al.

Table 1. Characteristics of the Internal Order in the PAMPS-CPC-Cu(II) Systems d h L rm qmax (nm-1) (nm) (nm) (nm) ∆/d h

sample PAMPS-0.01 M CPC PAMPS-CPC (0.01 M)-Cu(II) PAMPS-CPC (0.005 M)-Cu(II)

1.37 1.22 1.15

4.6 5.1 5.5

96.3 152.1 0.07 47.2 77.5 0.11

individual Bragg peaks in the SAXS patterns. Theoretically, the average long-range order dimension L representing the size of quasi-crystalline regions in the sample, and the radius of interaction rm reflecting maximum separation between spatially correlated structural motifs, should increase with increasing order in the system. In contrast, relative mean square deviation of the distance between the neighboring periodic motifs ∆/d h (degree of disorder) should decrease if the system becomes more ordered. The structural parameters are given in Table 1. The addition of the copper(II) salt changes the initial structure of the PSC and yields another one with one maximum (Figure 7, curve 4). For the PAMPS-CPCCu(II) complex the average distance between the ordered structural motifs d h increases from 4.6 to 5.14 nm after the addition of the 0.2 M copper(II) salt. The decrease of the amount of CPC in the system leads to the relative decrease of the scale L over which long-range order is presented and of the value of the radius of interaction rm. At the same time the degree of disorder in the system ∆/d h increases at the same salt concentration (Figure 7, curves 3 and 4; Table 1). Thus, the main factor responsible for the formation of the ordered structure in the studied systems is CPC. However, the amount of CPC molecules has to be enough to form self-organized nanostructures. SAXS study shows that in the present of 0.2 M copper(II) salt the formation of the ordered structures starts from 0.005 M concentration of CPC. At lower concentration of the surfactant, ordering in the system has not been detected (Figure 7, curve 2). The inclusion of CPC into the system leads to formation of “bridges” between charged groups of the polymer and the headgroups of CPC molecules, that is, the formation of bilayers detected by the SAXS method. Hydrophobic tails of CPC overlap to reduce a contact area with water in the system. The van der Waals size of the CPC molecule is 2.6 nm. Taking into account that periodic structures formed by CPC bilayers have a period approximately equal to 4.6 nm, the overlap is about 0.6 nm. The addition of copper(II) nitrate leads to rearrangement of the internal structures and to the increase of the periodicity of the complexes from 4.6 to 5.1-5.5 nm (Table 1). The last result can be explained by the fact that ions of copper(II) nitrate can enter the structure of PSC. As has been found in our previous investigations, the ions of inorganic salts can be incorporated into the highly ordered structures of PSC.19,29,30,33 The analysis of the SAXS profiles demonstrates that the presence of Cu(II) and Cl- ions in the gel apparently rearranges the structure of the PSC. On the other hand, the copper(II) ions compete with the surfactant cations for the electrostatic favorable contacts with the sulfonate anions of the network. Discussion Interaction of PAMPS with Metal Salts. Let us analyze qualitatively the change in the free energy of the charged network after the addition of bivalent salt to the (48) Hansson, P.; Schneider, S.; Lindman, B. J. Phys. Chem. B 2002, 106, 9777.

Figure 8. Simplified scheme of sodium (a) and copper(II) (b)(e) counterion states in PAMPS gel.

solution. The change in the free energy of such system can be represented as a sum of four terms:

∆F ) ∆Fel + ∆Fint + ∆Ftran + ∆Fc

(4)

Here ∆Fel describes the change in the elastic free energy; ∆Fint is the change in the free energy of the interactions between polymer segments and solvent molecules; ∆Ftran is related to the change in the translational entropy of the counterions of the network, the charged network, and 1,1salt; ∆Fc describes the change in the free energy of Coulomb interaction between the ions.33 Let us discuss the possible effects of univalent and bivalent metal salts on the swelling behavior of anionic gel. The simplified models of the gel with counterions in salt-free solution are listed in Figure 8. The swelling ratio of the gel with mobile univalent counterions is very high due to osmotic pressure of the counterions of the network (Figure 8a). On the other hand, at a very high ionic strength created by sodium chloride, the swelling ratio of polyelectrolyte network approaches that of the neutral gel. In the presence of bivalent cations, which do not form complexes with anionic groups of the network (Figure 8b), the osmotic pressure in the gel is significantly lower but the term ∆Ftran is still predominant. The other limiting case, when all of the bivalent cations are attached to the charges of the gel, is shown in Figure 8c. Obviously, such structure is equivalent to the structure of polyampholyte gel in the isoelectric point. For this case the term ∆Ftran disappears and the gel shrinks due to Coulomb interaction of the oppositely charged polymer segments.26 The increase of the ionic strength leads to the swelling of such a collapsed gel because of the screening of the electrostatic interactions between oppositely charged polymer units. When there is a strong complex formation of two anionic ligands with one bivalent ion (Figure 8e), we should expect a collapse of the gel. Such a collapse is indeed observed when sodium poly(acrylate) reacts with copper(II) nitrate

Interaction of PAMPS Gels and Cu(NO3)2

(Figure 1, curve 4). The data of Tanaka and co-workers also have shown the strong collapse of polyacrylamide gels containing a small amount of carboxylic groups in the presence of Cu(II) salt.5 A simple model, which describes the state of Cu(II) ions in the PAMPS gel, is shown in Figure 8d. In this system some fraction of bivalent ions forms a complex with sulfonic groups of the polyelectrolyte network, inducing their overcharging, while the other fraction diffuses freely in the gel phase. This assumption is confirmed by the fact that complexes that correspond to the multiplet line in the ESR spectra are present in the gel at high enough concentration. If they were bifunctional, their formation would lead to the strong shrinking or collapse of the gel; that is not the case. On the other hand, Figure 8d formally describes the polyampholyte network in which only a part of the charges of one sign is neutralized by the charges of the opposite sign. As is known from theoretical and experimental studies, in the latter case in some regimes the swelling ratio of the gel passes through a minimum with the increase of the ionic strength.33 Such a minimum is indeed observed in Figure 1 (curve 3). Structure of Ternary PAMPS-CPC-Cu(II) Gel. The ESR data show that in the presence of CPC the fraction of “free” Cu(II) ions significantly decreases in comparison with another part of Cu(II) ions, which are coordinated with PAMPS. It should be noted that CPC replaces approximately half of the total amount of Cu(II) ions from the gel, for the case when its concentration in the solution is 4-8 times less than the concentration of copper(II) ions. As is well-known, the replacement of several low charged counterions by one multicharged ion from the gel is accompanied by a high gain in translational entropy. Thus, it can be assumed that effective replacement of Cu(II) ions by CPC cations from the gel is possible only because the ions of CPC are aggregated and the average summary charge of the aggregates is higher than the charge of the copper(II) ion. The absence of maxima on the scattering curves at these concentrations demonstrates that such aggregates are disordered and randomly distributed. It should be noted that already in this region of CPC concentration the initially transparent gel containing Cu(II) ions becomes strongly opaque. At higher concentration of CPC further growth and ordering of the surfactant aggregates occur, which is confirmed by the appearance of the maxima on the scattering curves. At

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the same time the gel becomes turbid. Thus, there is a strong tendency to segregation between the surfactantrich and the Cu(II)-rich areas in the gel phase. Here it should be noted that the value of F for the collapsed gel obtained in 0.01 M copper-free solution of CPC is equal to 3.8, while for the gel in 0.2 M copper(II) nitrate solution without CPC it is equal to 14.1. Obviously, the cross-links are freezing segregation between CPC and Cu(II) in the polymer on the microscopic level and retard the formation of the compact highly ordered polymer-surfactant complex. SAXS study shows that an interaction of the anionic gel and the cationic surfactant yields a nanocomplex with a complicated structure expressed by some central scattering and a sharp central peak with two shoulders. The shape of the scattering curve probably manifests the formation of cubic structure. Addition of copper nitrate to the complex leads to a significant rearrangement of the structure of the complex. Conclusion The results of this work demonstrate the two states of the ions of Cu(II) in the phase of PAMPS gel, those that form complexes with the charges of the network and the “free” aqua ions. The complex formation does not lead to the collapse of the network; however, it induces additional shrinking of the gel. This study also demonstrates microinhomogeneous structure of the anionic gel, which interacts at the same time with oppositely charged surfactant and with metal bivalent ions, which coordinate with the ions of the network. Previously, analogous behavior was observed for the cationic gels of poly(diallyldimethylammonium chloride), which interacted with anionic surfactant, sodium dodecyl sulfate, and iodide anions (which form a charge-transfer complex with the cations of the network).50 It can be suggested that the observed micro-inhomogeneous structure is rather common and will be detected in other ternary gel systems where one type of counterions corresponds to the charged surfactants and the other one tends to form complexes with the charges of the network. LA026626F (49) Dewar, M. J. S.; Zoebisch, E. F.; Healy, E. F.; Stewart, J. J. J. Am. Chem. Soc. 1985, 107, 3903. (50) Mironov, A. V.; Starodoubtsev, S. G.; Khokhlov, A. R.; Dembo, A. T.; Dembo, K. A. J. Phys. Chem. B 2001, 105, 5612.