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Langmuir 1998, 14, 252-259
Articles Complexes of Polyelectrolyte Gels with Oppositely Charged Surfactants: Interaction with Metal Ions and Metal Nanoparticle Formation L. M. Bronstein, O. A. Platonova, A. N. Yakunin, I. M. Yanovskaya, and P. M. Valetsky Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, 28 Vavilov St., Moscow 117813, Russia
A. T. Dembo Institute of Crystallography, Russian Academy of Sciences, 59 Leninsky Pr., 117333 Moscow, Russia
E. E. Makhaeva, A. V. Mironov, and A. R. Khokhlov* Physics Department, Moscow State University, Moscow 117234, Russia Received May 21, 1997. In Final Form: September 24, 1997X The interaction of polyelectrolyte gel/oppositely charged surfactant complexes with AgNO3 and H2PtCl6 was investigated. Three kinds of gel/surfactant complexes were studied: a complex of the anionic gel of poly(methacrylic acid) with the cationic surfactant cetylpyridinium chloride and complexes of the cationic gel of poly(diallyldimethylammonium chloride) with two anionic surfactants: sodium dodecyl sulfate and sodium dodecylbenzenesulfonate. After reduction of metal compounds by hydrazine-hydrate, sodium borohydride, or UV-irradiation, Pt and Ag metal particles embedded in the body of the hydrogel were formed. The degree of metal ion exchange was higher for the oppositely charged metal ion and the polyelectrolyte gel; i.e., Ag+ is strongly absorbed by the complex poly(methacrylic acid)/cationic surfactant, while PtCl62- ions are mainly consumed by the complex of poly(diallyldimethylammonium chloride) gel with anionic surfactants. Small-angle X-ray scattering data indicated different structural changes in the gel for the complex of an anionic gel with cationic surfactant and for complexes of cationic gel with anionic surfactants. The incorporation of the metal ions in the body of the hydrogel and the growth of metal nanoparticles was found to lead to the loss of order provided by surfactant aggregates if the distance between charged groups in the polyelectrolyte does not provide a strong hydrophobic interaction between surfactant molecules.
Introduction Complexes of polyelectrolyte gels with oppositely charged surfactants were intensively studied in recent years.1-9 This is largely because these complexes exhibit highly regular self-assembled nanostructures as revealed by X Abstract published in Advance ACS Abstracts, November 15, 1997.
(1) Khokhlov, A. R.; Starodubtsev, S. G.; Vasilevskaya, V. V. Adv. Polym. Sci. 1993, 109, 123. (2) Khokhlov, A. R.; Kramarenko, E. Yu.; Makhaeva, E. E.; Starodubtsev, S. G. Macromolecules 1992, 25, 4779. (3) Khokhlov, A. R.; Kramarenko, E. Yu.; Makhaeva, E. E.; Starodubtsev, S. G. Macromol. Theory Simul. 1992, 1, 105. (4) Khandurina, Yu.V.; Dembo, A. T.; Rogacheva, V. B.; Zezin, A. B.; Kabanov, V. A. Vysokomol. Soed. 1994, 36A, 235. (5) Okuzaki, H.; Osada, Y. Macromolecules 1995, 28, 380. (6) Chu, B.; Yeh, F.; Sokolov, E. L.; Starodubtsev, S. G.; Khokhlov, A. R. Macromolecules 1995, 28, 8447. (7) Yeh, F.; Sokolov, E. L.; Khokhlov, A. R.; Chu, B. J. Am. Chem. Soc. 1996, 118, 6615. (8) Sokolov, E. L.; Yeh, F.; Khokhlov, A. R.; Chu, B. Langmuir, in press. (9) Dembo, A. T.; Yakunin, A. N.; Zaitsev, V. S.; Mironov, A. V.; Starodubtsev, S. G.; Khokhlov, A. R.; Chu, B. J. Polym. Sci.: Phys. 1996, 34, 2893.
extensive small-angle X-ray scattering (SAXS).4-9 Although the physical reasons for the promotion of the regularity of the surfactant self-assembly in a statistically disordered polyelectrolyte gel are not yet clarified,7-9 it is expedient to use the fact of high spatial organization of the medium of gel/surfactant complexes for further applications. In particular, one of the interesting possibilities is to study the interaction of these complexes with noble or transition metal ions with the subsequent reduction of these ions to produce metal nanoparticles with high surface areas. One can then expect that the size, shape, and size distribution of the emerging nanoparticles will be controlled by the state of the surrounding polymer/ surfactant matrix, which can be easily varied by changing the external parameters. We plan subsequent studies of the catalytic properties of the resulting metal/polymer hydrogel systems which would be interesting from the viewpoint of obtaining polymer materials in an aqueous medium with regulated catalytic activity. The stability of the resulting metal colloidal particles will be ensured by embedding them in the body of hydrogel.
S0743-7463(97)00527-1 CCC: $15.00 © 1998 American Chemical Society Published on Web 01/20/1998
Complexes of Polyelectrolyte Gels
A variety of methods for metal colloid formation and stabilization exists, including formation of metal nanoparticles in polymer films,10-14 gels,15,16 microemulsions,17,18 polymer solutions,19,20 etc. A promising approach dealing with the formation of metal particles in amphiphilic block copolymer micelles has been recently proposed quite simultaneously in some research groups including ours.21-24 The well-characterized amphiphilic block copolymers of polystyrene (PS) and poly(4-vinylpyridine) (P4VP) formed micelles with a P4VP core in organic solvents, such as toluene and THF.21,22 The cores of these micelles intensively absorbed metal ions, stabilizing and controlling the size of metal colloids which were obtained upon reduction of the metal ions. This approach permitted us to prepare spherical metal particles of different size with a narrow size distribution and promising catalytic properties.22 In contrast to previous work,21,22 here we are dealing with aqueous systems. Water is the best choice as a solvent for a number of reasons, including environmental considerations, potential use for widespread catalytic reactions which take place in aqueous media, and the possibility of biological applications. In the present communication, which is the first in the series of papers implementing the outlined program, we report on the interaction of the noble metal compounds H2PtCl6 and AgNO3 with complexes of three kinds: a complex of the anionic gel of poly(methacrylic acid) (PMA) with the cationic surfactant cetylpyridinium chloride (CPC) and complexes of the cationic gel of poly(diallyldimethylammonium chloride) (PDADMACl) with two anionic surfactants: sodium dodecyl sulfate (SDS) and sodium dodecylbenzenesulfonate (SDBS). As a result of the subsequent reduction of the Pt compound by hydrazinehydrate and sodium borohydride or of the Ag salt upon UV-irradiation and reaction with sodium borohydride, we were able to obtain Pt and Ag metal particles embedded in the body of the hydrogel. For the system PMA/CPC some experiments were carried out also with CuCl2. Experimental Section Materials and Sample Preparation. PMA gel was prepared by radical polymerization of methacrylic acid in water fully neutralized with sodium hydroxide. The PDADMACl gel was prepared by radical polymerization of a 50 wt/wt % solution of diallyldimethylammonium chloride (DADMACl) in water. The cross-linking agent was N,N1-methylenebisacrylamide (one crosslink per 200 monomer units). Ammonium persulfate (concen(10) Kost, K. M.; Bartak, D. E.; Kazee, B.; Kuwana, T. J. Anal. Chem. 1990, 62, 151. (11) Saito, H.; Okamura, S.; Ishizu, K. Polymer 1992, 33, 1099. (12) Bronstein, L. M.; Mirzoeva, E. Sh.; Valetsky, P. M.; Solodovnikov, S. P.; Register, R. A. J. Mater. Chem. 1995, 5, 1197. (13) Chan, Y. N. C.; Craig, G. S. W.; Schrock, R. R.; Cohen, R. E. Chem. Mater. 1992, 4, 885. (14) .Yue, J.; Cohen, R. E. Supramol. Sci. 1994, 1, 117. (15) Snowden, M. J.; Thomas, D.; Vincent, B. Analyst 1993, 118, 1367. (16) Choi, J. M.; Shea, K. J. J. Am. Chem. Soc. 1994, 116, 9052. (17) Touroude, R.; Girard, P.; Maire, G.; Kizling, J.; Boutonet-Kisling, M.; Stenius, P. Colloids Surf. 1992, 67, 9. (18) Petit, C.; Lixon, P.; Pileni, M. J. Phys. Chem. 1993, 97, 12974. (19) Mayer, A. B. R.; Mark, J. E. Nanotechnology, Molecular Designed Materials; Chow, G.-M., Gonsalves, K. E., Eds.; ACS Symposium Series 622; American Chemical Society: Washington, 1995; p 137. (20) Hirai, H.; Nakao, Y.; Toshima, N. J. Macromol. Sci. Chem. 1978, A12, 1117. (21) Antonietti, M.; Wenz, E.; Bronstein, L.; Seregina, M. Adv. Mater. 1995, 7, 1000. (22) Seregina, M.; Bronstein, L.; Platonova, O.; Chernyshov, D.; Valetsky, P.; Hartmann, J.; Wenz, E.; Antonietti, M. Chem. Mater. 1997, 9, 923. (23) Spatz, J. P.; Sheiko, S.; Mo¨ller, M. Macromolecules 1996, 29, 3220.
Langmuir, Vol. 14, No. 2, 1998 253 tration 4.4 × 10-3 mol/L) and N,N1-tetramethylethylenediamine were the initiator and accelerator of the polymerization, respectively. After the polymerization the gel was washed with a large amount of distilled water for 3 weeks. For preparation of complexes gel/surfactant thin disks of the gel were immersed in surfactant-water solutions. The surfactants CPC (monohydrate, Aldrich), SDS (Serva), and SDBS (Fluka) were used without further purification. The molar ratio of the CPC to monomer units of PMA was 1.5. For SDS and SDBS, 2 mol of surfactant were used for each mole of PDADMACl monomer residue. For the systems PMA/CPC and PDADMACl/ SDBS 400 L of water was used per mole of the network monomer units. For the system PDADMACl/SDS 2000 L of water per mole of the network monomer units was used to provide uniform gel collapse. AgNO3, H2PtCl6‚6H2O, CuCl2‚2H2O (Reahim), NaBH4 (Reidelde Haen), and N2H4.H2O (Aldrich) were used as received. Interaction of Gel/Surfactant Complexes with Metal Ions. A piece of uniformly collapsed gel/surfactant complex was placed in a water solution of the metal compound (concentration of metal was 10-2 M; 1:1 molar ratio of metal ions to gel monomer units was used) for 3 days at room temperature. Reduction of Metal Ions in Gel/Surfactant System. Before reduction the piece of gel containing metal ions was washed more than once by water and then immersed in 1 mL of water placed into a Schlenk tube. Oxygen removal was achieved by a three-time degassing procedure in freezing/thawing cycles and finally filling the tube with argon. The reducing agent was then added. N2H4‚H2O in 10-fold excess toward metal ions was injected by a syringe through a rubber septum; NaBH4 (powder) (in the same excess) was added into the tube with argon counterflow. Reduced samples were kept in the reducing agent medium for a 24 h and washed with a large amount of distilled water for 3 days. The reduction of Ag+ ions in the complex PMA/CPC was carried out in a quartz reactor under UV-irradiation with a Hg lamp for 1 h in argon (after the degassing procedure described above). Measurements. The mass changes of gels upon a complex formation were defined as m/m0 where m is gel mass at equilibrium with the surfactant solution and m0 is mass of the initial gel sample swollen in pure water. For CPC and SDBS the amount of surfactant in the gel was characterized by θ, the molar ratio between the absorbed surfactant Ψ and the gel network monomer units. The Ψ values were calculated using the formula Ψ) (M - DVs-1), where M is the initial amount of surfactant (mol), is the extinction at the absorption maximum, Vs is the solution volume, and D is the optical density of the solution at equilibrium. The optical density was determined with a Hewlett-Packard 8452A spectrophotometer. The metal content was examined by X-ray fluorescence analysis (XRF). XRF measurements employed a VRA-30 spectrometer (Carl Zeiss Jena) with a Mo anode, a LiF crystal analyzer, and a SZ detector. The lines used for analysis were Pt LR, Ag KR, and Cu KR. Samples (for comparison and for analysis) were prepared by mixing 1 g of polystyrene with 10-20 mg of standard substances. The scattering measurements were carried out on a smallangle X-ray scattering diffractometer AMUR-K (made in the Institute of Crystallography, Russian Academy of Sciences25 ) with a linear position-sensitive detector (produced in the Institute of Nuclear Physics, Siberian Division of Russian Academy of Sciences26 ). The detector had a window of 10 × 100 mm, the range of measured angles was up to 8°. Monochromatization was achieved with a crystalline monochromator; the wavelength was 0.1542 nm. The sample holder was made from stainless steel and had windows made of poly(ethylene terephtalate) (thickness of 0.01 mm); the thickness of samples was ∼1 mm. The samples of (24) Moffit, M.; McMahon, L.; Pessel, V.; Eisenberg, A. Chem. Mater. 1995, 7, 1185. (25) Mogilevsky, L. Yu.; Dembo, A. T.; Svergun, D. I.; Feigin, L. A. Kristallographia 1984, 29, 587. (26) Aultchenko, V. M.; Baru, S. E.; Sidorov, V. A.; Savinov, G. A.; Feldman, I. G.; Khabakhpashev, A. G.; Yasenev, M. V. Nucl. Instrum. Methods 1983, 208, 443.
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complexes did not have the same thickness along the window of the holder (10 mm); therefore we could not calculate the desmeared curves.
Results and Discussion Complexes of Polyelectrolyte Gels with Oppositely Charged Surfactants. Both theoretical3 and experimental studies1,2 have been reported on the behavior of complexes of charged gels with the surfactants of opposite charge. If a polyelectrolyte gel swells in a large volume of a dilute solution of oppositely charged surfactant, the surfactant molecules are strongly absorbed by the gel due to ion exchange with the gel counterions. As a result, their concentration inside the gel can easily exceed the critical micelle concentration (CMC) which leads to the formation of hydrophobic surfactant aggregates in the gel. This effect is further enhanced by the fact3,27 that the CMC within the gel is much lower than in the outside solution. Due to the formation of the aggregates of the surfactants within the gel, the osmotic pressure which is normally imposed by the gel counterions drastically diminishes, and the gel collapses. All these conclusions which have a general character have been checked experimentally both for anionic gels interacting with cationic surfactant [poly(acrylic) or poly(methacrylic acid) interacting with alkyl(pyridinium chlorides)] and for cationic gels interacting with anionic surfactants [poly(diallyldimethylammonium chloride) interacting with sodium alkyl sulfates].1,2,6,27 Fluorescence spectroscopy studies have shown that the structure of hydrophobic surfactant aggregates within the gel is different from that in pure aqueous solution.27 The most detailed structure studies of the resulting gel/ surfactant complexes have been performed using the SAXS method.4-9 The main results obtained can be summarized as follows. Both the anionic gel/cationic surfactant and cationic gel/anionic surfactant complexes exhibit extremely sharp and intense SAXS peaks which manifest a high degree of order in the spatial arrangement of surfactants within the gel. The fact that the gel is statistically disordered does not seem to destroy the perfect regular arrangement of surfactant aggregates: it was shown that the average mesh size of the gel is much smaller than the spatial scale over which the surfactant aggregates are arranged in a perfect crystalline order.8,9 Moreover, the surfactants within the oppositely charged gels are better ordered than in the pure aqueous medium (at the same concentration of the surfactants).9 On the basis of several experimental observations we have formulated the hypothesis that the distribution of surfactants within the oppositely charged gel is highly inhomogeneous,9 in analogy with the heterogeneity of polyacrylamide gels.28 Indeed, in the statistically disordered gel there are more loosely crosslinked regions intercalated by more densely cross-linked areas and the change of gel properties from region to region can be very pronounced. Another important feature of gel/surfactant complexes is that gels with higher charge densities form better defined supercrystalline structures.7,8 On the other hand, if gels are fully charged, for instance, PMA and PDADMACl, better defined structures are formed in the gels where the distances between the charges on the chain and the charges in the surfactant self-assembly fit each (27) Philippova, O. E.; Starodubtsev, S. G. J. Polym. Sci.: Phys. 1993, 31, 1471. (28) Weiss, N.; Van Vliet, T.; Silberberg, A. J. Polym. Sci.: Phys. 1981, 19, 1505.
Figure 1. Dependence of relative mass of gel m/mo (curves 1, 2, 3) and the amount of absorbed surfactant (curves 4, 5) on the initial surfactant concentration (mole ratio between the added surfactant molecules and network monomer units [surf.]/ [monomer unit]) for complexes PMA/CPC (curves 1, 4), PDADMACl/SDS (curve 2), and PDADMACl/SDBS (curves 3, 5). The amount of the absorbed surfactant is characterized by the mole ratio between absorbed CPC or SDBS molecules and PMA or PDADMACl network monomer units ([surf.]/[monomer unit]).
other so that a stronger interaction between hydrophobic surfactants is provided.29 Interaction of Gel/Surfactant Complexes with Metal Ions. Figure 1 shows that both anionic and cationic gels collapse upon the addition of oppositely charged surfactants. It can be seen that the gel collapse is connected with the absorption of surfactants. In the collapsed phase the mole ratio of the absorbed surfactant molecules and network monomer units is close to 1:1. The elemental analysis data in Table 1 show that for the complex PMA/CPC this ratio reached 1:1; for the system PDADMACl/SDS the molar ratio was 1/0.92; for PDADMACl/SDBS it approached 1:1.2. Figure 2 (curve 1) demonstrates the SAXS results for the PMA/CPC complex. A similar system has been previously studied.4,5 As in those papers, we see here sharp peaks; i.e., self-assembly of surfactants within the gel results in the formation of ordered structure. The PMA/CPC complexes were allowed to swell in the aqueous solutions of the metal compounds H2PtCl6, AgNO3, and CuCl2, which dissociate in water giving the bivalent anion PtCl62-, the monovalent cation Ag(H2O)2+, and the bivalent cation Cu(H2O)42+, respectively. Copper chloride was employed for interaction with PMA/CPC complex to clarify the influence of a bivalent (as compared to a monovalent) cation on complex structure though it could not be reduced in this system for reasons explained below. A small fraction of bivalent anions of PtCl62- can penetrate into the gel/surfactant complex PMA/CPC. For a homogeneous distribution of all the components the content of Pt inside the gel would be 0.88 wt % that fits (29) Sokolov, E.; Yeh, F.; Khokhlov, A. R.; Chu, B. Macromolecules, in press.
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Langmuir, Vol. 14, No. 2, 1998 255
Table 1. Elemental Analysis Data for Complexes PMA/CPC, PDADMACl/SDS, and PDADMACl/SDBS in the Collapsed Phase PMA-CPC,a wt % element
PDADMACl-SDS,b wt %
PDADMACl-SDBS,c wt %
found
calcd
found
calcd
found
calcd
C H N S Cl Na
70.32 10.41 3.64
70.54 10.90 3.29
64.86 9.82 2.55 7.01
0.84 0.54
57.16 10.50 3.50 7.36 0.71
65.22 9.97 2.36 7.30
1.35 0.30
57.70 9.69 3.30 6.69 0.50
0.60
0.84
composition
(PMAn + CPCat) +d 0.1NaCl + 1.7H2O
(PDADMACat + 0.08Cl- +e 0.92DSAn) + 1.5H2O
(PDADMACat +f 1.2DBSAn + 0.2Na+) + 1.5H2O
a Surfactant concentration in outer solution is 5 × 10-3 M for CPC. b Surfactant concentration in outer solution is 5 × 10-4 M for SDS. Surfactant concentration in outer solution is 5 × 10-3 M for SDBS. d PMAn: poly(methacrylic acid) anion. CPCat: cetylpyridinium cation. e PDADMACat: poly(diallyldimethylammonium) cation. DSAn: dodecyl sulfate anion. f DBSAn: dodecyl benzenesulfonate anion. c
Figure 2. SAXS profiles of the PMA/CPC complex (curve 1), of PMA/CPC + H2PtCl6 (curve 2), and after subsequent reduction of PtCl62- ions by N2H4‚H2O (curve 3).
well with Pt content found (Table 2). Therefore, in this case there is no driving force for the penetration of PtCl62inside the gel. Moreover from the elemental analysis data in Table 2 it can be noted that the H2PtCl6 in the outer solution induces a slight release of the CPC by the gel: after exposure of the PMA/CPC complex to the H2PtCl6 solution the molar ratio PMA:CPC becomes 1:0.9. This can be explained by increase of acidity of the medium (pH of H2PtCl6 solution containing a gel with PMA/CPC complex was equal 3.9, while pH of initial solution of H2PtCl6 at concentration 10-2 mol/L was 2.7) that depresses the charge of the PMA carboxylic groups. In the case of AgNO3, the molar ratio in complex PMA/CPC was not disturbed and the amount of Ag+ ions penetrating the gel was much higher. CuCl2 incorporation results in the 30% replacement of surfactant molecules for metal cations. This result shows that bivalent cation can successfully compete with CPC molecules for interaction with gel units. The elemental analysis of the resulting PMA/CPC + H2PtCl6, PMA/CPC + AgNO3, and PMA/CPC + CuCl2 systems shows that both anions and cations enter the gel (Table 2); however, the content of the PtCl62- anion is extremely low. The ratio of the charge of the absorbed metal compound ions per one charged monomer unit of the gel was 1:25, 1:5, and 1:2.5, respectively (Table 2 lists the molar ratios of gel units to metal atoms). From these data it can be surmised that the interaction of both monovalent Ag+ and bivalent Cu2+ cations with the anionic hydrogel is a more important factor than the interaction of PtCl62- with CPC cations.
On the other hand, for Ag+ ions the CPC molecules do not leave a gel; therefore, Ag+ cations enter the gel together with their NO3- counterions. Contrary to this, bivalent cations Cu2+ are able partly to replace surfactant molecules (Table 2). Thus, it can be supposed that the driving forces for the incorporation of Cu2+ and Ag+ cations into anionic gel are different. Cu2+ ions compete effectively with CPC for the ion pairs with negative charges of the gel, while Ag+ ions (together with NO3- counterions) most probably act as an ionic diluent for the most polar parts of the resulting PMA/CPC microstructure. Another noteworthy feature for the case of Cu2+ cations is that the ratio of charges of cations (both Cu2+ and cetylpyridinium) to monomer charged units increases up to 1.1:1, that shows that some copper ions penetrate in the gel together with their counterions (Cl-). Probably, an extra amount of low molecular ions are attracted by the most polar regions of the resulting microstructure, similar to AgNO3 case. Figures 2-4 (curves 2) demonstrate the SAXS profiles of complexes PMA/CPC after introduction of metal compounds. The SAXS curves after the reduction of metal ions with the formation of metal nanoparticles are shown as curves 3 in Figures 2 and 3. The most striking observation is the emergence of additional peak for the system PMA/CPC + H2PtCl6. This effect can be explained as follows. Within the gel there are always free surfactant ions in addition to those involved in the complex. The slight amount of the PtCl62- ions entering the gel should, first of all, replace the Clcounterions of these free CPC molecules and, therefore, change the ability of CPC surfactants to self-assemble within the gel. It is known30 that the addition of bivalent counterions to the aqueous solutions of charged surfactants decreases very significantly the CMC. Therefore, the PtCl62- counterions penetrating in the gel might induce additional self-aggregation of the free CPC surfactants which are located there, originating a new structural pattern for ordering which should be different from the structural pattern of the gel/surfactant complex. As to the regular gel/surfactant structure, because of the low content of PtCl62- it is mainly unaffected by the additional type of ordering. For comparison in Figure 5 we present also the SAXS results for the precipitate obtained from the aqueous solutions of CPC upon the addition of H2PtCl6. The comparison of curves 1 and 2 in Figure 5 clearly shows that the new family of peaks which appear in the SAXS picture after the addition of H2PtCl6 to the gel/surfactant (30) Shinoda, K.; Nakagawa, T.; Tamamushi, B.; Isemura, T. Colloidal Surfactants. Some Physical Properties; Academic Press: New York, London, 1963.
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Table 2. Elemental Analysis Data for PMA/CPC Complex after Interaction with H2PtCl6, AgNO3, and CuCl2a H2PtCl6, wt % element C H N Cl Met Na charged gel units/ metal ion (mol) composition a
AgNO3, wt %
CuCl2, wt %
found
calcd
found
calcd
found
calcd
70.82 10.42 3.26 2.21 1.06
70.68 10.73 3.24 2.01 1.01
67.64 10.08 3.84 0.73 4.03 0.20
67.41 9.91 3.77 0.79 4.81 0.10
63.00 9.92 2.40 2.28 3.34 0.40
63.01 9.99 2.75 2.02 3.59 0.65
50:1
5:1
5:1
(PMAn + 0.9CPCat + 0.1H+) + 0.1HCl + 0.02H2PtCl6 + H2O
(PMAn + CPCat) + 0.01NaCl + 0.2AgNO3 + 1.1H2O
(PMAn + 0.7CPCat + 0.2Cu2+ + 0.1Cl- ) + 0.1NaCl + 2H2O
Salt concentration in outer solution is 10-2 M.
Figure 3. SAXS profiles of the PMA/CPC complex (curve 1), of PMA/CPC + AgNO3 (curve 2), and after subsequent reduction of Ag+ cations with UV irradiation (curve 3).
Figure 5. SAXS profiles of the PMA/CPC + H2PtCl6 system (curve 1) and the CPC + H2PtCl6 precipitate (curve 2).
Figure 6. SAXS profiles of the PDADMACl/SDS complex (curve 1), of PDADMACl/SDS + H2PtCl6 (curve 2), and after its subsequent reduction by NaBH4 (curve 3). Figure 4. SAXS profiles of the PMA/CPC complex (curve 1) and of the PMA/CPC + CuCl2 system (curve 2).
complex corresponds to the CPC precipitates in pure water induced by the platinum compound. From curve 3 in Figure 2 it can be seen that after the reduction of H2PtCl6 with hydrazine-hydrate in the PMA/CPC complex, additional ordering disappears. On the other hand, the peak
characterizing the ordering of the surfactant molecules in the gel mainly remains unaffected upon the addition of H2PtCl6 and after reduction. This means that the formation of metal nanoparticles occurs outside the most ordered regions of the gel/surfactant complexes. The addition of monovalent (AgNO3) or even bivalent (CuCl2) cations also does not destroy the ordering in the
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Langmuir, Vol. 14, No. 2, 1998 257
Table 3. Elemental Analysis Data for PDADMACl/SDS Complex after Interaction with H2PtCl6 and AgNO3 H2PtCl6, wt % element C H N S Cl Met Na
AgNO3, wt %
found
calcd
found
calcd
52.20 9.20 3.16 6.23 5.80 4.20
51.73 9.56 3.21 6.61 5.61 4.47
60.54 10.58 3.52 7.61 0.61 0.90
60.02 10.38 3.70 7.65 0.73 0.56
charged gel units / metal ion (mol)
10:1
50:1
(PDADMACat + 0.9DSAn + 0.1Cl-) + 0.1H2PtCl6 + 1.5H2O
composition
(PDADMACat + 0.92DSAn + 0.08Cl-) + 0.02AgNO3 + 0.5H2O
Table 4. Elemental Analysis Data for PDADMACl/SDBS Complex after Interaction with H2PtCl6 H2PtCl6, wt % element C H N S Cl Pt Na charged gel units/ metal ion (mol) composition
found
calculated
63.16 9.60 2.32 7.03 2.16 2.08
63.03 9.39 2.48 6.81 2.27 2.02 0.81 17:1
(PDADMACat + 1.2DBSAn + 0.2Na+) + 0.06H2PtCl6 + H2O
system, despite the high concentration of cations in hydrogel/surfactant complexes. The main reason for such a behavior can be the following. The cationic aggregates inside PMA gel are so well-defined due to strong hydrophobic attraction and electrostatic interaction with PMA that they cannot be disturbed significantly by penetration of other cations into gel even when some more loosely ordered CPC molecules are partially released from the gel. Small similar changes of SAXS images (Figure 3 and 4, curves 2) observed for systems PMA/CPC + AgNO3 and PMA/CPC + CuCl2 can be explained by interaction of metal cations with PMA gel. The subsequent reduction of PtCl62- ions was performed with N2H4‚H2O; Ag+ ions were reduced by UV irradiation. For Ag+ ions N2H4‚H2O cannot be used as a reducing agent because the complexes AgNO3‚N2H4 are formed without reduction.31 NaBH4 reduces Ag+ ions; however, it evolves atomic hydrogen due to hydrolysis:
NaBH4 + 3H2O f 4H2 + NaH2BO3 (Na2HBO3, Na3BO3) Therefore the hydrogenation of pyridine cycle of CPC to piperidine takes place and leads to the formation of tertiary saturated amine and a complete loss of surfactant ordering. Cu2+ ions cannot be reduced32 in PMA/CPC complexes because of reasons mentioned for Ag+ and also because UV irradiation is not effective for Cu2+ reduction, so copper particles were not prepared in PMA/CPC complex. The SAXS curves for the resulting metal colloids embedded in the body of the PMA hydrogel are shown in Figures 2 and 3 (curves 3). One can see that both for Pt and Ag particles (31) Gmelin Handbuch der Annorganischen Chemie; Keim, R., Ed.; Springer-Verlag: Berlin, Heidelberg, Germany, New York, 1975; Vol. B6. (32) Comprehensive Coordination Chemistry; Wilkinson, G., Ed.; Pergamon Press: New York, 1987, Vol.1.
Figure 7. SAXS profiles of the PDADMACl/SDBS complex (curve 1), of the PDADMACl/SBDS + H2PtCl6 system (curve 2), and after its subsequent reduction by N2H4 (curve 3) and NaBH4 (curve 4).
the initial ordering is mainly preserved. The significant increase of scattering in reduced samples in the region of very small angles (0° < 2θ < 1°) is due to the formation of metal nanoparticles. However in the case of Ag+ the increase of scattering intensity when 2θ varies from 0 to 1° was also observed for the system PMA/CPC + AgNO3 (without metal reduction), which might be explained by the partial reduction of Ag+ ions under X-ray irradiation during SAXS examination. Along with the PMA/CPC system, the complexes of cationic gels of PDADMACl with anionic surfactants SDS and SDBS were also studied. Figure 6 (curve 1) shows the SAXS profile for the complex PDADMACl/SDS. In complete agreement with previous findings6-9 we see here sharp peaks corresponding to hexagonal ordering. Then the samples of the complex PDADMACl/SDS were placed in the solutions of AgNO3 and H2PtCl6, correspondingly. The elemental analysis (see Table 3) shows that Ag+ and PtCl62- ions penetrate to some extent inside the gels. In the final state, the content of Ag matches to one Ag+ ion per 50 charged gel units. The low degree of cation incorporation in such a system can be explained by the fact that Ag+ ions are charged similarly to the gel units. In the case of PtCl62-, added anions can interact with cationic gels providing a higher degree of the incorporation (Table 3). This trend is similar to that observed for anionic gel/cationic surfactant complexes (see above). The incorporation of Ag+ ions and subsequent reduction of them by NaBH4 does not lead to distortion of the
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Figure 8. Schematic representation of the polyelectrolyte gel/surfactant complex (L is a distance between charges on the chain in cis position; l is a distance between surfactant tails) (a) and schematic images of the PMA/CPC, PDADMACl/SDS, and PDADMACl/ SDBS complexes (b).
surfactant ordering. This is also completely analogous to PMA/CPC + H2PtCl6 system, with the change of sign of all components. However the incorporation of PtCl62- ions which can interact with gel charged units results in the partial disordering of the complex (Figure 6, curve 2). The subsequent reduction of the PtCl62- ions with sodium borohydride or hydrazine-hydrate (similar results) leads in this case to the complete disordering of the system (Figure 6, curve 3). For comparison, the incorporation of PtCl62- ions in the complex PDADMACl/SDBS was also studied. Similarly to the PDADMACl/SDS system, by elemental analysis data (Table 4) SDBS molecules do not leave a gel (the molar ratio [gel charged units]/[SDBS] is preserved) and PtCl62- ions penetrate into the gel together with H+ counterions; in the final state, the ratio of the charge of the absorbed metal ions per one charged monomer unit of the gel is 1:8.5. For this complex the distortion of lamellar ordering9 after the penetration of bivalent anions is practically unnoticeable, however the growth of Pt particles in the gel destroys the self-assembly (Figure 7, curves 3 and 4). Thus, considering all systems studied we can conclude that for polyelectrolyte gel/surfactant complexes the incorporation of ions of the same charge (with respect to the gel units) does not lead to any distortion of ordering; however, the degree of metal ion incorporation is also low. The incorporation of oppositely charged ions (with respect to charged groups of hydrogel) results in different changes for cationic and anionic gels. For PMA/CPC system, incorporation of oppositely charged metal ions (both monovalent and bivalent ones) and metal particle formation in the gels does not destroy the surfactant ordering, despite the high degree of penetration of metal cations in the gel. This means that the PMA/CPC complex produces very well-defined and stable structures. Alternatively, the PDADMACl/SDS complex is rather unstable, and welldefined nanostructures can be easily spoiled by growing metal nanoparticles. On the other hand, highly ordered nanostructures formed by SDBS in PDADMACl gel are more stable and Pt particle growth only partially disturbs
them. The cause of such a different behavior may be connected with different possibilities for the realization of hydrophobic interactions between surfactant molecules in each system (this idea was proposed in ref 29). The optimum situation corresponds to the exact fitting of the distance between the charges on the chain (L) and the distance between hydrocarbon tails (l) in the surfactant layers (Figure 8a). In the orthorhombic unit cell of high density polyethylene the two main distances between polymer chains are l1 ) 4.45 Å and l2 ) 4.93 Å.33 On the other hand, from Figure 8b one can see that the distance between two PMA carboxylic groups in the cis position, a, is 5.06 Å, while the distance between two neighboring charged groups in PDADMACl, b, is about 7.2 Å. One can see that for the PMA/CPC complex we are closer to the exact fitting situation shown in Figure 8a. In contrast, for PDADMACl/SDS the values l2 ) 4.93 Å and b ) 7.2 Å are far from any fitting, and therefore the optimum complex providing maximum hydrophobic interactions of hydrocarbon surfactant tails cannot be realized. Therefore, it is not surprising that PMA/CPC structure is more stable. An additional fact which should be taken into account is that the tail of CPC as compared to SDS is longer, also leading to stronger hydrophobic interactions for this case. As to the PDADMACl/SDBS complex, it is possible to assume that a strong π-π interaction between benzene rings of SDBS molecules located mainly in parallel to each other results in additional hydrophobic interaction between surfactant molecules, providing the relatively higher stability of the PDADMACl/SDBS complex. Thus, the PDADMACl/SDS complex should be the weakest one among all the systems studied, which is indeed observed. Conclusions The interaction of PMA/CPC, PDADMACl/SDS, and PDADMACl/SDBS complexes with metal cations and (33) Encyclopedia of Polymer Science and Engineering; Kroschwitz, J. I., Ed.; John Wiley & Sons: New York, 1986; Vol. 6.
Complexes of Polyelectrolyte Gels
anions has been studied. The incorporation of metal ions in polyelectrolyte gels containing monomer units of the same charge was found not to lead to any loss of ordering; however, the degree of metal ion incorporation is rather low. In contrast to this, the incorporation of oppositely charged ions (with respect to charged groups of hydrogel) for all systems is noticeable, but it results in different changes for cationic and anionic gels. The structure of the PMA/CPC complex is not destroyed despite the incorporation of metal ions and even metal particle formation. The SDS and SDBS ordering in PDADAMCl is destroyed partly by incorporation of PtCl62- ions and by Pt nanoparticle formation. The more stable structure
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of CPC ordering in PMA gels is explained by optimum fitting of the distance between charged groups in cis position on PMA chain to the distance between hydrocarbon tails in the surfactant layers that provides the stronger hydrophobic interaction in self-assembly. Acknowledgment. The authors acknowledge the financial support provided by the Russian Foundation for Basic Research (Grant No. 96-03-32335 and Grant No. 96-03-32732) and the Volkswagen Foundation. O.A.P. is grateful for a fellowship granted by Haldor Topsoe A/S. LA970527Y
