Langmuir 1998, 14, 4427-4434
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Preferential Binding with Regard to Chain Length and Chemical Structure in the Reactions of Formation of Quasi-Soluble Polyelectrolyte Complexes N. Karibyants and H. Dautzenberg* Max-Planck-Institut fu¨ r Kolloid- und Grenzfla¨ chenforschung, Kantstrasse 55, D-14513 Teltow-Seehof, Germany Received March 12, 1998. In Final Form: May 18, 1998 Viscometry, UV spectroscopy, and analytical ultracentrifugation (AUC) were applied to study preferential binding of poly(diallyldimethylammonium chloride) added to mixtures of sodium poly(styrenesulfonates) (PSS) of quite different molecular weights and PSS/sodium poly(methacrylate) (PMA). In salt-free solutions the decrease of the relative viscosity during complex formation was compared with model systems containing amounts of the free PSS components and NaCl, corresponding to special cases of preferential binding. The analysis suggests some preference for binding of the shorter polyanions. These findings were supported by AUC measurements of the composition of supernatant solutions after spinning down the polyelectrolyte complexes (PECs). In the presence of low molecular salt, the systems exhibit even binding at first (at ionic strengths of about 5 × 10-3 N NaCl) and at higher salt contents a pronounced binding of the high molecular component. Even an exchange low molecular weight (LMW) PSS for high molecular weight (HMW) PSS was found in long-term experiments at higher ionic strength. In the case of polyanions of different natures, only a weak preference of PSS was observed by AUC and UV spectroscopy in salt-free media, whereas in the presence of NaCl, PSS molecules were strongly favored. PSS was also demonstrated to substitute PMA in the complexes at I >7.5 × 10-3 N NaCl. The peculiarities of preferential binding in salt-free solutions can be explained by thermodynamic nonequilibrium conditions.
1. Introduction The selectivity of interpolymer reactions, i.e., their ability to “recognize” a certain partner has a dominant role in living organisms. This ability is typical not only for biopolymers but also for simple synthetic polymers in various mechanisms of complexation. The knowledge of the basic principles of macromolecular recognition on the level of simple polymers can play a decisive role in understanding the biological evolution of macromolecules. Baranovsky et al.1 have developed the basic theory of intermolecular complexation of any nature predicting conversion of these reactions and stability of the complexes according to chain lengths and chemical structure. Theoretical considerations provide an experimental approach for fractionation of reacting polymers by molecular weight and chemical structure due to selectivity of interpolymer reactions. Experimental results of recognition in interpolymer complex (IPC) formation P + P1 + P2 f IPC(P/P1) + P2 or substitution reactions IPC(P/P1) + P2 f IPC(P/P2) + P1 are well-reviewed by Papisov and Litmanovich.2 Whereas interpolymer recognition reactions with hydrogen bonding between polycarbon acids and poly(ethylene glycol) were studied comprehensively,3 there are only some publications concerning these processes in polyelectrolyte complexes (PECs).4-6 (1) ) Baranovsky, V. Yu.; Litmanovich, A. A.; Papisov, I. M. Eur. Polym. J. 1981, 17, 969. (2) Papisov, I. M.; Litmanovich, A. A. Adv. Polym. Sci. 1989, 90, 139. (3) Osada Y.; Antipina, A. D.; Papisov, I. M.; Kabanov, V. A.; Kargin V. A. Dokl. Akad. Nauk SSR, 1970, 232, 485; Antipina, A. D.; Baranovsky, V. Yu.; Papisov, I. M.; Kabanov, V. A. Vysokomol. Soedin. 1972, A14, 941; Papisov, I. M.; Antipina, A. D.; Sergeeva, E. I.; Antipina, A. D.; Kabanov, V. A. Vysokomol. Soedin. 1974, A16, 1133; Papisov, I. M.; Litmanovich, A. A.; Vysokomol. Soedin. 1977, A19, 716; Tsusida, E.; Osada I. Makromol. Chem. 1971, 175, 593; Kokufuta, E.; Yokota, A. Polymer 1983, 24, 1031. (4) Kikushi, Y.; Kubota, N. Makromol. Chem., Rapid. Commun. 1985, 6, 387.
The question of complex stability and the ability of exchange processes were intensively investigated for complexes of polyelectrolytes with micellar surfactants and proteins, which are of great practical importance in pharmaceutics and as highly effective flocculants.7 In the present paper, we used a combination of experimental methods to investigate recognition reactions of oppositely charged polyelectrolytes, where P1 and P2 are polyanions of different molecular weights (MW) and of different chemical natures (weak and strong polyelectrolytes), respectively. Complex formation was carried out between poly(diallyldimethylammonium chloride) (PDADMAC) as the polycation and the polyanions sodium poly(styrenesulfonate) (PSS) and sodium poly(methacrylate) (PMA), leading to highly aggregated quasi-soluble complexes as described in detail in our previous publications.8-11 The UV-active PSS offers the possibility of determining quantitatively the amount of bound sulfonate groups from the shift in the spectrum caused by complex formation.9 Therefore, UV spectroscopy allows information about the relative conversion of the ionic binding reaction of these molecules with PDADMAC in the presence of PMA polyions to be obtained. Analytical ultracentrifugation (AUC) monitors all species individu(5) Izumrudov, V. A.; Bronich, T. K.; Saburova, O. S.; Zezin, A. B.; Kabanov, V. A. Vysokomol. Soedin. 1974, A16, 1133. (6) Dautzenberg, H.; Linow, K.-J.; Rother, G. Acta Polym. 1990, 41, 98. (7) Gulyaeva, Zh. G.; Zansokhova, M. F.; Razvodsky, Ye. F.; Yefimov, V. S.; Zezin, A. B.; Kabanov, V. A. Vysokomol. Soedin. 1983, A25, 1238. Izumrudov, V. A. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 1017. (8) Dautzenberg, H.; Rother, G.; Hartmann, J. In Macro-ion Characterization from Dilute Solutions to Complex Fluids; Schmitz, K. S. Eds.; ACS Symposium Series 548; American Chemical Society: Washington, DC, 1994. (9) Dautzenberg, H.; Hartmann, J.; Grunewald, S.; Brand, F. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 1024. (10) Dautzenberg, H. Macromolecules 1997, 30, 7810. (11) Karibyants, N.; Co¨lfen, H.; Dautzenberg, H. Macromolecules 1997, 30, 7803.
S0743-7463(98)00301-1 CCC: $15.00 © 1998 American Chemical Society Published on Web 07/15/1998
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Table 1. Molecular Characteristics of Samples Employed in PEC Formation polyanion/polycation
specification
poly(styrenesulfonate), sodium salt
PSS-8t PSS-66t PSS-356t PMA PDADMAC
poly(methacrylic acid), sodium salt poly(diallyldimethylammonium chloride) a
10-3 × Mw, g/mol
Mw/Mn
s20,w, Svb
0.8. While X relates to the ratio of PDADMAC to PSS units only but not to all anionic charges, the entire mixing ratio is about 0.4. The earlier onset of turbidity can be explained by the formation of mixed three-component complexes, as described in ref 17. AUC measurements of the amounts of free PSS in the supernatant solutions of PECs formed in the presence and absence of PMA support the findings of UV spectroscopy. In Figure 8, the stoichiometric factors calculated from these measurements are represented for the complex PSS-66t/PDADMAC and (PSS-66t + PMA)/ PDADMAC. To understand Figure 8, it should be taken into consideration that, in the case of polyanion mixtures per given definition, the stoichiometric factor f does not reflect the stoichiometry of anionic to cationic groups but only PSS to PDADMAC. The same is valid for the mixing ratio X.
Figure 7. (a) Peak heights at 203 nm in the difference spectra of the complexes PSS-66t/PDADMAC (closed symbols) and (PSS-66t/PMA, 1:1 monomolar mixture)/PDADMAC (open symbols) as a function of the initial molar mixing ratios at various ionic strengths and (b) the relative degree of binding γN of PSS calculated from these values.
While the UV-spectroscopic results reflect the ionic binding mainly in the cores of the PECs, AUC also monitors the attached chains of the shell as belonging to the complex. In the salt-free system, f is higher than γΝ, but smaller than in the system without PMA. This indicates the incorporation of both polyanions in the cores and shells of the PECs. At X ) 1, the value f ) 1 means that all free PSS is consumed and probably also most of the PMA. Since light scattering8,9 studies led to the conclusion that even at high mixing ratios new PEC particles are formed under detaching the excess component from the shells of the previously built ones, this process comes into play above X ) 1. The rapid increase of γΝ to 1 reveals the strong preferential binding of PSS in this range. With rising salt content, the f values with and without PMA approach each other, suggesting strong preferential binding of PSS also in the shells. Concerning the polyelectrolyte exchange reaction of components with different natures, systems of weak polyacid/polybase are well-known to be able to show these reactions in salt media (usually salt concentrations above
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Figure 8. Stoichiometric factor of PSS binding (f) versus initial molar mixing ratios for PSS/PDADMAC complexes (closed symbols) and (PSS-66t/PMA)/PDADMAC complexes at various ionic strengths, obtained by AUC measurements (open symbols).
0.02 N have been used).17,18 We examined these phenomena by reacting PMA/PDADMAC, X ) 0.68, with PSS66t at different ionic strengths. Figure 9 illustrates the data of the AUC analysis of the supernatant solutions of these complexes. One can see that at concentrations up to 7.5 × 10-3 N NaCl, only small changes of the sedimentation coefficient distribution of PSS-66t occurred, i.e., exchange reactions did not take place. At salt concentrations higher than 0.01 N NaCl, most of the PSS is included in highly aggregated polydisperse PEC particles (see the inset, obtained by the classical approach for sedimentation coefficient distributions). This was proven up to I ) 1 mol/L. Unfortunately, this reaction leads to the formation of very highly aggregated products or macrophase separation, impeding the use of such methods as UV and polarization luminescence spectroscopy, developed for exchange reactions of soluble complexes. Exchange reactions PSS-66t/PDADMAC for PMA were not observed. Conclusions Preferential binding between PDADMAC and PSS of different molecular weights as well as between PSS and PMA, relative to the ionic strength of the medium, was (18) Bakeev, K.; Izumrudov, V.; Kuchanov, S.; Zezin, A.; Kabanov V. Macromolecules 1992, 25, 4249.
Karibyants and Dautzenberg
Figure 9. Sedimentation coefficient distributions (not corrected for diffusion) of pure PSS in 5 × 10-3 N NaCl (solid line) and of the products of the exchange reaction PSS-66t S PMA/ PDADMAC in 5 × 10-3 N (dashed line), in 7.5 × 10-3 N (dotted line) and in 0.01 N (inset) NaCl.
studied by various methods. In contradiction to theoretical expectations, at extremely low ionic strengths in the systems of LMW and HMW PSS, the short chain PSS is preferred in complexation. In competition with PMA, only a slight favoring of PSS was observed. These findings were explained by the fact that, under such conditions, complex formation takes place far from thermodynamic equilibrium, mainly governed by the kinetics of the process. With increasing ionic strength, the binding of PSS in comparison to PMA and of HMW PSS in the mixture with LMW PSS dominates. In contrast to the common assumption that complexation in highly aggregated complexes of strong polyelectrolytes is irreversible, exchange reactions could be observed in long-term experiments. At higher ionic strength, a complete exchange of PMA for PSS and of LMW PSS for HMW PSS was found. Acknowledgment. N.K. cordially thanks Dr. H. Co¨lfen for the introduction into several skills needed for the AUC technique. We also thank him for reading the manuscript and for fruitful discussions. S. Mackowiak and B. Zilske are thanked for careful technical assistance. Financial support of the Deutsche Forschungsgemeinschaft and the Max Planck Society is gratefully acknowledged. LA980301A
