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Water-Soluble Complexes between Cationic Surfactants and Comb-Type Copolymers Consisting of an Anionic Backbone and Hydrophilic Nonionic Poly(N,N-dimethylacrylamide) Side Chains I. Balomenou and G. Bokias* Department of Chemistry, University of Patras, GR-26504 Patras, Greece Received February 7, 2005. In Final Form: March 30, 2005 The formation of complexes between the cationic surfactant dodecyl trimethylammonium bromide (DTAB) and the comb-type anionic polyelectrolytes poly(sodium acrylate-co-sodium 2-acrylamido-2-methylpropane sulfonate)-g-poly(N,N-dimethylacrylamide) (P(NaA-co-NaAMPS)-g-PDMAMx) was investigated in dilute aqueous solutions by means of turbidimetry, pyrene fluorescence probing, viscometry, z-potential measurements, and dynamic light scattering. The comb-type copolymers consist of an anionic copolymer backbone, P(NaA-co-NaAMPS), containing 84 mol % NaAMPS units, while the weight percentage, x, of the PDMAM side chains varies from x ) 12% (w:w) up to x ) 58% (w:w). It was found that, contrary to the water-insoluble complexes formed between the linear polyelectrolyte P(NaA-co-NaAMPS) and DTAB, the solubility in water of the complexes formed between the comb-type copolymers and DTAB is significantly improved with increasing x. The complexation process starts at the same critical aggregation concentration (about 2 orders of magnitude lower than the critical micelle concentration of DTAB), regardless of x, and it is accompanied by charge neutralization and appearance of hydrophobic microdomains. Both effects lead to the substantial collapse of the polyelectrolyte chain upon addition of DTAB. However, the complexes of the comb-type copolymers with DTAB are stabilized in water as nanoparticles, and probably consisted of a water-insoluble core (the polyelectrolyte/surfactant complex), protected by a hydrophilic nonionic PDMAM corona. The size of the nanoparticles varies from ∼35 nm up to ∼120 nm, depending on x.
Introduction The interactions between polyelectrolytes and oppositely charged surfactants1-3 have been extensively investigated, due to its importance in both fundamental and applied fields. In water, surfactant molecules bind to the oppositely charged polyelectrolyte chains, forming the so-called polyelectrolyte/surfactant complexes. Usually, binding, initiated by the saltlike bridging between the oppositely charged polyelectrolyte and surfactant molecules and stabilized by hydrophobic interaction of the bound surfactant tails,4 is characterized by an important cooperativity and starts at a critical surfactant concentration, termed critical aggregation concentration (cac), at concentrations well below the critical micelle concentration of the surfactant (cmc).2 The phase behavior of the oppositely charged polyelectrolyte/surfactant complexes strongly depends on their composition. As a general rule, when the charge ratio approaches stoichiometry, waterinsoluble polyelectrolyte/surfactant complexes separate out from water,1,2,5-12 usually leading to three-dimensional highly ordered structures or nanostructures.13-21 * Corresponding author. Phone: +30 2610 997 102. Fax: +30 2610 997 122. E-mail:
[email protected]. (1) Hayakawa, K.; Kwak, J. In Cationic Surfactants: Physical Chemistry; Rubingh, D., Holland, P. M., Eds.; Surfactant Science Series; Marcel Dekker: New York, 1991. (2) Lindman, B.; Thalberg, K. In Interactions of Surfactants with Polymers and Proteins; Goddard, E. D., Ananathapadmanabhan, K. P., Eds.; CRC Press: Boca Raton, FL, 1993. (3) Langevin, D. Adv. Colloid Interface Sci. 2001, 89-90, 467. (4) Gong, J. P.; Osada, Y. J. Phys. Chem. 1995, 99, 10971. (5) Dubin, P. L.; Oteri, R. J. Colloid Interface Sci. 1983, 95, 453. (6) Dubin, P. L.; Rigsbee, D. R.; McQuigg, D. W. J. Colloid Interface Sci. 1985, 105, 509. (7) Thalberg, K.; Lindman, B.; Bergfelt, K. Langmuir 1991, 7, 3. (8) Piculell, L.; Lindman, B. Adv. Colloid Interface Sci. 1992, 41, 149.
On the other hand, the development of stoichiometric oppositely charged polyelectrolyte/surfactant complexes maintaining water-solubility even at electroneutrality has attracted a great interest in past years, due to their potential applications in aqueous milieu. The first and most studied approach to achieve this goal is based on the pioneering work of Kabanov and co-workers,22-25 who introduced the use of block ionogenic copolymers, synthetic copolymers where a polyelectrolyte is covalently attached to a nonionic hydrophilic block. Polyelectrolytes of a combtype structure provide an alternative approach for preparing water-soluble polyelectrolyte/surfactant complexes (9) Piculell, L.; Lindman, B.; Karlstrom, G. In Polymer-Surfactant Systems; Kwak, J. C. T., Ed.; Marcel Dekker: New York, 1998. (10) Yoshida, K.; Sokhakian, S.; Dubin, P. L. J. Colloid Interface Sci. 1998, 205, 257. (11) Wang, Y.; Kimura, K.; Dubin, P. L.; Jaeger, W. Macromolecules 2000, 33, 3324. (12) Hansson, P. Langmuir 2001, 17, 4167. (13) Antonietti, M.; Conrad, J.; Thunemann, A. Macromolecules 1994, 27, 6007. (14) Okuzaki, H.; Osada, Y. Macromolecules 1995, 28, 380. (15) Antonietti, M.; Burger, C.; Effing, J. Adv. Mater. 1995, 7, 751. (16) Yeh, F.; Solokov, E. L.; Khokhlov, A. R. J. Am. Chem. Soc. 1996, 118, 6615. (17) Chen, L.; Yu, S.; Kagami, Y.; Gong, J.; Osada, Y. Macromolecules 1998, 31, 787. (18) Zhou, S.; Burger, C.; Yeh, F.; Chu, B. Macromolecules 1998, 31, 8157. (19) Thunemann, A.; Lockhaas, K. H. Langmuir 1998, 14, 4898. (20) Kim, B.; Ishizawa, M.; Gong, J.; Osada, Y. J. Polym. Sci., Part A: Polym. Chem. 1999, 37, 635. (21) Thunemann, A.; Lockhaas, K. H. Langmuir 1999, 15, 4867. (22) Bronich, T. K.; Kabanov, A. V.; Kabanov, V. A.; Yu, K.; Eisenberg, A. Macromolecules 1997, 30, 3519. (23) Kabanov, A. V.; Bronich, T. K.; Kabanov, V. A.; Yu, K.; Eisenberg, A. J. Am. Chem. Soc. 1998, 120, 9941. (24) Bronich, T. K.; Nehls, A.; Eisenberg, A.; Kabanov, V. A.; Kabanov, A. V. Colloids Surf., B 1999, 16, 243. (25) Bronich, T. K.; Popov, A. M.; Eisenberg, A.; Kabanov, V. A.; Kabanov, A. V. Langmuir 2000, 16, 481.
10.1021/la0503505 CCC: $30.25 © 2005 American Chemical Society Published on Web 08/27/2005
Complexes between Cationic Surfactants and Copolymers Scheme 1. Chemical Structure of the Comb-Type Copolymers P(NaA-co-NaAMPS)-g-PDMAMxa
a From the characterization of the products, the relative amounts of the repeating units are estimated as: y ) 0.84, z ) 0.16 - w, and n ≈ 110. The value of w is w ) 0 for x ) 0, w ) 0.003 for x ) 12, w ) 0.008 for x ) 29, and w ) 0.03 for x ) 58.
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of cationic surfactants to polymer gels based on NaAMPS has been investigated aiming to the preparation of potential “smart” hydrogels.4,14,17,20,35-39 The comb-type structures based on NaAMPS-containing backbone offer an additional possibility. It is shown that the comb-type P(NaA-co-NaAMPS)-g-PDMAMx copolymers can form water-soluble complexes with cationic surfactants, like dodecyl trimethylammonium bromide (DTAB). Due to the rather high molecular weight of the PDMAM side chains and their important hydrophilicity, water-solubility of the polyelectrolyte/surfactant complexes formed is maintained even for PDMAM-poor copolymers. The properties of the water-soluble polyelectrolyte/surfactant complexes formed are investigated in aqueous solution by a variety of techniques, like turbidimetry, viscometry, pyrene fluorescence probing, zpotential, and dynamic light scattering measurements. Experimental Section
of opposite charge. However, until now, just a few examples have been reported in the literature. Thus, Kabanov et al.24,26 reported the formation of pH-dependent micellelike aggregates between alkyl sulfates and the comb-type copolymer poly(ethylene oxide)-graft-poly(ethyleneimine), consisting of a nonionic poly(ethylene oxide) backbone and the weak polybase poly(ethyleneimine) as the side chains. Moreover, the preparation of water-soluble complexes between sodium dodecyl sulfate27 or bile salts28 and combtype copolymers consisting of a cationic backbone and oligo(ethylene glycol) as side chains has been recently reported by Maiti et al. In the present study, we focus on comb-type anionic polyelectrolytes, consisting of a highly charged poly(sodium acrylate-co-sodium 2-acrylamido-2-methylpropane sulfonate) backbone (P(NaA-co-NaAMPS)) and the highly hydrophilic nonionic poly (N,N-dimethylacrylamide) (PDMAM) side chains. The chemical structure of these products, denoted P(NaA-co-NaAMPS)-g-PDMAMx, x being the weight percentage of the PDMAM side chains, is shown in Scheme 1. Some aspects of the multifunctional character in water of these comb-type products have already been demonstrated. Thus, it has been shown that they can form water-soluble interpolymer complexes either through hydrogen bonding with weak polyacids at low pH29 or through electrostatic interaction with cationic homopolymers.30 Furthermore, their ability to form watersoluble polyelectrolyte/protein complexes at pH lower than the isoelectric point of the protein has been recently demonstrated, using bovine serum albumin as a model protein.31 The monomer sodium 2-acrylamido-2-methylpropane sulfonate (NaAMPS) is often used to introduce permanent negative charges in the polymer chain and control the structural parameters of the polymer chain. Thus, it has been shown that the association of linear polymers based on NaAMPS with oppositely charged micelles leads to associative phase separation.32-34 In addition, the binding
Materials. The monomers, acrylic acid (AA), 2-acrylamido2-methyl-1-propanesulfonic acid (AMPSA), and N,N-dimethylacrylamide (DMAM), were purchased from Aldrich. Ammonium persulfate (APS, Serva), potassium metabisulfite (KBS, Aldrich), 2-aminoethanethiol hydrochloride (AET, Aldrich), and 1-(3(dimethylamino) propyl)-3-ethyl-carbodiimide hydrochloride (EDC, Aldrich) were used for the synthesis of the comb-type copolymers. Dodecyl trimethylammonium bromide (DTAB, Aldrich) was used as received. Water was purified by means of a Seralpur Pro 90C apparatus combined with an USF Elga water purification unit. Synthesis and Characterization of the Copolymers. The synthetic procedure for the preparation of the P(NaA-coNaAMPS)-g-PDMAMx copolymers has been described elsewhere.29 Briefly, amine-terminated PDMAM chains were first synthesized via free radical telomerization in water at 35 °C, using APS as initiator and AET as telogen. The number average molecular weight of PDMAM was determined to be 11 000 by acid-base titration in water. As a second step, the backbone P(NaA-co-NaAMPS) was synthesized via free radical copolymerization of AA and AMPSA in water, using the redox couple APS/KBS as initiator. The composition of the product, recovered in the sodium salt form, was determined by acid-base titration and 1H NMR analysis. It was found that the copolymer contains 84 mol % NaAMPS units. The molecular weight of the backbone P(NaA-co-NaAMPS) was determined by static light scattering measurements in 0.1 M NaCl. Finally, the amine-terminated PDMAM chains were grafted to the carboxylic groups of the copolymer by a grafting reaction in water at room temperature, using the water-soluble EDC as condensing agent. 1H NMR analysis was performed to characterize the composition of the three comb-type copolymers P(NaA-co-NaAMPS)-g-PDMAMx prepared. The molecular weight of the comb-type products was determined from their composition and the molecular weight of the backbone. The results of the characterization are reported in Table 1. The chemical composition of the products in monomer units, reported in Scheme 1, is estimated from the results of Table 1 and the molecular weight of PDMAM. We should note that the values of n and w in Scheme 1 are just rough estimations, because the samples are polydisperse as a result of the synthetic method applied. Methods. Turbidimetry. The optical density of dilute aqueous polymer-surfactant solutions at 25 °C was monitored at 490 nm by means of a Hitachi spectrophotometer, model U 2001.
(26) Bronich, T. K.; Cherry, T.; Vinogradov, S. V.; Eisenberg, A.; Kabanov, V. A.; Kabanov, A. V. Langmuir 1998, 14, 6101. (27) Nisha, C. K.; Basak, P.; Manorama, S. V.; Maiti, S.; Jayachandran, K. N. Langmuir 2003, 19, 2947. (28) Nisha, C. K.; Manorama, S. V.; Kizhakkedathu, J. N.; Maiti, S. Langmuir 2004, 20, 8468. (29) Sotiropoulou, M.; Bokias, G.; Staikos, G. Macromolecules 2003, 36, 1349. (30) Sotiropoulou, M.; Cincu, C.; Bokias, G.; Staikos, G. Polymer 2004, 45, 1563. (31) Sotiropoulou, M.; Bokias, G.; Staikos, G. Biomacromolecules 2005, 6, 1835.
(32) Yoshida, K.; Morishima, Y.; Dubin, P. L.; Mizusaki, M. Macromolecules 1997, 30, 6208. (33) Mizusaki, M.; Morishima, Y.; Dubin, P. L. J Phys. Chem. B 1998, 102, 1908. (34) Kayitmazer, A. B.; Seyrek, E.; Dubin, P. L.; Staggemeier, B. A. J. Phys. Chem. B 2003, 107, 8158. (35) Okuzaki, H.; Osada, Y. Macromolecules 1994, 27, 502. (36) Okuzaki, H.; Osada, Y. Polym. Gels Networks 1994, 2, 267. (37) Okuzaki, H.; Osada, Y. Macromolecules 1995, 28, 4554. (38) Matsukata, M.; Hirata, M.; Gong, J. P.; Osada, Y.; Sakurai, Y.; Okano, T. Colloid Polym. Sci. 1998, 276, 11. (39) Travas-Sejdic, J.; Easteal, A. J. Polymer 2000, 41, 7451.
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Table 1. Chemical Characterization of the Anionic Polyelectrolytes
a
copolymer
feed composition
P(NaA-co-NaAMPS) P(NaA-co-NaAMPS)-g-PDMAM12 P(NaA-co-NaAMPS)-g-PDMAM29 P(NaA-co-NaAMPS)-g-PDMAM58
80 mol % NaAMPS 10% (w:w) PDMAM 25% (w:w) PDMAM 50% (w:w) PDMAM
1H
NMR analysis
84 mol % NaAMPS 12% (w:w) PDMAM 29% (w:w) PDMAM 58% (w:w) PDMAM
molecular weight ×10-5 3.2a 3.6b 4.5b 7.6b
From static light scattering measurements. b Calculated from the molecular weight of the backbone and the composition of the copolymer.
Viscometry. Reduced viscosity studies were carried out at 25 ( 0.02 °C with an automated viscosity measuring system (SchottGera¨te AVS 300, Germany), equipped with a micro-Ostwald viscometer. Pyrene Fluorescence Probing. Steady-state fluorescence spectra of pyrene were recorded on a Perkin-Elmer LS50B luminescence spectrometer, equipped with a circulating water bath. A stock ethanolic solution, containing 1 × 10-3 M pyrene, was used. The final concentration of the probe was 6 × 10-7 M, and the excitation wavelength was 334 nm. The intensity ratio (I1/I3) of the first (I1) over the third (I3) vibronic band of the emission spectrum of pyrene, at 373 and 384 nm, respectively, was used to detect the formation of hydrophobic microdomains.40,41 z-Potential Measurements. The z-potential of dilute aqueous polymer-surfactant solutions at room temperature was measured by means of a Zetaseizer 5000 (Malvern Instruments Ltd.), equipped with a cell type ZET 5104 (cross beam mode). Dynamic Light Scattering. The dynamic light scattering measurements were performed using a thermally regulated ((0.1 °C) spectrogoniometer, model SEM RD (Sematech France), equipped with an R.T.G. correlator (Sematech France) and a He-Ne laser, emitting a vertically polarized light at a wavelength λ ) 633 nm. The pinhole was set at 200 nm, and the scattering angle was set at 90°. The autocorrelation functions were analyzed using a second-order cumulants analysis, which leads to the determination of the hydrodynamic diameter of the nanoparticles formed. Sample Preparation. Parent solutions of the cationic surfactant (DTAB), the backbone (P(NaA-co-NaAMPS)), and the comb-type copolymers (P(NaA-co-NaAMPS)-g-PDMAMx) were prepared by dissolution of the materials in water under gentle agitation for 24 h. The parent solutions were filtered through a membrane filter with a pore size of 5 µm, before preparing the polyelectrolyte/surfactant mixtures. The mixtures were prepared by dropwise addition of the DTAB solution into the corresponding polymer solution under agitation. The solutions of the mixtures were let under gentle agitation for 24 h before performing the measurements. Some of the solutions were tested for several days under gentle agitation. No important changes of their appearance (turbid, clear) were detected visually. The polymer concentration was kept constant for all of the polyelectrolyte/surfactant series studied and selected adequately, so that the concentration of the anionic units of the polyelectrolytes was always of the order of 1 mM. The contribution of both the sodium acrylate (NaA) and the sodium 2-acrylamido2-methyl-1-propanesulfonate (NaAMPS) units was taken into account for the calculation of the polyelectrolyte concentration in anionic units. In fact, for the comb-type products, some of the NaA units have reacted with the amine end-groups of the PDMAM side chains. This fraction has been neglected in consequent calculations. Nevertheless, as seen from the copolymers compositions reported in Scheme 1, this fraction is just the 3% or less of the total charged units. As a result, the error in the calculation of Z+/- is estimated to be of the order of 3% or less, depending on the sample.
Results and Discussion Solubility of the Complexes. The optical density of dilute aqueous solutions of mixtures of DTAB with either the backbone P(NaA-co-NaAMPS) or the comb-type co(40) Kalyanasundaram, K.; Thomas, J. K. J. Am. Chem. Soc. 1977, 99, 2039. (41) Lianos, P.; Viriot, M.-L.; Zana, R. J. Phys. Chem. 1984, 88, 1098.
Figure 1. Variation of the optical density of aqueous solutions of mixtures of DTAB with (a) P(NaA-co-NaAMPS) (O), (b) P(NaA-co-NaAMPS)-g-PDMAM12 (0), (c) P(NaA-co-NaAMPS)g-PDMAM29 (4), and (d) P(NaA-co-NaAMPS)-g-PDMAM58 (3), as a function of the concentration of DTAB. The polymer concentrations are 0.025% (w:w) (O), 0.029% (w:w) (0), 0.033% (w:w) (4), and 0.050% (w:w) (3), respectively. T ) 25 °C.
polymers P(NaA-co-NaAMPS)-g-PDMAMx is presented in Figure 1 as a function of the DTAB concentration. As noted, the study was performed at a fixed polymer concentration, adequately chosen so that the molar concentration of anionic units is similar for all of the copolymers. As seen, at low DTAB concentration, all of the solutions are clear. However, as the charge neutralization point (1.2 mM for P(NaA-co-NaAMPS)) is approached upon addition of surfactant, the P(NaA-coNaAMPS)/DTAB complex formed becomes water-insoluble and the solution turns turbid. The onset for the appearance of turbidity, as seen more clearly in the inset of Figure 1, is observed at a DTAB concentration ∼0.8 mM, corresponding to a surfactant-to-polyelectrolyte charge ratio Z+/- ) 0.7. It is noteworthy that just a smooth turbidity increase is observed initially and strongly turbid solutions, accompanied by the formation of large P(NaA-coNaAMPS)/DTAB complex precipitates, are obtained only when the concentration of DTAB exceeds 5 mM, for example, at large charge ratios, Z+/- > 4. On the other hand, when the comb-type copolymers are used, instead of the linear polyelectrolyte, no large precipitates are formed and the level of turbidity (if any) remains very low, regardless of the charge ratio. In fact, the turbidity of the polyelectrolyte/surfactant complex formed decreases with increasing of the content x of the comb-type copolymer in hydrophilic nonionic PDMAM side chains: the system P(NaA-co-NaAMPS)-g-PDMAM12/DTAB is slightly turbid, whereas the solutions of the system P(NaA-coNaAMPS)-g-PDMAM29/DTAB are almost clear and those of the system P(NaA-co-NaAMPS)-g-PDMAM58/DTAB are completely transparent (Figure 1, inset). Although an accurate determination of the turbidity onset for the P(NaA-co-NaAMPS)-g-PDMAM12/DTAB and P(NaA-co-
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Figure 2. Variation of the ratio I1/I3 of aqueous DTAB solutions (b) or of aqueous mixtures of DTAB with (a) P(NaA-co-NaAMPS) (O), (b) P(NaA-co-NaAMPS)-g-PDMAM12 (0), (c) P(NaA-coNaAMPS)-g-PDMAM29 (4), and (d) P(NaA-co-NaAMPS)-gPDMAM58 (3), as a function of the concentration of DTAB. The polymer concentrations are as in Figure 1. T ) 25 °C.
Figure 3. Variation of the reduced viscosity of aqueous solutions of mixtures of DTAB with (a) P(NaA-co-NaAMPS) (O), (b) P(NaA-co-NaAMPS)-g-PDMAM12 (0), (c) P(NaA-coNaAMPS)-g-PDMAM29 (4), and (d) P(NaA-co-NaAMPS)-gPDMAM58 (3), as a function of the concentration of DTAB. The polymer concentrations are as in Figure 1. T ) 25 °C.
NaAMPS)-g-PDMAM29/DTAB systems is not easy due to the small turbidity changes, the trends are similar to those found for the P(NaA-co-NaAMPS)/DTAB system. Thus, the initial onset of turbidity is observed at Z+/- ) 0.7 for both systems. Furthermore, for the system P(NaAco-NaAMPS)-g-PDMAM12/DTAB a more significant turbidity increase is observed at Z+/- > 4, whereas this is not observed for the system P(NaA-co-NaAMPS)-g-PDMAM29/ DTAB. It seems that this last system is the limit, as far as it concerns the composition x of the comb-type copolymers, for obtaining water-soluble P(NaA-co-NaAMPS)g-PDMAMx/DTAB complexes. It is noteworthy that solubility in water is maintained even if the content x in PDMAM is considerably low, x ) 29% (w:w). As mentioned, the appearance of turbidity is accompanied by the formation of large precipitates, for the mixtures of the linear polyelectrolyte with DTAB. As a consequence, the rest studies for this system were limited only in the transparent region, for example, for DTAB concentrations lower than 0.8 mM. On the contrary, the rest systems were studied in the whole surfactant concentration regime, 0-50 mM, as they were either fully transparent or slightly turbid, with no evidence of precipitation. Critical Aggregation Concentration of the Complexes. Pyrene fluorescence probing studies were performed for the determination of the critical aggregation concentration (cac) of the polyelectrolyte/surfactant complexes formed. The variation of the characteristic ratio I1/I3 of the emission spectrum of pyrene is presented in Figure 2, as a function of the DTAB concentration. In the absence of polymer, with an increasing of the surfactant concentration the ratio I1/I3 decreases from the value ∼1.9, characteristic of a polar aqueous microenvironment, to the value ∼1.4, characteristic of a less-polar micellar microenvironment.40 The critical micelle concentration (cmc) of DTAB, corresponding to the onset from the lower “plateau”, is found to be ∼14 mM, in a rather good agreement with the value 16 mM, reported in the literature.2 Similar sigmoidal curves are also found in the presence of polymer, but they are less sharp and they are displaced to a much lower DTAB concentration region, as compared to the curve obtained for pure DTAB. As seen, all data coincide regardless of the composition x of the
copolymer used. The inflection points of the curves, that is, the critical aggregation concentration (cac) for the formation of mixed polyelectrolyte/surfactant aggregates, are found to be ∼0.2 mM in all cases, about 2 orders of magnitude lower than the cmc of DTAB. In addition, we see that the value of the ratio I1/I3 at the lower “plateau” is ∼1.4, similar to the value found for pure DTAB micelles. This is an indication that the mixed polyelectrolyte/ surfactant aggregates are characterized by a microenvironment of a polarity similar to that of the DTAB micelles. Reduced Viscosity Study. Usually, when the main driving force is electrostatic attraction, the association between charged homopolymers and oppositely charged surfactants is accompanied by a dramatic viscosity decrease.42-45 In Figure 4, we present the variation of the reduced viscosity of the backbone and the comb-type polyelectrolytes upon addition of DTAB, as a function of the surfactant concentration. The observed reduced viscosity differences of the pure polymers are probably the combined result of their polyelectrolyte nature and the difference in their molar masses. However, it is observed that the trends of the reduced viscosity changes upon addition of DTAB are similar, regardless of the copolymer used. Thus, at low surfactant concentration the reduced viscosity of the aqueous polyelectrolyte/ surfactant mixtures is hardly influenced by the surfactant addition. On further addition of surfactant, for example, within the 0.1-1 mM concentration region, a drastic decrease of the reduced viscosity is observed. This drastic reduced viscosity decrease, indicating a corresponding significant shrinkage of the polymer chain, is the result of binding of surfactant molecules to the polyelectrolyte chain in this concentration region, leading to the enhancement of the hydrophobic character (Figure 2) and to the decrease of the net charge (Figure 3) of the polyelectrolytes/surfactant complexes. Finally, it is in(42) Abuin, E. B.; Scaiano, J. C. J. Am. Chem. Soc. 1984, 106, 6274. (43) Asnacios, A.; Langevin, D.; Argillier, J. F. Macromolecules 1996, 29, 7412. (44) Fundin, J.; Brown, W.; Iliopoulos, I.; Claesson, P. M. Colloid Polym. Sci. 1999, 277, 25. (45) Asnacios, A.; Klitzing, R.; Langevin, D. Colloids Surf., A 2000, 167, 189.
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concentration c, ηsp(c), can be calculated from the following equation:
ηsp(c) ) ηsp(0)(1 - c/cp)[1 + 2c/(fcp(1 - c/cp))]-3/4
Figure 4. Variation of the specific viscosity, ηsp, of aqueous solutions of mixtures of DTAB with (a) P(NaA-co-NaAMPS) (O), (b) P(NaA-co-NaAMPS)-g-PDMAM12 (0), (c) P(NaA-coNaAMPS)-g-PDMAM29 (4), and (d) P(NaA-co-NaAMPS)-gPDMAM58 (∇), as a function of the concentration of DTAB. The polymer concentrations are as in Figure 1. T ) 25 °C. The curves correspond to the predictions of eq 1.
Figure 5. Variation of the z-potential of aqueous solutions of mixtures of DTAB with P(NaA-co-NaAMPS)-g-PDMAM58 (∇), as a function of the concentration of DTAB. The polymer concentration is 0.050% (w:w). T ) 25 °C.
teresting that the reduced viscosity of the complexes formed for Z+/- g 1 is very low and close to zero, indicating that the interaction of the polyelectrolyte with the surfactant leads to the formation of very compact particles. However, contrary to the system P(NaA-co-NaAMPS)/ DTAB, the attachment of the hydrophilic PDMAM side chains in the comb-type copolymers prevents the further aggregation of the compact particles, stabilizing them to a very small or nonvisible size. An alternative representation of the previous viscosity results is found in Figure 5, where the specific viscosity, ηsp, of the polyelectrolyte/surfactant solutions is plotted against the concentration of DTAB. In this representation, the results can be compared to the theoretical predictions of Colby et al.,46 concerning the rheological properties of aqueous polyelectrolyte solutions with oppositely charged surfactant. According to this model, the specific viscosity of a polyelectrolyte in a solution of a surfactant of a molar (46) Plucktaveesak, N.; Konop, A. J.; Colby, R. H. J. Phys. Chem. B 2003, 107, 8166.
(1)
where ηsp(0) is the specific viscosity of the polyelectrolyte in the absence of surfactant, cp is the molar concentration of the charged units of the polyelectrolyte, and f is the fraction of the effectively charged monomers of the polyelectrolyte. For c > cac, the formation of aggregates should be taken into account and the term c/cp should be replaced by the term (c - cac)/cp. The curves in Figure 5 are calculated according to eq 1, applying the value f ) 0.5, as reported by Gong et al.47 for PNaAMPS hydrogels. It seems that our results support the theoretical predictions, as a rather good agreement is observed for all of the systems. z-Potential Measurements. The formation of mixed polyelectrolyte/surfactant aggregates is a charge neutralization process, as it is shown in Figure 5, where the z-potential of the system P(NaA-co-NaAMPS)-g-PDMAM58/DTAB is presented as a function of the surfactant concentration. As seen, upon addition of surfactant up to e ∼0.01 mM, the z-potential remains almost constant and close to the value obtained for the anionic P(NaAco-NaAMPS)-g-PDMAM58 copolymer (z-potential ) -53 mV). On further addition of DTAB, binding of surfactant molecules to the anionic backbone of the comb-type copolymer occurs and the z-potential of the system increases toward zero. We have to note that at a surfactant concentration of 1.2 mM, for example, at Z+/- = 1, the z-potential of the polyelectrolyte/surfactant complexes formed is still negative, while electroneutral complexes are obtained at a surfactant concentration of ∼4-5 mM, for example, at Z+/- = 4. For Z+/- > 4, the z-potential of the polyelectrolyte/surfactant complex becomes slightly positive, possibly due to an enrichment of the mixed aggregates with positively charged DTAB molecules. Size of the Complexes. The size of the water-soluble complexes formed was determined by dynamic light scattering measurements. In this study, we focus on high charge ratios Z+/-, to ensure that the complex is fully formed and no further binding occurs. In Figure 6, the mean hydrodynamic diameter of the complexes formed between the comb-type copolymers and DTAB is presented as a function of the surfactant concentration. As seen, contrary to the large particles formed when the backbone P(NaA-co-NaAMPS) is mixed with DTAB, the PDMAM side chains attached to the polyelectrolyte stabilize the comb-type polyelectrolyte/surfactant complex particles formed in the nanoscale regime. Probably, the nanoparticles adopt a core-corona structure, as it is schematically presented in Scheme 2. The hydrophobic waterinsoluble core consisted of the complex formed between the cationic surfactant and the oppositely charged backbone, while the hydrophilic nonionic PDMAM chains form the protecting hydrophilic corona. Such a structure has been also proposed for other similar systems.27,28 We should note that these polymer/surfactant nanoparticles are much larger than the pure DTAB micelles (mean diameter ∼4.8 nm48). The stabilizing efficiency of the hydrophilic corona depends on the number of the PDMAM side chains per polyelectrolyte chain, for example, on the weight percent(47) Gong, J. P.; Komatsu, N.; Nitta, T.; Osada, Y. J. Phys. Chem. B 1997, 101, 740. (48) Matzinger, S.; Hussey, D. M.; Fayer, M. D. J. Phys. Chem. B 1998, 102, 7216.
Complexes between Cationic Surfactants and Copolymers
Figure 6. Variation of the mean hydrodynamic diameter of the water-soluble nanoparticles formed in aqueous solution between DTAB and (a) P(NaA-co-NaAMPS)-g-PDMAM12 (0), (b) P(NaA-co-NaAMPS)-g-PDMAM29 (4), and (c) P(NaA-coNaAMPS)-g-PDMAM58 (w:w) (3), as a function of the concentration of DTAB. The polymer concentrations are as in Figure 1. T ) 25 °C. Scheme 2. Schematic Representation of the Structure of the Water-Soluble Polyelectrolyte/ Surfactant Complexes Formed between DTAB and the Comb-Type Copolymers
age x of the copolymers in PDMAM. Indeed, the mean diameter increases as the content x of the copolymers in PDMAM side chains decreases. Thus, the size of the nanoparticles formed for x ) 12 (the mean hydrodynamic diameter is ∼80-110 nm) is 2-3 times the size of the nanoparticles formed for x ) 58 (the mean hydrodynamic diameter is ∼35-40 nm). Moreover, for this high concentration region of DTAB, the size of the nanoparticles formed is practically independent of DTAB concentration for x ) 58, it increases slightly for x ) 29, while it increases substantially reaching a “plateau” for x ) 12. Apparently, as x decreases, the number of PDMAM side chains per polyelectrolyte chain does not suffice to form a well hydrophilic corona for the protection of the nanoparticles. As a consequence, the number (and, thus, the size) of macromolecules involved in each nanoparticle increases with decreasing x. Conclusions In the present work, we explored some basic characteristics of the polyelectrolyte/surfactant complexes formed between the cationic surfactant DTAB and the comb-type copolymers P(NaA-co-NaAMPS)-g-PDMAMx, consisting of the anionic polyelectrolyte backbone P(NaA-co-NaAMPS) onto which a weight percentage x of the hydrophilic nonionic PDMAM chains were grafted. Upon addition of
Langmuir, Vol. 21, No. 20, 2005 9043
DTAB to the linear backbone or the comb-type polyelectrolyte solution, binding of the surfactant molecules to the oppositely charge polyelectrolyte gradually occurs, provided that the concentration of DTAB exceeds the cac. The complexes formed have a very compact structure, as the reduced viscosity studies revealed, due to their increased hydrophobicity (pyrene fluorescence probing study) and to the corresponding charge neutralization (zpotential study). The cac of all of the polyelectrolyte/ surfactant complexes formed is similar, indicating that the attached hydrophilic nonionic PDMAM side chains are not involved in the complexation process. On the contrary, the existence of the PDMAM side chains is crucial, as it concerns the solubility of the complexes in water. Thus, the turbidity studies revealed that, contrary to the complexes formed between DTAB and the linear polyelectrolyte P(NaA-co-NaAMPS), the solubility of the complexes formed between DTAB and the comb-type copolymers P(NaA-co-NaAMPS)-g-PDMAMx is greatly enhanced. In this case, the complexes are probably stabilized in water as core-corona nanoparticles, the core being the water-insoluble complex between the backbone and the surfactant, surrounded by a protective hydrophilic nonionic PDMAM corona. The stabilizing efficiency and, consequently, the size of the nanoparticles formed depend on the content of the copolymers in PDMAM, x, as found by the dynamic light scattering study. Apart from the aforementioned basic information on the characteristics of the water-soluble complexes formed between comb-type polyelectrolytes and oppositely charged surfactants, several other questions should also be addressed for a deeper understanding of these systems, such as: what is the aggregation number of the mixed aggregates, is there any polymer concentration influence, what are the properties of the complexes in solid state, etc. The synthesis of comb-type copolymers through a grafting reaction is a rather facile procedure, performed under mild conditions. However, the great advantage of this procedure is that it can be used with a large variety of monomers, offering the possibility of designing, more or less, at will the composition of the comb-type products. For instance, the cac of the polyelectrolyte/surfactant complexes may be tuned by introducing a nonionic monomer during the preparation of the anionic backbone. Furthermore, the number and the length of the hydrophilic side chains may vary significantly, controlling this way the efficiency of the protective corona. Also, new properties such as thermosensitivity may be introduced, by simply choosing the side chains to be thermosensitive. These are just a few examples demonstrating the large possibilities of using comb-type structures for the introduction of new functionalities and characteristics or for the at will modulation of the properties of water-soluble polyelectrolyte/surfactant nanoparticles. Acknowledgment. We thank the European Social Fund (ESF), the Operational Program for Educational and Vocational Training II (EPEAEK II), and particularly the Program PYTHAGORAS for funding the present work. We acknowledge ICE/HT-FORTH for the use of the z-potential instrument. Prof. G. Staikos is gratefully acknowledged for useful discussions and suggestions. LA0503505
