Polyelectrolyte Block

May 2, 2008 - The salt effect on the complex formation of poly(acrylamide)-block-poly(sodium acrylate) (PAM-b-PAA) as a neutral-anionic block copolyme...
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Langmuir 2008, 24, 5707-5713

5707

Salt Effect on Complex Formation of Neutral/Polyelectrolyte Block Copolymers and Oppositely Charged Surfactants Tsuyoshi Matsuda and Masahiko Annaka* Department of Chemistry, Kyushu UniVersity, Fukuoka 812-8581, Japan ReceiVed December 28, 2007. ReVised Manuscript ReceiVed March 6, 2008 The salt effect on the complex formation of poly(acrylamide)-block-poly(sodium acrylate) (PAM-b-PAA) as a neutral-anionic block copolymer and dodecyltrimethylammonium bromide (DTAB) as a cationic surfactant at different NaBr concentrations, CNaBr, was investigated by turbidimetric titration, steady-state fluorescence spectroscopy, and dynamic light scattering. At CNaBr < 0.25 M, DTAB molecules may form micelle-like aggregates on PAM-b-PAA chains to form a PAM-b-PAA/DTAB complex above the critical surfactant concentration Ccritical for the onset of complex formation. In the region of relatively high turbidity, a larger complex is likely to form a core-shell structure, of which the core is a dense and disordered microphase made of surfactant micelles connected by the PAA blocks. The corona was a diffuse shell of PAM chains, and it ensured steric stability. At CNaBr ) 0.25 M, a higher electrostatic intermicellar repulsion and intercomplex repulsion induced by a large amount of bound DTAB micelles may lead to a redissolution of large colloidal complexes into intrapolymer complexes. Moreover, a salt-enhancing effect on the complex formation was observed in the PAM-b-PAA/DTAB system; the critical surfactant concentration decreased with increasing salt concentration at CNaBr < 0.10 M. The salt-enhancing effect is due to the larger increase of interaction in comparison to the screening of the interaction.

Introduction Complexes involving oppositely charged polyelectrolytes or polyelectrolytes and low-molecular weight surfactant constitute a very active field of research in recent years.1–6 These systems have attracted scientific attention due to a number of interesting characteristics. From a scientific point of view, such systems present the possibility of the formation of various self-assembled nanostructures, whose characteristics can be tuned by a large number of parameters, including total concentration, charge ratio, ionic strength, and pH. These systems also show similarities in structure and behavior with more complex biological macromolecular self-assembled systems, such as lipoproteins and protein/DNA complexes.7–9 Since the interaction between polyelectrolyte and oppositely charged surfactant is primarily electrostatic in nature, electrostatic factors, such as macromolecular charge densities and ionic strength, are the most important factors. As to whether or not association takes place seems to depend mainly on the micelle surface charge density, the polymer linear density, and the Debye-Hu¨ckel ion atmosphere thickness.10,11 The addition of salt, therefore, should have a significant effect on the complex formation between polyelectrolyte and oppositely charged surfactant. * To whom correspondence should be addressed. E-mail: annaka@ chem.kyushu-univ.jp. (1) Kogej, K.; Skerjanc, J. In Physical Chemistry of Polyelectrolytes; Radeva, T., Eds.;Surfactant Science Series Vol. 99; Marcel Dekker: New York, 1991; pp 798-828. (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) Thalberg, K.; Lindman, B.; Bergfelt, K. Langmuir 1991, 7, 3. (4) Klotz, J.; Kosmella, S.; Beitz, T. Prog. Polym. Sci. 2001, 26, 1199. (5) Kabanov, V. A. Russ. Chem. ReV. 2005, 74, 1. (6) Pispas, S. J. Phys. Chem. B 2007, 111, 8351. (7) Woodcock, C. L. F.; Frado, L. L. Y.; Rattner, J. B. J. Cell Biol. 1984, 99, 42. (8) Schiessel, H. J. Phys.: Condens. Matter 2003, 15, 699. (9) Harada, A.; Kataoka, K. Macromolecules 1997, 30, 7810. (10) Hansson, P.; Almgren, M. J. Phys. Chem. 1995, 93, 16684. (11) Wang, C.; Tam, K. C. Langmuir 2002, 18, 6484.

The addition of salt, thus far, is considered to weaken the strength of complex formation; that is, the critical surfactant concentration for the onset of complex formation increases with increasing salt concentration.10–15 Furthermore, the addition of an excess amount of salt completely suppresses the formation of complexes.16–19 These salt-reducing effects on complex formation are generally explained in terms of the reduction or complete screening of the electrostatic attraction between polyelectrolyte and surfactant. The effect of salt on the polyelectrolyte/surfactant complexes is opposite from the influence of salt in micellar systems, where stabilization occurs manifested by a lowering of the critical micelle concentration (cmc). The effect of salt is two-fold: (i) reduction in the electrostatic interaction between polyelectrolyte and surfactant and (ii) stabilization of the surfactant aggregates. The first mechanism will dominate at lower ionic strengths, while at higher ionic strengths, the second mechanism will take over. A decrease in critical surfactant concentration for the onset of complex formation with the addition of salt in the oppositely charged polyelectrolyte/surfactant system at high salt concentrations, similar to the cmc behavior for the pure surfactant system, can therefore be expected.2,20 Although this prediction has been put forward, such a salt-enhancing effect on complex formation is not yet well-understood. A more precise study, therefore, must be needed to reveal the salt effect on complex formation between polyelectrolyte and oppositely charged surfactant. Stoichiometric polyelectrolyte/surfactant complexes usually precipitate from aqueous solution. More recently, however, (12) Hayakawa, K.; Kwak, J. C. T. J. Phys. Chem. 1982, 86, 3866. (13) Hayakawa, K.; Kwak, J. C. T. J. Phys. Chem. 1983, 87, 506. (14) Malovikova, A.; Hayakawa, K.; Kwak, J. C. T. J. Phys. Chem. 1984, 88, 1390. (15) Wang, C.; Tam, K. C.; Jenkins, R. D.; Tan, C. B. J. Phys. Chem. B 2003, 107, 4667. (16) Thalberg, K.; Lindman, B. J. Phys. Chem. 1989, 93, 1478. (17) Herslo¨f-Bjo¨rling, Å.; Bjo¨rling, M.; Sundelo¨f, L. Langmuir 1999, 15, 353. (18) Thalberg, K.; Lindman, B.; Bergfelt, K. Langmuir 1991, 7, 2893. (19) Villetti, M. A.; Borsali, R.; Crespo, J. S.; Soldi, V.; Fukuda, K. Macromol. Chem. Phys. 2004, 205, 907. (20) Wang, X.; Li, Y.; Li, J.; Wang, J.; Wang, Y.; Guo, Z.; Yan, H. J. Phys. Chem. B 2005, 109, 10807.

10.1021/la704054h CCC: $40.75  2008 American Chemical Society Published on Web 05/02/2008

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Kabanov and co-workers21 began to explore complexes formed by block ionomer and oppositely charged surfactants, in which the segments of the blocks could be either hydrophilic or hydrophobic. Therefore, this type of complex represents a special class of colloids that exhibit combined properties of amphiphilic copolymers and polyelectrolyte complexes. Colloidal complexes result from a self-assembly mechanism between polyelectrolyte/ neutral block copolymer and oppositely charged surfactant. The block copolymer, also called the double hydrophilic copolymer, is the key feature of the electrostatic self-assembly. The overall size and stability of the colloid depends on the nature of the electrostatic charges, on the molecular weight, and on the flexibility of the chains. Berret and co-workers22–24 performed small-angle scattering (small-angle neutron scattering and smallangle X-ray scattering) studies of colloidal complexes resulting from the self-assembly of poly(acrylamide)-block-poly(sodium acrylate) (PAM-b-PAA) as a neutral-anionic block copolymer and dodecyltrimethylammonium bromide (DTAB) as a cationic surfactant. They proposed that the complex exhibited a core-shell structure, of which the core is a dense and disordered microphase made of surfactant micelles connected by the polyelectrolyte blocks. The corona of the large core-shell complex was a diffuse shell of the neutral chains, and it ensured steric stability. In this study, we focused on the salt effect on the complex formation of PAM-b-PAA as a neutral-anionic block copolymer and DTAB as a cationic surfactant under the presence of various amount of sodium bromide (NaBr). The effect of concentration of added NaBr on the complex formation was investigated by turbidimetric titration, steady-state fluorescence spectroscopy, and dynamic light scattering (DLS).

Experimental Procedures Materials. Acrylamide (AAm) was recrystallized from ethyl acetate and dried in vacuo. Acrylic acid (AA, Wako Pure Chemicals) was distilled under reduced pressure prior to use. 4,4′-Azobis(4cyanopentanoic acid) (ABCA, Wako Pure Chemicals) was recrystallized from methanol. Dimethylsulfoxide (DMSO, Wako Pure Chemicals) and methanol (Wako Pure Chemicals) were distilled over a drying agent under a dry nitrogen atmosphere prior to use. A reversible addition-fragmentation chain transfer (RAFT) agent, 2-cyanopropyldithiobenzoate (CPDB), was prepared according to literature procedure.25–27 N-Phenyl-1-naphthylamine (PNA, Aldrich) was used without further purification. Preparation of PAM-b-PAA. PAM was prepared by RAFT polymerization using CPDB as a chain transfer agent (CTA). AAm, CPDB, and ABCA as initiators were dissolved in DMSO. The solution was deoxygenated by three freeze-pump-thaw cycles, and polymerization was carried out at 70 °C. The polymer was isolated by precipitation in an excess of diethyl ether and purified by repeated precipitations, followed by drying in vacuo. RAFT polymerization of diblock copolymers, PAM-b-PAA, was performed with PAM as the macro-CTA. AA, AIBN, and PAM were dissolved in acetate buffer (pH 5.0), solutions were deoxygenated by three freezepump-thaw cycles, and polymerization was carried out at 67 °C. After polymerization, polymers were precipitated in excess diethyl ether and then dried in vacuo. (21) Kavanov, A. V.; Boronich, T. K.; Kabanov, V. A.; Yu, K.; Eisenberg, A. J. Am. Chem. Soc. 1998, 120, 9941. (22) Herve´, P.; Destarac, M.; Berret, J.-F.; Lal, J.; Oberdisse, J.; Grillo, I. Europhys. Lett. 2002, 58, 912. (23) Berret, J.-F.; Herve´, P.; Aguerre-Chariol, O.; Oberdisse, J. J. Phys. Chem. 2003, 107, 8111. (24) Berret, J.-F.; Vigolo, B.; Eng, R.; Herve´, P.; Grillo, I.; Yang, L. Macromolecules 2004, 37, 4922. (25) Chiefari, J.; Chong, Y. K.; Elcore, F.; Krstina, J.; Jeffery, J.; Le, T. P. T.; Mayadunne, R. T. A.; Meijs, G. F.; Moad, C. L.; Moad, G.; Rizzardo, E.; Thang, S. H. Macromolecules 1998, 31, 5559. (26) Perrier, S.; Barner-Kowollik, C.; Quinn, J. F.; Vana, P.; Davis, T. P. Macromolecules 2002, 35, 8300. (27) Nagasawa, M.; Murase, T.; Kondo, K. J. Phys. Chem. 1965, 69, 4005.

Matsuda and Annaka Preparation of PAM-b-PAA/DTAB Complexes. The colloidal complexes in salt-free solutions might be out of the equilibrium state. Stop-flow experiments conducted by Dautzenberg28 showed that polyelectrolyte complex formation takes place in less than 5 µs, nearly corresponding to the diffusion collision of the polyion coils. In salt-free solutions, frozen structures in a nonequilibrium state are considered to form. In this study, as a standard condition, 0.25 g/L PAM-b-PAA in NaBr was placed in the cell, and an equal volume of DTAB solution and a solution of 0.50 g/L PAM-b-PAA containing the same NaBr concentration were added to the desired mixing ratio. The dosage was carried out continuously at a slow rate under gentle stirring at 25 °C. Solutions were made dust-free by filtration through a Millipore membrane (0.45 µm pore size). Characterization. Gel Permeation Chromatography (GPC). The molecular weight distributions of PAM (macro-CTA) and PAMb-PAA were determined by GPC. GPC was performed using a TOSOH LC-8020 system with TSKgel G2500PWXL + G4000PWXL + G5000PWXL columns using 50 mM NaHCO3/100 mM NaNO3/ 20 mM triethanol amine/0.03% NaN3 as the mobile phase at a flow rate of 1.0 mL/min at 40 °C. Monodisperse poly(ethylene glycol) standards was used for calibration. 1H NMR. The number-average molecular weight, M , of PAM n (macro-CTA) was determined by 1H NMR spectra using a JEOL JNMAL300 spectrometer operating at a frequency of 300 MHz in CDCl3. The probe temperature was kept constant at (0.5 °C by the passage of thermostatically regulated air during accumulation. The temperature was measured with a calibrated thermocouple. Mn was estimated by comparing the 1H NMR peak areas of phenyl protons in the terminal dithiobenzoate with methine protons of the PAM main chains. Potentiometric Titration. Potentiometric titration was carried out to determine the Mn of the PAA block in a titration vessel in which the temperature was controlled to within (0.1 °C of 25 °C under a nitrogen atmosphere. Titration of PAM-b-PAA in its acidic form was carried out with a 200 mM NaOH aqueous solution as a titrant at a 1.0 g/L polymer concentration, and the solution pH was monitored with an ORION 720A pH meter with an ORION 8102BN electrode precalibrated with pH 4.01, 7.0, and 10 buffers. The titration was carried out slowly for several hours to allow for proper equilibration. The equivalence point of the titration was set as the point of intersection of the inflection tangents of the titration curve at high pH values. The degree of polymerization of the PAA block was calculated from the added amount of NaOH at the equivalence point of the titration. Turbidimetric Titration. The transmittance of the solution, %T ) It/I0, was measured at a wavelength of 500 nm using a JASCO V-660 spectrophotometer with a thermostatically controlled 1 cm cuvette. Here, It is the transmitted light intensity, and I0 is the incident light intensity. The turbidity, τ, was determined by using the expression τ ) 100 - %T. To fix the concentration of PAM-b-PAA to be 0.25 g/L, turbidimetric titrations were performed by adding equal volumes of 60 mM DTAB and a solution of 0.50 g/L PAMb-PAA in 0.00, 0.01, 0.02, 0.05, 0.10, 0.20, and 0.25 M NaBr to a solution of 0.25 g/L PAM-b-PAA containing the same NaBr concentration. The final turbidimetric titration curves were corrected by subtracting the turbidity curve for the polymer-free titration. All measurements were conducted at 25 ( 0.5 °C. The sample was equilibrated for 5 min before the data were collected. Laser Light Scattering. DLS experiments were conducted with an ALV DLS/SLS-5000 light scattering system equipped with an ALV-5000 multiple τ digital correlator for the collective diffusion coefficient D. The wave vector q is defined as q ) (4nπ/λ)sin(θ/2), where n is the refractive index of the solution, λ is the wavelength of the incident beam (λ ) 632.8 nm), and θ is the scattering angle. The autocorrelation functions were analyzed by performing an inverse Laplace transform using the routine CONTIN assuming the superposition of exponentials for the distribution of relaxation times. The relaxation rate Γ of each process was calculated as the inverse (28) Dautzenberg, H. In Data EValuation in Light Scattering of Polymers; Helmstedt, M., Gast, K., Eds.; Wiley-VCH: Weinheim, Germany, 2000; p 1.

Complex Formation of Block Copolymer and Surfactant

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Table 1. Characterization of PAAm (Macro-CTA) and Diblock Copolymers

c

code

[M] (mol L-1)

feed [M]/[CTA]/[I]a

reaction time (h)

Mnb

monomer unit [AAm]/[AAc]c

PDId

CPDB-PAM PAM92PAA156

3.80 3.13

5000:10:1 3000:10:2

6 6

6540 18000

92:92:156

1.15 1.18

a Concentrations of monomer [M], RAFT agent or macro-CTA [CTA], and initiator [I]. b Number-averaged molecular weight Mn estimated by 1H NMR. Estimated by 1H NMR and potentiometric titration. d Polydispersity index determined by GPC.

Scheme 1. Preparation of PAM-b-PAA

of its relaxation time 1/Γ. In the case of a diffusive process, its diffusion coefficient D was obtained from the slope of Γ versus q2 by Γ ) Dq2. From this value, the hydrodynamic radius was calculated according to the Stokes-Einstein equation: RH ) kBT/6πη0D, where kB is the Boltzmann constant, η0 is the viscosity of the solvent, and T is the temperature of the sample. The polymer solutions were clarified by filtering through a Millipore membrane (0.45 µm pore size). The samples were equilibrated at the measurement temperature for at least 1 h. Steady-State Fluorescence Measurements. Steady-state fluorescence spectra were recorded on a HITACHI F-2500 fluorescence spectrometer in right-angle geometry (90° collecting optics). All spectra were run on an air-liquid equilibrated solution. Temperature control of the samples was achieved using a water-jacketed cell holder connected to a LAUDA circulating bath. The excitation and emission slit were set at 1.5 nm; the excitation wavelength was 340 nm. A typical procedure for preparing 2 × 10-6 mol/L N-phenyl1-naphthylamine (PNA) in an aqueous solution of PAM-b-PAA was as follows: 10 µL of a 2 × 10-3 mol/L PNA in methanol solution was added to a 10 mL volumetric flask and then dried under a mild flow of N2 gas. After the solvent was evaporated, a calculated volume of PNIPAM-b-PAA solution was added, diluted to 10 mL with deionized water, and stirred for 2 h.

Figure 1. GPC trace for macro-CTA (CPDB-PAM) and PAM92PAA156 in 50 mM NaHCO3/100 mM NaNO3/20 mM triethanol amine/0.03% NaN3 measured at 40 °C.

Results and Discussion Polymerization and Molecular Characterization. PAMb-PAA was prepared by RAFT polymerization using 2-cyanopropyldithiobenzoate (CPDB) as a CTA as shown in Scheme 1. The molecular characteristics of macro-CTA and block copolymers, as determined by 1H NMR, GPC, and potentiometric titration, are presented in Table 1. The obtained polydispersity was low. The living/controlled character of the polymerization was supported by the appearance of a characteristic UV signal at 500 nm due to the absorbance of the dithiobenzoate PhsS(CdS)s chromophore of the CTA for macro-CTAs and block copolymers. Another evidence of this feature was given by characteristic 1H NMR signals for both dithiocarbenzoate and 2-cyanopropyl end groups of the polymer. The use of CPDB led to the formation of ω-dithiobenzoate homopolymers. These subsequently were utilized as macro-CTAs to prepare diblock copolymers. The representative GPC trace of PAM-b-PAA clearly shows the formation of block copolymers: the GPC trace shifted to the higher molecular weight region after polymerization of AA from PAM macro-CTA (Figure 1). This indicates that the PAM precursor efficiently participated as a macro-CTA via

Figure 2. Potentiomatric titration curve for 1.0 g/L PAM92PAA156 in the presence of 100 mM NaCl at 25 °C.

RAFT; therefore, well-defined AB-type block copolymers were produced. Potentiometric Titration. The pH dependence on the degree of neutralization, R ) [Na+]/[COOH]0, where [COOH]0 is the total concentration of carboxyl and carboxylate groups and [Na+] assigns the amounts of added NaOH, for PAM92PAA156 under the presence of 100 mM NaCl at 25 °C is presented in Figure 2. From the titration curve, the apparent pKa value was found to be 5.4. This value agrees well with that obtained for PAA titrated in the presence of 100 mM NaCl (pKa ) 5.4).27 This

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Matsuda and Annaka Table 2. Critical Micelle Concentration for Pure DTAB and Critical DTAB Concentration and Apparent Critical Charge Ratio for PAM-b-PAA/DTAB Complex Formation at Various NaBr Concentrations CNaBr (M)

cmc (mM)a

Ccritical (mM)b

ZCappb

0.00 0.01 0.02 0.05 0.10 0.20 0.25

14.9 11.5 9.26 7.00 4.48 2.60 2.21

2.89 2.50 2.22 2.22 2.56 3.21 5.54

0.95 0.87 0.71 0.71 1.03 1.48 1.63

a

Refs ,20, 31 and 32.

b

Determined by turbidimetric titration.

Figure 3. Turbidimetric titration curves, 100 - %T, for 0.25 g/L PAM92PAA156 with DTAB at different CNaBr values measured at 25 °C.

value is considerably lower than that for linear PAA in the absence of salt, reported to be 6.3.29 This may be explained by the fact that, in the absence of salt, the negative charges created upon deprotonation of the same AA units make the subsequent deprotonation of AA units more difficult. In the presence of sufficient amounts of salt, those charges are screened, facilitating the deprotonation of all AA units. In this study, we investigated the sample, PAM92PAA156, with a degree of neutralization of R ) 1. Critical Surfactant Concentration for Complex Formation. Figure 3 shows the turbidimetric titration curves for PAM92PAA156 with DTAB at CNaBr ) 0.00, 0.01, 0.02, 0.05, 0.10, 0.20, and 0.25 M. The concentration of PAM92PAA156 is fixed at 0.25 g/L. As for the titration curves for CNaBr ) 0.00-0.20 M, the turbidity is constant and very low at low CDTAB values.The CDTAB of the initial turbidity increase is designated as Ccritical, above which the turbidity increases gradually up to a maximum value at C1. Beyond C1, the turbidity remains almost constant. For the system at CNaBr ) 0.25 M, the turbidity decreases sharply with a further increase in CDTAB above C1 and finally becomes small and constant beyond C2. The abrupt change in turbidity arises mainly from the change in mass and size of aggregates in the solution; therefore, the previously mentioned changes in turbidity are the result of the formation of the PAM92PAA156/DTAB complex, which is similar to other oppositely charged polymer/surfactant systems.22–24,30,31 Here, Ccritical values are considered to correspond to the DTAB concentration for the onset of complex formation. Below Ccritical, there exists no micelle-like aggregation in the system. Above Ccritical, PAM92PAA156/DTAB complexes are formed. The concentration at which the micellization begins in the polyelectrolyte solution, denoted as the critical aggregation concentration (cac),32 is generally much lower than the cmc as demonstrated by Hansson33 but increases with increasing concentration of polyelectrolyte and simple salt.10,12,34,35 The Ccritical values obtained by tubidimetruc titration are orders of magnitude larger than cac values for the poly(sodium acrylate)/ DTAB complex under the presence of NaBr as investigated by Hansson and Almgen.10 The multimicelle clusters are considered to be formed at CNaBr ) Ccritical. The cac/cmc ratio is a measure (29) Plamper, F. A.; Becker, H.; Lanzendo¨rfer, M.; Patel, M.; Wittermann, A.; Ballauff, M.; Mu¨ller, A. H. E. Macromol. Chem. Phys. 2005, 206, 1813. (30) Dubin, P. L.; Vea, M. E. Y.; Fallon, M. A.; The´, S. S.; Rigsbee, D. R.; Gan, L. M. Langmuir 1990, 6, 1422. (31) Xia, J.; Zhang, H.; Rigsbee, D. R.; Dubin, P. L.; Shaikh, T. Macromolecules 1993, 26, 2759. (32) Chu, D.; Thomas, J. K. J. Am. Chem. Soc. 1986, 108, 6270. (33) Hansson, P. Langmuir 2001, 17, 4167. (34) Hayakawa, K.; Snterre, J. P.; Kwak, J. C. T. Macromolecules 1983, 16, 1642. (35) Konop, A. J.; Colby, R. H. Langmuir 1999, 15, 58.

Figure 4. Critical DTAB concentrations for the onset of complex formation, Ccritical, at different CNaBr values. Inset: cmc of DTAB as a function of CNaBr. The dashed line is a guide for the eye.

of the strength of the interaction between micelle and polyions. To develop a more thorough understanding of the formation of the complex, a determination of cac is warranted under various conditions, which will be the subject of future studies. The turbidity for the polymer in 0.2 M NaBr approaches a value of 60-65% in 15 mM DTAB. This seems to suggest that the solution is rather cloudy; however, no macrophase separation instead of micellization was observed. It should be noted that this solution showed no precipitate formation even after 1 month of storage at room temperature, which is in sharp contrast to the obvious and prompt precipitation observed in a mixture of DTAB with PAA homopolymers. In the case of complexes in 0.25 M NaBr aqueous solution, they began to redissolve into smaller complexes above C1. Beyond the level-off concentration C2, the turbidity is higher than the value below Ccritical, indicating that smaller complexes may still exist in the system. The critical DTAB concentrations Ccritical at various CNaBr values are listed in Table 2 and plotted as a function of CNaBr in Figure 4 together with cmc values for pure DTAB for comparison.20,36,37 When CNaBr < 0.1 M, Ccritical decreases with the addition of NaBr, which indicates that the addition of salt enhances complex formation between PAA blocks and DTAB at these NaBr concentrations. At CNaBr g 0.1 M, the shift of Ccritical to higher CDTAB with increasing CNaBr is consistent with the generally observed shielding effect of salt on the complex formation. From Figure 3, it also is noted that the turbidity change in the DTAB concentration range of Ccritical < CDTAB < C1 becomes steeper when CNaBr increases from 0.00 to 0.20 M. At CNaBr ) 0.25 M, the turbidimetric titration curve shows an initial slow increase (5.5 e CDTAB e 8.0 mM) and a very steep increaase (8.0 < CDTAB e 10 mM), indicating a weaker tendency for complex (36) Kresheck, G. C.; Haragraves, W. A. J. Colloid Interface Sci. 1974, 48, 481. (37) Andersson, B.; Olofsson, G. J. Chem. Soc., Faraday Trans. 1 1988, 84, 4087.

Complex Formation of Block Copolymer and Surfactant

formation in the range of CNaBr ) 5.5-8.0 mM. DTAB micelles can be formed prior to their aggregation with PAM92PAA156 chains. The observed change in the turbidity can be plausibly explained in terms of competition between the increase of interaction and the screening of interaction. Probably because of the high screening of electrostatic attraction, the PAM92PAA156/ DTAB complex is difficult to form in the NaBr concentration range between 5.5 and 8.0 mM, which leads to a slow increase in turbidity. Above CDTAB ) 8.0 mM, DTAB micelles aggregate with PAM92PAA156 chains to form complexes. In the region CNaBr ) 8.0-10 mM, owing to the increase of interaction being larger than the screening interaction, DTAB molecules may bind to polymer chains to form PAM92PAA156/DTAB complexes. The DTAB concentration at which the complex begins to form in terms of the charge ratio between PAM92PAA156 and DTAB is important to study since it would allow us to know if complexation occurred below or above the charge stoichiometry. Z is defined as Z ) [S]/nPAA[P], where [S] and [P] are the molar concentrations for DTAB and PAM92PAA156, respectively. nPAA denotes the average number of monomers in the PAA block. Z ) 1 describes the isoelectric solution, characterized by the same number densities of positively (DTA+) and negatively (COO-) charged ions. With increasing Z, there exists a critical charge ratio denoted as ZC, above which the turbidity increases, indicating the formation of colloidal complexes. The critical charge ratio, ZC, is that between PAM92PAA156 and DTAB at CDTAB ) Ccritical, and those at different CNaBr values are listed in Table 2. For the system at CNaBr ) 0.00 M, the critical charge ratio is found to be ZC ) 0.95, which is close to 1 but slightly below the stoichiometric charge condition. This may be explained by the model predicting overcharging.38 When a polyelectrolyte adsorbs onto an oppositely charged colloid surface, the positive and negative charges do not compensate. The amount of polymers involved in the adsorption process is such that the initial charge of the colloid is reserved. The change in the ZC value with increasing CNaBr is consistent with that for Ccritical. When CNaBr < 0.1 M, ZC decreases with the addition of NaBr, which indicates that the addition of salt enhances the complex formation. Above CNaBr ) 0.1 M, however, the ZC value increases with CNaBr. At higher salt concentrations, the screening of interaction plays a dominant role, leading to the whole interaction being reduced. From Figure 4, we note the two contrary salt effects depending on the NaBr concentration: the salt-enhancing effect on complex formation between PAM92PAA156 and DTAB in lower salt concentration ranges (CNaBr < 0.1 M) and the salt-reducing effect in higher salt concentration ranges (0.1 M e CNaBr). On one hand, the addition of salt favors the formation and growth of surfactant micelles.39–44 Therefore, in the polyelectrolyte/ surfactant system, surfactant molecules tend to form larger micelles at higher salt concentrations, which have a strong tendency to bind to the polymer chain.10,12,34,35 This salt effect can be designated as an increasing interaction. On the other hand, the addition of salt screens the electrostatic attraction between PAM92PAA156 and DTAB, which weakens the interaction. This salt effect can be designated as the screening interaction. Thus, the overall effect of salt depends on the competition of the increasing interaction with the screening interaction. The (38) Berret, J.-F. J. Chem. Phys. 2005, 123, 164703. (39) Imae, T.; Ikeda, S. J. Phys. Chem. 1986, 90, 5216. (40) Roelants, E.; Schryver, F. C. D. Langmuir 1987, 3, 209. (41) Bo¨hmer, M. R.; Koopal, L. K.; Lyklema, J. J. Phys. Chem. 1991, 95, 9569. (42) Lindman, B.; Wennerstro¨m, H. Top. Curr. Chem. 1980, 84, 744. (43) Hayashi, S.; Ikeda, S. J. Phys. Chem. 1980, 84, 744. (44) Quina, F. H.; Nassar, P. M.; Bonilha, J. B. S.; Bales, B. L. J. Phys. Chem. 1995, 99, 17028.

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Figure 5. (a) Change in turbidity, 100 - %T, for a mixture of 0.25 g/L PAM92PAA156 and 2.5 mM DTAB as a function of CNaBr. (b) Change in maximum emission wavelength, λmax, of the PNA probe in a mixture of 0.25 g/L PAM92PAA156 and 2.5 mM DTAB (solid circles) and in aqueous solution of pure 2.5 mM DTAB (open circles) at various CNaBr values.

promotion of micelle formation induced by added NaBr is more marked at CNaBr values lower than 0.10 M in comparison to higher CNaBr values. In this concentration range, therefore, the attractive interaction may exceed the screening interaction, which leads to an enhancement of complex formation. At higher NaBr concentrations, however, the screening of interaction is dominant, which results in the whole attractive interaction being reduced (CNaBr > 0.10 M). From the turbidimetric titration presented in Figure 3, we note that the mixture of 0.25 g/L PAM92PAA156 and 2.5 mM DTAB behaves differently depending on different CNaBr values, which suggests that the formation of the PAM92PAA156//DTAB complex is likely to be induced by the addition of salt. Therefore, we focused on the change in the turbidity for a mixture of 0.25 g/L PAM92PAA156 and 2.5 mM DTAB as a function of CNaBr (Figure 5a). With increasing CNaBr, the turbidity increases gradually to a maximum value at CNaBr ) 0.10 M and then decreases gradually to a small value and remains constant at CNaBr > 0.20 M. This change in turbidity apparently suggests that the addition of salt to a mixture of PAM92PAA156 and DTAB induces complex formation. In the region of CNaBr e 0.10 M, due to the increasing interaction being larger than the screening interaction, DTAB molecules may bind to polymer chains to form complexes. In contrast, in the region of 0.10 M < CNaBr < 0.20 M, the screening interaction may become dominant, leading to the redissolution of complexes. Above CNaBr ) 0.20 M, the excess amount of salt completely dissociated PAM92PAA156/DTAB complexes into single polymer chains and micelles. Insight into the structure of the PAM92PAA156/DTAB complex may provide help in understanding the salt-enhanced complex formation. The micelle formation of DTAB in aqueous solutions was examined using PNA as a fluorescent probe. This method is based on the sensitivity of the probe to the hydrophobicity and polarity of its environment. In the presence of micelles or similar aggregates, PNA is solubilized within the interior of the hydrophobic part (core) of such aggregates. As a result, significant changes in the spectroscopic properties are observed upon transfer of the probe from an aqueous environment to the nonpolar environment of the core. Figure 5b shows the maximum emission

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Figure 7. Evolution of hydrodynamic radius RH for 0.25 g/L PAM92PAA156/ DTAB in different concentrations of aqueous NaBr solutions as a function of CDTAB. Figure 6. Normalized intensity autocorrelation functions of 0.25 g/L PAM92PAA156 in a CNaBr ) 0.10 M aqueous solution with CDTAB ) 1, 3, 5, 10, 12, and 20 mM at q ) 1.87 × 10-3 Å-1 (θ ) 90°) and 25 °C. The relaxation time distributions are shown in inset for the respective CDTAB values.

wavelength, λmax, of the PNA probe in the mixture of 0.25 g/L PAM92PAA156 at various CNaBr values in the presence of 2.5 mM DTAB together with the curve for the 2.5 mM DTAB aqueous solution for comparison. For the pure DTAB aqueous solution, a sharp blue shift of λmax provides strong evidence that the addition of salt enhances the formation of DTAB micelles. In the range of CNaBr e 0.10 M, the λmax value for the PAM92PAA156/DTAB mixture is lower than that for pure DTAB. This difference becomes marked by the addition of NaBr. It may be due to the binding of micelle-like DTAB aggregates to PAM92PAA156 chains and that hydrophobic microdomains of PAM92PAA156/DTAB complexes are more hydrophobic and compact as compared to pure DTAB micelles. At CNaBr ) 0.10 M, the PAM92PAA156/ DTAB complexes have a similar micropolarity as the pure DTAB micelles, suggesting that PAM92PAA156 chains are located at the surface rather than the interior of the micelle-like DTAB aggregates in PAM92PAA156/DTAB complexes. Above CNaBr ) 0.10 M, however, the difference in the λmax values between the two systems with or without PAM92PAA156 tends to be smaller, owing to the redissolution. The redissolution of the complexes makes the micelle-like DTAB aggregates bound to the PAM92PAA156 chains much more like free DTAB micelles, and finally, the micelle-like aggregates become free micelles when the complexes are completely redissolved. To further investigate the salt effect on complex formation, DLS was conducted to characterize the PAM92PAA156 /DTAB complex in various concentrations of NaBr. Figure 6 shows the normalized intensity autocorrelation functions of 0.25 g/L PAM92PAA156 in a CNaBr ) 0.10 M aqueous solution with CDTAB ) 1, 3, 5, 10, 12, and 20 mM at q ) 1.87 × 10-3 Å-1 (θ ) 90°) and 25 °C. The distributions of relaxation times are shown in the inset of Figure 6. At CDTAB ) 1 mM, two relaxation processes are observed. The fast process with diffusivity Dfast ) 2.7 × 10-7 cm2/s, which corresponds to a hydrodynamic radius RH,fast ) 91 ( 1.1 Å, is attributed to a single polymer chain, while a slower process with diffusivity Dslow ) 0.33 × 10-7cm2/s, which corresponds to a hydrodynamic radius RH,slow ) 740 ( 25 Å, also is observed. This process may be attributed to some kind of polymer aggregate. Both processes exhibit low scattering intensities; this is expected for the unimer chains due to their small size, whereas for the polymer aggregates, it is suggested that they are either very few in number or highly hydrated and, therefore, have a low scattering contrast. Caution is necessary, however. Since the single molecule correlation function is rather

noisy due to the low scattering intensity, the presence of noise on the correlation function (as well as dust in the solutions) can lead to spurious peaks. In addition, it is not possible to apportion the total scattering intensity between the individual components. However, when peaks are well-separated, as in Figure 6 (CDTAB ) 1 mM), the overall trends in the data become systematic, and the sizes of the components match those expected from the structures of the free chain and complex. Since the CONTIN analysis renders an intensity weighted distribution, the proportion of the large particle is strongly exaggerated, as the scattering intensity is dependent on the particle radius (∼R6 for spherical particles). The formation of these aggregates is still not wellunderstood. Above CDTAB ) 3 mM, DLS for solutions shows a remarkable change in the relaxation process as shown in Figure 6. Here, the normalized intensity autocorrelation function exhibits a single exponential function with diffusive character of the concentration fluctuations. For CDTAB ) 3 mM, the single diffusive process with diffusion coefficient D ) 6.0 × 10-8 cm2/s, which corresponds to hydrodynamic radius RH ) 410 ( 5 Å, is observed. Above CDTAB ) 5 mM, the diffusion coefficient D ) 5.1 × 10-8 cm2/s is observed, which corresponds to the hydrodynamic radius RH ) ∼480 Å. It is interesting to note that this polydispersity is narrower than that of a single diblock copolymer chain. Figure 7 exhibits the evolution of the hydrodynamic radius RH for 0.25 g/L PAM92PAA156/DTAB in different concentrations of aqueous NaBr solutions as a function of CDTAB. Three scattering regimes can be distinguished. At low CDTAB (