Langmuir 1998, 14, 6669-6675
6669
Interactions of a Pyrene-Labeled Polyelectrolyte with Oppositely Charged Rodlike Micelles Monitored by Fluorescence Quenching with Thallium Ions Tatsuyoshi Kawamoto and Yotaro Morishima* Department of Macromolecular Science, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan Received June 25, 1998. In Final Form: September 11, 1998 Interactions between poly(sodium 2-(acrylamido)-2-methylpropanesulfonate) labeled with 1 mol % pyrene (PyPAMPS) and rodlike micelles of dimethyloleylamineoxide (DMOAO) were studied by a fluorescence quenching technique using thallium cations (Tl+) as a quencher. The micelle surface charge density was varied by solubilizing n-hexadecyltrimethylammonium chloride (CTAC) in the DMOAO micelle in varying mole fractions or by varying the pH of DMOAO micelle solutions. Macroscopic phase separation was observed when the mole fraction of solubilized CTAC (Y) was increased to a critical level (Yp) in the PyPAMPS-DMOAO/CTAC system or the pH was decreased to a critical value (pHp) in the PyPAMPSDMOAO system. All fluorescence experiments were performed at Y < Yp or at pH > pHp. When Y is sufficiently low or the pH is sufficiently high, the fluorescence of pyrene (Py) labels was efficiently quenched by Tl+ ions and there was no effect of the presence of the micelles on the quenching. As Y was increased or the pH was decreased to a certain critical level (Yc or pHc), the quenching was abruptly diminished, corresponding to the onset of soluble polymer-micelle complex formation. This is due to the protection of Py labels solubilized in the micelle upon polymer-micelle complex formation. The critical charge densities of the micelles calculated from Yc and pHc were in good agreement. All the critical values of Yc, pHc, Yp, and pHp depend on the ionic strength (µ) (e.g., Yp ) 0.04 and Yc ) 0.008 at µ ) 0.10, while Yp ) 0.11 and Yc ) 0.021 at µ ) 0.30). For comparison, interactions of PyPAMPS with spherical micelles of dimethyldodecylamineoxide were investigated by varying Y with CTAC. It was found that Yc values for the rodlike and spherical micelles were virtually identical at a given ionic strength, although Yp for the latter was approximately 1.5 times larger than that for the former.
Introduction Interactions between polyelectrolytes and oppositely charged surfactant micelles normally lead to irreversible bulk-phase separation, arising form strong electrostatic attractions. However, if the electrostatic attractions are attenuated by proper adjustment of the polyion linear charge density (ξ), the micelle surface charge density (σ), or the ionic strength (µ), then soluble polyelectrolytemicelle complexes are formed.1 For example, such soluble polymer-micelle complexes are formed when an ionic surfactant is added to a nonionic surfactant micelle and σ is increased to an adequate level.1 The appearance of the complexed state is so abrupt that one can identify a micelle critical surface charge density (σc) for the onset of complex formation. Extensive studies by Dubin et al.1-6 have shown that the magnitude of this critical value varies directly with µ1/2 (∝κ) and inversely with ξ such that the foregoing observations may be expressed as
σc ∝ ξ-1κ
(1)
where κ is the Debye-Hu¨ckel parameter. Such phasetransition-like behavior is consistent with theoretical (1) See, for example: Dubin, P. L.; Rigsbee, D. R.; Gan, L. M.; Fallon, M. A. Macromolecules 1988, 21, 2555. (2) McQuigg, D. W.; Kaplan, J. I.; Dubin, P. L. J. Phys. Chem. 1992, 96, 1973. (3) Li, Y.; Dubin, P. L.; Dautzenberg, H.; Luck, U.; Hartmann, J.; Tuzar, Z. Macromolecules 1995, 28, 6795. (4) Li, Y.; Dubin, P. L.; Habel, H. A.; Edwards, S. L.; Dautzenberg, H. Macromolecules 1995, 28, 3098. (5) Xia, J.; Zhang, H.; Rigsbee, D. R.; Dubin, P. L.; Shaikh, T. Macromolecules 1993, 26, 2759. (6) Yoshida, K.; Morishima, Y.; Dubin, P. L.; Mizusaki, M. Macromolecules 1997, 30, 6208.
predictions for the interaction of polyelectrolytes with oppositely charged flat,7-9 cylindrical,10 or spherical11 surfaces. If σ is further increased, optically clear solutions of polyelectrolyte-micelle complexes abruptly become strongly turbid at a critical micelle surface charge density (σp) due to irreversible bulk-phase separation. Therefore, soluble complexes are formed only in the region σc < σ < σp. Surfactant molecules form micelles with a range of dimensions and shapes, depending on conditions such as the temperature, surfactant concentration, ionic strength, and additives. Some surfactant molecules form a rodlike structure under proper conditions,12-15 which has been observed by static light scattering12-15 and transmission electron microscopy.16-18 Solutions of very long rodlike micelles may exhibit a viscoelasticity, arising from their entanglement or network structures.18-21 (7) Wiegel, F. W. J. Phys. A: Math. Gen. 1977, 10, 299. (8) Evers, O. A.; Fleer, G. J.; Scheutjens, J. M. H. M.; Lyklema, J. J. Colloid Interface Sci. 1986, 111, 446. (9) Muthukumar, M. J. Chem. Phys. 1987, 86, 7230. (10) Odjik, T. Langmuir 1991, 7, 1991. (11) Von Goeler, F.; Muthukumar, M. J. Chem. Phys. 1994, 100, 7796. (12) Imae, T.; Ikeda, S. J. Colloid Interface Sci. 1984, 98, 363. (13) Imae, T.; Ikeda, S. Colloid Polym. Sci. 1984, 262, 497. (14) Imae, T. J. Phys. Chem. 1990, 94, 5953. (15) Imae, T.; Ikeda, S. Colloid Polym. Sci. 1985, 263, 756. (16) Lin, Z. Langmuir 1996, 12, 1279. (17) Imae, T.; Kamiya, R.; Ikeda, S. J. Colloid Interface Sci. 1984, 99, 300. (18) Shikata, T.; Hitrata, H.; Kotaka, T. Langmuir 1987, 3, 1081. (19) Imae, T.; Ikeda, S. Langmuir 1991, 7, 1734. (20) Kern, F.; Lequeux, F.; Zana, R.; Candau, S. J. Langmuir 1994, 10, 1714. (21) Hoffmann, H.; Rauscher, M.; Gradzielski, M.; Schulz, S. F. Langmuir 1992, 8, 354.
10.1021/la9807659 CCC: $15.00 © 1998 American Chemical Society Published on Web 10/24/1998
6670 Langmuir, Vol. 14, No. 23, 1998 Chart 1
Studies on polymer-micelle interactions have so far focused mainly on spherical micelles, and there has been no report concerning interactions of polyelectrolytes with long rodlike or wormlike micelles. An objective in the present study is to clarify whether polyelectrolytes can form soluble complexes with rodlike micelles and, if so, whether the onset of the complex formation can be described with a distinct micelle surface charge density (i.e., σc). The present paper reports on the interactions of pyrene (Py)-labeled poly(sodium 2-(acrylamido)-2-methylpropanesulfonate) (PyPAMPS) (Chart 1) with rodlike micelles of dimethyloleylamineoxide (DMOAO)12,13,15,19 compared to the interactions of the Py-labeled polymer with spherical micelles of dimethyldodecylamineoxide (DMDAO).2,22 DMOAO forms long rodlike micelles (e.g., 575 nm in contour length13) in wide ranges of the surfactant concentration (3-190 mM) and ionic strength (0 < µ e 2),13,19 whereas DMDAO forms spherical micelles with a hydrodynamic mean diameter of 6-7 nm in a range of the ionic strength 0.05 e µ e 0.20.22 The hydrodynamic diameter of PyPAMPS employed in the present study (12 nm in hydrodynamic diameter at µ ) 0.10) is much smaller than that of DMOAO rodlike micelles but larger than that of DMDAO spherical micelles. We previously reported on the interaction of PyPAMPS with mixed micelles of n-dodecyl hexa(oxyethylene) glycol monoether (C12E6) and n-hexadecyltrimethylammonium chloride (CTAC).6,23,24 We monitored the polymer-micelle interaction via fluorescence quenching using N,N-dimethylaniline (DMA)6,23 or cetylpyridinium chloride (CPC)24 as a quencher solubilized in the micelles. We were able to monitor the polymer-micelle complex formation through fluorescence quenching due to the fact that fluorescence of Py labels is quenched when the labels come into the micelle surface area. However, there are some drawbacks in the use of DMA and CPC as the quencher. DMA is not completely solubilized in C12E6 micelles, and a small fraction of DMA remaining in the bulk aqueous phase quenches the fluorescence of Py labels in uncomplexed polymers.6,23 Being a cationic surfactant, solubilized CPC inevitably causes an increase in σ, although CPC is thoroughly micellized into C12E6 micelles. In the present study, we successfully employed thallium ions (Tl+) for the quencher of Py fluorescence. Because the cationic quencher exists only in the bulk aqueous phase, fluorescence of Py labels is effectively protected from the quenching when the labels are locally solubilized in the micelle. Thus, we were able to detect polymermicelle interactions via the suppression of fluorescence quenching. (22) Dubin, P. L.; Chew, C. H.; Gan, L. M. J. Colloid Interface Sci. 1989, 128, 566. (23) Mizusaki, M.; Morishima, Y.; Yoshida, K.; Dubin, P. L. Langmuir 1997, 13, 6941. (24) Mizusaki, M.; Morishima, Y.; Dubin, P. L. J. Phys. Chem. 1998, 102, 1908.
Kawamoto and Morishima
Experimental Section Materials. Pyrene(Py)-labeled poly(sodium 2-(acrylamido)2-methylpropanesulfonate) (PyPAMPS) (Chart 1) was prepared by copolymerization of 2-(acrylamido)-2-methylpropanesulfonic acid (AMPS) and N-(1-pyrenylmethyl)methacrylamide as reported previously.25 The content of the Py unit in the copolymer was determined to be 1 mol % by UV absorbance at 343 nm. This low mole percent of Py in the polymer ensures the existence of isolated Py chromophores with minimal alteration of polyelectrolyte properties. The weight-average molecular weight of PyPAMPS was estimated to be 4 × 104 by GPC on a Superose 6 column relative to pullulan standards. The apparent Stokes mean diameter of PyPAMPS was estimated by dynamic light scattering to be 12 nm in 0.10 M NaCl. Dimethyloleylamineoxide (DMOAO) was a gift from the Lion Corp. Dimethyldodecylamineoxide (DMDAO) (Fluka Chemie AG) and n-hexadecyl trimethylammonium chloride (CTAC) (Wako Pure Chemicals) were recrystallized twice from ethanol/ acetone (1/50, v/v). TlNO3 and NaCl (Wako Pure Chemical) were used without further purification. Water was purified with a Milli-Q SP system to a specific resistance higher than 14 MΩ cm-1. Fluorescence. Steady-state fluorescence spectra were recorded on a Hitachi F-4500 spectrometer with excitation at 343 nm. A mixture of 0.05 g/L of polymer and 5 or 15 mM DMOAO (or DMDAO) was weakly sonicated for 2 h and magnetically stirred for an additional 12 h at room temperature and stored as a stock solution. The mole fraction of CTAC in the mixed micelles (Y), where Y is defined as Y ) {[CTAC]/([CTAC] + [DMOAO])} or Y ) {[CTAC]/([CTAC] + [DMDAO])}, was adjusted by adding a 22.8 mM CTAC aqueous solution to the polymer/ micelle stock solution with a Gilmont microburet. The mixture was stirred for 30 min in an Ar atmosphere prior to fluorescence measurement. The pH values were adjusted by adding an HCl or NaOH aqueous solution to the mixture by a Gilmont microburet. The ionic strength (µ) was adjusted with NaCl. Fluorescence quenching experiments were performed by adding a 20.0 mM TlNO3 aqueous solution to thus prepared mixture at a predetermined Y, pH, and µ. Turbidimetry. Turbidity was measured at 450 nm on a JASCO V-520 spectrophotometer with a 1-cm path length quartz cuvette at 25 °C. The values of Y and pH were adjusted as described above. The mixtures were stirred overnight under ambient conditions, followed by additional stirring for 30 min in an Ar atmosphere prior to measurement. Transmittance (T) values were corrected by subtracting the turbidity values of polymer-free blank solutions of DMOAO (or DMDAO) at the same Y, pH, and µ. The turbidity values thus corrected are reported in the form of 100 - %T.
Results Fluorescence Quenching for PyPAMPS-DMOAO/ CTAC. Fluorescence quenching of chromophore-labeled polyelectrolytes by Tl+ ions provides a useful tool for studies of the behavior of polyelectrolytes in aqueous solution.26-30 Fluorescence of polyanion-bound aromatic chromophores is efficiently quenched by Tl+ because of the electrostatic attraction of Tl+ by polyanions. When polyanions labeled with aromatic chromophores form complexes with cationic micelles, the polymer-bound chromophore sites may be solubilized into micelles. This situation may lead to a protection of the chromophores from quenching by Tl+ because the quenching of aromatic (25) Morishima, Y.; Tominaga, Y.; Kamachi, M.; Okada, T.; Hirata, Y.; Mataga, N. J. Phys. Chem. 1991, 95, 6027. (26) Morishima, Y.; Nomura, S.; Ikeda, T.; Seki, M.; Kamachi, M. Macromolecules 1995, 28, 2874. (27) Cao, T.; Yin, W.; Webber, S. E. Macromolecules 1994, 27, 7459. (28) Cao, T.; Yin, W.; Armstrong, J. L.; Webber, S. E. Langmuir 1994, 10, 1841. (29) Procha´zka, K.; Kiserow, D. J.; Webber, S. E. Acta Polym. 1995, 46, 277. (30) Kramer, M. C.; Welch, C. G.; Steger, J. R.; McCormick, C. L. Macromolecules 1995, 28, 5248.
Polyelectrolyte-Micelle Interactions
Langmuir, Vol. 14, No. 23, 1998 6671
Figure 1. Stern-Volmer plots for fluorescence quenching by Tl+ for mixtures of PyPAMPS (0.05 g/L) and DMOAO (15 mM) in 0.20 M NaCl at pH 11 at varying Y. Quenching data of PyPAMPS (0.05 g/L) in 0.20 M NaCl at pH 11 in the absence of DMOAO are also presented.
fluorophores by Tl+ requires a short-range interaction due to an external heavy-atom effect31 and Tl+ ions can exist only in the bulk aqueous phase. Therefore, one may be able to monitor polymer-micelle interactions via the suppression of the Tl+ quenching. In fact, our earlier studies of interactions between PyPAMPS and C12E6/ CTAC mixed micelles revealed that Py labels were locally solubilized in the surface area of the micelle when polymer-micelle complexes were formed.6,23,24 Figure 1 shows Stern-Volmer plots for Tl+ quenching for PyPAMPS in the absence and presence of 15 mM DMOAO/CTAC mixed micelles with varying Y (see Experimental Section for the definition of Y) at pH 11 in 0.20 M NaCl. Here, I0 and I are the steady-state fluorescence intensities in the absence and presence of Tl+, respectively. At pH 11, the amineoxide headgroups of DMOAO micelles are not protonated, and thus pure DMOAO micelles are nonionic. The quenching is not affected at all by DMOAO micelles and DMOAO/CTAC mixed micelles of Y ) 0.005 present in the solution. The linear relationship in the Stern-Volmer plot suggests that the quenching is virtually dynamic due to collisions between Py sites and Tl+. When Y is increased to 0.018, the quenching begins to be slightly suppressed. As Y is further increased, the quenching is markedly suppressed and the plot exhibits a strongly downward curvature. These observations suggest that the mixed micelles are electrostatically bound to PyPAMPS such that Py labels are protected by the micelle from encountering Tl+ ions. In the case where there are two fluorophore sites, one accessible to quenchers and the other not, the SternVolmer equation can be modified as31
I0/(I0 - I) ) 1/φK[Tl+] + (1/φ)
(2)
where K is the Stern-Volmer constant for the accessible chromophore, and φ is the fraction of the accessible chromophores. In Figure 2a, the quenching data in Figure 1 were plotted according to eq 2. At Y e 0.018, the plots follow a straight line with the intercept of unity. At Y g 0.031, however, the plots exhibit linear lines with intercepts > 1, both the intercept and slope increasing with an increase in Y. Figure 2b shows modified Stern-Volmer plots at a lower micelle concentration (5 mM DMOAO/CTAC) and at a lower ionic (31) Hashimoto, S.; Thomas, J. K. J. Am. Chem. Soc. 1985, 107, 4655.
Figure 2. Modified Stern-Volmer plots for fluorescence quenching by Tl+ for mixtures of (a) 0.05 g/L of PyPAMPS and 15 mM DMOAO in 0.20 M NaCl at pH 11 at varying Y and (b) 0.05 g/L of PyPAMPS and 5 mM DMOAO in 0.05 M NaCl.
Figure 3. Dependence of accessible fraction φ on Y for mixtures of 0.05 g/L of PyPAMPS and 15 mM DMOAO at pH 11 at varying ionic strengths (µ). The dependence of φ on Y for a mixture of 0.05 g/L of PyPAMPS and 5 mM DMOAO at pH 11 at µ ) 0.05 is also presented.
strength (µ) (0.05 M NaCl). At Y e 0.005 the plots follow a straight line with the intercept of unity, indicative of all fluorophores being accessible to the quencher. The intercept and slope increase with increasing Y at Y g 0.009. From these modified Stern-Volmer plots, we can estimate the fractions of fluorophores that are accessible to the quencher, φ, at varying Y values. We performed Tl+ quenching experiments at varying µ at 15 mM DMOAO/CTAC and estimated φ values as a function of Y. The results are plotted in Figure 3 along with the result at 5 mM DMOAO/CTAC at µ ) 0.05 (Figure 2b). The plots show somewhat clear breaks (i.e., φ is essentially
6672 Langmuir, Vol. 14, No. 23, 1998
Kawamoto and Morishima
Table 1. Critical Y Values for the Mixture of DMOAO(15 mM)/CTAC and PyPAMPS (0.05 g/L) at Varying Ionic Strengths (µ) Determined by Tl+ Quenching, I3/I1 Ratio, and Turbidimetry µ
Yc by Tl+ quenching
Yc by I3/I1 ratio
Yp by turbidimetry
0.05 0.10 0.20 0.30
0.004 0.008 0.014 0.021
0.003 0.008 0.015 0.022
0.03 0.04 0.09 0.11
Figure 5. Turbidity as a function of Y for mixtures of PyPAMPS (0.05 g/L) and DMOAO (15 mM) at pH 11 at varying ionic strengths (µ).
Figure 4. I3/I1 ratios as a function of Y for mixtures of 0.05 g/L of PyPAMPS and 15 mM DMOAO at pH 11 at varying ionic strengths (µ).
unity at lower Y values but commences to decrease at a certain Y value at a given ionic strength). This onset Y value at which φ commences to decrease depends on µ. Since the decrease in the fraction of accessible fluorophores arises from the protection of Py labels upon polymermicelle complexation, this onset Y for φ to commence to decrease may be defined as Yc at which polymer-micelle complexation commences to occur. It is to be noted that Yc for 5 mM DMOAO/CTAC is in fair agreement with that for 15 mM DMOAO/CTAC at µ ) 0.05, although the quenching is more suppressed at the higher concentration of the micelle at Y > Yc. These observations indicate that Yc is dependent on µ but independent of the concentration of DMOAO. Values of Yc estimated from the plots in Figure 3 are listed in Table 1. It is clearly seen that Yc increases as µ is increased, suggesting that the polymer-micelle complexation is driven by electrostatic forces. I3/I1 ratio for PyPAMPS-DMOAO/CTAC. Spectral patterns of pyrene fluorescence provide information about its local environments because the intensity ratio of the third to first vibrational peaks, I3/I1, in pyrene fluorescence spectra is sensitive to the environmental polarity.32 It is generally known that I3/I1 is larger in less polar microenvironments. Figure 4 shows plots of I3/I1 as a function of Y for mixtures of PyPAMPS and DMOAO at varying µ at pH 11. The I3/I1-Y plots in Figure 4 somewhat resemble the φ-Y plots in Figure 3. It appears that I3/I1 commences to decrease near Yc observed by Tl+ quenching (Figure 3), although breaks in the I3/I1-Y plots are not as clear as those in the φ-Y plots. We had anticipated that I3/I1 would have increased upon polymer-micelle complexation but the results were counterintuitive ones. In an earlier work, we encountered a similar unexpected tendency that I3/I1 decreased when PyPAMPS formed complexes with C12E6/ CTAC mixed micelles.23 In a parallel experiment in our previous work, we found that I3/I1 for PyPAMPS dissolved (32) Kalyanasundaram, K.; Thomas, J. K. J. Am. Chem. Soc. 1977, 99, 2039.
in a mixture of water and low-molecular-weight poly(ethylene glycol) (PEG) decreased as the fraction of PEG in the mixture was increased, I3/I1 decreasing from ca. 0.6 in pure water to ca. 0.5 in a PEG/water (80/20, w/w) mixture.23 Therefore, we concluded that the decrease in the I3/I1 ratio upon complexation with the micelle was an indication that Py labels were locally solubilized in the hexa(oxyethylene) layer in C12E6/CTAC mixed micelles.23 These results indicate that the polymeric (methacrylamido)methyl-substituted pyrene moiety in PyPAMPS is not a straightforward polarity indicator. Nevertheless, the I3/I1 ratio reports a change in microenvironments around the Py label. From these considerations, the decrease in I3/I1 observed in the present study suggests that Py labels are locally solubilized in DMOAO/CTAC micelles upon polymer-micelle complexation and that the microenvironment near the micelle surface is somewhat similar to that of the hexa(oxyethylene) layer in C12E6 micelles, bringing about a similar effect on I3/I1. We were able to roughly estimate Yc from changes in I3/I1 in Figure 4. The values of Yc estimated from I3/I1 are in fair agreement with those determined by Tl+ quenching, as compared in Table 1. Turbidimetry for PyPAMPS-DMOAO/CTAC. It is known that there is a critical micelle surface charge density (Yp) at which optically clear solutions of polyelectrolytes and spherical mixed micelles abruptly become strongly turbid.2,33-36 The turbidity of the polymermicelle solution at Y > Yp arises from the formation of insoluble complexes between polyelectrolytes and micelles, leading to macroscopic phase separation. Between Yc and Yp, soluble polyelectrolyte-micelle complexes are formed.1,3-5 In the present study, we found that there was well-defined Yp for solutions of PyPAMPS and DMOAO/CTAC rodlike mixed micelles. In Figure 5, the turbidities are plotted as a function of Y for mixtures of PyPAMPS and DMOAO/CTAC at varying µ at pH 11. As can be seen in Figure 5, Yp strongly depends on µ, Yp increasing with increasing µ. The values of Yp are much larger than those of Yc at a given ionic strength, as can be seen in Table 1. PyPAMPS-DMDAO/CTAC Interactions. DMDAO, a nonionic surfactant with the same hydrophilic headgroup (33) Dubin, P. L.; Rigsbee, D. R.; McQuigg, D. W. J. Colloid Interface Sci. 1985, 105, 509. (34) Dubin, P. L.; Oteri, R. J. Colloid Interface Sci. 1983, 95, 453. (35) Dubin, P. L.; Vea, M. E. Y.; Fallon, M. A.; The´, S. S.; Rigsbee, D. R.; Gan, L. M. Langmuir 1990, 6, 1422. (36) Dubin, P. L.; Davis, D. Colloids Surf. 1985, 13, 113.
Polyelectrolyte-Micelle Interactions
Langmuir, Vol. 14, No. 23, 1998 6673 Table 2. Critical Y Values for the Mixture of DMDAO(15 mM)/CTAC and PyPAMPS (0.05 g/L) at Varying Ionic Strengths (µ) Determined by Tl+ Quenching, I3/I1 Ratio, and Turbidimetry µ
Yc by Tl+ quenching
Yc by I3/I1 ratio
Yp by turbidimetry
0.05 0.10 0.20 0.30
0.003 0.007 0.015 0.023
0.003 0.004 0.015 0.024
0.05 0.07 0.12 0.17
Figure 7. Turbidity as a function of Y for mixtures of 0.05 g/L of PyPAMPS and 15 mM DMDAO at pH 11 at varying ionic strengths (µ).
Figure 6. (a) Modified Stern-Volmer plots for fluorescence quenching by Tl+ for mixtures of PyPAMPS (0.05 g/L) and DMDAO (15 mM) in 0.20 M NaCl at pH 11 at varying Y. (b) Dependence of accessible fraction φ on Y for mixtures of PyPAMPS (0.05 g/L) and DMDAO (15 mM) at pH 11 at varying ionic strengths (µ).
as DMOAO, forms spherical micelles in aqueous solution over a range of pH and µ.22,37 We examined the complex formation of PyPAMPS and spherical DMDAO/CTAC mixed micelles for comparison with rodlike DMOAO/CTAC mixed micelles. Figure 6a shows modified Stern-Volmer plots for the Tl+ quenching of PyPAMPS in the presence of DMDAO/ CTAC mixed micelles at varying Y at µ ) 0.20 at pH 11. The fraction of Py labels accessible to the quencher was estimated from Figure 6a with use of eq 2 in a manner analogous to the case of the rodlike DMOAO/CTAC mixed micelle system. The dependence of φ on Y is plotted in Figure 6b. There are reasonably well-defined breaks in the φ-Y plot, from which Yc values were estimated. As in the rodlike micelle case, Yc values for the spherical DMDAO/CTAC micelle system were able to be estimated from I3/I1-Y plots. These results are listed in Table 2. Dubin et al.2,22 investigated interactions of polyAMPS with DMDAO spherical micelles by turbidimetry at varying ionic strengths. They determined the degree of protonation (β) of micellar amineoxide groups by potentiometric titration.22 By varying β via pH, they observed a critical pH (i.e., a critical micelle surface charge density) at which the turbidity abruptly increases, the critical pH decreasing with an increase in the ionic strength. The critical pHs that they observed were 6.98 and 6.77 at µ ) 0.10 and 0.20, respectively, which were converted into (37) Ikeda, S.; Tsunoda, M.-A.; Maeda, H. J. Colloid Interface Sci. 1979, 70, 448.
the critical degrees of protonation (βc) of 0.08 and 0.12, respectively, using the Henderson-Hasselbach equation.2 With respect to the micelle surface charge density, β for DMDAO micelles corresponds to Y for DMDAO/CTAC mixed micelles. These βc values for the polyAMPSDMDAO system2 are much larger than Yc values obtained in the present study for those of the PyPAMPS-DMDAO/ CTAC system listed in Table 2. In attempt to elucidate this large difference, we performed turbidimetry for the PyPAMPS-DMDAO/CTAC mixed micelle system by varying Y. Figure 7 shows plots of the turbidity as a function of Y for mixtures of PyPAMPS and DMDAO at varying µ at pH 11. There is a critical Y at which an abrupt increase in the turbidity is observed. We define this critical Y as Yp as in the case of the DMOAO/CTAC rodlike micelle system (Figure 5). The values of Yp were estimated to be 0.07 and 0.12 at µ ) 0.10 and 0.20, respectively (Table 2). These Yp values coincide with the βc values for the polyAMPS-DMDAO system reported by Dubin and co-workers.2 Therefore, we conclude that βc reported by Dubin et al.2 corresponds to Yp in the present study, a micelle surface charge density at which bulk phase separation abruptly occurs. We carefully examined the turbidity data for the PyPAMPS-DMDAO/CTAC mixed micelle system in a regime Y < Yp, but there was no sign of an increase in the turbidity near Yc. Therefore, Yc, corresponding to the onset of the formation of soluble complexes, can only be observed by fluorometry. In contrast to Yc, the Yp value for the DMDAO/CTAC spherical micelle system is ca. 1.5 times larger than that for the DMOAO/CTAC rodlike micelle system at a given ionic strength. In other words, the spherical micelles need ca. 1.5 times more charge than the rodlike micelles for macroscopic phase separation to occur. Since the macroscopic phase separation is considered to arise from the aggregation of polymer-micelle complexes which occurs as the net charge of the complex becomes near neutral. PyPAMPS-DMOAO Interactions at Varying pH. The surface charge density of rodlike DMOAO micelles
6674 Langmuir, Vol. 14, No. 23, 1998
Kawamoto and Morishima
PyPAMPS-DMOAO solutions at µ ranging from 0.05 to 0.30 and found a sharp increase in the turbidity in a pH region from 7.2 to 7.6, depending on the ionic strength. The pH at which a sharp increase is observed is defined as pHp which corresponds to the onset of macroscopic phase separation. The values of pHp thus estimated and the critical degree of protonation for the onset of macroscopic phase separation (βp) calculated from pHp are listed in Tables 3 and 4, respectively. The values of βp is in good agreement with the Yp values listed in Table 1.
Figure 8. Dependence of accessible fraction φ on pH for mixtures of PyPAMPS (0.05 g/L) and DMOAO (15 mM) at varying ionic strengths (µ). Table 3. Critical pH Values for the Mixture of DMOAO(15 mM)/CTAC and PyPAMPS (0.05 g/L) at Varying Ionic Strengths (µ) Determined by Tl+ Quenching, I3/I1 Ratio, and Turbidimetry µ
pHc by Tl+ quenching
pHc by I3/I1 ratio
pHp by turbidimetry
0.05 0.10 0.20 0.30
8.5 8.2 8.1 7.9
8.3 8.1 8.0 7.9
7.6 7.5 7.3 7.2
Table 4. Critical Degrees of Protonation for the Mixture of DMOAO(15 mM)/CTAC and PyPAMPS (0.05 g/L) at Varying Ionic Strengths (Μ) Determined by Tl+ Quenching, I3/I1 Ratio, and Turbidimetry µ
βc by Tl+ quenching
βc by I3/I1 ratio
βp by turbidimetry
0.05 0.10 0.20 0.30
0.003 0.008 0.013 0.021
0.005 0.010 0.015 0.021
0.03 0.04 0.07 0.10
can be varied through β of the amineoxide headgroups by adjusting the pH. As the pH is decreased, the surface charge density of the micelle increases, owing to an increase in β. We carried out Tl+ quenching experiments on the PyPAMPS-DMOAO system at varying pHs in a regime 7.3 e pH e 8.9 at varying ionic strengths and analyzed quenching data using eq 2. In Figure 8, thus estimated φ is plotted as a function of pH. Although the plots show some scatter, there seems to be a pH at which φ commences to deviate downward from the line of φ ) 1. We define this pH as pHc which corresponds to the onset of polymer-micelle complex formation. We also attempted to estimate pHc through I3/I1 of Py fluorescence; I3/I1 commenced to decrease at a pH corresponding to the pH at which φ commences to decrease (Figure 8). Values of pHc thus estimated are presented in Table 3. Dubin et al.2,22 reported apparent pKβ (the logarithmic protonation constant) as a function of β at various ionic strengths. Since the protonation takes place at the amineoxide headgroup, it is reasonable to assume that the acid-base equilibrium constant for DMOAO rodlike micelles is nearly equal to that for DMDAO spherical micelles if β is sufficiently low. Using the titration data for DMDAO micelles reported by Dubin et al.,2,22 we calculated βc from pHc. The results of the calculation are presented in Table 4. The βc values are in good agreement with the Yc values listed in Table 1. We measured the turbidity as a function of pH for
Discussion It is an interesting question whether the critical charge density of micelles for the onset of complex formation with polyelectrolytes depends on the micelle dimension. There is a great difference in the shape and dimension between the DMOAO and DMDAO micelles under conditions employed in the present study. An important conclusion in this work is that the values of Yc for the rodlike DMOAO/ CTAC and spherical DMDAO/CTAC micelle systems are virtually identical within the range of ionic strengths employed. This is an experimental manifestation that the critical surface charge density of a rodlike micelle for the onset of soluble polymer-micelle complex formation is the same as that for a spherical micelle if the surfactant headgroups are the same and the tails are similar in length. The surface charge density of amineoxide-type surfactant micelles can be controlled either by adjusting pH or by incorporating an ionic surfactant into the nonionic micelle. However, there is a concern that the incorporation of an ionic surfactant may perturb the surface of nonionic micelles. The good agreement observed both between βc and Yc and between βp and Yp in the present study indicates that the incorporation of trimethylammonium-type surfactants into amineoxide-type micelles causes no particular perturbation of micelles except for the alteration of the surface charge density. In general, a potential problem with polyelectrolytes labeled with hydrophobic fluorescent chromophores is a possibility of perturbation by the label. Earlier studies of the interaction of PyPAMPS with mixed micelles of C12E6/CTAC revealed that, while the binding was predominantly driven by electrostatic forces, micelles bind preferentially to Py sites, indicating a hydrophobic interaction between Py labels and micelles.6 A number of studies by Dubin and co-workers2,33-36,38 have shown that there is a linear relationship between Yc and µ1/2 in complex formation between polyelectrolytes and oppositely charged surfactant micelles. The micelle critical charge density σc is proportional to Yc, and κ -1 (the Debye length) varies with µ-1/2. Thus, from eq 1, the slope of the Yc-µ1/2 plot is proportional to ξ-1 because
dσc/dκ ∝ ξ-1
(3)
A polyelectrolyte with a larger ξ binds more strongly to micelles, the Yc-µ1/2 plot exhibiting a smaller slope (i.e., dσc/dκ). Thus, a smaller slope reflects a stronger binding affinity of a polymer. In Figure 9, Yc, βc, Yp, and βp for the interactions of PyPAMPS with DMOAO rodlike micelles and DMDAO spherical micelles are plotted as a function of µ1/2. All these critical charge density parameters appear to depend linearly on µ1/2, suggesting that the interaction is primarily controlled by electrostatic forces. However, it should be noted that the slope for the Yc and βc plots are much smaller (38) Dubin, P. L.; The´, S. S.; McQuigg, D. W.; Chew, C. H.; Gan, L. M. Langmuir 1989, 5, 89.
Polyelectrolyte-Micelle Interactions
Figure 9. Plots of Yc, βc, Yp, and βp as a function of the square root of the ionic strength (µ1/2) for mixtures of PyPAMPS (0.05 g/L) and DMOAO (15 mM). Plots of Yc and Yp as a function of µ1/2 for mixtures of PyPAMPS (0.05 g/L) and DMDAO (15 mM) are also presented.
than those for the Yp and βp plots. This much smaller slope for Yc and βc may suggest that the micelle binding occurs preferentially to pyrene sites prior to the binding to sulfonate sites in PyPAMPS, as discussed by Yoshida et al.6 for the interaction of PyPAMPS with mixed micelles of C12E6/CTAC. Therefore, Yc and βc values observed in the present study may represent a critical micelle charge density at which interaction between Py sites and the micelles begins to occur as a result of an interplay of electrostatic and hydrophobic interactions. It is important to compare Yc values in the present polymer-micelle system with those reported for other systems. The PyPAMPS-C12E6/CTAC system shows Yc ≈ 0.05 and 0.10 at µ ) 0.20 and 0.30, respectively.6,7 These values are much larger and much more dependent on the ionic strength than those for the present PyPAMPSDMOAO/CTAC system. This difference in the two systems may be attributed to the steric effect of the nonionic headgroups.22 It is likely that, in the mixed micelles of C12E6/CTAC, the cationic headgroups of CTAC are held in the corona layer of hexa(oxyethylene) groups near the interface between the corona and core. Therefore, the mean separation of polyelectrolyte segments from the cationic headgroups is increased, and thus the electrostatic interaction between the polyelectrolyte chain and the micelle charge is diminished. In contrast, in DMOAO micelles, cationic charge is not shielded by the nonionic corona layer and is located on the surface of the micelle, allowing stronger electrostatic interactions with polyelectrolytes.
Langmuir, Vol. 14, No. 23, 1998 6675
Although Yc values for the rodlike and spherical micelles are virtually the same, Yp values for the spherical micelle were found to be ca. 1.5 times larger than those for the rodlike micelle (i.e., the rodlike micelle needs less charge than the spherical micelle for the macroscopic phase separation to occur). The phase separation is considered to be due to the formation of multipolymer higher order aggregates of primary (soluble) polymer-micelle complexes. This may occur when the net charge of the complex becomes near neutral. As Y is increased to Yp, tight ion pairing between a polymer and micelle occurs with the concomitant loss of osmotic swelling of the complex, leading to an insoluble amorphous precipitate.34 Dubin et al.35 discussed a possible effect of micelle size and shape on the higher order association of the primary complexes. A larger (spherical) micelle can lead to tighter binding of a polyion chain to the micelle surface with less osmotic hydration, making the primary complexes easier to aggregate.35 This consideration could explain the observations in the present work. Namely, the PyPAMPS chain can bind to a very long rodlike micelle of DMOAO/CTAC with a more extended conformation than in the binding to a spherical micelle of DMDAO/CTAC of a much smaller size. Thus, the binding to the rodlike micelle may be tighter with less osmotic hydration than the binding to the spherical micelle, the former leading to the primary complex aggregation at a lower micelle charge. However, there is a possibility that a polymer chain may link two or more micelles together and that a network may be formed with the rodlike micelle system. In this situation, the system may form a gel state at Y g Yp. However, this is not the case for the present PyPAMPS-DMOAO/CTAC system. Conclusions Complex formations of a pyrene-labeled polyanion, PyPAMPS, with DMOAO rodlike micelles and with DMDAO spherical micelles were studied by Tl+ quenching of pyrene fluorescence. The micelle surface charge density was varied either by solubilizing CTAC in the micelles in varying mole fractions or by varying the solution pH. When the mole fraction of CTAC in the mixed micelles, Y, is sufficiently low or the pH is sufficiently high, pyrene fluorescence is efficiently quenched by Tl+, indicating no effect of the presence of the micelles on the quenching. As Y is increased or the pH is decreased to a critical level, Yc or pHc, the quenching is abruptly diminished, corresponding to the onset of soluble polymer-micelle complex formation. This arises from the protection of Py labels locally solubilized in the micelles upon polymer-micelle complex formation. The surface charge densities calculated from Yc and pHc agreed well, implying that no particular perturbation is caused by solubilizing CTAC. For comparison, interactions of PyPAMPS with spherical micelles of DMDAO were investigated by varying Y with CTAC. Values of Yc for the rodlike micelle and the spherical micelle are virtually identical at a given ionic strength, although macroscopic phase separation occurs at a lower charge density for the rodlike micelle system. Acknowledgment. This work was supported in part by a Grant-in-Aid for Scientific Research No. 10450354 and by a Grant-in-Aid on Priority-Area-Research, “New Polymers and Their Nano-Organized Systems” (No. 277/ 08246236) from the Ministry of Education, Science, Sports, and Culture, Japan. LA9807659
