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Photophysical Study of the Interactions of Charged Copolymers with Surfactants of Opposite Charge Eduardo T. Iamazaki, Carla C. Schmitt, and Miguel G. Neumann* Instituto de Quı´mica de Sa˜ o Carlos, Universidade de Sa˜ o Paulo, Caixa Postal 780, 13560-970 Sa˜ o Carlos SP, Brazil Received August 21, 2000. In Final Form: March 9, 2001 The interactions between PSS-nBVE copolymers and oppositely charged ionic surfactants (CTAC) in aqueous solution have been investigated using pyrene as a photophysical probe. Static and dynamic fluorescence determinations have been used to obtain information about the microenvironments formed. Micropolarity studies using the I1/I3 ratio of the vibronic bands of pyrene show the formation of hydrophobic domains. Early formed premicelles due to the electrostatic interactions between the copolymers and the surfactant (having I1/I3 ratios around 1.5-1.6) attract initially the free probes that will migrate from the more aqueous environment to these new hydrophobic sites. The cac’s at which these premicelles are formed are found at 0.27 ( 0.2 mM. Pyrene excimers are detected in the same concentration range. At higher surfactant concentrations free micelles are formed with the same properties of micelles in solutions without polymers, presenting I1/I3 ratios around 1.3 and cmc’s around 1.45 mM. Also new excimers are formed that disappeared during the progressive addition of the surfactant.
Introduction Photochemical methods are used extensively in the study of microheterogeneous systems.1,2 The investigation of amphiphilic polymers is an area where great progress has been made using fluorescence techniques.3 Amphiphilic polyelectrolytes, as well as micelle-forming surfactants, lipids, and proteins, form hydrophobic structures in aqueous solutions where fluorescent probes can be placed.4,5 Information about these systems can be obtained from the spectral behavior of solubilized or attached probes, combined with results from quenching experiments. After extensive studies on the properties and characteristics of micelles, formed by the spontaneous aggregation of surfactants in aqueous solutions, attention began to fall on the properties of polymers able to form hydrophobic domains with properties similar those of classical micelles.6 Polymers of this type could be obtained by chemical modification of normal polymers7-9 or by copolymerization of monomers with hydrophobic and hydrophilic groups.10-12 The hydrophobic domains formed * To whom correspondence should be addressed. E-mail:
[email protected]. (1) Kalyanasundaram, K. Photochemistry in Microheterogeneous Systems; Academic Press: New York, 1987. (2) Winnik, F. M. Chem. Rev. 1993, 93, 587. (3) Nivaggioli, T.; Alexandridis, P.; Hatton, T. A.; Yekta, A.; Winnik, A. M. Langmuir 1995, 11, 730. (4) Anthony, O.; Zana, R. Langmuir 1996, 12, 1967. (5) Winnik, F. M.; Regismond, S. T. A.; Goddard, E. D. Colloid Surf. 1996, 243. (6) Dubin, P. L. Microdomains in Polymer Solution; Plenum Press: New York, 1985. (7) Chandar, P.; Somasundaran, P.; Turro, N. J. Macromolecules 1988, 21, 950. (8) Soldi, V.; Erismann, N. M.; Quina, F. H. J. Am. Chem. Soc. 1988, 110, 5137. (9) Chu, D. Y.; Thomas, J. K. Macromolecules 1987, 20, 2133. Chu, D. Y.; Thomas, J. K. Adv. Chem. Ser. 1989, 223, 325. (10) Lacik, I.; Selb, J.; Candau, F. Polymer 1995, 36, 3197. (11) Dowling, K. C.; Thomas, J. K. Macromolecules 1990, 23, 1059. (12) Zhao, C. L.; Winnik, M. A.; Riess, G.; Croucher, M. D. Langmuir 1990, 6, 514. Wilhelm, M.; Zhao, C. L.; Wang, Y.; Xu, R.; Winnik, M. A.; Mura, J. L.; Riess, G.; Croucher, M. D. Macromolecules 1991, 24, 1033. Cao, T.; Munk, P.; Ramireddy, C.; Tuzar, Z.; Webber, S. E. Macromolecules 1991, 24, 6300.
in aqueous solutions of polyelectrolytes can be ascribed to interactions similar to those found in the formation of surfactant micelles, that is, the simultaneous existence of hydrophobic and hydrophilic groups on the chains. Experiments using fluorescent probes with hydrophobic monomers containing long alkyl chains, especially with a low degree of dissociation of the hydrophile, indicated the lack of a point that could be considered equivalent to the critical micelle concentration, cmc, even at moderately high concentrations. In these cases, where the systems can be considered polysoaps, the aggregation processes are assumed to occur intramolecularly.13 At higher concentrations, the primary intramolecular aggregation of these polymers may be superposed by a secondary intermolecular aggregation. Under these conditions, critical concentrations for the associations (point for intermolecular aggregations) have been reported, as well as cmc-like breaks in the surface tension curves.14 The latter processes seem to depend on the molecular weight and the length of the hydrophobic chains.15 Recent studies indicate that the distribution of the hydrophobic groups along the polymer chain also affects significantly this type of association.16-18 Initial studies showed that, in the presence of polyelectrolytes, surfactants form aggregates with properties similar to those of micelles, but in general of smaller size.13 These induced micelles were formed at surfactant concentrations lower than that of the cmc found for the same compounds in pure aqueous solutions. These micelles have been described to be situated along the main polymer chain like a string of pearls.13,19-21 Up to now it does not seem (13) Zana, R. Polyelectrolyte-Surfactant Interactions: Polymer Hydrophobicity, Surfactant Aggregation Number, and Microstructure of the Systems. In Polymer-Surfactant Systems; Kwak, J. C. T., Ed.; Surfactant Science Series Vol. 77, Ch. 10; Marcel Dekker: New York, 1998. (14) Tandford, C. The Hydrophobic Effect; Wiley & Sons: New York, 1980. (15) Oliveira, V. A. de; Tiera, M. J.; Gehlen, M. H.; Neumann, M. G. Photochem. Photobiol. 1996, 63, 779. (16) Chang, Y.; McCormick, C. L. Macromolecules 1993, 26, 6121. (17) Hill, A.; Candau, F.; Selb, J. Macromolecules 1993, 26, 4521. (18) Neumann, M. G.; Sena, G. de Polymer 1999, 40, 5003. (19) Abuin, E. A.; Scaiano, J. C. J. Am. Chem. Soc. 1984, 106, 6274.
10.1021/la001204w CCC: $20.00 © 2001 American Chemical Society Published on Web 04/27/2001
Interactions of Charged Copolymers with Surfactants
Langmuir, Vol. 17, No. 11, 2001 3487 Table 1. Polymerization Conditions and Properties of Copolymers of SS-BVE Obtained by Micellar Copolymerization
copolymer
mole % in mole % in the chargea copolymerb [BVE]/ SS BVE SS BVE [CTAB]
coBVE1 coBVE17 coBVE38 coBVE45 coBVE48 coBVE53
97.0 90.0 50.0 60.0 95.0 85.0
3.0 10.0 50.0 40.0 5.0 15.0
98 82 62 55 52 47
2 18 38 45 48 53
0.05 0.17 0.88 0.70 0.09 0.27
Mwc
yield %
121 000 157 000 204 000 230 000 228 000 374 000
13 49 68 67 68 29
a Experimental concentration ratios used in the synthesis. Obtained by NMR analysis ((3%). c Determined by GPC using NaPSS as standard.
b
Figure 1. Illustration of the aggregation of cationic surfactants induced by copolymers.
to be clear if the interactions that initially bring together both species are of electrostatic character, when the charges on the polyelectrolyte and the surfactant are opposite. On the other hand, in the same situation, as well as for species with the same sign, hydrophobic interactions between the more organic portions of the molecules may induce the approach (Figure 1). Interaction free energies for the formation of induced micelles for acrylic acid-ethyl methacrylate copolymers were calculated by comparing the change in the cmc of similar size surfactants, carrying positive or negative charges.21-23 In this study we want to present results of the interaction between BVE-PSS copolymers and the CTAC surfactant in the low concentration regime of copolymers. These polymers do not have intrinsic pH-dependent configurations, as the hydrophilic part of the polyelectrolyte carries the sulfonate group which is practically completely dissociated at all conditions, so that they can be considered real polyelectrolytes and not polysoaps. The behavior of the solutions was investigated using the intrinsic fluorescence of the polymers and the timeresolved and steady-state fluorescence of pyrene. From the results it was possible to propose that the initial interaction for all the copolymers studied (with hydrophobic content between 0 and ∼50%) was always of electrostatic character. Experimental Details Chemicals. Styrene-4-sulfonate, sodium salt (SS, Aldrich), cetyltrimethylammonium bromide (CTAB, Sigma), n-butylvinyl (20) Shirahama, K.; Tsujii, K.; Tagaki, T. J. Biochem. 1974, 75, 309. Cabane, B.; Duplessix, R. J. Phys. (France) 1982, 43, 1529. (21) Neumann, M. G.; Tiera, M. J. Pure Appl. Chem. 1997, 69, 791. (22) Tiera, M. J.; Neumann, M. G. J. Braz. Chem. Soc. 1995, 6, 191. (23) Oliveira, V. A. de; Tiera, M. J.; Neumann, M. G. Langmuir 1996, 12, 607.
ether (BVE, Polyscience, 99%), and potassium persulfate (KPS, Merck) used for the synthesis of the polymers were used as received. Poly(styrene-4-sulfonate sodium salt) (PSS, Aldrich) was precipitated twice from acetone before use. Hexadecyltrimethylammonium chloride (CTAC, Kodak, 99%) was used as received. Pyrene (Aldrich) was recrystallized twice from ethanol before use. Milli-Q purified water was used throughout. Copolymers. The copolymers were prepared by an aqueous micellar copolymerization technique and characterized by NMR techniques.18 The concentrations of the monomers incorporated in the copolymers, as well as other parameters related to the copolymers used in this work, are shown in Table 1. Fluorescence Measurements. Steady-state fluorescence measurements were performed on air equilibrated solutions using a Hitachi F-4500 spectrofluorimeter. Excitation of the pyrene probe was performed at 334 nm, and the detection wavelengths were 373 nm for I1 and IM, 384 nm for I3, and 475 nm for IE. Fluorescence lifetimes were measured by the time-correlated single-photon counting method using an Edinburgh CD-900 instrument. Samples were excited at 334 nm, and the detection was made at 374 nm. The fluorescence decay profiles were analyzed with a nonlinear least-squares iterative reconvolution method. All samples were examined at room temperature (25 ( 0.5 °C). The solutions containing the probe were prepared by transferring a sufficient amount of a methanolic stock solution of pyrene to a flask under a stream of nitrogen, after which the polymer solution was added. The final pyrene concentration was 5 × 10-7 mol/L for the steady-state fluorescence measurements and 1 × 10-6 mol/L for the fluorescence lifetime measurements. Solutions were allowed to equilibrate for at least 4 h prior to fluorescence measurements. The excitation wavelength was 334 nm; and the intensities of the first and third peaks of the pyrene emission were measured at 373 and 384 nm, respectively. The emission spectra due to the phenyl moieties of the SS monomer, the PSS homopolymer, and the copolymers were measured at concentrations between 0.01 and 6.0 g/L, exciting at 255 nm. No time-dependent intermolecular associations, as reported by McCormick and Chang24 in concentrated solutions, were observed here. All measurements were repeated after 24 h using the same solutions, with identical results.
Results and Discussion The behavior of aqueous solutions of polyelectrolytes when CTAC is added to them can be deduced from the results shown in Figure 2, which illustrates the evolution of the I1/I3 ratio of the peaks of the vibronic structure of pyrene and the ratio between the monomeric and excimer emissions of pyrene IE/IM, in coBVE45 copolymer solutions. The I1/I3 ratio shows an initial rapid decay from values near to 1.8, corresponding to the probe placed in the practically “aqueous” microenvironment around the polyelectrolyte, to values around 1.5, attaining a first plateau at surfactant concentration of ∼0.3 mM. This more hydrophobic domain is ascribed to the formation of (24) McCormick, C. L.; Chang, Y. Macromolecules 1994, 27, 2151.
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Iamazaki et al. Table 2. Critical Aggregation Concentration for CTAC in the Presence of Copolymers Obtained by the Fluorescence Method FVE
[coBVE]
caca
cmcb
cac ∆Gmic
cmc ∆Gmic
copolymer
%
×10-5 mol/L
mM
mM
kJ/mol
kJ/mol
PSS coBVE1 coBVE17 coBVE38 coBVE45 coBVE48 coBVE53
2 18 38 45 48 53
0.98 1.07 1.21 1.27 1.29 1.34
0.25 0.28 0.25 0.29 0.28 0.26 0.26
1.30 1.43 1.46 1.44 1.47 1.46 1.45
-20.6 -20.3 -20.6 -20.2 -20.3 -20.5 -20.5
-16.5 -16.2 -16.2 -16.2 -16.2 -16.2 -16.2
a Critical aggregation concentration. b Critical micellar concentration.
Figure 2. Dependence of the I1/I3 and IE/IM ratios of pyrene on CTAC concentration in the presence of coBVE45 (2.0 mg/L). λexc ) 334 nm. Detection wavelengths: I1 ) IM, 373 nm; I3, 384 nm; IE, 475 nm.
premicelles induced by the (probably electrostatic) interactions between the CTAC and the copolymer. The plateau continues up to concentrations around 1.0 mM, after which the ratio falls again. The second decrease of the I1/I3 ratio is ascribed to the formation of free micelles in the bulk of the solution, as probably the capacity of placing induced micelles on the macromolecular chain is completed at these concentrations (it has to be remembered that the concentration of charged monomers incorporated in the macromolecule, under the working conditions, is about 10-5 M). This decrease characterizes a cmc between 1.3 and 1.5 mM, which can be assigned to the formation of “free” CTAC micelles in aqueous solution (compared with the cmc of 1.27 mM found for the surfactants in pure aqueous solution25). The I1/I3 ratio reaches a new plateau with a value of 1.2, typical of pyrene in CTAC micelles.26 As can be observed in Figure 2, the IE/IM ratio shows a behavior parallel to that of the I1/I3 ratio. The decrease of the I1/I3 ratios is concomitant with the peaks of the IE/IM ratio. This proves that the changes in the system induced by the increase of the concentration of the detergent create new micellelike microdomains with higher hydrophobic character toward which the probes will migrate preferentially. Further addition of CTAC will increase the amount of these “micellelike” structures, generating a redistribution of the probe molecules between a larger number of these microenvironments, thus increasing the amount of emitting monomeric pyrene, until practically all the probe molecules are place in premicelles. In this situation there will be no more than one probe per premicelle, and according to the I1/I3 ratio the hydrophobicity will be less than that of pure water but still larger than that of free CTAC micelles. The second decrease of the I1/I3 ratio is also accompanied by a peak of the IE/IM ratio. In this case, the probe molecules placed in the premicelle microdomains, as probably also the entire premicelles, will tend to migrate to the more hydrophobic free micelle environments, concentrating (25) Tadros, F. Surfactants; Academic Press: London, 1984. (26) Kalyanasundaram, K.; Thomas, J. K. J. Am. Chem. Soc. 1977, 99, 2039.
initially in a few micelles and again giving rise to an increase in the excimer emission. When the number of micelles is sufficiently large to redistribute the probes one-a-piece or less per micelle, a plateau is reached at ∼1.30, which corresponds quite well to the value for CTAC micelles in water.4 The same behavior is observed for all the copolymers used in this work, independently of the hydrophobic monomer content (ranging from pure PSS to 53% BVE). In all cases the value of the cac, which is considered here as the critical concentration for induced aggregates, is always around 0.27 ( 0.02 mM with no defined trend with the monomer composition in the polymer. It has to be noted the cmc for the surfactant in the presence of polyelectrolytes is in the range of 1.45 mM, again independent of the copolymer composition. These values are collected in Table 2. Furthermore, the same values for the cac and the cmc were obtained from conductivity measurements. The results suggest that the main initial interaction between the surfactant and the copolymers has electrostatic character and proceeds between the charged centers on the macromolecular chain and the charged head of the surfactant.4 If this interaction would be hydrophobic, a dependence of the cac on hydrophobic monomer content should be found. The major electrostatic character demonstrated by the effect of the ionic strength on the cac is shown in Figure 3. The addition of NaCl (1.0 × 10-3 M) to coBVE45 practically does not result in any noticeable change of the cac or the cmc, which remain around 0.28 and 1.45 mM, respectively. At higher concentrations of the salt, like 0.1 M, the effect on the cac is so large that the first decrease of the I1/I3 ratios or the IE/IM ratio peak decreases the CTAC concentrations to levels too low to be noticed experimentally. On the other hand, the cmc falls to values around 0.45 mM, as may be expected. Assuming in this case a detection limit of 0.10 mM, a value of ∼543 cal/mol can be ascribed to the difference in the aggregation free energy. [This amounts to the ionic strength effect for the screening of the electrostatic interaction between two charges placed at a distance of about 7.8 Å, which is a reasonable guess for this system.] At lower salt concentrations, like 10-5 M, no effect whatsoever could be noticed. The different microenvironments present in the solutions of the polyelectrolytes were also identified by the lifetimes of pyrene, used as a probe. The single photon counting decays of pyrene in solutions of PSS with various amounts of added CTAC are shown in Figure 4, and that of coBVE17 is shown in Figure 5. The decays can be approached by monoexponential functions for the polyelectrolytes without added surfactant and by biexponential functions after its addition. The corresponding lifetimes are collected in Tables 3 and 4, respectively. In the absence of surfactant, as shown for PSS and coBVE17, the single
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Langmuir, Vol. 17, No. 11, 2001 3489
Figure 5. Emission decays of pyrene in the presence of coBVE17 (2 mg/L) and various concentrations of CTAC. λexc ) 334 nm; λdet ) 374 nm. Table 3. Pyrene Lifetimes in the Presence of PSS (2 mg/L) and CTAC
Figure 3. Dependence of the I1/I3 and IE/IM ratios of pyrene in coBVE-45 on CTAC concentration in the presence of NaCl (1 × 10-3 and 0.1 M): (b) I1/I3; (2) IE/IM. λexc ) 334 nm. Detection wavelengths: I1 ) IM, 373 nm; I3, 384 nm; IE, 475 nm.
[CTAC]
τ1
mM
ns
B1
ns
B2
SHISQ
0 0.1 0.5 1.0 1.7 2.3 3.0
135.1 125.6 120.5 122.8 156.8 172.2 171.8
0.37 0.31 0.29 0.30 0.23 0.31 0.31
40.7 38.0 36.7 44.5 46.2 40.8
0.09 0.13 0.12 0.17 0.09 0.09
1.155 1.022 1.000 1.047 1.102 1.098 0.970
τ2
Table 4. Pyrene Lifetimes in the Presence of coBVE17 (2 mg/L) and CTAC [CTAC]
τ1
mM
ns
B1
ns
B2
SHISQ
0 0.05 0.1 0.5 1.0 2.0
132.4 131.6 130.1 127.7 127.1 173.8
0.41 0.39 0.33 0.32 0.30 0.33
33.5 23.8 30.5 26.6 44.5
0.05 0.04 0.07 0.09 0.07
1.001 0.904 1.025 0.948 1.080 1.206
τ2
The same behavior is noticed in both systems. The contribution of each decay, measured by the factors in the biexponential function, decreased steadily until near the cmc. From this point on, the longer lifetime started to increase, as expected for pyrene free micelles. Conclusions
Figure 4. Emission decays of pyrene in the presence of PSS (2 mg/L) and various concentrations of CTAC. λexc ) 334 nm; λdet ) 374 nm.
monoexponential decay corresponded to a lifetime of around 130 ns, compatible with the decay time of pyrene in aqueous solutions. The addition of surfactant to any of those systems converted the decay to a biexponential one, in which one of the lifetimes remained on the order of 120-130 ns and the second stayed between 35 and 50 ns. Values similar to these were already assigned to pyrene excimers in several systems (including ethanol solutions).27 The latter could be assigned to the lifetime of the excimers.
In the presence of BVE-PSS copolymers, surfactants such as CTAC are aggregated, forming induced micelles at concentrations smaller than those needed to form micelles in aqueous solution. These micelles are formed at concentrations in the range 0.25-0.30 mM, seemingly independent of the composition of the copolymers. The microenvironments formed by these aggregates are less hydrophobic than that of free micelles (I1/I3 ≈ 1.45 against values of 1.3 for normal micelles). At larger surfactant concentrations normal micelles are formed at cmc values slightly larger than those for aqueous solution. The main initial interaction between the surfactant and the copolymers is of electrostatic character and proceeds (27) Wintgens, V. Representative kinetic behaviour of excited singlet states. In CRC Handbook of Organic Photochemistry; Scaiano, J. C., Ed.; CRC Press: Boca Raton, FL, 1989; Vol. II, p 96.
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between the charged centers on the macromolecular chain and the charged head of the surfactant. If it would not be so, a dependence of the cac with hydrophobic monomer content should be found. The major electrostatic character is demonstrated by the effect of the ionic strength on the critical aggregation concentration. The different microenvironments present in the solutions of the polyelectrolytes were also identified by the lifetimes of pyrene, used as a probe. The single photon counting decays can be approached by monoexponential functions for the polyelectrolyte systems without surfactant and by biexponential functions after addition of surfactant. The single monoexponential decay in PSS and coBVE17 corresponded to lifetimes of around 130 ns, compatible with the decay time of pyrene in aqueous solutions. The addition of surfactant to any of those systems converts the decays to biexponential ones, in which one of the lifetimes remains around 130 ns and the
Iamazaki et al.
shorter one falls in the range between 25 and 35 ns, up to the cmc in that medium. The latter could be assigned to the lifetime of the excimers (or of pyrene in the premicellar environment). Its contribution, measured by the factors in the biexponential function, decreased steadily until near the cmc. From this point on, the longer lifetime started to increase, as expected for pyrene in a solution of surfactant with no added polyelectrolyte, as the detergent concentration present as premicelles collapsed into free micelles. Acknowledgment. The authors thank FAPESP (Fundac¸ a˜o de Amparo a` Pesquisa do Estado de Sa˜o Paulo) for financial support. E.T.I. also thanks the same agency for a Graduate Fellowship. LA001204W
