© Copyright 1996 American Chemical Society
FEBRUARY 7, 1996 VOLUME 12, NUMBER 3
Articles Interaction of Cationic Surfactants with Acrylic Acid-Ethyl Methacrylate Copolymers Vera A. De Oliveira, Marcio J. Tiera, 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, Sa˜ o Paulo, Brazil Received October 4, 1994. In Final Form: September 25, 1995X The aggregation of alkyltrimethylammonium surfactants in the presence of several poly(acrylic acidco-ethyl methacrylate) copolymers has been studied using pyrene as a probe. The charges on the chain assist the formation of the micelle-like aggregates due the onset of electrostatic interactions, lowering the critical micelle concentration by a factor ∼20. The increase in the proportion of nonionic monomers lowers the critical aggregation concentration even more, due to the higher hydrophobic character of the polymer chain. The aggregation number for decyltrimethylammonium bromide (DeTAB) also decreases by a factor 3 when going from PAA to the copolymer with 25% of nonionic monomer.
Introduction Many studies have been performed lately on the interaction between polymers and surfactants.1 These studies were induced by several practical applications of systems of these type, like the recovery of oil spills, as well as due to the biological interest in systems which mimic the interaction of surfactants with proteins, with special interest in the influence of structure, conformation, and ordering of the macromolecules.2 A large number of studies have been done involving nonionic polymers such as poly(ethylene glycol), poly(ethylene oxide), and others.3 In the case of polyelectrolytes these studies are more difficult as in many cases precipitation occurs at low X Abstract published in Advance ACS Abstracts, December 1, 1995.
(1) (a) Dubin, P. L. Microdomains in Polymer Solution; Plenum Press: New York, 1985. (b) Zana, R.; Yiv, S.; Strazielle, C.; Lianos, P. J. Colloid Interface Sci. 1981, 80, 208. (c) Lissi, E. A.; Abuin, E. J. Colloid Interface Sci. 1985, 105, 1. (2) Balazs, A. C.; Hu, J. Y. Langmuir 1989, 5, 1230. (3) (a) Nagarajan, R.; Kalpakci, B. In Microdomains in Polymer Solution; Dubin, P. L., Ed.; Plenum Press: New York, 1985; p 369. (b) Brackman, J. C.; Engberts, J. B. F. N. Langmuir 1992, 8, 424. (c) Inoue, T.; Motoyama, R.; Totoki, M.; Miyakawa, K.; Shimozawa, R. J. Colloid Interface Sci. 1994, 164, 318. (d) Cartalas, A. S.; Iliopoulos, I.; Audebert, R.; Olsson, U. Langmuir 1994, 10, 1421. (e) Zhong, S. F.; Einsenberg, A. Macromolecules 1994, 27, 1751.
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concentrations of the compounds.4 The addition of high concentrations of surfactants may change the conformation of the polyelectrolyte. On the other hand, the presence of the polymer chains will induce surfactant microdomains that may be very different from the micelles formed in homogeneous solution. Apart from the hydrophobic interactions, electrostatic interactions will occur, which change some of the properties of the micelles such as critical micelle concentration (cmc), aggregation number, solubility of substrates, etc. Systems involving surfactants and oppositely charged polyelectrolytes have been studied using various techniques as equilibrium dialysis, conductivity and potentiometry, and fluorescence spectroscopy.5 Earlier work in this area was developed by Kwak and co-workers6,7 using surfactant-sensitive electrodes to study the binding of dodecyl- and tetradecyltrimethylammonium bromide by carboxylic polyelectrolytes. These studies indicate that the binding process is highly cooperative and dependent on the structure and charge density of the polyelectrolyte. For more hydrophobic polyelectrolytes, like poly(styre(4) Dubin, P. L.; Oteri, R. J. Colloid Interface Sci. 1983, 95, 453. (5) Goddard, E. D. Colloids Surf. 1986, 19, 301. (6) Hayakawa, K.; Kwak, J. C. T. J. Phys. Chem. 1982, 86, 3866. (7) Hayakawa, K.; Kwak, J. C. T. J. Phys. Chem. 1983, 87, 506.
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nesulfonate), the binding process has more cooperative character when compared to hydrophilic dextran sulfate and carboxymethyl cellulose.8 Photophysical methods employing fluorescence spectroscopy have been largely used to study the behavior of polyelectrolyte solutions, specially in relation to changes in the conformation and interaction with different cosolutes.9 These techniques, mainly involving fluorescence measurements have been used in various recent studies on polymer-surfactant systems.10 Chu and Thomas11 investigated the interaction between poly(methacrylic acid) and cationic detergents demonstrating a cooperative interaction and its dependence with the chain length of the detergent. Turro et al.12 studied the interaction between poly(acrylic acid) and dodecyltrimethylammonium bromide and found that the aggregation process depends on the flexibility of the chain. Photophysical methods have also been used by us13 and other authors14 to study the changes in the properties of polyelectrolytes when introducing neutral monomers to the polymer chain. Recently, the aggregation of alkyltrimethylammonium surfactants in aqueous poly(acrylic acid) solutions has been studied in order to evaluate the influence of the length of the alkyl chain on the aggregate properties.15 The polarity and rigidity of the new microenvironments were found to depend on the interchain H bonds. In this paper we want to present results on the interaction between cationic surfactants and acrylic acidco-ethyl methacrylate copolymers (I), using pyrene as a probe. Results are presented related to the formation process of microdomains in these systems, their structure and properties. These results are related to the dependence with the charge density on the polymer chain.
Experimental Details Synthesis of Copolymers. The copolymers used in this work were prepared by thermal polymerization of appropriate proportions of the monomers in dimethylformamide. The ampules with the mixture of monomers were degassed in high vacuum by four freeze-and-thaw cycles. Polymerizations were performed at 60 °C for 24 h, using 1% of AIBN as initiator. The polymers were precipitated from the reaction mixture by addition of ethyl ether. The amount of acrylic acid in the copolymers were determined by titration with freshly prepared NaOH. Molecular weights were determined by gel permeation chromatography on a Shimadzu C-R7A chromatograph using refractive index detection. These analyses were performed at fixed ionic strength (0.1 M (8) Hayakawa, K.; Santerre, J. P.; Kwak, J. C. T. Macromolecules 1983, 16, 1642. (9) (a) Kalyanasundaram, K. Photochemistry in Microheterogeneous Systems; Academic Press: New York, 1987. (b) Winnik, F. M. Chem. Rev. 1993, 93, 587. (10) (a) Zana, R.; Lang, J.; Lianos, P. In Microdomains in Polymer Solution; Dubin, P. L., Ed.; Plenum Press: New York, 1985; p 357. (b) Maltesh, C.; Somasundaran, P. J. Colloid Interface Sci. 1993, 157, 14. (c) Schild, G. H.; Tirrell, D. A. Langmuir 1990, 6, 1676. (d) Dualeh, J. A.; Steiner, C. A. Macromolecules 1990, 23, 251. (11) Chu, D.-Y.; Thomas, J. K. J. Am. Chem. Soc. 1986, 108, 6270. (12) Chandar, P.; Somasundaran, P.; Turro, N. J. Macromolecules 1988, 21, 950. (13) (a) Tiera, M. J.; Neumann, M. G.; Bertolotti, S. G.; Previtali, C. M. J. Macromol. Sci., Pure Appl. Chem. 1992, A29, 689. (b) Tiera, M. J.; Neumann, M. G.; Bertolotti, S. G.; Previtali, C. M. J. Macromol. Sci., Pure Appl. Chem. 1994, A31, 439. (14) (a) Maltesh, C.; Xu, C.; Somasundaran, P.; Benton, W. J.; Nguyen, H. Langmuir 1992, 8, 1511. (b) Limbele, W. B.; Zana, R. Macromolecules 1990, 23, 2731. (15) Choi, L.-S.; Kim, O.-K. Langmuir 1994, 10, 57.
De Oliveira et al. Table 1. Molar percentage of Ethyl Methacrylate and Molecular Weight of the Copolymers PAA
COP10
COP15
COP25
COP30
% EMA 11.6 17.4 24.0 29.2 in the copolymers Mw 8.1 × 104 1.24 × 105 1.45 × 105 9.6 × 104 1.1 × 105
NH4NO3 containing 1% of ethylene glycol), using poly(styrene sulfonate) standards. The proportions of ethyl methacrylate (EMA) and the molecular weights of the copolymers are shown in Table 1. These copolymers are expected to be mostly random as the product of the reactivity ratio for this system can be estimated as 0.7.16 Chemicals. The surfactants decyltrimethylammonium bromide (DeTAB, Eastman Kodak), dodecyltrimethylammonium bromide (DoTAB, Aldrich), and cetyltrimethylammonium bromide (CTAB, Sigma) were recrystallized from ethanol, and dodecylpyridinium chloride (DPyC, Aldrich) was recrystallized from ethanol/ethyl ether. Pyrene (Aldrich) was purified by two recrystallizations from ethanol. All the solutions were prepared in deionized water (Milli-Q). pH was adjusted by addition of appropriate amounts of NaOH or HCl. Fluorescence Measurements. Fluorescence measurements, using pyrene 10-6 M were made on an Aminco-Bowman spectrofluorometer. Pyrene was added to the polymer solutions, which were shaken vigorously and allowed to equilibrate for 24 h before measurement. Surfactants from concentrated stock solutions were added to the polymer solutions under magnetic stirring and the fluorescence spectra (λexc ) 310 nm, slits ) 2.0 and 0.2 mm) were recorded after each addition. The ratio between the intensities of peaks I (373 nm) and III (383 nm) of the emission spectrum of pyrene (I1/I3) was used to evaluate the polarity of the local environment.17 The symbols I and I0 are the intensities at 373 nm in the presence and absence of the surfactant, respectively. Fluorescence decay measurements were made by single photon counting with a CD-900 Edinburgh Instruments spectrometer. Excitation of the samples was performed at 330 nm and the pyrene monomer emission was measured at 380 nm. Absorption spectra were recorded on a Hitachi U-2000 spectrophotometer. Aggregation Numbers. The aggregation numbers for the surfactants in the presence of the copolymers were determined by dynamic and static methods. Dynamic quenching experiments of pyrene by dodecylpyridinium chloride in the presence of DeTABwere fitted to be Poisson distribution using18
I(t) ) I(0) exp[-k0t] + n j [exp(-kqt) - 1]
(1)
The curves so fitted provide values for k0, kq, and n j , where n j corresponds the average number of quencher molecules per aggregate and k0 and kq are the first-order constants for the decay of pyrene in the absence and presence of quencher, respectively. The concentration of aggregates can also be determined from static fluorescence quenching experiments assuming that the dodecylpyridinium molecules are distributed between aggregates according to the Poisson model. Then the steady-state fluorescence quenching behavior can be fitted with19
ln (I0/I) ) [Dpy]/[aggr]
(2)
where I0 and I are the intensities in the absence and presence of dodecylpyridinium ([Dpy]).
Results and Discussion Pyrene in the Presence of Copolymers. The fluorescence spectra of pyrene in water in the presence of COP15 and COP15 plus DeTAB at pH 4.0 are shown in (16) Braun, D.; Cherdron, H.; Kern, W. Practical Macromolecular Organic Chemistry; Harwood Academic Publ.: Chur, 1984; p 212. (17) Kalyanasundaram, K.; Thomas, J. K. J. Am. Chem. Soc. 1977, 99, 2039. (18) Tachiya, M. Chem. Phys. Lett. 1975, 33, 289. (19) Chu, D.-Y.; Thomas, J. K. Macromolecules 1987, 20, 2133.
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Figure 1. Fluorescence spectra of pyrene in water, pyrene in the presence of 1.0 g/L of COP15, and pyrene and COP15 with 2.0 × 10-3 M DeTAB, pH ) 4.0. Figure 3. Dissociation degree (R) of the polyelectrolytes as a function of pH for PAA (b), COP10 (O), COP15 (9), and COP25 (0). [COP] ) 1.0 g/L.
Figure 2. Changes in the ratio I1/I3 as a function of pH for PAA (b), COP10 (O), COP15 (9), COP25 (0), and COP30 (2). [COP] ) 1.0 g/L.
Figure 4. Fluorescence intensity of pyrene (10-6 M) in the presence of polyelectrolytes and different concentrations of DeTAB: (b) PAA (pH ) 8.63); (O) COP10 (pH ) 7.54); (9) COP15 (pH ) 7.04), (0) COP25 (pH ) 6.03).
Figure 1. In the presence of the polymer, the vibronic band 3 of pyrene increases significantly due to its incorporation in the hydrophobic domains of the chain. This incorporation also results in an increase of the emission quantum yield. The conformational changes of the polymer chain as a function of pH can be observed by monitoring the I1/I3 ratio of the emission spectra of pyrene, as shown in Figure 2. The general behavior depends on the composition of the polymer chains. The decrease of the I1/I3 ratio at low pH, for the copolymers having higher EMA content, shows that pyrene is increasingly protected from the aqueous medium with the increase of the proportion of the nonionic monomer. For COP30 and COP25 this value is close to that observed for ethyl acetate (1.45),17 indicating that water is almost excluded from the microdomains where pyrene is hosted. This effect can be ascribed to intramolecular hydrophobic interactions, that become more important for larger EMA-content polymers. For these copolymers the increase in pH causes more abrupt changes in the I1/I3 ratio, which may be related to the destruction of the intramolecular hydrophobic interactions, that are more important for the more hydrophobic polymers. However, there seem to remain low polarity domains even at relatively high pH (e.g., for COP30 at pH 6 I1/I3 is still 1.6). At low pH, the difference in I1/I3 between PAA and COP30 is about 0.3, compared with 0.15 at pH 8. The pH at which the conformational transition is observed also increases from PAA to COP30 and is better defined in the latter cases. This indicates that for the more hydrophobic copolymers, pyrene is not ejected completely to the aqueous
medium but remains incorporated in the low polarity sites still present in the macromolecular chain at high pH. Interactions between Polyelectrolytes and Cationic Surfactants. The interaction between cationic surfactants and ionic polyelectrolytes involves hydrophobic and electrostatic forces. In order to evaluate the effect of the hydrophobic monomer in the interaction polyelectrolyte-surfactant, experiments were performed at the same ionization degree (R). In Figure 3 are shown the degrees of ionization of PAA, COP10, COP15, and COP25 as a function of pH. Ionization seems to be easier for the polymers with lower charge density. All subsequent experiments were performed at pH values corresponding to R ) 0.6. The fluorescence intensity of pyrene in the presence of the various copolymers is shown in Figure 4 as a function of added DeTAB. The addition of small amounts of DeTAB to solutions of PAA and COP10 results in a small initial decrease of the fluorescence intensity of pyrene, until a first plateau is reached. Further addition of the surfactant leads to an increase, until a first plateau is reached. As can be seen from Figure 2, in solutions of PAA and COP10 at pH 8, pyrene is mostly located in the aqueous phase. Although the initial decrease has been ascribed exclusively to the quenching of pyrene by bromide counterions,11 the same effect, although slightly diminished, is observed when using the chlorine salt of the surfactant (Figure 5). A differential solubility of oxygen has also been ruled out by performing the same experiments in deareated solutions. Finally, the emission spectra in this range of surfactant concentration show an increasing amount of
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De Oliveira et al. Table 2. Critical Aggregation Concentrations and Mean Aggregation Numbers of DeTAB in the Presence of Acrylic Acid-Ethyl Methacrylate Copolymers polymers
pH
cac (M × 10-3)
(DeTAB) PAA COP10 COP15 COP25
7.00 8.63 7.54 7.04 6.02
65.0 3.63 2.25 2.10 1.85
N h agga
N h aggb ∆G (kcal/mol)
120 ((10) 48 ((8) 42 ((7)
48 109 73 46 32
-1.60 -3.45 -3.62 -3.66 -3.73
a From lifetime measurements. b From steady-state measurements.
Figure 5. Fluorescence intensity of pyrene (10-6 M) in the presence of PAA (1 g/L) and (9) DeTAB, (b) DeTACl, and (0) degassed DeTAB.
Figure 7. Fluorescence decays for pyrene in the presence of COP15 (1.0 g/L) and DeTAB (4.0 × 10-3 M) quenched by dodecylpyridinium chloride: (a) 0; (b) 7.13 × 10-5 M; (c) 1.06 × 10-4 M.
Figure 6. Fluorescence spectra of pyrene in the presence of PAA (1.0 g/L) and various amounts of DeTAB.
excimer emission (Figure 6). Furthermore, from the quenching of pyrene by bromide ions (NaBr) in polyelectrolytes without added surfactant, the contribution of the quenching by bromide can be estimated to be less than 20%. On the other hand, for the more hydrophobic polyelectrolytes (COP15, COP25, and COP30) a small increase of the intensity is observed before the first plateau. These results can be interpreted by assuming the initial formation of small premicellar aggregates involving a few molecules of the surfactant, to which the pyrene molecules will migrate. From the curves in Figure 4, critical aggregation concentrations (cac’s) can be defined in the range between the beginning of the rise of the fluorescence intensity and the leveling off of the curve. It is assumed that the pyrene probe is preferentially redistributed to the hydrophobic domains thus formed. The initial rise can be associated with the formation of these aggregates, which result from the interaction between the polymer chain and the surfactants. Previous studies20 confirmed the presence of these domains which also induce a contraction of the polymer chain. For the more hydrophobic polyelectrolytes the cac’s are displaced to lower surfactant concentrations, and the limiting intensities are higher, suggesting more hydrophobic and compact microdomains. The values of the cac’s, cmc, and aggregation numbers of DeTAB are shown in Table 2. (20) Abuin, E.; Scaiano, J. B. J. Am. Chem. Soc. 1984, 106, 6274.
The free energies were calculated using ∆Gcac ) RT ln(cac),11 and correspond to the transference of the detergent from the aqueous phase to the induced micelle in the polyelectrolyte domain. The differences between the cmc for the aggregation in pure water and in the presence of PAA are quite large (a factor of about 20), leading to a difference in free energies of 1.8 kcal/mol. This difference can be ascribed to the electrostatic and hydrophobic interactions between the polyelectrolyte and the surfactant. The incorporation of uncharged monomers to the polymer chain reduces the cac by an additional factor of 2, while the carboxylate groups decrease only 30%. The values in the table show that the increase in the hydrophobicity of the polyelectrolyte results in free energy changes of 200-300 cal/mol, when comparing PAA solutions with those of the copolymers. This variation is relatively small, possibly because the increase of the hydrophobic effect is compensated by a loss of electrostatic interactions. The aggregation numbers of the surfactants were determined from the dynamic decay curves of pyrene quenched by Dpy using eq 1 (a typical decay is shown in Figure 7). Static fluorescence measurements of the same system allowed an alternative way of doing these evaluations. Assuming that all of the DeTAB molecules are present as aggregates in the polymer domain and that the Dpy quencher is placed in the aggregates, the mean aggregation numbers (N) can be calculated from eq 3. On
N ) [DeTAB]/[aggr]
(3)
the other hand, from potentiometric studies of the interaction of carboxylic polyelectrolytes with cationic surfactants,8,21 it was found that only a very small concentration of the detergents remains in the bulk of (21) Kiefer, J. J.; Somasundaran, P. Langmuir 1993, 9, 1187.
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a
b
Figure 8. (a) Stern-Volmer plots for the quenching of pyrene by dodecylpyridinium chloride in the presence of DeTAB 1.0 × 10-3 M and (b) PAA, (O) COP10, (9) COP15, and (0) COP25. (b) Corresponding logarithmic plots.
solution. The quenching of the pyrene fluorescence by dodecylpyridinium in the presence of copolymers and DeTAB 4.0 × 10-3 M is shown in Figure 8. The N h values determined from static and dynamic experiments are shown in Table 2. The results from transient measurements are in good agreement with those determined from steady state. For PAA the aggregation numbers are about three times larger than those in aqueous solution and decrease for the more hydrophobic polyelectrolytes (larger proportion of EMA). The increase of the aggregation number for DeTAB in the presence of PAA and COP15 can be ascribed to the screening of the anionic charges by the polyelectrolyte, diminishing the electrostatic repulsion. In these cases higher ionic strength will result in an increase of the aggregation numbers, similar to what is observed when salt is added to aqueous solutions of surfactants. For the more hydrophobic copolymers the decrease of the aggregation number may be due to the aggregation process being induced at lower polarity sites, formed by the EMA monomers. This leads to smaller aggregates and to a lower quenching of the pyrene placed in them, as represented in Figure 6. The interaction between cationic surfactants and poly(styrenesulfonate) induces the formation of aggregates in a similar way. In these systems, the aggregation numbers are about two or three times smaller than those observed in pure surfactant solutions.22 A tentative description of these aggregates and the behavior of the probe are shown in Figure 9. For PAA (22) Almgren, M.; Hansson, P.; Mukhtar, E.; Stam, J. V. Langmuir 1992, 8, 2405.
Figure 9. Illustration of the aggregation of cationic surfactants induced by PAA and PAA-EMA copolymers.
and COP10, most of the probe molecules will be situated on the polymer chain in a quasi-aqueous environment. The addition of surfactant molecules will create an interaction with the chain that is mainly electrostatic (at least initially), forming premicelles with their hydrophobic chain away from the polymer backbone. As these premicelles are more hydrophobic than the polyelectrolyte chain, the probe molecules will migrate to them and create relatively high local concentrations, as evidenced by the increase in the excimer emission and decrease in the fluorescence yield. This behavior is similar to that observed for the interactions of dyes and surfactants.23 Further increase of the surfactant concentration leads to a larger number of aggregates, in which the probe may redistribute. Thus, the probability of finding more than one probe in one aggregate is reduced and excimer emission disappears gradually. For the higher concentrations of surfactant, where the upper intensity plateau is reached, the excimer emission is not detected anymore. The pyrene dimers may be formed, at least partially, before excitation as is evidenced by a decrease of the absorption band of pyrene on addition of the surfactant. This behavior (23) Neumann, M. G.; Gehlen, M. H. J. Colloid Interface Sci. 1990, 135, 209.
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Table 3. Critical Aggregation Concentrations for Alkyltrimethylammonium Bromides in the Presence of COP25 surfactant
cac (M)
∆GA (kcal/mol)
cmc (M)a
∆GM (kcal/mol)
DeTAB DoTAB CTAB
1.85 × 10-3 3.15 × 10-4 1.98 × 10-4
-3.73 -4.75 -5.03
6.5 × 10-2 1.5 × 10-2 9.2 × 10-4
-1.6 -2.4 -4.1
a
From ref 26.
is similar that observed by Herkstroeter et al.24 using pyrene derivatives dissolved and bound to anionic and cationic polyelectrolytes and Winnik et al.25 with pyrenelabeled derivatives of poly(N-isopropylacrylamide). Inversely, for COP15, COP25, and COP30, pyrene is placed in hydrophobic microdomains originally formed by the intramolecular interactions of the polyelectrolytes. The addition of surfactant will increase the hydrophobicity of these microdomains, due to their interaction with the alkyl chain of the molecule. Therefore, the probes situated in these regions will have a higher emission yield. It can be noticed that the increase in the fluorescence intensities occurs at surfactant concentrations well below the cmc of the free detergent. This clearly indicates a cooperative interaction between both components of the system that form hydrophobic domains. Dependence of cac with pH and Alkyl Chain Length. The cac values for DeTAB, DoTAB, and CTAB in the presence of COP25 are shown in Table 3. The free energy change for the aggregation process is always larger than that for aqueous solutions of the detergents. The increase for the surfactants with longer chains is ascribed to the larger hydrophobic interactions, originated by the additional methylene groups. Similar effects were observed for the cmc.26 A small increase of the cac values can be observed when increasing the pH of the solution, as seen in Figure 10. In the range shown, the conformation of the polymer chain changes very little with pH (Figure 2), so that higher pH will only increase the amount of anionic sites on which (24) Herkstroeter, W. G.; Martic, P. A.; Hartman, S. E.; Williams, J. R. L.; Farid, S. J. Polym. Sci., Polym. Chem. Ed. 1983, 21, 2473. (25) Winnik, F. M.; Ringsdorf, H.; Venzmer, J. Langmuir 1991, 7, 912. (26) Fendler, J. H.; Fendler, E. J. Catalysis in Micellar and Macromolecular Systems; Academic Press: New York, 1975; p 33.
Figure 10. pH dependence of the cac for the COP20-DeTAB system.
the surfactant may aggregate. Therefore, the variation of the cac may be ascribed to changes in the ionic strength of the solution. Conclusions Anionic polyelectrolytes induce aggregation of cationic surfactants and the presence of charges on the polyelectrolyte chain will assist further the formation of the aggregates due to the onset of favorable electrostatic interactions. The values of the cac are about a factor 1.5 lower than those of the cmc of the detergents used, but the aggregates formed are larger than those in aqueous solutions. The introduction of hydrophobic moieties in the polymer chain decreases the cac, as well as the aggregation numbers. This can be ascribed to the fact that the detergent will start the aggregation process preferentially on the polymer sites where hydrophobic domains are already formed. Furthermore, the existence of these additional hydrophobic interactions reduces the need for a larger number of surfactant molecules to stabilize the system, so that the aggregation numbers will be lower for the more hydrophobic polyelectrolytes. Acknowledgment. Financial support by FAPESP (Proc.91/0480-1), CNPq and FINEP-PADCT is gratefully acknowledged. V.A.O. and M.J.T. thank CAPES and CNPq for graduate fellowships. LA9407774
