Langmuir 1996, 12, 1967-1975
1967
Interactions between Water-Soluble Polymers and Surfactants: Effect of the Polymer Hydrophobicity. 1. Hydrophilic Polyelectrolytes Olivier Anthony and Raoul Zana* Institut Charles Sadron (CRM), CNRS, 6, rue Boussingault, 67083 Strasbourg, France Received September 29, 1995. In Final Form: January 25, 1996X The binding of DTAC (dodecyltrimethylammonium chloride) to two polyelectrolytes of differing hydrophobicity, the alternated copolymers poly(maleic acid-co-methyl vinyl ether) and poly(maleic acidco-butyl vinyl ether) referred to as PS1 and PS4, respectively, has been investigated by potentiometry with a surfactant ion-specific electrode, by time-resolved fluorescence quenching, by fluorescence anisotropy, and by viscosimetry. The surfactant binding isotherms, the number of surfactants (N) making up polymerbound aggregates, the lifetime of pyrene (τ2) in these aggregates, and their microviscosity (ηi) were thus obtained as a function of the surfactant concentration and copolymer neutralization degree R, which determines its electrical charge density. As observed in other studies, the binding is cooperative in the whole range of R for PS1 and for R G 0.5 for PS4. Aggregation numbers, pyrene lifetimes, and aggregate microviscosities were all found to be independent of the surfactant concentration but to depend strongly on R. N increased with R for PS1 but decreased for PS4. For PS1, τ2 and ηi increased when R decreased, i.e. with decreasing micelle size. The comparison of the τ2 values obtained with DTAC and dodecyltrimethylammonium bromide in the presence of PS1 revealed that the surfactant counterions are expelled from the surface of polymer-bound aggregates. This leads to a model where polymer chains tightly wrap around aggregates with their charged groups in contact with surfactant charged groups. The difference of behavior between PS1 and PS4 is attributed to the contribution of hydrophobic interactions between PS4 butyl side chains and surfactant alkyl chains. This contribution is small in the case of PS1, where binding is of essentially electrostatic nature. The high microviscosity of polymer-bound aggregates relative to free DTAC micelles is attributed to the electrostatic binding of surfactant ions to the polyelectrolyte and to the presence of the polymer main chain at the aggregate surface. The large values of the pyrene fluorescence lifetime are the result of the high microviscosity of the bound aggregates, which slows down the diffusive motion of reactants (probe and quencher, or any other reactant) in the aggregates and thus makes quenching (and other) processes much less efficient than in free micelles.
Introduction Polymer-surfactant interactions are currently the subject of extensive investigations in view of the number of formulations and processes where they are utilized simultaneously, and the topic has been recently reviewed.1-5 In most studies “interaction” is used to mean “binding” of surfactant to polymer. Nevertheless, recent studies have started dealing with systems where there is no binding but where interactions, of electrical origin for instance, can result in phase separation.4,5 Among the various polymer-surfactant systems, the study of the binding interaction between charged polymers and oppositely charged ionic surfactants holds a special place. Briefly, it has been observed that, as for neutral polymeranionic surfactant systems, the interaction takes place only when the surfactant concentration is larger than a value referred to as the cac (critical aggregation concentration, which is the surfactant critical micellization concentration (cmc) in the presence of polymer) and that the binding is cooperative.1-4 This means that surfactants bind to polyelectrolytes in the form of aggregates. The effect of the ionic strength, of the polyelectrolyte charge * To whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, March 15, 1996. (1) Goddard, E. D. Colloids Surf. 1986, 19, 301. (2) Goddard, E. D. In Interactions of Surfactants with Polymers and Proteins; Goddard, E. D. Ananthapadmanabhan, K. P., Eds.; CRC Press: Boca Raton, FL, 1993; p 171. (3) Hayakawa, K.; Kwak, J. In Cationic Surfactant: Physical Chemistry; Rubingh, D. Holland, P. M., Eds.; Marcel Dekker: New York, 1991; p 189. (4) Lindman, B.; Thalberg, K. In Interactions of Surfactants with Polymers and Proteins; Goddard, E. D., Ananthapadmanabhan, K. P., Eds.; CRC Press: Boca Raton, FL, 1993; p 203. (5) Thalberg, K.; Lindman, B. Colloids Surf., A: Physicochem. Eng. Asp. 1993, 76, 283.
0743-7463/96/2412-1967$12.00/0
density, and of the nature of both surfactant and polymer on the binding constant and cooperativity parameter has been investigated.1-4 However, there have been few studies of the aggregation number of polyion-bound surfactant aggregates6-11 and of their microviscosity,12-18 i.e. the viscosity that reactants solubilized in the aggregates sense in the diffusive motion which brings them in contact for the reaction to occur. Also, in general the effect of the polyelectrolyte hydrophobicity on surfactant binding has not been much investigated.8,19,20 Indeed such a study is possible only with a series of well defined and homologous polymers of increasing hydrophobicity, which are not easily available. (6) Abuin, E. A.; Scaiano, J. C. J. Am. Chem. Soc. 1984, 106, 6274. (7) Chu, D.; Thomas, J. K. J. Am. Chem. Soc. 1986, 108, 6270. (8) Kiefer, J. J.; Somasundaran, P.; Ananthapadmanabhan, K. P. Langmuir 1993, 9, 1187. (9) Kiefer, J. J.; Somasundaran, P.; Ananthapadmanabhan, K. P. In Polymer Solutions Blends and Interfaces; Noda, I., Rubingh, D. N., Eds.; Elsevier Science Publishers B. V.: Amsterdam, 1992; p 423. (10) Almgren, M.; Hansson, P.; Mukhtar, E.; van Stam, J. Langmuir 1992, 8, 2405. (11) Thalberg, K.; van Stam, J.; Lindblad, C.; Almgren, M.; Lindman, B. J. Phys. Chem. 1991, 95, 8975. (12) Chu, D. Y.; Thomas, J. K. Macromolecules 1991, 24, 2212. (13) Winnik, F. M.; Winnik, M. A.; Ringsdorf, H.; Venzmer, J. J. Phys. Chem. 1991, 95, 2583. (14) Yekta, A.; Duhamel, J.; Brochard, P.; Adiwidjada, H.; Winnik, M. A. Macromolecules 1993, 26, 1829. (15) Aizawa, M.; Komatsu, T.; Nakagawa, T. Bull. Chem. Soc. Jpn. 1977, 50, 3107. (16) Binana-Limbele´, W.; Zana, R. Macromolecules 1987, 20, 1331 and references therein. (17) Binana-Limbele´, W.; Zana, R. Macromolecules 1990, 23, 2731. (18) Cochin, D.; Candau, F.; Zana, R. Macromolecules 1993, 26, 5755. (19) Shimizu, T.; Kwak, J. C. T. Colloids Surf., A: Physicochem. Eng. Asp. 1994, 82, 163. (20) Benrraou, M.; Zana, R.; Varoqui, R.; Pefferkorn, E. J. Phys. Chem. 1992, 96, 1468.
© 1996 American Chemical Society
1968 Langmuir, Vol. 12, No. 8, 1996
The present investigation and preceding ones16,17,20 were undertaken with the aim of at least partially filling these gaps. For this purpose we used a series of alternated copolymers poly(maleic acid-co-alkyl vinyl ether), referred to as PSX, X being the carbon number of the alkyl group. The polymer hydrophobicity can be tuned by changing X and/or R, the degree of neutralization of the maleic acid moieties. In aqueous solution, these copolymers behave like polyelectrolytes and adopt an extended conformation, when X is low enough and/or R large enough.21-24 On the contrary when X is large enough and/or R low enough, the copolymers adopt a compact conformation and are then referred to as polysoaps,21-24 as their solutions have been shown to contain hydrophobic microdomains which, in some respects, behave similarly to micelles.16,17 Our first investigation20 concerned the binding of dodecyltrimethylammonium bromide (DTAB) to PSX, with 1 e X e 16 and 0 e R e 1.00, thus covering the range encompassing polyelectrolytes and polysoaps. The results clearly showed that the constant for DTAB binding to PSX increased while the cooperativity parameter decreased as the copolymer hydrophobicity was increased.20 The binding to polysoaps was very strong (large binding constants) but not cooperative. In a more recent study, the fraction of alkyl chains of PSX involved in forming hydrophobic microdomains was determined as a function of X and R for several PSX.25 The purpose of the present study was to investigate the aggregation numbers of PSX-bound surfactant aggregates, which were determined using the time-resolved fluorescence quenching method. The surfactant used was dodecyltrimethylammonium chloride (DTAC). In the course of this work we were led to determine the relative microviscosity of the aggregates in order to interpret some results concerning the rate constants of quenching in aggregates. The first part in this series reports our results concerning PS1 at all R’s and PS4 at R g 0.50, which behave like conventional polyanions. The second part reports our results for PSX of the polysoap type. Materials and Methods Materials. The samples of PS1 and PS4 were the same as in previous investigations.16,17,20,25 The polymerization degrees of the parent poly(maleic anhydride-co-alkyl vinyl ether)16,17 were determined by light-scattering measurements in tetrahydrofuran and found to be 3400 and 260 for PS1 and PS4, respectively. DTAC was purchased from Aldrich and recrystallized thrice from ethanol-ethyl acetate mixtures. The sample of pyrene used as fluorescent probe was the same as used previously.20,25 DMBP (3,4-dimethylbenzophenone, Aldrich 99%, recrystallized twice from ethanol) was used as quencher of the pyrene fluorescence.10,11 The polymer concentrations are expressed in moles of repeat unit per liter. The neutralization degree R is equal to 1 when all carboxylic acid groups are neutralized. Methods. Potentiometry. The cationic surfactant-specific electrode was prepared as described.20,26 The observed slopes of the linear calibration plots (emf vs log([surfactant]) in the submicellar range and in the absence of polymer20) were very close to the Nernst values, i.e. 59.1 ( 0.5 mV per concentration decade at 25 °C. The following experimental procedure was adopted for the addition of surfactant in the presence of copolymer. To the initial 2 mM polymer solution contained in the potentiometric cell was added a surfactant solution of low concentration (21) Pefferkorn, E.; Schmitt, A.; Varoqui, R. C. R. Acad. Sci., Ser. C 1968, 268, 349. (22) Strauss, U. P.; Varoqui, R. J. Phys. Chem. 1968, 72, 2657. (23) Dubin, P.; Strauss, U. P. J. Phys. Chem. 1967, 71, 2757; 1970, 74, 2482; 1973, 77, 1427. (24) Strauss, U. P.; Vesnaver, G. J. Phys. Chem. 1975, 79, 1558. (25) Anthony, O.; Zana, R. Macromolecules 1994, 27, 3885. (26) Shirahama, K.; Nishiyama, Y.; Takisawa, N. J. Phys. Chem. 1987, 91, 5928.
Anthony and Zana (3 mM) to avoid precipitating out a polymer-surfactant complex. The resulting dilution of the polymer solution was compensated by adding the appropriate amount of a concentrated polymer solution, typically 20 mM, in order to keep constant the polymer concentration. The emf reached a constant (equilibrium) value in less than 4 min after adding the surfactant solution. Spectrophotometry. The quencher used, DMBP, was not completely solubilized within the polymer-bound aggregates because of its low solubility in water and also because of the rather small volume fraction of hydrophobic pseudophase under the experimental conditions used. The concentration of quencher solubilized in aggregates is the relevant one for determinations of aggregation numbers and was obtained from spectrophotometric measurements. Thus the absorbance of DMBP in aqueous solutions, concentrated micellar solutions, and polymer-surfactant systems was measured at 300 nm as a function of the DMBP concentration. The plots of absorbance vs DMBP concentration were linear, and their slope S varied between Sm ) 3100 M-1 in a concentrated micellar solution of DTAC (fully micelle-solubilized DMBP) and Sw ) 5900 M-1 in water. The values of S in PXS-DTAC systems were all between these two limits. Assuming a linear variation of S between Sm and Sw with the fraction φ of DMBP solubilized in polymer-bound aggregates, φ was obtained from
φ)
Sw - S Sw - Sm
(1)
Time-Resolved Fluorescence Quenching (TRFQ). This well described method27-33 makes use of a fluorescent probe (pyrene in the present study) and of a quencher, both preferentially solubilized in micelles. It allows the measurement of the micelle aggregation number, N, i.e. the number of surfactants constituting a micelle, of the probe fluorescence lifetime, τ, and of the rate constant of intramicellar quenching of the probe by the quencher used, kQ, both in the absence and in the presence of polymer.10,11,34 In the absence of quencher and if all the pyrene is solubilized in one phase, hydrophobic or not, the fluorescence decay curve is monoexpotential and obeys the equation
( τt)
(2)
I(t) ) I(0) exp -
where I(0) is the fluorescence intensity at time t ) 0. However, in most PSX-DTAC systems investigated, the volume of hydrophobic pseudophase constituted by polymerbound surfactant aggregates was very small and, therefore, pyrene was partitioned between the aqueous phase and this hydrophobic pseudophase. For this reason, and because the pyrene residence time in micellar aggregates (about 1 ms)35 is much longer than the pyrene fluorescence lifetime in micelles (200-400 ns), the fluorescence decay curves were biexponential and obeyed the equation
( )
I(t) ) I(0)1 exp -
( )
t t + I(0)2 exp τ1 τ2
(3)
where τ1 and τ2 are the fluorescence lifetimes in water and in the (27) Zana, R. In Surfactant Solutions: New Methods of Investigation; Zana, R., Ed.; Marcel Dekker Inc.: New York, 1987; Chapter 5, p 241. (28) Infelta, P. Chem. Phys. Lett. 1979, 61, 88. (29) Tachiya, M. Chem. Phys. Lett. 1975, 33, 289. (30) Yekta, A.; Aikawa, M.; Turro, N. J. Chem. Phys. Lett. 1979, 63, 88. (31) Almgren, M. Adv. Colloid Interface Sci. 1992, 41, 9. (32) Gehlen, M.; De Schryver, F. C. Chem. Rev. 1993, 93, 199. (33) Binana-Limbe´le´, W. Doctorate Thesis, University Louis Pasteur, Strasbourg, 1991. (34) Zana, R.; Lang, L.; Lianos, P. In Microdomains in Polymer Solutions; Dubin, P., Ed.; Plenum Press: New York, 1985; p 257; J. Phys. Chem. 1985, 89, 41. (35) Almgren, M.; Grieser, F.; Thomas, J. K. J. Am. Chem. Soc. 1979, 101, 279.
Interactions between Polymers and Surfactants
Langmuir, Vol. 12, No. 8, 1996 1969
hydrophobic pseudophase, respectively, and I(0)1 and I(0)2 are the corresponding fluorescence intensities at time t ) 0. The presence of quencher is not expected to affect the partition of pyrene, and the decay equation in PSX-DTAC systems containing polymer-bound aggregates becomes
( )
I(t) ) I(0) exp(-A2t - A3[1 - exp(-A4t)]) + I(0)1 exp -
t τ1 (4)
where the first term on the right hand side (rhs) of eq 4 describes the fluorescence decay of pyrene solubilized in bound aggregates (This expression is formally identical to that when pyrene is fully solubilized in micelles, and the fitting parameters A2, A3, and A4 are expected to have the same expressions as those reported for various situations.28-30) and the second term describes the fluorescence decay of water-solubilized pyrene. Rather than fitting eq 4 to the experimental decay curves in the presence of quencher, these curves were first corrected for the fluorescence arising from water-solubilized pyrene, i.e. the term I(0)1 exp(- (t/τ1). For this purpose we checked that the amount of water-solubilized quencher was too small to affect I(0)1 and τ1. In aerated solutions, the fluorescence lifetime of pyrene in water was found to be τ1 ) 124 ns and I(0)1 was taken proportional to the counting time in the photon-counting experiments. Equation 3 was fitted to the decay curves in the absence of quencher to yield the required correcting term, which was multiplied by the ratio of the counting times in experiments with and without quencher and subtracted from the decay data in the presence of quencher. Equation 4 without the second term on its rhs was best-fitted to the corrected decay curves to yield the fitting parameter values from which were calculated the aggregation numbers. This procedure was found to be more accurate than a direct fit of eq 4 to the uncorrected curves which involves six parameters. In general, the fits yielded A2 ) 1/τ2. In such a case A3 ) [Q]/[micelle], the ratio of the molar concentrations of micellesolubilized quencher and of micelles, and A4 ) kQ, the intraaggregate quenching rate constant. The surfactant aggregation number was obtained from N ) (C - cmc)/[micelle], where C is the surfactant concentration. At the highest C values used, A2 was sometimes larger than 1/τ2. The full equations which take into account quencher migration between aggregates were then used to calculate the value of N. The TRFQ data have been further used to estimate a quantity proportional to the relative microviscosity, ηi, of the aggregates from the values of the intra-aggregate quenching rate constant, kQ, and the aggregation number, N. Indeed, for conventional spherical or spheroidal micelles, to a first approximation, kQ is inversely proportional to ηi and to the micelle volume V.36 In turn, V is proportional to N and to n + 1, n being the carbon number of the surfactant alkyl chain. For PSX-bound aggregates V must also include the polymer side chains of carbon number X and, therefore,
ηi ∝
1 ((n + 1)N + (X + 1)N′)kQ
fluorescence anisotropy r of a fluorescent probe solubilized in a homogeneous solution or in surfactant aggregates through37,38
ηi ∝
(36) Van der Auweraer, M.; De Schryver, F. C. Chem. Phys. 1987, 111, 105. (37) Shinitzky, M.; Inbar, M. Biochim. Biophys. Acta 1976, 433, 133.
(6)
where τ is the probe fluorescence lifetime in the medium under study and r0 is the fluorescence anisotropy measured in a medium of infinite viscosity. The present study used the fluorescent probe diphenylhexatriene (DPH), for which r0 ) 0.362 in glycerol at -60 °C.37,38 The fluorescence anisotropies were measured using an SLM 8000 fully computerized by Biologic (Grenoble, France). The microviscosities were all referred to that of free DTAC micelles. Viscosimetry. The measurements used a Contraves Low Shear 30 apparatus. All measurements were performed at 25 °C, using in most instances a copolymer concentration of 2 mM. This concentration is about ten times larger than those used in similar studies of polyelectrolyte-surfactant interactions.3,20 Indeed, surfactantspecific electrodes work more reliably in the 0.1-0.4 mM range and the precipitation of polymer-surfactant complexes is then easier to avoid. However, in this range, TRFQ determinations of aggregation numbers become nearly impossible, owing to the low concentration of probe which must then be used and the resulting very low photon-counting rates. The concentration used was a good compromise between these contradictory requirements.
Results Potentiometry. The potentiometric results are usually represented in the form of binding isotherms, i.e. concentration of bound surfactant, Cb, or fraction of occupied binding sites, β, vs log Cf (Cf ) free surfactant concentration).3,20 The analysis of these isotherms yields the critical aggregation concentration (cac), the surfactant binding constant (K), and the binding cooperativity parameter.1-3 For polycarboxylic acids, two different definitions of β have been used:8,19,39
β)
Cb
Cb
[COO]total
) [COOH] + [COO-]
(7)
and
βi )
Cb
Cb )
[COO ] -
i[COO]total
)
β i
(8)
where i is the ionization degree of the acid groups, given by
(5)
where N′ is the number of polymer side chains involved in a PSX-surfactant aggregate. The calculations assumed N′ ) N. This assumption may not be valid, but the resulting error is small as X e 4 while n ) 12. The decay curves were determined using the photon-counting apparatus already described, and the fluorescence decay curves were analyzed using a weighted least-squares procedure.18,25,33 The solutions used in the measurements were aerated, that is naturally air-saturated. Indeed deoxygenation, whether by argon bubbling or freeze-pump-thaw cycles, brought about the precipitation of a polymer-surfactant complex. Fluorescence Anisotropy. This technique permits the determination of the viscosity of the microenvironment of appropriate probes, and it has been used to obtain the microviscosity of surfactant aggregates.37,38 The microviscosity is related to the
τ r0 -1 r
i)
[COO-] [COO]total
(9)
The neutralization degree and the ionization degree, R and i, respectively, are equal for high values of R, but for R < 0.15, i is larger than R, owing to the self-ionization of the acid functions. i can be determined from pH measurements. The second definition of β is more used, as it is accepted that the real binding sites are the ionized carboxylic groups (COO-) rather than the unionized ones. Figure 1 shows binding isotherms of PS1-DTAC systems for several R values in the βi vs log Cf representation. The plots are typical for cooperative binding, with a rapid increase of βi above a surfactant concentration (38) Shinitzky, M. In Physical Methods On Biological Membranes and their Model Systems; Plenum Publishing Corp.: New York, 1984; p 227. (39) Shimizu, T. Colloids Surf., A: Physicochem. Eng. Asp. 1994, 84, 239.
1970 Langmuir, Vol. 12, No. 8, 1996
Figure 1. Binding isotherms of DTAC to PS1 at different degrees of neutralization, as indicated in the figure. “Corrected” and “noncorrected” refer to the correction for ionization (see text). The arrows indicate the onset of precipitation (visual inspection).
value which defines the cac, as shown in similar works.1-3,8-11,19,20,39 Also, as reported,39 the surfactant binding induced a decrease of pH and, thus, an increase of binding site concentration. This effect resulted in a change of i and required a correction of the βi values. Figure 1 shows the corrected and noncorrected isotherms for PS1 at R ) 0, where the correction is the largest. This correction remained significant at R ) 0.10, where i ) 0.114 in the absence of DTAC and i ) 0.163 in the presence of 1.29 mM DTAC. As reported,1-3,19,20,39-42 the cooperative binding range is restricted to the early stage of binding, since all isotherms show a plateau or an important decrease of slope. The βi values at this plateau or inflection are seen to be very close to 1 for 0 e R e 0.20, indicating that the maximum concentration of bound surfactant is equal to the concentration of COO- groups. For R > 0.20, the βi value at the plateau decreases, as reported.19 This decrease may arise from steric effects associated with crowding of surfactant ions on the polyelectrolyte. For the highest surfactant concentrations, a decrease of βi is sometimes observed which corresponds to the onset of precipitation of a polymer-surfactant complex, indicated by arrows in Figures 1 and 2. This complex apparently adsorbs on the surfactant electrode membrane and makes its response unreliable.10 This effect of precipitation on binding isotherms as well as on the other properties investigated is not discussed here. The data for precipitated systems are given only for the sake of illustrating the effect of precipitation on the measured quantities. Note however that cast films of such polyelectrolyte-surfactant complexes have been shown to constitute a new type of solid material capable of forming highly ordered mesophases.43 At this stage a remark must be made concerning the determination of cac values from binding isotherms as in Figure 1. The surfactant starts binding at very low surfactant concentration C (below 0.1 mM in our case). However, just above the cac, Cb is much smaller than the binding site concentration (which varies from 0.3 to 4 mM, depending on R). Therefore, βi remains very small after binding starts, its increase with Cf becoming visible on (40) Hayakawa, K.; Kwak, J. C. T. J. Phys. Chem. 1982, 86, 3866; 1983, 87, 506. (41) Hayakawa, K.; Santerre, J.; Kwak, J. C. T. Macromolecules 1983, 16, 1642. (42) Hayakawa, K.; Santerre, J.; Kwak, J. C. T. Biophys. Chem. 1983, 17, 175. (43) Antonietti, M.; Conrad, J.; Thunemann, A. Macromolecules 1994, 27, 6007.
Anthony and Zana
Figure 2. Variation of the fraction γ of DTAC bound to PS1 with the total surfactant concentration at different degrees of neutralization of PS1. The R ) 0.00 + HCl plot was obtained in the presence of 2 mM HCl, which completely inhibits the self-ionization of the acid groups. The arrows indicate the onset of precipitation (visual inspection). Table 1. Values of the cac in PS1-DTAC Systems in Watera and in Water + 20 mM NaClb R)0
R ) 0.05 R ) 0.10
R ) 0.20
R ) 0.40 R ) 1.00
cac (M)a 1.5 × 10-4 7 × 10-5 5 × 10-5 3.5 × 10-5 4 × 10-5 4 × 10-5 cac (M)b 6 × 10-3 5 × 10-3
the isotherms only when Cb represents a few percent of [COO-]. This effect leads to overestimated cac values in cases where binding cooperativity is not very strong. More accurate cac determinations can be achieved by plotting the fraction of bound surfactant, γ ) Cb/C, against C. Figure 2 shows examples of such plots, and Table 1 gives the cac values obtained from these data. The plot labeled R ) 0.00 + HCl refers to measurements in the presence of 2 mM HCl, which completely inhibits the COOH group self-ionization. PS1 is then uncharged and DTAC binding is seen to be weaker and to start taking place at a surfactant concentration about ten times larger, illustrating the importance of electrostatic interactions. At 25 °C, in the absence of polymer and salt, the cmc of DTAC is 20 mM. In the presence of 2 mM PS1, the cac is 130-500 times lower, depending on R (Table 1). The binding constants are very high but are not discussed in this study, which is essentially concerned with aggregation numbers. The decrease of cac when R increases from 0 to 0.20 is noteworthy. This decrease appears to be due to electrostatic interactions: the higher R or i, i.e. the polymer charge density, the stronger the binding and the lower the cac. However, increases of cac with R were reported8,19,39,44 in surfactant binding studies to poly(acrylic acid), poly(methacrylic acid), and alternated copolymers of maleic acid and ethylene or styrene, but no clear explanation was given for this result. Increased hydrophobic interactions between polymer and surfactant when R decreased19 and changes of polymer conformation and of backbone hydrophobicity8 were invoked. However, all these studies were carried out in the presence of a large amount of salt. As part of this work, the cac of DTAC in the presence of 2 mM PS1 was measured in water and in water + 20 mM NaCl. The results are listed in Table 1 and show that the effect of R on the cac nearly disappears in the presence of NaCl, in agreement with results from a previous study.20 Thus, in part at least, the difference between the effect of R in the present results and reported ones is due to the presence of salt: the cation of the added (44) Shimizu, T.; Seki, M.; Kwak, J. C. T. Colloids Surf., A: Physicochem. Eng. Asp 1986, 20, 89.
Interactions between Polymers and Surfactants
Figure 3. Binding isotherms of DTAC to PS4 at R ) 0.50 and 1.00.
Figure 4. Variation of the fraction γ of DTAC bound to PS4 with the total surfactant concentration at R ) 0.50 and 1.00. The arrows indicate the onset of precipitation (visual inspection).
salt competes with the dodecyltrimethylammonium ion (DTA+) for the PSX binding sites. For R g 0.40, the cac becomes independent of R (or i). A similar observation has been reported for other systems19,39 and explained on the basis of Manning’s theory of counterion condensation.45,46 For polyelectrolytes such as PSX, condensation of monovalent counterions (Na+ or DTA+, for instance) should occur at ic g 0.38, and this effect would leave the PSX effective charge density constant as i is increased above ic.45,46 Thus, both the constancy of the cac at R g 0.40 and the dependence of the cac on salt addition confirm that the binding of DTAC to the PS1 is mainly caused by electrostatic attractions. This interaction is rather short range, as the condensed counterions are close to the micelle surface, in the Stern layer, the thickness of which is close to that of the diameter of the hydrated counterion. Turning to PS4, recall that this copolymer does not form hydrophobic microdomains for R > 0.35, at least in dilute solution,25 and behaves at 0.35 e R e 1.00 as a typical polyelectrolyte. Figures 3 and 4 show the variations of βi with Cf and of γ with C at R ) 0.5 and 1.0. The surfactant binding is cooperative, but a binding plateau is rapidly reached for βi values well below 1 (Figure 3), as for PS1 at R > 0.20. The cac values obtained from the γ vs C plots are 4 × 10-6 M at R ) 0.50 and 9 × 10-6 M at R ) 1.00. The ratio of the cac values for PS1 and PS4 at R ) 1.00 is close to 7, corresponding to a decrease of 1.8 kT (4.5 kJ/mol) of the free energy of the system. This effect can only be explained by the occurrence and/or the increase of hydrophobic interactions between polymer and surfactant in going from PS1 to PS4. Another indication of the contribution of hydrophobic interactions to the binding of PS4 is provided by the value of the maximum fraction of bound surfactant, which increases from 0.90 for PS1 at R ) 1.00 to 0.97 for PS4 at R ) 0.50 (Figures 2 and 4). (45) Manning, G. S. J. Chem. Phys. 1969, 51, 924. (46) Manning, G. S. Q. Rev. Biophys. 1978, 2, 179.
Langmuir, Vol. 12, No. 8, 1996 1971
Figure 5. Variation of the aggregation number of DTAC aggregates bound to PS1 at different degrees of neutralization with the concentration of bound surfactant.
Figure 6. Variation of the aggregation number of DTAC aggregates bound to PS4 at R ) 0.50 and 1.00 with the concentration of bound surfactant.
While it is reasonable to assume that the major interaction between PS1 and DTAC is of electrostatic nature, it appears that, for PS4, the polymer-surfactant hydrophobic interaction is comparable to the electrostatic one. Such a behavior has already been pointed out for two other copolymers of different hydrophobicity, poly(maleic acidco-ethylene) and poly(maleic acid-co-styrene), interacting with dodecylpyridinium chloride.19 However no difference in cac value originating from hydrophobic interactions was observed in a study of binding of tetradecyltrimethylammonium to poly(acrylic acid) and poly(methacrylic acid).8 The comparison of Figures 1 and 3 shows that the increase of R from 0.50 to 1.00 leaves the cac unchanged with PS1 but induces a rise of cac with PS4. This increase probably reflects the decreased PS4 hydrophobicity when the polymer bears more charged groups. Time-Resolved Fluorescence Quenching: Aggregation Numbers. Various quenchers of the pyrene fluorescence have been used in order to determine aggregation numbers of surfactant micelles in the absence of polymer. The most widely used quenchers are cationic surfactants: alkylpyridinium or alkylcyanopyridinium chlorides.34 Unfortunately, for polyelectrolyte- or polysoap-surfactant systems, quenchers with a long linear hydrophobic chain were found to be very inefficient, with some of them not quenching at all the pyrene fluorescence.18 The diffusive motion of quencher and probe which permits the quenching reaction seemed to be very hindered in polymer-bound aggregates with respect to micelles of the same surfactant in the absence of polymer. However, quenchers having a somewhat spherical shape, like 3,4-dimethylbenzophenone10,11 or dibutylaniline,18 still quenched efficiently enough pyrene to allow micelle aggregation number determinations. These observations are further discussed below. Figures 5 and 6 show the variation of the DTAC aggregation number, N, in the presence of 2 mM PS1 and
1972 Langmuir, Vol. 12, No. 8, 1996
PS4, respectively. The N values are all below that for free DTAC micelles, 55 at C ) 0.1 M and 25 °C (see below). A similar observation was made in other studies.10,11 It is noteworthy that, at a given R, the values of N are nearly independent of the concentration of bound surfactant, Cb, irrespective of the investigated system, within the experimental error represented by the scatter of the results, which reaches 30%. The errors are so large for the investigated PSX-DTAC systems because the determination of N involves several independent steps, each affected by errors: determination of the fraction of micellized DMBP and of the concentration of bound surfactant and recording, correction for water-solubilized pyrene, and analysis of decay curves. A near constancy of N with the surfactant concentration was already reported8,10 and interpreted as additional evidence for aggregate formation. This result means that the concentration of polymer-bound aggregates increases nearly linearly with Cb. This behavior is opposite to that for POE-SDS systems, where N increases from 20 at the cmc to about 70 at polymer saturation,34,47 although a recent study disputed this finding.48 The results show an important effect of the neutralization degree R on N. Thus, in Figure 5, N increases from about 11 at R ) 0 (i.e. i ) 0.06) to 50 at R g 0.40. Rather large errors are anticipated for the lowest N values, due, in particular, to the presnce, on an averaged basis, of one quencher per aggregate (The aggregation number being about 10, the aggregate size is probably affected by the presence of quencher at a large mole fraction of about 0.1). This, however, should not affect much the qualitative aspect of the discussion below. For PS1, N increases with R but remains below the aggregation number of DTAC micelles in polymer-free solutions (N ) 55). For the cac and the cooperative part of binding isotherms, no difference is observed between the values of N at R ) 0.40 and R ) 1.00. The explanation for this result is again to be found in Manning counterion condensation theory.45,46 Note that very few studies examined the dependence of N on R. Kiefer et al.9 concluded for poly(acrylic acid)-tetradecyltrimethylammonium bromide systems that the effect of R was nearly inexistent, but they did not exclude a small effect at very low R values. A more precise conclusion was not reached because the experimental conditions used led to large errors in N. The results in Figure 6 for the PS4-DTAC systems show a behavior very different from that for PS1-DTAC systems. Thus, N depends on R in the range 0.50-1.00 and decreases from 35 at R ) 0.50 to 20 at R ) 1.00. These differences are probably caused by the increased polymer hydrophobicity and are discussed below. Microviscosity. Microviscosities were only determined for PS1-DTAC systems, and the results are presented as relative microviscosities, the ratio of the microviscosities for PS1-DTAC systems and for polymerfree DTAC micelles. Figure 7 shows the results obtained from TRFQ data. All ηi values are seen to be between 2 and 12 for the nonprecipitated systems. Thus PS1-DTAC aggregates are more viscous or rigid than free DTAC micelles. Like N, ηi depends little if at all on the surfactant concentration C, showing again that new aggregates of identical size, shape, and composition are formed upon increasing C. This conclusion is supported by the ηi data obtained from fluorescence anisotropy measurements listed in Table 2. (47) van Stam, J.; Almgren, M.; Linblad, C. Prog. Colloid Polym. Sci. 1991, 84, 13. (48) Nilsson, S.; Holmberg, C.; Sundelo¨f, L.-O. Colloid Polym. Sci. 1995, 273, 83.
Anthony and Zana
Figure 7. Variation of the microviscosity of surfactant-bound aggregates relative to that of free DTAC micelles (calculated from TRFQ data) with the total DTAC concentration for PS1DTAC systems at different degrees of neutralization. Table 2. Relative Microviscosities of PS1-Bound DTAC Aggregates 104C (M) ηi
3.6 3.2
4.9 3.05
5.9 2.95
6.7 2.9
8.0 2.8
8.8 2.8
Table 3. Values of the Aggregate Relative Microviscosity and Surfactant Aggregation Number for PS1-DTAC Systems ([PS1] ) 2 mM) system
ηi
N
polymer-free DTAC, 50 mM R ) 0.00, [DTAC] ) 0.6 mM R ) 0.20, [DTAC] ) 0.7 mM R ) 0.40, [DTAC] ) 0.8 mM R ) 1.00, [DTAC] ) 0.8 mM
1 25.2 12.4 5.6 2.7
55 11 31 50 50
Table 4. Micellar Characteristics of DTASuc and DTAC at 25 °C
cmc ionization degree aggregation number
DTASuc 0.132 M
DTAC 0.099 M
13 mM 0.27 34
20 mM 0.35 55
It is also noteworthy that ηi increases rapidly when R is decreased from 1.00 to 0, as seen in Figure 7 (TRFQ measurements) and Table 4 (fluorescence anisotropy measurements). Thus ηi depends strongly on the N value of bound aggregates: the smaller the aggregate, the larger its microviscosity. This result is used below to explain two previously insufficiently explained observations made in this work and others9-11 for aerated polymer-surfactant systems: (i) the presence of polymer decreases the efficiency of quenching processes, and (ii) the pyrene fluorescence lifetime is always larger in polymer-bound aggregates than in polymer-free micelles. Fluorescence Lifetime of Pyrene in Surfactant Aggregates (τ2). Figure 8 shows the variations of τ2 in PS1-DTAC aggregates. τ2 depends little, if at all within the experimental error, on the bound surfactant concentration, Cb, a result qualitatively similar to that for N (Figures 5 and 6) and ηi (Figure 7). For the lowest neutralization degrees, the increases of τ2 at high Cb reflect the onset of precipitation of a polymer-surfactant complex. Figure 8 shows that, for all polymer-surfactant systems investigated, τ2 is always larger than 165 ns, the value in polymer-free DTAC micelles (dotted line in Figure 8), reaching at low R values of 260-270 ns. τ2 increases to 330 ns when the solution is deaerated, thereby eliminating the strong quenching effect of solubilized oxygen. Thus, polymer-bound aggregates appear to shield pyrene from oxygen quenching better than polymer-free micelles. The following explanations have been proposed for this effect.
Interactions between Polymers and Surfactants
Figure 8. Variation of the pyrene fluorescence lifetime in PS1bound surfactant aggregates with the concentration of bound surfactant. The dotted line shows the pyrene lifetime in free DTAC micelles. All solutions were aerated.
(i) The polymer-bound surfactant aggregates would be more hydrophobic than polymer-free micelles, resulting in a larger value of τ2.9 However, as part of this work, we measured the same value of the pyrene polarity index, I1/I3 ) 1.40, for PS1-DTAC systems and polymer-free micellar solutions of DTAC. Since the value of the I1/I3 ratio reflects the polarity sensed by pyrene at its solubilization site,27,49 the proposed explanation in terms of hydrophobicity does not hold. (ii) It has been argued that, for cationic surfactants with bromide counterions (efficient quenchers of the pyrene fluorescence, which reduce its lifetime to 112 ns in aerated micellar solutions of dodecyltrimethylammonium bromide (DTAB)), the counterions are expelled from the surface of polymer-bound aggregates,9-11 resulting in larger values of τ2, contrary to polymer-free micelles, which bind about 75% of the bromide counterions. The results in Figure 8 at R ) 1.00 for DTAB and DTAC show that the values of τ2 for the two surfactants are identical at all C values. Thus the quenching effect of bromide ions has disappeared in the presence of PS1, indicating that these ions are now removed from the aggregate surface, as the quenching of pyrene occurs only when the quencher is very close to pyrene. However our results show the same increase of τ2 in the presence of PS1 with both the quenching bromide ions and the nonquenching chloride ions. Thus the high values of τ2 cannot be explained by the removal of counterions from the surface of polymer-bound aggregates. (iii) Almgren et al.10 concluded that polymer-bound aggregates shield pyrene better than free micelles against quenching by oxygen solubilized in aggregates or micelles but did not specify the origin of this effect. We show in the Discussion section that this effect is associated to the larger microviscosity of polymer-bound aggregates with respect to free aggregates, which makes quenching processes less efficient. Viscosity Measurements on the PS1-DTAC Systems. All the measurements described above concerned the surfactant. Viscosity measurements were then performed in order to gain information about the polymer behavior upon surfactant binding under the same experimental conditions as for potentiometric or fluorescence measurements, i.e. a polymer concentration of 2 mM. The measurements suffered from two limitations. First, such a low PS1 concentration prevents measurements on solutions of low R because their viscosity was very close to that of water. Second, measurements of intrinsic viscosity for polymer-surfactant systems are very complex because a change of polymer concentration induces a change of concentration of bound surfactant, rendering (49) Kalyanasundaram, K.; Thomas, J. K. J. Am. Chem. Soc. 1977, 99, 2039.
Langmuir, Vol. 12, No. 8, 1996 1973
Figure 9. Variation of the specific viscosity of PS1 solutions at R ) 1.00 with the concentration of added DTAC and tetramethylammonium chloride.
the extrapolation to low polymer concentration meaningless. Thus, the measurements presented below concern only the specific viscosity, ηsp, given by eq 10, of PS1DTAC systems at R ) 1.00, upon increasing DTAC concentration.
ηsp )
η - η0 η0
(10)
In eq 10, η is the viscosity of the polymer-surfactant system extrapolated to zero shear rate, and η0 is the viscosity of water (η0 ) 0.905 mPa‚s at 25 °C, measured as part of this work). The results are presented in Figure 9. Below the cac (4 × 10-5 M), there is no effect of C on ηsp (ηsp ≈ 1.7), because the surfactant does not bind. Above the cac, ηsp starts to decrease first slowly and then rapidly to reach a value about 8 times lower than the one below the cac, before the system starts precipitating out. This decrease of ηsp is not due to a screening of long range electrostatic repulsions between polymer charged groups caused by the surfactant. Indeed, Figure 9 shows that additions of tetramethylammonium chloride, a salt chemically identical to DTAC except for the dodecyl group which is replaced by a methyl group, induce a small decrease of ηsp, by a factor of 1.7 instead of 8, for the same concentration of DTAC. The decrease of ηsp upon DTAC additions thus reveals a true decrease of the hydrodynamic radius of PS1 with its bound DTA+ ions, despite the increase of the molecular weight of the polymer due to surfactant binding. It also means that the formation of polymer-bound surfactant aggregates does not result in intermolecular associations of polymer chains. On the contrary, the large decrease of ηsp indicates that the polymer interacts strongly with the surfactant aggregates, probably by wrapping around them. The slow change of ηsp just above the cac is due to the fact that the number of aggregates per polymer chain is then too small to result in a significant effect. Discussion It is clear from this study and a previous one20 that DTAC binds to PS1 and PS4 in the form of aggregates that have some common characteristics with free DTAC micelles. As for micellization, the surfactant aggregation on a polyelectrolyte is a cooperative process that occurs above a critical surfactant concentration or cac. As for free spherical micelles, the aggregation size does not depend on surfactant concentration, within the experimental accuracy. However, some major differences exist between polyelectrolyte-bound surfactant aggregates and free micelles. First, the surfactant counterions (here, Cl-
1974 Langmuir, Vol. 12, No. 8, 1996
or Br-) are expelled from the aggregate surface, whereas, for free DTAC micelles, the Cl- association degree to micelles is about 0.70. Second, the aggregate size seems to be determined by the electrical and hydrophobic characteristics of the polymer (See the effect of R in Figures 5 and 6 and the effect of polymer hydrophobicity by comparing the results in these two figures). Last, the microviscosity of polymer-bound aggregates is much higher than that of free micelles and depends much on aggregate size (Table 3). The electrostatic interactions invoked above are clearly very short range interactions, as indicated by the following results: (i) salt additions increase the cac; (ii) DTAC additions to PS1 solutions decrease ηsp; and mostly (iii) the presence of PS1 results in the removal of the surfactant counterions from the polymer-bound aggregate surface, as indicated by the absence of quenching of the pyrene fluorescence by Br- ions in the presence of PS1. Recall that quenching occurs only when pyrene and quencher are in close proximity. Consider the structure of the bound aggregates. The fact that the surfactant counterions are expelled from the aggregate surface indicates that an important fraction of PSX charged groups are in direct electrostatic interaction with bound surfactants. Indeed, the polymer-bound aggregates cannot be totally ionized, in particular when their aggregation number is close to that of free DTAC micelles (N ) 55), i.e. for high values of R. Polymer charged groups must replace surfactant counterions, so as to decrease the aggregate effective charge, to a value close to that of free DTAC micelles. The number of contacts between polymer and surfactant charged groups must therefore be large. This is possible only if the polymer main chain wraps around aggregates, in order that its charged groups be distributed over most of the aggregate surface, thereby explaining the large decrease of specific viscosity. The important increase of microviscosity ηi in going from free micelles to polymer-bound aggregates supports this idea. Indeed, the binding of surfactant ions to polymer charged groups induces a reduction of their mobility in bound aggregates, making these aggregates less fluid than free micelles. The presence of polymer main chains at the aggregate surface also makes the aggregates less fluid. Some surfactant ions may not be directly bound to PS1 charged groups, and their charge would thus remain unneutralized. A direct measure of the fraction of such surfactant ions is very difficult. This fraction was estimated by measuring the micelle ionization degree of dodecyltrimethylammonium succinate (DTASuc). This surfactant is chemically very close to the system formed by two dodecyltrimethylammonium ions and one PS1 repeat unit. The micellar characteristics of DTASuc and DTAC are compared in Table 4. The aggregation number of DTASuc micelles is close to that of PS1-bound DTA+ aggregates at R ) 1.00. It can be assumed that the ionization degree of DTASuc micelles, 0.27, is close to the fraction of unneutralized DTA ions in PS1-bound DTA+ aggregates. This fraction probably depends on N and R. Differences between PS1-DTAC and PS4-DTAC Systems. At a given R, an important decrease of cac was observed when replacing PS1 by PS4. Besides, the variations of cac, aggregation number N, and fluorescence lifetime τ2 as a function of R for PS1 were opposite to those for PS4. These opposite variations can be explained by considering PS1 as a purely hydrophilic polyelectrolyte, which interacts mainly via short range electrostatic interactions with DTA+ ions whereas PS4 acts also through hydrophobic interactions. The presence of a few COOgroups on PS1 at R ) 0 is sufficient to bring about a
Anthony and Zana
Figure 10. Schematic representation of the variation of the cac with the PSX degree of neutralization for PS1-DTAC and PS4-DTAC systems. With PS1 a large electrostatic contribution is responsible for the large decrease between cmc and cac values. The increase of R from 0.00 to 1.00 causes a small further decrease of the cac. With PS4, the existence of a hydrophobic interaction is responsible for the larger decrease from the cmc to the cac values. The decrease of the hydrophobic contribution upon increasing R is responsible for the increase of the cac with R, at large R.
decrease of the aggregation concentration (cac) by a factor of about 140. Changes of polymer charge fraction, i.e. of ionization degree i, only slightly further modify these low cac values. For instance, the increase of i from 0.06 to 1.00 reduces the cac by a factor of only about 4 (Table 1). PS4 is more hydrophobic than PS1, and the occurrence and/or enhancement of hydrophobic interactions between PS4 butyl side chains and surfactant alkyl chains is responsible for the larger decrease of cac caused by PS4. Thus, at R ) 1, the cac values are 9 × 10-6 M and 4 × 10-5 M for PS4 and PS1, respectively. For PS4, an increase of R induces a reduction of cac because it enhances electrostatic interactions between polymer and surfactant charged groups, as in PS1. But increasing R also makes PS4 less hydrophobic and should increase the cac. This second effect is probably responsible for the slight rise of cac, observed when R is increased from 0.5 to 1. Figure 10 gives a schematic representation of the effects just discussed and of the corresponding changes of cac for the two polymers. Effect of r on the Aggregation Number N. For PS1, most surfactant ions are electrostatically bound to the polymer, in the form of aggregates. Two main interactions are simultaneously at play: (i) short range electrostatic interactions between polymer and surfactant charged groups and (ii) hydrophobic interactions between surfactant alkyl chains. At high R many binding sites are available and the surfactant ions can bind to adjacent binding sites, with their alkyl chains in contact (favorable hydrophobic interaction). At lower R, the number of binding sites is lower and the average distance between successive binding sites on the polymer backbone larger. Hydrophobic interactions between alkyl chains of surfactant ions bound to adjacent binding sites can nevertheless occur because (i) the polymer charged groups may be displaced upon surfactant binding, resulting in a local reduction of distance between charged groups, and (ii) new charged groups are created upon surfactant binding, as indicated by the decrease of the solution pH (see above). For the PS4-DTAC systems the occurrence of a third interaction of a hydrophobic nature between polymer side chains and surfactant alkyl chains introduces an additional complexity. The surfactant binding can be looked at as a comicellization between DTAC and PS4 amphiphilic repeat units (butyl side chains). Recall that the comicellization of two surfactants with differing alkyl chains
Interactions between Polymers and Surfactants
usually results in micelles of aggregation number smaller than that of the surfactant with the longer alkyl chain. For instance, addition of alcohol to dilute micellar solutions, of SDS for instance, brings about a decrease of N.50-52 The same effect was observed upon addition of DTAC to hexadecyltrimethylammonium chloride solution.53 For the PS4-DTAC systems, the βi value at the plateau decreases when R increases (Figure 3). This indicates that the fraction of PS4 butyl side chains in PS4-bound aggregates increases. The comicellization model then predicts a decrease of N, in agreement with the experimental observation. Relationship between Fluorescence Lifetime τ2 and Microviscosity ηi. It was shown above that the long pyrene fluorescence lifetimes (>200 ns) measured for PSX-DTAC systems cannot be explained in terms of an increased hydrophobicity of the pyrene microenvironment in aggregates or by the substitution of surfactant counterions by polymer charged groups, contrary to what has been suggested.9-11 The results in Figure 7 and 8 show striking similarities. There are no variations of τ2 with the surfactant concentration but large increases when R is decreased. In all instances, large values of τ2 are associated with large values of ηi. This can be understood as follows. An increase of microviscosity reduces the diffusion rate of aggregate-solubilized pyrene and oxygen and, thus, the efficiency of quenching of pyrene by oxygen,54 thereby resulting in an increase of pyrene lifetime. Almgren et al.10 reported a decrease of the second-order quenching rate constant in poly(sodium styrenesulfonate)-alkyltrimethylammonium halide systems, with respect to polymer-free micellar solutions. The increased microviscosity of polymer-bound surfactant aggregates with respect to free micelles observed in this study and others12-15 may reflect a more complex motional pattern in the former, to which the physical binding of surfactant ions to the polymer, which reduces their motional freedom (50) Zana, R.; Yiv, S.; Strazielle, C.; Lianos, P. J. Colloid Interface Sci. 1983, 91, 256. (51) Lianos, P.; Lang, J.; Strazielle, C.; Zana, R. J. Phys. Chem. 1982, 86, 1019. (52) Almgren, M.; Swarup, S. J. Colloid Interface Sci. 1983, 91, 256. (53) Malliaris, A.; Binana-Limbe´le´, W.; Zana, R. J. Colloid Interface Sci. 1986, 110, 114. (54) Hirayama, S.; Yasuda, H.; Scully, A. D.; Okamoto, M. J. Phys. Chem. 1994, 98, 4609.
Langmuir, Vol. 12, No. 8, 1996 1975
(less fluid micelle core), and the presence of the polymer hydrophobic backbone on, and possibly somewhat below, the aggregate surface probably contribute. The microviscosity can also be related to the aggregation number. For the smallest aggregates (N ) 11 at R ) 0), interdigitation of the surfactant alkyl chains in the micelle core can slow down their motion and that of reactants (probe and quencher). These various effects explain the reduction of efficiency of quenchers (alkylpyridinium ions or DMBP), which had remained unexplained thus far. Conclusions The interaction between the surfactant DTAC and two polyelectrolytes, PS1 and the more hydrophobic PS4, has been investigated. The binding of the surfactant is cooperative at neutralization degrees R between 0 and 1.00 for PS1 and between 0.50 and 1.00 for PS4. The aggregation numbers of polymer-bound aggregates have been found to be independent of the concentration of bound surfactant. They increase with R for PS1 but decrease with PS4. This difference of behavior arises from the contribution of hydrophobic interactions in the case of DTAC binding to PS4 while the binding to PS1 appears to be essentially controlled by short range electrostatic interactions. The fluorescence lifetime of pyrene in polymer-bound aggregates is always larger than that in polymer-free micelles. These results have been explained in terms of the microviscosity of polymer-bound aggregates, which has been found to be significantly larger than that of polymer-free micelles, probably because of surfactant ion binding to the polymer and also the tight wrapping of the polymer main chain around aggregates with polymer and surfactant charged groups in contact. The surfactant counterions are expelled from the bound aggregate surface. The large microviscosity of bound aggregates is responsible for the low efficiency of quenching processes in these aggregates. Acknowledgment. The authors gratefully acknowledge Dr. G. Duportail (Faculty of Pharmacy, University Louis Pasteur, Strasbourg) for the use of the fluorescence anisotropy apparatus and for his help in the measurements and Drs. R. Varoqui and E. Pefferkorn for the gift of the polymer samples and stimulating discussions. LA950817J
