NMR Study of the Association of Anionic Surfactants with an Anionic

Monitoring the different micelle species and the slow kinetics of tetraethylammonium perfluorooctane-sulfonate by 19 F NMR spectroscopy. Xiaolin Wang ...
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Langmuir 2001, 17, 8049-8054

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NMR Study of the Association of Anionic Surfactants with an Anionic Polyelectrolyte Hydrophobically Modified with Perfluorinated Side Chains Ilias Iliopoulos*,†,§ and Istva´n Furo´‡ Laboratoire de Physico-chimie Macromole´ culaire, UMR-7615, ESPCI - CNRS - UPMC, 10, rue Vauquelin, F-75231 Paris Cedex 05, France, and Division of Physical Chemistry, Department of Chemistry, Royal Institute of Technology, SE-10044 Stockholm, Sweden Received June 4, 2001. In Final Form: September 5, 2001 Mixtures of surfactants with a hydrophobically modified polyelectrolyte in aqueous solution are investigated by 19F NMR spectroscopy performed on the fluorinated side chains of the polyelectrolyte component. The two added surfactants, one hydrogenated and one fluorinated, behave differently as manifested by the chemical shift effects in the obtained NMR spectra: the mixing of the hydrogenated one with the polyelectrolyte is clearly nonideal, while the mixing of the two fluorinated species is closer to ideal. The different aggregates dominated either by the polyelectrolyte or by the surfactant component share some dynamic features: the exchange of the polyelectrolyte molecules among them is slow, while that of the surfactant molecules is fast on the millisecond time scale.

Introduction Aqueous systems based on polymer/surfactant mixtures1,2 have attracted substantial interest. These complex fluids, besides having found important practical applications in detergency, cosmetics, food, paints, and so forth, also present some fundamental questions about the polymer-surfactant interactions that, to a large extent, control their observed behavior. The early studies have been devoted mainly to systems with attractive interactions, either weak (e.g., between nonionic polymers and anionic surfactants) or strong (between oppositely charged polyelectrolytes and surfactants) ones.1 In particular, the role of the attractive hydrophobic interactions, although it had been recognized a long time ago, has been studied systematically only in the past decade.2-5 In this context, the rapid development of amphiphilic polymers, such as hydrophobically modified associative water-soluble polymers and polysoaps6,7 (note that there is no sharp distinction between these materials: polysoap if the hydrophobic content is high, associative polymer if it is low), has been essential. These materials allow us to precisely tune the hydrophobic interactions with surfactants and thereby to change the structure of the obtained intermolecular complexes.8,9 Among associative polymers, the ones with fluorinated hydrophobic * Corresponding author. Tel: +33 140795114. Fax: +33 140795117. E-mail: [email protected]. † ESPCI - CNRS - UPMC. ‡ Royal Institute of Technology. § Present address: Matie ` re Molle et Chimie, UMR-167, ESPCI - CNRS - ATOFINA, 10 rue Vauquelin, F-75231 Paris cedex 05, France. (1) Goddard, E. D.; Ananthapadmanabhan, K. P. Interactions of Surfactants with Polymers and Proteins; CRC Press: Boca Raton, FL, 1993. (2) Kwak, J. C. T. Polymer-Surfactant Systems; Marcel Dekker: New York, 1998. (3) Hansson, P.; Lindman, B. Curr. Opin. Colloid Interface Sci. 1996, 1, 604-613. (4) Iliopoulos, I. Curr. Opin. Colloid Interface Sci. 1998, 3, 493-498. (5) Piculell, L.; Guillemet, F.; Thuresson, K.; Shubin, V.; Ericsson, O. Adv. Colloid Interface Sci. 1996, 63, 1-21. (6) Winnik, M. A.; Yekta, A. Curr. Opin. Colloid Interface Sci. 1997, 2, 424-436. (7) Laschewsky, A. Adv. Polym. Sci. 1995, 124, 1-86.

groups10-19 occupy a rather exceptional position with their strong, compared to their hydrogenated analogues, tendency to self-associate. Moreover, the presence of fluorine atoms opens a new window, via 19F NMR, onto the molecular details of the association mechanism. In the present study, we exploit this window. One particularly interesting question that stems from a long line of related studies of simple surfactants is the ability of such fluorinated polymers to form mixed micelles with hydrogenated surfactants. Most often, one found that hydrogenated and fluorinated surfactants do not mix ideally20-25 because of the mutual phobicity between hydrocarbons and fluorocarbons. In a previous work, we have reported rheological data on the association between hydrophobically modified poly(sodium acrylate), HMPA, and anionic surfactants.14 HMPAs with either hydrogen(8) Magny, B.; Iliopoulos, I.; Zana, R.; Audebert, R. Langmuir 1994, 10, 3180-3187. (9) Anthony, O.; Zana, R. Langmuir 1996, 12, 3590-3597. (10) Zhang, Y. X.; Zhang, F. S.; Hogen-Esch, T. E. Fluorocarbonmodified water soluble polymers; Dubin, P., Bock, J., Davis, R., Schultz, D. N., Thies, C., Eds.; Springer: Berlin, 1994; pp 95-116. (11) Hwang, F. S.; Hogen-Esch, T. E. Macromolecules 1995, 28, 3328. (12) Cochin, D.; Hendlinger, P.; Laschewsky, A. Colloid Polym. Sci. 1995, 273, 1138-1143. (13) Xie, X.; Hogen-Esch, T. E. Macromolecules 1996, 29, 1734-1745. (14) Petit, F.; Iliopoulos, I.; Audebert, R.; Szo¨nyi, S. Langmuir 1997, 13, 4229-4233. (15) Xu, B.; Li, L.; Yekta, A.; Masoumi, Z.; Kanagalingam, S.; Winnik, M. A.; Zhang, K.; MacDonald, P. M.; Menchen, S. Langmuir 1997, 13, 2447-2456. (16) Corpart, J. M.; Ernst, B.; Liu, F. S. Rev. Inst. Fr. Pet. 1997, 52, 254-257. (17) Cathebras, N.; Collet, A.; Viguier, M.; Berret, F. F. Macromolecules 1998, 31, 1305-1311. (18) Chen, J.; Jiang, M.; Zhang, Y.; Zhou, H. Macromolecules 1999, 32, 4861-4866. (19) Preuschen, J.; Menchen, S.; Winnik, M. A.; Heuer, A.; Spiess, H. W. Macromolecules 1999, 32, 2690-2695. (20) Mukerjee, P.; Yang, A. Y. S. J. Phys. Chem. 1976, 80, 13881390. (21) Shinoda, K.; Nomura, T. J. Phys. Chem. 1980, 84, 365-369. (22) Funasaki, N.; Hada, S. J. Phys. Chem. 1980, 84, 736. (23) Muto, Y.; Esumi, K.; Meguro, K.; Zana, R. J. Colloid Interface Sci. 1987, 120, 162-171. (24) Asakawa, T.; Mori, M.; Miyagishi, S.; Nishida, M. Langmuir 1989, 5, 343. (25) Kissa, E. Fluorinated Surfactants; Marcel Dekker: New York, 1994.

10.1021/la0108104 CCC: $20.00 © 2001 American Chemical Society Published on Web 12/01/2001

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ated or fluorinated hydrophobic groups were mixed with hydrogenated or fluorinated surfactants at constant polymer concentration. For polymers with low levels of hydrophobic modification, a rather straightforward indication of nonideal mixing between fluorinated and hydrogenated species (e.g., a fluorinated polymer and a hydrogenated surfactant) is the lack of any significant change of the viscosity as a function of surfactant concentration.14 In contrast, the viscosity has peaked in mixtures of polymers and surfactants with the same atoms in the alkyl chains; that was interpreted as a sign of more ideal mixing. For HMPAs with a higher level of hydrophobic modification (by fluorinated groups), association with hydrogenated surfactants is nevertheless feasible, as witnessed by viscometry.14 Nonideal mixing between fluorinated (hydrogenated) hydrophobically modified polymers and hydrogenated (fluorinated) surfactants has also been reported, based on rheological data, in some other systems.13,26 Hence, it might be judicious to investigate this problem by another technique, in our case by 19F NMR. The polymer used in the present investigation is the same poly(sodium acrylate) derivative as that in our previous studies.27,28 It contains 8 mol % of -CH2C7F15 side groups randomly anchored onto the polymer backbone. Hereafter, these hydrophobic groups will be referred to as C8F (for details, see Experimental Section).

Iliopoulos and Furo´ Table 1. Concentrations Cp of PA-8C8F Used in This Study and the Corresponding Molal Concentrations of the Monomer, Ionic, and Hydrophobic Unitsa polymer monomer units sodium acrylate C8F units concn (wt %) (mol/kg D2O) units (mol/kg D2O) (mol/kg D2O) 0.3 5

2.45 × 10-2 4.30 × 10-1

2.25 × 10-2 3.95 × 10-1

2.0 × 10-3 3.4 × 10-2

a The following molar masses were used to calculate the molal concentrations: 122.7 for the average repeating unit, 94 for the ionic sodium acrylate unit, and 453 for the fluorinated C8F unit.

with such spectra, it is therefore easy to measure the relative fractions of free and aggregated C8F groups by simple spectral integration. In addition, it has been demonstrated38,42 that the 19F chemical shift of the -CF3 group is also sensitive to the type of hydrophobic environment (hydrogenated or fluorinated): the maximum shift difference between these two environments is about 1.7 ppm with the fluorocarbon environment more upfield (larger negative ppm values). Hence, one may anticipate detecting the mixing, if any, of the C8F chains with hydrogenated surfactants via the NMR spectrum. Experimental Section

In the absence of surfactant, both NMR and rheology data allow us to conclude that HMPAs with C8F groups behave almost in the same manner as HMPAs with hydrogenated dodecyl (C12) groups.14,29 This equivalence is in line with the hydrophobicity of a CF2 group that is about 1.5-1.7 times higher than that for a CH2 group25 as demonstrated in hydrogenated and fluorinated surfactants.30 The aggregation of the C8F groups, in aqueous solution, can be easily followed by 19F NMR since the chemical shift of 19F is strongly sensitive to the molecular environment.31-33 The 19F NMR spectra of HMPAs with C8F side groups exhibited a characteristic doubling27 of the spectral peaks that was straightforward to interpret in terms of slow exchange of the C8F chains between associated (surrounded mainly by other fluoroalkyl groups) or free (surrounded by water molecules) states (see Figure 1 in ref 28). Such behavior has also been demonstrated in other fluorinated surfactant systems,34-38 usually via the peaks belonging to the terminal -CF3 group of the alkyl tail that is the most sensitive to association.39-41 For systems

The amphiphilic copolymer, with its formula above, was the same as the one used in previous studies.27-29 It was obtained by modification of a poly(sodium acrylate) precursor with a low molar mass (Mn ≈ 5600, Mw/Mn ≈ 2.3). Hereafter, this particular HMPA will referred to as PA-8C8F; PA stands for poly(sodium acrylate) and 8C8F for 8 mol % C8F groups (-CH2C7F15). Sodium dodecyl sulfate (SDS, Sigma, 99%) and D2O (Isotec, 99.8%) were used as obtained. The applied D2O was deoxygenated by bubbling N2 gas through it for several hours. Cesium perfluorooctanoate (PFO) was produced as given in ref 43. The solutions were prepared by dissolving the proper amount of polymer in deoxygenated D2O or in SDS or PFO solutions with deoxygenated D2O. All solutions were equilibrated for ≈4 h at 40 °C and then for at least 24 h at room temperature (≈22 °C). Concentrations are given in weight % and, if necessary, in molal units m (moles of solute per kg of D2O). Two polymer concentrations, Cp ) 0.3% and 5%, were studied; their concentration in molal units is given in Table 1. All spectra reported in this work display solely the -CF3 regions of the 19F NMR spectra recorded on a Bruker DMX500 spectrometer at 470 MHz resonance frequency for 19F. The chemical shift scale was set to provide -82 ppm for the -CF3 signal corresponding to groups in an aqueous environment; the position of this peak remained the same in all samples. Note that for the sake of a clearer distinction between the different spectral shapes, the spectra recorded in the PA-8C8F/SDS mixtures (Figures 2 and 3) are displayed with their F/P peaks normalized to identical amplitudes. The spectra recorded in the PA-8C8F/PFO mixtures (Figure 5) are displayed with unaltered relative amplitudes. The 90° pulse length for a conventional highresolution NMR probe was set to 10.5 µs and the sample temperature to 25 °C. The true 19F NMR spectral shapes were,

(26) Ka¨stner, U.; Hoffmann, H.; Do¨nges, R.; Ehrler, R. Colloids Surf., A 1994, 82, 279-297. (27) Petit, F.; Iliopoulos, I.; Audebert, R. Polymer 1998, 39, 751753. The random arrangement of the side groups was proved for derivatives with hydrogenated side groups (Magny, B.; Lafuma, F.; Iliopoulos, I. Polymer 1992, 33, 3151.). Since the present derivative was obtained by the same procedure, one expects a similar random structure. (28) Furo´, I.; Iliopoulos, I.; Stilbs, P. J. Phys. Chem. B 2000, 104, 485-494. (29) Petit-Agnely, F.; Iliopoulos, I. J. Phys. Chem. B 1999, 103, 48034808. (30) Ravey, J. C.; Ste´be´, M. J. Colloids Surf., A 1994, 84, 11. (31) Abraham, R. J.; Wileman, D. F. J. Chem. Soc., Perkin Trans. 1973, 1027-1035. (32) Abraham, R. J.; Wileman, D. F. J. Chem. Soc., Perkin Trans. 1973, 1521-1526. (33) Gerig, J. T. Prog. Nucl. Magn. Reson. Spectrosc. 1994, 26, 293370.

(34) Segre, A. L.; Proietti, N.; Sesta, B.; D’Aprano, A.; Amato, M. E. J. Phys. Chem. B 1998, 102, 10248-10254. (35) Bossev, D. P.; Matsumoto, M.; Nakahara, M. J. Phys. Chem. B 1999, 103, 8251-8258. (36) Huc, I.; Oda, R. Chem. Commun. 1999, 2025-2026. (37) Abe, M.; Tobita, K.; Sakai, H.; Kamogawa, K.; Momozawa, N.; Kondo, Y.; Yoshino, N. Colloids Surf., A 2000, 167, 47-60. (38) Oda, R.; Huc, I.; Danino, D.; Talmon, Y. Langmuir 2000, 16, 9759-9769. (39) Muller, N.; Simsohn, H. J. Phys. Chem. 1971, 75, 942-945. (40) Guo, W.; Brown, T. A.; Fung, B. M. J. Phys. Chem. 1991, 95, 1829-1836. (41) Guo, W.; Fung, B. M.; O’Rear, E. A. J. Phys. Chem. 1992, 96, 10068-10074. (42) Clapperton, R. M.; Ottewill, R. H.; Ingram, B. T. Langmuir 1994, 10, 51-56. (43) Jo´hannesson, H.; Furo´, I.; Halle, B. Phys. Rev. E 1996, 53, 49044917.

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Figure 2. 19F NMR spectra of PA-8C8F at Cp ) 0.3% with the molal concentration of added SDS given in the figure. The molal concentration of the C8F units is 2.0 × 10-3 (see Table 1). The F/P peak arises from free (F) and partially aggregated (P) HMPAs indicated in Figure 1. The S peak belongs to HMPAs associated to SDS micelles. The spectra are normalized to set the F peaks to identical amplitudes. Figure 1. Schematic illustration of the different states of the PA-8C8F polymer in the investigated systems. The free (F), partially aggregated (P), and aggregated (A) polymers all appear in pure aqueous solutions of PA-8C8F. SDS molecules are first embedded into the A-type aggregates, but at higher SDS concentrations SDS-rich micelles are formed into which a few C8F chains are involved (S). as previously,28 recorded by a 90°-τ-180°-τ spin-echo experiment with short (τ ) 20 µs) delay times and with cycling the rf phases according to the Exorcycle44 scheme. This procedure provides exact refocusing of the dephasing due to a chemical shift distribution.45 13C spectra of the SDS/PA-8C8F mixtures were also recorded under 1H decoupling by 48 µs long 180° pulses in the WALTZ-16 decoupling sequence46 (note that the spectra were not 19F-decoupled and, therefore, only the SDS and the PA backbone carbon signals were detected as narrow single lines). For all other experimental details, the reader is referred to ref 28.

Results and Discussion To aid the reader, in Figure 1 we depict the aggregate types in terms of which we explain below the spectral features. The existence of three (denoted by F, P, and A, signifying “free”, “partially aggregated”, and “aggregated”) states of HMPAs has been thoroughly established in our previous work.28 The S-type aggregate, in which a few C8F side chains are associating to an SDS micelle, is specific to the present study. As we shall discuss below, some surfactant molecules can also be incorporated in the A-type aggregates. Nonideal Mixing of PA-8C8F with Sodium Dodecyl Sulfate. At low concentrations, the side chains of the associative polymer do not form dense hydrophobic aggregates and the 19F spectrum in Figure 2 exhibits, for the -CF3 groups, a single asymmetric line contributed by the F- and P-type HMPA chains. The strongest proof for this is the position of the line that is very close to that of perfluorinated surfactants below the cmc.39 As discussed in our previous work,28 the narrow part, centered around -82 ppm, corresponds to free C8F groups in direct contact with the aqueous environment, while the “foot”, extending to about -83 ppm, belongs to partially aggregated C8F units (a kind of preaggregates). Note that such preaggre(44) Bodenhausen, G.; Freeman, R.; Turner, D. L. J. Magn. Reson. 1977, 27, 511-514. (45) Rance, M.; Byrd, R. A. J. Magn. Reson. 1983, 52, 221-240. (46) Waugh, J. S. J. Magn. Reson. 1982, 50, 30.

Table 2. Approximatea Critical Micelle Concentration (cmc) of the Added Surfactants at Ionic Strength Corresponding to the Different Concentrations Cp of PA-8C8F Used in This Studyb polymer concn (wt %)

cmc of SDS (mol/kg D2O)

cmc of PFO (mol/kg D2O)

0 0.3 5

8 × 10-3 6.5 × 10-3 a 1 × 10-3 a

3 × 10-3 c

a The addition of unmodified PA decreases the cmc of SDS (ref 47). However, due to counterion condensation on the polyelectrolyte only 1/3 of the ionic units of the PA contribute to the ionic strength of the solution, and therefore PA decreases the cmc of SDS less than equimolar simple salts such as NaCl. The approximate cmc values, quoted from ref 47, take into account this effect. b The molal concentrations of the ionic groups of the added polymer are given in Table 1. c For CsPFO (ref 51). The cmc of sodium perfluorooctanoate is higher (ref 39), about 3 × 10-2 m, in the absence of salt. In the presence of 5% polyelectrolyte, the cmc of PFO should be much lower than this value.

gates (or premicelles) have also been claimed to exist for some perfluorinated surfactants.35 On addition of 10-2 m SDS, which is a concentration that is above the critical micelle concentration (cmc) of SDS at the estimated47 ionic strength of the solution (see Table 2), the spectrum remains practically unchanged, with slightly increased P-type contribution. Further increase of SDS content results in the appearance of a new peak, the maximum of which is at about -82.7 ppm (peak S). We attribute this new peak to the incorporation of the C8F groups into the SDS micelles. These mixed micelles are expected to be rich in SDS and to contain only a small fraction of C8F groups. The C8F groups in the mixed aggregates experience a hydrocarbon environment instead of an aqueous one which moves their chemical shift upfield. Since the relative weight of the F peak decreases somewhat (recall that the spectra in Figure 2 are normalized to identical F amplitudes), a fraction of originally free HMPAs become associated to SDS micelles. The presence of two separate 19F peaks and, even more importantly, the constant chemical shift of the F peak show that the exchange of polymer chains between the S-type mixed aggregates and the F state is slow on the time scale that is set by the inverse of the frequency (47) Binana-Limbele, W.; Zana, R. Colloids Surf. 1986, 21, 483-494.

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Figure 4. The chemical shift of the A peak (b, HMPAs in C8F-rich aggregates) and of the S peak (9, HMPAs in SDS-rich micelles) as a function of the SDS molal concentration at the fixed concentration of Cp ) 5% of PA-8C8F.

Figure 3. 19F NMR spectra of PA-8C8F at Cp ) 5% with the molal concentration of added SDS given in the figure. The molal concentration of the C8F units is 3.4 × 10-2 (see Table 1). The F/P peak arises from free (F) and partially aggregated (P) HMPAs indicated in Figure 1. The A peak belongs to HMPAs forming C8F-rich aggregates, and the S peak belongs to HMPAs involved in SDS-rich micelles. The spectra are normalized to set the F peaks to identical amplitudes. The thick arrow in (a) indicates the appearance of SDS-rich micelles.

difference between the two observed peaks,48 that is, ∼4 ms. Since the spectral contribution of the P state is masked by the S peak, we cannot say if the P state remains intact or not. The spectra obtained at Cp ) 5% are shown in Figure 3. In the absence of surfactant (see Figure 3a), the spectrum exhibits two well-separated peaks, the first of which is essentially the same as the one observed at low polymer concentration and corresponds to the free and partially aggregated C8F groups (peak F/P). The second (48) Abragam, A. The Principles of Nuclear Magnetism; Clarendon: Oxford, 1961.

broad peak (peak A), centered around -84 ppm, represents the C8F groups involved in dense hydrophobic aggregates comparable to micelles of perfluorinated surfactants.28 When small amounts of SDS are added, up to ∼5 × 10-3 m (Figure 3a), the spectrum changes only slightly with peak A moving a bit downfield. At Cs ) 5 × 10-3 m, where Cs denotes the surfactant concentration, a “shoulder” appears on the left side of peak A (indicated by a thick arrow in Figure 3a) that is the forerunner of a new peak appearing at higher Cs. The new “intermediate” peak is clearly seen in Figure 3b for all Cs between 10-2 and 10-1 m. This peak increases at the expense of peak A, and as in the case of low polymer concentration, we attribute it to SDS-rich mixed aggregates (peak S). An isobestic point is observed at -83.4 ppm (Figure 3b) that indicates a kind of equilibrium between the A- and S-type aggregates. Finally, at the two highest concentrations, 0.1 and 0.33 m, peak A disappears and only peaks F and S can be seen (Figure 3b,c). Moreover, the S peak shifts downfield with increasing Cs and at 0.33 m its maximum is approximately at the same chemical shift, -82.7 ppm, as the S peak observed at low polymer concentration (Figure 2, Cs ) 0.1 m). The chemical shifts at the maxima of peaks A and S are summarized in Figure 4. Let us now discuss the observed spectral changes at the higher polymer concentration, Cp ) 5%. At this concentration, the ionic strength introduced by the polyelectrolyte is expected to reduce the cmc of SDS to ∼1 × 10-3 m (see Table 2). Hence, only a small fraction of the SDS molecules present in the solution are free. Up to Cs ) 5 × 10-3 m, the A-type aggregates of the polymer chains seem to coexist with molecular or micellar SDS (one should note that the presence of some SDS micelles that do not contain any PA-8C8F cannot be excluded based on our 19F spectral data). A part of the added SDS incorporated into the A-type aggregates slightly changes the average environment of the involved CF3 groups and induces the small downfield shift of the A peak. Further addition of SDS results in the formation of a new type of mixed aggregates, plausibly rich in SDS, into which a few C8F groups are involved (S peak). This point is supported by the appearance of the same peak at low polymer concentration (Figure 2), where no A-type aggregates are formed. The decreasing upfield shift of the S peak with increasing Cs, shown in Figure 4, is most probably caused by the decreasing average C8F content of the S-type aggregates. The most important conclusion we can extract from Figure 3 is that in the presence of SDS, PA-8C8F forms two types of aggregates,

NMR Study of Surfactant Association

one rich in C8F and the other rich in SDS. Thereby, 19F NMR spectroscopy gives the direct molecular proof of nonideal mixing between the fluorinated side groups of the polymer and the SDS molecules. This result is in line with some of our previous conclusions extracted from the viscosity behavior of this system14 and reiterates the behavior of hydrogenated/fluorinated surfactant mixtures.20 Note that the nonideal mixing between fluorinated and hydrogenated groups has also been elegantly exploited for the synthesis of amphiphilic copolymers with locally segregated hydrophobic aggregates.49,50 From the spectra in Figure 3 one can, by simple integration, extract information about the fraction of C8F groups contributing to the F/P peak. Since the F/P and S peaks are not sufficiently separated for Cs > 5 × 10-2 m, these estimates of the relative F/P population are less accurate (≈15% relative error for the two highest SDS concentrations). Nevertheless, the observed trends are clear. The most important conclusion is that the F/P fraction remains practically constant for SDS concentrations up to 5 × 10-2 m (see Figure 6). Therefore, in a first step the addition of SDS does not induce any extra aggregation of the C8F groups. The SDS-rich aggregates (S) are formed by picking up PA-8C8F chains from the C8F-rich aggregates (A). Only when all A-type aggregates have been consumed do the F/P polymer chains start to incorporate into the SDS micelles. A side effect of SDS addition is the increase in the total ionic strength of the solution. This contribution becomes substantial for the two highest SDS concentrations and can partly explain the decrease of the F/P fraction as already observed when adding salt.29 As indicated by the presence of three distinct peaks, the exchange process of the C8F groups between states A, S, and F is slow, on the time scale of ∼1 ms, even in the presence of high SDS concentration. An obvious question is, how does the SDS exchange between aggregates A and S? To answer this question, we recorded the 1H-decoupled 13C spectrum (not shown) of the mixture that contained 5% PA-8C8F and 5 × 10-2 m SDS. As has been demonstrated in hydrogenated HMPAs,29 the 13C lines split, just like the 19F lines in our case, because the alkyl chains exchange slowly between different environments such as A- and S-type aggregates. Here, only wellresolved single lines have been observed for the alkylchain carbons of SDS (methyl and methylene groups in the spectral region of 13-24 ppm). Therefore, we conclude that the SDS exchange is fast. The fast exchange of the surfactant and the slow exchange of the C8F groups of the polymer can be easily understood in the framework of the model proposed recently.28 This model (for all details, see ref 28) is based on the NMR relaxation and diffusion characteristics which show that the polymer chains exchange very slowly between the aggregated and the free states (to exchange a polymer chain, several C8F groups have to change environment simultaneously). On the other hand, NMR relaxation also provides that the internal dynamics of the aggregate is fast. In other words, any individual side group (C8F) of a polymer chain involved in the aggregate exchanges quickly between the hydrophobic core of the aggregate and the surrounding aqueous environment. However, such an individual C8F group cannot exchange completely with the free F state since it (49) Sta¨hler, K.; Selb, J.; Barthelemy, P.; Pucci, B.; Candau, F. Langmuir 1998, 14, 4765-4775. (50) Sta¨hler, K.; Selb, J.; Candau, F. Langmuir 1999, 15, 75657576. (51) Boden, N.; Jolley, K. W.; Smith, M. H. J. Phys. Chem. 1993, 97, 7678-7690.

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Figure 5. 19F NMR spectra of PA-8C8F at Cp ) 5% with the molal concentration of added PFO given in the figure. The molal concentration of the C8F units is 3.4 × 10-2 (see Table 1). The F/P peak arises from free (F) and partially aggregated (P) HMPAs indicated in Figure 1, and the A peak belongs to dense hydrophobic domains formed by PFO and C8F chains. The PFO peak is contributed by the PFO- ions that exchange quickly among different aggregates and the aqueous pool.

is connected to the polymer backbone. It is obvious that the dynamics of the SDS molecules in the aggregates is rather similar to that of the individual C8F groups, but with one important difference: there is no connecting polymer backbone to keep the SDS molecule in the vicinity of the aggregate from which it has departed. One should also note that the chemical shift of the S peak is 0.7 ppm upfield from the aqueous F peak at high SDS concentrations both at Cp ) 0.3% and 5%. This value is roughly 2/3 of the expected shift difference (∼1.1 ppm) between aqueous and hydrogenated hydrophobic environments.42 Hence, all C8F chains of S-state HMPAs cannot be simultaneously embedded in the interior of the SDS micelles but some of them must reside outside in the aqueous environment (see the schematic illustration in Figure 1). The exchange of the individual C8F chains between these two different environments is fast, just like the SDS exchange between aggregates. Mixing of PA-8C8F with Cesium Perfluorooctanoate. Complementary information on the behavior of mixed polymer/surfactant systems can be obtained when adding a perfluorinated surfactant instead of SDS. The spectrum of PA-8C8F (Cp ) 5%) in the presence of 5 × 10-3 and 2 × 10-2 m PFO is shown in Figure 5. The -CF3 group of the surfactant gives a separate narrow peak, between the F/P and A peaks, that shifts upfield with increasing surfactant concentration. Comparing the integral intensities of the peaks yields that the PFO chains contribute only to this narrow peak while the F/P and A peaks represent the total polymer contribution. Therefore, the PFO molecules exchange fast between the aqueous and the fluorocarbon environments while the polymer exchange, between the F/P and A states, remains slow. Hence, PFO behaves similarly to SDS. However, there are also some clear differences compared to the SDS case. First, the A peak shifts slightly upfield. This we explain by the PFO chains incorporating into the polymer aggregates, thereby making them more compact and reducing the contact of the C8F groups with water. Note that at our highest PFO content (10-1 m, spectrum not shown) the A-type peak expands up to -85 ppm; -CF3 groups with that chemical shift39 must be entirely devoid of water contact. Another difference compared to the system mixed with SDS is the higher affinity of the PFO micelles to incorporate the free C8F groups of the polymer (i.e., those

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Figure 6. The estimated fraction of unaggregated HMPAs obtained by spectral integration of the F/P peaks in the spectra of PA-8C8F/SDS (see spectra in Figure 3) and PA-8C8F/PFO (see spectra in Figure 5) mixtures as a function of surfactant molal concentration.

originally contributing to the F/P peak). Hence, the F/P fraction, obtained by spectral integration, decreases much faster upon PFO addition (Figure 6). In other words, the mixing of the fluorinated side groups of the polymer with the perfluorinated surfactant is closer to the ideal mixing case than the mixing with SDS. Note that in contrast to the SDS case we cannot, even if they exist, distinguish between PFO-rich and C8F-rich aggregates since the environment in both of these hypothetical situations is dominantly fluorocarbon. Consequently, we cannot prove, from the existing data, that the mixing is ideal. Conclusion 19

F NMR can give detailed molecular information on the association mechanism between associative polymers

Iliopoulos and Furo´

and surfactants. By using associative polymers with fluorinated side groups, it is possible to follow the formation of mixed aggregates involving surfactant molecules and the side groups of the polymer. Whatever the nature of the surfactant, SDS or PFO, its exchange between different environments (various aggregated states and water) is fast, while the polymer exchange is slow even at high surfactant/polymer ratios. In the presence of SDS, two types of mixed aggregates have been observed, one C8F-rich and one SDS-rich, owing to the nonideal mixing of the hydrogenated surfactant and the fluorinated groups of the polymer. On the other hand, the mixing of PFO with our fluorinated polymer seems to be closer to the ideal case, thereby reflecting the higher compatibility between the fluorinated surfactant and the C8F groups. These inferences are in good agreement with the conclusions of a previous viscometric study of similar polymers.14 Concerning the microscopic structure of the formed mixed aggregates, the shift effects observed in the present study seem to indicate, if any, no molecular-level segregation of the fluorinated and hydrogenated chains within the mixed aggregates. Additional studies are necessary to put this inference on a firmer ground. Further work is also in progress to probe in more detail the changes in the molecular dynamics of the polymer upon addition of surfactants. Acknowledgment. This work has been supported by the Centre National de la Recherche Scientifique CNRS and by the Swedish Natural Science Research Council NFR. I.I. is grateful to the Wenner-Gren Foundations and I.F. to the Ecole Supe´rieure de Physique et de Chimie Industrielles de la Ville de Paris (ESPCI) for supporting their respective stays in Stockholm and in Paris. Florence Petit-Agnely is thanked for preparing the polymer used in the present study. LA0108104