Langmuir 2000, 16, 9921-9927
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Hydrophobically Modified Sodium Polyacrylates in Aqueous Solutions: Association Mechanism and Characterization of the Aggregates by Fluorescence Probing F. Petit-Agnely,*,† I. Iliopoulos,† and R. Zana‡ Laboratoire de Physico-Chimie Macromole´ culaire, UMR 7615, ESPCI-CNRS-UPMC, 10 Rue Vauquelin, 75231 Paris Cedex 05, France, and Institut Charles Sadron, CNRS-ULP, 6 Rue Boussingault, 67083 Strasbourg Cedex, France Received May 23, 2000. In Final Form: September 6, 2000 The association mechanism of several hydrophobically modified sodium polyacrylates (HMPA) in aqueous solutions has been investigated by fluorescence probing with pyrene as the fluorescent probe, by electrical conductivity, and by 13C NMR. The investigated HMPAs differed by the molecular weight of the poly(acrylic acid) precursor, the degree of hydrophobe substitution, and the carbon number of the hydrophobe (alkyl chain). The results showed that the association of HMPAs is less cooperative than the micellization of conventional surfactants and that there is a concentration below which no micelle-like aggregates are present in the systems. 13C NMR studies showed that only a fraction Faggr of hydrophobes is involved in the formation of hydrophobic microdomains, and that Faggr increases with the hydrophobe length and degree of substitution. The combination of the values of Faggr with the hydrophobic microdomain concentration, obtained from steady state and time-resolved fluorescence quenching measurements, allowed the determination of the average number of alkyl chains making up a hydrophobic microdomain. The polydispersity of the aggregates was estimated from time-resolved fluorescence quenching measurements. These characteristics of the HMPA microdomains were compared to those of micelles of conventional surfactants and of microdomains present in systems of other amphiphilic polymers.
Introduction Associating polymers are amphiphilic water-soluble polymers mainly constituted of a hydrophilic backbone and a low fraction of very hydrophobic side or end groups as, for instance, long alkyl chains. In aqueous solution the hydrophobic moieties can associate and form hydrophobic aggregates. These aggregates act as reversible cross-links between the polymer chains provided the chain backbone is long enough.1-10 Hence very viscous solutions and even gels can be obtained. Many studies have been devoted, in recent years, to the characterization at the molecular level of the hydrophobic aggregates present in such systems. The aggregation number, N, i.e., the number of hydrophobic moieties making up an aggregate, is a key parameter in this molecular description. N has been * Current address: Laboratoire de Physique Pharmaceutique, UMR 8612, Faculte´ de Pharmacie, Universite´ Paris XI, 5 rue JeanBaptiste Cle´ment, 92296 Chaˆtenay-Malabry Cedex, France. † Laboratoire de Physico-Chimie Macromole ´ culaire. ‡ Institute Charles Sadron. (1) Wang, T. K.; Iliopoulos, I.; Audebert, R. In Water-Soluble Polymers. Synthesis, Solution Properties and Applications; Shalaby, S. W., McCormick, C. L., Butler, G. B., Eds.; ACS Symposium Series 467; American Chemical Society: Washington, 1991; p 218. (2) Biggs, S.; Hill, A.; Selb, J.; Candau, F. J. Phys. Chem. 1992, 96, 1505. (3) Chang, Y.; McCormick, C. L. Macromolecules 1993, 26, 6121. (4) Guillemet, F.; Piculell, L. J. Phys. Chem. 1995, 99, 9201. (5) Maechling-Strasser, C.; Franc¸ ois, J.; F., C.; Tripette, C. Polymer 1992, 33, 627. (6) Ringsdorf, H.; Venzmer, J.; Winnik, F. M. Macromolecules 1991, 24, 1678. (7) Sinquin, A.; Hubert, P.; Dellacherie, E. Langmuir 1993, 9, 3334. (8) Tanaka, R.; Meadows, J.; Williams, P. A.; Phillips, G. O. Macromolecules 1992, 25, 1304. (9) Zhang, Y. X.; Da, A. H.; Hogen-Esch, T. E. J. Polym. Sci. C: Polym. Lett. 1990, 28, 213. (10) Winnik, M. A.; Yekta, A. Curr. Opin. Colloid Interface Sci. 1997, 2, 424.
determined by several techniques, the most widely used being fluorescence quenching,11-14 pulsed field gradient spin-echo NMR15-17 and, to a lesser extent, electron spinecho resonance.18 These techniques allow the determination of the the aggregate concentration. The aggregation number was then obtained on the assumption that all hydrophobic moieties are in the associated state. This assumption is however a very approximate one. Indeed, we have recently studied the association of hydrophobically modified poly(sodium acrylate) (HMPA) by NMR spectroscopy.19-21 Since the aggregation had an effect on the chemical shift of the alkyl chains, it was possible to directly measure the fraction of aggregated alkyl chains. The results showed that in HMPA solutions a substantial fraction of alkyl chains remains free or make up small preaggregates. This behavior was attributed to the polyelectrolyte nature of the backbone and to the comblike structure of these copolymers. This paper reports a study of the association of HMPAs in aqueous solutions, performed mainly by fluorescence (11) Wang, Y.; Winnik, M. A. Langmuir 1990, 6, 1437. (12) Yekta, A.; Duhamel, J.; Adiwidjaja, H.; Brochard, P.; Winnik, M. A. Langmuir 1993, 9, 881. (13) Alami, E.; Almgren, M.; Brown, W.; Francois, J. Macromolecules 1996, 29, 2229. (14) Richey, B.; Kirk, A. B.; Eisenhart, E. K.; Fitzwater, S.; Hook, J. J. Coat. Technol. 1991, 63, 31. (15) Walderhaug, H.; Hansen, F. K.; Abrahmsen, S.; Persson, K.; Stilbs, P. J. Phys. Chem. 1993, 97, 8336. (16) Rao, B.; Uemura, Y.; Dyke, L.; Macdonald, P. M. Macromolecules 1995, 28, 531. (17) Uemura, Y.; McNulty, J.; Macdonald, P. M. Macromolecules 1995, 28, 4150. (18) Persson, K.; Bales, B. L. J. Chem. Soc., Faraday Trans. 1995, 91, 2863. (19) Petit, F.; Iliopoulos, I.; Audebert, R. Polymer 1998, 39, 751. (20) Petit-Agnely, F.; Iliopoulos, I. J. Phys. Chem. B 1999, 103, 4803. (21) Furo´, I.; Iliopoulos, I.; Stilbs, P. J. Phys. Chem. B 2000, 104, 485.
10.1021/la000709j CCC: $19.00 © 2000 American Chemical Society Published on Web 11/11/2000
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Petit-Agnely et al.
Scheme 1
spectroscopy and steady state and time-resolved fluorescence quenching. Electrical conductivity and 13C NMR were also used to a lesser extent. By combining the data on the fraction of aggregated alkyl chains, measured by 13 C NMR, to the aggregate concentration, obtained by fluorescence quenching, we determined the aggregation numbers and the polydispersity of the hydrophobic microdomains, without the assumption discussed above. The results also permitted us to infer information on the aggregate micropolarity and microviscosity. Experimental Methods Materials. The HMPAs studied are random copolymers with the structure shown in Scheme 1, where x is the degree of hydrophobic modification in mol % and n is the carbon number of the alkyl chain. These polymers are obtained via a modification reaction of poly(acrylic acid) samples of average molecular weight 150 000, 50 000, and 5000. Their synthesis and characterization have been described.1,22,23 In the following these polymers are referred to as PAy-xCn where y is the PA molecular weight in thousands. Thus PA5-10C12 denotes a polymer obtained from a precursor of average molecular weight 5000 bearing 10 mol % of dodecyl side chains. The molecular weight of the average repeat unit, Munit, and the concentration of alkyl chains, Calkyl (in mol/ L), in a solution of concentration Cp (in weight percent) are given by:
Munit ) [94(100 - x) + (71 + 14n)x]/100
(1)
Calkyl ) xCp/10Munit
(2)
The surfactant DTAC (dodecyltrimethylammonium chloride) was purchased from TCI (Tokyo Kasei Organic Chemicals) as >98% pure. It was dried under vacuum (10-2 mmHg) before use. Highly purified pyrene was obtained from TCI and used as received. The quencher DPC (dodecylpyridinium chloride) was obtained from Fluka and purified by two cristallizations from ethanol/ethyl acetate mixtures. Solution Preparation. For the fluorescence experiments, stock solutions were prepared by solubilizing the proper amount of polymer or DTAC in deionized water and were left to equilibrate at room temperature for at least 18 h. The stock solutions were diluted to obtain samples with concentrations ranging from 10-6 to 10 wt %. A pyrene stock solution (6 × 10-4 M) was prepared in ethanol. For steady-state fluorescence measurements an aliquot of this solution was added with a microsyringe to the sample to obtain a final pyrene concentration of 6 × 10-7 M, at least 18 h before the measurements. For time-resolved fluorescence experiments the proper amount of pyrene stock solution was introduced into empty vials and the ethanol was evaporated. The vials were then filled with the polymer solutions, which were gently stirred for 18 h. The final pyrene concentration was adjusted to be 2 × 10-5 M. Those two methods of solution preparation yielded the same results. For steady-state fluorescence quenching experiments (SSFQ) and time-resolved fluorescence quenching experiments (TRFQ), concentrated aqueous solutions of DPC were prepared and added in small aliquots to the polymer solutions. The concentrations were recalculated taking into account the volume of additive. For TRFQ experiments the samples were deoxygenated prior to use by three successive freeze-pump-thaw cycles. For the NMR experiments, the polymer solutions were prepared, at least 18 h before use, by dissolving the proper amount of polymer in D2O. (22) Wang, T. K.; Iliopoulos, I.; Audebert, R. Polym. Bull. 1988, 20, 577. (23) Magny, B.; Lafuma, F.; Iliopoulos, I. Polymer 1992, 33, 3151.
Apparatus and Methods. 13C NMR spectra were recorded on a Bruker ARX 250 spectrometer operating at 62.9 MHz according to the experimental conditions previously described.20 Care was taken to record fully relaxed spectra. The 13C NMR spectra exhibit the characteristic features of slow exchange between the free and aggregated states for the alkyl chains. It was therefore possible to determine the fractions of free and aggregated chains by direct integration of the NMR signals.20 agg The concentration of aggregated chains, Calkyl , was then calculated from these fractions. The electrical conductivity method was used in an attempt to determine the critical aggregation concentration (cac) of some HMPAs. The conductivity measurements were performed at 25 °C using a Radiometer CDM 92 conductimeter equipped with platinum electrodes and calibrated with aqueous KCl solutions. Fluorescence spectroscopy, with the fluorescent probe pyrene, was used to investigate the mechanism of association of HMPAs. Recall that I1/I3, the ratio of the intensities of the first and third peaks in the pyrene emission spectrum, depends greatly on the polarity of the pyrene microenvironment. Thus, I1/I3 is of about 1.90 in water and only 0.60 in an apolar environment, such as hexane. Besides, pyrene is preferentially solubilized by micelles and hydrophobic aggregates. The variation of I1/I3 with surfactant concentration was successfully used to determine the critical micelle concentration (cmc) of surfactants24 or the cac of mixed polymer/surfactant systems.25,26 The emission fluorescence spectra of pyrene were recorded using an Aminco 500 SPF spectrometer. The excitation wavelength was set at 334 nm and bandpasses were set at 5 and 0.5 nm for excitation and emission, respectively. The emission intensities were measured at 373 nm (I1) and 384 nm (I3). For the most concentrated polymer solutions the intensities were corrected by taking into account the absorbances A1 and A3 at these wavelengths. Absorbances were measured with a Perkin-Elmer 552 spectrometer. The correction used the Beer-Lambert law, applied to a 1 cm wide cell:
() I1 I3
I1 × 10(A1-A3)/2 I3
)
cor
(3)
Steady-state fluorescence quenching (SSFQ) was used to determine the aggregation number of the hydrophobic microdomains.27 This method was first used to obtain the aggregation numbers of surfactant micelles.24,27,28 It was later applied to determine the aggregation numbers of mixed micelles of HMPA and DTAC25 and more recently of hydrophobically end-capped poly(ethylene oxide).13 In the present study the intensity I1 of the first peak in the pyrene emission spectra was measured in the presence of increasing amounts of quencher (DPC). The data were analyzed using the equation:27
I0 [Q] ) I [M]
ln
(4)
where [M] is the molar concentration of hydrophobic microdomains; [Q] is the quencher molar concentration; I0 and I are the fluorescence intensities in the absence and in the presence of quencher. A straight line of slope 1/[M] is expected when plotting ln(I0/I) versus [Q]. The aggregation number is then obtained from: agg N ) Calkyl /[M]
(5)
For TRFQ, the fluorescence decay curves were obtained using the single-photon counting method.26,29 The TRFQ method was first developed for surfactant systems.24 It was later successfully used for the study of the hydrophobic microdomains in solutions (24) Zana, R. In Surfactant Solutions: New Methods of Investigation; Zana, R., Ed.; Surfactant Science Series 22; Marcel Dekker: New York, 1987; p 241. (25) Magny, B.; Iliopoulos, I.; Zana, R.; Audebert, R. Langmuir 1994, 10, 3180. (26) Binana-Limbele´, W.; Zana, R. Macromolecules 1987, 20, 1331. (27) Turro, N. J.; Yekta, A. J. Am. Chem. Soc. 1978, 100, 5951. (28) Malliaris, A. Int. Rev. Phys. Chem. 1988, 7, 95. (29) Binana-Limbele´, W.; Zana, R. Macromolecules 1990, 23, 2731.
Aggregation of Aq HMPA by Fluorescence Probing
Langmuir, Vol. 16, No. 25, 2000 9923
Figure 1. Variation of I1/I3 with the DTAC concentration. The cmc is obtained as the concentration at the intersection of the two lines. of polysoaps,26,29-31 in mixtures of HMPA and surfactants,25 and in associating polymers.32,33 In micellar systems, under such conditions that [Py] , [M] and [Q]/[M] = 1, the decay of the fluorescence intensity with time follows the equation:34,35
I(t) ) I(0)exp[-A2t - A3[1 - exp(-A4t)]}
(6)
The parameters A2, A3, and A4 are given by eq 7, provided that the distribution of probe and quencher among micelles is frozen on the fluorescence time scale:34,35
A2 )
1 τ0
A3 )
[Q] [M]
A4 ) kq
(7)
In eq 7, τ0 is the pyrene fluorescence lifetime and kq is the rate constant for intra-aggregate quenching. The parameters I(0), A2, A3, and A4 were obtained by nonlinear least-squares fitting of eq 6 to the experimental fluorescence decay curves. The values of [M] and, in turn, of N were obtained from the value of A3 using eqs 5 and 7. Moreover the determination of N at several quencher concentrations provided information on the polydispersity of the hydrophobic aggregates. Indeed, for moderately polydisperse micelles the following first-order approximation holds:36
1 [Q] Nq ) 〈N〉 - σ2 agg 2 Calkyl
(8)
where Nq is the aggregation number obtained at a quencher concentration [Q], 〈N〉 is the weight average aggregation number obtained by extrapolation of Nq to zero quencher concentration, and σ is the root-mean-square deviation of the micelle size distribution (supposed Gaussian). Equation 8 was fitted to the Nq vs [Q] data using a least-squares linear regression. The values of 〈N〉 and σ were obtained from the intercept and the slope of the plots. The micelle polydispersity was characterized by the ratio σ/〈N〉.
Results and Discussion Study of the Association Mechanism. Figure 1 illustrates the variation of I1/I3 with concentration for a conventional surfactant, DTAC. At low surfactant concentration I1/I3 is about 1.90-1.95, i.e., pyrene experiences an aqueous environment. At the cmc there is a cooperative (30) Hsu, J. L.; Strauss, U. P. J. Phys. Chem. 1987, 91, 6238. (31) Chu, D. Y.; Thomas, J. K. Macromolecules 1987, 20, 2133. (32) Yekta, A.; Xu, B.; Duhamel, J.; Adiwidjaja, H.; Winnik, M. A. Macromolecules 1995, 28, 956. (33) Xu, B.; Li, L.; Zhang, K.; Macdonald, P. M.; Winnik, M. A.; Jenkins, R.; Bassett, D.; Wolf, D.; Nuyken, O. Langmuir 1997, 13, 6896. (34) Infelta, P.; Gra¨tzel, M.; Thomas, K. J. Phys. Chem. 1974, 78, 190. (35) Tachiya, M. Chem. Phys. Lett. 1975, 33, 289. (36) Warr, G. G.; Grieser, F. J. Chem. Soc., Faraday Trans. 1 1986, 82, 1813.
Figure 2. Variations of I1/I3 with the PA5-10C12 concentration in water and in NaCl 0.1 M. The solid and broken lines are the fits of the pyrene partition model (ref 52) to the experimental data points (correlation coefficients > 0.992).
association of the surfactant molecules leading to the formation of micelles which are well-defined aggregates. Pyrene is then solubilized in the micelles where it senses a less polar environment and I1/I3 decreases to about 1.50. Above the cmc, I1/I3 remains constant, as there are enough micelles for solubilizing all pyrene molecules. The variation of I1/I3 is steep over a narrow range of surfactant concentration. It can thus be used to determine the cmc of DTAC taken as the intersection of the two lines, as shown in Figure 1.37 This procedure yields a cmc value of 2 × 10-2 M, which is in very good agreement with the values reported for this surfactant.38,39 Similar experiments were performed with HMPAs and the effect of several structural and experimental parameters was examined. Figure 2 displays the variation of I1/I3 with the polymer concentration, Cp, for PA5-10C12 in water and in aqueous 0.1 M NaCl solution. When Cp increases, a decrease of I1/I3 is observed, revealing the presence or formation of hydrophobic aggregates in those systems. However the variation of I1/I3 is much more progressive than with DTAC, stretching over almost 2 decades of polymer concentration. A similar behavior was observed with other associating polymers such as hydrophobically modified POE11,40-42 and polyacrylamides.43,44 It was attributed to a less cooperative association of the hydrophobic moieties, as compared to micellization of conventional surfactants, and also to the polydispersity of the polymers leading to the existence of a broad range of cac. Another explanation may be that pyrene is partitioned between the aqueous phase and the hydrophobic aggregates as with polysoaps.26,29,45 These possibilities are further discussed below. In the presence of salt the decrease of I1/I3 is shifted toward a lower polymer concentration. Indeed, salt favors the HMPA association by screening the electrostatic repulsive interactions along and between polymer backbones. (37) Frindi, M.; Michels, B.; Zana, R. J. Phys. Chem. 1992, 96, 6095. (38) Osugi, J.; Sato, M.; Ifuku, N. Rev. Phys. Chem. Jpn. 1965, 35, 32. (39) Weiner, N. D.; Zografi, G. J. Pharm. Chem. 1965, 54, 436. (40) Binana-Limbele´, W.; Clouet, F.; J., F. Colloid Polym. Sci. 1993, 271, 748. (41) Franc¸ ois, J. Prog. Org. Coat. 1994, 24, 67. (42) Alami, E.; Rawiso, M.; Isel, F.; Beinert, G.; Binana-Limbele, W.; Franc¸ ois, J. In Hydrophilic Polymers: Performance with Environmental Acceptance; Glass, J. E., Ed.; Advances in Chemistry Series 248; American Chemical Society: Washington, DC, 1996; p 343. (43) Hill, A.; Candau, F.; Selb, J. Macromolecules 1993, 26, 4521. (44) Varadaraj, R.; Branham, K. D.; McCormick, C. L.; Bock, J. In Macromolecular Complexes in Chemistry and Biology; Dubin, P., Bock, J., Davis, R., Schulz, D. N., Thies, C., Eds; Springer-Verlag: Heidelberg, 1994; p 15. (45) Anthony, O.; Zana, R. Macromolecules 1994, 27, 3885.
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Figure 3. Variations of I1/I3 with the alkyl chain concentration for PA5-5C12, PA5-10C12, and PA5-15C12.
Figure 4. Variations of I1/I3 with the alkyl chain concentration for PA5-5C12 and PA5-5C18.
Figure 3 displays the variation of I1/I3 with the alkyl chain concentration, Calkyl, for PA5-5C12, PA5-10C12, and PA5-15C12. A plot of the data as a function of Calkyl, instead of Cp, was used as it allows a direct comparison between polymers of different degrees of hydrophobic modification. The decrease of I1/I3 is centered at approximately the same value of Calkyl for PA5-5C12 and PA5-10C12, but at a lower Calkyl for PA5-15C12. For such a high modification degree the probability of finding two alkyl chains next to one another is higher, which in turn increases the association tendency as it was shown for PA modified with disubstituted amines.46 Figure 4 shows the effect of the alkyl chain length. As expected, the decrease of I1/I3 occurs at a lower Calkyl with PA5-5C18 as compared to PA5-5C12: the longer the alkyl chain, the higher the hydrophobicity of the polymer and thus the greater its tendency to associate and form hydrophobic microdomains. This behavior is qualitatively similar to that observed with conventional surfactants. However an important quantitative difference exists between the two types of systems. Thus, in Figure 4 the upward shift in concentration between the plots for PA55C18 and PA5-5C12 corresponds to a factor of about 5. For conventional ionic surfactants a decrease of the alkyl chain length from 18 to 12 carbon atoms would give rise to an increase of cmc by a factor close to 64.47 The influence of the molecular weight of the PA backbone was studied with PA5-10C12, PA50-10C12 and PA150-10C12. In water the three polymers exhibited a very similar behavior (not shown). However with PA150(46) Magny, B. Thesis; University Pierre et Marie Curie: Paris, 1992. (47) van Os, N. M.; Haak, J. R.; Rupert, L. A. M. Physicochemical Properties of Selected Anionic, Cationic and Nonionic Surfactants; Elsevier: Amsterdam, 1993.
Petit-Agnely et al.
Figure 5. Variations of I1/I3 with the polymer concentration for PA5-10C12, PA50-10C12, and PA150-10C12, in NaCl 0.1 M.
10C12 the end of the decrease of I1/I3 occurred at Cp > 2 wt % and could not be accurately determined because the corresponding samples were too viscous. To avoid this problem, the same experiments were repeated in 0.1 M NaCl so that the decrease of I1/I3 occurred at a lower polymer concentration (Figure 5). No significant difference between the three polymers was observed. This behavior means that, at a local level, the association mechanism of HMPAs is mainly governed by the alkyl chain concentration and length and is little influenced by the molecular weight of the polymer backbone. The influence of temperature was also studied by performing some experiments at 45 °C. The decrease of I1/I3 (not shown) occurred at about the same value of Cp as at 25 °C and the overall plot was shifted toward lower values of I1/I3. A decrease of I1/I3 upon increasing the temperature is observed for pyrene in solvents of widely differing polarities and reflects an intrinsic property of this probe.48,49 The Meaning of the Decrease of I1/I3. To get a deeper insight into the association mechanism of HMPAs, the meaning of the decrease of I1/I3 must be further examined. As it was stated above, two main explanations can be given to the more progressive decrease observed with HMPAs, as compared to surfactants: (1) association of the alkyl chains above a certain polymer concentration according to a less cooperative mechanism than surfactant micellization; (2) partition of pyrene between the aqueous phase and the hydrophobic microdomain pseudo-phase. The latter would exist even at the lowest polymer concentration. Decreases of I1/I3 due to partition are encountered with polysoaps,26,29,45 other associating end-modified polymers,50,51 and amphiphilic block copolymers.52,53 In polysoaps the density of hydrophobic moieties along the polymer backbone is quite high and hydrophobic microdomains exist even at infinite dilution. Polysoaps display I1/I3 curves with a sigmoidal shape which are very similar to the ones determined in this study with HMPAs. It was stressed that these curves looked like binding isotherms plotted on a semilogarithmic scale and a partition model of pyrene was developed to fit the experimental data. This (48) Marinov, G.; Michels, B.; Zana, R. Langmuir 1998, 14, 2639. (49) Zana, R.; Unpublished I1/I3 vs temperature data for cyclohexane. (50) Nishikawa, K.; Yekta, A.; Pham, H. H.; Winnik, M. A.; Sau, A. C. Langmuir 1998, 14, 7119. (51) Vorobyova, O.; Yekta, A.; Winnik, M. A.; Lau, W. Macromolecules 1998, 31, 8998. (52) Wilhelm, M.; Zhao, C. L.; Wang, Y.; Xu, R.; Winnik, M. A.; Mura, J. L.; Riess, G.; Croucher, M. D. Macromolecules 1991, 24, 1033. (53) Sczubialka, K.; Ishikawa, K.; Morishima, Y. Langmuir 1999, 15, 454.
Aggregation of Aq HMPA by Fluorescence Probing
Figure 6. Variations of the conductivity as a function of ionic units concentration for PA5 and PA5-10C12. The data for PA510C12 are shifted by +2 mS for the sake of clarity. Inset: variation of the conductivity as a function of DTAC concentration.
model makes use of a partition coefficient Kv, that is equal to the ratio of the effective concentrations of pyrene in the hydrophobic pseudo-phase and in water.52 This model may account for the I1/I3 vs concentration plots for HMPAs in water, as is illustrated in Figure 2. The extracted values of Kv are in the range 1.4-6 × 104. These values are only apparent values, as not all alkyl chains make up microdomains.20 Besides, the fraction of alkyl chains making up microdomains increases rapidly with the polymer concentration in the range where most of the decrease of I1/I3 occurs (see below). This precludes any meaningful correction of the fitted Kv values by the fraction of micellized alkyl chains. Nevertheless this fact probably explains why our Kv values are somewhat smaller than those generally reported for similar systems.51-54 This explanation is supported by the results for HMPAs in the presence of 0.1 M NaCl. There the Kv values are larger, between 1 × 105 and 5 × 105, i.e., rather close to those reported for amphiphilic block copolymers.51-54 In the presence of salt, the fraction of micellized alkyl chains is larger than in pure water. Our results for aqueous systems suggest a less pronounced hydrophobic character of the investigated HMPAs as compared to block copolymers with strongly hydrophobic blocks. Electrical conductivity and 13C NMR measurements were carried out in order to check the first hypothesis, i.e., the existence of a concentration below which no hydrophobic aggregates are detected. The electrical conductivity is a property that is often used to determine the cmc of charged surfactants. The occurrence of micelles in an ionic surfactant solution always results in a less rapid increase of the conductivity with concentration, mainly because a large fraction of counterions are bound to the micelles. With DTAC the variation of the conductivity with concentration, represented in Figure 6 (inset), shows a rapid and important change of slope at a surfactant concentration that is very close to the value of the cmc measured by fluorescence spectroscopy (see Figure 1). The electrical conductivity experiments were repeated with the unmodified PA5 and with PA5-10C12. The results are represented in Figure 6 as a function of the concentration of charged groups (-CO2-Na+) since the conductivity essentially arises from these groups. The conductivity of the PA5 solution varies linearly with concentration in the range investigated. On the contrary the conductivity of the PA5-10C12 solution shows a small but clear change (54) Jada, A.; Hoffstetter, J.; Siffert, B.; Dumas, P. Colloids Surf. A. 1999, 149, 315.
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Figure 7. Variations of I1/I3 and of the fraction of aggregated alkyl chains as a function of polymer concentration for PA510C12.
of slope which suggests that some association is taking place in the system. The change of slope occurs at a concentration of about 0.11 M (1.3 wt %). It thus falls in the range where I1/I3 varies little (see Figure 2). It is noteworthy that the change of slope is much less pronounced than in the case of the micellization of conventional surfactants. This difference reflects the fact that PA5-10C12 is a polyelectrolyte and that a significant fraction of counterions are condensed on the polymer backbone irrespective of the state of association of the alkyl chains, contrary to conventional surfactants. The change of slope upon self-association is thus more limited as it mostly arises from changes of particle size and of small changes of degree of counterion binding, if any. Note that the HMPA used has a low degree of polymerization (about 70) and several polymer chains contribute to the formation of a hydrophobic microdomain as shown in the next section. The second explanation of the I1/I3 results is that the association of HMPAs is more progressive and less cooperative than the micellization of conventional surfactants. An extensive 13C NMR study was performed on HMPAs and concluded that their aggregation is less cooperative than micellization.19,20 This explanation is supported by the variations of I1/I3 and of the fraction of aggregated alkyl chains, Faggr, with Cp, represented in Figure 7 for PA5-10C12. The fraction of alkyl chains making up long-lived micelle-like aggregates,21 Faggr, is seen to start increasing from a value close to zero, at Cp ) 0.3 wt %, up to a value of about 0.7, at 10 wt %, in a rather progressive manner. The concentration 0.3 wt % may be considered to correspond to the onset of aggregation of PA5-10C12 chains in water. This value is significantly lower than that at the change of slope in the conductivity plot (Figure 6) confirming the lack of sensitivity of the latter. Figure 7 also shows that I1/I3 already starts decreasing at Cp ) 0.03 wt %, i.e., at a concentration 10 times lower than that where 13C NMR detects aggregated HMPAs. This variation of I1/I3 at low concentration suggests the presence in the system, at concentrations between 0.03 and 0.3 wt %, of small preaggregates consisting, most probably, of only few alkyl chains (∼25). In fact, the formation of such preaggregates having short lifetimes was previously suggested19,20 and recently confirmed in a NMR study.21 The binding of pyrene to these small aggregates would partially shield the probe from contact with water and thus results in a decrease of I1/I3. The following picture for the self-association of HMPAs in aqueous solutions emerges from the above fluorescence probing, NMR, and electrical conductivity results. Already
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Petit-Agnely et al. Table 1: Values of the Fraction of Aggregated Alkyl Chains Faggr, Pyrene Fluorescence Lifetime τ0, Intramicellar Quenching Rate Constant kq, Aggregation Number (N: from SSFQ; 〈N〉: from TRFQ) and Root Mean Square Deviation of the Micelle Size Distribution σ, Determined for Different HMPAs in Aqueous Solution at 25 °Ca polymer
Faggr
τ0 (ns)
10-7 kq (s-1)
τ0kq
N
〈N〉
σ
PA5-5C12 PA5-10C12 PA5-15C12 PA5-5C18
0.54 0.74 0.88 0.72
300 315 332 305
3.0 2.0 1.1 1.5
9.0 6.3 3.6 4.5
44 39 45 38
48 41 47 38
25 17 18 18
a Polymer concentration: 10 wt %; uncertainty on the N and 〈N〉 values: 10-15%.
Figure 8. Variation of ln(I0/I) as a function of the DPC concentration for PA5-15C12 (Cp ) 10 wt %) in SSFQ experiments. A linear regression performed on the data points yielded a correlation coefficient of 0.999.
Figure 9. Variation of Nq as a function of the DPC concentration for PA5-10C12 (Cp ) 10 wt %) in TRFQ experiments. The solid line was obtained by fitting eq 8 to the data (correlation coefficient is 0.875).
at very low concentration, these solutions apparently contain alkyl chain preaggregates to which pyrene can bind, thus causing a decrease of I1/I3. These preaggregates appear to be formed preferentially intramolecularly, i.e., within the same HMPA chain.21 At higher concentration, 0.3 wt % for PA5-10C12, NMR reveals the formation of larger and long-lived aggregates formed intermolecularly. The fraction of alkyl chains making up such aggregates increases progressively with the polymer concentration indicating that the association process is much less cooperative than micelle formation in solutions of conventional surfactants. The concentration above which these aggregates form is not as well defined as the cmc of conventional surfactants.20 This discussion shows the necessity of using a variety of methods sensitive to the different stages of the selfassociation process when investigating such complex systems as HMPA solutions or other types of associating amphiphilic polymers. Aggregation Numbers and Polydispersity. Aggregation numbers were obtained for different HMPAs by SSFQ and TRFQ, at a polymer concentration Cp ) 10 wt %, i.e., where I1/I3 has become nearly constant and the aggregates are micelle-like. Figure 8 gives an example of variation of ln(I0/I) with the quencher (DPC) concentration obtained for PA5-15C12, by SSFQ. The TRFQ measurements were performed at different quencher concentrations in order to obtain information on the aggregate polydispersity. Figure 9 shows the variation of Nq with the quencher concentration for PA5-10C12. Table 1 lists the values of the fraction of aggregated alkyl chains in the
investigated systems, as well as those of the fluorescence lifetime τ0, the intra-aggregate quenching rate constant kq and the aggregation number N. For a given HMPA solution, τ0 was found to be essentially independent of the quencher concentration. This is a good indication that the probe and quencher distributions among micelles were frozen on the fluorescence time scale. Also the value of the product τ0kq was always much larger than 1 (see Table 1). These two results explain why the values of the aggregation numbers determined by SSFQ agree well with those from TRFQ.55 The values of N in Table 1 for PA5-5C12, PA5-10C12, PA5-15C2, and PA5-5C18 differ only little, being all between 38 and 48. Owing to the low degree of polymerization of the PA precursor (about 70) and to the low value of the degree of hydrophobic substitution, these values of N mean that several PA5-xCn molecules are required to form one hydrophobic aggregate. As indicated by the results in Figure 5 and as shown elsewhere,20 high and low molecular weight HMPAs have the same tendency to self-assemble. However, with low molecular weight polymers the hydrophobic aggregates are not connected via the polymer backbone over a wide range of polymer concentration, which explains the low viscosity of these systems. With high molecular weight HMPAs, the same macromolecular chain can participate in several hydrophobic aggregates, which are thus interconnected, and the viscosity of such systems is high.22,56 Note that the N values in Table 1 are all below those that can be calculated for the maximum spherical micelles formed by surfactants with dodecyl (Nmax ) 60) or octadecyl (Nmax ) 116) alkyl chains.57 It can thus be assumed that the hydrophobic microdomains present in these systems probably have a spheroidal shape. The effect of temperature on the value of N was investigated for PA5-10C12. At 45 °C, N was found to be about 5-7 units lower than at 25 °C. Thus the size of the hydrophobic microdomains decreases upon increasing temperature as in the case of conventional ionic surfactants. The results obtained in this study also permit one to infer information on the effect of HMPA structural characteristics on the micropolarity and microviscosity of the hydrophobic microdomains. Table 1 shows that the pyrene fluorescence lifetime τ0 slightly increases with the degree of hydrophobic modification. This indicates that pyrene senses a progressively less polar microenvironment. The slight decrease of the I1/I3 value upon increasing the degree of hydrophobic modification seen in Figure 3 at high polymer concentration also reveals a less polar environment of the probe. (55) Alargova, R. G.; Kochijashky, I. I.; Sierra, M. L.; Zana, R. Langmuir 1998, 14, 5412. (56) Petit, F.; Iliopoulos, I.; Audebert, R.; Szo¨nyi, S. Langmuir 1997, 13, 4229. (57) Tanford, C. J. Phys. Chem. 1972, 76, 3020.
Aggregation of Aq HMPA by Fluorescence Probing
Coming to microdomain microviscosity, the values of kq in Table 1 show a significant decrease as the modification degree is increased at constant alkyl chain length and also as the alkyl chain length is increased at constant degree of modification. For spheroidal micelles, the product Nkq is proportional to the reciprocal of the microviscosity sensed by the probe and the quencher in their intramicellar diffusive motion.58 As N varies little for the investigated HMPAs, the values of kq in Table 1 lead to the conclusion that the microviscosity of the HMPA hydrophobic microdomains increases with the degree of modification and also with the length of the alkyl chain. This last result is similar to that reported for micelles of conventional surfactants. Thus, for a series of homologous surfactants, the micelle microviscosity has been shown to increase with the alkyl chain length and with the micelle size.59 Another result shows that the microdomain microviscosity is probably high. Indeed, the values of the pyrene lifetime found for aerated HMPA solutions were rather high, up to 250-260 ns. Recall that the lifetime of pyrene in aerated micellar solutions of conventional surfactants is usually around 160 ns. Larger τ0 values have been found for pyrene in systems such as polysoaps and hydrophobically modified polymers that contain hydrophobic microdomains and for which other techniques indicated large values of the microviscosity.59,60 Comparison to Ionic Surfactants. As seen in the preceding section, the aggregation numbers obtained with PA-xC12 are lower than the ones reported for ionic surfactants with dodecyl chains (N values of 55 and 57 have been reported for sodium dodecyl sulfate61 and for DTAC,60 respectively). Moreover, for PA5-5C18, the aggregation number (N ) 38) is much lower than the one expected for corresponding ionic surfactants.60 The HMPAs may not be able to form aggregates of low curvature, as in conventional surfactant micelles, owing to the steric hindrance of the PA backbone. These polymers are therefore constrained to form smaller aggregates with a higher curvature, leaving more space for the polar groups (PA segments). That is especially clear for PA5-5C18. As regards the polydispersity of the aggregation number, the values of σ/〈N〉 that can be calculated from the data in Table 1 are all around 0.4 to 0.5. These values are smaller than those reported for cationic surfactants with a dodecyl chain (0.62-0.78) and close to those for cationic surfactants with a hexadecyl chain (0.35-0.42).60 No reliable results appear to exist for anionic surfactants. Comparison to Other Associating Polymers and to Polysoaps. The comparison of the aggregation numbers of HMPA to values reported for other hydrophobically modified polymers and for polysoaps is not straightforward. In fact, N seems to depend strongly on the structural details of the polymer: nature of the backbone, average distance between hydrophobic groups, type of link between backbone and hydrophobic tail, etc. A brief review of some available data is given below. A large number of N values have been reported for hydrophobically end-capped poly(ethylene oxide) (PEO).11-13,32,51,62 These values all range between 20 and 30, irrespective of the length of the hydrophobic chain, from dodecyl to octadecyl. In fact the reported value for (58) Lianos, P.; Lang, J.; Zana, R. J. Phys. Chem. 1982, 86, 1019. (59) Zana, R. J. Phys. Chem. B 1999, 103, 9117. (60) Almgren, M.; Hansson, P.; Mukhtar, E.; van Stam, J. Langmuir 1992, 8, 2405. and references therein. (61) Warr, G. G.; Grieser, F.; Evans, D. F. J. Chem. Soc., Faraday Trans. 1 1986, 82, 1829. (62) Pham, Q.; Russell, W.; Thibeault, J.; Lau, W. Macromolecules 1999, 32, 2996.
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the dodecyl-capped polymer is larger than for the hexadecyl-capped polymer. These results are thus quite similar to the ones found for HMPAs. In the case of end-capped PEO the polymer loops are rather long and seem to limit the size of the aggregates due to steric hindrance. The same explanation may account for the value N ) 15 obtained by Xu et al.33 for a comb associating polymer based on PEO. For this polymer each hydrophobe (tetradecyl group) is separated by ca. 185 EO units, which would create a higher steric hindrance than with HMPA. For polysoaps, the first reported values of aggregation numbers are for alternated copolymers poly(maleic acidalt-alkyl vinyl ether). The values range from 15 to 27 alkyl chains when the copolymer with a decyl chain is fully neutralized but are higher (between 60 and 80) when the ionization degree is lower.29-31 Values ranging from 10 to 29 were reported for different poly(dimethyldiallylammonium-co-methyl-N-dodecyldiallylammonium) salts.63 More recently values ranging between 150 and 320, at pH 2, and about 60, at pH 8, were obtained in a study of naphthyl and/or dansyl labeled N-octylamide-substituted poly(sodium maleate-alt-ethyl vinyl ether).64 Conclusion The association of HMPA was studied by fluorescence probing, electrical conductivity, and 13C NMR and compared to the association of conventional surfactants. Below a certain polymer concentration no well-defined hydrophobic aggregates were detected, by any of these techniques, contrary to what is observed with polysoaps. The results indicate that the association process of HMPA is less cooperative and more progressive than micellization of conventional surfactants. The increase of the alkyl chain length and of the modification degree and the presence of salt favor the association, whereas the molecular weight of the polymer backbone has little influence on the polymer tendency to self-associate. The combination of data from 13C NMR and fluorescence quenching made it possible to determine the aggregation number N and the polydispersity σ/〈N〉 of the hydrophobic microdomains present in HMPA solutions. Values of N of about 40 and of σ/〈N〉 of approximately 0.4-0.5 were thus found, irrespective of the modification degree, for polymers bearing alkyl (dodecyl or octadecyl) grafts. The microviscosity of the aggregates was found to be larger than that of the micelles of the corresponding conventional surfactants. The association mechanism seems to be mainly governed by the alkyl chain concentration. However, the PA backbone appears to limit the aggregate size compared to ionic surfactants with the same alkyl chain length, especially for octadecyl chains. Acknowledgment. This work was supported by the CNRS (DIMAT) under the program CPR “Associative Polymers”. We acknowledge stimulating discussions within the framework of this program. This paper is dedicated to the memory of Dr. Roland Audebert who passed away in July 1997. Most of this work was performed in his group before his untimely death. Dr. Audebert played a leading role in the studies on the association mechanism of hydrophobically modified polymers. The authors greatly benefited from his suggestions and comments. LA000709J (63) Kevelam, J.; Engberts, J. B. F. N. J. Colloid Interface Sci. 1996, 178, 87. (64) Hu, Y.; Smith, G. L.; Richardson, M. F.; McCormick, C. L. Macromolecules 1997, 30, 3526.
