Study of Sodium Dodecyl SulfateâPoly(propylene oxide) Methacrylate

Langmuir 2004, 20, 5759-5769

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Study of Sodium Dodecyl Sulfate-Poly(propylene oxide) Methacrylate Mixed Micelles Guillaume Bastiat, Bruno Grassl,* Abdel Khoukh, and Jeanne Franç ois Laboratoire de Physico-Chimie des Polyme` res (L.P.C.P.), C.N.R.S./U.P.P.A. UMR 5067, Helioparc PAU-PYRENEES, 2 Av. du Pre´ sident Angot, 64053 Pau Cedex 9, France Received January 13, 2004. In Final Form: March 29, 2004 Sodium dodecyl sulfate (SDS)-poly(propylene oxide) methacrylate (PPOMA) (of molecular weight Mw ) 434 g‚mol-1) mixtures have been studied using conductimetry, static light scattering, fluorescence spectroscopy, and 1H NMR. It has been shown that SDS and PPOMA form mixed micelles, and SDS and PPOMA aggregation numbers, Nag SDS and Nag PPOMA, have been determined. Total aggregation numbers of the micelles (Nag SDS + Nag PPOMA) and those of SDS decrease upon increasing the weight ratio R ) PPOMA/SDS. Localization of PPOMA inside the mixed micelles is considered (i) using 1H NMR to localize the methacrylate function at the hydrophobic core-water interface and (ii) by studying the SDS-PPO micellar system (whose Mw ) 400 g‚mol-1). Both methods have indicated that the PPO chain of the macromonomer is localized at the SDS micelle surface. Models based on the theorical prediction of the critical micellar concentration of mixed micelles and structural model of swollen micelles are used to confirm the particular structure proposed for the SDS-PPOMA system, i.e., the micelle hydrophobic core is primarily composed of the C12 chains of the sodium dodecyl sulfate, the hydrophobic core-water interface is made up of the SDS polar heads as well as methacrylate functions of the PPOMA, the PPO chains of the macromonomer are adsorbed preferentially on the surface, i.e., on the polar heads of the SDS.

Introduction In the past decade, water-soluble associating polymers have attracted a great deal of attention due to the interesting rheological behavior of their aqueous solutions and numerous applications as thickening agents and rheology modifiers in the petroleum industry (oil recovery, drilling fluids hydraulic fracturing, or drag reduction).1-3 These polymers are generally constituted by a main hydrophilic chain (charged or uncharged) which ensures the water solubility and a small amount of hydrophobic groups (pendant or terminal) which can strongly enhance viscosity of the aqueous solutions by autoassociation and formation of a transient network with hydrophobic nanodomains playing the role of temporary junctions. In a first class of polymers, the pendant or terminal groups considered were aliphatic,4-6 zwitterionic,7,8 perfluorated,9,10 or aromatic,11 but their hydrophobicity and tendency to self-aggregate do not change with temperature. The distribution of the pendant groups was also found to play an important role, and it has been demonstrated by several (1) Water soluble polymers. Synthesis, solution properties and applications; Shalaby, S. W., McCormick, C. L., Butler, G. D., Eds.; ACS Symposium series 467; American Chemical Society: Washington, DC, 1991. (2) Polmers in Aqueous Media: Performancee through Association; Glass, J. E., Ed.; Advances in Chemistry Series 223; American Chemical Society: Washington, DC, 1989. (3) Bock, J.; Valint, P. L., Jr.; Pace, S. J.; Siano, D. B.; Schulz, D. N.; Turner, S. R. In Water soluble Polymers for Petroleum Recovery; Stahl, G. A., Schulz, D. N., Eds.; Plenum Press: New York, 1988; Chapter 9, p 147. (4) Volpert, E.; Selb, J.; Candau, F. Polymer 1998, 39, 1025. (5) Candau, F.; Selb, J. Adv. Colloid Interface Sci. 1999, 79, 149. (6) McCormick, C. L.; Middleton, J. C.; Cummins, D. F. Macromolecules 1992, 25, 1201. (7) Kathmann, E. E.; White, L. A.; McCormick, C. L. Polymer 1997, 38, 871. (8) Kathmann, E. E.; McCormick, C. L. J. Polym. Sci., Part A: Polym. Chem. 1997, 35, 243. (9) Hwang, F. S.; Hogen-Esch, T. E. Macromolecules 1995, 28, 3328. (10) Xie, X.; Hogen-Esch, T. E. Macromolecules 1996, 29, 1734. (11) Winnik, F. M.; Adronov, A.; Kitano, H. Can. J. Chem. 1995, 73, 2030.

authors that random or blocky distribution is obtained by copolymerization in homogeneous medium or in micellar solution, respectively.12 Associating polymers of a new type have been recently developed: thermothickening polymers, whose aqueous solutions are able to undergo an abrupt viscosity increase (or even sol-gel transition) at a given temperature, Tt. This behavior is clearly of high importance for many industrial applications, particularly in the petroleum industry. The concept of thermothickening polymers has been described recently by Hourdet et al.13,14 Such systems are based upon a water-soluble macromolecular backbone containing some side chains of poly(ethylene oxide) (PEO) or poly(N-isopropylacrylamide) (PNIPAM).15,16 Aqueous solutions of PEO and PNIPAM have the remarkable property to phase separate upon heating above the lower critical solution temperature (LCST) of considered polymers. An abrupt jump of viscosity was observed with solutions of sodium poly(acrylate) (PAA) grafted with PEO chains (PAA-PEO) at a temperature, Tt, very close to the demixing temperature, Tp, of pure PEO solutions of the same PEO concentration. This is attributed as in the case of classical associative polymers to the formation of microdomains of PEO chains (when they become insoluble in water for T > Tp) which act as new junctions in the temporary polymer network. When PEO is used as pendant associating group, change in viscosity occurs around 100 °C, which is an interesting temperature range for application in oil recovery processes. For other applications, i.e., biomaterials, transition at lower temperature (20-40 °C) is preferable. PNIPAM and PPO are promising candidates as LCST values are in this temperature range. Nevertheless, there is a lack of information about the behavior of poly(12) Hill, A.; Candau, F.; Selb, J. Macromolecules 1993, 26, 4521. (13) Hourdet, D.; L’Alloret, F.; Audebert, R. Polymer 1994, 35, 2624. (14) Maroy, P.; Hourdet, D.; L’Alloret, F.; Audebert, R. Eur Patent 583814 A1, 1993. (15) Durand, A. PhD Dissertation University of Paris VI, 1998. (16) Durand, A.; Hourdet, D. Polymer 2000, 41, 545.

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Figure 1. Formula of poly(propylene oxide) methacrylate.

(propylene oxide) (PPO) based thermoassociative polymers. Moreover, to the best of our knowledge, such polymers were prepared by chemical modification of poly(acrylate) and a random grafting is expected. The effect of the distribution of pendant chains was not investigated, as in the case of the first class of polymers. We have undertaken synthesis of new PAM-based polymers decorated by PPO short chains. The samples can be prepared by copolymerization of acrylamide and poly(propylene oxide) methacrylate (PPOMA) (see Figure 1) in micellar solution. Therefore this approach enables investigation of the effect of the pendant group distribution, if the same blocky structure is obtained in the second way, as already described for classical associating PAM. This paper presents the results of a study of the SDSPPOMA micellar system. The micellar synthesis has been extensively studied5,12,18-22 with various types of hydrophobic monomers. It is generally assumed that the number of hydrophobic units per block in the copolymer is roughly equal to the initial number of hydrophobic monomers per micelle, NH. This number is simply calculated from the surfactant-monomer composition by assuming that hydrophobic monomers incorporate micelles without changes in the aggregation number Nag of the surfactant and in the critical micellar concentration (cmc).5 Such assumptions have not been really checked, and in order to determine without any ambiguity the length of the PPO blocks in polymers prepared in micellar solutions, a systematical study of the SDS-PPOMA mixture was performed. The PPOMA, insoluble in water, can be assumed to be an “amphiphilic” molecule: the PPO chain is soluble at room temperature (LCST property) and the methacrylate function confers the hydrophobic property to PPOMA. The SDS-PPOMA mixture could be regarded as a micellar mixture. In literature, different mixtures have been studied: anionic and nonionic surfactants,23-27 cationic and nonionic surfactants,27-29 ionic surfactants or nonionic surfactant mixtures.25,30 Solubilization of water-insoluble compounds in surfactant micelles has also been considered.31,32 These systems have been studied by fluorescence (17) L’Alloret, F.; Hourdet, D.; Audebert, R. Colloid Polym. Sci. 1995, 273, 1163. (18) Evani, S. Eur. Patent 57875, 1982. (19) Evani, S. US Patent 4432881, 1984. (20) Turner, S. R.; Siano, D. B.; Bock, J. US Patent 4520182, 1985. (21) Turner, S. R.; Siano, D. B.; Bock, J. US Patent 4528348, 1985. (22) McCormick, C. L.; Nonaka, T.; Johnson, C. B. Polymer 1988, 731, 9829. (23) Matsubara, H.; Muroi, S.; Kameda, M.; Ikeda, N.; Ohta, A.; Aratono, M. Langmuir 2001, 17, 7752. (24) Griffiths, P. C.; Whatton, M. L.; Abbott, R. J.; Kwan, W.; Pitt, A. R.; Howe, A. M.; King, S. M.; Heenan, R. K. J. Colloid Interface Sci. 1999, 215, 114. (25) Ghosh, S.; Moulik, S. P. J. Colloid Interface Sci. 1998, 208, 357. (26) Abe, M.; Tsubaki, N.; Ogino, K. J. Colloid Interface Sci. 1985, 107 (2), 503. (27) Alargova, R. G.; Kochijashky, I. I.; Sierra, M. L.; Kwetkat, K.; Zana R. J. Colloid Interface Sci. 2001, 235, 119. (28) Desai, T. R.; Dixit, S. G. J. Colloid Interface Sci. 1996, 177, 471. (29) Esumi, K.; Miyazaki, M.; Arai, T.; Koide, Y. Colloids Surf., A 1998, 135, 117. (30) Malliaris, A.; Binana-Limbela, W.; Zana, R. J. Colloid Interface Sci. 1986, 110 (1), 114.

Bastiat et al.

spectroscopy, static light scattering,33-38 and NMR.39-43 The cmc and the aggregation number of these ternary systems are strongly modified with respect to those characterizing the binary systems (surfactant-water), depending on (i) the hydrophilic-hydrophobic balance of the surfactants and (ii) their charges. At first, we focus our attention on the characterization of the micellar system SDS-PPOMA, using conductimetry and static light scattering (SLS) to construct the SDSPPOMA “phase diagram” and to determine the SDS critical micellar concentration. Moreover, steady-state fluorescence quenching was used to obtain the concentration of mixed micelle and then deduce the aggregation numbers. In a second part, the molar mass of mixed micelle was measured by SLS and compared with the values obtained by fluorescence spectroscopy. The localization of PPOMA inside the micelle was investigated by 1H NMR, and the structure of our mixed micelle was determined. Finally, we have used thermodynamics of mixed micelles to confirm all the previous studies. Experimental Section Materials. All solvents and reagents of the best reagent grade available were obtained from Aldrich, D2O was obtained from Euriso-top, and used as received. Pyrene was recrystallized from methanol. Water was three times distilled over quartz. Conductometry.44,45 The conductivity was measured with a Radiometer Copenhagen CDM92 conductometer using a twopole conductivity cell (constant ) 1 cm-1). Batch solutions concentrated in SDS (approximately 0.05 mol‚L-1) were prepared, and PPOMA was added at weight ratios, R ) PPOMA/SDS, equal to 0, 0.25, 0.5, 0.75, and 1. Small aliquots of these batch solutions were added in a volume of water contained in a double-walled glass vessel thermostated at 25 ( 0.1 °C. Conductivity was measured after each addition. Fluorescence Spectroscopy. The fluorescence spectra were obtained on a Perkin-Elmer LS50B spectrofluorometer. The excitation wavelength was fixed at 335 nm, and the band-passes were set at 2.5 nm for the excitation and the emission. For the determination of the micelle concentration, a small amount of a pyrene solution solubilized in methanol was introduced into an Erlenmeyer flask, the methanol was evaporated, and SDSPPOMA mixtures were prepared such as the concentration in SDS was 0.05 mol‚L-1 and weight ratios, R ) PPOMA/SDS, were 0, 0.25, 0.5, 0.75, and 1. These solutions, S1, were stirred for 1 day at room temperature. The final pyrene concentration is about 5 × 10-6 mol‚L-1. In a part of S1, a given amount of dodecylpyridinium chloride (quencher) was added and batch solutions S2 were obtained. S2 were diluted by the solutions S1 of the same ratio R in such a way that the quencher concentration was ranging between approximately [Mi]/5 and [Mi], [Mi] being the micelle concentration. (31) Lianos, P.; Lang, J.; Strazielle, C.; Zana, R. J. Phys. Chem. 1982, 86, 1019. (32) Almgren, M.; Swarup, S. J. Phys. Chem. 1982, 86, 4212. (33) Ikeda, S.; Ozeki, S.; Tsunoda, M. J. Colloid Interface Sci. 1980, 73, 27. (34) Abe, M.; Yamagushi, T.; Shibata, Y.; Uchiyama, H.; Yoshino, N.; Ogino, K.; Christian, S. D. Colloids Surf. 1992, 67, 29. (35) Pisarcik, M.; Devinsky, F.; Lacko, I. Colloids Surf., A 2000, 172, 139. (36) Parfitt, G. D.; Wood, J. A. Kolloid-Z. Z. Polym. 1968, 229 (1), 55. (37) Anacker, E. W.; Westwell, A. E. J. Phys. Chem. 1964, 68, 3490. (38) Kratohvil, J. P. J. Colloid Interface Sci. 1980, 75, 271. (39) Gjerde, M. I.; Nerdal, W.; Hoiland, H. J. Colloid Interface Sci. 1996, 183, 285. (40) Wang, T.-Z.; MAO, S.-Z.; Miao, X.-J.; Zhao, S.; Yu, J.-Y.; Du, Y.-R. J. Colloid Interface Sci. 2001, 241, 465-468. (41) Laguitton-Pasquier, H.; Pansu, R.; Chauvet, J.-P.; Collet, A.; Faure, J. Synth. Met. 1996, 81, 309-314. (42) Kim, B.-J.; Im, S.-S.; Oh, S.-G. Langmuir 2001, 17, 565-566. (43) Gao, H.-C.; Zhao, S.; Mao, S.-Z.; Yuan, H.-Z.; Yu, J.-Y.; Shen, L.-F.; Du, Y.-R. J. Colloid Interface Sci. 2002, 249, 200-208. (44) Rodenas, E.; Sierra, M. L. Langmuir 1996, 12, 1600. (45) Benkhira, A.; Franta, E.; Franç ois, J. J. Colloid Interface Sci. 1994, 164, 428.

Study of Mixed Micelles

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Figure 2. Variations of the scattered intensity at 90° as a function of SDS concentration for various weight ratios R ) PPOMA/SDS: 0.25 (9), 0.5 (2), 0.75 (]), and 1 (b). The intensity of the first peak (I1) at 373 nm of pyrene fluorescence spectrum was used to estimate the micelle concentration from the steady-state fluorescence quenching experiments in the presence of a fluorescence quencher, Q, the dodecylpyridinium chloride generally used for studying SDS micelles. We have obtained the molar concentration of the micelles [Mi] from the fluorescence intensity decrease of the probe (pyrene) as a function of [Q], and the relation is

I ) I0 exp(-[Q]/[Mi])

(1)

I0 is the fluorescence intensity in absence of quencher and [Mi] is the concentration of micelles.46-49 From [Mi], Nag SDS, Nag PPOMA, and Nag total, SDS, PPOMA, and total aggregation numbers, respectively, can be calculated

[SDS] - cmc Nag SDS ) [Mi]

(2a)

[PPOMA] Nag PPOMA ) [Mi]

(2b)

Nag total )

[SDS] - cmc + [PPOMA] [Mi]

(2c)

where [SDS] and [PPOMA] are known concentrations and cmc is determined by conductimetry. Static Light Scattering (SLS). The apparatus of static light scattering consists of a laser of power Spectra-Physics Stabilite2017 (wavelength λ ) 514.5 nm), adjustable in intensity between 0.1 and 2 W, and of a Sematech photogoniodiffusiometer. For the solubility diagram building, batch solutions are the same as for conductivity measurements. The solutions were filtered on a Millipore filter (0.1 µm) and then diluted with filtered three time distilled water in the scattering cell after each measurement of the scattered intensity at 90°. We have measured the scattering intensity for 30 s, 10 s after the immersion of the scattering cell in the toluene tank. For the determination of the micelle molar mass, the SLS apparatus was the same and the various refractive index increments (dn/d(∆C)) were measured with an on-line characterization system including a WATERS 2410 refractometer. Batch solutions concentrated in SDS (approximately 0.05 mol‚L-1) were prepared and PPOMA added such that weight ratios R ) PPOMA/ (46) Valeur, B. In Molecular Fluorescence Principles and Applications; Wiley-VCH: Weinheim, 2002; Chapter 4, p 84. (47) Turro, N. J.; Yekta, A. J. Am. Chem. Soc. 1978, 100, 5951. (48) Winnik, F. M.; Regismond, S. T. A. Colloids Surf., A 1996, 118, 1. (49) Benkhira, A. PhD Dissertation, University of Rabat, 1998.

SDS are equal to 0, 0.5, and 1. These solutions were filtered on a Millipore filter (0.1 µm) and then successively diluted with filtered three times distilled water. The scattered intensity at 90° can be expressed by:

1 K ∆C ) + 2A2 ∆C ∆I Mw mic

(3)

with Mw mic the weight average molar mass of micelle, ∆C ) C - Ccmc with C the SDS concentration and Ccmc the concentration (in g‚cm-3) at the cmc, ∆I ) I - Icmc with I and Icmc the scattered intensities at C and cmc concentrations, respectively, A2 the second virial coefficient, and

K)

4π2n02(dn/d(∆C))2 Naλ4(Rtol)90°

(Itol)90°

(4)

where n0 is the refractive index of the solvent, Na is the Avogadro number, and (Rtol)90° and (Itol)90° are the Rayleigh ratio and the scattered intensity at 90° of toluene, respectively, used as standard. Nuclear Magnetic Resonance (NMR). The 1H NMR experiments were carried out on a BRUCKER Avance 400 MHz apparatus. Initially, we studied the SDS in D2O with various concentrations. Then, we carried out mixtures of SDS with PPOMA with SDS concentration of 0.05 mol‚L-1 and with various weight ratios, R ) PPOMA/SDS, equal to 0, 0.25, 0.5, 0.75, 1, always in D2O.

Results and Discussion Solubility Diagram of Poly(propylene oxide) Methacrylate (PPOMA) in Sodium Dodecyl Sulfate (SDS) Aqueous Solutions. PPO, which has the same molecular weight as the PPOMA sample used in this study, is soluble in water in the concentration range studied. The presence of the polymerizable function CH2dC(CH3)COO- at the end of the chain strongly decreases the affinity of PPOMA for water. Then it was necessary to perform preliminary experiments in order to determine the composition range where PPOMA can solubilize in aqueous SDS solution. Static light scattering was used to establish a solubility diagram. Figure 2 shows the variation of scattered intensity I (at 90°) versus SDS concentration, the weight ratio R varying between 0.25 and 1. Let us note that these values of I correspond to measurements made after only 40 s of stabilization after each dilution (see Experimental Section). What we observe is a first domain where I increases,

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Bastiat et al. Table 1. Aggregation Numbers (total, for SDS and PPOMA), Critical Micellar Concentration (cmc), and Ionization Degree (r) versus the Weight Ratio, R ) PPOMA/SDS, for the Mixed Micelle, CSDS ) 0.05 mol‚L-1 conductometry fluorometry cmc (10-3 wt ratio R ) PPOMA/SDS Nag total Nag SDS Nag PPOMA mol‚L-1) 0 0.25 0.5 0.75 1

Figure 3. Variations of the conductivity as a function of SDS concentration for different weight ratios R ) PPOMA/SDS: 0 ([), 0.25 (9), 0.5 (2), 0.75 (]), and 1 (b). All the curves start from the origin. They have been shifted for better readability.

followed by a second one where an abrupt decrease of I occurs, and finally for a concentration CLS, I stabilizes at very low values. We will assume that CLS corresponds to the limit of solubilization of PPOMA in SDS aqueous solution. The first question that concerns the origin of a such solubilization: Is it due to the SDS micellization? In such a case, CLS should be confounded with cmc. Conductivity was used to determine cmc, because it is a method frequently used to determine the cmc of ionic surfactants in aqueous solution. The micellization appears as a break in the curve of variation of specific conductivity κ as a function of the surfactant concentration CSDS (Figure 3): For CSDS < cmc, all the ionic species DS- and Na+ are free and contribute to conductivity. The behavior is similar to that of a simple monovalent electrolyte

1000κ ) CSDS (λNa+ + λDS-)

(5)

where λNa+ and λDS- are the limiting equivalent conductivities of Na+ and DS-, respectively. When CSDS ) cmc, the micelles start to be formed, and for CSDS > cmc, the solution contains (i) free DS- ions with cmc concentration, (ii) micelles with large charge density, of molar concentration (CSDS - cmc)/Nag, which binds a part of the Na+ ions, and (iii) free Na+ ions whose concentration is cmc + (CSDS - cmc) R. One generally supposes that the equivalent conductivity of the DS- ions engaged in micelles is equal to that of the free ions, which makes it possible to write

1000κ ) (RCSDS + (cmc(1 - R))(λNa+ + λDS-)

(6)

where R is the average ionization degree of the SDS molecules in micelles. Equations 5 and 6 show that the ratio of the slopes of κ versus CSDS gives the value of R. The results which we obtained for pure SDS in aqueous solution are represented by curve 0 of the Figure 3. They lead to cmc ) 8.4 × 10-3 mol‚L-1 and R ) 0.371, in agreement with literature results.16,20-22 Figure 3 shows that the curves κ ) f(CSDS) obtained for the SDS-PPOMA system at various weight ratios R present the same features as those described for pure SDS. A break is observed in all the curves at CSDS ) cmc. For CSDS < cmc,

58 47 45 39 38

58 39 32 24 21

0 8 13 15 17

8.4 7.0 5.9 5.5 4.8

R 0.371 0.485 0.612 0.616 0.696

the slope does not depend on R: this implies that micellization is accompanied by ionic condensation and the number of ionic species and their mobility are the same as for pure SDS. cmc decreases when R increases; for R ) 1, cmc ) 4.8 × 10-3 mol‚L-1, a value appreciably lower than the cmc of pure SDS, which shows that micellization is influenced by PPOMA and that the micelles are probably mixed (SDS + PPOMA). For CSDS > cmc, the slope of the straight line κ ) f(CSDS) strongly increases with R, which indicates a larger ionization degree of SDS. One can imagine a spacing of the sulfate groups by the PPOMA molecules at the surface of the micelles, which would weaken the condensation phenomenon of the counterions and involve a higher average ionization degree. Values of cmc and R are reported in Table 1 versus the weight ratio PPOMA/SDS. From this study, three domains can be distinguished according to the system composition (Figure 4): Domain A: Small SDS and PPOMA Concentrations (CSDS < CLS): PPOMA is present under the form of aggregates on which probably some SDS molecules are adsorbed. This phenomenon does not disturb significantly conductivity of SDS. Domain B: Intermediate SDS and PPOMA Concentrations (CLS < CSDS < cmc): This domain corresponds to solubilization of PPOMA aggregates and the formation of SDS-PPOMA complexes when the SDS and PPOMA concentrations increase. With the conductivity not being disturbed in this regime, one can think that the formed complexes do not correspond to a large density of SDS molecules (lower than those of real mixed micelle) and thus to a large charge density. Domain C: Large SDS and PPOMA Concentrations (CSDS > cmc): This is the range of mixed micelles formation with a decrease in the average ionization degree of the micelle; this behavior is related to the partial condensation of the sodium counterions at the surface of the mixed micelle. One can think that in this domain, SDS-PPOMA mixed micelles are in equilibrium with free SDS molecules. It is probable that at large SDS excess compared to PPOMA, micelles of pure SDS also exist in the balance of the species, but we did not explore this domain. Formation of free SDS micelle should be revealed by a new decrease of the slope of the curves κ ) f(CSDS). In fact, we will be interested, for the following work, in the domain C (Figure 4) of mixed micelles, where copolymerization will be carried out. We have decided to keep constant the SDS concentration, i.e., 0.05 mol‚L-1, and to study the SDS-PPOMA mixed micelles. This SDS concentration will be the one that we will use for our copolymer synthesis by radical micellar copolymerization. Sodium Dodecyl Sulfate (SDS)-Poly(propylene oxide) Methacrylate (PPOMA) Mixed Micelle. The micelle concentration [Mi] is directly obtained as specified in experimental part starting from the slope of the straight line ln(I0/I) ) f([Q]) (Figure 5).


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Figure 4. “Phase diagram” of the SDS-PPOMA system, determined from conductometry and static light scattering measurements: cmc ([) and CLS (2).

Figure 5. Fluorescence measurements. Variations of ln(I0/I) as a function of quencher (dodecylpyridinium chloride) concentration for various weight ratios R ) PPOMA/SDS: 0 ([), 0.25 (9), 0.5 (2), 0.75 (]), and 1 (b). CSDS ) 0.05 mol‚L-1.

Values of the different aggregation numbers Nag SDS, Nag PPOMA, and Nag total are reported versus R in Table 1. We find for a pure SDS micelle an aggregation number of 58 in agreement with the literature.47,50 When R increases, a very marked decrease of Nag SDS down to 21 for SDS-PPOMA mixed micelles (for R ) 1) is observed. Nag PPOMA increases up to 17 (for the same ratio R). This perfectly explains the increase in the ionization degree with R observed in conductometry measurements. We have characterized the mixed micelles by static light scattering (SLS) to determine their weight average molar mass and to compare it with aggregation numbers determined by fluorometry measurements. Figure 6 presents the variation of the normalized scattered intensity I of aqueous solutions of SDS in pure water (normalized with a scattered intensity by toluene of 200) versus SDS concentration. Two quite distinct parts are found on the curve: a part for small SDS concentrations where I does not vary significantly with CSDS, and a second part where I increases with CSDS. The crossover between (50) Franç ois, J.; Dayantis, J.; Sabbadin, J. Eur. Polym. J. 1985, 21 (2), 165.

these two ranges is observed at CSDS ) 8.3 × 10-3 mol‚L-1, that correspond to the cmc of SDS. Figure 7 presents K ∆C/∆I versus (∆C) (see experimental part). For R ) 0, one obtains values of A2 ) 0.013 mol‚mL‚g-2 and Mw mic ) 20 100 g‚mol-1. We can consider that the micelles have a polydispersity very close to 1 and thus Mn mic ≈ Mw mic. From Mw mic, the intercept of the straight line K ∆C/∆I ) f(∆C) (relation 3), the aggregation number of micelle can be written as

Nag SDS )

Mw mic MSDS

(7)

where MSDS ) 288.38 g‚mol-1 is the SDS molar mass. Nag SDS ) 70, the value of the SDS aggregation number being slightly higher than the value determined by fluorescence spectroscopy (Nag SDS ) 58). For our measurements, we have used Isolvent instead of Icmc (very close values); we thus slightly overestimate ∆I and thus Mw mic. The value of the second virial coefficient is of the same order of magnitude as that determined by Parfitt et al.36 Parfitt et al. used another model to describe micelles from SLS experiments.36 In the case of charged systems,

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Figure 6. Variation of the normalized scattered intensity at 90° by a SDS solution versus SDS concentration (standardized with a scattered intensity by toluene of 200).

Figure 7. Variation of K∆C/∆I as a function of ∆C for SDS-PPOMA mixtures. Weight ratios R ) PPOMA/SDS ) 0 ([), 0.5 (9), and 1 (2).

the determined molar mass is in fact only an apparent mass. The real aggregation number Nag is given by the Mysels et al. relation which takes account of the charge effect in micelles51 Nag )

Mw mic + MSDS

4MSDSA2Ccmc + 8(MSDSA2Ccmc)1/2 - 2 4

MSDS (MSDSA2Ccmc)1/2 Mw mic

( )

MSDS MSDS -2 Mw mic Mw mic

2

(8)

The micellar charge is given by the following relation: zm )

4(MSDSA2Ccmc)1/2 + 4MSDSA2Ccmc MSDS 2 MSDS 2 Mw mic Mw mic

( )

(51) Princen, G.D.; Mysels, K. J. J. Colloid Sci. 1957, 12, 594.

(9)

By use of the Mysels relation (relation 8), Nag SDS ) 90, which moves away more from the generally allowed values of Nag SDS. On the other hand, the micelle charge, i.e., the product of the ionization degree (determined by conductivity measurements) by the SDS aggregation number (measured by fluorescence spectroscopy), is equal to 21.5, a value close to that found with the Mysels relation (relation 9), i.e., 20.9. For R ) 0.5 and 1, the apparent molar masses can be compared with the molar masses of micelles determined in fluorescence spectroscopy, Mmic f, from the aggregation numbers, i.e.

Mmic f ) Nag SDSMSDS + Nag PPOMAMPPOMA

(10)

with MPPOMA ) 434 g‚mol-1. All the results are reported in Table 2. The apparent molar mass of mixed micelles decreases when the ratio PPOMA/SDS increases. The micelles become smaller as already observed by fluorescence spectroscopy. The agreement between the values determined by the two measurements is rather good. Nevertheless, the discrepancy between these values can be attributed to the difference


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Table 2. Static Light Scattering: Refractive Index Increment (dn/d(∆C)), Weight Average Molar Mass Mw mic, and Second Virial Coefficient A2 of SDS-PPOMA Mixed Micelle for Various Weight Ratios R ) PPOMA/ SDS Determined by the Debye Relation (relation 3) and Comparison with the Fluorescence Spectroscopy Measurements static light scattering wt ratio dn/d∆C Mw mic A2 PPOMA/SDS (mL‚g-1) (g‚mol-1) (mol‚mL‚g-2) 0 0.5 1 a

0.121 0.122 0.123

20100 14000 9400

0.013 0.023 0.017

fluorometry Mmic f (g‚mol-1) 16700a 14900 13400

Calculated from relation 10.

Table 3. Chemical Shift Variation of the Various Groups of SDS versus SDS Concentration and Temperaturea CSDS (mol‚L-1)

temp (°C)

δ (-CH2- R) (ppm)

δ (-CH2- β) (ppm)

δ (-(CH2)9-) (ppm)

0.00464 0.00769 0.0234 0.0489 0.0489 0.0489

25 25 25 25 40 60

4.20 4.20 4.15 4.14 4.14 4.14

1.82 1.82 1.80 1.79 1.79 1.79

1.42 1.42 1.42 1.41 1.41 1.41

a

δ(-CH3) was normalized to 1 ppm.

of the concentrations: 0.05 mol‚L-1 for fluorescence spectroscopy and C ) cmc for SLS. Moreover, we did not take into account the preferential adsorption in the analysis of the SLS data. The second virial coefficient A2 increases when ratio PPOMA/SDS increases. The value of zm passes from 21.5 to 14.6 (values determined by the product of the SDS aggregation number with the ionization degree of micelle) when R varies from 0 to 1. Despite the weaker stabilization of the mixed micelles, related to a decrease of the micellar charge, PPOMA seems to improve the solubility of our system. In literature, the SDS aggregation number was supposed to be constant when hydrophobic monomers are solubilized inside SDS micelles.5,12,18-22 Considering a SDS concentration of 0.05 mol‚L-1, a constant aggregation number of 60 and a constant cmc of 0.0083 mol‚L-1 for the SDS, we would have obtained aggregation numbers for PPOMA equal to 12, 24, 36, and 47, for the weight ratios R ) PPOMA/SDS of 0.25, 0.5, 0.75, and 1, respectively. These values are very different from those found with the true values of cmc and Nag SDS. In our particular case, the study well demonstrates a change in the structure of the micelles when the PPOMA is solubilized inside: there is a decrease in the cmc, as well as a very marked decrease in the SDS aggregation number. Localization of PPOMA inside the SDS Micelle. Nuclear magnetic resonance was used to study the localization of PPOMA in micelles, as already made for many other surfactants.39-43 At first, this study was carried out on SDS solution (with CSDS ≈ 0.05 mol‚L-1) at three temperatures: 25, 40, and 60 °C. The results are reported in Table 3. Despite a decrease in the aggregation number of SDS with temperature as shown by various authors,52-54 there is no significant variation of the chemical shifts δ. δ of the CH3 methyl group of SDS was normalized at 1 ppm and CH2 groups in R and β position near sulfate function have a constant δ value at 4.14 and 1.79 ppm, (52) Hayashi, S.; Ikeda, S. J. Phys. Chem. 1980, 84, 749. (53) Mazer, N. A.; Benedek, G. B.; Carey, M. C. J. Phys. Chem. 1976, 80, 1075. (54) Missel, P. J.; Mazer, N. A.; Benedek, G. B.; Young, C. Y.; Carey, M. C. J. Phys. Chem. 1980, 84, 1044.

respectively. The δ variations described in the literature are only due to interactions between two surfactants.43 We can nevertheless notice a difference of δ for CH2 groups in R and β position according to the SDS concentration due to the micellar form of surfactant. In the micelle, the CH2 groups close to the sulfate function are more shielded than those in the free form, and this should be another way to determine surfactant cmc. A spectrum of SDS-PPOMA mixture in aqueous solution (D2O) for R ) 1 is presented in Figure 8. 1H NMR (D2O) δ (ppm): (i) for SDS 0.89 (CH3-), 1.29 (-(CH2)9-), 1.66 (-CH2- β) and 4.00 (-CH2- R); (ii) for PPOMA 6.16 and 5.66 (H2CdC-). Table 4 provides data on SDS-PPOMA mixtures. When R passes from 0 to 1, δ of the CH2 group in R position near the sulfate group decreases from 4.032 to 4.001 ppm, that of β position from 1.684 to 1.661 ppm, while δ of CH3 group remains constant (very weak variation from 0.892 to 0.888 ppm). δ of the nine CH2 groups decreases from 1.306 to 1.291 ppm for the same variation of R. We decided not to discuss this decrease: NMR peak being the superposition of nine CH2 groups, uncertainties of this result are appreciably increased. This result reflects the SDS-PPOMA interactions. Weak δ variations are observed for the protons of the double bond of the PPOMA: they pass from 6.165 to 6.158 ppm and 5.677 to 5.662 ppm. We do not have the δ values of PPOMA groups in pure solvent, and one would see certainly δ as significant as in the case of pure SDS. One can nevertheless notice that the δ variation of 0.015 ppm in the case of the protons of the double bond of the PPOMA is comparable to the δ variation of CH2 in R position of the sulfate group of the SDS, when R passes from 0.25 to 1. Figure 9 brings information on the interaction between the PPO chain of the macromonomer and the SDS micelle. Spectrum 1H NMR of the soluble PPOMA in CDCl3 presents a better resolution for the CH2 groups than that of SDS-PPOMA mixture in D2O. One also has a good resolution for the spectrum of PPO in the D2O. This loss of resolution on the spectrum can be explained by a partial immobilization of the chain. 1 H NMR study shows that the macromonomer is located at the surface of the micelle despite its insolubility in water and does not enter inside the micelle: the CH3 groups are not disturbed (there is a loss of 0.004 ppm when ratio PPOMA/SDS passes from 0 to 1). The variation of chemical shift δ is of 0.031 ppm for the CH2 in R position near sulfate group. For mixture SDS-poly(ethylene glycol) (23) lauryl ether described by Gao et al., δ values are much more significant: 0.025 ppm.43 On the other hand, for mixture SDS-anilinium chloride, Kim et al. have observed a decrease of δ for CH2 groups in R position from 3.856 to 3.571 ppm when the anilinium chloride concentration passes from 0 to 0.1 mol‚L-1, that is to say a very significant variation: approximately 10 times higher than our δ.42 No information is given on the molar fraction into surfactant or the aggregation numbers in this reference. In this last case, a very strong attraction between the sulfate groups of SDS and the ammonium groups of anilinium chloride is expected. This will bring closer the different components of the micelle and will increase the δ variations. Figure 10 gives the possible different models for the SDS-PPOMA mixed micelle: (i) model A, PPOMA is located at the core of the micelle, forming a droplet surrounded by SDS molecules; (ii) model B, PPOMA is extended inside the SDS micelle as for a mixed micelle of two surfactants; (iii) model C, only the methacrylate function of PPOMA lies inside the SDS micelle and the PPO chain of the macromonomer is pending in water; (iv)

5766


Figure 8.

Bastiat et al.

H NMR spectrum for the SDS-PPOMA system. CSDS ) 0.05 mol‚L-1 and the weight ratio R ) PPOMA/SDS ) 1.

1

Figure 9. (a) 1H NMR spectrum of the PPOMA soluble in the CDCl3. (b) 1H NMR spectrum for the SDS-PPOMA system (R ) 1) in D2O. (c) 1H NMR spectrum of the PPO Mw ) 400 g‚mol-1 in D2O (c). Table 4. Chemical Shift Variation of the Various Groups of SDS and H2CdC- Function of the PPOMA versus the Weight Ratio R ) PPOMA/SDS and CSDS ) 0.05 mol‚L-1 SDS

PPOMA

wt ratio PPOMA/SDS

δ (-CH2- R) (ppm)

δ (-CH2- β) (ppm)

δ (-(CH2)9-) (ppm)

δ (CH3-) (ppm)

δ (CH2dC) (ppm)

δ (CH2dC) (ppm)

0 0.25 0.5 0.75 1

4.032 4.018 4.008 4.003 4.001

1.684 1.667 1.666 1.663 1.661

1.306 1.300 1.294 1.292 1.291

0.892 0.889 0.885 0.887 0.888

6.165 6.159 6.158 6.158

5.677 5.668 5.664 5.662

model D, only the methacrylate function of PPOMA lies inside the SDS micelle and the PPO chain of the macromonomer is preferentially adsorbed on the surface, i.e., on the polar heads of the SDS. In a fluorescence quenching experiment, the decrease of fluorescence intensity of the probe is related to the formation of a complex between this probe and the fluorescence quencher, the complex not emitting fluorescence.46 With the quencher that we use being the dodecylpyridinium chloride, the dodecyl chain is incorporated

inside the micelle in the same way as the dodecyl chain of SDS; the pyridinium group forms a complex with pyrene near the surface of the micelle. Various authors specify that pyrene is located at the neighborhoods of the surface.32,46,55 It is to be noticed that all the molecules having a double bond behave like quenchers for fluorescent probes. Figure 11 presents the variation of the (55) Alargova, R. G.; Kochijashky, I. I.; Sierra, M. L.; Zana, R. Langmuir 1998, 14, 5412.


Langmuir, Vol. 20, No. 14, 2004 5767

Figure 10. Models proposed for SDS-PPOMA micellar system.

Figure 11. Variation of the fluorescence intensity of the first peak for the pyrene spectrum versus the weight ratio R ) PPOMA/ SDS. CSDS ) 0.05 mol‚L-1.

fluorescence intensity of the first peak of the pyrene spectrum versus R. Pyrene is quenched by PPOMA until a ratio R ) 0.2. For higher ratios, addition of PPOMA does not modify the fluorescence intensity of pyrene: pyrene-PPOMA complexes are not formed any more. PPOMA quenches pyrene and this shows that the double bond of the PPOMA is well located near the interface of hydrophobic core-water. Hence, the determination of the aggregation numbers by steady-state fluorescence quenching is reliable.56 To complete the study of the localization of PPOMA in micelle, it is interesting to compare the behavior of the PPOMA with that of PPO (whose the molar mass is equivalent) which does not have the hydrophobic double bond. It is well-known that polyethers and surfactants strongly interact in aqueous solution. It is the case for anionic surfactant and POE.44,45,51,57-62 The surfactant is fixed on polymer under the form of micelles rather as isolated molecules to form a “pearl necklace”, and the aggregation numbers are similar or slightly lower than they are without polymer. Consequently, the phase diagram of the SDS-PPO system should be comparable to those described for other polymer-surfactant mixture, with three domains: the first one where SDS remains free (56) Bastiat, G.; Borisov, O.; Lapp, A.; Grassl, B.; Franç ois, J. Submitted to Langmuir. (57) Benkhira, A.; Lachhab, T.; Bagassi, M.; Franç ois, J Polymer 2000, 41, 1471. (58) Witte, F. M.; Engberts, J. B. F. N. Colloids Surf. 1989, 36, 417. (59) Shirahama, K. Colloid Polym. Sci. 1974, 252, 978. (60) Zana, R.; Lianos, P.; Lang, J. J. Phys. Chem. 1985, 89, 41. (61) Binana-Limbele, W.; Zana, R. Colloid Surf. 1986, 21, 483. (62) Witte, F. M.; Engberts, J. B. F. N. J. Org. Chem. 1987, 52, 4767.

Table 5. Critical Micellar Concentration (cmc), Ionization Degree (r), and SDS Aggregation Number (Nag SDS) versus the Weight Ratio R′ ) PPO/SDS, Compared with the Values for the PPOMA-SDS System, CSDS ) 0.05 mol‚L-1 PPO PPOMA wt ratio PPO/SDS and cmc (R) cmc (R) PPOMA/SDS (10-3 mol‚L-1) Nag SDS (10-3 mol‚L-1) Nag SDS 0 0.25 0.5 1

8.4 (0.371) 6.9 (0.417) 6.6 (0.494) 6.4 (0.601)

58 51 47 37

8.4 (0.371) 7.0 (0.485) 5.9 (0.612) 4.8 (0.696)

58 39 32 21

in solution, the second one which correspond to SDS at cmc in equilibrium with SDS-PPO complexes, and the third one where PPO is saturated and free SDS micelles are formed. This system was studied by conductometry and fluorescence spectroscopy. The values of cmc, R, and Nag SDS are reported in Table 5 for SDS-PPO systems. The curves of conductivity that we obtained are completely similar to those of Figure 3 for the SDS-PPOMA system. cmc does not vary significantly with the weight ratio R′ ) PPO/SDS. Indeed, one passes from 8.4 × 10-3 to 6.4 × 10-3 mol‚L-1 when R′ increases from 0 to 1. The variations of cmc and R are less pronounced with this system than for SDS-PPOMA. For the same ratio R ) 1, the values of the ionization degree are 0.601 and 0.696 with PPO and PPOMA, respectively. One can thus think that the average distance between the charged groups is smaller in the case of PPO than in the case of the macromonomer; PPO chains do not modify the SDS micelle as significantly as PPOMA. Nag SDS values confirm

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Bastiat et al.

Figure 12. Variation of the cmc for the SDS-PPOMA mixture versus the PPOMA micellar composition for various β values: 0, -1, and -2; ([) experimental values, (s) theorical values.

the remarks on ionization degrees. Indeed, one passes from 58 to 37 chains of SDS when the ratio R′ increases from 0 to 1. This reduction is much less pronounced than in the case of the PPOMA for which one obtains a value of 21 SDS per micelle for R ) 1. We did not calculate the aggregation numbers of PPO and the total aggregation numbers because, due to PPO solubility, a fraction of PPO chains does not take part in association. We do not know the PPO concentration really implied in mixed micelles. This study well illustrates the role of the “amphiphilic character” of the PPOMA and in particular of the methacrylate function: the stronger affinity of the polymerizable function for the aliphatic chains tends to involve the molecules of PPOMA within the micelle by drawing aside the charged groups of surfactants. The results obtained with PPO clearly show that there are interactions with SDS. Nevertheless, one can suppose that because of the hydrophilic character of PPO, its molecules are not really integrated in micelles but that, while remaining on the surface, they anchor partially between the sulfate groups and decrease ionization. In fact, the PPO chain (Mw ) 400 g‚mol-1) must behave like portions of chain of a polyether of higher molar mass in interaction with SDS. We have determined the influence of the poly(propylene oxide) Mw ) 400 g‚mol-1 on the critical micellar concentration and the SDS aggregation number of micelle. The mass of this polymer being small compared to the mass of SDS micelle, the model of the “pearl necklace” will not be considered, but rather a model of polymer adsorbed on the micelle surface. In conclusion, the results shows that the PPOMA macromonomer does not penetrate inside micelles (only the CH2 in R position near sulfate group is disturbed by PPOMA) but only the double bond seems to be anchored between the SDS polar end groups. The PPO part of the macromonomer is on the surface of micelle (immobilization of the PPO chain) and does not remain mobile in solution. Thus we could propose the following model (model D in Figure 10): the C12 chains of the SDS form the hydrophobic core of micelle, the sulfate groups of SDS and the methacrylate functions of PPOMA form the interface hydrophobic core-water, and the PPO chains of the macromonomer are adsorbed on the surface of the micelle. Thermodynamics of a Mixed Micelle. Holmberg et al. have established laws to predict the critical micellar

concentration (cmc) of a surfactant mixture63

cmc ) x1mf1mcmc1 + x2mf2mcmc2

(11)

where x1m and x2m are the molar fractions of surfactant 1 and 2 in the micelle and cmc1 and cmc2 are the critical micellar concentrations of the pure surfactants 1 and 2. f1m and f2m are the activity coefficients of surfactants1 and 2 in the micelle, respectively. The activity coefficients are given by

ln f1m ) (x2m)2β

(12a)

ln f2m ) (x1m)2β

(12b)

and

where β is an interaction parameter quantifying the interactions between the two surfactants in micelles. Positive values of β indicate a repulsion between the two surfactants (the case of surfactants with a hydrocarboneous and fluorocarboneous chain). Negative values of β are most commonly found, meaning an attraction between the two species (the case of anionic and cationic surfactants). When β ) 0, the mixture is considered as ideal, with very weak interactions between the mixed surfactants (the case of a mixture of two surfactants with the same polar head but a different length of hydrophobic chain). By considering PPOMA as a surfactant (solubility limit of about 10-5 mol‚L-1), we have reported in Figure 12 the experimental and theoretical variation of cmc versus the SDS-PPOMA composition, and we can note that the ideal case (β ) 0) is in agreement with our SDS-PPOMA system. There are very weak interactions between SDS and PPOMA. This is not surprising since Holmberg et al. have shown that the concept of mixed micelles can apply to the amphiphilic molecules which do not form micelles in solution and present rather a phase separation (mixtures of surfactant and hydrophobic alcohol) where the experimental cmc variations are well described by the ideal case of mixed micelles (β ) 0).63 (63) Holmberg, K.; Jonsson, B.; Kronberg, B.; Lindman, B In Surfactants and Polymers in aqueous solution; J. Wiley & Sons: New York, 2003; Chapter 5, p 119.


Langmuir, Vol. 20, No. 14, 2004 5769

Table 6. Radius, rch, and Total Surface (Smicelle(1) and Smicelle(2)) of the Hydrophobic Core versus the Weight Ratio R ) PPOMA/SDS, CSDS ) 0.05 mol‚L-1 wt ratio PPOMA/SDS

Nag SDS/ Nag PPOMA

rch (nm)

Smicelle(2) (nm2)

Smicelle(1) (nm2)

0 0.25 0.5 0.75 1

58/0 39/8 32/13 24/15 21/17

1.693 1.483 1.388 1.261 1.206

36.0 27.6 24.2 20.0 18.3

36.0 26.5 23.5 19.1 17.8

However, let us note that we do not have a true mixed micelle but rather a micelle model surrounded by PPOMA as illustrated by the model D in Figure 10. The two forces which control the assembly of amphiphilic molecules in a well-defined structure such as a micelle are the attraction of the hydrophobic parts of surfactant which induces the association of the molecules and the repulsion of the heads (ionic or steric repulsion). These two opposite interactions act mainly at the interface of hydrophobic core-water: tending to decrease or to increase the optimal surface, noted a0, by molecule exposed to the aqueous phase. The micelle geometry is defined by the optimal surface a0 of the sulfate groups and the volume v of the hydrocarbon chain C12, located only in the micelle core, assumed to be fluid and incompressible. Volume v is a semiempirical parameter and for a hydrocarbon chain saturated with n carbons, it was shown by Israelachvili et al. that v ≈ (27.4 + 26.9n) × 10-3 nm3.64 For a spherical micelle, with rch the radius of the hydrophobic core, a0 the optimal surface per polar head, and Nag the aggregation number, we have

4πrch2 4πrch3 Nag ) ) a0 3v

(13)

From the data on pure SDS micelle, we obtain v ) 0.3502 nm3, rch ) 1.7 nm, and a0 ) 0.621 nm2 for Nag SDS ) 58. In the absence of SDS-PPOMA interaction (β ) 0), the total surface is given by the relation

Smicelle(1) ) Nag SDS a0 + Nag PPOMA aPPOMA (14) where aPPOMA is the surface of the methacrylate function. The methacrylate function with its conjugated double bond can be considered at first approximation as plane. This function fits in a circle of radius of roughly 0.3 nm and thus aPPOMA ) 0.283 nm2. The total surface of the interface hydrophobic core-water can be calculated in two ways: (i) from the aggregation number (Smicelle(1)) with relation 14 or (ii) from the radius of the hydrophobic core (Smicelle(2)) by considering that total surface is 4πrch2. These values are reported in the Table 6. The good agreement between the two ways to calculate surfaces confirms the values of aggregation numbers as (64) Israelachvili, J. N. In Physics of Amphiphiles: Micelles, Vesicles, and Microemulsions; Degiorgio, V., Corti, M., Eds.; North-Holland: Amsterdam, 1985; p 24.

determined through fluorescence measurements, and one can consider that the geometrical considerations well illustrate our mixed micelle model, schematized by model D in Figure 10. Conclusion This study leads to clear conclusions about a new monomer-surfactant system. The addition of PPOMA to SDS micellar solution leads to a decrease of the aggregation number of SDS, as shown by fluorometry and static light scattering. 1H NMR and fluorescence quenching allow proposal of a particular structure for the SDS-PPOMA mixed micelle: (i) the hydrophobic core of micelle is primarily composed of the C12 chains of the sodium dodecyl sulfate, (ii) the interface hydrophobic core-water is made up of the polar heads of the SDS as well as methacrylate functions of the poly(propylene oxide) methacrylate, and (iii) the PPO chain of the macromonomer is adsorbed preferentially at the surface, i.e., on the polar heads of the SDS. The thermodynamics of mixed micelles well confirms the micellar model described by Figure 10 (model D) and is consistent with the experimental values of the aggregation number. It is clear that these results also lead to a question about the hypothesis generally used to calculate the length of the hydrophobic sequences in associative copolymers prepared by micellar copolymerization. The conclusion of our systematic study of the mixed micelles SDS-PPOMA is that the true sequence length in copolymer will be much lower than that usually calculated by assuming incorporation of monomer without change of the SDS aggregation number. This means that an analogous study should be undertaken for every micellar copolymerization. It should be also useful to verify through a serious characterization of copolymers that the number of hydrophobic monomer in the initial micelles do really correspond exactly to the length of the sequences. Moreover, despite the great number of publications on micellar copolymerization using acrylamide as the main monomer, there is no information about interactions between this amphiphilic molecules and surfactants. Beside, parameters such as salinity or temperature should also be considered because they may be varied in the copolymerization process in order to promote particular microstructure of the copolymers. Such studies have already been undertaken in our laboratory (forthcoming paper),65 and the surprising results obtained show that the used description for associative copolymers prepared by this way must be renewed, as well as the conclusions concerning relation between microstructure and associative properties. Acknowledgment. We gratefully acknowledge G. Clisson for NMR studies. We also thank W. BinanaLimbele for the fruitful discussions about fluorometry experiments. LA049890C (65) Bastiat, G. PhD Dissertation, University of Pau, 2003.

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