Study of Sodium Dodecyl Sulfate−Poly(propylene oxide) Methacrylate Mixed Micelles

Study of Sodium Dodecyl Sulfate - Poly(propylene oxide) Methacrylate Mixed Micelles

Guillaume Bastiat, Bruno Grassl,* Abdel Khoukh, and Jeanne Franc ¸ ois

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, and¹H NMR. It has been shown that SDS and PPOMA form mixed micelles, and SDS and PPOMA aggregation numbers, Nag SDSand 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) using¹H 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 C12chains 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 nano- domains playing the role of temporary junctions. In a first class of polymers, the pendant or terminal groups con- sidered were aliphatic,^4-6zwitterionic,^7,8perfluorated,^9,10 or aromatic,¹¹but 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

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.^13,14Such systems are based upon a water-soluble macromolecular backbone containing some side chains of poly(ethylene oxide) (PEO) or poly(N-isopropylacrylamide) (PNIPAM).^15,16Aqueous solutions of PEO and PNIPAM have the remarkable property to phase separate upon heating above the lower critical solution temperature (LCST) of considered poly- mers. 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 ap- plications, i.e., biomaterials, transition at lower temper- ature (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- (1) Water soluble polymers. Synthesis, solution properties and ap-

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(propylene oxide) (PPO) based thermoassociative poly- mers. 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 distribu- tion, 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 SDS- PPOMA micellar system. The micellar synthesis has been extensively studied^5,12,18-22with various types of hydro- phobic 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 hy- drophobic monomers incorporate micelles without changes in the aggregation number Nagof the surfactant and in the critical micellar concentration (cmc).⁵Such assump- tions 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,^23-27cationic and nonionic surfactants,^27-29ionic surfactants or nonionic surfactant mixtures.^25,30Solubilization of water-insoluble compounds in surfactant micelles has also been consid- ered.^31,32These systems have been studied by fluorescence

spectroscopy, static light scattering,^33-38and 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 SDS- PPOMA “phase diagram” and to determine the SDS critical micellar concentration. Moreover, steady-state fluores- cence 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 by¹H 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.^44,45The conductivity was measured with a Radiometer Copenhagen CDM92 conductometer using a two- pole 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 SDS- PPOMA mixtures were prepared such as the concentration in SDS was 0.05 mol‚L^-1and 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^-6mol‚L^-1. In a part of S1, a given amount of dodecyl- pyridinium chloride (quencher) was added and batch solutions S2were obtained. S2were diluted by the solutions S1of 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.

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

The intensity of the first peak (I1) at 373 nm of pyrene fluorescence spectrum was used to estimate the micelle con- centration from the steady-state fluorescence quenching experi- ments 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

I0is 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

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 (wavelengthλ)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 (0.1µm) 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 charac- terization 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/

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

with Mw micthe weight average molar mass of micelle,∆C)C -Ccmcwith C the SDS concentration and Ccmcthe concentration (in g‚cm^-3) at the cmc,∆I)I-Icmcwith I and Icmcthe scattered intensities at C and cmc concentrations, respectively, A2the second virial coefficient, and

where n0is the refractive index of the solvent, Nais 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^-1and 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) Meth- acrylate (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 Sec- tion). What we observe is a first domain where I increases, (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.

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).

I)I₀exp(-[Q]/[Mi]) (1)

Nag SDS)[SDS]-cmc

[Mi] (2a)

Nag PPOMA)[PPOMA]

[Mi] (2b)

Nag total)[SDS]-cmc+[PPOMA]

[Mi] (2c)

K∆C

∆I ) 1

M_{w mic}+2A2∆C (3)

K)4π²n₀²(dn/d(∆C))²

N_aλ⁴(R_tol)_90° (Itol)90° (4)

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 CLScorresponds 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, CLSshould 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 conductivityκas 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

whereλNa⁺andλDS^-are the limiting equivalent conduc- tivities 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

where R is the average ionization degree of the SDS molecules in micelles. Equations 5 and 6 show that the ratio of the slopes ofκversus CSDSgives the value ofR. 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.^16,20-22Figure 3 shows that the curvesκ)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,

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^-3mol‚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 lineκ)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 andRare 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 Con- centrations (CLS < CSDS < cmc): This domain corre- sponds 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 Concentra- tions (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 curvesκ)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 concen- tration, 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).

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.

1000κ)C_SDS(λ_Na++λ_DS-) (5)

1000κ)(RC_SDS+(cmc(1- R))(λ_Na++λ_DS-) (6)

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

wt ratio

R)PPOMA/SDS Nag total Nag SDS Nag PPOMA

cmc (10^-3 mol‚L^-1) R

0 58 58 0 8.4 0.371

0.25 47 39 8 7.0 0.485

0.5 45 32 13 5.9 0.612

0.75 39 24 15 5.5 0.616

1 38 21 17 4.8 0.696

Values of the different aggregation numbers Nag SDS, Nag PPOMA, and Nag totalare 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 SDSdown to 21 for SDS-PPOMA mixed micelles (for R)1) is observed.

Nag PPOMAincreases 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

these two ranges is observed at CSDS)8.3×10^-3mol‚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^-2and 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

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 mea- surements, we have used Isolventinstead 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.³⁶In the case of charged systems, (50) Franc¸ois, J.; Dayantis, J.; Sabbadin, J. Eur. Polym. J. 1985, 21

(2), 165.

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.

N_{ag SDS})M_{w mic}

M_SDS (7)

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

The micellar charge is given by the following relation:

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 conduc- tivity 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.

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 agree- ment between the values determined by the two mea- surements is rather good. Nevertheless, the discrepancy between these values can be attributed to the difference (51) Princen, G.D.; Mysels, K. J. J. Colloid Sci. 1957, 12, 594.

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).

N_ag)M_{w mic} M_SDS +

4M_SDSA₂C_cmc+8(M_SDSA₂C_cmc)^1/2-2 M_SDS

M_{w mic}(M_SDSA₂C_cmc)^1/2 4 M_SDS

M_{w mic}-2

(

)

(8)

z_m)4(M_SDSA₂C_cmc)^1/2+4M_SDSA₂C_cmc 2 M_SDS

M_{w mic}-

(

)

M_{mic f})N_{ag SDS}M_SDS+N_{ag PPOMA}M_PPOMA (10)

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 zmpasses 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 sup- posed to be constant when hydrophobic monomers are solubilized inside SDS micelles.^5,12,18-22Considering a SDS concentration of 0.05 mol‚L^-1, a constant aggregation number of 60 and a constant cmc of 0.0083 mol‚L^-1for 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.^39-43At 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 shiftsδ.

δof the CH3methyl group of SDS was normalized at 1 ppm and CH2 groups in Rand β position near sulfate function have a constantδvalue at 4.14 and 1.79 ppm,

respectively. Theδvariations described in the literature are only due to interactions between two surfactants.⁴³ We can nevertheless notice a difference ofδfor CH2groups inRandβposition according to the SDS concentration due to the micellar form of surfactant. In the micelle, the CH2groups 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,δof the CH2group inRposition near the sulfate group decreases from 4.032 to 4.001 ppm, that ofβ position from 1.684 to 1.661 ppm, while δof CH3

group remains constant (very weak variation from 0.892 to 0.888 ppm).δof the nine CH2groups 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 CH2groups, uncertainties of this result are appreciably increased. This result reflects the SDS-PPOMA interactions.

Weakδvariations 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 theδvalues of PPOMA groups in pure solvent, and one would see certainlyδas significant as in the case of pure SDS. One can nevertheless notice that theδvariation of 0.015 ppm in the case of the protons of the double bond of the PPOMA is comparable to theδvariation of CH2inRposition 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 CH2groups 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.

1H 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 CH3groups 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 inR position near sulfate group. For mixture SDS-poly(ethylene glycol) (23) lauryl ether described by Gao et al.,δvalues are much more significant: 0.025 ppm.⁴³ On the other hand, for mixture SDS-anilinium chloride, Kim et al. have observed a decrease ofδfor CH2groups inRposition 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 ourδ.⁴²No 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 theδvariations.

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) (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.

Table 2. Static Light Scattering: Refractive Index Increment (dn/d(∆C)), Weight Average Molar Mass M_{w mic}, and Second Virial Coefficient A₂of 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 fluorometry wt ratio

PPOMA/SDS

dn/d∆C (mL‚g^-1)

Mw mic

(g‚mol^-1)

A2

(mol‚mL‚g^-2)

Mmic f

(g‚mol^-1)

0 0.121 20100 0.013 16700^a

0.5 0.122 14000 0.023 14900

1 0.123 9400 0.017 13400

aCalculated from relation 10.

Table 3. Chemical Shift Variation of the Various Groups of SDS versus SDS Concentration and Temperature^a CSDS

(mol‚L^-1) temp

(°C)

δ(-CH2- R) (ppm)

δ(-CH2-β) (ppm)

δ(-(CH2)9-) (ppm)

0.00464 25 4.20 1.82 1.42

0.00769 25 4.20 1.82 1.42

0.0234 25 4.15 1.80 1.42

0.0489 25 4.14 1.79 1.41

0.0489 40 4.14 1.79 1.41

0.0489 60 4.14 1.79 1.41

aδ(-CH₃) was normalized to 1 ppm.

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 fluores- cence.⁴⁶ With the quencher that we use being the do- decylpyridinium 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 fluores- cent probes. Figure 11 presents the variation of the

(55) Alargova, R. G.; Kochijashky, I. I.; Sierra, M. L.; Zana, R.

Langmuir 1998, 14, 5412.

Figure 8. ¹H NMR spectrum for the SDS-PPOMA system. CSDS)0.05 mol‚L^-1and the weight ratio R)PPOMA/SDS)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^-1in 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 4.032 1.684 1.306 0.892

0.25 4.018 1.667 1.300 0.889 6.165 5.677

0.5 4.008 1.666 1.294 0.885 6.159 5.668

0.75 4.003 1.663 1.292 0.887 6.158 5.664

1 4.001 1.661 1.291 0.888 6.158 5.662

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 quench- ing 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.44,45,51,57-62The 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

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 SDSare 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^-3to 6.4× 10^-3mol‚L^-1when R′increases from 0 to 1. The variations of cmc andRare 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 mi- celle as significantly as PPOMA. Nag SDSvalues confirm (56) Bastiat, G.; Borisov, O.; Lapp, A.; Grassl, B.; Franc¸ois, J.

Submitted to Langmuir.

(57) Benkhira, A.; Lachhab, T.; Bagassi, M.; Franc¸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.

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.

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

PPOMA/SDS

cmc (R)

(10^-3mol‚L^-1) Nag SDS

cmc (R)

(10^-3mol‚L^-1) Nag SDS

0 8.4 (0.371) 58 8.4 (0.371) 58

0.25 6.9 (0.417) 51 7.0 (0.485) 39

0.5 6.6 (0.494) 47 5.9 (0.612) 32

1 6.4 (0.601) 37 4.8 (0.696) 21

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 poly- merizable 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^-1on the critical micellar concen- tration 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 CH2inRposition 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 C12chains 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

where x1mand x2mare the molar fractions of surfactant 1 and 2 in the micelle and cmc1and cmc2are the critical micellar concentrations of the pure surfactants 1 and 2.

f1mand f2mare the activity coefficients of surfactants1 and 2 in the micelle, respectively. The activity coefficients are given by

and

where β is an interaction parameter quantifying the interactions between the two surfactants in micelles.

Positive values ofβindicate a repulsion between the two surfactants (the case of surfactants with a hydrocar- boneous 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). Whenβ)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^-5mol‚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 (mix- tures 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.

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

cmc)x₁^mf₁^mcmc₁+x₂^mf₂^mcmc₂ (11)

ln f₁^m)(x₂^m)²β (12a)

ln f₂^m)(x₁^m)²β (12b)

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 a0of 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^-3nm³.⁶⁴For a spherical micelle, with rchthe radius of the hydrophobic core, a0the optimal surface per polar head, and Nagthe aggregation number, we have

From the data on pure SDS micelle, we obtain v)0.3502 nm³, rch)1.7 nm, and a0)0.621 nm²for Nag SDS)58.

In the absence of SDS-PPOMA interaction (β)0), the total surface is given by the relation

where aPPOMAis 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

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 aggrega- tion 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 C12chains 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 ag- gregation 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 incor- poration of monomer without change of the SDS aggrega- tion number. This means that an analogous study should be undertaken for every micellar copolymerization. It should be also useful to verify through a serious charac- terization 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 copolymer- ization using acrylamide as the main monomer, there is no information about interactions between this am- phiphilic 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 under- taken 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. Binana- Limbele for the fruitful discussions about fluorometry experiments.

LA049890C (64) Israelachvili, J. N. In Physics of Amphiphiles: Micelles, Vesicles,

and Microemulsions; Degiorgio, V., Corti, M., Eds.; North-Holland:

Amsterdam, 1985; p 24. (65) Bastiat, G. PhD Dissertation, University of Pau, 2003.

Table 6. Radius, r_ch, and Total Surface (S_micelle(1) and Smicelle(2)) of the Hydrophobic Core versus the Weight

Ratio R)PPOMA/SDS, C_SDS)0.05 mol‚L^-1 wt ratio

PPOMA/SDS

Nag SDS/ N_{ag PPOMA}

rch

(nm)

Smicelle(2) (nm²)

Smicelle(1) (nm²)

0 58/0 1.693 36.0 36.0

0.25 39/8 1.483 27.6 26.5

0.5 32/13 1.388 24.2 23.5

0.75 24/15 1.261 20.0 19.1

1 21/17 1.206 18.3 17.8

N_ag)4πr_ch²

a₀ )4πr_ch³

3v (13)

S_micelle(1))N_{ag SDS}a₀+N_{ag PPOMA}a_PPOMA (14)