J. Phys. Chem. B 2001, 105, 9077-9082
9077
Comicellization of Polystyrene-block-Poly(ethylene oxide) with Cationic and Anionic Surfactants in Aqueous Solutions: Indications and Limits Lyudmila M. Bronstein* Chemistry Department, Indiana UniVersity, Bloomington, Indiana 47405
Dmitrii M. Chernyshov, Evgenii Vorontsov, Galina I. Timofeeva, Lydia V. Dubrovina, and Pyotr M. Valetsky NesmeyanoV Institute of Organoelement Compounds, Russian Academy of Sciences, 28 VaViloV St., Moscow 117813, Russia
Sergey Kazakov Department of Chemistry, Chemical Engineering, and Materials Science, Polytechnic UniVersity, 6 MetroTech Center, Brooklyn, New York 11201
Alexei R. Khokhlov Physics Department, Moscow State UniVersity, Moscow 117234, Russia ReceiVed: February 14, 2001; In Final Form: April 27, 2001
Interaction of polystyrene-block-poly(ethylene oxide) (PS-b-PEO) with cationic and anionic surfactants was studied using light scattering, sedimentation in ultracentrifuge, and 1H NMR. The complex structure of the mixed solution containing hybrid micelles, micellar clusters, and supermicellar aggregates was confirmed by independent experiments. By proton NMR, addition of anionic surfactant, sodium dodecyl sulfate (SDS), to PS-b-PEO solution results in loosening the micelle cores and an increase of the PS molecular mobility in the presence of surfactant. At the same time, incorporation of SDS into the block copolymer solution leads to noticeable broadening of the signals related to methylene protons of the surfactant, which can be assigned to a decrease of mobility of surfactant alkyl chains “immobilized” on the PS cores. These data along with ultracentrifugation of hybrid solutions containing PS-b-PEO and surfactant at various concentrations prove that comicellization occurs and mixed micelles are formed. However, this phenomenon is observed up to a certain limiting concentration of the surfactant. In the case of cetyl pyridinium chloride (CPC), increase of surfactant loading up to 0.05 mol/L results in the solutions containing CPC micelles along with hybrid micellar clusters and supermicellar aggregates.
Introduction The interaction of surfactants with block copolymers was extensively studied by several groups; while using different kinds of block copolymers, most attention was paid to the behavior of commercially available poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymers (pluronics).1-7 For all the block copolymer/surfactant systems, the complexation between one or both blocks and surfactants was established, but the structure of hybrid solutions was found to depend on the specific system. Another example of interaction is a spontaneous formation of well-defined block ionomer complexes between various block ionomers of poly(ethylene oxide)-block-poly(methacrylic acid) (PEO-b-PMA) and oppositely charged surfactant ions or polyions, which results in interesting morphologies.8-10 The evident driving force for formation of block ionomer complexes is the electrostatic interaction between oppositely charged groups, though hydro* To whom correspondence should be addressed: Chemistry Department, Indiana University, 800 E. Kirkwood Ave., Bloomington, IN 47405. Telephone: (812) 855-3727. Fax: (812) 855-8300. E-mail:
[email protected].
phobic interactions should be taken in consideration as well.11 In the case of uncharged block copolymers, the driving forces are even more complex and result in more uncertain structures. It is well-established that in the presence of PEO, surfactants form micelles at concentrations below critical micelle concentrations (cmc) and these micelles are connected by PEO chains.12-17 In the case of amphiphilic block copolymers, when another, more hydrophobic block is present in the system, surfactant molecules can compete for more preferable interaction sites.18 In so doing, the resulting structure of the complex should depend on the particular system. Several publications describing the interaction of sodium dodecyl sulfate (SDS) with pluronics (PEOx-PPOy-PEOx) reported decomposition of the block copolymer micelles upon complexation with surfactant molecules,1-3 which progressed with increase of the SDS concentration. Other publications reported the formation of mixed micelles.6,7,19,20 In recent publications,6,7 using a SDS surfactant selective electrode via electromotive force, isothermal titration calorimetry, DSC, and light scattering, concentration ranges for each composition of the mixed pluronic (F127)-SDS micelles were
10.1021/jp010565x CCC: $20.00 © 2001 American Chemical Society Published on Web 08/25/2001
9078 J. Phys. Chem. B, Vol. 105, No. 38, 2001 found. At the SDS concentration of 0.001-0.003 M, breakdown of the mixed micelles was observed. In our preceding papers we described the hybrid systems obtained by interaction of polystyrene-block-poly(ethylene oxide) (PS-b-PEO) with cationic surfactant, cetylpyridinium chloride (CPC), and anionic surfactant, SDS.21,22 Such hybrid systems were found to be successful stabilizing media for the synthesis of Pd, Pt, and Rh nanoparticles, which are promising catalysts for various catalytic reactions in aqueous media. By means of ultracentrifugation, the increase of micelle weight and size upon addition of surfactant was established; however, no clear evidence was obtained on comicellization in these particular systems. In the present paper we present a more detailed study of the interaction of PS-b-PEO with surfactants. Experimental Section PS-b-PEO (SE 1030) PS-1000, PEO-3000, Mn ) 4000, Mw ) 6200 (by ultracentrifugation), was received as a gift from Goldschmidt AG (Germany). Dimethylformamide (DMF) was distilled under KOH. CPC and SDS (Aldrich) were used as received. Water was purified with a Milli-Q water purification system. Two procedures were used to prepare micellar solutions of PS-b-PEO in water. In method 1, polymer samples were prepared by direct dissolution of PS-b-PEO block copolymer (concentration of 5-10 g/L or (0.8 x 10-3)-(0.16 x 10-2) mol/ L) in H2O under vigorous stirring for 24 h at room temperature. In method 2, PS-b-PEO was dissolved in DMF (concentration of 10 g/L) and then water was slowly added to the DMF solution with the simultaneous ultrafiltration of the resulting solution through a Millipore ultrafiltrating device. This procedure was stopped when the residual concentration of DMF in the stock aqueous solution did not exceed 100-200 ppm (measured with GC). For purification of PS-b-PEO, it was reprecipitated in diethyl ether from the THF solution. For this, 10 g of block copolymer were dissolved in 150 mL of freshly distilled THF. The resulting PS-b-PEO solution was precipitated dropwise in the excess (10fold by volume) amount of diethyl ether, isolated, and dried in a vacuum desiccator. CPC or SDS were added to the PS-b-PEO micellar solutions to reach a concentration of (0.8 x 10-3)-(0.2 x 10-2) mol/L. The viscosity of the solutions was measured with an automatic viscosimeter AVS-400 (Germany). Examination of molecular weight and micelle sizes was performed by static light scattering measurements with the Fica-50 (France) goniodiffusometer using vertically polarized light of wavelength λ ) 546 nm in the angle interval 30-150° by a standard procedure.23 The diffusion and sedimentation rates of the sedimenting species were studied by using an analytical ultracentrifuge MOM 3180 (Hungary) with Philpot-Svensson optics (T ) 25 ( 0.1 °C, λ ) 546 nm). The total diffusion coefficients D were calculated from the broadening rate of a synthetic boundary formed in the ultracentrifuge cell between the polymer solution and the pure solvent.24 For micelles and micellar clusters, diffusion coefficients were calculated from the value of the area under the appropriate peak. Having calculated diffusion coefficients, the micelle hydrodynamic radii Rh were calculated from the Stokes equation for the spherical particles.25 The sedimentation coefficients Sc were calculated from the rate of migration of the concentration zone in the force field of the ultracentrifuge.24 Values of molecular weights were calculated according to the first Svedberg formula.24 The specific partial volume V h and the solvent density F0 were measured by pycnometry (T )
Bronstein et al. 25 ( 0.05 °C). Before measurement, the pycnometer was calibrated with Hg. As a solvent, solution of surfactant in water for each particular concentration was taken. For V h determination, overall solution density (after addition of block copolymer) was found and then the solvent density was subtracted from the overall density. The difference was divided by the block copolymer concentration (g/mL).26 1H NMR spectra were recorded with a Bruker YMX-400 at a frequency of 400.13 MHz. Polymer samples for 1H NMR examination were prepared in D2O according to the modified method 1 described above. All spectra were recorded at 25 °C. For light scattering (LS) experiments, stock solutions of PSb-PEO and PS-b-PEO/CPC (0.0008M) were diluted to the concentration 0.8 mg/mL using pure water and water with CPC as solvents, respectively, and then filtered into the quartz 10mm diameter sample cells using 0.2-µm Sartorius Minisart filters. LS measurements were done with a Malvern 4700c system. An argon ion laser (Uniphase 2213-75 SL) operating at 488nm wavelength and 30-mW output power was used as a light source. The spectrometer was calibrated with distilled water and toluene to make sure that the scattering intensity from water and toluene had no angular dependence in the angular range employed. All measurements were performed at (25.0 ( 0.1) °C. The intensity-intensity time autocorrelation functions of the scattered intensity in the self-beating mode can be related to the normalized first-order electric field time correlation function g(1)(t,q) as
G(2)(t) ) ) B[1 + β|g(1)(t,q)|2]
(1)
where β is the parameter depending on the coherence of the detection, t is the delay time, B is the measured baseline, and q is the magnitude of the scattering vector related to the scattering angle Θ
q ) (4πn0/λ0)sin(Θ/2)
(2)
n0 is the refractive index of a solvent and λ0 is the wavelength of the light used. For polydisperse small particles g(1)(t,q) is given by g(1)(t,q) ) ∑ciMi exp(-Γit)/∑ ciMi. The initial slope in semilogarithmic coordinates yields the z-average inverse decay constant: ΓZt[d ln g(1)(t,q)/dt]tf0 ) ∑ciMiΓi/∑ciMi, where ci and Mi are the concentration and molecular weight of ith component, respectively. In integral form |g(1)(t,q)| can be connected with the normalized characteristic line-width distribution G(Γ) by
|g(1)(t,q)| )
∫0∞G(Γ)e-Γt dΓ
(3)
Each Z-average inverse decay time can be calculated by equilibrium statistical thermodynamics and at low q it is related to the apparent diffusion coefficient D: Γ ) Dq2. The “Z-average particle size” can be found by using the well-known Stokes-Einstein relationship, which is a definition for a hydrodynamically effective sphere diameter
〈dh〉 ) kbT/3πηD
(4)
where η is the viscosity of the solvent, T is the absolute temperature, and kb is the Boltzmann’s constant. A way to get the width distribution G(Γ) is to use CONTIN program, i.e., an inverse Laplace transformation.
Comicellization of PS-b-PEO with Ionic Surfactants
J. Phys. Chem. B, Vol. 105, No. 38, 2001 9079
TABLE 1: Characteristics of the PS-b-PEO Solutions Prepared under Various Experimental Conditionsa micelles
micellar clusters
media
10-6Mw(m)
w1, %
1013So, s
107Do, cm2/s
10-6MSD, Da
Rh, nm
w2, %
1013So, s
107Do, cm2/s
10-6MSD, Da
Rh, nm
H2O H2Ob H2O/DMFc
2.1 2.5 2.2
29 28 30
3.7 3.9 3.7
2.00 2.10 2.05
0.255 0.265 0.250
12.8 13.0 12.5
71 72 70
12.5 13.3 13.0
0.61 0.58 0.60
2.8 3.4 3.2
42.1 42.2 41.8
a Here and in Tables 2 and 3 S is the sedimentation coefficient, D is the diffusion coefficient, M o o SD is the micellar weight of fraction, and Rh is the hydrodynamic radius. b Block copolymer sample was reprecipitated from THF into diethyl ether prior analyzing. c Polymer solution was prepared via dialysis of PS-b-PEO diblock copolymer from DMF into water.
TABLE 2: Characteristics of the PS-b-PEO/CPC (0.8 x 10-3 mol/L) Solutions Prepared by Direct Dissolution in Water (1) or via Dialysis from DMF into Water (2) micelles media
supermicellar aggregates
micellar clusters
w1, % 1013So, s 107Do, cm2/s 10-6MSD, Da Rh, nm w2, % 1013So, s 107Do, cm2/s 10-6MSD, Da
H2O (1) H2O/D MF (2)
61 60
3.2 3.1
1.90 1.85
0.3 0.3
13.5 13.2
30 30
11.7 12.5
0.55 0.54
3.75 3.90
Rh, nm
w3, %
46.7 48.0
9 10
TABLE 3. Characteristics of Micellar Structures in the PS-b-PEO/CPC Aqueous Solutions Obtained from Sedimentation Data CPC loading mol/L 0 0.5 x 10-3 0.8 x 10-3 0.14 x 10-2 (cmc) 0.2 x 10-2 0.5 x 10-2 0.8 x 10-2 0.2 x 10-1
micelles micellar clusters V h ,a cm3/g w1, % 1013So, s 107Do, cm2/ s 10-6MSD, Da Rh, nm w2, % 1013So, s 107Do, cm2/ s 10-6MSD, Da Rh, nm 0.825 0.845 0.863 0.871 0.896 0.980 1.125 1.134
29 63 61 55 46 41 40 45
3.7 3.3 3.2 2.8 3.1 2.6 2.0
2.0 1.9 1.9 1.8 1.7 2.1 2.3 2.4
0.255 0.310 0.300 0.325 0.346 0.250 0.154b 0.148
12.8 13.5 13.5 14.2 14.3 11.6 10.6 10.2
71 37 30 34 36 33 39 37
12.5 11.6 11.7 13.2 17.2 5.7 8.2 6.9
0.62 0.59 0.55 0.50 0.48 2.1 1.2 1.2
2.80 3.5 3.75 4.3 4.50 3.3 1.4 1.02
42.1 43.6 46.7 48.8 50.8 41.0 20.1 20.4
supermicellar aggregates w3, % 0 0 9 11 18 26 21 18
V h is the specific partial volume. M h w is obtained by the method of approaching to sedimentation equilibrium (Lakatos, S.; Zavodszky, P. Analytical Ultracentrifugation; MOM: Budapest, 1982). a
b
Dynamic measurements were performed as follows. The scattering angle at 90° was kept constant. The quality of measurements was checked over the signal-to-noise ratio and the range of the correlation function. The intensity autocorrelation function was collected in 128 channels and, in a so-called far point, a special group of correlator channels pushed out in time by a special extension of the memory. The difference between the measured and the calculated baselines was taken into account. The particle size distribution and average particle size were obtained from the correlation function by fitting the data with CONTIN analysis, using PCS software programs (version 1.35) supplied by Malvern Instruments Ltd. Differential scanning calorimetry (DSC) was performed with a Perkin-Elmer DSC 7 with liquid N2 as a coolant. Polymer samples were sealed in reusable stainless steel high-pressure capsules, cooled to -85 °C and then run up to 120 °C. Results and Discussion 1. Structure of the PS-b-PEO/Surfactant Solutions. As shown by ultracentrifugation,27 PS-b-PEO forms two types of structures in aqueous solutions: micelles and micellar clusters. Incorporation of CPC or SDS results in three kinds of aggregates: in addition, larger structures appear which were called supermicellar aggregates (Tables 1-3). The structure of the solutions (fractions of all kinds of micellar structures) strongly depends on the surfactant loading. From Table 3 one can see that the fraction of micelles first strongly increases, while the fraction of micellar clusters decreases. This can be explained by incorporation of ions in the system, which suppresses the tendency of PEO chains to micellar cluster formation.21,22 Increase of surfactant loading to 0.0008 M is accompanied by formation of supermicellar aggregates, which slightly decreases
the fractions of both micelles and micellar clusters; the latter seem to be more involved in aggregate formation. Further increase of the supermicellar aggregate fraction goes along with a decrease of micelle fraction, while the micellar aggregate fraction is rather unchanged. This shows that in the hybrid PSb-PEO/CPC solution micelles are also actively involved in supermicellar aggregate formation. As seen from Table 3 for CPC and from the data on SDS presented in ref 21, the supermicellar aggregate fraction goes through a maximum at CPC concentration of 0.005 M and then slightly decreases. Apparently, higher concentration of surfactant favors partial disintegration of supermicellar aggregates. To confirm the complex structure of PS-b-PEO and PS-bPEO/surfactant solutions, independent experiments were carried out with LS technique. The field-field correlation functions (CF) g(1)(t) for PS-b-PEO and PS-b-PEO/CPC (0.0008 M, below cmc) samples are compared in Figure 1. One can see that CFs are quite different. In both cases it was impossible to fit CF in one exponential curve. For the PS-b-PEO/CPC sample, a wider CF (than for PS-b-PEO) was observed. The results of CONTIN analysis, i.e., the intensity size distributions, for the samples studied, are presented in Figure 2. The surfactant-free PS-bPEO shows a bimodal distribution of sizes with the hydrodynamic diameter maxima at ∼24 nm and 80-100 nm. An incorporation of CPC results in broadening of the above two peaks and appearance of a third one at larger sizes: ∼250300 nm. The LS data perfectly match those obtained by ultracentrifugation.22 2. Secondary Micellar Aggregates in the PS-b-PEO/ Surfactant Solutions. As discussed in our preceding papers,21,22,27 the secondary aggregates (micellar clusters and supermicellar aggregates) can be destroyed by addition of
9080 J. Phys. Chem. B, Vol. 105, No. 38, 2001
Figure 1. Field-field correlation functions for pure block copolymer PS-b-PEO (1) and PS-b-PEO/CPC sample (2) containing 0.0008 mol/L CPC.
Figure 2. Size distributions obtained by CONTIN analysis for pure block copolymer PS-b-PEO (a) and PS-b-PEO/CPC sample (b) containing 0.0008 mol/L CPC.
different kinds of additives influencing either the corona (interactions of PEO chains) or the core (PS mobility). Here, we explored two ways to prevent secondary aggregate formation, while avoiding additives. By DSC, Tg of PS with molecular weight 950 (Merck) is 17 °C. Tg of the PS block of PS-b-PEO is higher: glass transition was not observed below 40 °C and above 60 °C (in the interval 40-60 °C, a melting peak of the PEO crystalline phase appears). Although Tg of the PS block in the block copolymer is much higher than that of the homopolymer of the similar molecular weight, it demonstrates higher mobility of the PS block compared to high molecular weight PS (∼100 °C).28 In so doing, at room temperature this block is in a glassy state and has low mobility, also it is insoluble in water. Then, micellar cluster presence might be explained by incomplete dissolution of PS-b-PEO in water: low mobility of the PS block does not allow the aggregates to disintegrate and to form equilibrium micelles. To check this hypothesis, PSb-PEO was dialyzed from DMF to water, which provided slow formation of micellar structures from completely dissolved PS blocks. The data presented in Table 1 show that this procedure does not influence the micellar characteristics and fractions of both micelles and micellar clusters. When CPC is present in the system (Table 2), the micelles and micellar clusters are
Bronstein et al. slightly smaller for dialyzed block copolymer, though the fractions of all kinds of structures are the same. Thus, the presence of micellar aggregates cannot be explained by incomplete dissolution in water. Another possible cause of secondary aggregation is the presence of some impurities or inhomogeneities in the block copolymer (for example, mercaptoethanol, which is used as a chain-transfer agent in block copolymer synthesis), which might enhance interaction between PEO chains.29 To remove impurities, PS-b-PEO was dissolved in THF, filtered, and then precipitated in diethyl ether. As can be seen from Table 1, this procedure did not change the composition of PS-b-PEO solution and micellar structure characteristics. Thus, formation of micellar clusters in PS-b-PEO solution is determined neither by the incomplete dissolution of block copolymer nor by impurities: it is inherent to this particular system. As discussed in refs 3032, micellar clusters in aqueous solutions of block copolymers containing the PEO block are supposedly formed due to various interactions including hydrogen bonding between PEO chains and water, the structure of water, and hydrophobic interactions between PEO units of the neighboring chains. Based on our preceding papers21,22 and the present results, one can conclude that for the PS-b-PEO block copolymer in aqueous systems, only additives influencing either the ability of PEO chains to interact with each other (salts, alcohols) or the mobility of the PS core (toluene) allow the destruction of the secondary micellar aggregates. Thus, apparently, the interactions (hydrogen bonding, hydrophobic interactions, etc.) between PEO chains are responsible for micellar cluster formation, but these interactions are not a key factor if the micelle core is mobile. 3. Indications of Comicellization of PS-b-PEO Block Copolymer with Surfactants. When PS-b-PEO micelles interact with surfactant molecules three scenarios are possible: (i) comicellization of surfactant molecules and block copolymer micelles with formation of mixed micelles; (ii) no comicellization, but both surfactant and block copolymer form separate micelles in aqueous solution; (iii) decomposition of block copolymer micelles due to interaction of surfactant molecules with block copolymers micelles (this was reported for SDS and pluronics1-3). As seen from Table 3, increase of CPC loading in the PS-b-PEO solution first results in an increase of micelle size and weight and then in decrease of those parameters when surfactant loading strongly exceeds the cmc value. Similar data were presented for the system PS-b-PEO/SDS in ref 21. Such a dependence of micelle size and weight on surfactant loading can be explained by incorporation of surfactant molecules inside the block copolymer micelles, resulting in increase of micelle size and weight until some critical value when repulsion of surfactant headgroups and high osmotic pressure of counterions located in the micelles lead to diminishing the micelle size.21 Analogous decrease of the size of the mixed micelles with increase of surfactant loading was observed for the pluronic (F127)/SDS mixed micelles.6,7 At the same time, specific partial volume in our system increases with increase of surfactant loading (see Table 3), which is evidence of decreased density of the micellar structures. Thus, no decomposition of PS-b-PEO micelles occurred under incorporation of CPC up to concentration of 0.02 mol/L. 3.1. 1H NMR Study. To study the interaction of surfactant molecules with the PS cores, 1H NMR spectroscopy was employed. Because CPC molecules have pyridinium rings which 1H NMR signals interfere with signals of the PS phenyl protons, this particular study was carried out with SDS. Figure 3 shows 1H NMR spectra (in the spectral region characteristic of phenyl
Comicellization of PS-b-PEO with Ionic Surfactants
Figure 3. 1H NMR spectra of PS-b-PEO/SDS solutions in D2O containing various amounts of surfactant: (a) SDS-free, (b) 0.008 mol/ L, (c) 0.016 mol/L, and (d) 0.024 mol/L.
Figure 4. 1H NMR spectra of pure SDS (lower curve) and PS-b-PEO/ SDS hybrid system (upper curve). The SDS concentration in D2O is 0.008 mol/L.
protons) of surfactant-free PS-b-PEO and block copolymer solutions containing various amounts of SDS. One can see that aromatic proton signals stay nearly unresolved in the case of pure PS-b-PEO. This shows that at room temperature PS micellar core is likely in a glassy state,33 which corroborates the DSC data. Incorporation of SDS in block copolymer D2O solution results in improved resolution of aromatic proton signals. Increase of surfactant concentration from 0.008 to 0.016 mol/L leads to progressive narrowing of the peaks in the spectra and to an increase of their intensity. Further increase of SDS loading (to 0.024 mol/L) does not influence the shape and intensity of the signals: 1H NMR spectra of PS-b-PEO containing 0.016 and 0.024 mol/L SDS look nearly identical. This phenomenon, i.e., increase of resolution and intensity of the NMR signals characteristic of aromatic protons, can be attributed to loosening the micelle cores and increase of the PS molecular mobility in the presence of surfactant.34,35 This becomes possible if SDS molecules are inserted between PS chains, diminishing their interaction with each other. Formation of less compact block copolymer micellar structures under incorporation of surfactant molecules was also depicted in refs 19 and 36. Thus, NMR data give independent evidence of the comicellization of PS-b-PEO and SDS. Adsorption of surfactant by PS-b-PEO micelles can also influence molecular motion of SDS alkyl chains.37 As shown in Figure 4, incorporation of SDS into block copolymer solution results in noticeable broadening of the signals related to methylene protons of the surfactant in the spectral region 0.4-3.7 ppm. According to ref 37, such pronounced broadening might be assigned to a decrease of mobility of surfactant alkyl chains “immobilized” on the PS cores. As stated in ref 38, fluorescent surfactant 5-(N-(octadecanoyl)amino)fluorescein strongly binds at the core/shell
J. Phys. Chem. B, Vol. 105, No. 38, 2001 9081
Figure 5. Sedimentograms of the PS-b-PEO/CPC aqueous solution containing 0.02 (a), 0.04 (b), and 0.05 (c) mol/L CPC.
interface with its aliphatic tail buried in or located near the PS core. This corroborates our hypothesis of preferential incorporation of surfactant molecules into the hydrophobic core of the PS-b-PEO micelles. On the other hand, some decrease of SDS molecular motion in the PS-b-PEO solution might be a result of interaction with PEO corona as well.18,39 3.2. Ultracentrifugation Study. To get other independent proof of comicellization and to determine a range of surfactant concentration at which this phenomenon is applicable, ultracentrifugation of PS-b-PEO/CPC solutions was carried out by using CPC solution in a comparison cell. Because the CPC micellar solution has a negative refractive index increment, it appears as a negative part (IV) of the sedimentogram (Figure 5). When the concentration of CPC in the hybrid solution is 0.02 mol/L (which is 14 times higher than the cmc), no negative increment is observed on the sedimentogram; i.e., no CPC micelles are present in the solution. Doubling the CPC loading results in the appearance of a negative part (IV) on the sedimentogram along with block copolymer micelles (I), micellar clusters (II), and supermicellar aggregates (III). The concentration of 0.02 mol/L CPC can be called a critical comicellization concentration (ccc). Further increase of the surfactant loading (to 0.05 mol/L) results in disappearance of micelles, while micellar clusters with sedimentation coefficient S0 ) 7.8 x 10-13 s and supermicellar aggregates are present. Most likely at this surfactant concentration hybrid micelles interact with surfactant micelles forming supermicellar aggregates. Increase of CPC concentration to 0.06 mol/L results in crystallization of surfactant out of hybrid solution at room temperature, so higher concentration cannot be examined. Conclusions By means of light scattering and ultracentrifugation, it was confirmed that hybrid solutions containing PS-b-PEO and surfactant have a complex structure and contain micelles, micellar clusters, and supermicellar aggregates. Addition of cationic and anionic surfactants to PS-b-PEO block copolymer micellar solutions results in no decomposition of block copolymer micellar structures but in an increase of weight and size of micelles and micellar clusters up to certain concentration followed by some decrease of micelle size at concentrations much higher than cmc. As shown by proton NMR, significant increase of mobility in the PS micelle cores takes place under surfactant loading, which is unambiguous proof of comicellization, i.e., incorporation of surfactant molecules into the block copolymer micelles. At the same time, comicellization has a limit above which surfactant micelles appear in the solution. For CPC, this limit is 0.02 mol/L, which is higher than cmc by a factor of 14. Ultimately, increase of surfactant loading up to
9082 J. Phys. Chem. B, Vol. 105, No. 38, 2001 0.05 mol/L results in the solutions containing no block copolymer micelles but containing CPC micelles, hybrid micellar clusters, and supermicellar aggregates. Acknowledgment. We acknowledge financial support provided by the Russian Foundation for Basic Research (Grant 0103-32937) and NATO Science for Peace Program (SfP-974173, Colloidal Catalysts). References and Notes (1) Hecht, E.; Hoffmann, H. Langmuir 1994, 10, 86-91. (2) Hecht, E.; Mortensen, K.; Gradzielski, M.; Hoffmann, H. J. Phys. Chem. 1995, 99, 4866-4874. (3) Zhang, K.; Lindman, B.; Coppola, L. Langmuir 1995, 11, 538542. (4) Contractor, K.; Bahadur, P. Eur. Polym. J. 1998, 34, 225-228. (5) Almgren, M.; van Stam, J.; Lindblad, C.; Li, P.; Stilbs, P.; Bahadur, P. J. Phys. Chem. 1991, 95, 5677-5684. (6) Li, Y.; Xu, R.; Bloor, D. M.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 2000, 16, 10515-10520. (7) Li, Y.; Xu, R.; Couderc, S.; Bloor, D. M.; Wyn-Jones, E.; Holzwarth, J. F. Lanmuir 2001, 17, 183-188. (8) Bronich, T. K.; Kabanov, A. V.; Kabanov, V. A.; Yu, K.; Eisenberg, A. Macromolecules 1997, 30, 3519-3525. (9) Kabanov, A. V.; Bronich, T. K.; Kabanov, V. A.; Yu, K.; Eisenberg, A. J. Am. Chem. Soc. 1998, 120, 9941-9942. (10) Bronich, T. K.; Popov, A. M.; Eisenberg, A.; Kabanov, V. A.; Kabanov, A. V. Langmuir 2000, 16, 481-489. (11) Li, Y.; Ghoreishi, S. M.; Warr, J.; Bloor, D. M.; Wyn-Jones, E.; Holwarth, J. F. Langmuir 2000, 16, 3093-3100. (12) Veggeland, K.; Nilsson, S. Langmuir 1995, 11, 1885-1892. (13) Cabane, B. J. Phys. Chem. 1977, 81, 1639-1645. (14) Cabane, B.; Duplessix, R. J. Phys. 1982, 43, 1529-1542. (15) Brown, W.; Fundin, J.; Miguel, M. Macromolecules 1992, 25, 7192-7198. (16) Binanalimbele, W.; Clouet, F.; Francois, J. Colloid Polym. Sci. 1993, 271, 748-758. (17) Ramachandran, R.; Kennedy, G. J. Colloids Surf. 1991, 54, 261266. (18) Gao, Z.; Wasylishen, R. E.; Kwak, J. C. T. J. Colloid Interface Sci. 1990, 137, 137-146.
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