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Study of Mixed Micelles and Interaction Parameters for ABA Triblock Copolymers of the Type EOm-POn-EOm and Ionic Surfactants: Equilibrium and Structure T. Thurn,† S. Couderc,‡ J. Sidhu,† D. M. Bloor,† J. Penfold,§ J. F. Holzwarth,*,‡ and E. Wyn-Jones*,†,‡ Fritz-Haber Institut der Max-Planck Gesellschaft, Faradayweg 4-6, D-14195 Berlin-Dahlem, Germany, School of Sciences, Chemistry, University of Salford, Salford, M5 4WT, United Kingdom, and ISIS Facility, Rutherford Appleton Laboratory, Chilton Didcot, Oxfordshire, OX11 O QX, United Kingdom Received July 11, 2002. In Final Form: September 19, 2002 We applied isothermal titration calorimetry (ITC) and surface tension (ST) and electromotive force (emf) measurements using a coated wire sodium dodecyl sulfate membrane-selective electrode to measure the mixed micellar composition of various mixtures of the triblock copolymer EO97PO69EO97, a nonionic surfactant code-named Pluronic F127, with sodium dodecyl sulfate (SDS). In the region where mixed micelles are formed, the interaction between the two surfactants showed synergistic behavior and interaction parameters β, which characterize the nonideal interaction in the mixed micelles, could be calculated over a range of mole ratios. For several compositions, the critical micelle concentrations of the mixed micelles were determined using ITC and ST measurements. In addition, small-angle neutron scattering (SANS) experiments were carried out in order to investigate the structure and provide additional information about the composition of the mixed micelles, taking advantage of contrast variation between SDS-h12 and SDS-d12. Mixed F127/SDS aggregates could be confirmed, and from an examination of the results of all methods the mixed F127/SDS system can be explained in considerable detail.
Introduction Water-soluble poly(ethylene oxide)-poly(polypropylene oxide)-poly(ethylene oxide) triblock copolymers, known as Pluronics, are highly surface active compounds and as nonionic surfactants form micelles above their critical micelle concentration (cmc).1-4 The formation of triblock copolymer micelles is an extremely temperature-dependent entropy-driven process resulting in a large decrease of the cmc on increasing the temperature. This arises because of the temperature-dependent difference in the solvation of the ethylene oxide (EO) and propylene oxide (PO) blocks. As a result, the micellization is a thermally reversible process, and this has led to the widespread use of the critical micellar temperature (cmt) as a convenient micellar parameter. Structural studies have shown that the micelles form a hydrophobic core consisting mainly of weakly hydrated PO blocks which are surrounded by an outer shell of almost fully hydrated EO blocks.5-11 * To whom correspondence should be addressed. Mailing address: Professor J. F. Holzwarth and Professor E. Wyn-Jones, Physical Chemistry, Fritz-Haber-Institut, Faradayweg 4-6, D-14195 Berlin, Germany. Tel: +49 30 8413 55 16. Fax: +49 30 84 13 53 85. E-mail:
[email protected]. † University of Salford. ‡ Fritz-Haber Institut der Max-Planck Gesellschaft. § Rutherford Appleton Laboratory. (1) Chu, B. Langmuir 1995, 11, 414. (2) Alexandridis, P.; Holzwarth, J. F.; Hatton, T. A. Macromolecules 1994, 27, 2414. (3) Alexandridis, P.; Holzwarth, J. F. Curr. Opin. Colloid Interface Sci. 2000, 5, 312. (4) Alexandridis, P.; Hatton, T. A. Colloids Surf., A 1995, 96, 1. (5) Almgren, M.; Brown, W.; Hvidt, S. Colloid Polym. Sci. 1995, 273, 2. (6) Schmolka, I. R. J. Am. Oil Chem. Soc. 1977, 54, 110. (7) Bahadur, P.; Riess, G. Tenside, Surfactants, Deterg. 1991, 28, 173. (8) (a) Mortensen, K.; Pedersen, J. S. Macromolecules 1993, 26, 805. (b) Hecht, E.; Hoffmann, H. Colloids Surf., A 1995, 96, 181.
Triblock copolymers interact with ionic surfactants.12-17 In such mixtures, the block copolymers can play a dual role in the sense that below the cmt they behave like any other nonassociated neutral polymer, but above the cmt mixed micellar aggregates are formed. In the presence of the nonassociated triblock copolymer F127 with the composition EO97PO69EO97, sodium dodecyl sulfate (SDS) forms SDS aggregates13b,15 which are bound to the polymer chain; such aggregates are smaller in size than pure SDS micelles. This first mode of binding occurs when the ionic surfactant reaches a critical aggregation concentration (cac) denoted Conset (or T1)18a which occurs well below the cmc of pure SDS. As more SDS is added, these polymerbound aggregates grow in size (and number) until F127 becomes saturated with SDS aggregates. The second mode of SDS binding occurs when the ionic surfactant is added to F127 micelles being present above their cmt. This (9) Goldmints, I.; Yu, G. E.; Booth, C.; Smith, K. A.; Hatton, T. A. Langmuir 1999, 15, 1651 and references therein. (10) Kositza, M. J.; Bohne, C.; Alexandridis, P.; Hatton, T. A.; Holzwarth, J. F. Macromolecules 1999, 32, 5539. (11) (a) Yang, L.; Alexandridis, P.; Steytler, D. C.; Kositza, M. J.; Holzwarth, J. F. Langmuir 2000, 16, 8555. (b) Genz, A.; Holzwarth, J. F. Eur. Biophys. J. 1986, 13, 323. (12) Almgren, M.; Van Stam, J.; Linblad, C.; Li, P. Y.; Stilbs, P.; Bahadur, P. J. Phys. Chem. 1991, 95, 5677. (13) (a) Hecht, E.; Hoffmann, H. Langmuir 1994, 10, 86. (b) Hecht, E.; Mortensen, K.; Gradzielski, M.; Hoffmann, H. J. Phys. Chem. 1995, 99, 4866. (14) (a) Contractor, K.; Bahadur, P.; Eur. Polym. J. 1998, 34, 225. (b) Contractor, K.; Patel, C.; Bahadur, P. J. Macromol. Sci., Pure Appl. Chem. 1997, A34, 2497. (15) Li, Y.; Xu, R.; Bloor, D. M.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 2000, 16, 10515. (16) Li, Y.; Xu, R.; Couderc, S.; Bloor, D. M.; Holzwarth, J. F.; WynJones, E. Langmuir 2001, 17, 183. (17) Li, Y.; Xu, R.; Couderc, S.; Bloor, D. M.; Holzwarth, J. F.; WynJones, E. Langmuir 2001, 17, 5742. (18) (a) Jones, M. N. J. Colloid Interface Sci. 1967, 23, 36. (b) Couderc, S.; Li, Y.; Bloor, D. M.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 2001, 17, 4818.
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binding occurs at the lowest measured SDS concentration (C < 10-5 mol dm-3) and leads to the formation of F127/ SDS mixed micelles, which are formed over a wide concentration range.15,16 The most dramatic effect which happens during this binding process is the sequential breakdown of the F127/SDS mixed micelles into smaller mixed aggregates. The binding of SDS which follows the breakdown of the mixed micelles continues until all the aggregated F127 is dissociated. At an SDS concentration immediately below that at which the complete dissociation of the F127 aggregates takes place, the monomer SDS concentration reaches the value of Conset (or T1). This is the thermodynamic condition which triggers the formation of SDS aggregates bound to unassociated F127 monomers, which are in equilibrium with micellar F127. In this short concentration interval between Conset (or T1) and the complete dissociation of F127 micelles, mixed F127/SDS micelles coexist with SDS aggregates bound to monomeric F127. In our earlier work15,16 on the F127/SDS mixed system, we approached the problem from the perspective of the well-established polymer surfactant systems where experiments are based on the investigations of binding of surfactants (SDS) to the polymer (F127). However, F127 is a nonionic surfactant and the formation of mixed F127/ SDS aggregates raises the question of whether it is strictly correct to define the mixed aggregate as micellar considering that the two surfactants have very different dimensions. It is recognized in the literature that the selfassociative block copolymer F127 will form micelles above its cmt,2 and SDS will form micelles above its cmc. In our studies on the interaction of hexaethylene glycol monon-dodecyl ether (C12EO6) and F127,18b we found complete mixing of the two surfactants to form mixed aggregates whose cmc’s could be experimentally determined. Furthermore, the nonideal mixing of the two surfactants was satisfactorily treated on the basis of regular solution theory19,20 (RST) for mixed micelles. Taking the above into account in the present study, which deals with the application of RST to the mixed F127/SDS system, we therefore shall use the term “mixed micelles” to describe the mixed aggregates, as long as there is more than one F127 molecule involved. At SDS concentrations preceding the formation of polymer-bound SDS aggregates at Conset (or T1), SDS binds exclusively to aggregated F127 to form mixed micelles.15,16 During this second mode of binding, it was also observed that the cmt of the mixed aggregate is less than that of the pure F127.16 A consequence that is implicit in this result is the fact that the cmc of the mixed F127/SDS micelle is less than that of the pure components. This significant and potentially important result relates to the fact that the most successful practical application of surfactants invariably utilizes binary surfactant mixtures containing ionic and nonionic blends because of their improved performance in comparison to the single components. This synergistic (favorable) interaction between the two surfactants causes the decrease in the cmc of the mixed micelles in comparison with the pure components. A very useful formal handle on the extent of synergistic (or antagonistic) interaction is provided by the theoretical treatment of Rubingh developed on the basis of a regular solution theory19,20 with provisions for specific interaction between surfactants. In practice, when mixed micelles are formed from different surfactants, the monomer
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concentrations and micellar compositions usually do not follow ideal mixing thermodynamics. A nonideality parameter (β) is required in the RST description of a binary system due to specific molecular interactions in micelles. When the cmc’s of the mixed micelles are measured experimentally, this interaction parameter is related to the activity coefficients, fi, of the surfactants being part of the mixed micelle as formulated in eqs 1 and 2:19,20
f1 ) exp[β(1 - x1)2]
(1)
f2 ) exp(βx12)
(2)
where x1 is the mole fraction of surfactant 1 in the mixed micelle. x1 can be calculated from eq 3,
(
x12 ln 2
(
(1 - x1) ln
)
R1cmcmix x1cmc1
)
(1 - R1)cmcmix (1 - x1)cmc1
)1
(3)
by an iterative procedure using a computer; R1 is the mole fraction of component 1 in the whole solution, cmcmix is the cmc of the mixture, and cmc1 is the cmc of the pure component 1. The interaction parameter β then follows from eq 4.
ln β)
(
)
R1cmcmix x1cmc1
(1 - x1)2
(4)
In some mixtures, when one of the surfactants is ionic, its free monomer concentration m1 can be measured directly using a surfactant-selective electrode. In the simplest case, where mixed micelles show negative deviation from Raoult’s law, the free monomer concentration for the ionic component 1 (m1) at the cmc of the mixed micelle is given by19,20
m1 ) R1cmcmix ) cmc1x1 exp[β(1 - x1)2]
(5)
where cmc1 is the cmc of the pure ionic surfactant. Although the interaction parameter β is largely empirical, has little direct physical interpretation, and sometimes varies with concentration, it nevertheless provides a useful way to identify synergistic (β < 0) and antagonistic (β > 0) interactions between two surfactants in a mixture. The observation that the cmc’s of mixed F127/SDS micelles are less16 than those of the single components satisfies the criterion that their interaction is synergistic. We have shown recently that cationic surfactants also behave in a similar manner in the presence of both micellar and nonassociated single F127 molecules.17 In this study, we evaluated for the first time β parameters related to the synergistic interaction between SDS and F127 in the mixed micellar range. We also carried out small-angle neutron scattering measurements on various F127/SDS mixtures, using SDS-h12 and SDS-d12 for contrast variation to verify structural details. Experimental Section
(19) Rubingh, D. N. In Solution Chemistry of Surfactants; Mittal, K., Eds.; Plenum: New York, 1979; Vol. 1, p 337. (20) Holland, P. M. Adv. Colloid Interface Sci. 1986, 26, 111.
The Pluronic F127 with the composition EO97PO69EO97 and a molecular weight of 12 600 g mol-1 was a gift from BASF,
Interaction of Triblock Copolymers and Surfactants Mount Olive, NJ. SDS was purchased from Sigma-Aldrich and was purified by recrystallization with p.a. ethanol (three times). The SDS-d12 was supplied by R.K. Thomas,21 Oxford. The experimental methods used in this study were as follows. Differential Scanning Calorimetry (DSC). Measurements were made with a MicroCal MC-2 DSC instrument (MicroCal Inc., Northampton, MA) in the temperature range of 10-50 °C with the slow scanning rate of 0.5 °C/min. Almost all samples showed an endothermic transition peak of the heat capacity above the cmt. The cmt is defined as the temperature at the intersection with the baseline of the tangent drawn through the first inflection point of the heat capacity-temperature dependence curve.2-4,8b,13b,15 Isothermal Titration Calorimetry (ITC). The isothermal titration microcalorimeter used was the MicroCal Omega ITC instrument (MicroCal Inc., Northampton, MA). In ITC experiments, one measures directly the energetics (enthalpy changes) associated with processes occurring at constant temperature. Experiments were carried out by titrating micellar F127/SDS mixtures into water. An injection schedule (number of injections (50), volume of injection (5 µL with injection duration of 20 s), and time between injections (4 min and 400 rpm)) was set up using interactive software, and all data were stored on a hard disk. After each addition, the heat released or absorbed as a result of the various processes occurring in the investigated solution was monitored by the ITC microcalorimeter15-17,22 and recorded. Electromotive Force (emf) Measurements. A new coated wire surfactant membrane electrode23 selective to DS- was constructed at Salford and used to determine monomer surfactant concentrations below and above the cmc by measuring their emf relative to a commercial silver/bromide ion reference electrode. In practice, the emf data monitor the monomer surfactant concentration (m1) as a function of total surfactant concentration (C); when aggregation occurs, the difference (C - m1) represents the amount of associated surfactant. Light Scattering (LS). LS data were collected at 360 nm and 90° with a RF-5000 Shimadzu spectrofluorophotometer. A Haake F3-C bath was employed to control the sample temperature inside 0.1 °C, and the temperature was changed at a rate of 0.5 °C/min by using a Haake PG 20 programmable controller. No hysteresis was observed for the light scattering intensity when the sample was heated and then subsequently cooled. Surface Tension (ST). The axis-symmetric drop shape analysis (ADSA) technique24 (constructed in the Salford laboratories) was used to determine the surface tension of liquids from the shape of a pendant drop. The basic principle is to capture the drop image and detect the edge of the drop profile to find the initial parameters of the Laplace equation which are then used in a minimization procedure25 to evaluate the surface tension. The temperature-controlled chamber allows experiments to be performed within an accuracy of (0.1 °C. The system was tested with an ethanol-water mixture standard, established by Bircumshaw,26 and the surface tension results agreed very well in a range of (0.1 mN/m with the standard data. Dye Fluorescence. The fluorescence behavior of 1-3 diphenyl hexatriene (DPH), a non-water-soluble dye, was used to measure the cmc of the surfactant aggregates. When the surfactant forms micelles, the dye is solubilized in the hydrophobic part and exhibits a characteristic fluorescence spectrum which can be used to signal the occurrence of the cmc. DPH was supplied by Molecular Probes and used as received. A fresh solution of 0.4 mM DPH in methanol was prepared for each set of measurements. The final concentration of DPH in the polymer-surfactant mixture was 4 × 10-6 M corresponding to an amount of 1% v/v methanol.2,11a (21) Turner, S. F.; Clarke, S. M.; Rennie, A. R.; Thirtle, P. N.; Cooke, D. J.; Li, Z. X.; Thomas, R. K. Langmuir 1999, 15, 1017. (22) Olofsson, G.; Wang, G. In Polymer Surfactant Systems; Kwak, J. C. T., Ed.; Surfactant Science Series Vol. 77; Marcel Dekker: New York, 1998; pp 193-238. (23) Xu, R.; Bloor, D. M. Langmuir 2000, 16, 9555. (24) Susnar, S. S.; Neumann, A. W. Trans. Can. Soc. Mech. Eng. 2000, 24, 215. (25) del Rio, O. I.; Neumann, A. W. J. Colloid Interface Sci. 1997, 196, 136. (26) Bircumshaw, L. L. J. Chem. Soc., Faraday Trans. 1922, 1, 887.
Langmuir, Vol. 18, No. 24, 2002 9269 Small-Angle Neutron Scattering (SANS). The SANS measurements were performed on the LOQ diffractometer27 at the ISIS pulsed neutron source at the Rutherford Appleton Laboratory. The measurements were performed using the whitebeam time-of-flight method with a limited wavelength range at a source frequency of 50 Hz (giving a Q range of 0.02-0.15 Å-1). A beam aperture of 12 mm and a sample path length of 5 mm were used, for a sample temperature of 25 °C. Both the instrument configuration and sample geometry were optimized to give maximum sensitivity to low surfactant concentrations, over a limited Q range. The data were corrected for background scattering and detector response and converted to the scattering cross section (in absolute units of cm-1) using standard procedures.28 The SANS data were evaluated using established models for SDS micelles29,30 and F127 micelles.11a,30 For a solution of globular polydisperse interacting micelles, the coherent scattering cross section can be written by the so-called “decoupling approximation” (assuming that there are no correlations between position, orientation, and size):30,31
dσ (Q) ) Np[S(Q)|〈F(Q)〉Q|2 + 〈|F(Q)|2〉Q - |〈F(Q)〉Q|2] dΩ
(6)
Here the averages denoted by 〈 〉 are averages over particle size and orientation, Np is the particle number density, S(Q) is the structure factor, and F(Q) is the particle form factor. The micelles are modeled as a “core + shell”,29 and hence the form factor is given by eq 7:
F(Q) ) V1(F1 - F2)F0(QR1) + V2(F2 - Fs)F0(QR2)
(7)
where Vi ) (4πRi3)/3; F0(QR) ) 3j1(QR)/(QR); F1, F2, and Fs are the scattering length densities of the micelle core and shell and of the solvent; and j1(x) is the first-order spherical Bessel function. For the SDS micelles, the model comprises an inner core made up of the alkyl chains, constrained to space-fill a volume limited by a radius R1, and defined by the fully extended chain length of the surfactant. Remaining alkyl chains and headgroups with the corresponding hydration define the radius of the outer shell, R2. The interparticle interactions are included using the rescaled mean spherical approximation, RMSA, calculation32,33 for a repulsive (or attractive) Yukawa potential, where the surface potential is defined by the surface charge and the Debye screening length, κ-1. κ is represented in the usual form,
κ)
( ) 8πne2 kBT
1/2
(8)
and n is taken as the surfactant monomer concentration. The main adjustable model parameters are then the aggregation number (ν), surface charge (z), and polydispersity (σ). For the F127/SDS mixtures that are predominantly SDS micelles, an additional parameter, rp, which accounts for the polymer in the outer shell of the SDS micelles (as a fraction of the polymer chain), is included. The F127 micelles are described using a similar core-shell model, where R1 is defined by the PPO space-filling a spherical volume, with some fraction of hydration, wp, and a small fraction of the PEO chains, fe. R2, the outer radius, is then defined by space-filling the remaining PEO chains and associated hydration, (27) Heenan, R. K.; King, S. M.; Penfold, J. J. Appl. Crystallogr. 1997, 30, 1140. (28) Heenan, R. K.; King, S. M.; Osborn, R.; Stanley, H. B. RAL Internal Report; RAL-89-128; Rutherford Appleton Laboratory: Chilton Didcot, Oxfordshire, U.K., 1989. (29) Li, Y.; Xu, R.; Bloor, D. M.; Penfold, J.; Holzwarth, J. F.; WynJones, E. Langmuir 2000, 16, 8677. (30) Hayter, J. B.; Penfold, J. Colloid Polym. Sci. 1983, 261, 1022. (31) Hayter, J. B. In Physics of Amphiphiles, Micelles, Vesicles and Microemulsions; Degiorgio, V., Corti, M., Eds.; North-Holland: Amsterdam, 1992. (32) Hayter, J. B.; Penfold, J. Mol. Phys. 1981, 42, 109. (33) Hansen, J. P.; Hayter, J. B. Mol. Phys. 1982, 46, 651.
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we. Likewise for this model the main adjustable parameters are ν, z, σ, fe, wp, and we. The model was convoluted with the known instrument resolution and compared with the data on an absolute intensity scale on a least-squares basis. Acceptable model fits require not only that the shape of the scattering is reproduced but that the absolute value of the scattering cross section is reproduced; this is reflected in the value of the scale factor, f (data/theory), where an acceptable variation is ca. (10%.
Results The prerequisites in the application of RST to F127/ SDS mixed micelles using eqs 1-4 are values of the cmt and cmc of F127. The first parameter is required to plan the emf experiments and ensure that F127 micelles are present before adding SDS, and the cmc of F127/SDS mixtures is needed in the calculation of the interaction parameter β. Triblock copolymers are nonionic surfactants, which limits the number of techniques available to measure their cmc’s in comparison to those of ionic surfactants. In addition, the cmc of such nonionic surfactants occurs at very low concentrations and hence sensitivity is an important requirement. Moreover, these cmc’s are not as well-defined as those of conventional surfactants, in the sense that less sharp breaks or inflection points are observed; the cmc spans over a much larger concentration interval than observed with conventional surfactants. This is not surprising since the triblock copolymers are likely to contain small amounts of impurities like other triblock copolymers and in particular EO/PO diblock copolymers. We used commercially available F127, and therefore the batches can vary as well. In addition, the blocks in the copolymer are not completely monodisperse. Finally, their cmc’s are also sensitive to temperature, which is likely to extend the concentration range over which the cmc occurs. Despite these inherent problems, we have recently shown18b that ITC can be used successfully at 25 °C to measure the cmc of such nonionic block copolymer surfactants. From these data, a value of 0.563 × 10-3 mol dm-3 was obtained for F127 which can be compared with 0.479 × 10-3 mol dm-3 obtained in the present work from ST measurements. This difference is acceptable in the context of the above discussion. In an attempt to get a better estimate of the cmc, we prepared solutions of F127 in water around those critical concentrations and carried out DSC measurements. The best estimate for the cmc is 0.563 × 10-3 mol dm-3, which gives a cmt of 25 °C. In the following treatment of the interaction parameter β, we used this value for F127. Measurements Carried Out at 25 °C. (a) Critical Micelle Concentration Data. At 25 °C, the ITC and ST methods were used in a complementary fashion to measure the cmc’s of F127 and F127/SDS mixtures. In the ITC experiment, a mixture of F127/SDS of known composition (molar ratio) is titrated into water and the cmc is determined from the inflection point (corresponding to the maximum in the first derivative according to Blume et al.34) of the enthalpy profile shown in Figure 1. In a similar way, the ST is plotted against different concentrations of F127/SDS mixtures for which the molar ratio is kept constant. The cmc can be measured from the minimum in a typical curve plotted in Figure 2. The good agreement shown by the two methods is illustrated in Figure 3 in which the cmc’s of various mixtures are given. We also prepared some solutions at their measured cmc’s (34) Paula, S.; Su¨s, W.; Tuchtenhagen, J.; Blume, A. J. Phys. Chem. 1995, 99, 11742 and references therein.
Figure 1. Graph of the enthalpy change ∆Hi from ITC experiments as a function of total F127 concentrations for the F127/SDS mixture with the molar ratio XF127 ) 0.243 at 25 °C. The maximum in the first derivative curve corresponds to the cmc of the mixture (see ref 34).
Figure 2. Surface tension measurements for different SDS and F127 concentrations, the F127 molar ratio being constant and equal to 0.4, at 25 °C.
Figure 3. Critical micelle concentration data as a function of F127 mole fraction from ITC (9) and surface tension ([) measurements for F127-SDS mixtures at constant molar ratios; T ) 25 °C. The solid line corresponds to the calculated cmc’s according to RST (refs 19 and 20) with β ) -7.4.
and carried out DSC experiments. In all cases, the measured cmt’s were ca. 25 °C as expected (Figure 4). For F127/SDS mixtures exceeding an SDS mole fraction of 0.8, the ITC profiles display two separate stepwise changes in ∆Hi of different sign as shown in Figure 5. The first exothermic process involves a large change in ∆Hi and is associated with the dissociation of the F127/SDS mixed micelles.15 The second smaller endothermic process reflected in ∆Hi occurs at higher SDS concentration and
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Figure 4. Graph of the heat of capacity Cp (J/cal) as a function of temperature for the F127-SDS mixture at the F127 molar ratio of 0.2; scan rate ) 0.5 °C/min.
Figure 6. (a) Graph of the emf of the SDS-sensitive electrode (reference bromide ion (Br-) electrode) as a function of total SDS concentrations in 10-4 mol dm-3 NaBr: (O) pure SDS and (2) SDS/3% w/v F127 block copolymer in D2O at 15 °C; Conset (or T1) denotes the binding of SDS aggregates to single F127 units, and Csat (or T2) denotes the saturation of F127 by SDS micelles. (b) Binding isotherm for SDS to 3% w/v F127 at 15 °C in D2O using data from (a). Figure 5. Graph of the enthalpy change ∆Hi from ITC experiments as a function of total SDS concentrations for the F127-SDS mixture with the molar ratio XF127 ) 0.12 at 25 °C.
is associated with the tail end of the binding process involving the formation of SDS aggregates bound to single F127 units.16 In this range, DSC studies show that no F127 aggregates are present. The absence of this feature at F127/SDS molar ratios less than 0.8 in SDS is probably associated with the presence of F127 aggregates, which dominate the measured enthalpic process. From the data in Figure 3, we can evaluate the β parameter using eqs 1-4 as described previously.18b,19,20 The calculated values of the cmc using a β of -7.4 are shown as a solid line in Figure 3, and the points are experimental data. In this approach to evaluate β, the monomer concentration of the ionic surfactant is inferred from cmc data. On the other hand, if we use the surfactant-selective electrode we can measure this quantity directly from experiments as described in the following section. (b) Electromotive Force Data. Monomer surfactant concentrations of SDS in the presence of micellar F127 were measured using a new coated wire selective electrode23 whose emf is determined relative to a Br- electrode. First, the emf of the surfactant electrode relative to the reference was measured for pure SDS solutions at increasing surfactant concentrations well into the micellar range. All solutions were doped with 10-4 mol dm-3 NaBr to supply a constant reference voltage. At concentrations below the cmc of SDS, all of the surfactant is in monomeric
form and the respective emf’s yield good Nernstian responses according to eq 9: 0 EDS-/Br- ) EDS -/Br- -
RT ln m1 F
(9)
0 where EDS -/Br- is a constant and m1 refers to the free surfactant monomer concentration. In eq 9, DS- and Brhave the same charge and the ratio of their activity coefficients may be assumed to be unity. Typical emf plots of the SDS electrode in the presence and absence of F127 are shown in Figures 6a and 7a. At the cmc, a sharp break is observed in the data for SDS alone because the emf of the cell only monitors m1 which decreases with increasing surfactant concentration above the cmc. The experiment was then repeated by measuring the relative emf’s in the presence of a constant amount of F127. The existence of a binding process is clearly demonstrated in Figures 6a and 7a for 3% w/v F127/SDS in D2O at 15 and 27.2 °C, respectively, when the emf’s with and without the polymer are different (the cmt of a 3% w/v F127 aqueous solution is 21 °C).2 The binding isotherms are given in Figures 6b and 7b. They clearly show an increase in the SDS monomer concentration as more SDS is bound until the turning point (corresponding to Csat) of the curve, after which the SDS monomer concentration decreases while bound SDS is still increasing. In the presence of unassociated monomeric F127 at 15 °C, the data represent the binding of SDS aggregates to unassociated F127 units (the cmt of a
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Figure 7. (a) Graph of the emf of the SDS-sensitive electrode (reference bromide ion (Br-) electrode) as a function of total SDS concentrations in 10-4 mol dm-3 NaBr: (O) pure SDS and (2) SDS/3% w/v F127 block copolymer in D2O at 27.2 °C; Csat (or T2) denotes the saturation of F127 by SDS micelles. (b) Binding isotherm for SDS to 3% w/v F127 at 27.2 °C in D2O using data from (a).
3% w/v F127 aqueous solution is 21 °C).2 As we have shown previously,15,16 binding starts at an SDS concentration of Conset (or T1) when SDS aggregates start forming on the polymer and as the binding proceeds, the bound aggregates grow in size and number until the polymer is fully saturated with bound SDS aggregates at Csat (or T2). In the presence of micellar F127 (Figure 7a,b) at 27.2 °C, binding occurs even at the lowest SDS concentration measured, because the emf of the cell deviates from Nernstian behavior. This binding leads to the formation of F127/SDS mixed micelles, and at each total SDS concentration, C, in the mixed micellar range the free monomer SDS concentration, m1, can be evaluated from the emf data using eq 9. These data allow the interaction parameter β to be calculated from eq 5 in the mixed micellar range. In such a calculation, it is generally assumed that since the cmc of the block copolymer is extremely small the micellar concentration of F127 is the same as the weighed-in concentration. Unfortunately, this is not the case at 25 and 27.2 °C for F127/SDS since the cmc of F127 is 0.563 × 10-3 mol dm-3 at 25 °C and 0.248 × 10-3 mol dm-3 at 27.2 °C.2 We have therefore made an allowance for such a behavior in the β calculation by assuming that for any mixture of F127/SDS measured in the emf experiments the cmc can be interpolated from Figure 3. From these extrapolations, the amount of monomeric F127 at the estimated cmc follows. β is then plotted as a function of SDS concentration in the SDS concentration range where mixed micelles are exclusively formed (Figure 8a-c). In Figure 8a,b, the β parameters
Thurn et al.
Figure 8. (a) Graph of the interaction parameter β against total SDS concentrations ([) for F127 (3% w/v)/SDS in H2O at 25 °C and (0) for F127 (3% w/v)/SDS in D2O at 27.2 °C from emf. (b) Graph of the interaction parameter β against total SDS concentrations ([) for F127 (3% w/v)/SDS and (×) F127 (0.625% w/v)/SDS at 25 °C and (0) for F127 (3% w/v)/SDS in D2O at 27.2 °C (all emf), (4) for data from surface tension measurements at 25 °C, and (O) for data from ITC measurements at 25 °C. (c) Graph of the interaction parameter β against total SDS concentrations for (1) F127 (3% w/v)/SDS in D2O and (]) F127 (0.5% w/v)/SDS at 35 °C from emf.
calculated from emf electrode data on the following F127/ SDS mixtures are shown: 3% w/v F127/SDS in H2O at 25 °C, 0.625% w/v F127/SDS in H2O at 25 °C, and 3% w/v F127/SDS in D2O at 27.2 °C. The β values calculated at 25 °C from the cmc data described earlier are also included in the diagrams of Figure 8b. A mean β value for the F127/SDS system from emf data can be estimated as -4.5 at 25 °C, which indicates a strong synergy between the block copolymer and SDS. This synergy is stronger than that for the system F127/ C12EO6 (β ) -2.7).18b We can notice that the β values obtained from emf data are different from those from ITC and surface tension fittings as shown in Figure 8b. This might be caused by the fact that eq 5 is valid only very near to cmcmix and at higher F127/SDS molar ratios. As soon as SDS is causing mixed F127/SDS micelles to break down, β increases dramatically as seen in Figure 8b (×) for emf data and the application of eq 5 is no longer useful. Measurements Carried Out at 35 °C. We experienced serious difficulties in our attempt to measure the cmc of F127 at 35 °C. The cmc is too low to make any realistic progress with ITC or DSC. On the other hand, both ST and the dye fluorescence methods give values in the range of ca. 2 × 10-6 mol dm-3. This value is less than the 2 × 10-5 mol dm-3 which showed a cmt in the light scattering at ca. 35 °C. Finally, we have attempted to extrapolate a value for this cmc from cmt measurements using DSC in the range of 15-35 °C. Here we have assumed that ∆H for micellization is constant and plotted ln(1/Ci) against 1/(cmti + 273) on the assumption that the Van’t Hoff equation is valid. Here cmti is the cmt for F127 solutions
Interaction of Triblock Copolymers and Surfactants
Langmuir, Vol. 18, No. 24, 2002 9273 Table 1. First and Second Minima from Surface Tension Measurements for F127-SDS Mixtures with Different Molar Ratios at 35 °C
Figure 9. Surface tension measurement for the addition of SDS to 0.5% w/v micellar F127 at 35 °C (9) and to 0.5% w/v monomeric F127 at 15 °C (O) as a function of total SDS concentration.
of concentration Ci. This procedure gives an estimate of 2.5 × 10-5 mol dm-3 for the cmc of F127 at 35 °C. Since we also experienced similar difficulties with ITC for F127/ SDS mixtures at 35 °C, we have confined our determination of the β parameter exclusively to emf measurements at 35 °C. In the evaluation of the amount of F127 and other Pluronics in the mixed micelles, their cmc’s at 35 °C are negligible in comparison with the weighed-in concentrations. The β parameters obtained are shown in Figure 8c. A mean β value of -5.5 is obtained at 35 °C. It was not possible to calculate β from the ST data at 35 °C because of the large difference in the cmc’s (2 orders of magnitude between F127 and SDS at 35 °C). Surface Tension Measurements. We performed surface tension measurements for 0.5% w/v F127 with added SDS (shown in Figure 9) for both unassociated F127 (at 15 °C) and micellar F127 (at 35 °C) to compare information given by the shape of the curve with former ITC and emf results.15,16 Figure 9 shows the critical concentrations which are described in the preceding text. The two curves exhibit flat variations (max 6 mN/m) in surface tension when the SDS concentration increases, certainly due to the weak influence of SDS on the surface tension. The low values of surface tension (,72 mN/m for water) at an SDS concentration below 10-5 M also indicate the strong influence of the surface active F127. At 35 °C, the first measured surface tension is 37.5 mN/m which is lower than that at 15 °C, as expected. In the SDS range up to 10-3 mol dm-3, the ST data exhibit a different behavior for 15 and 35 °C. The ST decreases with increasing SDS concentration at 15 °C, but at 35 °C there is evidence of a first slight increase in ST, corresponding to the onset of breakdown of F127/SDS mixed micelles (see the LS experiment in Figure 4 of ref 15). At 15 °C, below Conset no interaction takes place between SDS and monomeric F127 and the decrease in ST is simply due to the increase in surface activity from the addition of monomeric SDS at the air/water interface. This decrease stops when the SDS concentration reaches Conset (or T1) equal to 0.35 × 10-3 mol dm-3. At 35 °C, most of the added SDS is used for the formation of F127/SDS mixed micelles. At 35 °C, the decrease in ST, which starts around 2.5 × 10-4 mol dm-3 SDS, is probably due to the increased surface activity resulting from F127 monomers being released into solution and hence the air/liquid interface, caused by the breakdown of the F127/SDS mixed micelles. The increase in ST following the binding of SDS aggregates to monomeric F127 at both 15 and 35 °C is due to the loss
XF127a
first minimum 10-4 mol/kg
[F127] % w/v
[F127] 10-4 mol/kg
[SDS] 10-4 mol/kg
0.05 0.1 0.2 0.4 0.6 0.8 1
1.69 1.09 1.20 0.278 0.494 0.697 0.0120
0.0105 0.0136 0.0301 0.0139 0.0371 0.0558 0.0015
0.084 0.109 0.241 0.111 0.297 0.446 0.0120
1.60 0.980 0.962 0.167 0.198 0.251 0
XF127a
second minimum 10-4 mol/kg
[F127] %w/v
[F127] 10-4 mol/kg
[SDS] 10-4 mol/kg
0.05 0.1 0.2 0.4 0.6 0.8 1
2.73 1.78 2.34 1.10 9.38 6.08 0.0259
0.0170 0.0223 0.0585 0.0549 0.703 0.608 0.0032
0.136 0.178 0.468 0.439 0.563 0.486 0.0259
2.59 1.60 1.87 6.59 3.75 1.22 0
a
F127 molar ratio.
in the surface activity of the F127 monomers which contain bound SDS aggregates and thus behave as polyelectrolytes, leaving the air/water interface. Finally, the decrease in ST is followed by the merging of the two curves at Csat (or T2) which is consistent with the formation of free SDS micelles in solution. We can notice a levelling off of the surface tension which does not represent the decrease of monomer SDS concentration observed in the binding isotherm after Csat (T2) in Figures 6b and 7b. We also now present the ST measurements for fixed F127/SDS molar ratios which were taken at 35 °C. The ST data are characterized by two minima which are listed in Table 1 for different F127/SDS molar ratios. We believe that the first minimum is associated with the cmc of F127/ SDS mixed micelles. On this basis, when solutions were prepared at these critical concentrations and their DSC was measured, one would expect that all the DSC data should have the same cmt, that is, 35 °C. When the DSC experiments were performed, all the cmt values were constant and equal to 28 °C. These data are consistent with the minimum in ST being a cmc for the mixed micelles, but we are unable to explain why the cmt is 28 °C and not 35 °C as expected. The other noteworthy feature is that the DSC peaks are very weak and a precise evaluation of the cmt is very difficult, with errors of several degrees Celsius. The second minimum in the ST always lies in the range (1-6) × 10-4 mol dm-3 SDS which is a good indication that it is associated with Conset (or T1), the onset of binding of SDS aggregates to the single F127 units. SANS Measurements. (a) F127/D2O. SANS measurements were made for 3% w/v F127/D2O at 25 and 27.5 °C and for 0.05% w/v F127/D2O at 15, 20, 30, and 35 °C. The core and shell model described earlier provided an adequate, but not perfect, fit to the 3% w/v F127/D2O data at 27.5 °C (see Figure 10), and the parameters are summarized in Table 2. The aggregation number and radii obtained are similar to those previously reported.9,11a We found that 20% of the PPO core is hydrated and ∼60% of the outer corona is hydrated, and similar values were obtained by Yang et al.11a A notable feature is the difference between the F127 solution concentration and that deduced in order to obtain the correct absolute scaling (0.6 × 10-3 M compared to 2.4 × 10-3 M or 3% w/v). We attribute this to a larger monomer concentration and still high degree of solvation due to the
9274
Langmuir, Vol. 18, No. 24, 2002
Thurn et al.
Figure 10. Scattering cross section, dσ/dΩ (cm-1), for 3% w/v F127/D2O at 27.5 °C. The insert corresponds to the scattering cross section, dσ/dΩ (cm-1), for 0.05% w/v F127/D2O at 30 °C; solid lines are calculated curves for model parameters described in the text.
Table 3. Model Parameters for 3% w/v F127/SDS Micelles from SANS Measurementsa
Table 2. Model Parameters for the F127/D2O System from SANS Measurementsa sample
ν
R1
R2
σ
fe
wp
we
f
3% w/v F127 at 27.5 °C 0.05% w/v F127 at 35 °C
20.0
35.7
57.7
0.3
0.2
50.0
800
1.1
37.0
43.5
73.2
0.3
0.1
50.0
800
0.97
a The main adjustable model parameters are the aggregation number (ν), surface charge (z), and polydispersity (σ). For the F127 micelles, R1 is defined by the PPO space-filling a spherical volume, with some fraction of hydration, wp, and a small fraction of the PEO chains, fe. R2, the outer radius, is then defined by space-filling the remaining PEO chains and associated hydration, we.
proximity of the cmt. This has not been reported in other related studies,9,11a but these data were measured at a temperature further from the cmt. The large polydispersity, σ ∼ 0.3, required to fit the data is an indication of the less well-defined aggregates. It did not prove possible to obtain a satisfactory model fit for the data at 25 °C, and this is probably due to the aggregation state and hence the structure being even less well-defined at a temperature so close to the cmt. Refinements of the model, such as those developed by Pedersen,35 did not improve the analysis. The data for 0.05% w/v F127/D2O showed no significant scattering (no aggregation) at 15, 20, and 30 °C (see insert in Figure 10), and the data are consistent with monomer scattering. However, at 35 °C there is a significant increase in the amount of scattering, indicating the presence of F127 aggregates (the core-shell model was consistent with the data; see also Table 2 for the model parameters). Compared to the parameters for 3% w/v F127 at 27.5 °C, R1, R2, and ν for 0.05% F127 at 35 °C are systematically larger, reflecting micelle growth due to the higher temperature relative to the cmt. (b) F127/SDS/D2O. At 3% w/v F127, data for a series of different SDS concentrations and temperatures were measured. For the measurements of 3% w/v F127/0.1 M SDS at 36.5 °C (see insert in Figure 11) and for 3% w/v F127/40 mM SDS at 25 °C, the scattering is predominantly from SDS micelles. The emf data15 at 25 °C suggest that these measurements were carried out in the SDS range where SDS aggregates are exclusively formed on monomeric polymer chains. The concentrations of the bound SDS are 95 and 37 mM for the 0.1 and 40 mM SDS solutions, respectively. (35) Pedersen, J. S. J. Chem. Phys. 2001, 114, 2839.
Figure 11. Scattering cross section, dσ/dΩ (cm-1), for 3% w/v F127/D2O/6 mM SDS ((b) h-SDS, (O) d-SDS) at 25 °C. The insert corresponds to the scattering cross section, dσ/dΩ (cm-1), for 3% w/v F127/0.1 M SDS at 36.5 °C; the solid line is the calculated curve for model parameters described in the text.
sample
ν
z
R1
R2
σ
rp
f
3% w/v F127/0.1 M SDS at 36.5 °C
46
12
15.3
18.9
0.12
0.5
1.0
a For the SDS/F127 mixtures that are predominantly SDS micelles, an additional parameter, rp, which accounts for the polymer in the outer shell of the SDS micelles (as a fraction of the polymer chain), is included.
For the combination 3% w/v F127/0.1 M SDS, the scattering is well described by the model for SDS micelles with some polymer incorporated into the outer shell of the micelle (see the value of rp in Table 3 and insert in Figure 10). Here the maximum of SANS is shifted to higher scattering vectors (0.075 Å-1). The aggregation number, ∼45, is consistent with previous observations36 but low compared to that of free SDS micelles at similar SDS concentrations. The amount of polymer in the outer shell of the micelles occupies 75% of the corona volume. The data for 3% w/v F127 and 6 mM (see Figure 11) and 10 mM SDS were measured at 25 °C, for both h-SDS and d-SDS. We noticed that for the mixture 3% w/v F127/6 mM SDS the scattering cross section in the presence of h-SDS (∼1.6 cm-1) is greater than in the presence of d-SDS (∼1.2 cm-1). The emf binding data at 25 °C and also the light scattering15,16 measurements showed that two binding processes are taking place simultaneously, namely, the formation of F127/SDS mixed micelles and also bound SDS aggregates to monomeric F127. The total amount of bound SDS is 5 and 9 mM, respectively. In addition to providing access to the structure of the mixed aggregates, the measurements with the two differently labeled SDS samples provided information about the composition. Analysis, following the procedure developed for mixed surfactant micelles by Staples et al.,37 gave polymer volume fractions of 0.89 (for 6 mM SDS) and 0.79 (for 10 mM SDS). The data are consistent with the presence of F127/ SDS mixed micelles. The simultaneous existence of F127/ SDS mixed micelles and some bound SDS aggregates to monomeric F127 prevented the perfect fitting of the SANS data. (36) Li, Y.; Xu, R.; Couderc, S.; Bloor, D. M.; Warr, J.; Penfold, J.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 2001, 17, 5657. (37) Penfold, J.; Staples, E.; Thompson, L.; Tucker, I.; Hines, J.; Thomas, R. K.; Lu, J. R. Langmuir 1995, 11, 2496.
Interaction of Triblock Copolymers and Surfactants
Langmuir, Vol. 18, No. 24, 2002 9275
Conclusion
Table 4. Model Parameters for 3% w/v F127-SDS Mixed Micelles at 27.5 °C from SANS Measurements sample
ν
R1
R2
σ
fe
wp
we
f
1 mM SDS 2 mM SDS
21 15
37.4 33.9
60.3 54.8
0.2 0.2
0.2 0.2
50 50
800 800
1.05 1.01
Similar measurements were made using h-SDS and d-SDS for 3% w/v F127 and SDS concentrations of 10-3 and 2 × 10-3 M at 27.5 °C. These data were taken in the region where SDS exclusively forms mixed micelles with F127. The maximum of the peak is shifted to higher scattering vectors but very near to that of pure F127 micelles (see Figure 10). From the emf data, we calculated the amount of SDS in these mixed micelles as 0.8 and 1.5 mM. The SANS data gave polymer volume fractions of 0.97 (for 10-3 M SDS) and 0.94 (for 2 × 10-3 M SDS). For these combinations, the scattering is dominated by F127 micelles, and the small volume fraction of SDS contributes little to the scattering. The model parameters are summarized in Table 4 and are similar to those reported for 3% w/v F127/D2O at 27.5 °C. Even at such low SDS concentrations as 1 and 2 mM, the addition of the SDS progressively reduces the F127 aggregation number. However, the SANS measurements are not sufficiently sensitive to indicate the location of the SDS within the predominantly F127 micelles, but it can be assumed that SDS is not aggregated with other SDS molecules. We believe that the absence of SDS scattering is a strong indication for this.
We applied four different methods (emf, ST, ITC, and SANS) to characterize the interaction of SDS with monomeric and micellar F127. Three modes of interaction could be identified: (i) binding of SDS aggregates to F127 after Conset (or T1) for SDS is reached; (ii) binding of SDS, most likely in monomeric form to F127 micelles, forming mixed aggregates; (iii) initially a reduction in F127 aggregate size and finally the complete dissociation of F127 micelles, which results in F127 monomers saturated with SDS aggregates. The emf measurements provided the free SDS monomer concentrations under the different conditions and clearly showed that after the onset of SDS binding to F127 either to its monomers or micellar aggregates, the SDS monomer concentration is continuously increasing until free SDS micelles are formed. A further increase of total SDS concentration results in a decrease of the free SDS concentration due to electrostatic shielding effects. The application of the regular solution theory in combination with surface tension and ITC results allowed the calculation of an interaction parameter β for F127/SDS mixed micelles of approximately -7.4, signaling strong synergistic effects in such mixed micelles. SANS experiments in the range of the mixed micelles allowed the evaluation of the composition of the aggregates showing no SDS aggregation but confirmed a micellar model with partly hydrated hydrophobic PPO in the core and almost fully hydrated PEO in the corona. LA020629A
