Langmuir 1998, 14, 4409-4414
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Interactions between Nonionic Surfactant Micelles Introduced by a Telechelic Polymer. A Small Angle Neutron Scattering Study Jacqueline Appell,*,† Gre´goire Porte,*,† and Michel Rawiso‡ Groupe de Dynamique des Phases Condense´ es, UMR5581 CNRSsUniversite´ Montpellier II, C.C. 26, 34095 Montpellier Cedex, France, and Institut Charles Sadron CNRS-ULP, 6 rue Boussingault, 67083 Strasbourg Cedex, France Received November 14, 1997. In Final Form: March 10, 1998 The nonionic surfactant hexa(ethylene glycol) mono-n-dodecyl ether (C12EO6) self-assembles in aqueous solutions to form small globular micelles. Progressive substitution of C12EO6 by a telechelic polymer (a water soluble poly(ethylene oxide) (PEO) chain grafted at both extremities with hydrophobic C12H 25 chains (PEO-2m)) is shown to preserve the initial C12 hydrophobic cores, at least up to 10 polymers per hydrophobic core, but to introduce additional interactions between the micelles. At low micellar concentrations (∼ 0.1 Å-1) where the intensity reflects essentially the form factor P(q) of the HC (S(q)f1). This provides the experimental evidence in agreement with our initial assumption that, for the spatial resolution of our scattering experiments (π/qmax = (π/0.2) ∼ 16 Å) the HCs remain unchangd upon substitution of C12EO6 by PEO-2m at least up to r ) 20; that is, 20 C12EO6 over 100 have been replaced in the solution by 10 PEO-2m. This is even clearer in Figure 4b where the product I(q) × q4 is plotted as a function of q and where the first oscillation clearly coincides for all four samples. From the maximum of the oscillation, which, for spheres, occurs at qR ) π(3/2)1/2, we can estimate R ) 21 ( 2 Å, in good agreement with the values derived above for the HCs of the C12E6 micelles. The HCs are thus found to remain identical over the explored r range. If we now examine the patterns especially at small q’s, they clearly indicate a dramatic change in the intermolecular correlation term even with the smallest r value (cf. Figure 4a). The structure factors have been extracted from the scattering patterns using relation 1 and, assuming that A × P(q) ) I(q), the scattering pattern from pure C12D-E6 solution. They are shown in Figure 5 and are characteristic of a repulsive interaction between the HCs. The correlation peak has the same position qmax for all three patterns, leading to the mean separation distance between HCs
d)
2π ∼ 110 qmax
From the observed decrease with r of S(qf0), which corresponds to a decrease of the osmotic compressibility (cf. relation 3), we can infer that the repulsive interaction becomes stronger with increasing r. However it is worth stressing that S(qmax) does not show such a drastic change as χT when r increases. This is in strong contrast to the observations and predictions on star polymers or flowerlike clusters, formed by associating diblock copolymers, where a strong increase of S(qmax) is found in correlation to the decrease of χT.26,27 From relations 3 and 5 we deduce A1 from the value of S(qf0). In Figure 6, A1 is plotted as (26) Witten, T. A.; Pincus, P. A.; Cates, M. E. Europhys. Lett. 1986, 2, 137. Witten, T. A.; Pincus, P. A. Macromolecules 1986, 19, 2509.
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Figure 6. Evolution of A1 )
1 -1 S(qf0)
with r. The straight line is the best fit to the points and corresponds to A1 ) 0.86r.
Figure 8. SANS patterns of the whole aggregates (i.e. C12EO6 + PEO-2m) in D2O (T ) 18 °C) for CC12 ) 0.023 M: (8a) crosses, r ) 0; triangles, r ) 1.2; squares, r ) 4. (8b) crosses r ) 0; circles, r ) 40.
Figure 7. SANS patterns of the whole aggregates (i.e. C12EO6 + PEO-2m) in D2O (T ) 18 °C) for CC12 ) 0.115 M: crosses, r ) 0; squares, r ) 4; circles, r ) 10; triangles, r ) 20; times signs, r ) 40.
a function of r. A1 is positive and increases linearly with r: A1 ) 0.86r. From the SANS patterns of the HC we thus obtain evidence that the HCs remain identical and that the PEO chains lead to an effective interaction between HCs which is repulsive for CC12 ) 0.115 M. SANS Patterns of the Complete Aggregates in D2O. As stated above we can only comment qualitatively on these patterns. We mainly want to stress here the differences observed in the scattering patterns of solutions with CC12 ) 0.023 M and with CC12 )0.115 M. The scattering patterns for solutions with CC12 ) 0.115 M shown in Figure 7 have roughly the same features as those presented and discussed above for the HC alone: at high q’s they superimpose reasonably well, indicating again the conservation of the HC. The continuous change in the corona of ethylene oxide groups is probably the main reason for the superposition to be less perfect than that in the previous patterns; at low q’s, the correlation (27) Richter, D.; Jucknischke, O.; Willner, L.; Fetters, L. J.; Lin, M.; Huang, J. S.; Roovers, J.; Toporouski, P. M.; Zhou, L. L. J. Phys. IV Suppl. 1993, 3, 3. Grest, G. S.; Fetters, L. J.; Huang, J. S.; Richter, D. in Advances in Chemical Physics; Prigogine, I., Rice, S. A., Eds.; Wiley & Sons: New York, 1969; Vol. XCIV, p 67.
peak becomes sharper with increasing r, showing qualitatively the same behavior as discussed above. The scattering curves for solutions with CC12 ) 0.023 M are shown in Figure 8. Here again at high q’s the patterns superimpose but at low q’s the evolution of the patterns with increasing r is very different. In fact the SANS measurements are done at T ) 18 °C, so that we could not study samples with 4 < r < 30 which are biphasic. In Figure 8a we compare the scattering pattern of pure C12EO6 with those for solutions corresponding to small r, very close to the phase separation described above (see Figure 2). Upon inspection of Figure 8a it is clear that the scattered intensity for the mixed solutions increases strongly at small q values as compared to that for the C12EO6 solution: this is characteristic of an effective attractive interaction which is presumably at the origin of the demixtion and is, at least partly, due to the bridging PEO chains, as described below. For samples with large r values, here r ) 40, we are about 10 °C below the temperature of phase separation but the SANS pattern (Figure 8b) reveals that the overall interaction is repulsive; then the attractive part of the interaction due to bridging PEO (and possibly the beginning of dehydration of the EO groups) is overwhelmed by an effective repulsive interaction due to the numerous surrounding PEO chains. Discussion The first conclusion that can be drawn from the experimental results is that in mixed solutions of C12EO6 and PEO-2m, at least in the range of composition explored, both constituents form mixed hydrophobic cores which
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remain essentially identical with on the order of 100 C12 chains. Thus the gain in free energy for the telechelic polymer to insert both its end chains in the HC apparently more than counterbalances the loss of entropy for the polymer. Before discussing the interactions introduced between the HCs by the decoration or bridging by the PEO chains, it is worthwhile to point out that unmodified PEO does not interact with C12EO6. In fact the existence of interactions between unmodified PEO and surfactant molecules depends on the nature of the surfactant. Binding interactions have been reported when the surfactant is an anionic one such as sodium dodecyl sulfate.13,28 But both a cationic surfactant such as cetyltrimethylammonium bromide and the nonionic surfactant C12EO6 have been found to have no interactions with PEO.13 This absence of interaction between a nonionic surfactant and PEO has also been confirmed in a recent study28 on mixed solutions of PEO and droplets of an oil in water microemulsion in which a mixture of TritonX is the surfactant (TritonX’s are commercial surfactants with small PEO chains as polar heads). As described above interactions are indeed introduced between the HCs when an increasing number of PEO chains are attached to them. The experimental evidences obtained are insufficient to give a complete and consistent picture of the situation. At the highest concentration explored the interaction is repulsive even with a very low number of PEO chains per HC. On the contrary at the lowest concentration a small number of PEO chains per HC leads to an effective attractive interaction. To describe qualitatively a possible origin for these interactions, it is interesting to compare the mean distance between the interacting aggregates with their dimension. As a first approximation we assume, as is customary, that the PEO chains linked by the two extremities to the HC retain a conformation close to that of a swollen coil of free PEO of half mass in water. The relation29,30 Rg ) (0.107 ( 0.001)M0.63(0.01 yields the radius of gyration of a coil of polymer with molar mass M. The interacting aggregates are HCs surrounded by a corona composed essentially of the PEO coils and we can then make a very rough estimate of the overall radius Rov ∼ 21 + 2 × 22 ∼ 65 Å. The mean distance between HCs can be evaluated from the concentration CC12 and from the aggregation number of HC ∼ 100 obtained for the samples with CC12 ) 0.115 M. The same aggregation number is assumed when CC12 ) 0.023 M (in fact this assumption is supported by the published studies (28) Filali, M.; Appell, J.; Aznar, R.; Porte, G. To be published. (29) Cabane, B.; Duplessix, R. J. Phys. (Paris) 1982, 43, 1529. (30) Benkhira, A.; Franta, E.; Rawiso, M.; Franc¸ ois, J. Macromolecules 1994, 27, 3963.
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of the C12EO6 micelles which are found to remain identical over a large range of concentration and also by a comparison of the patterns obtained for C12EO6 micelles at the two concentrations). We thus obtain d ∼ 110 Å for CC12 ) 0.115 M and ∼200 Å for CC12 ) 0.023 M. If we compare these values with the above estimation of the overall size of the aggregates pictured as “flower” type clusters, we note that the two concentrations of HCs studied correspond to two different situations: at CC12 ) 0.023 M the solutions are in the dilute regime, and at CC12 ) 0.115 M the solutions are at the overlap concentration c* for the polymer corona. If the PEO-2m decorates the micelles, forming flowerlike micelles, only an effective repulsive interaction should result, since the “flowers”, as star molecules, resist compression and overlap. The observation of an effective attractive interaction, when d ∼ 200 Å > 2Rov ∼ 130 Å, is probably an indication that PEO-2m bridges, at least partially, the micelles. When d ∼ 110 Å ∼ 2Rov, even bridging polymers will resist compression and the overall resulting interaction becomes repulsive. To speculate further, new experimental evidences are necessary and work is currently in progress to examine the interactions under other circumstances: varying d, r, and the mass of the PEO chain and also studying the interactions introduced by a polymer hydrophobically modified at one extremity alone so that bridging will be excluded. Finally we can emphasize that attempts to use a classical model potential, such as a repulsive Yukawa potential, to mimic the observed structure factors, for r > 0 and when the interactions are repulsive, proved unsuccessful. This is another indication that bridging does occur and plays a specific role, lowering the effective repulsive interaction. This attractive component cannot be neglected. Indeed simple flowers could be expected to behave like star molecules for which the Yukawa type screened potential is known to describe properly the osmotic repulsion.26 The observed facts ought to be a challenge for theoreticians, since the development of model potentials seems to be necessary in order to understand even such a simple situation as the one selected and described here. Acknowledgment. This work has received support from PIRMAT-CNRS. The authors thank Roland May, their local contact at ILL, for his help and precious advice during the SANS measurements. Thanks are also due to Ge´rard Beinert and Franc¸ ois Isel, who synthesized the PEO-2m and C12EO6 used in this study. Finally the authors gratefully acknowledge Jeanne Franc¸ ois for fruitful discussions. LA9712395
