Binding of Sodium Dodecyl Sulfate (SDS) to the ... - ACS Publications

(a) Li, Y.; Xu, R.; Bloor, D. M.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 2000, 16, 10515. ..... Ghoreishi, S. M.; Li, Y.; Holzwarth, J. F.; Khoshdel...
0 downloads 0 Views 204KB Size
Download PDF
Langmuir 2001, 17, 183-188

183

Binding of Sodium Dodecyl Sulfate (SDS) to the ABA Block Copolymer Pluronic F127 (EO97PO69EO97): F127 Aggregation Induced by SDS Y. Li,† R. Xu,‡ S. Couderc,† D. M. Bloor,‡ E. Wyn-Jones,*,†,‡ and J. F. Holzwarth*,† Fritz-Haber-Institut der Max-Planck Gesellschaft, Faradayweg 4-6, D-14195 Berlin-Dahlem, Germany, and School of Sciences, Chemistry, University of Salford, Salford M5 4WT, U.K. Received July 28, 2000. In Final Form: October 23, 2000 It has been established that sodium dodecyl sulfate (SDS) binds to the micelles and monomers of the block copolymer F127. SDS binds to the monomeric unassociated F127 in the form of polymer/bound SDS micellar complexes. SDS binds to F127 micelles first forming mixed micelles, which dissociate into smaller mixed aggregates and then to single F127 unassociated monomers. A third interaction of SDS, which involves promotion of F127 micelles at concentrations up to 3 °C below the critical micellar temperature of pure F127, was identified and is investigated in the present work. The formation of such SDS-induced mixed micelles was monitored using differential scanning calorimetry, light scattering, isothermal titration calorimetry, and a SDS selective electrode for electromotive force measurements. These investigations have shown how the different binding and aggregation processes between SDS and F127 involving induced micellization, growth of mixed micelles, breakdown of mixed micelles, and binding of SDS to monomeric F127 can be identified and characterized.

Introduction Water-soluble poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymers are high molecular weight nonionic surfactants. In water many of these polymers are known to aggregate to form micelles above their critical micellar concentrations (CMC). The formation of block copolymer micelles is an extremely temperature-dependent process resulting in a dramatic decrease of the CMC upon a small increase in temperature. This behavior has led to the widespread use of the critical micellar temperature (CMT) as a very useful micellar parameter.1-6 The temperature-dependent difference in solvation of the ethylene oxide (EO) and propylene oxide (PO) blocks especially in aqueous solutions makes these triblock copolymers particularly useful for thermally reversible micellization processes. Structural studies showed that the micelles possess a hydrophobic core consisting of the PPO blocks, which are surrounded by an outer shell, the corona containing hydrated PEO blocks.7,8,23 The molecular architecture of these polymers allows for extensive technological applications,5,6 which in turn has led to many fundamental studies of such systems. These surface active polymers are also known to interact with normal surfactants. The first reports by Hecht et al. were on the interactions of F127 having a structural formula EO97PO69EO97 (MW ) 12 600) with various * To whom correspondence may be addressed. † Fritz-Haber-Institut der Max-Planck Gesellschaf. ‡ University of Salford. (1) Chu, B. Langmuir 1995, 11, 414. (2) Alexandridis, P.; Holzwarth, J. F.; Hatton, T. A. Macromolecules 1994, 27, 2414. (3) Alexandridis, P.; Hatton, T. A. Colloids Surf., A 1995, 96, 1. (4) Almgren, M.; Brown, W.; Hvidt, S. Colloid Polym. Sci. 1995, 273, 2. (5) Schmolka, I. R. J. Am. Oil Chem. Soc. 1977, 54, 110. (6) Bahadur, P.; Reiss, G. Tenside, Surf., Deterg. 1991, 28, 173. (7) Mortensen, K.; Pederson, J. S. Macromolecules 1993, 26, 805. (8) Goldmints, I.; Yu, G E.; Booth, C.; Smith, K. A.; Hatton, T. A. Langmuir 1999, 15, 1651 and references therein.

surfactants.9-13 Using differential scanning calorimetry (DSC),10 it was shown that very small amounts of sodium dodecyl sulfate (SDS) interfered with the micelle formation of F127 and when the concentration of added SDS was sufficiently high the micellization of F127 was completely suppressed. The breakdown of F127 micelles by SDS was monitored using light scattering (LS) and small-angle neutron scattering (SANS) experiments.10,11 Following this work, a systematic study of the binding of SDS to both unassociated and micellar F127 was carried out by Li et al.12 Such data were measured at temperatures above and below the CMT, and two specific modes of SDS binding were identifiedsthe formation of mixed F127/SDS micelles and SDS micellar aggregates on single F127 chains. We report here experimental evidence showing that SDS can also interact with F127 at temperatures up to 3 °C below the CMT of pure F127 by first promoting the formation of F127 micelles followed by their subsequent breakdown, caused by a further SDS concentration increase. Experimental Section SDS was purchased from Fluka as purissimum and used as received. The block copolymer Pluronic F127, MW 12 600, was a donation by BASF, Mount Olive, NJ. Differential Scanning Calorimetry (DSC). Measurements were made with a MicroCal MC-2 DSC instrument (MicroCal, Northampton, MA) in the temperature range 10-50 °C with a slow scanning rate of 0.5 °C/min. All samples showed an endothermic transition peak of the heat capacity, Cp, on heating. The CMT is defined as the temperature at the intersection of the tangent drawn through the first inflection point of the heat capacity-temperature dependency curve with the baseline.13 (9) Hecht, E.; Hoffman, H. Colloids Surf. 1995, 96, 181. (10) Hecht, E.; Hoffman, H. Langmuir 1994, 10, 86. (11) Hecht, E.; Mortensen, K.; Gradzielski, M.; Hoffmann, H. J. Phys. Chem. 1995, 99, 4866 and references therein. (12) (a) Li, Y.; Xu, R.; Bloor, D. M.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 2000, 16, 10515. (b) Ghoreishi, S. M.; Fox, G. A.; Bloor, D. M.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 1999, 15, 5474. (13) Olofsson, G.; Wang, G. In Polymer Surfactant Systems; Kwak, J. C. T., Ed.; Surfactant Science Series; Marcel Dekker: New York, 1998; Vol. 77, pp 193-238.

10.1021/la001075j CCC: $20.00 © 2001 American Chemical Society Published on Web 12/08/2000

184

Langmuir, Vol. 17, No. 1, 2001

Figure 1. Light scattering intensity (b) and DSC Cp (s) as a function of temperature for block copolymer F127 (0.5% w/v) solutions. Isothermal Titration Calorimetry (ITC). The isothermal titration microcalorimeter used was the MicroCal Omega ITC instrument from MicroCal. In ITC experiments, one measures directly the energetics (enthalpy changes) associated with processes occurring at constant temperature. Experiments were carried out by first titrating micellar SDS into water and then into an aqueous solution containing a known amount of polymer. An injection schedule (number of injections, volume of injection, and time between injections) was set up using interactive software, and this schedule was automatically carried out with all data stored to disk. After each addition, the heat released or absorbed as a result of the various processes occurring in the solution was monitored by the ITC microcalorimeter.12,13 Electromotive Force (emf) Measurements. Surfactant membrane electrodes selective to SDS were constructed in the laboratory and used to determine monomer surfactant concentrations 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 added surfactant (C) and the difference (C - m1) represents the amount of bound surfactant. The cells used for these measurements and the procedures to calculate the respective monomer surfactant concentrations are described elsewhere.12 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 0.2 °C/min by using a Haake PG 20 controller. No hysteresis was observed for the light scattering intensities when the sample was heated and then subsequently cooled. In carrying out the different experiments, it was necessary to scale up or scale down the F127 concentration commensurate with the sensitivity and accuracy of the different techniques used. This was sensible because we could not detect any difference in the fundamental interaction of SDS with F127 between 0.05% and 3% w/v of F127. ITC allowed for the lowest concentrations used, and LS needed the highest concentrations with emf and DSC in between.

Results and Discussion (a) Micellization of F127. The CMT of F127 can be determined from DSC or LS experiments. In the DSC experiment a well-defined endothermic peak characteristic of micellization of the triblock copolymers is obtained. The CMT is defined as the temperature at the intersection of the tangent at the first inflection of the heat capacity peak with the baseline.10,11,13 In the LS experiments the CMT occurs at the sharp step-like increase in the LS intensity as a function of temperature. Typical measurements using LS or DSC are shown in Figure 1. The CMC of surfactants can also be measured using ITC and is usually associated

Li et al.

Figure 2. Isothermal titration calorimetry (ITC) profiles of ∆Hi for (b) surfactant C12EO6 at 15 °C and (9) block copolymer F127 at 30 °C in the concentration region of their CMCs.

with a sharp inflection point of the enthalpy (∆Hi) per injection as a function of surfactant concentration.12,13 Typical data for F127 are shown in Figure 2 at a temperature of 30 °C. The corresponding much sharper results for the nonionic surfactant hexa(ethylene glycol) mono-n-dodecyl ether (C12EO6) at 15 °C are also included in this figure for comparison. The formation of micelles in F127 is associated with large endothermic enthalpy changes as is observed in the ∆Hi values. These changes extend over a large interval in the F127 concentration range compared to the well-defined stepwise change for the CMC observed for C12EO6. Clearly the CMC of F127 is not as well defined as it is for C12EO6. The reason for this result is that triblock copolymers such as F127 are likely to contain impurities such as PEO, PPO, and other di- and triblock copolymers.14-19 In addition the blocks in the polymers are polydisperse. We have attempted to measure the CMC from the plot of the incremental enthalpy changes ∆H/F127 as a function of F127 concentration.20 These plots are shown in Figure 3, and the maximum in these graphs is defined as the CMT.20 The CMCs estimated compare favorably with previously published values for F127.2 There are differences between the CMT values measured using different techniquess these are to be expected because of the different nature of the experiments. In addition the methodologies described above to determine the CMT from experimental data also contribute to these differences. (b) Effect of SDS on Micelles of F127. (i) Differential Scanning Calorimetry (DSC). In the first instance the different interactions between F127 and SDS can be best understood by examining the DSC experiment first reported by Hecht et al.10,11 The DSC peaks, which are exclusively associated with the micellization of F127, were recorded for binary mixtures containing different constant concentrations of F127 and various amounts of added SDS. The CMT was measured as described above, and Figure 4 shows plots of the CMT of 0.05% and 0.625% (14) Yu, G. E.; Deng, Y L.; Dalton, S.; Wang, Q.-G.; Attwood, D.; Price, C.; Booth, C. J. Chem. Soc., Faraday Trans. 1992, 88, 2537. (15) Reddy, N. K.; Fordham, P. J.; Attwood, D.; Booth, C. J. Chem. Soc., Faraday Trans. 1990, 86, 1569. (16) Zhou, Z. K.; Chu, B. Macromolecules 1988, 21, 2548. (17) Kositza, J. M.; Rees, G. D.; Holzwarth, A.; Holzwarth, J. F. Langmuir, in press. (18) Kositza, M. J.; Bohne, C.: Hatton, T. A.; Holzwarth, J. F. Prog. Colloid Polym. Sci. 1999, 112, 146. (19) Olofsson, G.; Wang, G. J. Phys. Chem. 1995, 99, 5588. (20) Ghoreishi, S. M.; Li, Y.; Warr, J.; Bloor, D. M.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 1999, 15, 4380.

SDS Binding to a Triblock Copolymer

Langmuir, Vol. 17, No. 1, 2001 185

Figure 5. Light scattering intensity as a function of SDS concentration in block copolymer F127 (3% w/v) solutions at 22 °C.

Figure 3. Diagram of incremental enthalpy changes ∆(∆Hi)/ ∆C as a function of block copolymer F127 concentrations for (9) in pure water, (b) in 10-4 mol dm-3 SDS, and (2) in 10-3 mol dm-3 SDS, at (a) 27.5 °C and (b) 30 °C.

Figure 4. Tonset measured from DSC experiments as a function of SDS concentrations for block copolymer F127 (9) F127 (0.05% w/v) and (b) F127 (0.625% w/v).

w/v F127 as a function of SDS concentration. The CMT initially decreases as small amounts of SDS are added, goes through a minimum, and then increases sharply as more SDS is added. Eventually the DSC peak disappears when all the F127 micelles are broken down by the SDS, in the same way as the micellization of concentrated surfactants is often envisaged as a pseudo-phase-separation phenomenon. This diagram can be envisaged as a pseudophase diagram for F127, in the sense that at temperatures above the line connecting 9 or b points in Figure 4, F127 exists as micellar aggregates in equilibrium with some monomeric F127. Below the connecting lines F127 is not aggregated, and under these conditions the triblock copolymer is thought to exist as monomeric F127, like any other nonassociated linear polymer. These plots have similar shapes for different F127 concentrations up to 3% w/v but with the minimum dropping in temperature

and shifting to higher SDS concentrations as the F127 concentration is increased as shown in Figure 4. A minimum was also reported by Hecht et al.11 but apparently not discussed. We believe that these plots contain valuable information which can be used to develop a strategy to investigate different SDS/F127 interactions. As an illustration in 0.625% F127, take for example the straight line A-B in the SDS/F127 system as shown in Figure 4. Along this line we find SDS in various stages of aggregation, but only monomeric unassociated F127 is present. On the other hand the area above the straight line E-F′ represents SDS and mainly micellar F127, which is in equilibrium with F127 monomers, and the line F′-F represents SDS + monomeric F127. Finally let us now consider the line C-D in Figure 4. In the region C-C′ F127 remains in its monomeric form as SDS is added. However at C′ F127 micelles are formed, and as further SDS is added along C′-D′, these micelles first grow and eventually disappear at D′. From D′-D the copolymer exists in monomeric single polymer units and interacts with SDS in a similar way as found for nonionic polymers.12a,b (ii) Light Scattering (LS). At C the F127 is below its CMT (and CMC) and along the line C′-D′ it is evident that SDS actually promotes the formation of F127 micelles. The existence of these induced micelles in the region C′D′ can also be detected using light scattering, and the data are presented in Figure 5. Here a 3% w/v solution of F127 was investigated to gain enough sensitivity so that this result could be effectively monitored by light scattering at 22 °C, and at this concentration the experimental conditions are in principle equivalent to the line C-D in Figure 4, but the SDS and temperature scales are shifted, the latter down the former up. The scattered light intensity starts increasing as at C′, monitoring successively the onset and growth of F127 micelles. The different nature of this transition is consistent with other experimental determinations of CMC which have been mentioned previously. As further SDS is added the LS reaches a maximum indicating that the size and/or number of the micelles reach a maximum. Further addition of SDS starts to break down the micelles which eventually disappear as at D′. (iii) Isothermal Titration Calorimetry (ITC). Under isothermal conditions the above results mean that gradual addition of SDS initially depresses and then increases the CMC of F127. This effect is demonstrated in the ITC results presented in Figure 3a,b, when concentrated F127 is successively injected into water, containing 0, 10-4, or

186

Langmuir, Vol. 17, No. 1, 2001

Li et al.

Figure 6. A three-dimensional ITC diagram for ∆Hi as a function of SDS concentrations at eight different temperatures between 15 and 35 °C for block copolymer F127 0.05% w/v. Table 1. CMC of the Block Copolymer F127 at Various SDS Concentrations and Temperatures F127 CMC (% w/v)

T (°C)

0.09 0.04 0.06 0.02 0.16 0.13

27.5 30 27.5 30 27.5 30

SDS (mol dm-3) 0 1 × 10-4 1 × 10-3

10-3 mol dm-3 SDS. The ∆Hi/F127 plots which essentially monitor the endothermic enthalpy changes associated with the formation of F127 micelles shift to lower F127 concentrations in the presence of 10-4 mol dm-3 SDS indicating a lowering of the CMC. At SDS concentrations of 10-3 mol dm-3 the CMC is greater than that for pure F127. The CMCs were estimated as described previously20 and are listed in Table 1. Further information about the various processes involving SDS and F127 in various states can only be obtained by a technique which is sensitive to the details of F127/polymer interactions. We believe that the ITC method involving the injection of SDS into F127 at various temperatures spanning the lines A-B and E-F in Figure 4 satisfies these criteria.12,19-22 In Figure 6 a threedimensional plot of the ITC enthalpy profiles associated with the binding of SDS to F127 (0.05% w/v) at several temperatures and SDS concentrations over the same region as in Figure 4 is presented. The concentration of (21) Ghoreishi, S. M.; Li, Y.; Holzwarth, J. F.; Khoshdel, E.; Warr, J.; Bloor, D. M.; Wyn- Jones, E. Langmuir 1999, 15, 1938. (22) Li, Y.; Ghoreishi, S. M.; Warr, J.; Bloor, D. M.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 1999, 15, 6326.

F127 used was 0.05% w/v, and the relevant phase diagram is included in Figure 4. The importance of such plots becomes clear when considering Figure 7. Figure 7a, taken at 15 °C, is typical of the data for the binding of SDS to unassociated F127 single polymer units. The ∆Hi values for the same injection with and without the polymer are initially very close. Eventually as more SDS is added the ∆Hi values for the F127 solution start to deviate from the value in the SDS only aqueous solution at T1, which signifies the onset of the formation of SDS micelles on the polymer. As binding proceeds the ∆Hi values reach a maximum and then follow the characteristic steplike exothermic process associated with the cooperative formation of SDS micelles on the polymer. Finally the ∆Hi values merge again at T2 when the unassociated F127 is fully saturated with bound SDS micelles. These data are typical of SDS/neutral-polymer systems where T1 is associated with the onset of the maximum in ∆Hi.18-22 The ITC data in Figure 7c at 35 °C represent the enthalpy profile associated with the binding of SDS to micellar F127.12 The large difference between the ∆Hi with and without the polymer for the first SDS injection indicates strong binding involving a large endothermic process. As further SDS is added the ∆Hi becomes less endothermic as the bound SDS gradually breaks down the F127 micelles to smaller mixed SDS/F127 aggregates. According to the phase diagram in Figure 4 the F127 micelles are completely dissociated when the SDS concentration reaches ∼9 × 10-4 mol dm-3. Further addition of SDS results in the exclusive binding of SDS micelles to unassociated F127 as in Figure 7a. This latter process commences when the SDS monomer concentration reaches T1sthis is the thermodynamic condition12a that is required

SDS Binding to a Triblock Copolymer

Langmuir, Vol. 17, No. 1, 2001 187

Figure 8. Graph of the electromotive force (emf) of the SDSsensitive electrode (reference silver/Br-) as a function of total SDS concentrations in 10-4 mol dm-3 NaBr: (9) pure SDS; (b) SDS/0.625% w/v F127 block copolymer at 17 °C.

Figure 7. Graph of ∆Hi in ITC experiments as a function of SDS concentrations for the systems in the (0) presence and (b) absence of block copolymer F127 (0.05% w/v) at different temperatures: (a) 15 °C, (b) 27 °C, and (c) 35 °C.

to be satisfied before SDS micelles can bind to single F127 units. This occurs when the total SDS reaches ∼2 × 10-4 mol dm-3. Figure 7b at 27 °C is the ITC enthalpy profile along the dashed line in Figure 4 for 0.05% F127 and illustrates how ITC can be used to monitor the formation of F127 micelles induced as a result of the addition of SDS to unassociated F127 polymers at a few degrees (up to 3 °C) below the CMT of pure F127. Initially when SDS is added no binding takes place and the ∆Hi values with and without the polymer are close (not in the figure). As the SDS concentration corresponding to point C′ (dashed line cross solid line), in the phase boundary of Figure 4, where F127 micelles start forming is reached, the ∆His start deviating. At this point the ∆His for the polymer solution show the onset of an endothermic process consistent with the formation of F127 micelles. This process starts well below the T1 value for the formation of SDS micelles on unassociated F127.12a As more SDS is added, the ∆Hi values reach a maximum, then become less endothermic and decrease for a short period before increasing again by reaching a second endothermic maximum, and finally decrease until they merge with the SDS only values at T2. As stated above the first endothermic maximum in the ∆Hi values is the formation of F127 micelles induced by SDS. As these micelles are broken down by further SDS addition, this process is overlapping with a second process, i.e., the initial endothermic formation of SDS micelles binding to monomeric F127. When this latter interaction becomes the dominating enthalpic process, ∆Hi starts to be more endothermic again (second maximum) signaling the characteristic maximum associated with the formation of SDS micelles on F127 monomers as shown in Figure 7a. This changeover in the slope of the ∆His take place equivalent to the position D′ of the dashed line in the phase diagram of Figure 4, when SDS has broken down all the F127 micelles.

Figure 9. Graph of the electromotive force (emf) of the SDSsensitive electrode (reference silver/Br-) as a function of total SDS concentrations in 10-4 mol dm-3 NaBr: (9) pure SDS; (b) SDS/0.625% w/v F127 block copolymer at 25 °C.

It is clear that in addition to the endothermic processes associated with the binding of SDS to single (Figure 7a) and associated F127 (Figure 7c), there is a very specific interaction occurring where SDS actually induces the aggregate formation of F127. This process only occurs at temperatures 1-3 °C lower than the CMT of pure F127 (see Figures 4 and 6). (iv) Electromotive Force (emf) Measurements. We have also carried out emf measurements on the binding of SDS to 0.625% w/v F127 at several temperatures in the region 17-25 °C. These measurements span the region where the minimum in the phase diagram of Figure 4 is observed. emf measurements were taken below the line A-B of Figure 4. Data in Figure 8 are characteristic of the type of emf data that are obtained, when SDS binds to an unassociated neutral polymer, and were described previously.13,18,20-22 In summary SDS starts binding at T1 where the emf data with and without the F127 start diverging. The binding process which involves the formation of SDS micelles on the F127 chain continues with the addition of more SDS. These bound micelles increase in size and number until the polymer becomes fully saturated at T2, where the emf data with and without F127 merge again. The emf data along the line E-F in Figure 4 involve the binding of SDS to micellar F127. The data in Figure 9 show that SDS binds to F127 micelles at even the lowest SDS concentration used (T1 < 1 × 10-5 mol dm-3) and the process continues until T2 is reached when the emf’s with and without F127 merge. As we described previously,12 the initial part of the binding involves the formation of

188

Langmuir, Vol. 17, No. 1, 2001

Figure 10. Graph of the electromotive force (emf) differences, ∆emf, at (T - 17 °C) as a function of SDS concentrations in block copolymer F127 0.625% w/v solutions at 17 °C (9), 19 °C (b), 22 °C (2), and 25 °C (1).

mixed F127/SDS micelles and as further SDS is added the F127-rich mixed micelles start breaking down to smaller aggregates. The breakdown process continues until the point F′ in Figure 4 is reached, when all the F127 polymers are dissociated to monomers. The binding from this point to T2 is exactly the same as described for unassociated F127 above, i.e., the formation of SDS micelles on unassociated F127. Before the point F′ is reached, it is likely that both binding processes involving F127 micelles and SDS, as well as binding between F127 monomers and SDS micelles, are taking place over a small SDS concentration range. We also carried out a series of emf measurements on the system SDS/F127 over the intermediate temperature range between the above limits. To investigate what is taking place when the minimum in the phase boundary is reached, i.e., when SDS induces F127 micelles, we used the emf data at 17 °C as a reference and subtracted these data from the measurements at each subsequent temperature (19, 22, and 25 °C). The results are displayed in Figure 10. These data were measured over the SDS concentration range spanning the minimum in the phase diagram of Figure 4. Although the additional amount of SDS bound is very small, the data show clearly that over this region there is a small maximum in each curve, which progressively increases as the temperature is increased. The data also show that additional SDS is taken up by the F127 in the SDS concentration range of 5 × 10-5 to 3 × 10-3 mol dm-3, which is more than the amount of SDS expected to be bound on unassociated F127. The onset of the small maximum starts approximately where F127 micelles are induced by SDS (C′ Figure 4) and continues until all the induced F127 micelles are broken down to

Li et al.

F127 monomers (D′ Figure 4). Therefore at C′, SDS initially promotes F127 aggregates and for a small concentration range these aggregates are stable before further addition of SDS produces the reverse effect and simply breaks down the induced F127 aggregates. (c) Discussion of SDS/F127 Interaction. The exact mechanism which describes how SDS promotes the formation of F127 micelles is not clear at present. One could argue that because the measurements are close to phase boundaries, the single SDS monomers act as a seed or nucleus for the formation of F127 micelles. This seems reasonable since the effect only occurs at temperatures 3 °C or less below the CMT of pure F127, which can be considered analogous to a saturation phenomenon. It is likely that very small amounts of F127 aggregates are already present under these conditions. On the other hand it could be that SDS monomers added at concentrations or temperatures just before the appearance of F127 micelles may affect the solvent properties of water8 by the disturbance of its hydrogen bonds. It is well-known that the solubility of the PEO blocks is much better in water than that of PPO blocks. When solvent conditions become unfavorable for the PPO blocks, the triblock copolymer will aggregate to form micelles. On this basis one could argue that when SDS monomers induce F127 micelles, such unfavorable conditions for the solubility of single F127 copolymers exist. At concentrations close to the CMT, the PPO micellar core contains a significant amount of water.23 As we have mentioned previously the function of SDS in the context of the above observation is to initially decrease and then increase the CMC of F127. This is a phenomenon that was observed over 50 years ago in conventional ionic surfactant systems. In such systems the decrease in the CMC is normally associated with an additive interacting with the micelles. On the other hand the increase of CMCs is often the result of an additive interacting with the surfactant monomer and/or modifying the water structure. We would like to emphasize that the equilibrium type techniques used in this work do not give direct information on the structure of the various F127/SDS aggregates. We believe that this objective can be achieved using SANS measurements using deuterated SDS for contrast match purposes. Such data will also provide information on the mechanism of various F127/SDS interactions. Finally SANS is more sensitive than present light scattering experiments and will provide a powerful handle to probe the state of F127 close to the CMT. LA001075J (23) Yang, L.; Alexandridis, P.; Steytler, D. C.; Kositza, M. J.; Holzwarth, J. F. Langmuir 2000, 16, 8555.