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Articles Binding of Tetradecyltrimethylammonium Bromide to the ABA Block Copolymer Pluronic F127 (EO97 PO69 EO97): Electromotive Force, Microcalorimetry, and Light Scattering Studies Y. Li,† R. Xu,‡ S. Couderc,† D. M. Bloor,‡ J. F. Holzwarth,*,† and E. Wyn-Jones*,†,‡ Fritz-Haber Institut der Max-Planck Gesellschaft, Faradayweg 4-6, D-14195 Berlin-Dahlem, Germany, and School of Chemical Sciences, Science Research Institute, University of Salford, Salford, M5 4WT, United Kingdom Received January 2, 2001. In Final Form: June 27, 2001
The interaction between the cationic surfactant tetradecyltrimethylammonium bromide (TTAB) and the Pluronic triblock copolymer F127 was investigated. F127 is a nonionic surfactant with structural formula EO97PO69EO97, where EO represents the ethylene oxide block and PO represents the propylene oxide block. A combination of experiments involving a TTAB selective electrode (electromotive force), isothermal titration calorimetry (ITC), differential scanning calorimetry (DSC), and light scattering have shown that TTAB binds to both monomeric and micellar F127. TTAB forms a polymer/micellar TTAB complex with monomeric F127. In addition, TTAB binds to F127 micelles leading to the transformation of the aggregated F127 into mixed micelles followed by a breakdown of these aggregates into smaller mixed F127/TTAB aggregates as more TTAB is added. This process continues until all the aggregated F127 is dissociated. DSC measurements have also shown that small amounts of TTAB (typically 10-4 mol dm-3) can decrease the critical micelle temperature (cmt) of F127. This represents a third mode of binding in which TTAB induces F127 to form micelles at temperatures several degrees below its “pure” cmt. Possible mechanisms for these processes involving different modes of interaction of TTAB with F127 are introduced and discussed.
Introduction Water soluble poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymers are high molecular weight nonionic surfactants which are known to aggregate in water to form micelles above their critical micellar concentration (cmc). Structural studies have shown that the micelles form a hydrophobic core consisting mainly of the propylene oxide (PO) blocks which are surrounded by an outer shell, the corona, of hydrated ethylene oxide (EO) blocks.1-8a,b The temperature-de* To whom correspondence should be addressed: Professor J. F. Holzwarth and Professor E. Wyn-Jones, Physical Chemistry, FritzHaber-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]. † Fritz-Haber Institut der Max-Planck Gesellschaft. ‡ University of Salford. (1) Chu, B. Langmuir 1995, 11, 414. (2) (a) Alexandridis, P.; Holzwarth, J. F.; Hatton, T. A. Macromolecules 1994, 27, 2414. (b) Alexandridis, P.; Holzwarth, J. F. Curr. Opin. Colloid Interface Sci. 2000, 5, 312. (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.; Riess, G. Tenside, Surfactants, Deterg. 1991, 28, 173. (7) Mortensen, K.; Pederson, J. S. Macromolecules 1993, 26, 805. (8) (a) Goldmints, I.; Yu, G. E.; Booth, C.; Smith, K. A.; Hatton, T. A. Langmuir 1999, 15, 1651 and references therein. (b) Yang, L.; Alexandridis, P.; Steytler, D. C.; Kositza, M. J.; Holzwarth, J. F. Langmuir 2000, 16, 8555.
pendent difference in hydration of the EO and PO blocks, with the latter being more hydrophobic, causes these triblock copolymers to be particularly useful because the micellization process becomes strongly conditioned by temperature. This results in a decrease of the cmc of several orders of magnitude upon a small increase in temperature. The latter behavior has led to the widespread use of the critical micellar temperature (cmt) as a very useful and practical micellar parameter.2 These surface active block copolymers are also known to interact with “normal” surfactants such as sodium dodecyl sulfate (SDS). Hecht et al.9-11 first showed that gradual addition of SDS interfered with and eventually suppressed the micelle formation of the triblock copolymer code named by BASF as F127 having a structural formula EO97PO69EO97 (MW ) 12 500). The basic surface active components of most detergent products are ionic and nonionic surfactants. Most successful practical applications of surfactants utilize binary surfactant mixtures because of their improved performances in comparison to single components. This arises through synergistic (favorable) interactions between the surfactants.12 To assess the performance of new surfactant mixtures, it is (9) Hecht, E.; Hoffmann, H. Langmuir 1994, 10, 86. (10) Hecht, E.; Hoffmann, H. Colloids Surf., A 1995, 96, 181. (11) Hecht, E.; Mortensen, K.; Gradzielski, M.; Hoffmann, H. J. Phys. Chem. 1995, 99, 4866. (12) Rubingh, D. N. In Solution Chemistry of Surfactants; Mittal, K., Eds.; Plenum: New York, 1979; Vol. 1, p 337.
10.1021/la010004x CCC: $20.00 © 2001 American Chemical Society Published on Web 08/23/2001
Binding of TTAB to Block Copolymer Pluronic F127
necessary to investigate their mixed micellar behavior so that their compatibility as potential useful products can be determined.12-14 When considering mixtures of polymeric surfactants of block copolymers such as F127 and ionic surfactants, it is possible for the ionic surfactant to interact with both F127 micelles9-11 and also F127 monomers. In the latter case, it is well-known that ionic surfactants can interact strongly with nonassociated neutral polymers. In these circumstances, it is necessary to assess the extent of the above contributions before addressing the problem of mixed micelles. In this context, Li et al.15,16 using an SDS selective electrode (EMF, electromotive force) carried out systematic measurements of the binding isotherms of the anionic surfactant SDS to both monomeric and micellar F127. These data were complemented by isothermal titration calorimetry (ITC), light scattering (LS), and differential scanning calorimetry (DSC) measurements. As a result of their work, three different modes of SDS binding to F127 were identified: (i) SDS binds to monomeric F127 units in exactly the same way as it binds to normal unassociated water soluble polymers, that is, by the formation of polymer-bound SDS micellar aggregates in which a polymer chain interacts with the surface of the SDS micelle.15,16 (ii) SDS also interacts with micellar F127. This process involves the binding of SDS to F127 micelles followed by the breakdown of the F127/SDS mixed micelles to smaller mixed aggregates as more SDS is added and this process continues until all the aggregated F127 has been dissociated.15 (iii) At temperatures immediately below (1-4 °C) the cmt, small amounts of SDS (10-4-10-5 mol dm-3) can promote the formation of F127 micelles, which would not exist in pure F127 solutions under similar conditions.16 This essentially means that the SDS lowers the cmc of F127. As part of a comprehensive study of mixed conventional surfactant/triblock copolymer systems, we have carried out LS, EMF, ITC, and DSC measurements to examine how the cationic surfactant tetradecyltrimethylammonium bromide (TTAB) interacts with F127. In general, the interactions between cationic surfactants and watersoluble nonionic polymers are much less facile than the corresponding interaction between the same polymer and SDS.14 In a recent study, we showed that SDS binds strongly to a diverse range of neutral polymers.17 In comparison, cationic surfactants are more selective and only interact weakly with those polymers which have specific hydrophobic groups.18 For example, if we consider the separate EO and PO blocks of F127, SDS interacts strongly with PPO and PEO14 whereas TTAB only interacts weakly with PPO.18 On the other hand, a preliminary DSC measurement showed that cationic surfactants readily form mixed micelles with F127 and also moderate its micellar properties.19 This investigation should therefore clarify the binding behavior of TTAB to F127 in its monomeric and micellar form. (13) Holland, P. M. Adv. Colloid Interface Sci. 1986, 26, 111. (14) Goddard, E. D. In Interactions of Surfactants with Polymers and Protein; Goddard, E. D., Ananthapadmanabhan, K. P., Eds.; CRC Press: Boca Raton, FL, 1993. (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.; Ghoreishi, S. M.; Warr, J.; Bloor, D. M.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 1999, 15, 6326. (18) Ghoreishi, S. M.; Fox, G. A.; Bloor, D. M.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 1999, 15, 5474 and references quoted therein. (19) Kositza, M. J.; Holzwarth J. F. Unpublished results, 2000.
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Experimental Section Tetradecyltrimethylammonium bromide was purchased from Aldrich and used as received. The triblock copolymer with the trade name Pluronic F127 was a gift from BASF, Mount Olive, NJ. Electromotive Force Measurements. The surfactant membrane electrode used being selective for TTAB is a much improved version of our earlier electrodes and was described last year in detail in an already published article.20 Our electrode operates reliably below and above the cmc of TTAB; it possesses the same modified poly(vinyl chloride) membrane as previously used21,22 but is now constructed with a much improved mechanical design. The measurements were performed as described previously.18,20-22 The EMF data are presented by plotting EMF as a function of surfactant concentration. All EMF measurements were carried out in the presence of 0.1 mM NaBr. Since the surfactant ion and Na+ have the same charge, the concentration of free sodium ions in solution remains constant at 0.1 mM even when TTAB micelles are formed. In these circumstances, a sodium ion selective electrode is applied as a reference. Isothermal Titration Calorimetry and Differential Scanning Calorimetry. The microcalorimeters used here were the Microcal OMEGA ITC and the Microcal MC-2 DSC instruments, both from 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 first titrating micellar TTAB into water and then into an aqueous solution containing a known amount of F127 at different temperatures. An injection schedule (number of injections, volume of injections, and time between injections) is set up using interactive software, and this schedule is 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 is monitored by the calorimeter. In a fashion similar to the presentation of the EMF data, we plot the results of the ITC experiments in terms of the enthalpy per injection (∆Hi) as a function of surfactant concentration.15-18 DSC experiments measure the temperature dependence of the heat capacity Cp typically using a scan rate of 30 °C per hour. The ITC method can also identify the formation of bound surfactant micelles on the polymer.23 In principle, the enthalpies per injection are related to the equilibrium concentrations and mechanisms of the binding process. Although attempts have been made to formulate these relationships, we are not yet aware of a treatment which allows isotherms and mechanisms to be evaluated.24-27 This is particularly important since the enthalpies are extremely sensitive to changes in the system. In the present work, we use the ITC experiment as a method to monitor the binding process and have shown previously that the enthalpy profile hardly changes in the presence of 0.1 mM NaBr.15-18 Light Scattering. Light scattering data were collected at 360 nm and 90° with a RF-5000 Shimadzu spectrofluorometer. A Haake F3-C bath was employed to control the sample temperature, 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 intensity when the sample was heated and then subsequently cooled.19 This acts as a check on the thermal reversibility of the micellization of F127. (20) Xu, R.; Bloor, D. M. Langmuir 2000, 16, 9555. (21) Bloor, D. M.; Mwakibete, H. K. O.; Wyn-Jones, E. J. Colloid Interface Sci. 1996, 178, 334. (22) Painter, D. M.; Bloor, D. M.; Takisawa, N.; Hall, D. G.; WynJones, E. J. Chem. Soc., Faraday Trans. 1 1988, 84, 2087. (23) Ghoreishi, S. M.; Li, Y.; Khoshdel, E.; Warr, J.; Bloor, D. M.; Wyn-Jones, E. Langmuir, 1999, 15, 1938. (24) 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. (25) Blandamer, M. J.; Briggs, B.; Cullis, P. M.; Irlam, K. D.; Engberts, J. B. F. N.; Kevelam, J. J. Chem. Soc., Faraday Trans. 1998, 94, 259. (26) Torn, L. H.; de Keizer, A.; Koopal, L. K.; Lyklema, J. Colloids Surf., A 1999, 160, 237. (27) Wang, G.; Olofsson, G. J. Phys. Chem. B 1998, 102, 9276.
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Figure 1. Graph of the EMF of the TTAB sensitive electrode (reference, sodium ion (Na+) electrode) as a function of total TTAB concentration (C) in 10-4 mol dm-3 NaBr: (b) pure TTAB and (0) TTAB/0.5% w/v F127 block copolymer at 15 °C. T1 denotes the start of TTAB binding, and T2 denotes the saturation concentration.
Figure 2. Graph of the enthalpy change ∆Hi in ITC experiments as a function of total TTAB (C) concentration in the absence (9) and presence (0) of the block copolymer F127 (0.5% w/v) at 15 °C. T1 denotes the start of TTAB binding, and T2 denotes the saturation concentration for TTAB.
Results and Discussion Previous LS and DSC measurements have shown that the cmt of 0.5% w/v F127 is 27.5 °C.12,13,15,16 Thus, at 35 °C F127 exists predominantly in micellar form in equilibrium with a small amount of F127 monomers. At 15 °C, which is well below the cmt, we have every reason to believe that no micelles are present in a 0.5% w/v solution of F127. At a temperature of 15 °C, the solution of F127 is expected to behave like a solution of any conventional water-soluble nonionic polymer which exists in unassociated single monomer units. (a) Interaction of TTAB with Unassociated F127 at 15 °C. The EMF and ITC data for monomeric F127 in the presence of TTAB were measured at 15 °C and are shown in Figures 1 and 2, respectively. Both these experiments were carried out in a similar way such that a concentrated TTAB solution containing 0.5% w/v F127 is titrated into an aqueous solution containing the same amount of polymer. The respective EMFs and ∆Hi’s are then plotted as a function of added TTAB for solutions with and without the F127, the latter being the control experiment; in the case of the electrode data, the cmc of TTAB is at the maximum of the EMF plot. In theory, binding of surfactants to the polymer is taking place when the EMF and ∆Hi values with and without the polymer are different for each corresponding titration.
Li et al.
Figure 3. Binding isotherm for the TTAB/monomeric F127 (0.5% w/v) system at 15 °C shown in form of the amount of bound TTAB (C - m1) as a function of free monomeric TTAB concentration (m1) at 15 °C.
As shown in Figure 1, the EMF data exhibit the characteristic features of a binding process and the two EMFs diverge at a TTAB concentration denoted T1 when TTAB starts binding to F127. The binding proceeds as long as the EMFs of the electrode with and without the polymer are different. When the polymer is fully saturated with bound TTAB, no further binding occurs and the two EMFs merge again at a TTAB concentration denoted T2. The corresponding binding isotherm is shown in Figure 3. The symbols T1 and T2 are historical in the sense that they were first introduced by M. N. Jones28 in his pioneering studies on polymer/surfactant mixtures as characteristic transitions for the state of surfactants. In some cases, T1 is called the “critical aggregation concentration”, cac, which is synonymous with the formation of bound micellar type aggregates on the polymer. T1 in the EMF experiment and T2 from both EMF and ITC are estimated from the point where two experimental curves diverge or merge, respectively. From an inspection of Figures 1, 2, 5, and 6, it is evident that the process of actually pinpointing the position of both T1 and T2 on the graphs is not a 100% exact exercise and consequently these binding parameters are subject to error. We estimate that (10% would be a reasonable estimate of this error. The error in estimating T1 via ITC from the inflection point of the onset of the maximum in the ∆Hi curve is slightly less than ca. (8%. In the ITC enthalpy profiles of Figure 2, T1 occurs at the inflection point before the maximum in ∆Hi and T2 occurs when the ∆Hi’s with and without the polymer merge. Both EMF and ITC data display the characteristic features associated with the binding of surfactants to nonassociated neutral polymers. It is now accepted that the bound TTABs exist as micellar type aggregates which grow in size and number during the binding process.18 The noticeable change in EMF at T1 (Figure 1), the steplike endothermic enthalpy change in the ITC data in Figure 2, and the variation of bound TTAB as displayed in the binding isotherm of Figure 3 provide evidence for the cooperative nature of the formation of TTAB aggregates during the binding process to F127. The T1 and T2 values from our measurements at 15 °C are quoted in Table 1. The data of TTAB presented in Table 1 show clearly that T1 is independent of the F127 concentration and that T2 increases as the polymer concentration increases. This is exactly the type of behavior that is expected when ionic surfactants bind to unassociated neutral polymers.18,21,22 (28) Jones, M. N. J. Colloid Interface Sci. 1967, 23, 36.
Binding of TTAB to Block Copolymer Pluronic F127
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Table 1. Values of T1 and T2 from ITC and EMF Experiments for the TTAB/F127 System at 15 °C and for Two F127 Concentrationsa T1 × 103 (mol dm-3) TTAB
T2 × 103 (mol dm-3) TTAB
F127 concn (w/v)
EMF
ITC
EMF
ITC
0.1% 0.5%
not measurable 0.8
0.8 0.8
not measurable 25
10 27
a
Errors are 8-10%.
The corresponding values of T1 and T2 for 0.5% w/v F127 with SDS15 are 0.35 and 120 × 10-3 mol dm-3, respectively. These numbers demonstrate conclusively that F127 interacts much more strongly with SDS than TTAB, first in stabilizing the bound aggregates (lower T1), and also the maximum capacity for F127 to bind SDS aggregates is about 4 times greater than that for TTAB aggregates. These numbers are based on the premise that the aggregation numbers of bound SDS and TTAB at T2 are close to the aggregation numbers of their free micelles.29a,b At T2, the SDS/F127 complex contains on average four bound SDS micelles per polymer;15,16 for TTAB/F127, the complex at T2 is 1:1 meaning one TTAB micelle per F127 polymer molecule. The interaction between TTAB and F127 takes place via the methyl substituent on the PPO blocks of the F127 chain and the TTAB micelles and is hydrophobic in the sense that it involves the removal of hydrocarbon contact with water. Specifically, the interaction involves direct contact between the CH3- groups in the PPO blocks and the area between the headgroups on the micellar surface where the hydrocarbon chains are in contact with water. This interaction also involves the removal of bound water from the micellar surface and possibly from hydrated parts of the polymer and results in a reduction of the micelle/ water interfacial tension.30 In comparison with SDS/F127, these interactions are less facile because of the bulky headgroups in the TTAB micelles. For SDS/F127, there is also an additional strong electrostatic attraction between SDS micelles and both PEO and PPO blocks of F127 involving the etheric oxygen linkage1 in the chain and the SO4- headgroup; the exact nature of this interaction however is still a subject of debate.31,32 This interaction involves the removal of bound water from the micellar surface and the polymer chain. (b) Binding of TTAB to Micellar F127 at 35 °C. At 35 °C, 0.5% w/v F127 exists as an equilibrium mixture of F127 micelles with a small amount of its monomers, because this temperature is well above the corresponding cmt of 27.5 °C. It has previously been shown15,16 that F127 micelles can be detected by light scattering. Figure 4 shows the light scattering, measured in arbitrary units, of a 0.5% w/v solution of micellar F127 plotted as a function of total TTAB concentration at 35 °C. When a small amount of TTAB is initially added, the amount of light scattering remains high and constant. However, addition of TTAB in excess of 10-5 mol dm-3 results in a dramatic steplike decrease in the scattered light until LS remains constant again and equal to its background value. The light scattering technique used here can only detect the presence of F127 aggregates, and when the scattered light reaches (29) (a) Evans, D. F. Langmuir 1988, 4, 3. (b) Turro, N. J.; Yekta, A. J. Am. Chem. Soc. 1978, 100, 5951. (30) Nagarajan, R. Colloids Surf. 1985, 13, 1. (31) (a) Xia, J. L.; Dubin, P. L.; Kim, Y. S. J. Phys. Chem. 1992, 96, 6805. (b) Holzwarth, J. F.; Jobe, D.; Dunford, H. B. Czech. J. Phys. 1991, 41, 293. (32) Wang, Y. L.; Han, B. X.; Yan, H.; Cooke, D. J.; Lu, J. R.; Thomas, R. K. Langmuir 1998, 14, 6054.
Figure 4. Light scattering intensity as a function of total TTAB concentration (C) in solutions of the block copolymer F127 (0.5% w/v) at 35 °C. Areas A, B, C, and D denote different binding areas for TTAB.
Figure 5. Graph of the EMF of the TTAB sensitive electrode (reference, sodium ion (Na+) electrode) as a function of total TTAB concentration (C) in 10-4 mol dm-3 NaBr: (b) pure TTAB and (0) TTAB /0.5% w/v F127 block copolymer at 35 °C. T1 denotes the start of TTAB binding, and T2 is the saturation concentration of TTAB on F127. Areas A, B, C, and D denote different binding areas for TTAB.
its background value it is assumed that all the F127 aggregates have dissociated into F127 monomers. These data clearly show that addition of TTAB breaks down the F127 micelles first into smaller aggregates and eventually into F127 monomers. The corresponding EMF and ITC plots of 0.5% w/v F127 at 35 °C as a function of added TTAB covering the same TTAB range as the light scattering in Figure 4 are shown in Figures 5 and 6, respectively. These data clearly show that TTAB is binding to the micellar F127 even at the lowest TTAB concentration measured and continues to bind until the respective EMFs and ∆Hi’s with and without the F127 merge at T2 ) 2.5 × 10-2 mol dm-3. The combined EMF/LS data also show that following the binding of TTAB to micellar F127 the mixed F127/TTAB micelles break down to smaller mixed aggregates and finally to F127 monomers (Figure 4). This process continues until all the aggregated F127 has dissociated and the F127 monomers are saturated with TTAB. As the mixed micelles break up, these smaller aggregates must also become richer in their TTAB content because of its progressive binding to F127. However, during the breakdown of F127/TTAB mixed micelles, more monomer units of F127 will also be released into the solution. From the data in the preceding section (Figures 1-3), TTAB also binds to monomeric F127. In the presence of only monomeric F127, T1 is the TTAB concentration at which TTAB micelles start binding to F127. In the presence of both monomeric and micellar F127, that is, the conditions described above for 35 °C, T1
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Figure 6. Graph of the enthalpy change ∆Hi from ITC experiments as a function of total TTAB concentration (C) in the (b) absence and (0) presence of the block copolymer F127 (0.5% w/v) at 35 °C. T1 denotes the start of TTAB binding, and T2 denotes the saturation concentration for TTAB. Areas A, B, C, and D denote different binding areas for TTAB.
is the monomer concentration which TTAB must reach before TTAB micelles can start binding to unassociated F127. This is basically the thermodynamic condition for bound TTAB micelles to be formed on F127 monomers. Unfortunately, this value of T1 cannot be measured independently at 35 °C. This arises because the concentration of F127 required so that the corresponding cmt exceeds 35 °C (i.e., for monomer F127 only to exist at 35 °C) is too small (
