TX-100 Complex

Formation and structure transition of the complex composed of triblock copolymer F127 and nonionic surfactant TX-100 have been investigated by 1H NMR ...
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14566

J. Phys. Chem. B 2008, 112, 14566–14577

Formation and Microstructure Transition of F127/TX-100 Complex† Lingling Ge, Rong Guo,* and Xiaohong Zhang School of Chemistry and Chemical Engineering, Yangzhou UniVersity, Jiangsu ProVince 225002 People’s Republic of China ReceiVed: March 29, 2008; ReVised Manuscript ReceiVed: May 19, 2008

Formation and structure transition of the complex composed of triblock copolymer F127 and nonionic surfactant TX-100 have been investigated by 1H NMR spectroscopy, dynamic light scattering (DLS), and isothermal titration calorimetry (ITC). Three TX-100 concentration regions are identified, within which TX-100/20 mg/mL F127 complex undergoes different temperature-induced structure transitions. In low concentration region (157.57 mM), and free TX-100 micelles coexist with larger clusters of F127/TX-100 complexes. In addition, TX-100-induced F127/TX-100 complex formation and structure transition are also investigated at constant temperatures. The results show that within 5-10 °C, F127 unimers mainly adsorb on the surface of TX-100 micelles just like normal water soluble polymers; in the temperature region of 15-25 °C, TX-100 micelles prompts F127 micelle formation. Within 30-40 °C, TX-100 inserts into F127 micelles leading to the breakdown of F127 aggregates at higher TX-100 concentrations, and the obtained unimers thread through TX-100 micelles forming complex with TX-100 micelle as skeleton. Introduction Triblock copolymer consisting of a centered block of poly(propylene oxide) and two end blocks of poly(ethylene oxide) (PEO-PPO-PEO) exhibits very interesting physical properties. In general, PEO-PPO-PEO copolymer is in a unimer state in aqueous solution at low temperature and concentration since both blocks are soluble.1,2 Increasing temperature improves the hydrophobility of the PPO block and leads to micellization, where the transition temperature is denoted as critical micellization temperature (CMT). They would also aggregate above their critical micellization concentration (CMC) at a certain temperature.3 The structure of copolymer micelle was well described by the core-corona model, in which a spherical core composed of PPO is surrounded by a corona composed of Gaussian chains of strongly hydrated PEO.4–7 The PEO-PPOPEO copolymers have been widely used in detergency, pharmaceutics, bioprocess, etc.8–11 because of their low toxicity and higher biocompatibility. Besides, the solubilization capacity of copolymer micelles for hydrophobic compounds can be adjusted easily by temperature and copolymer content. However, temperature and copolymer concentration cannot be changed unboundedly in some applications, e.g., pharmaceutics or bioprocess. Thus, it is necessary to employ some additives, such as traditional surfactants, to control the aggregation process and to improve the properties of the polymer solutions.12–15 In recent years, much attention has been focused on the interactions between triblock copolymer and surfactant.16–22 One of the fundamental requirements in understanding the † Part of the “Janos H. Fendler Memorial Issue”. * Corresponding author. E-mail: [email protected]. Fax: (+86)-51487311374.

behavior of these mixed systems is the knowledge of the interaction mechanism and binding mode which are critical for solution properties and subsequently their application. The observed interaction modes, such as string-of-beads and coralshape usually in the system of water soluble polymer and surfactant,23,24 may probably exist in the system of PEO-PPOPEO copolymer and surfactant, because the copolymer behaves like normal water soluble polymer at lower temperatures.2 What is more, some more special modes may appear in copolymer/ surfactant system because of the temperature-induced larger copolymer aggregations, and the concentration of each component in the system and the temperature of solution may be the main factors to realize structural transition of these complexes. A decomposition of the copolymer aggregates induced by surfactant was detected in F127/SDS system.25 This kind of breakdown process of polymer micelles was also found in other similar system of E71/G7/E71 (EO71(OCH2CH(CH2OC6H5))7EO71)/SDS. Besides, three surfactant concentration regions were observed, within which the interaction mechanism and microstructure of complex were different.27 In addition, temperature-induced sphere-to-rod transitions of P123/C12EO6 mixed micelles have been observed.28 In the present work, the systemic measurements were carried out to explore temperature and surfactant-induced structural transition of complex formed by widely used triblock copolymer F127 and nonionic surfactant TX-100. The concentration of F127 is kept constant at 20 mg/mL as the CMT is near room temperature (about 26 °C).29 The temperature is increased from 5 to 40 °C at a constant TX100 concentration, or TX-100 concentration is gradually increased up to about 250 mM at a certain temperature.

10.1021/jp802717p CCC: $40.75  2008 American Chemical Society Published on Web 07/08/2008

Formation and Microstructure Transition of F127/TX-100 Complex

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Experimental Section

Results

Materials and Sample Preparation. Pluronic F127 of average composition EO97PO68EO97 was obtained from Sigma Corp. and used without further purification. Poly(ethylene glycol) (PEG4000, MW ) 4000) was an Aldrich product. The nonionic surfactant Triton X-100 (TX-100) was also purchased from Sigma and used as received. The molecular structure of TX-100 was listed as the following:

NMR Measurements. To study the effect of TX-100 upon micellization of F127, temperature-dependent aggregation behavior of 20 mg/mL F127 aqueous solution in absence of TX100 is first investigated. Figure 1A shows the 1H NMR spectra of all the protons of F127 at various temperatures. The triplet at ∼1.16 ppm, the broad peaks from 3.75-3.45 ppm and the sharp singlet at ∼3.70 ppm are attributed to the protons of POCH3, PO-CH- and EO-CH2- respectively.32,33 At lower temperatures, the triplet of PO-CH3 groups and hyperfine structure of PO-CH2- groups are clearly observed. When the temperature rises above a certain value, they disappear in a small temperature interval, and the resonance peeks are broadened. Meanwhile, a new resonance signal “g” appears at ∼3.42 ppm and becomes progressively larger with increasing temperature, which is attributed to the PO-CH2- protons by the 2D heteronuclear single-quantum coherence (HSQC)-resolved 1H NMR spectra. 34 And the growth of “g” signal suggests the breakdown of the intramolecular attraction between the POCH2- protons and the ether oxygens at higher temperatures. All these features described above indicate the dehydration of PO block and subsequently micellization of F127. Figure 1B-E shows the temperature-dependent 1H NMR spectra of F127 in presence of TX-100 with different concentrations. None of the resonance peaks at each temperature are changed upon addition of TX-100 at very low concentration (0.36 mM, Figure 1B). But all the resonance peaks show upfield shifts, and the changes in the spectral profile associated with micelle formation move to lower temperature when the concentration of TX-100 is over 2.44 mM (Figure 1C). Besides, the hyperfine of PO-CH2- in 3.60-3.50 ppm shifts to 3.65-3.60 ppm. When the concentration of TX-100 is larger than 27.85 mM, the spectral profiles of PO protons change drastically (Figure 1D). The changes in the spectral profile associated with micelle formation start at 5 °C. And the hyperfine structure of PO-CH2- protons within 3.75-3.60 ppm remains over the entire temperature range investigated, possibly due to the overlap with EO-CH2- signals of TX-100. What is more, the resonance peaks of PO-CH3 protons experience downfield shifts at higher temperatures, and a shoulder peak appears which may arise from the interaction of TX-100 and PO block. Between the spectra in Figure 1E, where TX-100 concentration is higher (157.57 mM), and those in Figure 1D, no difference is observed except that the triplet of PO-CH3 signal disappears at 5 °C and the resonance signal “g” is much smaller. However, as far as the signal of EO segments is concerned, it remains relatively sharp over both the entire temperature and TX-100 concentration ranges investigated, suggesting that EO chains keep in contact with water and move freely. The observed chemical shifts (δ) of both PO-CH3 and EOCH2- protons as a function of temperature are presented in Figure 2A,B, respectively. For the system in absence of TX100, the chemical shift of PO-CH3 protons experiences slight linear decrease with temperature within 5-26 °C, and then it starts to decrease dramatically at 26 °C until about 35 °C, after which the decrease becomes slow again. The temperature 26 °C has been determined as CMT, and this value is in accordance with that reported.29 After the introduction of TX-100 with low concentrations (99.5%) was used as external standard, and the presaturation method was used to further suppress the proton signal of the solvent. Dynamic Light Scatter, DLS. The DLS experiments have been described in detail in our previous paper.30 In this study, all the samples were heated from lower temperature to higher to eliminate the difference caused by heating and cooling during the temperature effect study,31 and the samples were thermostatted at desired temperature for at least 10 min. Experiment duration was 10 min, and each experiment was repeated two or more times. The error of Rh value obtained was within ( 0.1 nm. Isothermal Titration Calorimetry, ITC. ITC measurements were performed using a VP-ITC titration microcalorimeter (MicroCal Inc., Northampton, MA) at desired temperature. The removable integrated injection-stirrer (250 µL) was filled with 250 mM TX-100 aqueous solution, which was titrated into the calorimeter vessel that initially contained 1.438 mL of 20 mg/ mL F127 aqueous solution in portions of 3 µL at the constant stirring speed of 502 ram to ensure thorough mixing. The duration of each injection was 10 s, and the time delay (to allow equilibration) between successive injections was 240 s. Doubly distilled water was filled in the reference cell. Raw data were obtained as a plot of heating rate (µcal/s) against time (min). These raw data were then integrated to obtain a plot of observed enthalpy change per mole of injected TX-100 (∆Hobs, kJ · mol-1) against TX-100 concentration (mM). All experiments were repeated twice to achieve the reproducibility within (2%.

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Figure 1. 1H NMR signals of EO-CH2-, PO-CH2-, and PO-CH3 in 20 mg/mL F127 aqueous solution in presence of TX-100 with different concentrations/(mM): (A) no TX-100, (B) 0.36, (C) 2.44, (D) 27.85, (E) 157.57 at various temperatures.

Formation and Microstructure Transition of F127/TX-100 Complex

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Figure 2. Temperature-dependent 1H NMR chemical shifts of PO-CH3 (A) and EO-CH2- (B) signals in 20 mg/mL F127 aqueous solution with different TX-100 concentrations.

temperature, after which it increases to a certain value, and then decreases again. We denote this range as intermediate TX-100 concentration region. When the concentration of TX-100 is larger than 157.57 mM, the interaction of F127 with TX-100 is saturated as indicated by both of the DLS results and the NMR signals of TX-100 shown below, and this range is referred to as high TX-100 concentration region. As for the chemical shift of EO-CH2- protons (Figure 2B), it shows a slight linear decrease with rising temperature at a certain TX-100 concentration, and it increases with the concentration of TX-100 at a certain temperature. The slight decrease of chemical shift with rising temperature indicates that EO blocks experience a small degree of dehydration upon heating, and it is not disturbed by the addition of TX-100. In addition, TX-100 concentration dependent CMT values of F127 are plotted in Figure 3, from which we can find that the CMT value only decreases slightly at lower TX-100 concentrations (3.2 mM), indicating that the addition of TX-100 promotes F127 micelle formation, which is not prominent at lower TX-100 concentrations. The similar phenomenon has also been observed in P123/ C12EO6 system.28 It should be noted that CMT values of F127 in the presence of TX-100 with higher concentrations (>94.85 mM) are hard to estimate. But it is certain that the CMT values keep decreasing until the concentration of TX-100 increases to 157.57 mM, because the decreasing rate of chemical shift increases with TX-100 concentration as seen in Figure 2A. The spectral profiles of TX-(CH3)3 and phenyl protons of TX100 in F127/TX-100 systems investigated above are shown in Figure S1 in the Supporting Information, and the chemical shifts obtained are presented in Figure 4. The dashed lines correspond to the TX-100/H2O system for comparison, where the chemical shifts of both TX-(CH3)3 and phenyl protons undergo slight linear decrease with rising temperature in all the three TX-100 concentration regions. The main reason is that water is continuously excluded from TX-100 micelles upon heating30,35 and the deshielding effect of water on these protons is impaired, which can be verified by the reducing size of TX-100 micelles with rising temperature (Table 1). However, the change of chemical shift with temperature strongly depends on TX-100 concentra-

Figure 3. TX-100 concentration dependence of CMT of F127 (0) and of the temperature where F127/TX-100 complex with F127 micelle as skeleton starts breaking down (O).

tion after the addition of F127. In low TX-100 concentration region (curves a-c in Figure 4A), the chemical shift of TX(CH3)3 protons increases slightly first at lower temperatures followed by an abrupt increase within a small temperature interval, after which the increase rate slows down and the chemical shift even decrease at higher TX-100 concentration (curve c in Figure 4A). As for phenyl protons in this region (curve a in Figure 4B), the chemical shift decreases slightly at lower temperatures first, and then decreases abruptly until another temperature region within which there is a final slower decrease. In intermediate TX-100 concentration region, the chemical shift of TX-(CH3)3 protons increases first (Figure 4A curve d), and then decreases with further increasing temperature. But in the high TX-100 concentration region (curve e in Figure 4A and curve c in Figure 4B), the chemical shifts of both TX(CH3)3 and phenyl protons show linear decrease within the entire

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Figure 4. Temperature-dependent 1H NMR chemical shifts of TX-(CH3)3 (A) and phenyl proton (B) signals of TX-100 aqueous solution in absence (hollow symbols) and presence (filled symbols) of 20 mg/mL F127 at different TX-100 concentrations.

TABLE 1: Temperature Effect on the Apparent Hydrodynamic Radius (Rapp,h/nm) of 20 mg/mL F127 in Absence and Presence of TX-100 with Different Concentration/(mM) No TX-100 T/°C

F127 fast

5 10 15 20 25 30 35 40

4.7 4.1 3.7 3.3 2.2

F127 middle

31.4 23.7 14.0 10.0 8.8 8.0

0.35 F127 fast 4.5 3.9 3.4 2.9 3.0

2.44

F127 slow

157.57

No F127

F127

No F127

F127

No F127

F127 fast

F127 slow

6.4

5.4 5.8 10.2 12.2 10.2 8.5 7.7 7.0

5.9

8.0 7.2 5.9 5.6 5.1 4.9 4.6 4.4

5.4

4.4 3.9 3.6 3.2 3.3 3.0 3.0 3.2

22.6 22.9 16.4 21.1 14.3 21.8 17.5 16.9

4.8 14.8 13.9 9.8 8.3 7.6

27.85

3.7 3.1

temperature range investigated. In addition, comparing with the systems with and without F127, we can find that the addition of F127 leads to upfield shift of TX-(CH3)3 protons and downfield shift of phenyl protons in low TX-100 concentration region (hollow and filled squares in Figure 4A,B), and these chemical shift differences become significant with rising temperature. That the chemical shift difference caused by F127 addition decreases with increasing TX-100 concentration, and it is even not affected in high TX-100 concentration region (hollow and filled diamonds in Figure 4A,B). It should be noted that the change features of spectrum profiles of TX-(CH3)2 and TX-CH2- protons are similar to these of TX-(CH3)3, so the results are not listed. Figure 5 shows the effect of TX-100 concentration on the chemical shift of both PO-CH3 and EO-CH2- protons of F127 at constant temperatures. As for PO-CH3 protons, the change feature of chemical shift strongly depends on temperature (Figure 5A): in lower the temperature range (5-10 °C), the chemical shift increases slightly, and then increases significantly with further addition of TX-100; within 15-25 °C, the addition of TX-100 leads to a slight decrease of chemical shift first, and then it decreases dramatically until a minim value, after which it increases with further addition of TX-100, and two inflections in the curve have been denoted in Figure 5A; at higher temperatures (30-40 °C), the second inflection still exits while the first one disappears. The molar ratios of TX-100 to F127 corresponding to these two inflections vs temperature are plotted in Figure 6. The ratio corresponding to the first inflection decreases linearly with temperature, and that of the second also

4.9 4.3 4.2

5.1 4.8 4.8

decreases linearly until a relatively constant value at higher temperatures (30-40 °C). As for EO-CH2- protons of F127 (Figure 5B), the chemical shift increases exponentially with TX100 concentration at all temperatures, which is a manifestation of increased hydration of the EO segments. Similar experiments have been conducted on PEG4000/TX-100 system to explore whether TX-100 interacts separately with the individual end EO blocks of F127. PEG4000 is chosen because its molecular weight is close to that of EO block of F127 (3800). As shown in Figure 5B, the chemical shift of EO-CH2- of PEG 4000 also increases in the exponential fashion. This suggests direct interaction between TX-100 and EO chain which has no relevance to the special configuration or aggregation of F127. DLS Measurements. DLS measurements have been performed at all the systems investigated by NMR, and the relaxation time distributions (τA(τ) versus log(τ/µs)) obtained are shown in Figure 7. In F127 aqueous solution in absence of TX-100 (Figure 7A), the relaxation distributions are bimodal at 5 °C. The fast mode corresponds to F127 unimers,32 and the broad slow mode has been attributed to large clusters of unimers induced by more hydrophobic components in the sample.27 At higher temperatures (10-20 °C), the relaxation time distribution of F127 micelle appears and hence, unimers, micelles, and larger clusters coexist in the solution. At 30 °C, micelles become the main scattering species, resulting in the monomodal distribution, and it becomes narrower as the temperature increases. Figure 7B,C shows typical relaxation time distribution of F127/TX100 aqueous solution in low TX-100 concentration region. At very low concentration (0.35 mM), the relaxation time distribu-

Formation and Microstructure Transition of F127/TX-100 Complex

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Figure 5. Effect of TX-100 concentration on the 1H NMR chemical shifts of PO-CH3 (A), EO-CH2- (B) of F127 in 20 mg/mL F127/TX-100 system at various temperatures. The chemical shift of EO-CH2- in 14 mg/mL PEG4000/TX-100 system is included in Figure B (9) for comparison.

Figure 6. Effect of temperature on the critical molar ratios of TX100 to F127 corresponding to the first and second inflections denoted in the curves in Figure 5, respectively.

tion varies with temperature similarly to that in absence of TX100 except that F127 micelle mode does not appear until 20 °C (Figure 7B). This suggests that the effect of TX-100 with lower concentration on the aggregation of F127 is negligible, which is in accordance with the result of NMR discussed above. But when the concentration of TX-100 is over 2.44 mM (Figure 7C), only a broad unsymmetrical signal mode is observed at low temperatures (5-10 °C), which may indicate two unsolved peaks that are observed at higher temperatures (15-25 °C). The fast mode is ascribed to F127 unimer and the slow one to F127/ TX-100 complex. At temperatures higher than 30 °C, the unimer mode disappears and the relaxation time distribution becomes single again, the single mode becomes narrower and the relaxation time shifts to faster time with rising temperature. When the concentration of TX-100 reaches 9.42 mM, the

intermediate TX-100 concentration region is entered (Figure 7D). A single mode covers the entire temperature range investigated. It is broad at lower temperatures and becomes narrower and shifts to faster time with rising temperature. Interestingly, two relaxation modes are observed at all temperatures when TX-100 concentration is larger than 157.57 mM (Figure 7E). The slow mode is assumed to the diffusion of a larger F127/TX-100 complex, and the fast mode is attributed to free TX-100 micelles as it has a Rapp,h value close to that of free TX-100 micelles (Table 1), from which we can deduce that the interaction of F127 with TX-100 is saturated. Thus, we denote this range (>157.57 mM) as higher TX-100 concentration region. In order to follow TX-100-induced microstructure changes of F127/TX-100 complex, the concentration of TX-100 is increased gradually and the temperature is kept constant. Typical results at 5, 20, and 40 °C are shown in Figure 8A-C, respectively. At 5 °C (Figure 8A), the bimodal relaxation time distribution of F127 solution is not affected upon addition of small amount of TX-100 ( 2000) may interact with several TX-100 micelles forming larger coral-shape clusters. Here, the molecular weight of each EO blocks of F127 is about 4300, which is long enough to wrap about several complexes. Hence we can safely assume that the larger clusters in the solution correspond to clusters of F127/TX-100 complex. Besides, the inner skeleton of the cluster still transfers from TX-100 micelle successively into F127 micelle and TX-100 again with rising temperature just as in the case of intermediate TX-100 concentration region. This is manifested by the same spectral change feature of all protons of F127 with temperature in high and intermediate region (Figure 2A). However, the chemical shifts of TX-100 protons are almost the same with those in the system without F127 because large amounts of free TX-100 micelles contribute to apparent signals of TX-100 (Figure 4). The structure illustrations in high TX-100 concentration region are shown in Figure 11K-M. TX-100-Induced Structure Transition of F127/TX-100 Complex. Different types of F127 aggregations, such as unimers, the mixture of unimers and micelles, and well-defined micelles, can be obtained by keeping the temperature at a certain value. The focus of this section is on the TX-100-induced formation and structural transition of complex formed by these different types of F127 aggregations and TX-100. At lower temperatures (5-10 °C), unimers are the main type of diffusion objects in solution. And for the copolymer of ABA type, the relatively hydrophobic block B usually winds to prevent direct contact with water.27,41 The fast narrow mode in the relaxation time distribution of F127 solution at low temperature arises from the random coil of single copolymer chains. When a small amount of TX-100 micelles are initially added, the random coil of F127 unimer swells,42 and the stretched polymer chains partly insert into the hydrophilic layer of TX-100 micelles or wrap around them30 forming F127/TX100 complex (Figure 11D). The monomodal of relaxation time distribution at lower TX-100 concentrations is narrow (Figure 8A) and the apparent radius is small (Figure 9), because the main type of aggregation in solution is still F127 random coil. As the chains of F127 unimer are stretched upon addition of TX-100, the contact of PO blocks of F127 with water is improved resulting in initial slight increase in chemical shift of PO-CH3 protons (Figure 5A). Addition of TX-100 in excess of a certain value, large amounts of F127 random coils stretch and adsorb on TX-100 micelles resulting in a sudden increase in the chemical shift of PO-CH3 protons (Figure 5A). In addition, as the amount of the larger F127/TX-100 complex with longer F127 chains adsorbing on TX-100 micelles increases with TX100 concentration, the apparent hydrodynamic radius increases (Figure 9). The broad relaxation time distribution at higher TX100 concentration may possibly arise from the contribution of free TX-100 micelles or the varying number of F127 unimers in the complex. In short, F127 interact with TX-100 micelles much in the same way as most normal unassociated polymers, such as PEG at lower temperatures. 30 Within higher temperature range (15-25 °C), the hydrogen bond between PO block of F127 and water is weaker than that at lower temperatures (5-10 °C). Some of the PO blocks dehydrate and aggregate, thus, unimers, micelles, and larger clusters coexist in the system (Figure 11B), resulting in the three mode distribution as shown in Figure 8B. Upon addition of small amounts of TX-100 micelles, the hydrophobic interaction between the PO blocks and TX-100 micelles drive F127 unimers to aggregate on TX-100 micelles forming F127/TX-100 complex. This TX-100-induced aggregation of F127 tends to occur

14576 J. Phys. Chem. B, Vol. 112, No. 46, 2008 within intermediate temperature range (15-25 °C) because the hydrogen bond between PO blocks and water is weak and easy to disturb. The PO blocks transfer from water to the hydrophobic core of TX-100 micelles manifested by the slight decrease of chemical shift of PO-CH3 protons (Figure 5A). However, the effect of TX-100 at lower concentration on the aggregation behavior of F127 is small, which is confirmed by the slight decrease of CMT upon addition of small amounts of TX-100 (Figure 3). With further addition of TX-100, large amounts of F127 unimers aggregate with TX-100 micelles leading to dramatic decrease of the chemical shift until a minim value, where most of the F127 unimers finish aggregation. At this point, the molar ratio of TX-100 to F127 in the complex is estimated to be about 24.6. This aggregation process induced by TX-100 is also verified by the relaxation time distribution in Figure 8B. The unimer mode shrinks continuously with TX100 concentration until 9.89 mM where it disappears, indicating that most of the F127 monomers finish aggregation. And the middle mode of F127/TX-100 complex grows to be the only single mode in solution. As the concentration of F127 in solution is fixed, further addition of TX-100 may result in increasing amount of TX-100 in F127/TX-100 complex. This will cause the distance between PO blocks in the core of complex to be enlarged, and the structure of the complex to become looser. Therefore, the contact of PO blocks with water is improved, which results in the increase of chemical shift (Figure 5A). In addition, within the temperature range of 15-25 °C, the equilibrium of F127 unimer and micelle shifts to micelle with rising temperature. Therefore, the amounts of TX-100 decrease with rising temperature which cause either large amounts of F127 unimers to begin to aggregate, or the complex structure to start to become loose. So the molar ratios of TX-100/F127 corresponding to both of the two inflections denoted in Figure 5A decrease with temperature (Figure 6). In brief, gradual addition of TX-100 in F127 solution prompts the aggregation of F127 forming complex with F127 micelle as skeleton (Figure 11E), but further addition of TX-100 may leads to the loose of F127 micelles for the “spacer effect” of TX-100 which causes the separation of EO chains as mentioned (Figure 11H). This process occurs only at temperatures near CMT because the hydrogen bond of PO block and water is weak and easy to break. At higher temperatures (>30 °C), F127 exists predominantly in the form of micelle, and small amounts TX-100 may penetrate into the F127 micelles forming rich-in-F127 complex as shown in Figure 11F. The penetration of TX-100 may possibly extract some water from the hydrophobic core, because it is composed of PO block and contains a significant amount of water.43 Thus, the deshielding effect of water on PO protons is weakened, which leads to the initial increase of chemical shift as seen in Figure 5A. With addition of TX-100 of a certain excessive amount, F127 aggregations start to disassociate because of the increased TX-100 “spacer effect” mentioned above, and the structure of complex transfers from rich-in-F127 to rich-in-TX100. The observed change in Rapp,h value from 10.0 to 4.7 nm supports this assumption. And the rehydration of PO blocks causes the increase of the chemical shift of PO-CH3 protons (Figure 5). The breakdown process of F127/TX-100 complex is demonstrated in Figure 11 C-I. It should be noted that, the breakup of the complex composed of copolymer and ionic surfactant has also been observed by Jansson et al.27 and it is mainly attributed to the electrostatic repulsion between copolymer micelles and ionic surfactant. And the “spacer effect” of surfactant plays an important role in the breakdown of copolymer micelles in the system of nonionic copolymer and nonionic

Ge et al. surfactant. In addition, previous studies in P123/C12EO6 system show that the overall average of EO area of P123 micelle is reduced upon addition of the nonionic surfactant C12EO6, which facilitates the temperature-induced sphere-to-rod transition of P123 micelle.28 But the hydrophobic chain of TX-100 is shorter than C12EO6 and the hydrophobic chain is longer than C12EO6, so TX-100 monomer may probably locates in the outer side of copolymer micelles, and the overall average area of EO chains increased, and this “spacer effect” results in the breakdown of F127 micelle. TX-100-induced structure transitions of F127/TX-100 complex at 5, 20, and 40 °C are confirmed by ITC results (Figure 10). Possible contributions to thermal profiles in Figure 10 include the dilution of TX-100 micelles from the injected concentrated TX-100 solution, the conformation changes in F127 chains, the dehydration of F127 and the binding between F127 and TX-100.26,44 Considering the difference between the enthalpy profiles of systems in absence and presence of F127, only the latter three of the contributions mentioned above are positive and dominant. At a lower temperature (5 °C), the addition of TX-100 causes the random coil of F127 unimer to break, and the chains to stretch and combine with TX-100 micelles. The conformation change of unimers and their combination with TX-100 micelles bring about endothermic enthalpy changes. However, the contact of both PO and EO blocks of F127 unimer with water is improved as indicated by NMR results, and this exothermic hydrate process also contributes to the apparent enthalpy. So the enthalpy values observed are relatively small (0.10-0.16 kJ/mol, Figure 10A). However, the enthalpy per injection at 20 °C shown in Figure 10B is several kilojoules higher than that at 5 °C. The possible reason is that all the dehydration of PO blocks, the conformation changes of F127 chains, and the combination of F127 with TX100 micelles contribute to the large apparent endothermic enthalpy during TX-100-induced aggregation of F127, from unassociated unimers to F127/TX-100 complex. At 40 °C, F127 unimers have completed aggregation, and only the combination of F127 micelles with TX-100 and subsequently the dehydration of PO blocks in the micelle core contribute to the endothermic, so the initial enthalpy values at 40 °C (0.88 kJ/mol) is lower than that at 20 °C (4.57 kJ/mol). The endothermic enthalpy decreases with TX-100 concentration because the dehydration process is hindered by the loosening of complex structure for the “spacer effect” of TX-100. And the skeleton of F127 micelle is broken down at a certain TX-100 concentration, the subsequent rehydration of PO chains causes the high exothermic of the system. In addition, we have to note that calorimetry measurements cannot span the entire TX-100 concentration region covered by NMR and DLS measurements because TX100 solution at a desired concentration (about 600 mM) for titration is too viscous for ITC instruments, but it is efficient for low and part of intermediate TX-100 concentration region. Conclusions Synergetic interaction between triblock copolymer F127 and nonionic surfactant TX-100 is observed, and the microstructure of F127/TX-100 complex can be controlled by both the temperature and the concentration of TX-100. Temperature-induced structure transition: At low TX-100 concentration region (157.57 mM), and larger clusters of F127/TX-100 complex coexist with free TX-100 micelles. TX-100-induced structure transition: At lower temperatures (5-10 °C), gradual addition of TX-100 micelles provides the interface for the adsorption of F127 unimers, and complexes are formed with F127 unimer wrapping around TX-100 micelles. At 15-25 °C, the addition of TX-100 micelles prompts the aggregation of F127 unimers forming complexes with skeleton of loosened F127 micelle; at 30-40 °C, TX-100 monomers insert into F127 micelles, which leads to the breakdown of the copolymer aggregates, and finally the obtained F127 unimers interact with TX-100 micelles forming complex with TX-100 micelle as skeleton. Acknowledgment. This work was supported by the National Nature Science Foundation of China (Nos. 20633010 and 20773106). Supporting Information Available: 1H NMR spectra of TX-(CH3)3 and phenyl protons of the TX-100 system of 20 mg/ mL F127/TX-100 with increasing TX-100 concentrations (mM): (A) 0.36; (B) 2.44; (C) 27.85; (D) 157.57 at various temperatures. 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