Article pubs.acs.org/Langmuir
Effects of Cationic Ammonium Gemini Surfactant on Micellization of PEO−PPO−PEO Triblock Copolymers in Aqueous Solution Ruijuan Wang, Yongqiang Tang, and Yilin Wang* Key Laboratory of Colloid and Interface Science, Beijing National Laboratory for Molecular Sciences (BNLMS), Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China S Supporting Information *
ABSTRACT: Effects of cationic ammonium gemini surfactant hexamethylene-1,6-bis(dodecyldimethylammonium bromide) (12−6−12) on the micellization of two triblock copolymers of poly(ethylene oxide)−poly(propylene oxide)−poly(ethylene oxide), F127 (EO 97 PO 69 EO 97 ) and P123 (EO20PO70EO20), have been studied in aqueous solution by differential scanning calorimetry (DSC), dynamic light scattering (DLS), isothermal titration calorimetry (ITC), and NMR techniques. Compared with traditional single-chain ionic surfactants, 12−6−12 has a stronger ability of lowering the CMT of the copolymers, which should be attributed to the stronger aggregation ability and lower critical micelle concentration of 12−6−12. The critical micelle temperature (CMT) of the two copolymers decreases as the 12−6−12 concentration increases and the ability of 12−6−12 in lowering the CMT of F127 is slightly stronger than that of P123. Moreover, a combination of ITC and DLS has shown that 12−6−12 binds to the copolymers at the temperatures from 16 to 40 °C. At the temperatures below the CMT of the copolymers, 12−6−12 micelles bind on single copolymer chains and induce the copolymers to initiate aggregation at very low 12−6−12 concentration. At the temperatures above the CMT of the copolymers, the interaction of 12−6−12 with both monomeric and micellar copolymers leads to the formation of the mixed copolymer/12− 6−12 micelles, then the mixed micelles break into smaller mixed micelles, and finally free 12−6−12 micelles form with the increase of the 12−6−12 concentration.
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Hoffmann and co-workers6,7 studied the influence of SDS on the aggregation of F127 (EO97PO69EO97) and found that a very small amount of SDS interfered with the micelle formation of F127, and the micellization of F127 was completely suppressed when the SDS concentration was sufficiently high. Holzwarth et al.8−10 found that the binding of SDS or tetradecyltrimethylammonium bromide (TTAB) to monomeric F127 resulted in the formation of micelles on single F127 chains, while the binding of SDS or TTAB to micellar F127 caused the formation of mixed F127/SDS or F127/TTAB micelles, followed by the dramatic breakdown of mixed F127/SDS or F127/TTAB micelles into smaller mixed aggregates and then to complete dissociation of F127 micelles. In addition, Olofsson et al.11,12 studied the dissociation of the copolymer micelles with different PO/EO ratios by SDS and hexadecyltrimethylammonium chloride (CTAC). The result showed that the copolymers with higher PO/EO ratio possessed a significantly higher stability of micelles against surfactants. Jansson et al.13 found that the CTAC monomers of low concentration associated with the P123 (EO20PO69EO20) micelles and led to larger
INTRODUCTION Poly(ethylene oxide)−poly(propylene oxide)−poly(ethylene oxide) (PEO−PPO−PEO) triblock copolymers are watersoluble nonionic macromolecular surfactants, which can selfassemble into micelles above their critical micelle concentration (CMC) or above their critical micelle temperature (CMT) in dilute aqueous solutions. The self-assembly of PEO−PPO− PEO copolymers mainly arises from limited and temperaturedependent solubility of hydrophobic PPO block in water,1,2 which forms the hydrophobic core of micelles surrounded by a water-swollen corona of PEO blocks.3,4 The formation of copolymer micelles is strikingly dependent on temperature, concentration, and composition of the copolymers. In many applications of PEO−PPO−PEO triblock copolymers, low molecular weight ionic surfactants are always used to enhance surface activity or reduce CMT and so on. The interactions of traditional single-chain ionic surfactants with the triblock copolymers have been widely studied. Almgren et al.5 applied the NMR technique to study the association process of copolymers L64 (EO13PO30EO13) and F68 (EO78PO30EO78) with sodium dodecyl sulfate (SDS). The addition of SDS initially induced the formation of mixed copolymer/SDS micelles at a much lower temperature than its CMT, but pure SDS at high SDS concentrations formed micelles. © 2014 American Chemical Society
Received: January 3, 2014 Revised: February 4, 2014 Published: February 14, 2014 1957
dx.doi.org/10.1021/la500025k | Langmuir 2014, 30, 1957−1968
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Article
Above the CMT of the copolymers, the continuous addition of 12−6−12 into the monomeric and micellar copolymers or micellar copolymers leads to the formation of the mixed copolymer/12−6−12 micelles; then the micelles break into smaller mixed micelles and eventually free 12−6−12 micelles appear in the system.
complexes, while at intermediate CTAC concentrations P123 micelle/CTAC complexes and CTAC-rich complexes were observed, and only CTAC-rich complexes were formed at high CTAC concentrations. Bharatiya et al.14 observed that the addition of SDS or cetylpyridinium chloride (CPC) to P123 micelles caused micelle size to become smaller and then larger with a double relaxation mode; however, the effect of SDS or CPC on F127 micelles did not display double relaxation at high surfactant concentrations. In the past two decades, gemini surfactants have been attracting great attentions due to their strong self-assembly ability.15 Their strong self-aggregation ability can be applied to effectively influence the aggregation behavior of copolymers. Therefore, some recent studies have reported the interactions between cationic ammonium gemini surfactants [CmH2m+1(CH3)2N(CH2)sN(CH3)2CmH2m+1]Br2 (m−s−m) and triblock copolymers. Bakshi et al.16 studied the interaction of copolymers (EO 1 8 PO 3 1 E O 1 8 , E O 2 PO 1 5 . 5 EO 2 , and EO2.5PO31EO2.5) with TTAB, hexadecyltrimethylammonium bromide (CTAB), and a cationic ammonium gemini surfactant 10−2−10. It was found that the interaction parameter β values were always negative for the mixtures of the copolymers with TTAB or CTAB, and accordingly the mixtures showed synergistic interaction. However, the β values were always positive for the copolymer/10−2−10 mixture, and thus the mixture showed the antagonistic interaction. The difference has been attributed to the dimeric nature of 10−2−10, which may create steric hindrances to the triblock copolymers at the headgroup regions. Bakshi et al.17 also studied the mixed systems of F127/12−2−12 and P103 (EO16PO56EO16)/12−2− 12 in the temperature range of 21−40 °C. They found that the mixed micelles formed due to synergistic interaction. The synergistic interaction of F127/12−2−12 was much stronger than that of P103/12−2−12 and increased with increase in temperature. Verrall et al.18 studied the mixed systems of gemini surfactants 12−3−12 and 12−6−12 with F108 (EO112PO56EO112), P103, and F68 (EO79PO30EO79). The most hydrophobic copolymer P103 showed the strongest interaction with the gemini surfactants and the variations of the surfactant headgroup size only slightly affected their interactions. Verrall et al.19,20 continued their study and found that the interaction of monomeric and micellar copolymers with gemini surfactants was similar to with conventional single-chain ionic surfactants; however, the critical aggregation concentrations of gemini surfactants in the presence of monomeric copolymers were significantly reduced. In the present work, the interactions of cationic ammonium gemini surfactant [C12H25(CH3)2N(CH2)6N(CH3)2C12H25]Br2 (12−6−12) with two triblock copolymers F127 (EO97PO69EO97) and P123(EO20PO70EO20) have been investigated. These two copolymers have the similar PPO block lengths but have different PEO block lengths. The critical micelle temperatures (CMT) of F127 and P123 in the concentration range of 0.01−1.00 wt % were determined by differential scanning calorimetry (DSC) at first. Then, the effects of 12−6−12 on the micellization of the two copolymers were investigated by DSC, NMR, isothermal titration calorimetry (ITC), and dynamic light scattering (DLS). The results indicate that 12−6−12 has a much stronger ability of lowering the CMT of copolymers than traditional single-chain ionic surfactants. Moreover, below the CMT of the copolymers, the binding of 12−6−12 to monomeric copolymers results in the formation of 12−6−12 micelles on copolymer chains.
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EXPERIMENTAL SECTION
Materials. Two triblock copolymers of EO97PO69EO97 (F127) and EO20PO70EO20 (P123) were purchased from Aldrich. The nominal molar masses of F127 and P123 are 12 600 and 5800 g/mol, respectively. Their cloud points of 1.00 wt % solution are 100 and 90 °C, respectively. Cationic ammonium gemini surfactant hexamethylene-1,6-bis(dodecyldimethylammonium bromide) [C12H25(CH3)2N(CH2)6N(CH3)2C12H25]Br2 (12−6−12) was synthesized and purified according to the method of Zana et al.21 Its structure was confirmed by mass spectroscopy and 1H NMR, and the purity was verified by elemental analysis and surface tension. Triply distilled water (18.2 MΩ·cm) was used in all experiments from Milli-Q equipment. Differential Scanning Calorimetry (DSC). The DSC thermograms of F127, P123, F127/12−6−12, and P123/12−6−12 solutions were obtained using a VP-DSC microcalorimetric system (MicroCal). The solutions were degassed for 20 min, while pure water was degassed for 5 min. Each of the solutions was transferred into a Tantaloy 61 alloy sample cell of 500 μL, and the final weight of the solution was taken. The sample cell containing the copolymer solution or mixed copolymer/surfactant solution and the reference cell filled with water were cooled down to 1.0 °C for 15 min before the temperature scan started. The DSC thermograms were recorded at a heating rate of 30 °C/h. Isothermal Titration Microcalorimetry (ITC). The calorimetric measurements were taken in a TAM 2277-201 microcalorimetric system (Thermometric AB, Järfälla, Sweden) with a 1 mL stainless steel sample cell at the desired temperature T ± 0.01 °C. The cell was initially loaded with 0.8 mL of water, 1.00 wt % F127, or P123 solution. The 12−6−12 solution was injected consecutively into the stirred sample cell in each aliquot of 5 μL via a 500 μL Hamilton syringe controlled by a 612 Thermometric Lund pump until the desired concentration range was covered. During the whole titration process, the system was stirred at 75 rpm with a gold propeller, and the interval between two injections was long enough for signals to return to the baseline. The observed enthalpy (ΔHobs) was obtained by integrating the areas of the peaks in the plot of thermal power against time. Dynamic Light Scattering (DLS). The measurements of F127, P123, F127/12−6−12, and P123/12−6−12 solutions were carried out at the desired temperature T ± 0.1 °C with an LLS spectrometer (ALV/SP-125) which employs a multi-τ digital time correlator (ALV5000). A solid-state He−Ne laser (output power of 22 mW at λ = 632.8 nm) was used as a light source, and the measurements were conducted at a scattering angle of 90°. All solutions made freshly were filtered through a 0.45 μm membrane filter of hydrophilic PVDF prior to measurements. The correlation function of scattering data were analyzed via the CONTIN method to obtain the distribution of diffusion coefficients (D) of the solutes. The apparent hydrodynamic radius (Rh) was deduced from D by the Stokes−Einstein equation Rh = kT/6πηD, where k is the Boltzmann constant, T is the absolute temperature, and η is the solvent viscosity. 1 H NMR Measurements. 1H NMR spectra of the copolymer solutions and copolymer/surfactant solutions were recorded using a Bruker AV400 FT-NMR spectrometer operating at 400.1 MHz and at the desired temperatures. Deuterium oxide (99.9%) was purchased from CIL (Cambridge Isotope Laboratories) and used to prepare the stock solution of F127/12−6−12 or P123/12−6−12. About 600 μL solution was transferred to a 5 mm NMR tube for each measurement. Chemical shifts were given on the δ scale. The center of the HDO signal was used as the reference in the D2O solutions.22 The digital resolution of 1H NMR spectra was 0.04 Hz/data point. 1958
dx.doi.org/10.1021/la500025k | Langmuir 2014, 30, 1957−1968
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Figure 1. DSC curves for (a) F127 and (b) P123 in pure water at copolymer concentrations of 0.01, 0.05, 0.20, 0.50, and 1.00 wt %. Spin−spin relaxation time (T2) and 2D NOESY NMR experiments were performed on a Bruker Avance 600 spectrometer. The T2 values were measured by the Carr−Purcell−Meiboom−Gill (CPMG) sequence (PD-90°x-[τ-180°y-τ]2n-AC). The 2D NOESY experiments were carried out with the standard three-pulse sequence with a mixing time of 800 ms.
a similar variation trend. Without 12−6−12, the endothermic peaks of 1.00 wt % F127 and P123 are respectively located at 24.7 and 17.4 °C. When a small amount of 12−6−12 is added, the endothermic peak shifts to lower temperature and meanwhile the endothermic peak intensity decreases. When the 12−6−12 concentration reaches 3.00 mM in F127 or 5.00 mM in P123, another endothermic peak appears at much lower temperature, and the two endothermic peaks overlap with no distinct demarcation line. However, further adding 12−6−12 to 5.00 mM in F127 or 20.00 mM in P123 causes the first original endothermic peak to disappear, only leaving the second endothermic peak at the lower temperature. The overlapping of the endothermic peaks at intermediate 12−6−12 concentrations suggests that two different micelles coexist in solution and the intermediate 12−6−12 concentration region is the transition period. In summary, with increase of the 12−6−12 concentration, the CMT of the two copolymers decreases and becomes lower than 5 °C at high 12−6−12 concentrations. The lower CMT cannot be precisely determined because the temperature is lower than the detection limit of the DSC instrument. The decreasing of the CMT upon increasing the 12−6−12 concentration results from the enhanced selfassembling ability of the mixture due to the addition of more surfactant molecules. The trends are quite different from those in conventional single-chain ionic surfactant/PEO−PPO−PEO systems. The CMT of F127 initially decreases about 2−3 °C by 2 mM SDS, then moves upward, and finally disappears as the SDS concentration increases,6,11 while the addition of SDS always enhances the CMT of P123.11 In particular, compared with traditional single-chain ionic surfactants, cationic gemini surfactant 12−6−12 has a much stronger ability of lowering the CMT of the PEO−PPO−PEO copolymers no matter what the PO/EO ratio the copolymer has. The CMT and the temperature corresponding to the DSC peak maximum Tm of the two copolymers at various 12−6−12 concentrations are summarized in Table 2. Compared F127/ 12−6−12 with P123/12−6−12, the ability of 12−6−12 in lowering the CMT of F127 is slightly stronger than that of P123, and P123/12−6−12 shows two overlapping endothermic peaks in a much wider 12−6−12 concentration region than F127/12−6−12. As mentioned above, the precise CMT values of the copolymers in the 12−6−12 concentration range from 3.00 to 30.00 mM cannot be determined by using the available
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RESULTS AND DISCUSSION Micellization of F127 and P123. Figure 1 presents the DSC curves of F127 and P123 in the concentration range from 0.01 to 1.00 wt %. All the DSC curves show an endothermic peak with a rise of temperature, which is attributed to the micellization of the triblock copolymers.2,23−25 As the copolymer concentration increases, the endothermic peak gradually shifts to lower temperature, and the peak intensity evidently enhances. According to previous reports, there are three different methods determining the CMT: the onset temperature (Tonset),26 at which the curve starts to deviate from the baseline; the inflection point temperature (Tinf),27 corresponding to the intersection of the tangent of the first inflection point of the peak with the baseline; the temperature of peak maximum (Tm).28−30 Here, the second method is adopted. The values of CMT for the two copolymers are summarized in Table 1. As the copolymer concentration Table 1. Critical Micelle Temperature (CMT) of F127 and P123 CMT (°C) wt %
0.01
0.05
0.20
0.50
1.00
F127 P123
34.0 21.8
30.2 20.5
27.4 19.3
25.7 18.2
24.7 17.4
increases, the CMT decreases and tends to be more sensitive to lower concentrations, consistent with previous reports.2,31,32 In addition, the CMT of F127 is larger than that of P123 because the PEO−PPO−PEO triblock copolymer with more hydrophilic group is more difficult to form micelles. Effect of 12−6−12 on Micellization of F127 and P123. Figure 2 shows the DSC curves of 1.00 wt % F127 and P123 with an increase in the 12−6−12 concentration, and the corresponding DSC curves with representative shapes are presented in Figure S1 (Supporting Information). The DSC curves of F127/12−6−12 and P123/12−6−12 mixtures display 1959
dx.doi.org/10.1021/la500025k | Langmuir 2014, 30, 1957−1968
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Figure 2. DSC curves for (a) F127/12−6−12 and (b) P123/12−6−12 at the copolymer concentration of 1.00 wt % as the 12−6−12 concentration increases.
Table 2. Micelle Transition Temperatures (CMT, Tm) of 1.00 wt % F127 and P123 with Various 12-6-12 Concentrations F127 C12−6−12 (mM)
CMT (°C)
Tm1 (°C)
0 0.01 0.05 0.10 0.30 0.50 1.00 3.00 5.00 7.00 10.00 15.00 20.00 30.00
24.7 24.4 24.0 23.7 22.9 22.8 22.6
27.9 27.9 27.8 27.0 26.4 26.4 26.1 6.0 7.2 8.4 9.1 11.1 11.3
DSC instrument, so we just give the Tm values of the two copolymers at these 12−6−12 concentrations. To understand the transition region at the intermediate surfactant concentration where two endothermic peaks are overlapped, two representative mixtures of 1.00 wt % F127/ 3.00 mM 12−6−12 and 1.00 wt % P123/10.00 mM 12−6−12 are chosen to be studied by the following DLS and NMR measurements. Figure 3 shows the micelle size distribution of 1.00 wt % F127/3.00 mM 12−6−12 and 1.00 wt % P123/10.00 mM 12− 6−12 from DLS in the temperature range of 5−40 °C. At 5 °C, small aggregates with Rh of ∼3 nm exist in the two systems. Because the DSC curves of the pure F127 and P123 (Figure 1) and 12−6−12 (Figure S2) do not present endothermic peaks at 5 °C, the endothermic peak around 5 °C in the DSC curves of the mixtures (Figure 2) indicates that the copolymer/12−6−12 mixed micelles are already formed at 5 °C. That is to say, F127 and P123 form micelles at the temperature greatly lower than the CMT of the copolymers themselves. The possible reason is that the addition of 12−6−12 would bring about hydrophobic interaction between the hydrophobic PPO blocks of the copolymers and the alkyl chains of 12−6−12, which results in
P123 Tm2 (°C)
CMT (°C)
Tm1 (°C)
17.4 17.4 17.3 17.2
20.7 20.7 20.5 20.3
17.1 16.9
19.4 19.2
Tm2 (°C)
25.7 5.1 6.1 7.6 9.3 10.8 11.1
19.3 19.6 19.3 14.7
Figure 3. Size distributions of aggregates from DLS for (a) 1.00 wt % F127/3.00 mM 12−6−12 and (b) 1.00 wt % P123/10.00 mM 12−6−12 at different temperatures. 1960
dx.doi.org/10.1021/la500025k | Langmuir 2014, 30, 1957−1968
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Figure 4. (a) Chemical structures and 1H NMR signal assignments of 12−6−12 and the copolymers. (b) 1H NMR spectrum of 1.00 wt % F127/3.00 mM 12−6−12 in D2O at 25 °C. 2D NOESY spectra of 1.00 wt % F127/3.00 mM 12−6−12 at 12 °C (c) and 40 °C (d). 2D NOESY spectra of 1.00 wt % P123/10.00 mM 12−6−12 at 11 °C (e) and 40 °C (f).
than that at 5 °C. The possible reason is that the effects of temperature on the dehydration of hydrophobic PPO blocks and the hydrogen bonds between hydrophilic PEO blocks and water may be very weak in such a narrow and low temperature range. In addition, the temperature increase in this range may affect these interactions in different extents. It is well-known that 2D NOESY is effective to study interactions between two different components in close proximity (
