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Cite this: Chem. Commun., 2014, 50, 14233 Received 22nd August 2014, Accepted 24th September 2014
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CO2 as a smart gelator for Pluronic aqueous solutions† Chengcheng Liu,‡a Qingqing Mei,a Jianling Zhang,*a Xinchen Kang,a Li Peng,a Buxing Han,a Zhimin Xue,a Xinxin Sang,a Xiaogan Yang,a Zhonghua Wu,b Zhihong Lib and Guang Mob
DOI: 10.1039/c4cc06623e www.rsc.org/chemcomm
It was found that CO2 could induce the gelation of Pluronic aqueous solutions, during which the microstructure of the solution transforms from randomly dispersed spherical micelles to cubic close packed micelles. The gelation switched by compressed CO2 has many advantages and can be applied in the synthesis of porous materials.
In recent years, special attention has been paid to the selfassemblies of amphiphilic block copolymers, PEO (poly ethylene oxide)–PPO(poly propylene oxide),1 mainly because of their biocompatibility, stability, designability and ease of application.2,3 The copolymers can form various kinds of self-assemblies in water, including micelles,4 liquid crystals,5 niosomes6 and gels.7 In particular, the gels formed by the PEO–PPO–PEO tri-block copolymer (usually known as a Pluronic copolymer) have attracted much interest due to their diverse uses as controlled release drug carriers,8 templates in mesoporous material synthesis,8a,9 and as electrolytes10 and hydrogel actuators.11 To induce the gelation of Pluronics in water is a very interesting topic. In general, the gelation is induced by changing the surfactant concentration,12 varying temperature,13 imposing irradiation,14 tuning pH,15 and adding different kinds of additives (i.e. gelators). The commonly used gelators are inorganic salts,16 organic solvents,8a,17 and polymers.8a,18 These solid or liquid gelators usually suffer from high economic cost, environmental burden, and post-treatment difficulties, and the gelation cannot be switched reversibly. a
Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China. E-mail: [email protected]; Fax: +86-10-62559373; Tel: +86-10-62562821 b Beijing Synchrotron Radiation Facility (BSRF), Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China † Electronic supplementary information (ESI) available: Materials, experimental details and characterization. See DOI: 10.1039/c4cc06623e ‡ Chengcheng Liu performed the whole experiment. Qingqing Mei and Li Peng worked on the UV-vis spectra characterization. Xinchen Kang and Zhimin Xue worked on the silica synthesis and characterization. Xinxin Sang, Xiaogan Yang, Zhonghua Wu, Zhihong Li, and Guang Mo worked on the small angle X-ray scattering experiment. Buxing Han and Jianling Zhang analyzed the data and proposed the mechanism. Jianling Zhang conceived the project and designed the experiments.
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Compressed CO2 has received much interest because it is nontoxic, inexpensive, tunable and nonflammable, and can be easily recaptured and recycled after use. To date, compressed CO2 has been used in different fields, including extraction and fractionation,19 materials science,20 chemical reactions,21 microelectronics,22 etc. Especially, CO2 has been found to be versatile in tuning the properties of a variety of surfactant assemblies,23 such as micelles,24 reverse micelles,25 vesicles,26 liquid crystals,27 microemulsions,28 and emulsions.29 Usually, CO2 has a viscosity-lowering effect on the surfactant solutions. For example, CO2 can cause the transition from liquid crystals to micellar solutions.27 Herein we used gaseous CO2 to induce the gelation of a Pluronic aqueous solution at room temperature. The gels with microstructures of face-centered cubic packed micelles were formed at a certain CO2 pressure. In comparison with the conventional solid or liquid gelators, the use of compressed CO2 has many advantages: (1) the gel properties can be tuned by controlling CO2 pressure; (2) CO2 can be easily removed by depressurization and recycled, which makes the post-treatment easier and more economical; and (3) the CO2-induced gelation is reversible. Fig. 1 shows the photographs of 20 wt% F87 aqueous solution with and without CO2 at 25 1C. In the absence of CO2, the solution appears to be a transparent fluid (photograph a). Upon adding CO2, no obvious changes were observed at lower CO2 pressure (photograph b). Interestingly, at 3.90 MPa, the viscosity of the system increased, but the solution was still fluidic (photograph c). As the pressure reached 4.06 MPa, the sample completely lost its
Fig. 1 Photographs of 20 wt% F87 aqueous solution without CO2 (a) and in the presence of CO2 at 2.26 MPa (b), 3.90 MPa (c), 4.06 MPa (d and e) at 25 1C.
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Table 1 Structural parameters for various phases extracted from SAXS measurements
P/MPa
Phase
Structure
Rg a/nm
0 2.26 3.80 4.02 4.10
Liquid Liquid Liquid Liquid Gel
Spherical Spherical hex hex fcc
4.0 3.6
q*b/nm1
dc/nm
ad/nm
0.495 0.497 0.471
12.7 12.6 13.3
14.7 14.6 23.1
a Rg: the gyration radius of micelles. b q*: the value of the scattering vector at the first-order reflection. c d: the d spacing was calculated as 40 d d = 2p/q*. pffiffiffiparameter of the hex and fcc structure is pffiffiffi a: the lattice a ¼ 4p= 3q and a ¼ 2 3p=q , respectively.41
Fig. 2 SAXS spectra of 20 wt% F87 aqueous solution without CO2 (a) and at CO2 pressures of 2.26 MPa (b), 3.80 MPa (c), 4.02 MPa (d), and 4.10 MPa (e). The inset shows the cubic packing of micelles, corresponding to spectrum e.
fluidity and presented a gel-like or semi-solid appearance (photographs d and e). The phase transition from fluid to gel was reversible and could be repeated by controlling pressure. After releasing CO2, the gel shown in photograph d changed into fluid as shown in photograph a, and the gel formed again after CO2 was recharged. The gel can maintain its stability for more than two months. Small angle X-ray scattering (SAXS) was used to characterize the microstructure of the F87 aqueous solution at different pressures.30 The SAXS curves of 20 wt% F87 aqueous solution in the presence of CO2 at 25 1C are shown in Fig. 2. In the absence of CO2 or at lower pressures, the SAXS spectra are typically of isotropic solution (spectra a and b).24,31 The generalized indirect Fourier transformation gives the pair-distance distribution function, p(r), which is usually utilized to characterize the basic geometry of the aggregates (e.g. spherical, cylindrical, planar, etc.) in micellar systems.32 From the pair-distance distribution function curves (Fig. S1, ESI†), it can be deduced that the micelles exhibit a sphere-like microstructure.33 The gyration radii (Rg) of micelles are 4.0 and 3.6 nm in the absence of CO2 and at a CO2 pressure of 2.26 MPa, while the true radii calculated are 5.2 and 4.6 nm, respectively.34 The slight decrease of the micellar size may be due to the penetration of CO2 into the tail region of the surfactant, causing a larger interfacial curvature and a smaller micelle size.35 For the SAXS curves of the F87 aqueous solution in the presence of CO2 at higher pressures, interestingly, a series of peaks appear with different ratios of peak positions depending on pressure. This indicates that ordered structures were formed.36 The peak positions for the ordered structures extracted from SAXS measurements are listed in Table S1, ESI.† The SAXS intensities at 3.80 and 4.02 MPa are consistent with the hexagonal (hex) packing of micelles (spectra c and d),37 the d spacing and the lattice constant being B12.7 nm and B14.7 nm, respectively. As the pressure reaches 4.10 MPa, the SAXS intensities correspond to the face-centered cubic (fcc) phase (spectrum e),36,38 with a d spacing of 13.3 nm and a lattice constant of 23.1 nm. The structural parameters of the phases at different pressures obtained from SAXS measurements are shown in Table 1.
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The SAXS results reveal the microstructure change during the liquid-to-gel transition. In the absence of CO2, F87 molecules assemble into sphere-like micelles in water randomly. The hydrophilic PEO blocks form the hydrated coronas while dense PPO blocks form the hydrophobic core. Upon addition of CO2, the micelle spheres arrange themselves first in a hex packing and finally in a fcc array, in which the micelles sit on a fcc lattice and are closely packed (inset in Fig. 2). The formation of the fcc structure locks the micelles into a crystalline structure of hard spheres, thus causing the gelation of the F87 aqueous solution.39 The micropolarity of the F87 aqueous solution was investigated by UV-vis spectroscopy using MO as a probe, which is a commonly used solvatochromic hydrophilic probe to study the micropolarity of micelles.24,42 Its maximum absorption wavelength (lmax) shifts to a longer wavelength upon increasing the polarity of its environment.43 Fig. 3 shows the UV-vis spectra of MO in 20 wt% F87 aqueous solution at different CO2 pressures. In the absence of CO2, the lmax of MO absorption centered at 462 nm (spectrum a), similar to that in bulk water.24,44 This indicates that most MO is located in bulk water of F87 solution. In the lower pressure range (2.50 MPa), the lmax remained nearly unchanged (spectrum b). As the pressure was higher than 3.07 MPa, obviously, the lmax abruptly blue-shifted to 420 nm (spectrum d). As is well known, an absorption peak of MO centered at such a low wavelength usually corresponds to a confined domain, for example, the inner water core of reverse micelles.45,46 The dependence of lmax on pressure can be clearly seen from the inset of Fig. 3.
Fig. 3 UV-vis spectra of MO in 20 wt% F87 aqueous solution without CO2 (a) and at CO2 pressure of 2.50 MPa (b), 3.07 MPa (c), 3.48 MPa (d), 4.64 MPa (e), 5.35 MPa (f). The inset shows the dependence of the lmax on pressure.
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Fig. 4 The variation of gelation pressure with the surfactant concentration for F87 aqueous solution at 20 1C (a), 22 1C (b), 25 1C (c), 30 1C (d) and 35 1C (e).
In combination with the results of phase behavior and SAXS, it is clear that the pressure range of the sudden drop of lmax is consistent with that of the gelation. It means that MO molecules transfer from bulk water to the micellar core, most likely due to the formation of hydrophilic domains in micelles. After the sudden drop, the lmax blue-shifted slightly upon further addition of CO2 (spectra e and f). At this stage, MO are retained at the surfactant interface and the slight decrease can be attributed to the insertion of hydrophobic CO2 into the micellar core.45 We also studied the gelation of F87 aqueous solution induced by CO2 at different surfactant concentrations and temperatures. The results show that CO2 is versatile in inducing the gelation of the solution under different experimental conditions. The pressure at which the fluidic solution transforms into a gel is defined as the gelation pressure (Pg). The Pg of the solution decreases upon increasing the surfactant concentration and temperature (Fig. 4). It means that less CO2 is needed to induce the gelation of F87 aqueous solution at high surfactant concentrations and temperatures, since the CO2 concentration is increased upon increasing pressure. Moreover, the possibilities of inducing gelation in 20 wt% F87 aqueous solution by other compressed gases (e.g. ethylene, nitrogen, hydrogen and oxygen) were studied. Similar to CO2, compressed ethylene could induce the liquid-to-gel transition at certain pressure (Fig. S2, ESI†). However, the gelation was not observed in the 20 wt% F87 aqueous solution upon the addition of compressed hydrogen, oxygen and nitrogen in the pressure range of 0–12.4, 0–7.2, and 0–7.3 MPa, respectively. It means that the lipophilic feature of gas is dominant to the liquid-to-gel transition.47 The mechanism of CO2-induced gelation is very interesting. It has been reported that the CO2 dissolved in aqueous solution can change the pH value of the aqueous solution to about 3.0.46b,48 In order to clarify whether the pH changes caused by the addition of CO2 plays a key role in the gelation, we used hydrochloric acid to fix the pH values of 20 wt% F87 aqueous solution at 3.0 in the absence of CO2. No obvious viscosity change was observed, proving that the acidity caused by the carbonation of CO2 in water contributes little to the liquid-togel transition. In addition, the contribution of the pH change to
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the gelation can be neglected due to the fact that ethylene, which can also induce gelation of F87 aqueous solution, has no acidification effect in water. The micelle volume fraction is the critical parameter that determines the liquid-to-gel transition in the micelle system of Pluronic aqueous solution.8a,49 As reported in previous work, the addition of CO2 can increase the hydrophobicity of PPO blocks and thus promote the micellization of Pluronic monomers in water.24 Herein it is expected that the presence of CO2 (or ethylene) would increase the micelle volume fraction in F87 aqueous solution, which facilitates or enhances the structural ordering resulting in the cubic close packing of micelles. Therefore, the Pluronic aqueous solution transforms into a gel. The gel created in this work can find applications in materials synthesis.50 As an example of its utilization, we explored the formation of porous silica in the CO2-induced gel. Tetraethoxysilane (TEOS) was used as the silica precursor. Since CO2 can dissolve in water and form carbonic acid, which can be used as the catalyst for the hydrolysis of TEOS, no additional acid was needed for the silica formation. The transmission electron microscopy (TEM) images showed that the silica has a mesoporous structure (mesopore size: B3 nm, Fig. S3, ESI†). The N2 adsorption–desorption isotherms (Fig. S4, ESI†) reveal that the BET (Brunauer, Emmett, and Teller) surface area and the total pore volume of the silica are 105.4 m2 g1 and 0.368 cm3 g1, respectively. The mesopore size of the as-synthesized silica (Fig. S4, ESI†), calculated from Barrett–Joyner–Halenda analysis, is centered at 3.8 nm, which is well consistent with that determined from TEM images. The small-angle XRD data (Fig. S5, ESI†) reveal that the mesopores of the silica are disordered. In summary, it was found that compressed CO2 could induce the liquid-to-gel transition in the PEO–PPO–PEO tri-block copolymer aqueous solution. During the gelation process, the microstructure of the solution transforms from randomly dispersed spherical micelles to cubic close packed micelles. The gelation switched by compressed CO2 has many advantages and can be applied in the synthesis of different kinds of porous materials.
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Cite this: Chem. Commun., 2014, 50, 14233 Received 22nd August 2014, Accepted 24th September 2014
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CO2 as a smart gelator for Pluronic aqueous solutions† Chengcheng Liu,‡a Qingqing Mei,a Jianling Zhang,*a Xinchen Kang,a Li Peng,a Buxing Han,a Zhimin Xue,a Xinxin Sang,a Xiaogan Yang,a Zhonghua Wu,b Zhihong Lib and Guang Mob
DOI: 10.1039/c4cc06623e www.rsc.org/chemcomm
It was found that CO2 could induce the gelation of Pluronic aqueous solutions, during which the microstructure of the solution transforms from randomly dispersed spherical micelles to cubic close packed micelles. The gelation switched by compressed CO2 has many advantages and can be applied in the synthesis of porous materials.
In recent years, special attention has been paid to the selfassemblies of amphiphilic block copolymers, PEO (poly ethylene oxide)–PPO(poly propylene oxide),1 mainly because of their biocompatibility, stability, designability and ease of application.2,3 The copolymers can form various kinds of self-assemblies in water, including micelles,4 liquid crystals,5 niosomes6 and gels.7 In particular, the gels formed by the PEO–PPO–PEO tri-block copolymer (usually known as a Pluronic copolymer) have attracted much interest due to their diverse uses as controlled release drug carriers,8 templates in mesoporous material synthesis,8a,9 and as electrolytes10 and hydrogel actuators.11 To induce the gelation of Pluronics in water is a very interesting topic. In general, the gelation is induced by changing the surfactant concentration,12 varying temperature,13 imposing irradiation,14 tuning pH,15 and adding different kinds of additives (i.e. gelators). The commonly used gelators are inorganic salts,16 organic solvents,8a,17 and polymers.8a,18 These solid or liquid gelators usually suffer from high economic cost, environmental burden, and post-treatment difficulties, and the gelation cannot be switched reversibly. a
Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China. E-mail: [email protected]; Fax: +86-10-62559373; Tel: +86-10-62562821 b Beijing Synchrotron Radiation Facility (BSRF), Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China † Electronic supplementary information (ESI) available: Materials, experimental details and characterization. See DOI: 10.1039/c4cc06623e ‡ Chengcheng Liu performed the whole experiment. Qingqing Mei and Li Peng worked on the UV-vis spectra characterization. Xinchen Kang and Zhimin Xue worked on the silica synthesis and characterization. Xinxin Sang, Xiaogan Yang, Zhonghua Wu, Zhihong Li, and Guang Mo worked on the small angle X-ray scattering experiment. Buxing Han and Jianling Zhang analyzed the data and proposed the mechanism. Jianling Zhang conceived the project and designed the experiments.
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Compressed CO2 has received much interest because it is nontoxic, inexpensive, tunable and nonflammable, and can be easily recaptured and recycled after use. To date, compressed CO2 has been used in different fields, including extraction and fractionation,19 materials science,20 chemical reactions,21 microelectronics,22 etc. Especially, CO2 has been found to be versatile in tuning the properties of a variety of surfactant assemblies,23 such as micelles,24 reverse micelles,25 vesicles,26 liquid crystals,27 microemulsions,28 and emulsions.29 Usually, CO2 has a viscosity-lowering effect on the surfactant solutions. For example, CO2 can cause the transition from liquid crystals to micellar solutions.27 Herein we used gaseous CO2 to induce the gelation of a Pluronic aqueous solution at room temperature. The gels with microstructures of face-centered cubic packed micelles were formed at a certain CO2 pressure. In comparison with the conventional solid or liquid gelators, the use of compressed CO2 has many advantages: (1) the gel properties can be tuned by controlling CO2 pressure; (2) CO2 can be easily removed by depressurization and recycled, which makes the post-treatment easier and more economical; and (3) the CO2-induced gelation is reversible. Fig. 1 shows the photographs of 20 wt% F87 aqueous solution with and without CO2 at 25 1C. In the absence of CO2, the solution appears to be a transparent fluid (photograph a). Upon adding CO2, no obvious changes were observed at lower CO2 pressure (photograph b). Interestingly, at 3.90 MPa, the viscosity of the system increased, but the solution was still fluidic (photograph c). As the pressure reached 4.06 MPa, the sample completely lost its
Fig. 1 Photographs of 20 wt% F87 aqueous solution without CO2 (a) and in the presence of CO2 at 2.26 MPa (b), 3.90 MPa (c), 4.06 MPa (d and e) at 25 1C.
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Table 1 Structural parameters for various phases extracted from SAXS measurements
P/MPa
Phase
Structure
Rg a/nm
0 2.26 3.80 4.02 4.10
Liquid Liquid Liquid Liquid Gel
Spherical Spherical hex hex fcc
4.0 3.6
q*b/nm1
dc/nm
ad/nm
0.495 0.497 0.471
12.7 12.6 13.3
14.7 14.6 23.1
a Rg: the gyration radius of micelles. b q*: the value of the scattering vector at the first-order reflection. c d: the d spacing was calculated as 40 d d = 2p/q*. pffiffiffiparameter of the hex and fcc structure is pffiffiffi a: the lattice a ¼ 4p= 3q and a ¼ 2 3p=q , respectively.41
Fig. 2 SAXS spectra of 20 wt% F87 aqueous solution without CO2 (a) and at CO2 pressures of 2.26 MPa (b), 3.80 MPa (c), 4.02 MPa (d), and 4.10 MPa (e). The inset shows the cubic packing of micelles, corresponding to spectrum e.
fluidity and presented a gel-like or semi-solid appearance (photographs d and e). The phase transition from fluid to gel was reversible and could be repeated by controlling pressure. After releasing CO2, the gel shown in photograph d changed into fluid as shown in photograph a, and the gel formed again after CO2 was recharged. The gel can maintain its stability for more than two months. Small angle X-ray scattering (SAXS) was used to characterize the microstructure of the F87 aqueous solution at different pressures.30 The SAXS curves of 20 wt% F87 aqueous solution in the presence of CO2 at 25 1C are shown in Fig. 2. In the absence of CO2 or at lower pressures, the SAXS spectra are typically of isotropic solution (spectra a and b).24,31 The generalized indirect Fourier transformation gives the pair-distance distribution function, p(r), which is usually utilized to characterize the basic geometry of the aggregates (e.g. spherical, cylindrical, planar, etc.) in micellar systems.32 From the pair-distance distribution function curves (Fig. S1, ESI†), it can be deduced that the micelles exhibit a sphere-like microstructure.33 The gyration radii (Rg) of micelles are 4.0 and 3.6 nm in the absence of CO2 and at a CO2 pressure of 2.26 MPa, while the true radii calculated are 5.2 and 4.6 nm, respectively.34 The slight decrease of the micellar size may be due to the penetration of CO2 into the tail region of the surfactant, causing a larger interfacial curvature and a smaller micelle size.35 For the SAXS curves of the F87 aqueous solution in the presence of CO2 at higher pressures, interestingly, a series of peaks appear with different ratios of peak positions depending on pressure. This indicates that ordered structures were formed.36 The peak positions for the ordered structures extracted from SAXS measurements are listed in Table S1, ESI.† The SAXS intensities at 3.80 and 4.02 MPa are consistent with the hexagonal (hex) packing of micelles (spectra c and d),37 the d spacing and the lattice constant being B12.7 nm and B14.7 nm, respectively. As the pressure reaches 4.10 MPa, the SAXS intensities correspond to the face-centered cubic (fcc) phase (spectrum e),36,38 with a d spacing of 13.3 nm and a lattice constant of 23.1 nm. The structural parameters of the phases at different pressures obtained from SAXS measurements are shown in Table 1.
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The SAXS results reveal the microstructure change during the liquid-to-gel transition. In the absence of CO2, F87 molecules assemble into sphere-like micelles in water randomly. The hydrophilic PEO blocks form the hydrated coronas while dense PPO blocks form the hydrophobic core. Upon addition of CO2, the micelle spheres arrange themselves first in a hex packing and finally in a fcc array, in which the micelles sit on a fcc lattice and are closely packed (inset in Fig. 2). The formation of the fcc structure locks the micelles into a crystalline structure of hard spheres, thus causing the gelation of the F87 aqueous solution.39 The micropolarity of the F87 aqueous solution was investigated by UV-vis spectroscopy using MO as a probe, which is a commonly used solvatochromic hydrophilic probe to study the micropolarity of micelles.24,42 Its maximum absorption wavelength (lmax) shifts to a longer wavelength upon increasing the polarity of its environment.43 Fig. 3 shows the UV-vis spectra of MO in 20 wt% F87 aqueous solution at different CO2 pressures. In the absence of CO2, the lmax of MO absorption centered at 462 nm (spectrum a), similar to that in bulk water.24,44 This indicates that most MO is located in bulk water of F87 solution. In the lower pressure range (2.50 MPa), the lmax remained nearly unchanged (spectrum b). As the pressure was higher than 3.07 MPa, obviously, the lmax abruptly blue-shifted to 420 nm (spectrum d). As is well known, an absorption peak of MO centered at such a low wavelength usually corresponds to a confined domain, for example, the inner water core of reverse micelles.45,46 The dependence of lmax on pressure can be clearly seen from the inset of Fig. 3.
Fig. 3 UV-vis spectra of MO in 20 wt% F87 aqueous solution without CO2 (a) and at CO2 pressure of 2.50 MPa (b), 3.07 MPa (c), 3.48 MPa (d), 4.64 MPa (e), 5.35 MPa (f). The inset shows the dependence of the lmax on pressure.
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Fig. 4 The variation of gelation pressure with the surfactant concentration for F87 aqueous solution at 20 1C (a), 22 1C (b), 25 1C (c), 30 1C (d) and 35 1C (e).
In combination with the results of phase behavior and SAXS, it is clear that the pressure range of the sudden drop of lmax is consistent with that of the gelation. It means that MO molecules transfer from bulk water to the micellar core, most likely due to the formation of hydrophilic domains in micelles. After the sudden drop, the lmax blue-shifted slightly upon further addition of CO2 (spectra e and f). At this stage, MO are retained at the surfactant interface and the slight decrease can be attributed to the insertion of hydrophobic CO2 into the micellar core.45 We also studied the gelation of F87 aqueous solution induced by CO2 at different surfactant concentrations and temperatures. The results show that CO2 is versatile in inducing the gelation of the solution under different experimental conditions. The pressure at which the fluidic solution transforms into a gel is defined as the gelation pressure (Pg). The Pg of the solution decreases upon increasing the surfactant concentration and temperature (Fig. 4). It means that less CO2 is needed to induce the gelation of F87 aqueous solution at high surfactant concentrations and temperatures, since the CO2 concentration is increased upon increasing pressure. Moreover, the possibilities of inducing gelation in 20 wt% F87 aqueous solution by other compressed gases (e.g. ethylene, nitrogen, hydrogen and oxygen) were studied. Similar to CO2, compressed ethylene could induce the liquid-to-gel transition at certain pressure (Fig. S2, ESI†). However, the gelation was not observed in the 20 wt% F87 aqueous solution upon the addition of compressed hydrogen, oxygen and nitrogen in the pressure range of 0–12.4, 0–7.2, and 0–7.3 MPa, respectively. It means that the lipophilic feature of gas is dominant to the liquid-to-gel transition.47 The mechanism of CO2-induced gelation is very interesting. It has been reported that the CO2 dissolved in aqueous solution can change the pH value of the aqueous solution to about 3.0.46b,48 In order to clarify whether the pH changes caused by the addition of CO2 plays a key role in the gelation, we used hydrochloric acid to fix the pH values of 20 wt% F87 aqueous solution at 3.0 in the absence of CO2. No obvious viscosity change was observed, proving that the acidity caused by the carbonation of CO2 in water contributes little to the liquid-togel transition. In addition, the contribution of the pH change to
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the gelation can be neglected due to the fact that ethylene, which can also induce gelation of F87 aqueous solution, has no acidification effect in water. The micelle volume fraction is the critical parameter that determines the liquid-to-gel transition in the micelle system of Pluronic aqueous solution.8a,49 As reported in previous work, the addition of CO2 can increase the hydrophobicity of PPO blocks and thus promote the micellization of Pluronic monomers in water.24 Herein it is expected that the presence of CO2 (or ethylene) would increase the micelle volume fraction in F87 aqueous solution, which facilitates or enhances the structural ordering resulting in the cubic close packing of micelles. Therefore, the Pluronic aqueous solution transforms into a gel. The gel created in this work can find applications in materials synthesis.50 As an example of its utilization, we explored the formation of porous silica in the CO2-induced gel. Tetraethoxysilane (TEOS) was used as the silica precursor. Since CO2 can dissolve in water and form carbonic acid, which can be used as the catalyst for the hydrolysis of TEOS, no additional acid was needed for the silica formation. The transmission electron microscopy (TEM) images showed that the silica has a mesoporous structure (mesopore size: B3 nm, Fig. S3, ESI†). The N2 adsorption–desorption isotherms (Fig. S4, ESI†) reveal that the BET (Brunauer, Emmett, and Teller) surface area and the total pore volume of the silica are 105.4 m2 g1 and 0.368 cm3 g1, respectively. The mesopore size of the as-synthesized silica (Fig. S4, ESI†), calculated from Barrett–Joyner–Halenda analysis, is centered at 3.8 nm, which is well consistent with that determined from TEM images. The small-angle XRD data (Fig. S5, ESI†) reveal that the mesopores of the silica are disordered. In summary, it was found that compressed CO2 could induce the liquid-to-gel transition in the PEO–PPO–PEO tri-block copolymer aqueous solution. During the gelation process, the microstructure of the solution transforms from randomly dispersed spherical micelles to cubic close packed micelles. The gelation switched by compressed CO2 has many advantages and can be applied in the synthesis of different kinds of porous materials.
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