Phase Separation of Mixed Micelles and Synthesis of Hierarchical

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Phase Separation of Mixed Micelle and Synthesis of Hierarchical Porous Materials Fei Gao, Cheng Lian, Lihui Zhou, Honglai Liu, and Jun Hu Langmuir, Just Accepted Manuscript • DOI: 10.1021/la501648j • Publication Date (Web): 03 Sep 2014 Downloaded from http://pubs.acs.org on September 10, 2014

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Phase Separation of Mixed Micelle and Synthesis of Hierarchical Porous Materials Fei Gao, Cheng Lian, Lihui Zhou, Honglai Liu * and Jun Hu * State Key Laboratory of Chemical Engineering and Department of Chemistry, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, China. KEYWORDS. phase separation, mixed micelles, hierarchical porous materials, Flory-Huggins theory ABSTRACT. Mixed micelle template approach is one of the most promising synthesis methods for hierarchical porous materials. Although considerable research efforts have been made to explore the formation mechanism, an explicit theoretical guidance for appropriately choosing templates is still not available. We found that the phase separation occurring in the mixed micelles would be the key point for the synthesis of hierarchical porous materials. Herein, the pseudo-phase separation theory for the critical micelle concentration (cmc) combining with the Flory-Huggins theory for the chain molecular mixture were employed to investigate the properties of mixed surfactant aqueous solution. The cmc values of mixed surfactant solutions were experimentally determined to calculate the Flory-Huggins interaction parameter between two surfactants, χ. When χ is larger than the critical value, χc, the phase separation would occur within the micellar phase, resulting in two types of mixed micelles with different surfactant compositions, and hence different sizes, which could be used as the dual-template to induce

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bimodal pores with different pore sizes. Therefore, the Flory-Huggins theory could be a theoretical basis to judge whether the mixed surfactants were the suitable templates for inducing hierarchical porous materials. We chose cetyltrimethylammonium bromide (CTAB) and noctylamine (OA) as a testing system. The phase separation behavior of the mixed solutions, as well as the successful synthesis of hierarchical porous materials by this dual-template indicated the feasibility of preparing hierarchical porous materials based on the concept of phase separation of the mixed micelles. INTRODUCION In the last few years, considerable research efforts were devoted to the synthesis of hierarchical porous materials with structures that exhibit pores on different length scales from micro- (50 nm).1-2 Hierarchical porous materials such as macromicroporous,3 macro-mesoporous,4-6 meso-microporous,7-8 and meso-mesoporous materials9 have particular properties including specific pore structures, reduced transport limitations and multiple functions. They are of great potential in the fields of catalysis, sorption, separation, and biomedicine.10-15 Surfactant templating is a very important strategy in the synthesis of ordered mesoporous materials.16-17 Dual-template approach based on two different structure-directing agents is an effective method to fabricate the hierarchical porous materials.18-20 By carefully choosing suitable templates among different cationic, anionic, or nonionic surfactants, the bimodal pores can be induced. Sel et al. systematically studied the combinations of various block copolymers (Pluronics F127, Pluronics P123, and SE (PS-co-PEO)) with the surfactant of CTAB, and revealed that hierarchical bimodal mesoporous architectures could be obtained by the usage of block copolymers with a strong hydrophilic-hydrophobic contrast.21 Antonietti et al. reported the combination of special small fluorosurfactants with “KLE” block copolymers (KLE


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= Kraton- Liquid- b-poly(ethylene oxide)), which introduced small mesopores between larger spherical mesopores in a certain range of concentrations.22 Smått et al. reported the combination of poly(ethylene oxide) and CTAB to template monolithic silica, which exhibited interconnected porosity on several length scales.23 Niu et al. synthesized a novel kind of dual-mesoporous coreshell silica spheres by using an amphiphilic block copolymer (polystyrene-b-poly (acrylic acid), PS-b-PAA) and CTAB as the dual-template.24 In most cases, however, simply mixing two different templates won't necessarily result in favorable bimodal porous materials with well-defined pore structures because of the complexity of the mixed micelle system. The most fundamental thermodynamic relationship governing the mixing behavior is, ∆Gm = ∆H m − T∆S m , where ∆Gm , ∆H m and ∆S m are the free energy, enthalpy and entropy of mixing, respectively. Miscibility occurs when ∆Gm < 0 . Only when ∆Gm becomes positive, the phase separation driven by the thermodynamic force may occur. For the mixed micelle system, as illustrated in Scheme 1, it usually tends to form either a type of homogeneous micelle (a) or two types of separated micelles (b). Consequently, to be a successful dual-template for fabricating hierarchical porous materials, a suitable interaction between two template molecules which can lead to the phase separation is the prerequisite, just like the formation of an organized “alloy” phase. Therefore, further understanding the relationship between the molecular interaction and the phase separation behavior of the mixed micelle template system would be meaningful for designing and preparing novel hierarchical porous materials.


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Scheme 1. Illustration of different phase separation behaviors of mixed micelles, (a) one type of homogeneous mixed micelles; (b) two types of mixed micelles induced by the phase separation The regular solution theory25-27 was usually used to study the phase behaviors of mixed micelles, such as binary mixtures of cationic and nonionic surfactants,28 anionic and anionic surfactants,29 cationic gemini and zwitterionic surfactants,30 anionic and zwitterionic surfactants,31 cationic gemini surfactants and triblock polymers.32 However, treating the surfactant molecule as a rigid sphere, the regular solution theory considers little about the effect of surfactant structures, and can hardly tell the detailed information about the compositions of micelles when phase separation happens. The Flory-Huggins theory,33-34 which takes account of the great dissimilarity in molecular sizes, has been widely applied in polymer systems. Especially, for blends of polymers, it can successfully predict the phase separation behavior.35-38 The surfactant molecules, with hydrophilic heads and long hydrophobic tails, are somehow similar to the polymer molecules, while some nonionic surfactants are actually real polymer molecules. The Flory-Huggins theory considers the contribution of combination entropy attributed to the arrangement of the surfactant molecules during the mixing process. As illustrated in Scheme 1(b), assuming the number of surfactant molecules in each micelle is statistically similar, because of different chain lengths of surfactant molecules, the different compositions of the micelles would result in different sizes of the micelles if the phase separation occurs. Therefore,


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the Flory-Huggins theory would be more applicable for investigating the phase separation behavior of mixed micelle template systems and predicting the synthesis of hierarchical porous materials differing in pore size. Herein, for the first time we investigated the phase separation behaviors of dual-template system based on the Flory-Huggins theory. The solution of CTAB and OA, which was studied in early reports,39-40 was selected as the dual-template system. The phase separation properties of the mixed CTAB/OA solutions, as well as the successful synthesis of hierarchical porous materials by this dual-template indicated the feasibility of preparing hierarchical porous materials based on the concept of phase separation of the mixed micells. Therefore, the Flory-Huggins theory could provide a theoretical basis for judging whether the mixed surfactants were the suitable templates for hierarchical porous materials. EXPERIMENTAL SECTION Preparation of the mixed surfactants solutions. CTAB and Tetraethyl orthosilicate (TEOS) were purchased from Sinopharm Chemical Reagent Co., Ltd. OA was purchased from Aldrich. All the substances were used as received without further purification. A series of CTAB/OA solutions were obtained by dissolving CTAB and OA in deionized water at 40 °C. The total molar concentration of CTAB and OA was fixed as 0.02 mol/L, with the molar fraction of CTAB, x1(x1=nCTAB/( nCTAB+nOA)) varied as 0, 0.2, 0.4, 0.6, 0.8, and 1.0, respectively. Synthesis of porous materials. The porous materials were synthesized according to the conventional preparation approach of mesoporous MCM-41. Typically, CTAB and OA were


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dissolved in deionized water at 40 °C, and then ammonia was added to control pH. TEOS was added dropwise into the solution under vigorous stirring. The molar ratio of all compositions is TEOS:0.14(CTAB+OA):5NH3:150H2O, in which the molar fraction of CTAB, x1,varied from 0 to 1.0. After stirring for 2 h, the solution was heated and kept at a specific temperature under continuous stirring for 48 h. The reaction mixture was filtered, washed, and dried. Finally, the powder was calcined at 550 °C for 6 h in ambient air, with a heating rate of 2 °C/min. Characterizations. The critical micelle concentration (cmc) of surfactant solutions at different x1 was determined by plotting the specific conductivity against the total concentration of CTAB and OA. The conductivity measurements were taken on a DDSJ-308A conductivity meter. The size distribution of micelles was determined by the dynamic light scattering (DLS) technique on a Malvern Nano-ZS, the backscatter detection was used with a detecting angle of 173°. The powder X-ray diffraction (XRD) patterns were recorded on a D/Max2550 VB/PC spectrometer using Cu Kα radiation (40 kV and 200 mA). The transmission electron micrographs (TEM) were taken on a JEOL JEM-2010. Nitrogen adsorption measurements were conducted at 77.4 K on a Micrometrics-ASAP-2020 sorptionmeter. The total surface area was determined by the BET (Brunauer-Emmett-Teller) model, the microporous area was determined by the t-plot method, and the mesoporous area was calculated by the total BET surface area minus the microporous one. The size distribution of the mesopores was determined by the BJH (BarrettJoyner-Halenda) model, while the size distribution of the micropores was determined by the HK (Horvath–Kawazoe) model.

THEORETICAL APPROACH


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For a mixed surfactant solution consisting of n10 mol surfactant 1 and n20 mol surfactant 2, the 0 0 0 0 0 0 apparent molar fractions of the surfactants are x1 = n1 / (n1 + n2 ) and x2 = 1 − x1 , respectively.

When the total concentration of mixed surfactants reaches its cmc, mixed micelles made up of surfactant 1 and 2 will form in the solution. Assuming that there are no solvent molecules in the m

micellar phase, the molar fractions of the two surfactants in the micellar phase are x1m and x2 ( x1 + x2 = 1 ), respectively. m

m

According to the general criterion of the phase equilibrium, for a mixed surfactant solution at its cmc, the chemical potential of surfactant i in the micellar phase, µim (the subscript i represents surfactant i), equals that of the bulk phase, µib , which can be expressed as

µ1m = µ1b , µ 2m = µ 2b

(1)

Then the cmc of the mixed micelles can be calculated as a function of that of the single surfactant in aqueous solution by 23 1 x0 x0 = m 1 0+ m 2 0 cmc γ 1 cmc1 γ 2 cmc2

(2)

where cmci0 is the cmc of single surfactant in aqueous solution, and γ im is the activity coefficient of the surfactant in mixed micelles, which is a measure of the mixing nonideality. When γ im =1, the Eq. 2 can be simplified as Clint model41 on the assumption of ideal mixing. The molar fraction of surfactant 1 in the micellar phase can be described by


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x1m =

x10 / γ 1m cmc10 x10 / γ 1m cmc10 + x20 / γ 2m cmc20

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(3)

Different models can be adopted to calculate the activity coefficient. If there is little difference in size between the molecules of surfactant 1 and 2, the activity coefficient of the micellar phase can be calculated by the regular solution theory (RST).

lnγ 1m = β x2m , lnγ 2m = β x1m

(4)

where β is the interaction parameter based on RST model which represents the excess Gibbs free energy of mixing. However, composing of a hydrophilic head group and a hydrophobic long tail, the size effect of the surfactant molecule can not be neglected. By introducing the volume fraction

φ im , the

activity coefficient, γ 1m and γ 2m can be expressed by the Flory-Huggins Theory 33-34:

lnγ 1m = ln(φ1m / x1m ) + φ2m (1 − r1 / r2 ) + r1 χ (φ2m ) 2  m lnγ 2 = ln(φ2m / x2m ) + φ1m (1 − r2 / r1 ) + r2 χ (φ1m ) 2

(5)

m m m m where ri is the length of the surfactant i, φi = ri xi / ( r1 x1 + r2 x2 ) the volume fraction of

surfactant i in the micellar phase, and χ the interaction parameter. χ < 0 means attractive χ 23

χ 23

interaction between two surfactants, while χ > 0, on the contrary, repulsive interaction. χ 23

Particularly, when the repulsive interaction χ is larger than a critical parameter χc,

χ c = (1 + r1 / r2 ) 2 / 2r1 , the mixed micelle will separate into two types of micelles, m1 and m2, m1

and the corresponding compositions of surfactant 1 in mixed micelles of m1 and m2, x1


and

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x1m 2 , can be deduced from the equivalent chemical potential of each component in the two microscopic micellar phases,

µ1m1 = µ1m2 , µ 2m1 = µ 2m2

(6)

therefore,

ln(φ1m1 ) + φ2m1 (1 - r1/r2 ) + r1 χ (φ2m1 ) = ln(φ1m2 ) + φ2m2 (1 - r1/r2 ) + r1 χ (φ2m2 )  ln(φ2m1 ) + φ1m1 (1 - r2 /r1 ) + r2 χ (φ1m1 ) = ln(φ2m2 ) + φ1m2 (1 - r2 /r1 ) + r2 χ (φ1m2 )

(7)

Take an arbitrary dual-surfactant solution for example, when we set r1=15, r2=5, cmc10 = 1 mmol/L and cmc20 = 2 mmol/L, for any given χ, the plot of cmc- x10 can be obtained from equation (2). Figure 1(a) reveals the variation of cmc of the mixed surfactants with x10 at different χs . When χ < 0, the attractive force predominates between two surfactants, and the χ 23

value of cmc is lower than that of ideal mixed system. The negative deviation is enhanced with decreasing χ. When χ > 0, the repulsive force predominates between two surfactants, and the χ 23

value of cmc is larger than that of ideal mixed system. Moreover, in such a case, there are two possible situations, that χ > χc and χ < χc, corresponding to the states of with and without phase separation, respectively, and hence significantly different cmc- x10 curves. When χ > χc, the repulsive force between two surfactants is so large that it could form two different types of separated micelles in some composition regions, which is presented as dot segments in cmc- x10 curves. Moreover, a maximum peak appears on the cmc- x10 curve and grows larger when χ increases. The compositions of the mixed micelles at different χ can be calculated by the FloryHuggins Theory. As shown in Figure 1(b), the curve describes the compositions of surfactant 1


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in the mixed micelles as a function of χ. When χ > χc, drawing a horizontal line at a specific χ, it m1

intercepts with the curve at two points of x1

m2

and x1 , which represent the compositions of

two conjugated separated micelles of surfactant 1-rich (m1) and surfactant 2-rich (m2), respectively. The compositions of the conjugated micelles are closer and closer with decreasing

χ, As χ decreases to χc = 0.2488, the two micellar phases converge into a single one. Below the curve, the surfactant 1 and surfactant 2 are mutually miscible, and there is only one type of mixed micelle. In the phase separation region, suppose the number of surfactant molecules in each micelles (Nm) is the same, then the micelle volume ( Vi m ) can be described by

Vi m = N m v * ( x1mi r1 + x2mi r2 )

(8)

Where v* is the volume of each unit in the surfactant chain.

1.0

χ

(a)

(b)

0.8 0.5 0.4 0.3 0.2488 0.1 0 -0.25 -0.5 -1

-3

4.0x10

-3

2.0x10

r1=15, r2=5

0.8 0.6

Double phase

χ

-3

6.0x10

cmc (mol/L)

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0.4

m2

χ=0.5

m1

x1

0.2

x1 Single phase

0.0 0.0

0.2

0.4 0 0.6

x1

0.8

1.0

0.0

0.2

0.4 m 0.6

x1

0.8

1.0

Figure 1. (a) theoretical cmc variation of mixed surfactants with x10 at different χs; (b) Theoretical phase diagram, in which the black and red symbols and curves represent the compositions of surfactant 1 in the micelle phase 2, x1m 2 , and the micelle phase 1, x1m1 , respectively. For the mixed micellar system, we set r1=15, r2=5, while cmc10 = 1 and cmc20 = 2 mmol/L, respectively


10

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Therefore, if the repulsive interaction predominant between two surfactants, it might result in two kinds of separated micelles with different sizes. According to the soft template mechanism of porous materials, it is possible to synthesize hierarchical porous materials by using this kind of surfactant couple as the dual-template. When the cmc values of mixed surfactant solutions are measured experimentally, the interaction parameter χ can be calculated by the combination of Eq. 2, 3 and 5. Accordingly, the Flory-Huggins theory can present the judgment whether the phase separation can occur in the mixed surfactant system and thus predict the possibility of inducing hierarchical porous materials. RESULTS AND DISCUSSION Solution properties. The values of cmc of the mixed CTAB/OA micelles as a function of CTAB molar fraction, x10 , obtained by experiment and simulation are compared in Figure 2(a), where the circle symbols are the experimental results, obtained by the conductivity measurement at 25 °C (as shown in Figure S1, Supporting Information), the solid line is the prediction by the FloryHuggins theory, and the dashed line is the prediction of the Clint model of ideal mixing. It indicates that at a lower x10 , the experimental cmc values deviate from the ideal ones significantly. The much larger values indicate the positive deviation, suggesting the repulsive interaction dominated between CTAB and OA molecules. Only when x10 is larger than 0.8, the experimental cmc values are almost in accordance with that of the ideal mixing system. When we adopted the UNIFAC model to calculate the group volume parameters, the values of r1 and r2 are estimated as 14.0058 and 6.3167 for CTAB and OA, respectively. We tested a series of FloryHuggins interaction parameter χs to simulate the experimental data, and found whenχ = 0.23, the simulated curve by the Flory-Huggins theory is quite consistent with the experimental results.


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More importantly, as discussed above in the theoretical section, when the repulsive force between two surfactants is large enough, i.e., χ >χc, the phase separation would occur. The experimental results of cmc at low x10 show great deviation from the ideality, thus, the cmc- x10 curve simulated by the Flory-Huggins theory shows a maximum peak, which is the characteristic of the phase separation. The phase diagram of the mixed CTAB/OA micelles calculated by the Flory-Huggins theory can give a better understanding of the phase separation behaviors. As shown in Figure 2(b), the critical interation parameter χc of CTAB/OA mixed solution is calculated as 0.221. There will be a phase separation occurring when χ = 0.23, and two types of micelles with the compositions of x1m 2 = 0.18 and x1m1 = 0.53 co-existing. Therefore, when x10 is in the range of 0.18-0.53, the x m1

phase separation will result in two different types of micelles in the mixed CTAB/OA solution.

2.4

0.40

Experimental Ideal F-H simulation

(a)

0.35

1.6

0.30

Double phase m2

m1

x1

χ

2.0

(b)

x1

0.25

1.2

χ = 0.23

3

10 cmc (mol/L)

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0.20

0.8 0.0

0.2

0.4

0

0.6

0.8

1.0

C (0.2325, 0.2212)

0.0

0.2

x1

Single phase

0.4 m 0.6

0.8

1.0

x1

Figure 2. (a) experimental results and predictions of variations of cmc (a) with the molar ratio of CTAB/OA, x10 , (b) phase diagram of CTAB/OA mixed micellar system predicted by the FloryHuggins theory.


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The particle size distributions of the micelles in pure CTAB, pure OA and CTAB/OA mixed solutions at various molar fractions are shown in Figure 3(a). The evolvement of the phase separation with variations of x10 is illustrated. The average diameter of pure OA micelle with x10 =0 is 38 nm, and that of pure CTAB micelle with x10 =1 is 0.8 nm, respectively. Because of

the complexity of the real micelle structures, such as the surfactant capability of lowering the surface tension, solvent molecules involved in the micelles, and various aggregation states of surfactant molecules, the surfactant with shorter chain length may form larger micelles. Mixing CTAB with OA, when x10 =0.2, there is only one peak in the size distribution curves, with the size of 15 nm, smaller than that of pure OA, suggesting there is only one type of mixed micelle, no phase separation occurred. When x10 is in the range of 0.4 to 0.6, there are two peaks in the size distribution curves, suggesting the phase separation occurred in the CTAB/OA mixed solution, and two types of micelles with different sizes co-existing. The smaller peak at 1.1 nm is in accordance with CTAB-rich micelles, while the larger one at 17 nm represents the opposite OA-rich micelles. With increasing x10 , such as x10 =0.8, again, there is only one peak in the size distribution curve, suggesting only CTAB-rich micelles existing in the mixed solution. The temperature can also affect the phase separation in the mixed CTAB/OA solution. As shown in Figure 3(b), when we set x10 = 0.4, the size of the CTAB-rich micelles almost keeps as a constant at about 0.75 nm with increasing temperature; but it has significant effects on the size of the OA-rich micelles, which decreases from 7.5 nm (25 °C ) to 3.8 nm (50 °C). Moreover, the peak intensity of OA-rich micelles decreases, suggesting the OA-rich micelles are less dominated in the solution. When the temperature is above 55 °C, the peak of the OA-rich micelles disappears. OA molecules are possibly incorporated into CTAB-rich micelles to make


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two different micelles coalesced together. Therefore, the interaction parameter χ decreases with increasing temperature. When the temperature is high enough, χ < χc, there will be no phase separation occurring, and only one type of micelle existing.

0

o

(b)

x1

(a)

Temperature / C

0

25

0.2

35

0.4 0.6

Intensity

30

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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40 45 50 55

0.8

60

1.0 1

10

100

70

1

Size (nm)

10

100

Size (nm)

Figure 3. Particle size distributions of CTAB/OA mixed solutions (a) with different molar fractions of CTAB at 25 °C, (b) at various temperatures with x10 = 0.4. Characterizations of hierarchically porous materials. Since CTAB and OA mixed solutions showed specific micellization characteristics, their dual-template effect for inducing the porous materials was further investigated. After calcination, the organic templates were removed, and then, the samples were characterized by XRD. Figure 4 shows the XRD patterns of the samples induced by the CTAB/OA dual-template at different molar ratios of x10 = 0.2, 0.4, 0.6, 0.8, 1.0, respectively. The sample synthesized with the single template of CTAB ( x10 = 1) is the conventional mesoporous MCM-41, and the XRD pattern is a typical hexagonal arrangement, with an intense reflection peak and two small peaks at 2 θ = 2.25, 3.95, and 4.54o, corresponding to the (100), (110) and (200) reflections, respectively. The samples prepared with x10 = 0.8 and 0.6 have the XRD patterns similar to MCM-41, but with relatively lower diffraction intensities, indicating the ordered hexagonal structure is still preserved but less ordered. However, further


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reducing the molar fraction of CTAB leads to the disappearance of the three distinguished peaks on the XRD patterns, suggesting the formation of disordered structure.

5

4.0x10

5

3.2x10

5

2.4x10

5

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1.6x10

4

8.0x10 0.0

1.0 0.8 0.6 0.4

x01

0.2 1

2

3

4

2θ

5

6

7

Figure 4. XRD patterns of the samples induced by CTAB/OA dual-template with the molar ratio x10 = 0.2, 0.4, 0.6, 0.8, and 1.0 at 373 K, respectively.

Based on the analysis of N2 adsorption-desorption isotherms of the samples induced by CTAB/OA dual-template (as shown in Figure S2, Supporting Information), their pore size distributions are shown in Figure 5(a). When x10 = 0.2, there are two broad peaks on the pore size distribution curve, in which the smaller one is averagely at 2.5 nm, and the larger one at 3.73 nm. The bimodal mesopores can be induced by the phase separation of the dual-template micelles in the synthesis process. Because the size of the mesopore induced by pure CTAB is 2.4 nm, the smaller one is mainly induced by the CTAB-rich micelles. This bimodal mesopores induced by the phase separation phenomenon is even enhanced when x10 = 0.4, which is in good agreement with the predictions of the Flory-Huggins theory, as well as the properties of the mixed CTAB/OA solutions due to the strong repulsion between CTAB and OA. So CTAB-rich micelles induce the smaller mesopores of 2.74 nm and OA-rich micelles induce the larger one of


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3.86 nm. When x10 > 0.6, only one type of mesopores existed in the obtained porous material. As shown in Figure 5(b), the BET surface area of the samples increases significantly with x10 , because the number of the micelles would increase with increasing x10 . As a result, the BET surface area of the samples are above 1200 m2/g when x10 = 0.8, even higher than the BET surface area of pristine MCM-41 induced by CTAB alone, indicating OA molecules have

2

BET surface area (m /g)

synergic effect with CTAB molecules on increasing the BET surface area.

2.0 (a) 1.5

0.2 0.4 0.6 0.8 1.0

1.0 0.5

1000

0.0 2.0

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Figure 5. (a) Pore size distributions of the samples induced by CTAB/OA dual-template with the molar ratio x10 = 0.2, 0.4, 0.6, 0.8, and 1.0 at 373 K, respectively, calculated by BJH model based on desorption curves. (b) BET surface areas of each sample. TEM images in Figure 6 give further intuitive evidence of the porous materials induced by CTAB/OA dual-template at different x10 s. Coincided with the results of the Flory-Huggins theory prediction, the XRD patterns and the pore distributions, the resulted product at x10 = 0.4 illustrates a heterostructure, with one type of worm-like mesopores and the other type of relatively ordered mesopores, as shown in Figure 6(a). In contrast, as shown in Figure 6(b), when x10 = 0.8, the highly ordered hexagonal symmetry domain presents in its TEM image. As illustrated above in Figure 2(a), the mixed CTAB/OA solution behaves like an ideally mixed


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micelle when x10 > 0.8, and results in the ordered mesoporous structure. More peculiar hierarchical structures are found when x10 = 0.5 (Figure 6 (c) and (d)). There are many flower shape particles with a vesica core and worm-like mesoporous petals around. The central portion and edge portions of particles have different pore structures, which are induced by different micelles, accordingly, indicating the phase separation of the dual-template micelles.

Figure 6. TEM images of the samples induced by CTAB and OA dual-template at 100 °C with different CTAB molar fractions. (a) x10 = 0.4, (b) x10 = 0.8, (c) and (d) x10 = 0.5. When we fixed x10 =0.5, the influence of the hydrothermal temperature on the pore structure was further investigated. Figure 7 shows that the pore structure changes significantly with temperature. The sample obtained at 25 °C, as shown in Figure 7(a), contains both ordered and worm-like mesopores in disparate regions. When the hydrothermal temperature increases to 40 °C, the sample with core-shell structure can be obtained (Figure 7(b)), in which the core is full of


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relatively ordered concentric channels and the outside shell is constructed by worm-like mesopores. When the hydrothermal temperature reaches 100 °C, as shown in Figure 6(d) and 7(c), the central part converts to multi-layer vesicle with worm-like mesopores surrounded. As we illustrated above in Figure 3(b), increasing temperature causes the coalescent of two micelles together in the CTAB/OA mixed solution, however, when CTAB/OA mixed micelles are adopted as the dual-template, the temperature effect on the porous structure would be a little different.42 During the mixing and initial assembly process at 40 °C, the hydrolyzed TEOS would wrap the CTAB/OA mixed micelles, which preserve the original configuration of the mixed micelles. Then, during the high-temperature hydrothermal process, such as 100 °C, the OA-rich micelles collapse. However, the sol-gel SiO2 wall would hinder the transfer of OA molecules into the bulk solution; instead, OA and CTAB molecules might assemble themselves in the loose amorphous SiO2 wall to form the multi-layer vesicles. N2 sorption isotherms and pore size distributions of the obtained samples induced at different hydrothermal temperatures (as shown in Figure S3, Supporting Information) are similar with each other, suggesting the similar pore size and BET surface area, although the morphologies of the pore structure show significant difference at various temperatures. Therefore, the phase separation behavior of mixed surfactant solutions would be a good reference for predicting the porous structure when the hydrothermal temperature is similar to the temperature at the initial mixing stage, and hence, the Flory-Huggins theory can be well used to predict the possibility of formation hierarchal pores. On the contrary, when the hydrothermal temperature is much higher, the Flory-Huggins theory can hardly give an exact prediction for the hierarchal structure.


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Figure 7. TEM images of the samples induced by CTAB and OA dual-template at x10 = 0.5 with different hydrothermal temperatures. (a) 25 °C, (b) 40 °C, and (c) 100 °C. Combining the theoretical predictions, the experimental results of the mixed surfactant solutions and the hierarchical porous structures induced by the dual-template, the mechanism of preparing hierarchical porous materials via phase separation strategy described by Flory-Huggins theory was displayed in Scheme 2. According to the Flory-Huggins theory, when χ < χc, there is only one type of mixed micelles, and the system can be simulated into a binary phase model, the bulk (b) and the mixed-micelle (m); when χ > χc, there are two types of mixed micelles co-existing, and the system can be simulated into a triple phase model, the bulk, the mixed-micelle (m1) and the mixed-micelle (m2). Moreover, when the homogeneous mixed micelles are used as the template, the highly ordered porous material can be obtained, while the bimodal mixed micelle solution will lead to the formation of hierarchical porous material. So the Flory-Huggins theory would be very effective for understanding and predicting the formation of hierarchical porous silica induced by the mixed-surfactant solution.


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Scheme 2. Mechanism of the formation of hierarchical porous silica induced by the dualtemplate through the prediction of the Flory-Huggins theory CONCLUSIONS The micellization properties of CTAB and OA mixed solutions and their dual-template effect on the hierarchical porous structure via the phase separation strategy were investigated. The FloryHuggins theory presented good simulation results in accordance with the nonideal micellization properties of CTAB/OA mixed micelles, which clearly elucidated the repulsive interaction between CTAB and OA, the phase separation phenomena, and the compositions of the mixed micelles at different molar ratios of CTAB/OA, as well as the temperatures. Using CTAB/OA as the dual-template, we successfully synthesized hierarchical mesoporous silica. The resulted mesoporous structures highly depended on the properties of mixed micelles of the dual-template, which confirmed the evidence of the phase separation of CTAB and OA mixed micelles


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predicted by the Flory-Huggins theory. Therefore, the Flory-Huggins theory would provide a facilitate way for the design and synthesis of the desired hierarchical porous materials. ASSOCIATED CONTENT Supporting Information. The supporting information contains the conductivity measurement results, N2 adsorption-desorption isotherms and pore size distributions of some of the prepared samples. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing ﬁnancial interest. ACKNOWLEDGMENTS Financial support for this work is provided by the National Basic Research Program of China (2013CB733501), the National Natural Science Foundation of China (No. 91334203, 21176066), the 111 Project of China (No.B08021), the Fundamental Research Funds for the Central Universities of China and the project of FP7-PEOPLE-2013-IRSES (PIRSES-GA-2013-612230).


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Table of Contents Graphic

The Flory-Huggins theory can effectively simulate the phase separation occurring in mixed micelles, and therefore can provide a facilitate way for predicting the formation of desired hierarchical porous materials by the phase separation of the dual-template.


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