pubs.acs.org/Langmuir © 2010 American Chemical Society
Chemical Interactions of Surface-Active Agents with Growing Resorcinol-Formaldehyde Gels Hana Jirglova† and Francisco J. Maldonado-Hodar* Department of Inorganic Chemistry, Faculty of Sciences, University of Granada, Granada, Spain. Current address: J. Heyrovsk y Institute of Physical Chemistry v. v. i., Academy of Sciences of the Czech Republic, Prague, Czech Republic
†
Received June 21, 2010. Revised Manuscript Received July 22, 2010 The influence of cationic, anionic, and nonionic surfactants (S) on the characteristics of carbon xerogels was analyzed. The polymerization of resorcinol (R) and formaldehyde (F) was developed in an aqueous solution of S without any additional catalyst. The gels obtained were dried in air to obtain organic xerogels and then carbonized to carbon xerogels. The prepared samples were characterized by FTIR, TG, SEM, and N2 and CO2 adsorption. The formation of RF-S copolymers was observed for cationic and nonionic surfactants, but this was not observed for anionic S, probably because of repulsive electrostatic interactions between the two organic phases. Nevertheless, anionic S leads to a greater morphological transformation with the formation of nonporous needle particles associated with the higher pH induced by this S. Carbon xerogels are microporous materials with interesting molecular sieve behavior. The RF-S composites undergo greater shrinkage than do the pure RF xerogel; consequently, a narrower microporosity is obtained.
Introduction Cheap activated carbons with different applications can be prepared from various sources such as coals, agricultural wastes, and used tires.1-3 However, nowadays there is great interest in the production of more specific carbon materials for particular applications, such as molecular sieves, membranes, fuel cells, and energy storage. Moreover, substantial attention is focused on nanosized or nanostructured carbons. Various nanocasting techniques and the use of templates were employed to fit the porosity of nanocarbons.4,5 The direct synthesis of organic gels, which can be transformed into nanocarbon materials by pyrolysis, is an interesting alternative. Among other monomers, the polymerization of resorcinol (R) and formaldehyde (F) yields one of the most promising synthetic materials for this purpose. This procedure was developed initially by Pekala,6 and various kinds of nanocarbons7 can be prepared by varying the experimental conditions.8 Together with purity and homogeneity, the main characteristics of these gels lie in the ability to control their porosity. The resulting nanostructure is very sensitive to the various synthesis and processing conditions employed, and much effort has been made to control and tailor the shape, particle size, pore-size distribution, and surface chemistry of carbon gels, which are properties that finally determine their applications. *Corresponding author. E-mail:
[email protected]. Tel: þ34 958240444. Fax: þ34 958248526.
(1) Liu, Ch.; Tang, Z.; Chen, Y.; Su, S.; Jiang, W. Bioresour. Technol. 2010, 101, 1097–1101. (2) Betancur, M.; Martı´ nez, J. D.; Murillo, R. J. Hazard. Mater. 2009, 168, 882– 887. (3) Perez-Cadenas, A. F.; Maldonado-Hodar, F. J.; Moreno-Castilla, C. Carbon 2003, 4, 473–478. (4) Wu, D.; Liang, Y.; Yang, X.; Li, Z.; Zou, Ch.; Zeng, X.; Lu, G.; Fu, R. Microporous Mesoporous Mater. 2008, 116, 91–94. (5) Lu, An-H.; Li, W. C.; Schmidt, W.; Schuth., F. Microporous Mesoporous Mater. 2005, 80, 117–128. (6) Pekala, R. W.; Alviso, C. T.; LeMay, J. D. J. Non-Cryst. Solids 1990, 125, 67–75. (7) Inagaki, M.; Radovic, L. R. Carbon 2002, 40, 2279–2282. (8) Al-Muhtaseb, A.; Ritter, J. A. Adv. Mater. 2003, 15, 101–114.
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To optimize the textural properties, much work has dealt with the influence of the pH, concentrations of the reactants, drying, carbonization, and activation conditions.8-10 The drying process of hydrogels defines the obtained raw polymer: aerogels, cryogels, or xerogels depending on whether supercritical drying, freezedrying, or simple solvent evaporation is employed. In general, supercritical drying preserves to a substantial degree the original porosity of the hydrogel, but it is not easy to employ on an industrial scale. Surfactants (S) are widely used to prepare structured inorganic metal oxides.11,12 Some papers also deal with the use of surfactants for the preparation of R-F gels,13-16 but there is substantial controversy about the effect of surfactants on the properties of RF gels. Sample preparation usually entails a sol-emulsion-gel method13 in which a previously prepared R/F sol is suspended and stabilized in a solvent by the addition of an appropriate surfactant. Thus, Tonanon14 also prepared carbon cryogels by using inverse emulsions. The results greatly depend on the nature of the surfactants and solvents. The morphology of cryogels can be changed from microspherical to microcellular (spongelike) structures by using a nonionic SPAN80 or FC4430 surfactant, respectively. The pore volume and BET surface of the samples also increased when cyclohexane/water mixtures were used as the solvent in the emulsion instead of pure water. Horikawa15 also prepared RF gels by the inverse emulsion method in cyclohexane/ (9) Zhang, R.; Li, W.; Li, K.; Lu, Ch.; Zhan, L.; Ling, L. Microporous Mesoporous Mater. 2004, 72, 167–173. (10) Czakkel, O.; Marthi, K.; Geissler, E.; Laszlo, K. Microporous Mesoporous Mater. 2005, 86, 124–133. (11) Mishra, T.; Hait, J.; Aman, N.; Gunjan, M.; Mahato, B.; Jana, R. K. J. Colloid Interface Sci. 2008, 32, 377–383. (12) Neves, S.; Canobre, S. C.; Oliveira, R. S.; Polo-Fonseca, C. J. Power Sources 2009, 189, 1167–1173. (13) Lee, H. J.; Song, J. H.; Kim, J. H. Mater. Lett. 1998, 37, 197–200. (14) Tonanon, N.; Tanthapanichakoon, W.; Yamamoto, T.; Nishihara, H.; Mukai, S. R.; Tamon, H. Carbon 2003, 41, 2981–2990. (15) Horikawa, T.; Ono, Y.; Hayashi, J.; Muroyama, K. Carbon 2004, 42, 2683– 2689. (16) Matos, I.; Fernandes, S.; Guerreiro, L.; Barata, S.; Ramos, A. M.; Vital, J.; Fonseca, I. M. Microporous Mesoporous Mater. 2006, 92, 38–46.
Published on Web 09/22/2010
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Table 1. Molar Ratios and pH Values of the Starting Solutions sample
R
F
W
S
t-BuOH
TMB
pH
pure RF CTAB-RF DCBS-RF SPAN80-RF
1 1 1 1
2 2 2 2
180 180 180 180
0 0.5 0.5 0.5
0 1 1 1
0 1 1 1
5.2 4.5 8.4 6.8
nonionic surfactant mixtures. Spherical particles are always obtained, but a relationship between the surface-active agents and the pore structures of carbon aerogel particles was found: a lower hydrophile-lipohile balance (HLB) leads to large micropore and mesopore volumes of the spherical carbon aerogel particles. Matos et al.16 prepared RF xerogels under static conditions using sealed molds containing the aqueous R/F solutions together with nonionic, cationic, or anionic surfactants. They found that although a nonionic surfactant does not influence the morphology of carbon xerogels, a cationic surfactant enhances the particle size and an anionic surfactant promotes large mesopore size and a bimodal mesopore size distribution. They all previously described work using alkali carbonates as polymerization catalysts. However, Wu et al.17 prepared RF aerogels by the inverse emulsion method using RF solutions containing a cationic surfactant but without any additional catalyst. The particle size and degree of shrinkage of the samples can be modified by adjusting the S/R ratio. Thus, it is difficult to obtain a clear conclusion from information about the influence of surfactants in the synthesis because the results were obtained under very different experimental conditions. However, it is clear that surfactants become an interesting and alternative tool for modifying gel properties. In our case, although the use of both water as a solvent in the emulsions and conventional drying at atmospheric pressure (preparation of xerogels) clearly seems to exert a negative effect on the textural characteristics of the obtained samples,8,15 we will use these experimental conditions in an attempt to obtain cheap materials, thus avoiding the use of additional organic solvents in an exchange process, added catalysts, or long synthesis processes. The aim of this work is to study the influence of the nature of the surfactant (anionic, cationic, or nonionic) on the characteristics of carbon xerogels prepared by a modified emulsion method. The interactions of the surface-active agents with the growing RF polymers in aqueous solutions and the formation of surfactant-RF gel copolymers and their thermal decomposition during carbonization were analyzed. The morphological, chemical, and porous transformations from the organic to the carbon xerogels were studied by various techniques.
Experimental Section Synthesis. A starting solution was prepared by mixing resorcinol (R), surfactant (S), tert-butanol (t-BuOH), and 1,3,5-trimethylbenzene (TMB) in water (W). The molar ratios employed are given in Table 1. Three different surfactants;cationic, anionic, or nonionic (Table 2);and cosurfactants t-BuOH and TMB were used in this work. A pure RF polymer (without any S) was prepared as a reference. During the synthesis, the solution was stirred vigorously and refluxed. Initially, the solution was heated in an oil bath to 50 °C and then formaldehyde (F) was added dropwise to elicit the polymerization reaction. The R-F polymerization reaction takes place immediately at this temperature, and the appearance of new solids is observed after each drop of F. (17) Wu, D.; Fu, R.; Dresselhaus, M. S.; Dresselhaus, G. Carbon 2006, 44, 675– 681.
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Table 2. Three Types of Surfactants Used
Then the suspension that formed was aged for 2 h at 50 °C, after which the temperature was increased to 90 °C, and these conditions were maintained for another 24 h to complete the polymer curing. Thus, the RF polymer is always formed in the presence of S molecules, whereas in a previous paper the RF gels were first synthesized and then dispersed on the S solution. Because the rheological behavior of RF gels strongly depends on the synthesis conditions (concentration, temperature, etc.) and it sharply changes at the gelation time,18 the redispersed gels can be very different in the typical synthesis process. The “one-pot” synthesis method presented in this article attempts to avoid these differences because the RF gels are grown in situ and simultaneously to save materials (only water is used as solvent and the use of catalysts is avoided) and eliminate processes that are technologically difficult and time-consuming (solvent exchange and supercritical drying). Furthermore, the suspension was cooled to room temperature and the solid product was filtered and washed with water and ethanol, respectively, to remove unreacted compounds. The wet product was dried at atmospheric pressure, first at 50 °C and finally at 120 °C. The resulting organic xerogel was carbonized in a nitrogen flow (100 cc/min) in a tube furnace using a heating rate of 2 °C/min up to 900 °C and a soaking time of 5 h. Characterization. The textural and chemical characterization of the samples was performed by various techniques. The chemical nature of the organic xerogels was analyzed by FTIR spectroscopy, thermogravimetric analysis (TG-DTG), and X-ray diffraction (DRX). The IR absorption spectra were recorded on a Nicolet 20SXB FTIR spectrometer using KBr pellets. TG experiments were performed in a He flow (30 cm3.min-1) at a heating rate of 10 °C min-1 using a model TGA-50H thermobalance. A DRX measurement between 2θ = 2 and 80° at a scan rate of 0.02° was carried out using a Bruker D8 Advance diffractometer. The morphology of the carbons was observed using a scanning electron microscope (Leo, Carl Zeiss, Geminy-153). The textural properties of carbon xerogels were determined by the adsorption of N2 (77 K) and CO2 (273 K) using Autosorb-1 Quantachrome. The corresponding adsorption isotherms were analyzed using the BET and Dubinin-Radushkevich (DR) equations to obtain the surface area and micropore volume values according to the methodology described previously.19,20 (18) Job, N.; Panariello, F.; Crine, M.; Pirard, J. P.; Leonard, A. Colloids Surf., A 2007, 293, 224–228. (19) Stoeckli, F. In Porosity in Carbons: Characterization and Applications; Patrick, J. W., Ed.; Edward Arnold: London, 1995; pp 67-97. (20) Jirglova, H.; Perez-Cadenas, A. F.; Maldonado-Hodar, F. J. Langmuir 2009, 25, 2461–2466.
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Figure 1. TG profiles obtained in a N2 stream at a heating rate of 10 °C min-1.
Results and Discussion The first task was to analyze the interactions between the S molecules and the growing RF gels in aqueous solutions and the possible incorporation of S into the RF chemical structure. This last fact should be related to the nature and strength of the interactions between the different organic species occurring during the synthesis. The polycondensation of resorcinol with formaldehyde4 progresses in two main steps: the first consists of the addition of formaldehyde to resorcinol, leading to the formation of hydroxymethyl derivatives. The deprotonation of resorcinol generates R anions, which are more active than uncharged R molecules toward the F addition reaction.4 The second step is the condensation of these methylol groups, which is favored by acid and leads to the formation of the 3D RF chemical network. According to this mechanism, a greater interaction of cationic surfactants with anionic RF species is expected. The formation of RF-surfactant composites by electrostatic interactions between both phases was previously described by Nishiyama.21 The first indication of the different behavior of S and the possible incorporation of their molecules into the RF organic xerogel structure was indicated directly by weighing the obtained solid after drying. Thus, the synthesis yield, expressed as grams of organic xerogel obtained after drying, was greater (around 50% by weight) when the synthesis occurred in the presence of CTAB or SPAN80 than for the pure RF xerogel. However, the reaction yield is similar when the synthesis progresses without S or in the presence of the anionic DCBS surfactant. The incorporation of the surfactant molecules into the RF chemical network of the gels will modify the decomposition processes of these organic xerogels occurring during carbonization. The carbonization was simulated by TG. Figure 1 compares the TG profiles obtained for pure CTAB, pure RF xerogel, and CTAB-RF xerogel. Thus, it was pointed out that pure CTAB decomposes from 200 °C in only one step and without the formation of any solid residue (100% weight loss attained). The DTG profile shows that the maximum decomposition rate occurs at 250 °C. The TG profile of the pure RF xerogel exhibits two slope changes at around 350 and 550 °C corresponding to the breakage of C-O and C-C bonds in the structure of RF xerogels, respectively.22,23 However, the TG profile of CTAB-RF exhibits intermediate behavior between the two pure phases, indicating that there is a certain incorporation of surfactants molecules into (21) Nishiyama, N.; Zheng, T.; Yamane, Y.; Egashira, Y.; Ueyama, K. Carbon 2005, 43, 269–274. (22) Lin, C.; Ritter, J. A. Carbon 1997, 35, 1271–1278. (23) Maldonado-Hodar, F. J.; Ferro-Garcı´ a, M. A.; Rivera-Utrilla, J.; MorenoCastilla, C. Carbon 1999, 37, 1199–1205.
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Figure 2. FTIR spectra of the pure CTAB surfactant and S-RF organic xerogels.
Figure 3. XRD patterns of the pure CTAB surfactant and the CTAB-RF organic and carbon xerogels.
the RF chemical structure. Thus, the TG profile of this sample indicates that carbonization proceeds in three steps; the first one at low temperature (at around 250 °C) can be related to the decomposition of surfactant from the xerogel structure, and the other two changes at higher temperatures correspond, as described, to the typical carbonization of RF polymers. The slope of the first step in the TG profile of CTAB-RF is smaller than for pure CTAB; this means that the decomposition rate is also slower as a consequence of the interaction of CTAB with the RF phase. The weight loss (WL) also increases from 45 to 75%, but this difference cannot be directly assigned to the CTAB content in the xerogel, which can be greater if some part of the S molecule remains in the solid after carbonization. The nitrogen content in the organic xerogel as determined by elemental analysis is about 2%, also indicating a high incorporation of CTAB molecules into the organic RF polymer. The FTIR spectra (Figure 2) of the pure CTAB surfactant exhibits two characteristics bands at 2920 and 2850 cm-1 associated with the vibrational modes of the -CH2- groups. In the FTIR spectra of the organic CTAB-RF xerogel, the presence of these two large bands confirms that the CTAB molecules were incorporated into the RF structure, providing long aliphatic chains to the chemical structure of the polymer. Both bands are also visible in this region in the case of the blank (pure RF) DOI: 10.1021/la102499h
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Figure 4. SEM images of pure RF and CTAB-RF xerogels.
because the cross linking between aromatic rings to form the 3D chemical network is carried out through -CH2- or methylene ether -CH2-O-CH2- bridges;4,20 however, the intensity observed in this case is weaker than that of sample CTAB-RF because here the concentration of aliphatic structures is clearly smaller. The XRD patterns of the CTAB surfactant (Figure 3) show that this powder is highly crystalline, showing two main narrow diffraction peaks at 3.5 and 6.9°. However, these XRD peaks are not observed for the organic CTAB-RF polymers. Nonetheless, TG, nitrogen chemical analysis, and FTIR indicated the presence of the surfactant molecules in the solid. These results suggest that the surfactant molecules are integrated into the RF chemical structure, forming a copolymer, because the precipitation of CTAB from a CTAB aqueous solution also yields crystalline solids. The presence of a band at a low diffraction angle (2θ = 2.7°) in the XRD pattern of the organic CTAB-RF xerogel indicates a certain lamellar organization of the structure; however, the broad band at approximately 2θ = 20° is typical of amorphous carbon materials. After carbonization, the first band is reinforced and slowly shifted to higher 2θ values, indicating greater lamellar ordering and a certain decrease in the interlamellar spacing. Morphologically, the SEM images (Figure 4) also exhibit important changes. Thus, the pure RF xerogel is clearly composed of spherical particles with a diameter of about 1 μm. These rounded particles coalesce together mainly during the drying process; longer aging periods that mechanically reinforce the gel or more dilute solutions should probably be used to obtain more independent spherical particles in the solid. For the CTAB-RF xerogel, this 3D structure seems to be based on flaky or laminar sheets instead of spherical particles. The influence of the anionic and nonionic surfactants on the resulting S-RF xerogel characteristics was also analyzed. The TG profiles of the organic S-RF xerogels prepared in the presence of the three different surfactants are depicted in Figure 5. It is noteworthy that only in the case of the organic S-RF xerogel synthesized with an anionic surfactant (DCBS-RF xerogel) is the WL during carbonization similar to the blank sample (the pure RF xerogel), which is in agreement with the observed reaction yield. This can be related to a repulsive interaction between the RF and the anionic surfactant, preventing the formation of the S-RF composite. However, in spite of the nonionic character of SPAN80 (Table 2), the SPAN80-RF xerogel presents a total WL that is 16106 DOI: 10.1021/la102499h
Figure 5. Comparison of the thermal behavior of the various S-RF and pure RF organic xerogels during carbonization.
similar to the cationic CTAB-RF, also indicating the possible formation of RF-S composites. The SPAN80 molecules present different reactive sites such as CdC and CdO double bonds, -OH, and even the furanic ring (Table 2), which can easily interact with the RF chemical structure. However, the nature of the established links between S and RF gels is different in the case of cationic and nonionic surfactants. In fact, the composite of nonionic S-RF is stronger because it is decomposed at higher temperature. The thermal behavior of the SPAN80-RF xerogel and the pure RF xerogel is very similar up to 370 °C. At this temperature, the decomposition of the organic structure of the SPAN80-RF organic xerogel is faster, with the maximum decomposition rate occurring at 400-410 °C. On the basis of the decomposition temperature range previously mentioned, it is pointed out that nonionic S and RF can be linked to the RF structure through multiple-interaction-type hydrogen bonds and C-O or C-C bonds, whereas in the case of CTAB-RF, weaker interactions probably based only on electrostatic interactions occur. These interactions for the different S-RF copolymers are tentatively represented in Figure 6A,B. The formation of SPAN80-RF organic composites is clearly shown by FTIR analysis. The FTIR spectra (Figure 7) of the pure RF xerogel and DCBS-RF xerogels are quite similar, and the spectra of SPAN80-RF (S6) and CTAB-RF (included for comparison) exhibit very intense bands corresponding to the -CH2- vibrational modes. An analysis of the FTIR region between 900 and 1900 cm-1 (Figure 8) also revealed additional differences in the chemical Langmuir 2010, 26(20), 16103–16109
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Figure 6. Proposed model for the formation of (A) CTAB-RF and (B) SPAN80-RF copolymers.
Figure 7. FTIR spectra of pure RF and S-RF organic xerogels.
Figure 8. FTIR spectra of S-RF organic xerogels.
structure of the organic xerogels. FTIR bands located at approximately 1640, 1477, and 1100 cm-1 were detected in all cases.
The band in the region around 1600 cm-l has been observed by many authors24 and has been generally assigned to CdC
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Figure 9. SEM images of SPAN80-RF and DCBS-RF xerogels.
stretching in the aromatic rings. In the literature,20,25 the bands at 1480 and 1280 cm-1 are associated with the formation of methylene and methylene ether bridges between the aromatic rings in RF gels. These bands correspond to the C-H bending and C-O stretching vibrations in C-O-C structures, respectively. The band at 1470 cm-1 can also be attributed to either an O-H deformational vibration or to C-H bending vibrations. The bands at around 1100 and 1380 cm-1 are assigned to C-OH stretching and deformation in phenolic groups.24,26 The main difference is related to the presence of a strong band at 1740 cm-1 in the FTIR spectra of the SPAN80-RF organic xerogel, which is assigned to the CdO stretching vibration in carboxyl, ketone, anhydride, and aldehyde groups. This means that the CdO bonds in the SPAN80 molecule are largely preserved in the RF-S composite. Therefore, the SPAN80 molecules are probably linked to the RF structure through aliphatic CdC bonds or -OH groups. Hydrogen bond interactions between the different oxygenated groups of both phases can also largely stabilize the chemical structure of SPAN80-RF copolymers. Morphologically, greater differences are observed when the anionic DCBS surfactant is used (Figure 9). The morphology of sample SPAN80-RF resembles that previously described for CTAB-RF; nonetheless, in this case the solid is more compact and the lamellar structures are denser and highly overlapped. Although the DCBS molecule does not seem to be able to incorporate into the RF chemical structure, SEM analysis revealed that the DCBS-RF sample is formed from a needle structure rather than the from spherical particles obtained in the absence of surfactants. The textural characteristics of the final carbon xerogels (Table 3) also strongly depend on the presence of S in the synthesis. In the absence of S, the obtained RF carbon xerogel is clearly a microporous material, as shown by the high SCO2 surface area and micropore volume. In relation to the N2 adsorption, it is noteworthy that the SBET value is significantly smaller than SCO2, indicating serious diffusion restrictions on the interior of the narrowest micropores.27 The N2 adsorption isotherms correspond to type I of the IUPAC classification, and they do (24) Moreno-Castilla, C.; Carrasco-Marı´ n, F.; Maldonado-Hodar, F. J.; Rivera-Utrilla, J. Carbon 1998, 36, 145–151. (25) Wu, D.; Fu, R.; Sun, Z.; Yu, Z. J. Non-Cryst. Solids 2005, 351, 915–92. (26) Zawadzki, J. In Chemistrv and Phvsics of Carbon; Thrower, P. A., Ed.; Marcel Dekker: New York, 1989; Vol. 21, p 147. (27) Cazorla-Amoros, D.; Alcaniz-Monge, J.; De la Casa-Lillo, M. A.; LinaresSolano, A. Langmuir 1998, 14, 4589–4596.
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Table 3. Textural Characteristics of Carbon Xerogels carbon xerogel SBET (m2g-1) SCO2 (m2g-1) W0 (cm3g-1) L0 (nm) pure RF CTAB-RF DCBS-RF SPAN80-RF
622 null null 86
1133 893 null 832
0.33 0.22 null 0.22
0.58 0.51 null 0.52
not exhibit a hysteresis loop, which is typical of microporous materials. Both carbon xerogels obtained by the carbonization of CTAB-RF and SPAN80-RF composites are also microporous materials. Nevertheless, the micropore volume and the surface area decrease to approximately 25% compared to that of the blank sample. The mean micropore width (L0) also decreases. This fact produces a greater restriction on the diffusion of N2 into the micropores, and thus very low SBET values are obtained for the SPAN80-RF carbon xerogel. This effect is stronger for CTAB-RF, where N2 adsorption is completely prevented. These samples therefore present interesting properties to be used as molecular sieves. The separation of mixtures such as N2/H2, CH4/ H2, CO/H2, and O2/N2 is very important in applications such as fuel cells and oxygen enrichment of air for combustion applications.28,29 Again, greater differences are observed when the DCBS-RF carbon xerogel is analyzed. This sample behaves as a nonporous solid; no CO2 or N2 adsorption was observed. Thus, in this case, the presence of anionic S molecules does not modify the chemical nature of the RF gels but strongly modifies the polymer morphology, inducing the formation of needle particles instead of spherical ones, which also behave as a nonporous material. This lack of porosity can be related to the synthesis conditions. The presence of S molecules in the starting solution strongly modifies the initial pH of the reaction. The pH values increase in the order cationic < nonionic < anionic, changing from acidic to basic media (Table 1). RF carbon xerogels synthesized in slightly acidic media (around pH 5.5) with high surface areas and pore volumes were described,21 and both parameters decrease substantially at pH >7. In previous papers, various authors correlated the decomposition of the surfactant molecules in the surfactant-RF copolymers with the textural characteristics of the samples. They prepared (28) Li, Y.; Chung, T. S. Microporous Mesoporous Mater. 2008, 113, 315–324. (29) Moon, J. H.; Bae, J. H.; Bae, Y. S.; Chung, J. T.; Lee, C. C. J. Membr. Sci. 2008, 318, 45–55.
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carbon materials with tailored pore size using a cationic surfactant molded sol-gel process. Matos12 described a micropore volume increase using a high S concentration (15 wt %) associated with S decomposition during carbonization. Along these lines, Lee30 indicated than when the surfactant template is used, the RF xerogel exhibits a rather stable pore structure that avoids collapse during the drying process. However, Wu13 described a progressive radial shrinkage upon increasing the CTAB/R ratio, leading to a loss of porosity and a density increase in the organic and carbonized aerogels. In our case, the cationic and nonionic S molecules form composites with the RF gels, but the porosity of the carbon xerogels is unequivocally closer than for the pure RF sample, which is in agreement with the results obtained by Wu.
Conclusions The use of S in the synthesis of carbon xerogels is certainly another powerful tool in tailoring the characteristics of this kind of material. However, the synthesis conditions should be carefully (30) Lee, K. T.; Oh, S. M. Chem. Commun. 2002, 2722–2723.
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fitted because it is clear that the result is a function of many variables such as concentration, pH, agitation, solvent, and so forth. In our case, the direct polymerization of RF in aqueous solutions containing the cationic, anionic, or nonionic surfactant is clearly influenced by the presence of S. CTAB-RF copolymers seem to be formed by attractive interactions, and SPAN80-RF copolymers are formed by chemical bonds. The formation of DCBS-RF copolymers is not observed, probably because of repulsive interactions between the two phases. Nevertheless, DCBS induces a substantial increase in the pH regarding the other S, which favors the formation of nonporous needle particles. The decomposition of S-RF composites yields microporous carbon xerogels, but in both cases, the microporosity is narrower than that of the pure RF carbon xerogel, possibly because of greater shrinkage of the samples during carbonization. Acknowledgment. H.J. acknowledges the Spanish AECI for an MAE fellowship. These investigations were supported by the MEC-FEDER, project CTQ2007-61324.
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