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Notes Solubilization in Confined Surfactant Mesophases R. Denoyel*,† and E. Sabio Rey‡ Centre de Thermodynamique et Microcalorime´ trie du CNRS, 26, rue du 141 RIA, 13003 Marseille, France, and Escuela de Ingenieras Industriales, Universidad de Extremadura Carretera de Olivenza, Apartado de Correos 382, 06011 Badajos, Spain Received December 19, 1997. In Final Form: September 7, 1998
Introduction MCM-41 type mesoporous materials are synthesized by using a micelle-based crystal templating method:1 for example, with a source of silica and a cationic alkyltrimethylammonium halide surfactant, in conditions where a hexagonal organic/inorganic mesophase is obtained. Regardless of the synthesis mechanism which has been a subject of debate,2,3 it is evident that the surfactant plays a vital role in the formation of MCM-41 since not only does it determine the pore size but also it has a catalytic action on the silicate polymerization.4 After elimination of the surfactant, for example, by calcination, a material is obtained with very uniform pore size and shape and high thermal stability. These materials are potentially useful for catalytic, electronic, optical, and magnetic applications.5,6 Their modification by functionalized molecules can lead to adsorbents with specific properties.7 Although MCM-41 was only discovered in 1992, many of these applications have already been explored and the commercial application of some of them is not too distant. However, an application which has not been reported until now, to our knowledge, is the possibility of sorption of hydrophobic organic molecules from aqueous solution into the noncalcined mesophase. There is, indeed, an increasing interest in studying the removal of organic pollutants from water by using different biological and physicochemical techniques.8 Among the latter methods, the use of different sorbents seems to be one of the most effective technologies. Activated carbon filters are com† ‡
CNRS. Universidad de Extremadura Carretera de Olivenza.
(1) Kresge, C. T.; Leonovitcz, M. E.; Roth, C. T.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (2) Chen, C. Y.; Burkette, S. L.; Li, H. X.; Davis, M. E. Microporous Mater. 1993, 2, 27-36. (3) Firouzi, A.; Kumar, D.; Bull, L. M.; Besier, T.; Sieger, P.; Huo, Q.; Walker, S. A.; Zasadzinski, J. A.; Glinka, C.; Nicol, J.; Margolese, D.; Stucky, G. D.; Chmelka, B. F. Science 1995, 267, 1138-1143. (4) Cheng, C. F.; Luan, Z.; Klinowski, J. Langmuir 1995, 11, 28152819. (5) Tanev, P. T.; Chibwe, M.; Pinnavaia, T. J. Nature 1994, 368, 321-323. (6) Wu, C. G.; Bein, T. Chem. Mater 1994, 6, 1109-1112. (7) Feng, X.; Fryxell, G. E.; Wang, L. Q.; Kim, A. Y.; Liu, J.; Kemner, K. M. Science 1997, 276, 923. (8) Andrews, R. C.; Daignault, S. A.; Laverdure, C.; Anderson, W. B.; Rector, D. W.; Willian˜s, D. T.; Huck, P. M. In Fundamentals of Adsorption; Meersmann, A. B., Scholl, S. E., Eds.; AIChE: New York, 1991; p 97.
monly used in water treatment for the removal of organic pollutants, which are adsorbed onto the large surface area of the carbon.9 More recently, organically modified clays were proposed as sorbents to remove organic pollutants from aqueous industrial waste streams.10 When the clay surface is covered by long-chain cationic surfactants, such as hexadecyltrimethylammonium bromide, the hydrophobic chains act as a solvent, removing the organic pollutant from the aqueous solution by a process often termed “adsolubilization”.11-13 The adsolubilization of organic compounds was also carried out by using surfactant aggregates on porous or colloidal particles.11,14 Ultracentrifugation techniques may be needed for the final separation.15 However, all these sorption processes are very complex and are influenced by many parameters which are not easily standardized, such as the degree of surface coverage in the organoclays or the characteristics of the colloidal system in the colloidal surfactants. In the noncalcined MCM-41 mesophase, the mesoporous channels of silica are filled by the nonpolar chain of the surfactant. Hence, the idea is that, as in micelles, nonpolar molecules might be dissolved in the hydrophobic core, where the hydrocarbon tails probably act as a solvent. In a recent paper,16 hydrophobic probes were shown to be incorporated in the channels of MCM-41 during the synthesis. It would, therefore, appear feasible to use noncalcined MCM-41 as a sorbent for removing organic pollutants from the aqueous solution. The well-defined structure of this inorganic/organic system should make the standardization of the removal conditions easier than that with other sorbents. Experimental Section The first aim of the present work is to determine whether these organic/inorganic mesophases are effectively able to sorb small hydrophobic molecules. Second, we want to study the influence of the alkyl chain length on the sorption capacity and sorption mechanism. With that aim, we prepared three mesophases from different length alkyl chain surfactants as templates: dodecyltrimethylammonium bromide, tetradecyltrimethylammonium bromide, and hexadecyltrimethylammonium bromide. These mesophases will be referred to as C12, C14, and C16, respectively. They were prepared by using TEOS (tetraethoxysilane) as a silica source.17 Pure TEOS was injected on a stirred mixture of water, surfactant, and ammonia. The (9) Kira´ly, Z.; De´ka´ny, I. Langmuir 1996, 12, 423-430. (10) Boyd, S. A.; Jaynes, W. F.; Ross, B. S. In Organic Substances and Sediments in Water; Baker, R. S., Ed.; Lewis Publishers: Chelsea, MI, 1991; pp 181-200. (11) O’Haver, J. H.; Harwell, J. In Surfactant Adsorption and Surface Solubilization; Sharma, R., Ed.; American Chemical Society: Washington, DC, 1995; p 49. (12) Schieder, D.; Dobias B.; Klumpp, E.; Schwuger M. J. Colloids Surf. A 1994, 88, 103. (13) Dekany, I.; Farkas, A.; Regdon, I.; Klumpp, E.; Narres, H. D.; Schwuger M. J. Colloid Polym. Sci. 1996, 274, 981. (14) Treiner, C.; Monticone, V. In Surfactant Adsorption and Surface Solubilization; Sharma, R., Ed.; American Chemical Society: Washington, DC, 1995; p 36. (15) Jonsson, A. S.; Jonsson, B. J. Colloid Interface Sci. 1996, 180, 504-518. (16) Ohtaki, M.; Inata, K.; Eguchi, K.; Arai, H. Sogo Rikogaku Kenkyuka Hokoku 1997, 19, 9-14. (17) Gru¨n, M.; Lauer, I.; Unger, K. Adv. Mater. 1997, 9, 254.
10.1021/la971394j CCC: $15.00 © 1998 American Chemical Society Published on Web 11/13/1998
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Figure 1. Sorption isotherms (expressed as the excess sorbed amount versus equilibrium concentration) of 3-chlorophenol in the three silica mesophases. Curves are fitted by a Langmuir equation. precipitate produced is washed with water until a constant pH (around 6-7) is obtained and then dried at 90 °C. X-ray diffraction (XRD) patterns show the three Bragg peaks typical of MCM-41. The d100 values of C12, C14, and C16 are 3.16, 3.72, and 4.16 nm, respectively. The sorption isotherms of 3-chlorophenol were determined by the solution depletion method in stoppered tubes. For each point of the adsorption isotherm, 50 mg of solid was shaken with 15 mL of solution with the desired concentration in a thermostatic bath. After reaching equilibrium, the tubes were centrifuged and the equilibrium concentration of chlorophenol in each supernatant was determined by using a UV-visible spectrometer (at 274 or 290 nm, depending on the concentration). The structure and chemical compositions of the prepared mesophases and their stabilities before and after sorption of the solute were investigated by FTIR (Nicolet 205) and XRD techniques (STOE Stadi P transmission X-ray diffractometer). For FTIR, the samples were diluted in potassium bromide (around 5% by weight) and analyzed with a diffuse reflectance cell. To get a first insight into the mechanism of sorption, the corresponding enthalpies were measured by using already described procedures:18 a stock solution of chlorophenol is added step by step to the titration cell of a Tian-Calvet type microcalorimeter where the solid is maintained in suspension by stirring. Since the heat of dilution is negligible, the integral enthalpy of adsorption is obtained by dividing the cumulative measured heat by the corresponding sorbed amount.
Results and Discussion Adsorption isotherms are shown in Figure 1. A preliminary kinetic evaluation has shown that equilibrium is achieved in less than 3 h of contact between the 3-chlorophenol and the mesophase, even for high concentrations of the organic. The initial slope of the adsorption isotherms increases from C12 to C16. This indicates that the affinity of 3-chlorophenol for the mesophase increases in the same order. If adsorbed amounts were reported compared to the mass of confined surfactant, the same order of affinity would be obtained since the surfactant molar content is the same for the three mesophases and the variation of molar mass is 15% from C12 to C16 (this is confirmed by the chemical analysis, which shows that the surfactant contents of the mesophases are 34, 38, and 40% for C12, C14, and C16, respectively). As the solute equilibrium concentration increases, the slope of the sorption isotherms decreases, indicating a progressive saturation of the sorption process until a pseudoplateau is reached. The adsorption level at this pseudoplateau is not very different between the (18) Denoyel, R.; Rouquerol, F.; Rouquerol, J. In Fundamentals of adsorption; Liapis, A. I., Everett, D. H., Sing, K. S. W., Eds.; Engineering Foundation: New York, 1987; pp 199-210.
Notes
Figure 2. Sorption enthalpies of 3-chlorophenol in the three silica mesophases.
mesophases. At higher concentration, there is a crossing point between these sorption isotherms, but the mechanism of sorption is then different. Indeed, IR analysis of the samples after adsorption shows that above 10 mmol/L (equilibrium concentration of the monochlorophenol) the surfactant is displaced out of the mesophase whereas in the concentration range of Figure 1 the sorption process can be considered as a solubilization of the molecule in the confined surfactant phase. Also, XRD analysis of the mesophases before and after adsorption shows that the hexagonal structure is preserved (since the position of the peaks remains the same and the peak intensities are only very slightly modified). The question of the state of the solubilized molecule was analyzed by microcalorimetry, which is sensitive to any change in the energetical state of sorbed molecules as a function of coverage (or filling). If all sorbed molecules had the same localization in the mesophase, sorption enthalpies should be constant over a large range of filling. This is not the case here, as shown in Figure 2, where sorption enthalpies are plotted as a function of the sorbed amount. At low concentration (in the mesophase) the difference is large between C12-C14 on the one side and C16 on the other side. It is generally admitted that two kinds of sites are available when a molecule is solubilized inside micelles or in surfactant surface aggregates: one near the polar head and the other between alkyl chains in the hydrophobic core.19 It is too early to distinguish between these sites from the calorimetric results, but it is clear that different sites are available. At a first glance, it is surprising to see that at low filling, as the slope of the isotherm (and therefore the affinity) increases (from C12 to C16), the enthalpy of sorption decreases in absolute value. As is usual in systems where hydrophobic interactions are present, entropic effects are important. The higher enthalpies are obtained for the shortest chains, which may be an indication of a higher ordering of the system due to the presence of the solute molecules in the surfactant channels. The details of the sorption mechanism are not known: is there a displacement of residual water which could be present in the channels, what is the influence of the silica surface, etc.? Nevertheless, since the filling is made at constant volume, which is imposed by the inorganic matrix, and without surfactant displacement, the sorption leads to an increase in the concentration of organic components in the channel and this effect is more pronounced for the shortest chains. Indeed, for an equal sorbed amount, the local concentration increases when the channel diameter decreases. These variations (19) Nagarajan, R. Curr. Opin. Colloid Interface Sci. 1996, 1, 391; 1997, 2, 282.
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Table 1. Sorption Capacity (for a Chlorophenol Equibrium Concentration of 3 × 10-3 mol‚L-1) and Gibbs Energy of Adsorption of Monochlorophenol on the Three Studied Mesophase Samples (C12, C14, and C16), on a Modified Clay (Hexadecyltrimethylammonium-Treated Montmorillonite (21)) and a Carbon (Coconut-Shell Activated Carbon (20)) sample
∆G/(kJ‚mol-1)
sorption capacity/(mmol‚g-1)
C12 C14 C16 modified clay carbon
-4.0 -6.0 -8.2 -3.2 -9.8
0.91 1.2 1.35 0.24 2.7
of local parameters with pore size are strong enough to vary the enthalpy values from one sample to another. Finally, this preliminary study shows that confined surfactant mesophases are good sorbents for organic (20) Wang, R. C.; Kuo, C. C.; Shyu, C. C. J. Chem. Technol. Biotechnol. 1997, 68, 187-194. (21) Kim, Y. S.; Song, D. I.; Jeon, Y. W.; Choi, S. J. Separation Sci. Technol. 1996, 31 (20), 2815-2830.
molecules. Probably they can also be used to solubilize molecules from the gas phase. A comparison of the present results with those in the literature20,21 (we have compared the Gibbs energy of sorption derived from a Langmuir fit of the adsorption isotherms; results are presented in Table 1) shows that the efficiency of these mesophases to remove chlorophenol from water ranks them between activated charcoals (the most efficient adsorbents) and pillared clays. Despite a number of questions pending on the mechanism of sorption, these samples should offer new possibilities both for applications (for example, the treatment of water, micellar catalysis, analysis by solid-phase extraction, etc.) and for fundamental research on solubilization and reactivity in liquid confined systems, since these mesophases have a well-defined geometry. Finally, because of the displacement of surfactant molecules under some conditions, there is also a need to prepare mesophases with irreversibly bound surfactants. Acknowledgment. We thank M. Linden (University of Frankfurt) for the determination of XRD spectra. LA971394J
