Preparation of Spherical Magnetic Mesoporous Silica Containing

Magnetite-containing spherical mesoporous silica (M-SMS) has been prepared using the phase transfer method between an aqueous phase containing cationi...
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Ind. Eng. Chem. Res. 2009, 48, 2577–2582

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MATERIALS AND INTERFACES Preparation of Spherical Magnetic Mesoporous Silica Containing Magnetite Nanoparticles by Phase Transfer Masayuki Asada,† Yohei Hara,† Yoshitaka Kuroda,† Shunsuke Tanaka,†,‡ Toshihide Horikawa,‡,§ and Yoshikazu Miyake*,†,‡ Department of Chemical Engineering, and High Technology Research Center, Kansai UniVersity, 3-3-35, Yamate-cho, Suita, Osaka 564-8680, Japan, and Institute of Technology and Science, The UniVersity of Tokushima, 2-1 Minamijosanjima-cho, Tokushima 770-8506, Japan

Magnetite-containing spherical mesoporous silica (M-SMS) has been prepared using the phase transfer method between an aqueous phase containing cationic surfactant and magnetite nanoparticles and an oil phase consisting of hydrophobic tetrabutoxysilane (TBOS) as the silica source under stirring conditions. The M-SMS was 200-300 µm in diameter and possessed mesopores with a mean diameter of ca. 2 nm and a high BET surface area of over 960 m2/g. M-SMS could be easily collected and separated from the solution using a magnet. The following mechanism is proposed for the formation of the M-SMS: the magnetite nanoparticles are transferred with a cationic surfactant (cetylpyridinium chloride, CPC) and water molecules from the aqueous phase into the oil phase of TBOS. The W/O-type microemulsions are formed in the TBOS phase, and the hydrolysis and condensation of TBOS proceed subsequently at the interface and/or in the water pools of the microemulsions; thus, magnetite nanoparticles are loaded in a silica matrix. In this system, CPC acted as a structure-directing agent for formation of mesopores and as a phase transfer agent for magnetite nanoparticles. Butanol molecules, released by hydrolysis of TBOS, are thought to behave as a cosolvent for the phase transfer of magnetite nanoparticles. Introduction Since the development of ordered mesoporous silica in 1992,1 its technological importance has sparked extensive research interests in materials science and engineering. Mesoporous silica materials have been of interest in the field of catalysis, adsorption, and separation because of their high specific surface area, large pore volume, and controllable uniform pore size. Recently, they have been used for drug delivery,2,3 protein encapsulation,4,5 and immobilization of cells,6 because silica is nontoxic and highly biocompatible. Furthermore, to develop new functions for silica, many studies have been carried out on the loading of nanoparticles such as titania,7 Au,8-10 Pt,8,10 and Ag10 in the mesostructured silicas. Since magnetite nanoparticles are well-known as versatile carrier materials that can be easily separated from a complex multiphase system by an external magnetic device, for magnetic carrier technology,11,12 several researchers have attempted to combine magnetite nanoparticles with porous materials.13-17 Introduction of nanoparticles inside the mesostructure have been carried out by several methods as follows: (i) wet impregnation,8 (ii) impregnation using a supercritical fluid,14,18 (iii) chemical vapor deposition9 of the metal precursor into the preformed mesoporous silica and subsequent reduction or decomposition of the precursor to form metal or metal oxide nanoparticles, (iv) preparation of mesoporous silica in the presence of nanoparticles,10 (v) preparation of mesoporous silica in the presence of the metal precursor and * Corresponding author. Tel: +81-6-6368-0950. Fax: +81-6-63888869. E-mail: [email protected]. † Department of Chemical Engineering, Kansai University. ‡ High Technology Research Center, Kansai University. § The University of Tokushima.

subsequent reduction,16,17 and (vi) layer-by-layer method using nanosized mesoporous silica and metal oxide particles.15 On the other hand, in water-oil two-phase systems, nanoparticles tend to assemble at the water-oil interface19-21 and are transferred to the oil phase in the presence of surfactant.22-24 The hydrophilic group of the surfactant is adsorbed on the particle surface by electrostatic and/or hydrogen-bonding interactions, and the hydrophobic chain of the surfactant was directed to the outside, so the hydrophobic particles modified with surfactant could be extracted and dispersed in the oil phase. Phase transfer processes involving nanoparticles have been studied for applications involving size dependent classifications, because the characteristics of nanoparticles are dependent on the particle size. The phase transfer method can operate with low energy and can disperse the nanoparticles into various liquid phases, in which liquid dispersed nanoparticles can be used for potential applications of catalysts, sensors, and optics under various circumstances. In our previous studies,25-27 spherical mesoporous silica (SMS) particles with reverse mesostructure were prepared in the water/oil (W/O) phases, which consisted of an aqueous phase with a dissolved cationic surfactant and sodium hydroxide as a catalyst and an oil phase consisting of tetrabutoxysilnae (TBOS) as a silica source. The process of SMS formation was determined as follows: a cationic surfactant was extracted into the TBOS phase with water molecules and the W/O-type microemulsions formed in the TBOS phase. Since the hydrolysis and condensation of TBOS proceed in the water pools in self-assembled W/O-type microemulsions, the reverse of MCM-41 structure, in other words the continuous mesoporous structure, is developed. Here we report a novel type of composite material consisting of magnetite nanoparticles and reverse MCM-41 mesoporous

10.1021/ie8012133 CCC: $40.75  2009 American Chemical Society Published on Web 01/28/2009

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silica. The magnetite nanoparticles are directly incorporated into SMS using a phase transfer method. This synthetic process has the advantage that it involves a one-step process to prepare magnetite-containing spherical mesoporous silica (M-SMS), in which nanoparticles can be dispersed and loaded into the SMS. Characteristics of M-SMS such as the morphology, pore structure, magnetite contents, and the adsorption properties of cationic dyes were investigated by comparing the properties of SMS.25 The process of formation of M-SMS is discussed by measuring the concentration changes of the cationic surfactant, water, and magnetite nanoparticles. Furthermore, the effect of butanol, which is released with hydrolysis and condensation of TBOS, on the process of formation of M-SMS was evaluated by adding butanol at the initial stage of the reaction. Experimental Section Chemicals. Cetyl pyridinium chloride monohydrate (CPC, Wako), tetrabutoxysilane (TBOS, Shin-etsu), and sodium hydroxide (Wako) were used as received. The magnetite nanoparticles [iron(II), -(III), nanopowder, Aldrich], with a primary particle size of 25 nm, were used as received. 1-Butanol (Wako) was used as a cosurfactant for phase transfer of the magnetite nanoparticles. A 5 N HCl solution was used to dissolve the magnetite nanoparticles dispersed in M-SMS. All reagents were used without further purification. Preparation of M-SMS. CPC (3.7 g) and NaOH (2.0 g) were dissolved in 250 cm3 of distilled water, yielding a transparent yellow solution of pH 13, and then 0-1.2 g of the magnetite nanoparticles was dispersed in the solution. Subsequently, 32 g of TBOS (density 0.90 g/cm3, volume 35.6 cm3) was dropped into the solution, and the mixture was vigorously stirred using the agitator for 10 h. After about 10 h, dark gray silica particles (200-300 µm) were obtained dispersed in the aqueous phase. The solid products were filtered, washed with deionized water, and dried at room temperature. The samples were calcined under a nitrogen atmosphere at 773 K for 5 h to remove the surfactant. Characterization of M-SMS. M-SMS was characterized by X-ray powder diffraction (XRD, JDX-3530, JEOL), scanning electron microscopy (SEM, VE-9800, KEYENCE), transmission electron microscopy (TEM, JEM 2010, JEOL), and N2 adsorption measurement at 77 K (BELSOAP-mini, Bell Japan). The BrunauersEmmettsTeller (BET) method was utilized to calculate the specific surface area using the relative pressure from 0.05 to 0.3. The pore volume was calculated as the amount of nitrogen adsorbed at a relative pressure of 0.99. The pore size distributions were calculated by the DollimoresHeal method using the adsorption branch for the mesopore analysis. In order to evaluate the magnetite contents in the M-SMS, the magnetite nanoparticles in the silica matrix were dissolved with 5 N HCl solution, and the concentration of iron ions was determined using an inductive-coupled plasma analyzer (ICP, ICPS-7510, Shimadzu). In order to demonstrate the importance of the magnetic separation, a magnet was placed close to the side of the samples. The adsorption properties of M-SMS for an organic dye, rhodamine B, were evaluated using the following experiment. The weighted M-SMS was dispersed in rhodamine B solution (initial concentration 5 mM) at 303 K. The initial and equilibrium (after 24 h) concentrations of rhodamine B solution were measured using an UV-visible spectrophotometer (UV-2450, Shimadzu). Then, after adsorbing the solute on M-SMS, the M-SMS were collected using a magnet. The extinction coefficient of rhodamine B is 96 (cm mM)-1 at 553 nm. The adsorption quantity of rhodamine B was calculated using the following mass balance equation

Table 1. Properties of M-SMS Fe3O4 sample

g/La

wt %b

SBETc (m2/g)

pore volume (cm3/g)

pore size (nm)

pure SMS 4-M-SMS 12-M-SMS 17-M-SMS

0 1.2 2.4 4.8

0 4.0 12 17

850 960 1300 1300

0.53 0.62 0.88 0.81

2.1 2.2 2.3 2.4

a Concentration of magnetite in the water phase of synthetic mixture. b Weight percentage of magnetite in mesoporous silica. c BrunauersEmmetsTeller (BET) surface area.

q)

(C0 - Ceq)V W

(1)

where q (mmol/g) is the adsorption quantity, C0 and Ceq (mM) are initial and equilibrium concentration, respectively, and V (dm3) and W (g) are the volume of the solution and the weight of M-SMS, respectively. Extraction Process of Magnetite Nanoparticles. After TBOS was dropped in the reactor, CPC, water, and magnetite nanoparticles were extracted from the aqueous phase to the oil phase of TBOS. The time courses of the CPC concentration in the water phase and the water concentration in the oil phase were evaluated using an UV-visible spectrophotometer and a Karl Fischer moisture titrator (MKS-500, KEM), respectively. The extinction coefficient for CPC is 4.2 (cm mM)-1 at 269 nm. The magnetite contents extracted into the TBOS phase at each sampling time were evaluated as follows. The TBOS phase was in the liquid state until it had been reacted for 8 h, and upon contact with the 5 N HCl solution, the mixing solution TBOS and HCl turned into a gel. Therefore, only the TBOS liquid phase was gelled at room temperature in the subsequent several hours. The gel dispersed the magnetite nanoparticles and was calcined in order to remove the surfactant at 773 K for 5 h in a nitrogen atmosphere. Thereafter, the weighted silica powder was placed in contact with the 5 N HCl solution in order to resolve the magnetite from the silica powder. The iron concentration was determined by ICP. Further experiments were conducted in order to investigate the role of butanol in the transfer process of magnetite nanoparticles in the TBOS phase. Five cubic centimeters of water, 0.072 g of CPC, 0.04 g of NaOH, 0.64 g of TBOS, 0.012 g of magnetite nanoparticles, and an equivalent amount of butanol was mixed in a 10 cm3 test tube. The reagents consisted of 1/50 of the volume used for preparation of M-SMS, but the molar ratios of the mixture were the same. After the two-phase system was sonicated for 5 min and shaken for 5 min, the TBOS phase was separated from the system. The CPC and water concentrations in the TBOS phase and magnetite contents in the silica particle prepared in the sampled TBOS phase were evaluated by the method described above. Note that the butanol released by hydrolysis of TBOS could be neglected because of the short reaction time. Results and Discussion Characteristics of M-SMS. Four kinds of M-SMS were prepared with different concentrations (0, 1.2, 2.4, and 4.8 g/L) of magnetite nanoparticles in the water phase. The concentration of magnetite nanoparticles in water phase and the amount of magnetite in the mesoporous silica particles are summarized in Table 1. The product prepared without magnetite nanoparticles is referred to as pure-SMS. The final products prepared with magnetite nanoparticles are designated as X-M-SMS, where X represents the weight percent of magnetite in the M-SMS. The

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Figure 1. SEM images for (a) pure-SMS and (b) 12-M-SMS. TEM images of (c) magnetite nanoparticles and (d) magnetite in 12-M-SMS.

magnetite content in M-SMS increased upon increasing the concentration of magnetite in the water phase. SEM images of pure-SMS and 12-M-SMS are shown in Figure 1, parts a and b, respectively. In spite of the inclusion of the magnetite, the obtained M-SMS particles were spherical as in pure-SMS. The particle size of 12-M-SMS ranged from 200 to 300 µm in diameter, and it was similar to that of pureSMS. The particle size may be controlled by agitation conditions such as agitation speed, type of agitator and the volume ratio of TBOS to aqueous phases. TEM images of the initial magnetite nanoparticles and incorporated particles in 12-M-SMS are shown in Figure 1, parts c and d, respectively. The initial magnetite nanoparticles were nonspherical and approximately 25 nm in size, as shown in Figure 1c. The dark regions in Figure 1d indicate the magnetite nanoparticles dispersed in the silica matrix. It is concluded from the TEM observation for several parts in M-SMS that the magnetite nanoparticles are almost dispersed uniformly in the silica matrix, although the initial magnetite nanoparticles were partially aggregated. Low-angle XRD patterns of the pure-SMS and 12-M-SMS before calcination are shown in Figure 2a. Wide-angle XRD patterns of the magnetite nanoparticles and 12-M-SMS before calcination are shown in Figure 2b. As shown in Figure 2a, diffraction peaks at about 2θ ) 2.5° (d-space was 3.5 nm) that correspond to that of pure-SMS26-28 in position were observed in the low-angle XRD pattern of both SMS. These results indicate that the mesostructure of the M-SMS was similar to that of the pure-SMS. It was concluded that the formation of W/O-type microemulsions of CPC in the hydrophobic TBOS precursor was not inhibited by the presence of magnetite nanoparticles. As shown in Figure 2b, a sharp peak at 2θ ) 35.5°, which could be ascribed to the crystal of magnetite, was observed, indicating that magnetite nanoparticles were incorporated in M-SMS. N2 adsorption-desorption isotherms of M-SMS are shown in Figure 3. All of the isotherms show curves without hysteresis loops. There is no large difference in the adsorbed N2 volumes among the four samples at lower relative pressure, which correspond to the micropore volume. However, the N2 adsorption volume at high relative pressure (below 0.5), corresponding

Figure 2. (a) Low-angle XRD patterns of pure-SMS and 12-M-SMS and (b) wide-angle of XRD patterns of magnetite nanoparticles and 12-M-SMS.

Figure 3. N2 adsorption and desorption isotherms at 77 K for pure-SMS and M-SMS.

to mesopores, was increased with increasing magnetite nanoparticle content, with the exception of 17-M-SMS. The N2 adsorption volume reached a maximum for 12-M-SMS. Table 1 summarizes the results of pore structure analysis. The BET specific surface areas and the pore volumes of M-SMS increased with an increase in magnetite content. On the other hand, there is no large difference in the mesopore sizes between pure-SMS and M-SMS. These results indicate that the large surface area of M-SMS was derived from the porosity formed by selfassembly of surfactant and attributed to the interparticle porosity between the magnetite nanoparticles and between the silica matrix and magnetite nanoparticles. An adsorption isotherm of rhodamine B in the aqueous phase for M-SMS is shown in Figure 4. The adsorption isotherm can be correlated by the Langmuir-type isotherm, as follows: q)

q∞KLCeq 1 + KLCeq

(2)

Here, q∞ (mmol/g) is the maximum adsorption quantity and KL (1/mM) is the Langmuir equilibrium constant. As shown in Figure 4, the solid lines represent the values calculated to fit the experimental data using eq 2 with the following parameters: q∞ parameters of the silica particle, 0.75 mmol/g for pure-SMS and 0.62 mmol/g for 12-M-SMS; KL, 1.2 1/mM for pure-SMS and 1.3 1/mM for 12-M-SMS. The maximum adsorption quantity of pure-SMS was larger than that of 12-M-SMS,

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Figure 4. Adsorption isotherm of rhodamine B with pure-SMS (b) and 12-M-SMS (O). The solid lines represent the results of eq 2, and the broken line is the adsorption isotherm per unit weight of silica for 12-M-SMS.

Figure 5. Image of collected M-SMS dispersed in rhodamine B solution using a magnet.

Figure 7. (a) Time course of magnetite nanoparticle content, (b) time-course of CPC concentration in aqueous phase, and (c) time course of water concentration in oil phase.

Figure 6. Images of formation processes for pure-SMS and 12-M-SMS in reactor.

although the specific surface area of pure-SMS was lower than that of 12-M-SMS. The broken line in Figure 4 shows the revaluated adsorption quantity per unit of silica weight of 12M-SMS, 1.1 times the solid line for 12-M-SMS. The broken line was nearly equal to the solid line for pure-SMS. Since the cationic dye, rhodamine B, had a strong electrostatic interaction with silanol groups on the silica surface, it was deduced that rhodamine B molecules could be adsorbed on the silica surface of the mesopore formed by the surfactant, but rhodamine B might not be adsorbed on the magnetite surface. As shown in Figure 5, M-SMS adsorbed rhodamine B in aqueous solution could be attracted by the magnet. The magnetite content of M-SMS was 12 wt %, so it was thought that they had a strong magnetic power. This will provide an easy and efficient means to separate and collect the M-SMS from the aqueous phase. These results suggested that the pore structure of M-SMS was analogous to that of pure-SMS. Thus, the M-SMS can be used as an adsorbent, similar to SMS. Moreover, this process can facilitate preparation of SMS, which loaded other nanoparticles such as Au, Pt, and TiO2 while maintaining the mesostructure of SMS. Extraction of Magnetite Nanoparticles. Figure 6 shows the change of the synthetic solution on the process involved in preparing pure-SMS and 12-M-SMS. Prior to reaction (0 min),

the TBOS phase was transparent and colorless. The aqueous phase of pure-SMS synthetic mixture was yellow due to CPC, and that of 12-M-SMS synthetic mixture was brown due to the dispersed magnetite nanoparticles and dissolved CPC. The magnetite nanoparticles and CPC were transferred from the aqueous phase to the oil phase as the stirring time proceeds. After 30 min, the color of the TBOS phase turned yellow in the case of pure-SMS and turned dark brown for 12-M-SMS. As the interface between TBOS and the aqueous phases could not be distinguished, CPC and magnetite nanoparticles were not completely transferred to TBOS phase. After 60 min, as the color of TBOS phase became deeper than that of the aqueous phase, the CPC and magnetite nanoparticles were almost transferred from the aqueous phase to TBOS phase. After 10 h, orange particles in SMS and black particles in 12-M-SMS were generated in the upper portion of the reactor and the aqueous phase was transparent and colorless. This was because the dispersed magnetite nanoparticles and the dissolved CPC were removed from the aqueous phase. In order to evaluate the transfer process quantitatively, both TBOS and the aqueous phases were sampled at various time points during stirring. The weight fractions of magnetite nanoparticles in the sampled TBOS liquid phase at each time point was determined by the method described in the previous section. Furthermore, the CPC concentration in the aqueous phase was measured by diluting with water so as to neglect the turbidity by magnetite nanoparticles dispersed in the aqueous phase. The water concentrations in the TBOS phase were also measured. These results are shown in Figure 7a-c. As shown in Figure 7a, the magnetite nanoparticles were not extracted until 30 min of stirring. At 60 min, some magnetite nanoparticles

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were extracted, and amount of magnetite nanoparticles transferred reached an equilibrium at 120 min. The magnetite contents in the silica particles prepared for 120 min was 14 wt %. The value was larger than that in the M-SMS prepared after 10 h stirring, which was 12 wt %, as shown by the solid circle in Figure 7a. In phase transfer of nanoparticles such as gold and silver, all nanoparticles could not be transferred into the oil phase, however, as they tended to aggregate at the oil-water interface.28,29 Therefore, the difference in the magnetite concentration may be attributed to adsorption of magnetite nanoparticles at the interface of M-SMS and on the wall of the reactor. Furthermore, in order to investigate the role of the oil phase, the same amount of pure 1-butanol generated by hydrolysis of TBOS was used in the oil phase. As shown by triangle symbols in Figure 7a, no transfer of magnetite nanoparticles occurred, although there were some magnetite nanoparticles at the interface between the butanol phase and the aqueous phase. Phase transfer of nanoparticles was affected by the type of oil phase. In this case, it was deduced that the TBOS phase was appropriate for phase transfer of magnetite nanoparticles. As shown in Figure 7b,c, CPC and water were also gradually extracted into the TBOS phase. The concentration of CPC in the aqueous phase and water in the TBOS phase reached equilibrium at the same time as extraction of magnetite nanoparticles reached equilibrium. At equilibrium, almost all of the CPC was extracted into the TBOS phase, and the water concentration in the TBOS phase became 10 M. Since the value of [H2O]/[CPC] in the TBOS phase was larger than 35, the W/Otype microemulsions were formed in the TBOS phase.26 The extraction rate of CPC in the presence of magnetite nanoparticles was faster than that without magnetite. This was thought to be due to the extraction of magnetite nanoparticles that adsorbed the CPC. In the butanol system, extraction of these substances reached equilibrium after 15 min. The equilibrium concentration of CPC was only half that in the TBOS phase. The water concentration in the butanol phase was about 20 M, which was twice the concentration of the TBOS phase, because water could be solved in butanol phase without surfactant. Magnetite nanoparticles were transferred to the oil phase in the TBOS system; however, this trend was not observed for the butanol system. It was thought that the partition of CPC affected the phase transfer of magnetite nanoparticles because they were made hydrophobic with CPC, which was adsorbed on the surface of magnetite by electrostatic interactions. In the TBOS system, the partition of CPC to the TBOS phase resulted in phase transfer of magnetite nanoparticles and in the formation of W/O-type microemulsions. Magnetite nanoparticles dispersed in the TBOS phase were loaded in the silica matrix with condensation of TBOS. In the butanol system, only half of the CPC partitioned to the butanol phase because of the high polarity, so magnetite nanoparticles could not be dispersed in the butanol phase. Effect of Butanol on Phase Transfer of Magnetite Nanoparticles. The butanol was released by the hydrolysis and condensation of TBOS molecule, therefore; this extraction system is not a steady-state and the butanol concentration in this system increases. It is noted that if 0.64 g of TBOS was perfectly hydrolyzed, 0.7 mL of butanol was stoichiometrically released in the system. At the initial stages of formation of M-SMS, additional effects of butanol on the extraction of magnetite nanoparticles, CPC, and water were observed, as shown in Figure 8a,b. Magnetite nanoparticles and water could not be transferred to the TBOS phase under the free butanol

Figure 8. (a) Effect of additional butanol volume on magnetite contents in TBOS phase and (b) effect of additional butanol volume on the CPC concentration in aqueous phase and water concentration in TBOS phase.

system, but a subtle amount of CPC was extracted to the TBOS phase. The TBOS phase initially consisted of hydrophobic media; therefore, almost all of the CPC was not partitioned into the TBOS phase at the initial stage of the formation process of M-SMS, due to the absence of butanol. Upon addition of butanol, magnetite contents in the TBOS phase increased and the extracted amounts of magnetite nanoparticles were a maximum at 0.2 mL of butanol. The concentrations of CPC and water in the TBOS phase also increased upon addition of butanol. Upon addition of more than 0.2 mL of butanol, almost all CPC was extracted to the TBOS phase, and the water concentration increased reached 10 M, which corresponds to the equilibrium value. It was deduced that, because the TBOS phase was hydrophobic, water molecules could not be extracted into the pure TBOS phase in the absence of CPC. The TBOS phase becomes hydrophilic upon addition of butanol; butanol could be partitioned to both the aqueous and TBOS phases. Therefore, CPC and water were extracted into the TBOS phase. Furthermore, since the butanol concentration increased as hydrolysis and condensation of TBOS progressed, the CPC and water concentrations increased, and as the result, the magnetite nanoparticles were extracted. Butanol functioned as a cosolvent for the extraction system; more CPC, water, and magnetite nanoparticles could be extracted to TBOS phase. At the initial stage of preparation of M-SMS, magnetite and CPC were not extracted to the TBOS phase because there is not the butanol produced by the hydrolysis of TBOS. As adequate amounts of butanol were released upon hydrolysis of TBOS, magnetite and therefore nanoparticles could be extracted to the oil phase. Conclusion Magnetite-containing spherical mesoporous silica was prepared by the nanoparticle phase transfer method in a two-phase system consisting of TBOS and aqueous phases. M-SMS had a large specific surface area; therefore, it may be useful as an adsorbent and can be collected easily with a magnet after dispersion in the phase. Magnetite nanoparticles were transferred to the TBOS phase by hydrophobic modification to adsorb CPC on the surface after an adequate amount of butanol was released by hydrolysis of TBOS. In this system, CPC worked as a structure-directing agent and as a phase transfer agent for magnetite nanoparticles. Magnetite nanoparticles extracted and

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ReceiVed for reView August 6, 2008 ReVised manuscript receiVed December 11, 2008 Accepted December 13, 2008 IE8012133