Isothermal Carbon Dioxide Sorption in Poly (ionic liquid) s

Sep 23, 2009 - Jianbin Tang,† Youqing Shen,*,†,‡ Maciej Radosz,‡ and Weilin Sun§ ... CO2 sorption in the PILs fitted the dual-mode model very...
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Ind. Eng. Chem. Res. 2009, 48, 9113–9118

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Isothermal Carbon Dioxide Sorption in Poly(ionic liquid)s Jianbin Tang,† Youqing Shen,*,†,‡ Maciej Radosz,‡ and Weilin Sun§ Department of Chemical & Biochemical Engineering, State Key Lab of Chemical Engineering, Zhejiang UniVersity, Hangzhou 310027, China, Department of Chemical & Petroleum Engineering, UniVersity of Wyoming, Laramie, Wyoming 82071, and Department of Polymer Science & Engineering, Zhejiang UniVersity, Hangzhou 310027, China

The low-pressure isothermal sorption of CO2 in poly(ionic liquid)s (PILs) with varied structures including different cations, anions, backbones, and substituents was investigated to probe structure effects on the CO2 sorption. An ammonium cation with short alkyl group, BF4 anion, and polystyrene backbone was found to favor CO2 sorption in PILs. CO2 sorption in the PILs fitted the dual-mode model very well, suggesting that the CO2 sorption consists of dissolution in the polymer matrix and Langmuir sorption in the microvoids. 1. Introduction Carbon dioxide (CO2) was found to be highly soluble in an ionic liquid in 1999.1 For example, at 50 bar of CO2 pressure and 40 °C, the CO2 mole fraction in 1-butyl-4-methylimidazolium hexafluorophosphate ([bmim][PF6]) reached as high as 0.5.2 The CO2 solubilities in ionic liquids with different structures at varied pressures and temperatures were subsequently investigated.2-19 Experimental20,21 and theoretical22-30 studies on the mechanism of the CO2 dissolution in ionic liquids concluded that the interactions of CO2 with ionic liquids, especially with their anions, played a major role.22 The high solubility of CO2 in ionic liquids was proposed for many potential applications,31,32 for example, CO2 as a solvent to extract organic products or contaminants from ionic liquids.1 Ionic liquids were also reported as sorbents and membrane materials for CO2 separation.33-38 Supported ionic liquid membranes showed high permeability and permselectivity.39-41 Recently, we found that poly(ionic liquid)s (PILs), the polymers made from the monomers with ionic liquid moieties, had higher CO2 sorption capacities and faster sorption/desorption rates than room-temperature ionic liquids.42-45 For instance, poly[1-(pvinylbenzyl)trimethylammonium tetrafluoroborate] (P[VBTMA][BF4]) had a CO2 sorption capacity of 10.2 mol % in terms of the monomer unit of the polymer at as low as 0.78 bar of CO2 pressure (22 °C), which is 6.6 times higher than that in a room-temperature ionic liquid, 1-butyl-4-methylimidazolium tetrafluoroborate ([bmim][BF4]). Furthermore, the gas sorption of the PILs was very selective. They did not show any measurable sorption of N2 or O2 at ambient pressure.42,43 We previously reported the kinetics of CO2 sorption in imidazolium- and ammonium-based PILs.46,47 In this paper, we systematically examined the isothermal CO2 sorption of the PILs with varied structures to elucidate the structure effects on the sorption. The isothermal CO2 sorption was also fitted to a dual-mode model to investigate the mechanism of CO2 sorption in the PILs.

[1-(para-vinylbenzyl)-4-methylimidazolium tetrafluoroborate] (P[VBMI][BF4]), poly[2-(methacryloyloxy)ethyl]trimethylammonium tetrafluoroborate (P[MATMA][BF4]), poly[1-(para-vinylbenzyl)trimethylammonium hexafluorophosphate] (P[VBTMA][PF6]), and poly[(1-vinylbenzyl)trimethylammonium trifluoromethanesulfonamide] (P[VBTMA][Tf2N]) were synthesized according to our previous reports.46,47 Their structures are shown in Figure 1. paraVinylbenzyl chloride, pyridine, triethylphosphine, 6-di-tert-butyl4-methylphenol, NaBF4, acetone, acetonitrile, dimethylformamide (DMF), and ether were purchased from Aldrich and used as received. Poly[1-(para-vinylbenzyl)pyridinium tetrafluoroborate] (P[VBP][BF4]) and poly[1-(para-vinylbenzyl)triethylphosphonium tetrafluoroborate] (P[VBTEP][BF4]) were synthesized by free radical polymerization of ionic liquid monomers 1-(para-vinylbenzyl)pyridinium tetrafluoroborate ([VBP][BF4]) and 1-(para-vinylbenzyl)triethylphosphonium tetrafluoroborate ([VBTEP][BF4]), respectively. The synthesis procedures are as follows. [VBP][BF4]. para-Vinylbenzyl chloride (9.76 g, 0.064 mol), a small amount of 2,6-di-tert-butyl-4-methylphenol, and pyridine (5.05 g, 0.064 mol) were added to a 50-mL flask and heated at 45 °C under N2 protection. After heating overnight, NaBF4 (7.4 g, 0.067 mol) and dry acetone (30 mL) were added and the resulting mixture was stirred at room temperature. While the viscous liquid dissolved gradually, white solids precipitated out from the solution. After a 12 h reaction time, the precipitate

2. Experimental Section 2.1. Materials. P[VBTMA][BF4], poly[1-(para-vinylbenzyl)triethylammonium tetrafluoroborate] (P[VBTEA][BF4]), poly* To whom correspondence should be addressed. Tel./Fax: +86571-87953993. E-mail: [email protected]. † Department of Chemical & Biochemical Engineering, State Key Lab of Chemical Engineering, Zhejiang University. ‡ University of Wyoming. § Department of Polymer Science & Engineering, Zhejiang University.

Figure 1. Structures of the poly(ionic liquid)s.

10.1021/ie900292p CCC: $40.75  2009 American Chemical Society Published on Web 09/23/2009

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Table 1. Physical Properties and Parameters of CO2 Sorption in the Poly(ionic liquid)s CO2 sorption parameters (at 22 °C) no.

poly(ionic liquid)

F2 (g · cm-3)

Tg (°C)

kD (bar-1)

cH′ (bar-1)

b

ca (cm3/cm3)

cDa (cm3/cm3)

cHa (cm3/cm3)

1 2 3 4 5 6 7 8 9

P[VBTEA][BF4] P[VBTEP][BF4] P[VBP][BF4] P[VBMI][BF4] P[VBTMA][BF4] P[VBTMA][PF6] P[VBTMA][Tf2N] P[MATMA][BF4] cross-linked P[VBTMA][BF4]

1.225 1.268 1.382 1.379 1.035 1.028 1.294 1.184 1.270

185 187 155 110 235 255 74 216 220

3.75 3.10 3.40 2.59 6.30 5.00 2.29 4.60 4.35

3.20 2.70 3.25 2.12 5.40 5.30 0.79 4.50 7.50

2.04 1.51 2.00 1.83 3.83 2.74 0.50 3.34 1.78

5.90 4.77 5.62 3.99 10.68 8.95 2.59 8.13 9.23

3.80 3.14 3.45 2.61 6.38 5.06 2.32 4.66 4.40

2.10 1.63 2.18 1.38 4.29 3.89 0.27 3.47 4.82

a

Calculated at 22 °C and 1 atm.

was removed by filtration. The filtrate was concentrated under vacuum and then poured into ether to precipitate out the product. The product was collected by filtration, washed with water and ether, and dried under vacuum at room temperature, producing 15.2 g of white crystals (yield 84%). 1H NMR (DMSO-d6, 400 MHz, ppm): δ 9.18 (m, 2H), 8.63 (m, 1H), 8.19 (m, 2H), 7.55 (m, 4H), 6.75 (m, 1H), 5.89 (d, 1H), 5.85 (s, 2H), 5.32 (d, 2H). mp: 65 °C. [VBTEP][BF4]. 4-Vinylbenzyl chloride (6.1 g, 0.04 mol) and triethylphosphine (4.96 g, 0.042 mol) were mixed in a 50-mL flask and heated at 50 °C under a N2 atmosphere for 2 days. The formed solid was collected and washed with diethyl ether. The resultant white solid (8.5 g, 0.033 mol) was mixed with NaBF4 (3.8 g, 0.035 mol) in 50 mL of acetonitrile and stirred at room temperature for 2 days. The salt precipitate was removed by filtration. The filtrate was concentrated and poured into 200 mL of diethyl ether to precipitate out the product. The white product was collected by filtration and dried under vacuum. The total yield was 8.3 g (65%). 1H NMR (DMSO-d6, 400 MHz, ppm): δ 7.53 (d, 2H), 7.34 (d, 2H), 6.75 (m, 1H), 5.88 (d, 1H), 5.31 (d, 1H), 3.81 (d, 2H), 2.20 (s, 6H), 1.11 (d, 9H). mp: 133 °C. The polymers were synthesized via free radical polymerization using 2,2′-azobis(isobutyronitrile) (AIBN) as initiator. A typical example is as follows. [VBP][BF4] (3 g), AIBN (30 mg), and DMF (3 mL) were charged into a reaction tube. The tube was tightly sealed and degassed. The sealed tube was immersed in an oil bath at 60 °C for 8 h. The polymerization solution was poured into methanol to precipitate out the polymer. The polymer was collected by filtration and dried under vacuum at 100 °C. The yield was 2.85 g (95%). 1H NMR (DMSO-d6, 400 MHz, ppm): 9.02 (br, 2H), 8.55 (br, 1H), 8.10 (br, 2H), 7.18 (br, 2H), 6.40 (br, 2H), 5.68 (br, 2H), 0.5-2.2 (br 3H). Anal. Calcd for (C14H15NBF4)n: C, 59.20; H, 5.29; N, 4.93. Found: C, 59.42; H, 5.30; N, 4.94. The P[VBTEP][BF4] was synthesized in the same way with a yield of 90%. 1H NMR (DMSO-d6, 400 MHz, ppm): 7.01 (br, 2H), 6.37 (br, 2H), 2.09 (br, 6H), 1.39 (br, 3H), 0.97 (br, 9H). Anal. Calcd. for (C15H24PBF4)n: C, 55.94; H, 7.46; N, 0. Found: C, 56.22; H, 7.49; N, 0. All polymers used in this paper were characterized by 1H NMR and elemental analysis. Both characterization methods indicated that pure polymers were obtained. The estimated purities of the polymers were higher than 98%. 2.2. Measurements. 1H NMR spectra were measured on a Bruker Advance DRX-400 spectrometer using deuterated dimethyl sulfoxide (DMSO) as solvent. Elemental analyses were done by Midwest Microlab LLC (Indianapolis, IN). Differential scanning calorimetric (DSC) experiments were performed on a TA Instruments DSC Q900 differential scanning calorimeter. The samples were heated to 100 °C and then cooled to room temperature. The glass-transition temperatures (Tg’s) and melting

points (mp’s) were calculated in the second scan at a heating rate of 10 °C/min. The densities of the PILs were measured using a calibrated density bottle according to our previous report.45 The measured Tg’s and densities of the polymers are shown in Table 1. 2.3. Isothermal CO2 Sorption Measurements. The CO2 isothermal sorptions in the PILs were measured using a Cahn 1000 electrobalance. The measurement system is described in our previous reports.46,47 As shown in Figure 2, the pressure in the chamber was monitored by a barometer, and the weight change of the sample after CO2 sorption was recorded by a Cahn 1000 electrobalance. Before the measurement, the fine powder of a PIL was dried and degassed at 70 °C under vacuum for 12 h to remove any moisture or other volatile contaminants. It was further dried in the balance by evacuating the chamber at high vacuum until its weight remained constant for at least 30 min. Dried CO2 was then introduced into the chamber to reach a certain pressure and the chamber was sealed. The weight increase of the sample was recorded until the weight did not change significantly in 30 min. The actual weight increase (m) was the sum of the observed weight increase recorded by the balance (mabs) and the weight loss due to buoyancy (mb) as shown in eq 1. The buoyancy in these measurements was corrected according to the literature48 and calculated from eq 2, where F is the density of CO2 under the measurement conditions, Vas is the volume of the sample after sorbing CO2, Vc is the volume of the counterweight, and VI and VII are the volume of the balance components on the sample and reference sides, respectively. The VI - VII - Vc value was measured by running a blank experiment under vacuum and the ambient CO2 pressure and was found to be zero for the balance used. Since the experiments were carried out under low pressures, we assumed that there was no volume change during the CO2 sorption, and hence the Vas was equal to the volume of the polymer sample (V). Thus, eq 2 can be simplified into eq 3.

Figure 2. Schematic diagram of the gas sorption apparatus: (1) drying column packed with P2O5, (2) three-way valve, (3) two-way valve, (4) Cahn 1000 balance, and (5) vacuum pump.

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The volume was determined from the weight of polymer sample (M) and its density (F2). Therefore, the weight of the sorbed CO2 in a polymer sample can be calculated from eq 4. m ) mabs + mb

(1)

mb ) F(Vas + VI - VII - Vc)

(2)

m ) mabs + FV

(3)

M F2

(4)

m ) mabs + F

The sorption capacities (c) are presented as mL(STP)/mL of the polymer, which were calculated by eq 5 m F1 c) M F2

(5)

where m is the weight increase in mg, F1is the density of CO2 at standard conditions (0 °C, 1 atm) in mg/mL, M is the weight of the polymer sample in mg, and F2 is the density of the polymer in mg/mL. An approximately 1 g sample was used to measure the CO2 sorption in the poly(ionic liquid)s, and the weight of the sorped CO2 in most polyionic liquids (m) was in a range of 1-15 mg, while the resolution of the Cahn 1000 electrobalance used to measure m was 0.001 mg, and thus the measurement error of m was negligible. The densities of the polymers that were measured using a calibrated density bottle had an accuracy of (3%. According to eq 5, the main system error came from the measurement error of F2 and the sorption capacities (c) in mL · STP/mL consequently had an accuracy of (3%. All the sorption data with and without buoyancy correction are tabulated and available as Supporting Information. 2.4. Dual-Mode Model Correlation. The experimental data were correlated using a dual-mode sorption model (eq 6),49-51 which can describe the gas solubility in glassy polymers. According to this model, the gas sorption in the glassy polymers can be divided into two idealized populations as shown by eq 6. One population is the Henry dissolution population (cD), which is viewed as the gas dissolved in the polymer matrix, and can be described by Henry’s law (eq 7). The second population is the Langmuir sorption population (cH), which is described as a Langmuir “hole-filling” process (eq 8). The total sorption is the sum of these two populations as shown by eq 6: c ) kDp +

cH′ bp 1 + bp

c D ) kDp cH )

cH′ bp 1 + bp

Figure 3. CO2 sorption in the poly(ionic liquid)s with different cations as a function of the CO2 pressure at 22 °C: (a) P[VBTEA][BF4], (b) P[VBP][BF4], (c) P[VBTEP][BF4], and (d) P[VBMI][BF4]. The symbols are the experimental data; the solid lines are dual-mode model values.

(6) (7) (8)

where c is the gas solubility in mL(STP)/mL, where STP stands for standard temperature and pressure (0 °C and 1 atm), p is the gas pressure in bar, kD is Henry’s law constant, and cH′ and b are the saturation capacity and the affinity constant for the Langmuir mode, respectively.50 The parameters kD, cH′ , and b were obtained from the dual-mode model correlation (eq 6) with an accuracy of (3%. When the parameters were beyond this range, the correlation would significantly deviate from the

Figure 4. CO2 sorption in the poly(ionic liquid)s with different anions as a function of the CO2 pressure at 22 °C: (a) P[VBTMA][BF4], (b) P[VBTMA][PF6], and (c) P[VBTMA][Tf2N]. The symbols are the experimental data; the solid lines are dual-mode model values.

experimental data. cD and cH were calculated from eqs 7 and 8, respectively. 3. Results 3.1. CO2 Sorption in the PILs with Different Cations. Figure 3 shows the isothermal CO2 sorption in the PILs with different cations. As shown in Figure 1, P[VBTEA][BF4], P[VBTEP][BF4], P[VBP][BF4], and P[VBMI][BF4] had the same anion (BF4) and polystyrene backbone but different types of cations: ammonium, phosphonium, pyridinium, and imidazolium, respectively. As shown in Figure 3, the experimental data fit the dual-mode model very well. The CO2 sorption in P[VBTEA][BF4], P[VBTEP][BF4], P[VBP][BF4], and P[VBMI][BF4] had a kD of 3.75, 3.10, 3.40, and 2.59 bar-1, cH′ of 3.20, 2.70, 3.25, and 2.12 bar-1, and b of 2.04, 1.51, 2.00, and 1.83, respectively. Their CO2 sorption increased as the pressure increased and the rate of the increase slowed down with increasing pressure. The CO2 sorption capacities (mL(STP)/mL) of the PILs were in the order of P[VBTEA][BF4] > P[VBP][BF4] > P[VBTEP][BF4] > P[VBMI][BF4]. 3.2. CO2 Sorption in the PILs with Different Anions. P[VBTMA][BF4], P[VBTMA][PF6], and P[VBTMA][Tf2N] had the same ammonium cation and polystyrene backbone but different anions. Their isothermal CO2 sorption values at low pressures are shown in Figure 4. The CO2 sorption in these PILs could also be fitted by the dual-mode model, and the CO2 sorptions in P[VBTMA][BF4], P[VBTMA][PF6], and P[VBTMA][Tf2N] had kD values of 6.30, 5.00, and 2.29 bar-1,

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Figure 5. CO2 sorption in the poly(ionic liquid)s with different backbones as a function of the CO2 pressure at 22 °C: (a) P[VBTMA][BF4] and (b) P[MATMA][BF4]. The symbols are the experimental data; the solid lines are dual-mode model values.

Figure 6. CO2 sorption in the poly(ionic liquid)s with different substituents as a function of the CO2 pressure at 22 °C: (a) P[VBTMA][BF4] and (b) P[VBTEA][BF4]. The symbols are the experimental data; the solid lines are dual-mode model values.

cH′ of 5.40, 5.30, and 0.79 bar-1, and b of 3.83, 2.74, and 0.50, respectively. The CO2 sorption of P[VBTMA][Tf2N] had a almost linear relationship with CO2 pressure. Their CO2 sorption capacities followed the order P[VBTMA][BF4] > P[VBTMA][PF6] . P[VBTMA][Tf2N]. 3.3. CO2 Sorption in the PILs with Different Backbones. P[VBTMA][BF4] and P[MATMA][BF4] had the same anion and cation, but different backbones. Their CO2 sorptions are shown in Figure 5. Similarly to P[VBTMA][BF4], the CO2 sorption in P[MATMA][BF4] could also be fitted into the dual-mode model. The resulting kD, cH′ , and b for the CO2 sorption in P[MATMA][BF4] were 4.60 bar-1, 4.50 bar-1 and 3.34, respectively. P[VBTMA][BF4] had a higher CO2 sorption capacity than P[MATMA][BF4]. 3.4. CO2 Sorption in the PILs with Different Substituents. P[VBTMA][BF4] and P[VBTEA][BF4] had the same backbone, anion, and cation, but different substituents on the cations. As shown in Figure 6, P[VBTMA][BF4] with three methyl substituents had a higher CO2 sorption capacity than the polymer with three ethyl groups on the cation at the same pressure. 3.5. Cross-linking Effect on the CO2 Sorption. Figure 7 compares the CO2 sorption in un-cross-linked and cross-linked P[VBTMA][BF4]. Cross-linking decreased the CO2 sorption capacity. The kD, c′H, and b obtained from the dual-mode model correlation for the CO2 sorption in the cross-linked P[VBTMA][BF4] were 4.35 bar-1, 7.50 bar-1, and 1.78, respectively.

Figure 7. Cross-linking effect on CO2 sorption in the poly(ionic liquid) as a function of the CO2 pressure at 22 °C: (a) without cross-linking; (b) 5% cross-linked. The symbols are the experimental data; the solid lines are dual-mode model values.

Figure 8. Sketch on CO2 sorption into a glassy polymer containing a matrix and microvoids.

4. Discussion The physical properties including Tg’s and densities are summarized in Table 1. The Tg’s of the PILs indicate that all the PILs were glassy polymers at room temperature. As shown in Figures 3-7, the CO2 sorption in the PILs fitted into the dual-mode model very well, suggesting that the CO2 sorbed into the PILs indeed had two fractions: the fraction dissolved into the matrix phase (cD) and that adsorbed into the microvoids via a Langmuir hole-filling process (cH), as sketched in Figure 8. The cD fraction can be described by Henry’s law relation (eq 7), and the resulting cD is mainly determined by the interaction between CO2 and the polymer. The cH (eq 8) mainly reflects the microvoid volume in a polymer. A high Tg for a polymer may indicate that it has more thermally stabilized microvoids. The fitting parameters and the cD and cH in the PILs at a pressure of 1 atm of CO2 are also summarized in Table 1. The cations of the PILs strongly affected the CO2 sorption capacity (Table 1, no. 1-4). With similar structures and the same anions, their CO2 sorption capacities decreased in the order ammonium > pyridinium > phosphonium > imidazolium cations. Their kD and b values were in the same order of magnitude, suggesting that the ammonium cation had the strongest interaction with CO2 and that the imidazolium cation had the weakest. The PIL with ammonium also had a high cH, suggesting that it contains a high fraction of microvoids. The low sorption capacity of the imidazolium-based PIL appears to be due to its weak interaction with CO2 and low microvoid volume fraction. The anion in the PILs was also an important factor affecting the CO2 sorption capacity (Table 1, no. 5-7). P[VBTMA][BF4] had strong interaction with CO2 in terms of kD and b and a high fraction of microvoids and thereby high CO2 sorption

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capacity. When the anion was replaced with the [PF6] anion, the interaction of the PIL with CO2 decreased slightly, but the microvoid fraction remained similar. However, with [Tf2N] as the anion, both the interaction with the CO2 and the microvoid fraction of the PIL decreased substantially, resulting in a very low CO2 sorption capacity. The low interaction of the [Tf2N] anion with CO2 was in contrast to the observation in the roomtemperature ionic liquids in which those with this anion showed the highest CO2 absorption capacities and it was concluded that the fluorine atoms in the anion played a key role.22 The low microvoid volume of P[VBTMA][Tf2N] may be because of its low Tg due to the plasticization by the anion. The PIL with a polystyrene backbone had higher CO2 sorption capacity than that with polymethylmethacrylate backbone, probably because the polystyrene backbone was more rigid than the polymethylmethacrylate backbone. Long alkyl substituents on the cation reduced the CO2 sorption capacity (P[VBTEA][BF4] vs P[VBTMA][BF4]) because the long alkyl groups on the cation (P[VBTEA][BF4]) may hinder the interaction between CO2 and the cation due to steric effects. They also plasticized the polymer, resulting in a low microvoid volume fraction in the PIL. Both of these effects reduced the CO2 sorption in the PILs. Cross-linking may also hinder the interaction between CO2 and the PIL but increase the microvoid volume due to the rigidity. The overall effect of cross-linking decreased CO2 sorption capacity. Conclusion The isothermal CO2 sorptions in PILs with different chemical structures at low pressures were investigated. CO2 sorption into PILs fitted the dual-mode model very well. The CO2 sorption capacities of the PILs with different cations decreased in the order ammonium > pyridinium > phosphnium > imidazolium, and those of the PILs with different anions were in the sequence BF4 > PF6 . Tf2N. P[VBTMA][BF4] with a polystyrene backbone had a higher CO2 sorption capacity than P[MATMA][BF4] with a polymethylmethacrylate backbone. Long alkyl substituents on the cation and cross-linking decreased the CO2 sorption capacity. These factors affected the CO2 sorption capacity of a polymer by changing its interaction with CO2 and its microvoid volume. Acknowledgment We thank the State Key United Laboratory of Chemical Engineering - Laboratory of Polymer Reaction Engineering, the National Science Fund for Distinguished Young Scholars (50888001), and the state of Wyoming (EORI) for financial support. Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org. Literature Cited (1) Blancard, L. A.; Hancu, D.; Beckman, E. J.; Brennecke, J. F. Green processing using ionic liquids and CO2. Nature 1999, 399, 28–29. (2) Blanchard, L. A.; Gu, Z.; Brennecke, J. F. High-pressure phase behavior of ionic liquid/CO2 Systems. J. Phys. Chem. B 2001, 105, 2437– 2444. (3) Soriano, A. N.; Doma, B. T.; Li, M. H. Carbon dioxide solubility in 1-ethyl-3-methylimidazolium trifluoromethanesulfonate. Ind. Eng. Chem. Res. 2009, 48, 2739–2751. (4) Raeissi, S.; Peters, C. J. A potential ionic liquid for CO2-separating gas membranes: selection and gas solubility studies. Green Chem. 2009, 11, 185–192.

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ReceiVed for reView February 20, 2009 ReVised manuscript receiVed September 2, 2009 Accepted September 8, 2009 IE900292P