Synthesis and Properties of a Novel Class of Gemini Pyridinium

Gemini pyridinium surfactants were synthesized according to the following procedures. General Procedure for the Synthesis of 1-3. 1,4-Dibromobutane. (...
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Langmuir 2007, 23, 11404-11408

Synthesis and Properties of a Novel Class of Gemini Pyridinium Surfactants Limei Zhou,† Xiaohui Jiang,*,† Yintao Li,† Zhi Chen,† and Xingqi Hu‡ College of Chemistry and Chemical Engineering, China West Normal UniVersity, Nanchong, Sichuan, PR China 673002, and College of Chemistry and Chemical Engineering, Southwest Petroleum UniVersity, Chengdu, Sichuan, PR China 610500 ReceiVed April 26, 2007. In Final Form: July 12, 2007 A novel class of gemini pyridinium surfactants with a four-methylene spacer group was synthesized, and their surface-active properties and interactions with polyacrylamide (PAM) were evaluated by surface tension, fluorescence, and viscosity measurements. A comparison between the gemini pyridinium surfactants and their corresponding monomers was also made. The cmc’s of gemini pyridinium surfactants are much lower than those of the corresponding monomeric surfactants. The C20 value is about one order of magnitude lower than that of corresponding monomers, and the longer the hydrophobic chains of the surfactants, the lower the cmc value. Surface tension measurements of the surfactantPAM mixed systems show that the critical aggregation concentration (cac) value is much lower than the cmc value of the surfactant system alone. Viscosity measurements of the surfactant-PAM mixed systems show that the relative viscosity of the surfactant-PAM system decreased with increasing concentration of surfactant. Additionally, fluorescence measurements of the surfactant-PAM mixed system suggest the formation of surfactant-polymer aggregates, and the gemini pyridinium surfactant with longer hydrophobic chains have a stronger interaction with PAM, owing to the stronger hydrophobic interaction.

Gemini surfactants are made up of two identical amphiphilic moieties connected at the level of head groups by a spacer. These surfactants are superior to corresponding conventional surfactants in a number of aspects such as a lower critical micelle concentration (cmc), a higher efficiency in reducing the oil/ water interfacial tension, unusual aggregation morphologies, and better wetting, solubilizing, and foaming properties,1-2 which make them potentially useful in many applications such as enhanced oil recovery, drug entrapment and release, gene therapy, and the construction of high-porosity materials. Therefore, gemini surfactants are considered to be a new generation of surfactants. In the area of gemini surfactants, cationic alkanediyl-R,ωbis(alkyldimethylammonium) dibromide has been widely studied.3-5 However, there are a few reports6-11 about other cationic gemini surfactants. For example, Quagliotto et al.9 synthesized a series of pyridinium cationic gemini surfactants by quaternization of the 2,2′-(R, ω-alkanediyl)bispyridines with N-alkylating agents and studied their Krafft points, cmc’s, and degree of counterion binding. They found that the peculiar behavior of some structures is due to the existence of pyridinium headgroups. To study further the relationship between the structures of gemini pyridinium surfactants and their various * To whom correspondence should be addressed. E-mail: lmz860@ 126.com. † China West Normal University. ‡ Southwest Petroleum University. (1) Menger, F. M.; Keiper, J. S. Angew. Chem., Int. Ed. 2000, 39, 1906. (2) Kim, S. S.; Zhang, W. Z.; Pinnavaia, T. J. Science 1998, 282, 1302. (3) Zana, R.; Benrraou, M.; Rueff, R. Langmuir 1991, 7, 1072. (4) Yoshimura, T.; Nagata, Y.; Esumi, K. J. Colloid Interface Sci. 2004, 275, 618. (5) Zana, R.; Benrraou, M. J. Colloid Interface Sci. 2000, 226, 286. (6) Pe´rez, L.; Pinazo, A.; Rosen, M. J.; Infante, M. R. Langmiur 1998, 14, 2307. (7) Seredyuk, V.; Alami, E.; Nyde´n, M.; Holmberg, K.; Peresypkin, A. V.; Menger, F. M. Langmuir 2001, 17, 5160. (8) Wang, X.; Wang, J.; Wang, Y.; Ye, J.; Yan, H.; Thomas, R. K. J. Phys. Chem. B 2003, 107, 11428. (9) Quagliotto, P.; Viscardi, G.; Barolo, C.; Barni, E.; Bellinvia, S.; Fisicaro, E.; Compari, C. J. Org. Chem. 2003, 68, 7651. (10) Buwalda, R. T.; Engberts, J. B. F. N. Langmuir 2001, 17, 1054. (11) Stathatos, E.; Lianos, P.; Rakotoaly, R. H.; Laschewsky, A.; Zana, R. J. Colloid Interface Sci. 2000, 227, 476.

Figure 1. Chemical structures of the studied gemini pyridinium surfactants and their monomeric analogues.

properties, in the present work a series of novel gemini pyridinium surfactants (Figure 1) were synthesized by using 1,4-dibromobutane and R-alkylpyridine, and their surface-active properties and interaction with polyacrylamide (PAM) were investigated. Experimental Section Materials. Pyrene was purchased from a commercial supplier and used as received. Polyacrylamide (PAM) was purchased from a commercial supplier, and the molecular weight was 12 000 000 and the degree of hydrolysis was e5%. Triply distilled water was used in all experiments. Gemini pyridinium surfactants were synthesized according to the following procedures. General Procedure for the Synthesis of 1-3. 1,4-Dibromobutane (10 mmol, prepared in our laboratory) and R-alkyl pyridine (24 mmol, 2.4 equiv prepared in our laboratory) were dissolved in ethanol and refluxed for 72 h. The solvent was evaporated (low vacuum is recommended to avoid foaming), and the residue was recrystallized from acetone/ethanol at least three times. The product was dried in vacuum over P2O5. All the products’ structures were confirmed by an XT-4 melting-point apparatus, a Bruker-300 NMR spectrometer, a Nicolet FTIR spectrometer, an LC-QDECA mass spectrometer, and a Carlo-Erba 1106 elemental analysis apparatus. 1,4-Bis(R-octylpyridinium)butane Dibromide (1). White sheet crystals were obtained according to the general procedure. Yield, 30%. mp 218-219 °C. 300 MHz 1H NMR (CDCl3, TMS): δ 0.81 (t, 3 H), 1.02-1.22 (m, 8 H), 1.47 (m, 2 H), 1.70-1.78 (m, 2 H), 2.47 (m, 2 H), 3.38 (t, 2 H), 4.96 (t, 2 H), 7.73-7.80 (m, 2 H), 8.22-8.27 (m, 1 H), 9.94 (d, 1 H). 75.5 MHz 13C NMR (CDCl3): δ 158.8, 146.4, 144.5, 128.3, 125.5, 56.1, 32.7, 31.4, 29.0, 28.6, 28.6, 28.4, 27.7, 22.2, 13.7. FTIR (KBr): ν 3407, 3038, 2920, 2853, 1629, 1577, 1513, 1465, 1413, 1167, 774 cm-1. ESI-MS (m/z) :

10.1021/la701154w CCC: $37.00 © 2007 American Chemical Society Published on Web 10/12/2007

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517.1 (M-Br-)+, 437.2, 219.4 (100). Anal. Calcd for C30H50N2Br2: C, 60.20; H, 8.42; N, 4.68. Found: C, 59.92; H, 8.64; N, 4.79. 1,4-Bis(R-decylpyridinium)butane Dibromide (2). White sheet crystals were obtained according to the general procedure. Yield, 36%. mp 225-227 °C. 300 MHz 1H NMR (CDCl3, TMS): δ 0.83 (t, 3 H), 1.21 (m, 12 H), 1.48 (m, 2 H), 1.76 (m, 2 H), 2.47 (m, 2 H), 3.38 (t, 2 H), 4.97 (t, 2 H), 7.73-7.80 (m, 2 H), 8.25 (m, 1H), 9.95 (d, 1 H). 75.5 MHz 13C NMR (CDCl3): δ 158.9, 146.4, 144.4, 128.3, 125.5, 56.1, 32.7, 31.5, 29.2, 29.1, 29.0, 28.9, 28.7, 28.5, 27.7, 22.3, 13.8. FTIR (KBr): ν 3409, 3039, 2920, 2849, 1630, 1578, 1514, 1466, 1413, 1167, 774 cm-1. ESI-MS (m/z) : 573.1(M-Br-)+ ,493.3 (100). Anal. Calcd for C34H58N2Br2: C, 62.38; H, 8.93; N, 4.28. Found: C, 60.63; H, 8.89; N, 4.58. 1,4-Bis(R-Dodecylpyridinium)butane Dibromide (3). White sheet crystals were obtained according to the general procedure. Yield, 40%. mp 226-228 °C. 300 MHz 1H NMR (CDCl3, TMS): δ 0.86 (t, 3 H), 1.23-1.28 (m, 16H), 1.53 (m, 2 H), 1.73-1.79 (m, 2 H), 2.51 (m, 2 H), 3.41(t, 2 H), 4.98 (t, 2 H), 7.73-7.80 (m, 2 H), 8.24 (m, 1H), 10.0 (d, 1 H). 75.5 MHz 13C NMR (CDCl3): δ 158.9, 146.5, 144.4, 128.3, 125.6, 56.2, 32.8, 31.6, 29.3, 29.3, 29.2, 29.1, 29.0, 29.0, 28.8, 28.5, 27.8, 22.4, 13.8. FTIR (KBr): ν 3411, 3039, 2921, 2851, 1632, 1518, 1466, 1357, 1158, 774 cm-1. ESI-MS (m/z) : 629.1(M-Br-)+, 549.3 (100). Anal. Calcd for C38H66N2Br2: C, 64.21; H, 9.36; N, 3.94. Found: C, 62.22; H, 9.09; N, 4.01. Compounds 4 and 5 were synthesized and purified according to ref 12. Methods. Surface Tension Measurement. Surface tension was measured on a JYW-200C Processor tensiometer using the ring method. Surfactant solutions were kept for 15 min and surfactantPAM solutions were kept for 1 to 2 days to equilibrate. All measurements were performed at room temperature. Viscosity Measurement. Viscosity was measured with a Brookfield DV-III viscometer at room temperature. Surfactant-PAM solutions were stirred for 30 min to allow equilibration. Fluorescence Measurement. The ratio of the intensities of the first and the third vibronic peaks in the fluorescence spectrum of pyrene (I1/I3) was used to estimate the micropolarity sensed by pyrene in its solubilization site.13 The fluorescence intensities were measured using a Shimadzu RF-5301PC spectrofluorophotometer. The excitation wavelength was 336 nm, and the emission spectra were scanned over the spectra range of 350-450 nm. The slit widths of excitation and emission were fixed at 1.5 and 3.0 nm, respectively. SurfactantPAM solutions were prepared by mixing the surfactant solutions and the PAM solutions, and they were kept for 1 to 2 days to allow equilibration. The surfactant solutions and PAM solutions were prepared with a pyrene stock solution. The pyrene stock solution was prepared by dissolving pyrene in hot water up to saturation and then cooling to 25 °C. All measurements were performed at room temperature.

Results and Discussion Surface Activities of Pyridinium Surfactants. The surface tension measurement is a classical method of studying the cmc of surfactants. The surface-active behavior of the new gemini pyridinium surfactants is illustrated in Figure 2a. For comparison, the surfactants with the (CH2)6 spacer (prepared by a colleague) are also listed in Figure 2b. Experts pointed out that the ratio cmcmonomer/cmcdimer is around 10 or so for surfactants with dodecyl alkyl chain.3 The surface activity parameters of the pyridinium surfactants are listed in Table 1, and those of corresponding monomers are also listed there for comparison. All studied surfactants, with the exception of py-8-4-8 and py-8-6-8, show a sharp break in the surface tension versus concentration (mM) isotherm, which is indicative of the cmc and the formation of micelles. py-8-4-8 (12) Li, Y.; Jiang, X.; Chen, Z.; Zhou, L. Chem. Res. Appl. (in Chinese) 2005, 17, 411. (13) Kalyanasundaram, K.; Thomas, J. K. J. Am. Chem. Soc. 1977, 99, 2039.

Figure 2. Surface tension vs concentration isotherm of gemini pyridinium surfactants.

and py-8-6-8 cannot form micelles in the experimental range because their hydrophobic chains are too short to induce aggregation. From Figure 2 and Table 1, it can be concluded that the gemini pyridinium surfactant with longer hydrophobic chains has a lower cmc value. The cmc values of gemini pyridinium surfactants are much lower than those of the corresponding monomeric surfactants. For example, as seen in Table 1, the cmc value is 0.56 mM for py-12-4-12 but 3.62 mM for py-12-2, which is almost 6 times higher than that of py12-4-12. The C20 value of the gemini pyridinium surfactant is about one order of magnitude lower than that of the corresponding monomer; for example, the C20 value is 0.23 mM for py-12-4-12 and 2.90 mM for py-12-2. The packing densities of surfactants at the air/aqueous solution interface are important for the interpretation of the surface activities of the various series of surfactants. The surface areas Amin occupied by the surfactant molecules should reflect their packing densities.14 The saturation adsorption values Γmax at the air/aqueous solution interface can be calculated using the Gibbs absorption equation.6,15,16

Γmax )

∂γ T -1 2.303nRT ∂ log C

(

)

Amin) (NAΓ)-1 × 1016 where n represents the number of species at the interface whose concentration changes with surfactant concentration, R is the gas (14) Laschewsky, A.; Wattebled, L.; Arotcare´na, M.; Habib-Jiwan, J. L.; Rakotoaly, R. H. Langmiur 2005, 21, 7170. (15) Alami, E.; Beinert, G.; Marie, P.; Zana, R. Langmiur 1993, 9, 1465. (16) Song, L. D.; Rosen, M. J. Langmiur 1996, 12, 1149.

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Table 1. Surface Activity of Single-Surfactant and Surfactant-PAM Systems single system surfactant py-8-4-8 py-10-4-10 py-12-4-12 py-8-6-8 py-10-6-10 py-12-6-12 py-10-2 py-10-3 py-12-2 py-12-3 a

mixed system with PAM 6

γcmc (mN/m)

cmc (mM)a

cmc (mM)b

C20 (mM)

10 Γmax (mol/m2)

Amin (nm2/mol)

38.5 40.5

2.69 0.56

2.42 0.56

0.76 0.23

2.76 1.77

0.60 0.94

41.6 43.7 54.7 53.7 47.3 47.2

2.00 0.54 8.06 6.09 3.62 2.81

2.16 0.64

0.74 0.21

2.25 1.68

0.74 0.99

γcac (mN/m)

cac (mM)a

cac (mM)b

52.1 45.4 43.5 51.1 46.0 44.1

1.65 0.53 0.13 1.28 0.60 0.10

1.65 0.60 0.07 1.30 0.69 0.12

2.90 2.25

Measured by tensiometry. b Measured by spectrofluorometry using pyrene as a fluorescent probe.

Figure 3. Variation of pyrene emission with varying concentrations of py-10-4-10, [py-10-4-10] (M): (1) 0, (2) 2.9 × 10-4, (3) 5.8 × 10-4, (4) 9.3 × 10-4, (5) 1.5 × 10-3, (6) 1.7 × 10-3, (7) 2.1 × 10-3, (8) 2.4 × 10-3, (9) 2.7 × 10-3, and (10) 3.1 × 10-3.

constant (8.31 J‚mol-1‚K-1), T is the absolute temperature in K, C is the surfactant concentration, and NA is Avogadro’s number (6.02 × 1023 mol-1). The literature17 reported that for gemini surfactants with flexible methylene spacers the value of n ) 2 was found to agree better with the neutron reflectivity data than the value of n ) 3. In this study, the n value was assumed to be 2 for the used pyridinium surfactants with flexible methylene spacers. The values of Γmax and Amin are listed in Table 1. The Amin value increases as the hydrophobic chain length is increased from 10 to 12, which suggests that gemini pyridinium surfactants with a hydrophobic chain length of 10 carbon atoms have higher packing densities at the air/aqueous solution interface than do those with a hydrophobic chain length of 12 carbon atoms. A possible explanation is that the longer hydrophobic chains are more prone to curl and thus make the Amin value larger. The cmc values of gemini pyridinium surfactants have also been investigated by steady-state fluorescence using the emission pyrene. Figure 3 presents the variation of pyrene emission with varying concentrations of py-10-4-10. The cmc values obtained by spectrofluorometry are listed in Table 1, which are in good agreement with the values obtained by tensiometry. Interactions between Gemini Pyridinium Surfactants and Polyacrylamide (PAM). Gemini surfactant-polymer interactions have attracted much interest over the last few years.18-26 (17) Li, Z. X.; Dong, C. C.; Thomas, R. K. Langmiur 1999, 15, 4392. (18) Ka¨stner, U.; Zana, R. J. Colloid Interface Sci. 1999, 218, 468. (19) Wettig, S. D.; Verrall, R. E. J. Colloid Interface Sci. 2001, 244, 377. (20) Bai, G.; Wang, Y.; Yan, H.; Thomas, R. K.; Kwak, J. C. T. J. Phys. Chem. B 2002, 106, 2153. (21) Wang, X.; Li, Y.; Wang, J.; Wang, Y.; Ye, J.; Yan, H.; Zhang, J.; Thomas, R. K. J. Phys. Chem. B 2005, 109, 12850. (22) Pisa´rcˇik, M.; Solda´n, M.; Bakosˇ, D.; Devı´nsky, F.; Lacko, I. Colloids Surf., A 1999, 150, 207.

Polymer, surfactant, and alkali have been widely used in enhanced oil recovery. However, there have been few reports about the interaction of PAM with gemini surfactants,21 especially with gemini pyridinium surfactants. Thus, it is very useful to study the interactions between PAM and gemini pyridinium surfactants. In the present work, the properties of solutions of fixed PAM concentration (0.5 wt %) and variable gemini pyridinium surfactants were investigated. Precipitates were often observed when the surfactant concentration exceeded a certain value, especially in py-12-4-12-PAM and py-12-6-12-PAM systems. All measurements were carried out at concentration with no precipitate present. Figure 4 presents the variation of surface tension (mN/m) with the surfactant concentration (mM). The surface tension of surfactant (py-12-4-12 or py-12-6-12 or py-10-4-10 or py-106-10)-PAM systems decreased with increasing concentration of surfactant and reached a clear break point that is taken as the critical aggregation concentration (cac). In the case of surfactant (py-8-4-8 or py-8-6-8)-PAM systems, a very light break point is observed. All of the cac values are listed in Table 1. The cac value of a surfactant-PAM system is about 4 times smaller than the cmc value of the corresponding single surfactant in water, whereas γcac is little higher than γcmc (Table 1). These results indicate the strong interactions between gemini pyridinium surfactants and PAM. In addition, they indicate that both the gemini pyridinium surfactant and PAM absorb at the air/aqueous solution interface and form surface-active complexes. Similar behavior is reported in the literature.4 From Figure 4, we also can see that the cac value and the γcac value of surfactant-PAM systems decrease as the hydrophobic chain length of gemini pyridinium surfactants increases, which suggests that the cac and surface tension of surfactant-PAM systems are influenced by the hydrophobic chain length of the surfactants. Figure 5 shows the variations of relative viscosity (relative viscosity is understood here as the viscosity of the surfactantPAM system divided by the viscosity of the free surfactantPAM solution at the same PAM) with the surfactant concentration (mM). It can be found that the relative viscosity of the surfactantPAM system decreased with increasing concentration of the surfactant. A possible explanation of the observed viscosity changes is that, with the increase in surfactant concentration, surfactants may form micelle-like aggregates on the PAM chains21 and make PAM chains coil, resulting in a decrease in surfactant(23) Pisa´rcˇik, M.; Imae, T.; Devı´nsky, F.; Lacko, I.; Bakosˇ, D. J. Colloid Interface Sci. 2000, 228, 207. (24) Pisa´rcˇik, M.; Imae, T.; Devı´nsky, F.; Lacko, I. Colloids Surf. A 2001, 183-185, 555. (25) Wang, X.; Wang, J.; Wang, Y.; Yan, H. Langmiur 2004, 20, 9014. (26) Pi, Y.; Shang, Y.; Liu, H.; Hu, Y.; Jiang, J. J. Colloid Interface Sci. 2007, 306, 405.

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Figure 4. Surface tension vs concentration isotherm of gemini pyridinium surfactants.

Figure 5. Relative viscosity vs concentration of gemini pyridinium surfactants.

PAM system The relative viscosity changes of surfactant-PAM systems are influenced by the hydrophobic chain length of the surfactants. The shorter the hydrophobic chain length of the surfactant, the weaker the interactions between the surfactants and PAM and the more slowly the relative viscosity decreases (as seen in Figure 5). Fluorescence measurements have been used for further investigations of the interaction between gemini pyridinium surfactants and PAM. Pyrene is often used as a probe to investigate the interaction between surfactant and polymer.4,25,27 The shape and intensity of the fluorescence emission of this probe are sensitive to its microenvironment at the site of solubilization of the fluorophore. The intensity ratio of the first and third vibronic bands I1/I3 can be taken as a measure of the polarity of the microenvironment, being high in polar media and low in hydrophobic environments. In Figure 6, the I1/I3 plots for all surfactants in the absence of PAM have usual sigmoidal shapes except for py-8-4-8 and py-8-6-8, with an obvious decrease at a concentration slightly below the cmc and with a plateau at high concentration, which indicates the formation of micelles. Thus, the cmc value can be taken as the concentration that corresponds to the intercept between linear extrapolations of the rapidly varying portion of the curve and of the almost-horizontal portion at high concentration.8 The cmc values obtained by this means are listed in Table 1 and are in very good agreement with the values obtained by tensiometry. The I1/I3 value in water is about 1.90, whereas the values decreased along with the solubilization of pyrene molecules into the micelles. When the concentration of surfactant is above

the cmc, the I1/I3 values are 1.56, 1.58, 1.39, and 1.41 for gemini pyridinium surfactants py-12-4-12, py-10-4-10, py-12-6-12, and py-10-6-10, respectively. These values suggest that the spacer length has an obvious effect on the micelle micropolarity for the gemini pyridinium surfactants. This can be explained by the fact the pyrene is preferentially located in the palisade layer of micelles.28 This palisade layer is composed of pyridinium headgroups, Br- counterions, H2O molecules, and spacers. The longer the hydrophobic spacer, the lower the micropolarity in the palisade layer. Thus, the micelles of gemini pyridinium surfactants with a (CH2)6 spacer have a lower micropolarity than those with a (CH2)4 spacer. When the gemini pyridinium surfactants are added to PAM, the values of I1/I3 decrease remarkably toward a plateau region, except for the py-8-4-8-PAM and py-8-6-8-PAM systems. The changes in I1/I3 reflect the formation of surfactant-polymer aggregates, similar to the observation in gemini surfactants C12-s-C122Br and PAM systems.21 The end of transition is designated as the cac. Beyond the cac, gemini pyridinium surfactants start to form micelle-like aggregates on PAM chains, as indicated by lower I1/I3 values in the plateau region. py-8-4-8 and py-8-6-8 cannot form micelles in the experimental range. A similar result has been inferred in a surface tension study. However, py-8-4-8 (or py-8-6-8) and PAM can form a mixed micelle, as indicated by the lower I1/I3 value than in the absence of PAM. The cac values obtained by fluorescence measurements and by surface tension measurement are in good agreement, and they are summarized in Table 1. The cac values decrease with the increasing length of the hydrophobic chains. For example, as seen in Table 1, cac values decrease in the order py-12-4-

(27) Panmai, S.; Prud’homme, R. K.; Peiffer, D. G.; Jockusch, S.; Turro, N. J. Langmiur 2002, 18, 3860.

(28) Zana, R. In Surfactant Solutions. New Method of InVestigation; Zana, R., Ed.; Marcel Dekker: New York, 1987; Chapter 5.

viscosity.23

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Figure 6. I1/I3 vs concentration of gemini pyridinium surfactants.

12-PAM < py-10-4-10-PAM < py-8-4-8-PAM. This result indicates that surfactants with longer hydrophobic chains have stronger interactions with PAM. This is due to the stronger hydrophobic interaction between hydrophobic chains.

Conclusions (1) A novel class of gemini pyridinium surfactants were synthesized, and their surface-active properties were determined by surface tension and fluorescence measurements. (2) The results show that the gemini pyridinium surfactant with longer hydrophobic chains has a lower cmc value, the cmc’s of gemini pyridinium surfactants are much lower than those of the corresponding monomeric surfactants, and C20 is about one order of magnitude lower than that of corresponding monomers.

(3) Fluorescence measurements indicate that the spacer length has an obvious effect on micelle micropolarity for the gemini pyridinium surfactants. (4) Gemini pyridinium surfactants can form hydrophobic coaggregates with PAM and can cause the PAM chains to be wrapped and the viscosity of PAM to decreased. Also, the longer hydrophobic chains of surfactants have stronger interactions with PAM owing to the stronger hydrophobic interaction. Acknowledgment. We are grateful for financial support from the Science & Technology Department of Sichuan Province (no. 03JY029-021-1). LA701154W