Colloids and Surfaces B: Biointerfaces 123 (2014) 809–813
Contents lists available at ScienceDirect
Colloids and Surfaces B: Biointerfaces journal homepage: www.elsevier.com/locate/colsurfb
Hydrophilic modification of PVDF microfiltration membranes by adsorption of facial amphiphile cholic acid Meng-Xin Hu a,∗ , Ji-Nian Li a , Shi-Lin Zhang b , Liang Li a , Zhi-Kang Xu b a
Department of Applied Chemistry, School of Food Science and Biotechnology, Zhejiang Gongshang University, Hangzhou 310018, China Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science & Engineering, Zhejiang University, Hangzhou 310027, China b
a r t i c l e
i n f o
Article history: Received 14 August 2014 Received in revised form 9 October 2014 Accepted 11 October 2014 Available online 20 October 2014 Keywords: PVDF microfiltration membrane Cholic acid Facial amphiphile Adsorption Hydrophilicity
a b s t r a c t Amphiphilic molecules have been widely used in surface modification of polymeric materials. Bile acids are natural biological compounds and possess special facial amphiphilic structure with a unusual distribution of hydrophobic and hydrophilic regions. Based on the facial amphiphilicity, cholic acid (CA), one of the bile acids, was utilized for the hydrophilic modification of poly(vinylidene fluoride) (PVDF) microfiltration membranes via the hydrophobic interactions between the hydrophobic face of CA and the membrane surfaces. Ethanol, methanol, and water were respectively used as solvent during CA adsorption procedure. Their polarity affects the CA adsorption amount, as similar to CA concentration and adsorption time. There are no changes on the membrane surface morphology after CA adsorption. The hydrophilicity of PVDF membranes is greatly enhanced and the water drops permeates into the CA modified membranes quickly after modification. All these factors benefit to the permeation flux of membrane for water. When CA concentration is higher than 0.088 M, the water permeation flux is doubled as compared with the nascent PVDF membrane and shows a good stability during filtration procedure. These results reveal the promising potential of facial amphiphilic bile acids for the surface modification of polymeric materials. © 2014 Elsevier B.V. All rights reserved.
1. Introduction During the past several decades, significant milestones in the membrane technologies have been scientifically and commercially achieved [1]. Owing to the good membrane-forming and excellent physicochemical properties, polymeric membranes have been widely used in environmental, electronic energy, food, chemical, and biotechnologies areas. Therein, poly(vinylidene fluoride) (PVDF) membranes have attracted great attentions with regard to its outstanding properties such as high mechanical strength, thermal stability, and chemical resistance. However, the hydrophobicity of PVDF membranes makes membranes susceptible to fouling. Therefore, hydrophilic modification of PVDF membranes was widely investigated with chemical and physical methods, such as coating [2], blending [3], and grafting [4] with hydrophilic or amphiphilic polymers. Blending is one of the most popular methods for surface modification of PVDF membranes, but the mechanical properties of membrane are deteriorated after blending with other polymers. Grafting is relatively complex and costly
∗ Corresponding author. Tel.: +86 571 28008976; fax: +86 571 28008900. E-mail address: [email protected] (M.-X. Hu). http://dx.doi.org/10.1016/j.colsurfb.2014.10.019 0927-7765/© 2014 Elsevier B.V. All rights reserved.
for surface modification. By comparison, coating is cost-effective, energy-efficient and relatively simple for surface modification, which makes no effect on the mechanical properties of membranes. Polydopamine [5], chitosan [6], poly(vinyl alcohol) [7], amphiphilic random copolymers [8] and block copolymers [9] have been used for surface modification of PVDF membranes through coating. Therein, the amphiphilic molecules with self-assembled properties show the potential to achieve a higher adsorbed amount and a higher density on the hydrophobic PVDF membranes [10]. Different from other amphiphilic molecules used for surface modification [11–13], bile acids are natural amphiphilic compounds produced by all the vertebrates [14]. These molecules consist of a chiral saturated tetracyclic steroidal skeleton containing one or more hydroxyl groups (OH), connected to a short chiral aliphatic chain with a carboxylic end group. They exhibit facial amphiphilic properties via a hydrophilic concave face with hydroxyl groups and a carboxylic acid group and a hydrophobic convex face with methyl groups. Due to this facial amphiphilicity, bile acids can self-assemble in solutions driven by both hydrophobic interactions of the nonpolar faces and H-bonding interactions from the OH groups on the polar faces and acid groups [14,15]. Therefore, bile acids can form micelles and other supramolecular structures in a stepwise manner by self-assembly [15–18]. The
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M.-X. Hu et al. / Colloids and Surfaces B: Biointerfaces 123 (2014) 809–813
(a)
2.0
properties of bile acids, such as facial amphiphilicity, chirality, selfassembly, biocompatibility, and chemical stability of the steroid nucleus, lead to a wide range of potential applications in chemistry [19,20], material [14,21], biomedical [22], superstructures [23,24], and so on. However, it has not been valued till now to utilize bile acids for the surface modification of materials. Therefore, exploiting potentials of bile acids on surface modification of materials needs further developments. In this work, cholic acid (CA) (see Fig. 1), one of the bile acids, was chosen to modify PVDF membranes by adsorption through hydrophobic interactions based on the facial amphiphilicity and self-assembly properties of bile acids. The effects of solvent polarity and CA concentration on the adsorption amount were investigated. In terms of hydrophilic properties, water contact angle and filtration properties were analyzed. The results demonstrate that the hydrophilicity of PVDF membranes was enhanced after CA adsorption. This work is the continuation of our previous study, where the self-assembly behaviors of CA on microporous polypropylene membranes were studied in detail.
1.6
1.4
1.2
1.0
0.8 10
15
20
25
30
35
40
0.08
0.10
0.12
Time (h)
3.0
(b)
2.5
adsorption amount (mg)
Fig. 1. Molecular structure of cholic acid (CA).
Adsorption amount (mg)
1.8
2.0
1.5
1.0
0.5
0.0
2. Experimental
0.00
2.1. Materials
0.02
0.04
0.06
CA concentration (M)
Commercial PVDF microfiltration membranes (Cobetter filtration, China) were used in the experiments with an average pore size of 0.20 m. This membrane was cut into rotundity with a diameter of 25 mm and washed with methanol for 24 h to remove additives and impurities on the membrane surface. After dried under reduced pressure at 40 ◦ C to constant weight (m0 ), the PVDF microfiltration membranes were weighted with an analytical balance (METTLER TOLEDO) to a precision of 0.01 mg and stored in a desiccator. Choilc 0.30
Fig. 3. (a) Effect of adsorption time on the adsorption amount of CA on PVDF membranes with 0.072 M CA solutions in methanol; (b) effect of CA concentration on the adsorption amount on PVDF membranes in methanol solution for 24 h.
acid (CA, Aladdin, 98%), ethanol, and methanol were used without purification. 2.2. CA adsorption on PVDF membranes CA was dissolved in a solvent to prepare CA solution with different concentration. Thereafter, PVDF membranes were soaked
Adsorption amount (mg)
0.25
F
0.20
0.15
C
0.10
O
N b
0.05
a 0.00
ethanol
methanol Solvent
water
Fig. 2. Effect of solvent on the adsorption amount of CA on PVDF membranes in 0.008 mol/L CA solutions for 20 h.
200
300
400
500
600
700
800
Binding energy (eV) Fig. 4. Surver XPS spectra of the membranes: (a) nascent PVDF membrane; (b) CA modified PVDF membrane with 0.072 M CA solution.
M.-X. Hu et al. / Colloids and Surfaces B: Biointerfaces 123 (2014) 809–813
811
Fig. 5. SEM images for (a) nascent PVDF membrane; (b)–(d) PVDF membranes modified with 0.024, 0.040, and 0.088 M CA solutions, respectively.
in a fresh CA solution for some time. Then, the membranes were taken out of the solution, rinsed with 10 ml deionized water for 3 times and dried in a vacuum oven at 40 ◦ C to constant weight (m1 ). The adsorption amount of CA on PVDF membranes (mg) was determined as follows: = m1 − m0
hydrocarbon peak at 284.7 eV in order to compensate for surface charging effect. Scanning electron microscopy (SEM) images were taken on a Field Emission SEM (SIRION, FEI, USA). Samples were dried under reduced pressure at 40 ◦ C to constant weight, and then coated with a 20 nm gold layer before SEM analysis.
(1) 2.4. Water contact angle measurement
2.3. Membrane characterizations Spectra of X-ray photoelectron spectroscopy (XPS) were recorded on a PHI-5000C ESCA system (Perkin-Elmer, USA) with Al K␣ excitation radiation (1486.6 eV). The pressure in the analysis chamber was maintained at 10−6 Pa during measurements. All survey and core-level spectra were referenced to the C 1s
A CTS-200 contact angle system (Mighty Technology Pvt. Ltd., China) was used for the determination of water contact angles at room temperature in air. Static contact angles were measured by a sessile drop method. First, a 1 L drop of water was set onto the dry membrane surface with a microsyringe. Digital images for the droplet were then recorded. Contact angles were calculated from
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these images with software. Each reported value was an average of at least 5 independent measurements.
Table 1 Chemical composition of the nascent and CA adsorbed PVDF membranes and CA calculated from XPS scan spectra.
2.5. Permeation properties
3.2. Characterization of the CA modified PVDF membranes The CA modified PVDF membranes were studied with XPS as shown in Fig. 4. For the unmodified PVDF membranes, peaks of O1S and N1S belong to the additives existing in the commercial PVDF membranes. After CA adsorption, O and C contents on PVDF membrane increases, while F content reduces (shown in Table 1). These changes are due to the adsorbed CA molecules on membrane surface with high C and O contents. However, N content is little changed after CA adsorption. This phenomenon is due to the migration of hydrophilic additives from inside to surface of membranes in polar solvent during the adsorption procedure. The XPS result means that CA molecules have been adsorbed on PVDF membranes. The surface morphologies of membranes before and after CA adsorption were observed by SEM to gain more information of
2.28 3.58
83.29
16.71
F (mol%)
N (mol%)
44.52 38.78
1.51 1.58
–
–
the CA modified PVDF membranes. Fig. 5 presents SEM pictures of PVDF membranes with different adsorption conditions. Even though the CA concentration increases to 0.088 M, the membrane pore structures show little change. Blocked membrane pores are not observed. 3.3. Hydrophilicity of the CA modified PVDF membranes The surface hydrophilicity of membranes was characterized with water contact angle measurements to demonstrate the change of membranes after CA modification. Fig. 6(a) shows the static contact angle of PVDF membranes after CA adsorption with different concentrations. It is worth noting that the water contact angle increases slightly at first as the CA concentration increases
110
(a) 100
90
80
70 0.00
0.02
0.04
0.06
0.08
0.10
0.12
CA concentration (M) 120
(b)
nascent 0.024 M 0.056 M 0.104 M
100
o
Virgin PVDF membranes were separately soaked in deionized water, ethanol, and methanol solutions with different CA concentrations. The loosely adsorbed CA molecules were washed away with water before vacuum drying. Fig. 2 shows the effect of solvent on the adsorption amount of CA on PVDF membranes. The result shows that the adsorption amount of CA on membrane in the methanol solution is the highest. The solvent polarity parameter of the used solvents ranks in the ascending order as follow: ethanol, methanol, and water. The results demonstrate that adsorption of the amphiphilic CA on hydrophobic PVDF membranes is affected by the solvent polarity. When the solvent polarity is low, the hydrophobic face of CA is prior to bond solvent molecules, which leads less CA molecules bound to hydrophobic PVDF membranes. Accordingly, when the solvent polarity increases, CA is prior to bond onto the hydrophobic membrane surface. However, when CA is dissolved in water with the highest polarity, CA molecules tend to form aggregates (micelles) in solution. Therefore, the adsorption amount of CA on PVDF membranes in the water solution is slightly lower than that in the methanol solution. However, the adsorption amount of CA cannot be further increased in water since the CA concentration cannot be raised due to the limited solubility of CA in water. Compared the three solvents, methanol is the most suitable solvent used for CA adsorption. Effect of adsorption time on the adsorption amount of CA on PVDF membranes in methanol solution was studied. As shown in Fig. 3(a), the adsorption amount of CA increases and then reaches equilibrium after 24 h of incubation. Besides, CA concentration in solution greatly affects the adsorption amount of CA on membranes. As shown in Fig. 3(b), it is clear that the adsorption amount increases with CA concentration quickly. Therefore, the adsorption amount of CA on membranes can be expediently controlled by adjusting the CA concentration. Therefore, it is a relatively simple method for membrane surface modification.
O (mol%)
o
3.1. CA adsorption on PVDF membranes
51.69 56.07
Water contact angle ( )
3. Results and discussion
C (mol%)
PVDF PVDF modified with 0.072 M CA CA
Water contact angle ( )
The permeation properties of the unmodified and modified PVDF membranes were examined with a Millipore pressure vessel under constant pressure. All the membranes were wetted with ethanol before flux measurements. After prepressing at 0.12 MPa for 30 min, the flux of pure water (Jw ) was measured at 0.10 MPa.
Sample
80
60
40
20
0 0
5
10
15
20
25
30
Time (s) Fig. 6. (a) Effect of CA concentration on the water contact angle of the PVDF membrane; (b) time dependence of water contact angle for the PVDF membranes modified with different CA concentration.
M.-X. Hu et al. / Colloids and Surfaces B: Biointerfaces 123 (2014) 809–813
3.4. Filtration of the CA modified PVDF membranes
9000
(a)
The water permeation fluxes of PVDF membranes were shown in Fig. 7. It is clear that CA adsorption markedly improves the water flux of membrane. The water flux of the CA modified membranes increases with the CA concentration. When the CA concentration is higher than 0.088 M, the water flux is doubled as compared with the nascent PVDF membrane. During the whole test procedure, the water fluxes of all the membranes keep constant. Therefore, CA adsorption greatly enhances the water fluxes of PVDF membranes and the water fluxes of the modified membranes are very stable during the filtration procedure.
8000
2
Flux (L/m h)
7000
6000
5000
4000
4. Conclusions 0.00
0.02
0.04
0.06
0.08
0.10
0.12
CA concentration (M) 9000
(b)
nascent 0.024 M 0.040 M 0.056 M 0.072 M 0.088 M 0.104 M
8000
7000
2
Flux (L/m h)
813
6000
We demonstrate a simple and effective surface modification method to improve the hydrophilicity of PVDF microfiltration membranes through adsorption of facial amphiphilic CA. This adsorption technique used for membrane surface modification has little influence on the membrane morphology. The surface hydrophilicity of PVDF membranes is improved obviously after CA adsorption. The CA molecules are adsorbed on membrane surfaces in a stable state and grant the PVDF membranes to be suitable for long-time running for water filtration. Compared with the nascent PVDF membrane, the water flux of the CA modified membranes is doubled as the CA concentration is high than 0.088 M.
5000
Acknowledgements
4000
The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (Grant no. 21004051 and no. 51103130) and the Educational Commission of Zhejiang Province of China (Grant no. Y201018929) for this work.
3000 0
50
100
150
200
250
Time (min) Fig. 7. Pure water fluxes through the nascent and CA adsorbed PVDF membranes with different CA concentrations.
to 0.024 M and then decreases near linearly with the increase of CA concentration. As the CA concentration keeps increasing to 0.104 M, the water contact angle decreases to 82◦ . The PVDF membranes are commercial products with some hydrophilic additives blending in the membranes demonstrated by the XPS results. Due to the selfassembly ability of the hydrophilic face of CA [15], the N and O atoms of the hydrophilic additives exposed on the membrane surface can form H-bonding with the OH groups on the hydrophilic face of CA. So the hydrophobic face of CA exposes outside and the water contact angle increases slightly. After that, facial amphiphilic CA molecules are adsorbed on the surface of PVDF membranes through hydrophobic interactions. Therefore, more and more hydrophilic faces of the adsorbed CA molecules expose outside on membrane surface and the water contact angle of the membranes accordingly decreases. Time dependence of water contact angle for the CA modified membranes was also studied. As shown in Fig. 6(b), the water contact angle of the CA modified PVDF membranes quickly declines and the water drop rapidly permeates into the membrane pores. Although the initial water contact angles are almost the same, the time dependences of water contact angle are obviously different for the nascent membrane and the CA modified membrane with 0.024 M solution. The higher the CA concentration is used, the faster the water contact angle declines. The fast rate of the water permeation benefits to the filtration properties of membranes.
References [1] F. Liu, N.A. Hashim, Y.T. Liu, M.R.M. Abed, K. Li, J. Membr. Sci. 375 (2011) 1. [2] A. Venault, Y. Chang, H.S. Yang, P.Y. Lin, Y.J. Shih, A. Higuchi, J. Membr. Sci. 454 (2014) 253. [3] W.Z. Lang, Y.J. Guo, L.F. Chu, Polym. Adv. Technol. 22 (2011) 1720. [4] Y.P. Zhao, H.Y. Zhao, L. Chen, X. Feng, Q.S. Zhang, J.M. Wang, R. Zhang, J. Polym. Res. 20 (2013) 58. [5] H.L. Wei, J. Ren, B. Han, L. Xu, L.L. Han, L.Y. Jia, Colloid Surf. B 110 (2013) 22. [6] S. Boributh, A. Chanachai, R. Jiraratananon, J. Membr. Sci. 342 (2009) 97. [7] J.R. Du, S. Peldszus, P.M. Huck, X.S. Feng, Water Res. 43 (2009) 4559. [8] B. Peng, Y.Y. Li, Z.G. Zhao, Y.M. Chen, C.C. Han, J. Appl. Polym. Sci. 130 (2013) 3112. [9] N.J. Lin, H.S. Yang, Y. Chang, K.L. Tung, W.H. Chen, H.W. Cheng, S.W. Hsiao, P. Aimar, K. Yamamoto, J.Y. Lai, Langmuir 29 (2013) 10183. [10] R.W. Guo, Y.F. Liu, Y. Zhang, A.J. Dong, J.H. Zhang, Macromol. Res. 21 (2013) 1127. [11] O. Theodoly, M. Jacquin, P. Muller, S. Chhun, Langmuir 25 (2009) 781. [12] N.A. Lockwood, J.K. Gupta, N.L. Abbott, Surf. Sci. Rep. 63 (2008) 255. [13] N. Erdinc, S. Gokturk, M. Tuncay, Colloid Surf. B 75 (2010) 194. [14] X.X. Zhu, M. Nichifor, Acc. Chem. Res. 35 (2002) 539. [15] D. Madenci, S.U. Egelhaaf, Curr. Opin. Colloid Interface Sci. 15 (2010) 109. [16] L. Schefer, A. Sanchez-Ferrer, J. Adamcik, R. Mezzenga, Langmuir 28 (2012) 5999. [17] S. Mukhopadhyay, U. Maitra, Curr. Sci. 87 (2004) 1666. [18] J.V. Trillo, F. Meijide, A. Jover, V.H. Soto, S. de Frutos, M.C. di Gregorio, L. Galantini, J.V. Tato, RSC Adv. 4 (2014) 5598. [19] S. Nakamura, F. Imashiro, K. Takegoshi, T. Terao, J. Am. Chem. Soc. 126 (2004) 8769. [20] S. Pal, S.G. Roy, P. De, Polym. Chem. 5 (2014) 1275. [21] Y. Shao, C. Lavigueur, X.X. Zhu, Macromolecules 45 (2012) 1924. [22] M. Posa, J. Csanadi, K.E. Kover, V. Guzsvany, G. Batta, Colloid Surf. B 94 (2012) 317. [23] L. Travaglini, A. D’Annibale, M.C. di Gregorio, K. Schillen, U. Olsson, S. Sennato, N.V. Pavel, L. Galantini, J. Phys. Chem. B 117 (2013) 9248. [24] L. Travaglini, A. D’Annibale, K. Schillén, U. Olsson, S. Sennato, N.V. Pavela, L. Galantini, Chem. Commun. 48 (2012) 12011.
Contents lists available at ScienceDirect
Colloids and Surfaces B: Biointerfaces journal homepage: www.elsevier.com/locate/colsurfb
Hydrophilic modification of PVDF microfiltration membranes by adsorption of facial amphiphile cholic acid Meng-Xin Hu a,∗ , Ji-Nian Li a , Shi-Lin Zhang b , Liang Li a , Zhi-Kang Xu b a
Department of Applied Chemistry, School of Food Science and Biotechnology, Zhejiang Gongshang University, Hangzhou 310018, China Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science & Engineering, Zhejiang University, Hangzhou 310027, China b
a r t i c l e
i n f o
Article history: Received 14 August 2014 Received in revised form 9 October 2014 Accepted 11 October 2014 Available online 20 October 2014 Keywords: PVDF microfiltration membrane Cholic acid Facial amphiphile Adsorption Hydrophilicity
a b s t r a c t Amphiphilic molecules have been widely used in surface modification of polymeric materials. Bile acids are natural biological compounds and possess special facial amphiphilic structure with a unusual distribution of hydrophobic and hydrophilic regions. Based on the facial amphiphilicity, cholic acid (CA), one of the bile acids, was utilized for the hydrophilic modification of poly(vinylidene fluoride) (PVDF) microfiltration membranes via the hydrophobic interactions between the hydrophobic face of CA and the membrane surfaces. Ethanol, methanol, and water were respectively used as solvent during CA adsorption procedure. Their polarity affects the CA adsorption amount, as similar to CA concentration and adsorption time. There are no changes on the membrane surface morphology after CA adsorption. The hydrophilicity of PVDF membranes is greatly enhanced and the water drops permeates into the CA modified membranes quickly after modification. All these factors benefit to the permeation flux of membrane for water. When CA concentration is higher than 0.088 M, the water permeation flux is doubled as compared with the nascent PVDF membrane and shows a good stability during filtration procedure. These results reveal the promising potential of facial amphiphilic bile acids for the surface modification of polymeric materials. © 2014 Elsevier B.V. All rights reserved.
1. Introduction During the past several decades, significant milestones in the membrane technologies have been scientifically and commercially achieved [1]. Owing to the good membrane-forming and excellent physicochemical properties, polymeric membranes have been widely used in environmental, electronic energy, food, chemical, and biotechnologies areas. Therein, poly(vinylidene fluoride) (PVDF) membranes have attracted great attentions with regard to its outstanding properties such as high mechanical strength, thermal stability, and chemical resistance. However, the hydrophobicity of PVDF membranes makes membranes susceptible to fouling. Therefore, hydrophilic modification of PVDF membranes was widely investigated with chemical and physical methods, such as coating [2], blending [3], and grafting [4] with hydrophilic or amphiphilic polymers. Blending is one of the most popular methods for surface modification of PVDF membranes, but the mechanical properties of membrane are deteriorated after blending with other polymers. Grafting is relatively complex and costly
∗ Corresponding author. Tel.: +86 571 28008976; fax: +86 571 28008900. E-mail address: [email protected] (M.-X. Hu). http://dx.doi.org/10.1016/j.colsurfb.2014.10.019 0927-7765/© 2014 Elsevier B.V. All rights reserved.
for surface modification. By comparison, coating is cost-effective, energy-efficient and relatively simple for surface modification, which makes no effect on the mechanical properties of membranes. Polydopamine [5], chitosan [6], poly(vinyl alcohol) [7], amphiphilic random copolymers [8] and block copolymers [9] have been used for surface modification of PVDF membranes through coating. Therein, the amphiphilic molecules with self-assembled properties show the potential to achieve a higher adsorbed amount and a higher density on the hydrophobic PVDF membranes [10]. Different from other amphiphilic molecules used for surface modification [11–13], bile acids are natural amphiphilic compounds produced by all the vertebrates [14]. These molecules consist of a chiral saturated tetracyclic steroidal skeleton containing one or more hydroxyl groups (OH), connected to a short chiral aliphatic chain with a carboxylic end group. They exhibit facial amphiphilic properties via a hydrophilic concave face with hydroxyl groups and a carboxylic acid group and a hydrophobic convex face with methyl groups. Due to this facial amphiphilicity, bile acids can self-assemble in solutions driven by both hydrophobic interactions of the nonpolar faces and H-bonding interactions from the OH groups on the polar faces and acid groups [14,15]. Therefore, bile acids can form micelles and other supramolecular structures in a stepwise manner by self-assembly [15–18]. The
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M.-X. Hu et al. / Colloids and Surfaces B: Biointerfaces 123 (2014) 809–813
(a)
2.0
properties of bile acids, such as facial amphiphilicity, chirality, selfassembly, biocompatibility, and chemical stability of the steroid nucleus, lead to a wide range of potential applications in chemistry [19,20], material [14,21], biomedical [22], superstructures [23,24], and so on. However, it has not been valued till now to utilize bile acids for the surface modification of materials. Therefore, exploiting potentials of bile acids on surface modification of materials needs further developments. In this work, cholic acid (CA) (see Fig. 1), one of the bile acids, was chosen to modify PVDF membranes by adsorption through hydrophobic interactions based on the facial amphiphilicity and self-assembly properties of bile acids. The effects of solvent polarity and CA concentration on the adsorption amount were investigated. In terms of hydrophilic properties, water contact angle and filtration properties were analyzed. The results demonstrate that the hydrophilicity of PVDF membranes was enhanced after CA adsorption. This work is the continuation of our previous study, where the self-assembly behaviors of CA on microporous polypropylene membranes were studied in detail.
1.6
1.4
1.2
1.0
0.8 10
15
20
25
30
35
40
0.08
0.10
0.12
Time (h)
3.0
(b)
2.5
adsorption amount (mg)
Fig. 1. Molecular structure of cholic acid (CA).
Adsorption amount (mg)
1.8
2.0
1.5
1.0
0.5
0.0
2. Experimental
0.00
2.1. Materials
0.02
0.04
0.06
CA concentration (M)
Commercial PVDF microfiltration membranes (Cobetter filtration, China) were used in the experiments with an average pore size of 0.20 m. This membrane was cut into rotundity with a diameter of 25 mm and washed with methanol for 24 h to remove additives and impurities on the membrane surface. After dried under reduced pressure at 40 ◦ C to constant weight (m0 ), the PVDF microfiltration membranes were weighted with an analytical balance (METTLER TOLEDO) to a precision of 0.01 mg and stored in a desiccator. Choilc 0.30
Fig. 3. (a) Effect of adsorption time on the adsorption amount of CA on PVDF membranes with 0.072 M CA solutions in methanol; (b) effect of CA concentration on the adsorption amount on PVDF membranes in methanol solution for 24 h.
acid (CA, Aladdin, 98%), ethanol, and methanol were used without purification. 2.2. CA adsorption on PVDF membranes CA was dissolved in a solvent to prepare CA solution with different concentration. Thereafter, PVDF membranes were soaked
Adsorption amount (mg)
0.25
F
0.20
0.15
C
0.10
O
N b
0.05
a 0.00
ethanol
methanol Solvent
water
Fig. 2. Effect of solvent on the adsorption amount of CA on PVDF membranes in 0.008 mol/L CA solutions for 20 h.
200
300
400
500
600
700
800
Binding energy (eV) Fig. 4. Surver XPS spectra of the membranes: (a) nascent PVDF membrane; (b) CA modified PVDF membrane with 0.072 M CA solution.
M.-X. Hu et al. / Colloids and Surfaces B: Biointerfaces 123 (2014) 809–813
811
Fig. 5. SEM images for (a) nascent PVDF membrane; (b)–(d) PVDF membranes modified with 0.024, 0.040, and 0.088 M CA solutions, respectively.
in a fresh CA solution for some time. Then, the membranes were taken out of the solution, rinsed with 10 ml deionized water for 3 times and dried in a vacuum oven at 40 ◦ C to constant weight (m1 ). The adsorption amount of CA on PVDF membranes (mg) was determined as follows: = m1 − m0
hydrocarbon peak at 284.7 eV in order to compensate for surface charging effect. Scanning electron microscopy (SEM) images were taken on a Field Emission SEM (SIRION, FEI, USA). Samples were dried under reduced pressure at 40 ◦ C to constant weight, and then coated with a 20 nm gold layer before SEM analysis.
(1) 2.4. Water contact angle measurement
2.3. Membrane characterizations Spectra of X-ray photoelectron spectroscopy (XPS) were recorded on a PHI-5000C ESCA system (Perkin-Elmer, USA) with Al K␣ excitation radiation (1486.6 eV). The pressure in the analysis chamber was maintained at 10−6 Pa during measurements. All survey and core-level spectra were referenced to the C 1s
A CTS-200 contact angle system (Mighty Technology Pvt. Ltd., China) was used for the determination of water contact angles at room temperature in air. Static contact angles were measured by a sessile drop method. First, a 1 L drop of water was set onto the dry membrane surface with a microsyringe. Digital images for the droplet were then recorded. Contact angles were calculated from
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M.-X. Hu et al. / Colloids and Surfaces B: Biointerfaces 123 (2014) 809–813
these images with software. Each reported value was an average of at least 5 independent measurements.
Table 1 Chemical composition of the nascent and CA adsorbed PVDF membranes and CA calculated from XPS scan spectra.
2.5. Permeation properties
3.2. Characterization of the CA modified PVDF membranes The CA modified PVDF membranes were studied with XPS as shown in Fig. 4. For the unmodified PVDF membranes, peaks of O1S and N1S belong to the additives existing in the commercial PVDF membranes. After CA adsorption, O and C contents on PVDF membrane increases, while F content reduces (shown in Table 1). These changes are due to the adsorbed CA molecules on membrane surface with high C and O contents. However, N content is little changed after CA adsorption. This phenomenon is due to the migration of hydrophilic additives from inside to surface of membranes in polar solvent during the adsorption procedure. The XPS result means that CA molecules have been adsorbed on PVDF membranes. The surface morphologies of membranes before and after CA adsorption were observed by SEM to gain more information of
2.28 3.58
83.29
16.71
F (mol%)
N (mol%)
44.52 38.78
1.51 1.58
–
–
the CA modified PVDF membranes. Fig. 5 presents SEM pictures of PVDF membranes with different adsorption conditions. Even though the CA concentration increases to 0.088 M, the membrane pore structures show little change. Blocked membrane pores are not observed. 3.3. Hydrophilicity of the CA modified PVDF membranes The surface hydrophilicity of membranes was characterized with water contact angle measurements to demonstrate the change of membranes after CA modification. Fig. 6(a) shows the static contact angle of PVDF membranes after CA adsorption with different concentrations. It is worth noting that the water contact angle increases slightly at first as the CA concentration increases
110
(a) 100
90
80
70 0.00
0.02
0.04
0.06
0.08
0.10
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CA concentration (M) 120
(b)
nascent 0.024 M 0.056 M 0.104 M
100
o
Virgin PVDF membranes were separately soaked in deionized water, ethanol, and methanol solutions with different CA concentrations. The loosely adsorbed CA molecules were washed away with water before vacuum drying. Fig. 2 shows the effect of solvent on the adsorption amount of CA on PVDF membranes. The result shows that the adsorption amount of CA on membrane in the methanol solution is the highest. The solvent polarity parameter of the used solvents ranks in the ascending order as follow: ethanol, methanol, and water. The results demonstrate that adsorption of the amphiphilic CA on hydrophobic PVDF membranes is affected by the solvent polarity. When the solvent polarity is low, the hydrophobic face of CA is prior to bond solvent molecules, which leads less CA molecules bound to hydrophobic PVDF membranes. Accordingly, when the solvent polarity increases, CA is prior to bond onto the hydrophobic membrane surface. However, when CA is dissolved in water with the highest polarity, CA molecules tend to form aggregates (micelles) in solution. Therefore, the adsorption amount of CA on PVDF membranes in the water solution is slightly lower than that in the methanol solution. However, the adsorption amount of CA cannot be further increased in water since the CA concentration cannot be raised due to the limited solubility of CA in water. Compared the three solvents, methanol is the most suitable solvent used for CA adsorption. Effect of adsorption time on the adsorption amount of CA on PVDF membranes in methanol solution was studied. As shown in Fig. 3(a), the adsorption amount of CA increases and then reaches equilibrium after 24 h of incubation. Besides, CA concentration in solution greatly affects the adsorption amount of CA on membranes. As shown in Fig. 3(b), it is clear that the adsorption amount increases with CA concentration quickly. Therefore, the adsorption amount of CA on membranes can be expediently controlled by adjusting the CA concentration. Therefore, it is a relatively simple method for membrane surface modification.
O (mol%)
o
3.1. CA adsorption on PVDF membranes
51.69 56.07
Water contact angle ( )
3. Results and discussion
C (mol%)
PVDF PVDF modified with 0.072 M CA CA
Water contact angle ( )
The permeation properties of the unmodified and modified PVDF membranes were examined with a Millipore pressure vessel under constant pressure. All the membranes were wetted with ethanol before flux measurements. After prepressing at 0.12 MPa for 30 min, the flux of pure water (Jw ) was measured at 0.10 MPa.
Sample
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60
40
20
0 0
5
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25
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Time (s) Fig. 6. (a) Effect of CA concentration on the water contact angle of the PVDF membrane; (b) time dependence of water contact angle for the PVDF membranes modified with different CA concentration.
M.-X. Hu et al. / Colloids and Surfaces B: Biointerfaces 123 (2014) 809–813
3.4. Filtration of the CA modified PVDF membranes
9000
(a)
The water permeation fluxes of PVDF membranes were shown in Fig. 7. It is clear that CA adsorption markedly improves the water flux of membrane. The water flux of the CA modified membranes increases with the CA concentration. When the CA concentration is higher than 0.088 M, the water flux is doubled as compared with the nascent PVDF membrane. During the whole test procedure, the water fluxes of all the membranes keep constant. Therefore, CA adsorption greatly enhances the water fluxes of PVDF membranes and the water fluxes of the modified membranes are very stable during the filtration procedure.
8000
2
Flux (L/m h)
7000
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4. Conclusions 0.00
0.02
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CA concentration (M) 9000
(b)
nascent 0.024 M 0.040 M 0.056 M 0.072 M 0.088 M 0.104 M
8000
7000
2
Flux (L/m h)
813
6000
We demonstrate a simple and effective surface modification method to improve the hydrophilicity of PVDF microfiltration membranes through adsorption of facial amphiphilic CA. This adsorption technique used for membrane surface modification has little influence on the membrane morphology. The surface hydrophilicity of PVDF membranes is improved obviously after CA adsorption. The CA molecules are adsorbed on membrane surfaces in a stable state and grant the PVDF membranes to be suitable for long-time running for water filtration. Compared with the nascent PVDF membrane, the water flux of the CA modified membranes is doubled as the CA concentration is high than 0.088 M.
5000
Acknowledgements
4000
The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (Grant no. 21004051 and no. 51103130) and the Educational Commission of Zhejiang Province of China (Grant no. Y201018929) for this work.
3000 0
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Time (min) Fig. 7. Pure water fluxes through the nascent and CA adsorbed PVDF membranes with different CA concentrations.
to 0.024 M and then decreases near linearly with the increase of CA concentration. As the CA concentration keeps increasing to 0.104 M, the water contact angle decreases to 82◦ . The PVDF membranes are commercial products with some hydrophilic additives blending in the membranes demonstrated by the XPS results. Due to the selfassembly ability of the hydrophilic face of CA [15], the N and O atoms of the hydrophilic additives exposed on the membrane surface can form H-bonding with the OH groups on the hydrophilic face of CA. So the hydrophobic face of CA exposes outside and the water contact angle increases slightly. After that, facial amphiphilic CA molecules are adsorbed on the surface of PVDF membranes through hydrophobic interactions. Therefore, more and more hydrophilic faces of the adsorbed CA molecules expose outside on membrane surface and the water contact angle of the membranes accordingly decreases. Time dependence of water contact angle for the CA modified membranes was also studied. As shown in Fig. 6(b), the water contact angle of the CA modified PVDF membranes quickly declines and the water drop rapidly permeates into the membrane pores. Although the initial water contact angles are almost the same, the time dependences of water contact angle are obviously different for the nascent membrane and the CA modified membrane with 0.024 M solution. The higher the CA concentration is used, the faster the water contact angle declines. The fast rate of the water permeation benefits to the filtration properties of membranes.
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