Characterization of Novel Weak Amphoteric Charged Membranes

Langmuir 2001, 17, 3375-3381

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Characterization of Novel Weak Amphoteric Charged Membranes Using ζ-Potential Measurements: Effect of Dipolar Ion Structure Hidetoshi Matsumoto,† Yoshiyuki Koyama,‡ and Akihiko Tanioka*,† Department of Organic and Polymeric Materials and International Research Center of Macromolecular Science, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8552, Japan, and Department of Home Economics, Otsuma Women’s University, 12 Sanban-cho, Chiyoda-ku, Tokyo 102-8357, Japan Received December 6, 2000. In Final Form: March 22, 2001 Weak amphoteric charged membranes were prepared by the radiation-induced graft copolymerization of the weakly charged poly(ethylene glycol) derivatives onto high-density polyethylene porous membranes. In this study, two kinds of amphoteric charged membranes, which are the amphoteric ion-pair side chain type and the mixed grafted chain type, were prepared and then characterized using the ζ-potential measurements. For the amphoteric ion-pair type membrane, the ζ-potential/pH profile showed a plateau in the range of pH 6-11 which was characteristic of the dipolar ion structure of an amphoteric ion-pair. On the other hand, the ζ-potential for the mixed grafted chain type membrane showed an intermediate pH dependence between the separate positively charged chain- and negatively charged chain-grafted membranes. The apparent surface charge density on these membranes was obtained from the ζ-potential. For the amphoteric ion-pair side chain type membrane, the experimental results could be explained by the theoretical model based on the dissociation of charge groups on the pore surface. For the mixed graft chain type amphoteric charged membrane, however, the difference between the experimental result and the calculated one appears in the pH range of 4-9. This would be caused by the change in the surface states with pH, that is, a conformational change in the grafted polyelectrolyte chains with the change in the charging property. Finally, the unique charging property of the amphoteric ion-pair became apparent in this study.

1. Introduction Weak amphoteric charged membranes have attracted attention for the following two reasons. One is the controllability of the charging property by changing the pH of the external solution,1-10 and the other is the potential as an antifouling material which reduces the specific adsorption of organic molecules on the surface.11,12 These membranes are expected to be applied to medical devices, to drug delivery systems, for the separation of ionic drugs and proteins, and so forth.9,12 It is widely recognized that poly(ethylene glycol) (PEG) has biocompatibility and protein and cell resistance characteristics.13-17 Many researchers, such as Harris et al., have reported surface modifications by PEG.18-24 Thus, * To whom correspondence should be addressed. Telephone: +81-3-5734-2426. Fax: +81-3-5734-3659. E-mail: [email protected]. titech.ac.jp. † Tokyo Institute of Technology. ‡ Otsuma Women’s University. (1) Elmidaoui, A.; Boutevin, B.; Belcadi, S.; Gavach, C. J. Polym. Sci. B 1991, 29, 705. (2) Ramı´rez, P.; Alcaraz, A.; Mafe´, S. J. Electroanal. Chem. 1997, 436, 119. (3) Saito, K.; Tanioka, A. Polymer 1996, 37, 2299. (4) Saito, K.; Ishizuka, S.; Higa, M.; Tanioka, A. Polymer 1996, 37, 2493. (5) Jimbo, T.; Higa, M.; Minoura, N.; Tanioka, A. Macromolecules 1998, 31, 1277. (6) Jimbo, T.; Tanioka, A.; Minoura, N. Langmuir 1998, 14, 7112. (7) Jimbo, T.; Tanioka, A.; Minoura, N. Colloids Surf., A 1999, 159, 459. (8) Jimbo, T.; Tanioka, A.; Minoura, N. Langmuir 1999, 15, 1829. (9) Jimbo, T.; Ramı´rez, P.; Tanioka, A.; Mafe´, S.; Minoura, N. J. Colloid Interface Sci. 2000, 225, 447. (10) Uematsu, I.; Jimbo, T.; Tanioka, A. J. Colloid Interface Sci., submitted. (11) Burns, L. N.; Holmberg, K.; Brink, C. J. Colloid Interface Sci. 1996, 178, 116. (12) Ishihara, K.; Shinozuka, T.; Hanazaki, Y.; Iwasaki, Y.; Nakabayashi, N. J. Biomater. Sci., Polym. Ed. 1999, 10, 271.

it is very interesting that the surface modification using PEG can be applied to a porous membrane for the purpose of creating an antifouling or antithrombotic membrane. We have recently synthesized PEG derivatives having weakly charge groups by adding ionizable groups to the double-bond side chains of copoly(allyl glycidyl ether/ ethylene oxide) (copoly(AGE/EO))25,26 and then prepared the amphoteric charged membranes by the radiationinduced graft copolymerization of the polymer onto porous membranes.27 Electrokinetic measurements have been theoretically established and widely utilized for the characterization (13) Harris, J. M., Ed. Poly(Ethylene Glycol) Chemistry: Biotechnical and Biomedical Applications; Plenum Press: New York, 1992. (14) Harris, J. M., Zalipsky, S., Eds. Poly(Ethylene Glycol): Chemical and Biological Applications; ACS Symposium Series Vol. 680; American Chemical Society: Washington, DC, 1997. (15) Jeon, S. I.; Lee, J. H.; Andrade, J. D.; De Gennes, P. G. J. Colloid Interface Sci. 1991, 142, 149. (16) Prime, K. L.; Whitesides, G. M. J. Am. Chem. Soc. 1993, 115, 10714. (17) Gingell, D.; Owens, N. J. Biomed. Mater. Res. 1994, 28, 491. (18) Bergstro¨m, K.; Holmberg, K.; Safranji, A.; Hoffman, A. S.; Edgell, M. J.; Kozlowski, A.; Hovanes, B. A.; Harris, J. M. J. Biomed. Mater. Res. 1992, 26, 779. (19) Van Alstine, J. M.; Burns, N. L.; Riggs, J. A.; Holmberg, K.; Harris, J. M. Colloid Surf., A 1993, 77, 149. (20) O ¨ sterberg, E.; Bergstro¨m, K.; Holmberg, K.; Riggs, J. A.; Van Alstine, J. M.; Schuman, T. P.; Burns, N. L.; Harris, J. M.Colloid Surf., A 1993, 77, 159. (21) Burns, N. L.; Van Alstine, J. M.; Harris, J. M. Langmuir 1995, 11, 2768. (22) O ¨ sterberg, E.; Bergstro¨m, K.; Holmberg, K.; Schuman, T. P.; Riggs, J. A.; Burns, N. L.; Emoto, K.; Van Alstine, J. M.; Harris, J. M. J. Biomed. Mater. Res. 1995, 29, 741. (23) Emoto, K.; Van Alstine, J. M.; Harris, J. M. Langmuir 1998, 14, 2722. (24) Burns, N. L.; Emoto, K.; Holmberg, K.; Van Alstine, J. M.; Harris, J. M. Biomaterials 1998, 19, 423. (25) Koyama, Y.; Umehara, M.; Mizuno, A.; Itaba, M.; Yasukouchi, T.; Natsume, K.; Suginaka, A. Bioconjugate Chem. 1996, 7, 298. (26) Koyama, Y. Macromolecules, submitted.

10.1021/la001706+ CCC: $20.00 © 2001 American Chemical Society Published on Web 05/03/2001

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Figure 1. Schematic representation of the ideal morphological model for the grafted polymer chains on the pore surface of (a) the ASC type and (b) the MGC type amphoteric charged membranes at a pH near the isoelectric point.

of polymer surfaces,11,19,21-24,28 particularly for the surfaces of porous polymeric membranes.5-7,27,29 While the ζ-potential is the electrokinetic potential that is generated at the hydrodynamic slipping plane, strictly different from the pore surface, it provides qualitative information related to complex surface changes.5-7,11,21,27,30 We deduced the ζ-potential from the observed streaming potentials to estimate the charging property on the pore surface. These measurements are also sensitive to the pH dependence of the dissociation of the surface ionizable groups.5-7,11,21,27,30 Healy and White have suggested that surface charge can be described in terms of a site dissociation model (SDM) for surface ionizable groups.31 This model seems to be suitable for analyzing the charging properties on the pore surface such as surface charge density and dissociation constant of the charge group. We have adapted this semiquantitative methodology to characterize the amphoteric charged porous membranes.5-7,11,21,27 In this study, we prepared two kinds of amphoteric charged membranes, which were the amphoteric ion-pair side chain (ASC) type and the mixed grafted chain (MGC) type, by the radiation-induced graft copolymerization of weakly charged PEG derivatives onto high-density polyethylene (HDPE) porous membranes. The concept of these amphoteric charged membranes is schematically illustrated in Figure 1. These membranes were characterized using ζ-potential measurements over the pH range of 2-12. The streaming potential method was used for the ζ-potential measurements. The pH dependence of the apparent surface charge density derived from the ζ-potential was compared to the calculated results using the SDM. The amphoteric ion-pair, which consists of an amino group and a carboxyl group, forms the complex where a proton is mediated between the amino group and the carboxyl group (Figure 2). This complex is called a dipolar ion structure and appears as amphoteric charging states over a wide range of pH, as typical in amino acids.32 We took note of this character obtained only in the ASC type membrane but not in the MGC type one. 2. Experimental Section 2.1. Materials. Copoly(allyl glycidyl ether/ethylene oxide) (copoly(AGE/EO),25 AGE/EO ) 13.6/86.4 in molar ratio, Mn ) 3260, Mw/Mn ) 1.05) was provided by NOF, Japan. L-Cysteine hydrochloride monohydrate, 2-aminoethanethiol hydrochloride, 3-mercaptopropionic acid, methanol, chloroform, diethyl ether, (27) Matsumoto, H.; Koyama, Y.; Tanioka, A. J. Colloid Interface Sci., in press. (28) Jacobasch, H.-J.; Schurz, J. Prog. Colloid Polym. Sci. 1988, 77, 40. (29) Caussreand, C.; Nystro¨m, M.; Aimar, P. J. Membr. Sci. 1994, 88, 211. (30) Hunter, R. J. Zeta Potential in Colloid Science; Academic Press: London, 1981; Chapter 7. (31) Healy, T. W.; White, L. R. Adv. Colloid Interface Sci. 1978, 9, 303. (32) Stryer, L. Biochemistry, 3rd ed.; W. H. Freeman and Company: New York, 1988.

Figure 2. Dipolar ion structure of an amphoteric ion-pair. Scheme 1. Synthesis of the PEG Derivatives Having Weak Charge Groups

acetone, and potassium chloride were purchased from Wako Pure Chemical, Japan. These reagents were of extrapure grade. The 0.1 mol/L hydrochloric acid and 0.1 mol/L potassium hydroxide were from Wako Pure Chemical, Japan. These reagents were of analytical grade. All reagents were used without further purification. High-density polyethylene (HDPE) porous membranes (Hipore N720, 25 µm thick and 0.05 µm average pore size) were obtained from Asahi Kasei, Japan. 2.2. Synthesis of PEG Derivatives Having Weak Charge Groups. The synthesis of PEG derivatives having weak charge groups is shown in Scheme 1. We synthesized the PEG derivatives by adding weak electrolytes having a highly reactive mercapto group to the double-bond side chains of copoly(AGE/ EO). The PEG derivatives having amino groups (PEG-A), carboxyl groups (PEG-C), and the amphoteric ion-pair cysteine (PEGCys) were synthesized using 2-aminoethanethiol, 3-mercaptopropionic acid, and L-cysteine, respectively. 2.2.1. Addition of 2-Aminoethanethiol to Copoly(AGE/EO). Copoly(AGE/EO) (5.02 g) was dissolved in methanol (7 mL), and it was added dropwise at room temperature to the solution of 2-aminoethanethiol hydrochloride (10.01 g) in methanol (12 mL). After standing at room temperature for 2 days, the reaction mixture was evaporated to remove the methanol. The residual syrup was dissolved in chloroform, neutralized by sodium hydroxide, and washed two times with water. It was further purified two times with diethyl ether and one time with acetone/ diethyl ether. Removal of the solvent under reduced pressure gave a syrupy product (4.13 g). 2.2.2. Addition of 3-Mercaptopropionic Acid to Copoly(AGE/ EO). Copoly(AGE/EO) (5.03 g) and the solution of 3-mercaptopropionic acid (5 mL) in methanol (7 mL) were mixed. After standing at room temperature for 2 days, the polymer was purified three times by washing with diethyl ether. Removal of the solvent under reduced pressure gave a syrupy product (5.82 g). 2.2.3. Addition of Cysteine to Copoly(AGE/EO). Copoly(AGE/ EO) (5.0 g) in methanol solution (20 mL) was added dropwise at room temperature to the L-cysteine hydrochloride monohydrate (15 g) in methanol solution (20 mL). After standing at room

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Scheme 2. Graft Copolymerization of PEG Derivatives on the Pore Surface of Porous Membranes by γ-ray Irradiation

∆E ζ ) ∆P ηλ

(2)

where η is the solution viscosity and  and λ are the permittivity and electrical conductivity of the solution, respectively. The apparent surface charge density of the pores was calculated from the ζ-potential using the Gouy-Chapman equation:5-7,27

σs )

temperature for 2 days, the polymer was purified by repeated reprecipitation with methanol/diethyl ether. Removal of the solvent under reduced pressure gave a syrupy product in theoretical yield. 2.3. Membrane Preparation. Two types of amphoteric charged membranes and two types of singly-charged membranes were prepared by the radiation-induced graft copolymerization. The HDPE porous membranes were used as the substrate on which the PEG derivatives were graft-polymerized. A schematic illustration of grafting the PEG derivatives onto the HDPE porous membranes by γ-ray irradiation is shown in Scheme 2. The circular HDPE membrane samples (area ) 17.35 cm2) were extracted in methanol for 12 h, dried, and weighed before use. After soaking in methanol and deionized water, these membranes were immersed in a 16 wt % polymer aqueous solution on a glass plate. For the MGC type amphoteric charged membrane, the mixture ratio of NH2 in PEG-A to COOH in PEG-C was 1:1 in moles. This glass plate was covered with another one, and then the plates were sealed with cellophane tape. The plates were irradiated by 1.17 and 1.33 eV γ-rays using a 60Co generalpurpose irradiation device (Model RE-1012, TOSHIBA, Japan) at dose rates of 2.47 kGy/h and total doses of 50 kGy. The grafted membranes were washed and then rinsed for 12 h with deionized water to sufficiently remove the nongrafted polymers. Thereafter these membranes were dried at 60 °C under vacuum for 12 h. The degree of graft copolymerization (DG) was calculated using the following equation:

DG )

W2 - W1 W1

(1)

where W1 is the weight of the dried membrane before graft copolymerization and W2 is that of the dried membrane after graft copolymerization. 2.4. Measurements. 2.4.1. Nuclear Magnetic Resonance (NMR) Measurements. The completion of the addition reaction of each mercaptan to the double-bond side chain of copoly(AGE/EO) was confirmed by NMR measurements. The 1H NMR spectra of the prepared polymer were measured with a 300 MHz FT-NMR instrument (JNM-AL300, JEOL, Japan) in CDCl3 (for copoly(AGE/EO)) and DMSO-d6 (for the other polymers) at 40 °C. 2.4.2. Hydraulic Permeability Measurements. The hydraulic permeability of the membranes was calculated from the flow rate of deionized water, which was eluted for 20 min under an applied constant hydrostatic pressure difference of 75.0 cmH2O across the membrane. The area of the membrane exposed to the flow was 4.9 cm2. All measurements were done at 25 ( 0.1 °C. 2.4.3. ζ-Potential from Streaming Potential Measurements. The experimental setup is the same as previously mentioned.5,6 By varying the applied pressure (∆P) ranging from 15 to 120 cmH2O, the streaming potential (∆E), which had been generated by a flow of ions due to ∆P, was measured with a digital multimeter (HP3458A, Hewlett-Packard, U.S.) and recorded using a microcomputer. The ζ-potential was obtained from the slope of a ∆E-∆P plot using the following Helmholtz-Smoluchowski equation:5-7,27

( )

z+eζ 2kTκ sinh z+e 2kT

(3)

where k is the Boltzmann constant, T is the temperature, κ is the reciprocal of the electrical double-layer thickness, z+ is the valence of the counterion, and e is the Coulombic charge. The KCl concentration in the outer solution was 1, 10, or 100 mmol/L throughout the measurements. The pH of the outer solution was regulated from 2 to 12 by adding 0.1 mol/L HCl or 0.1 mol/L KOH. All measurements were carried out in a stirred solution thermostated at 25 ( 0.1 °C. The measurements were carried out three times for each experimental point, and the mean value (( standard deviation) of each experimental point is indicated.

3. Results and Discussion 3.1. Synthesis of Ionic PEG Derivatives and Preparation of Membranes. The ionic PEG derivatives were synthesized by the addition of the corresponding mercaptan to the double-bond side chains of copoly(AGE/ EO) in the previous paper.25 The reaction was completed in methanol at room temperature without a catalyst. The 1H NMR spectra of all the PEG derivatives having weak charge groups showed no residual double-bond peaks, which were observed near 5.2 and 5.8 ppm with copoly(AGE/EO), and the spectrum indicated that PEG derivatives having weakly charge groups with a definite structure were obtained (Figure 3). The degree of graft polymerization (DG) and water permeability of the prepared membranes are listed in Table 1. The PEG-A, PEG-Cys, and mixed PEG-A/PEG-C grafted membranes had similar DG values of about 5%, while the PEG-C grafted membrane had a 2.7% DG. All the grafted membranes also have a smaller water permeability than the unmodified HDPE membrane. Here, the HDPE membrane is so hydrophobic that the water permeability measurement was carried out after immersion in MeOH. The results in Table 1 indicated that the pore surfaces of the HDPE porous membranes are covered with the ionic PEG derivatives. The difference in DG between PEG-C and the other PEG derivatives would be caused by the difference in the grafting efficiency during graft copolymerization. 3.2. ζ-Potential Measurements of Prepared Membranes. The slope of the ∆E-∆P plot displayed good linearity for each membrane under various pH and concentration conditions. The ζ-potential was calculated using eq 2. In dealing with a porous membrane, we need to consider the effects of pore diameter and surface roughness on the ζ-potential.5-7,27 The pore size of the unmodified HDPE membrane, an average 0.05 µm pore diameter, appears to be large enough to permit ignoring the electrical conductance effect in the diffuse part of the double layer (the Debye screening length is 10 nm long in 1 mmol/L KCl solution).7 Therefore, we did not take into account the surface conductance and double-layer overlapping effects in both the unmodified and grafted HDPE membranes. However, because it is impossible to determine the real state of the pore surface (i.e., whether it is ideally smooth), the ζ-potential obtained in this study must be regarded as an apparent value. Figure 4 shows the pH dependence of the ζ-potential in 10 mmol/L KCl of unmodified HDPE membrane and

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Figure 3. 1H NMR spectra of (a) copoly(AGE/EO), (b) the addition product of copoly(AGE/EO) with aminoethanethiol (PEG-A), (c) the addition product of copoly(AGE/EO) with mercaptopropionic acid (PEG-C), and (d) the addition product of copoly(AGE/EO) with cysteine (PEG-Cys). Table 1. Pore Surface Radiation-Induced Grafting Modification of HDPE Porous Membranes membrane PEG-A PEG-C PEG-Cys PEG-A/PEG-C unmodified PE

DGa (w/w %)

water permeability (mL/m2‚h‚H2O)

5.7 2.7 4.5 4.5

34 52 36 47 72b

a DG ) (W - W )/W , where W ) the weight of the dried 2 1 1 1 membrane before graft copolymerization and W2 ) the weight of the dried membrane after graft copolymerization. b Measured after immersion in MeOH.

prepared amphoteric charged membranes. In Figure 4a, the ζ-potential of the unmodified HDPE membrane had a near zero value at low pH ( 6. This negative value indicated that the HDPE membrane used in this study had some charge groups on the pore surface initially. The ζ-potential of the unmodified HDPE membrane did not change before and after irradiation. Therefore, there was no possibility that charge groups generated on the pore surface during γ-ray irradiation. Here, the unmodified and irradiated HDPE membrane is so hydrophobic that ζ-potential measurements were carried out after immersion in MeOH and sufficient washing with deionized water. As can be seen from Figure 4b, the ζ-potential for the ASC type (PEGCys grafted) amphoteric charged membrane changed from a negative to a positive value, across the plateau, which showed a constant potential in the range of pH 6-11. This behavior is characteristic of the ASC type amphoteric charged membranes, and the constant potential in the intermediate pH region would be the evidence of the dipolar ion structure in the cysteine groups.27 In Figure 4c, the PEG-A grafted membrane showed a positive potential at low pH ( 5. The ζ-potential for the MGC type (mixed PEG-A/PEG-C grafted) amphoteric charged membrane showed an intermediate pH dependence behavior between the PEG-A and PEG-C separately grafted membranes.

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While the ζ-potential of the plateau region in the ASC type amphoteric charged membrane was expected to be zero, a negative ζ-potential (about -15 mV) was obtained in this measurement. The PEG-A grafted membrane also had a negative ζ-potential (about -15 mV) at high pH, where the potential should ideally be zero. This negative value would be originated from the contribution of the negative charge on the pore surface of the HDPE membrane mentioned above. When the thickness of the effective electrokinetic layer (approximately 3/κ, where 1/κ is the Debye screening length) of charged substrate is greater than the thickness of the grafted polymer chain on the substrate, it is known that the effect of charged substrate for electrokinetic phenomena is not negligible.22 3.3. Theoretical Analysis According to a Site Dissociation Model. To explain the pH dependence of the experimental results mentioned above, we simulated the apparent surface charge density/pore surface pH profiles using a site dissociation model.5-7,27,30,31 Apparent surface charge density/pH profiles at constant ionic strength are particularly useful, since they reflect the ionizable character of the pore surface. The experimental data of the ζ-potential in 10 mmol/L KCl solution were used for the SDM analysis in this study, because the contribution of the pH regulator, HCl or KOH, was too large for the ζ-potential measurements in 1 mmol/L KCl and the contribution of the electrolyte ion adsorption was not negligible for the ζ-potential in 100 mmol/L KCl.28 When the effect of the specific adsorption of electrolyte ions is not considered, the equilibrium between charge groups and the solution at the interface is represented as

AH T A- + H+

(4)

BH+ T B + H+

(5)

where AH and B are the acidic and basic groups, respectively. These equilibria have the following dissociation constants, K:

Ka ) Kb )

[A-][H+]s [AH] [B][H+]s [BH+]

(6)

(7)

where the subscripts a and b stand for the acidic site and basic site, respectively. [H+]s is the hydronium ion concentration at the pore surface, which is based on the hypothesis that the ion concentration in the electrolyte solution follows the Boltzmann distribution outside the plane where the ζ-potential is generated, and is written in the form

(-eζ kT )

[H+]s ) [H+]0 exp

(8)

where [H+]0 is the hydronium ion concentration in the bulk solution. The total site density of the pore surface, N, is denoted by

Na ) [A-] + [AH] +

Nb ) [BH ] + [B]

∑i [A-]i + e∑j [BH+]j

σs ) -e )

∑i

[

-eNa

] [

1 + 10(pKa-pHs)

+

∑j

eNb

]

1 + 10(pHs-pKb)

(11)

where pKa and pKb are the equilibrium acidic dissociation constants of the acidic and basic sites, respectively, and pHs is the surface pH. The experimental value of the apparent surface charge density obtained by eq 11 is plotted as a function of surface pH in Figure 5. The solid line in Figure 5 represents the calculated value according to a nonlinear regression method based on a SDM. The pK values, pKa and pKb, which are in ref 33, and the fitting parameters, Na and Nb, which were determined by implementing the nonlinear parameter estimation program UNCMIN,34 are summarized in Table 2. In this calculation, we added the effect of charge groups originated from a HDPE membrane to a SDM in order to consider the effect of the unexpected negative charge groups as mentioned above. The pK value of this negative charge group is 4.3, which was obtained from the fitting parameter on calculating with the SDM based on the assumption that there is one type of acidic group on the pore surface of a HDPE membrane. The fitting curve using this constant agreed well with the experimental data (Figure 5a). The experimental results of the ASC type (PEG-Cys grafted) amphoteric charged membrane could be explained by a SDM using the pK value of cysteine in ref 33, pKa ) 1.9 and pKb ) 10.8, which involved the effect of the dipolar ion structure (Figure 5b). Therefore, it is postulated that the plateau region over pH 6-11 in the ζ-potential/pH profile would originate in the dipolar ion structure of the cysteine residues on the side chain of the grafted polymer chains. The plateau region disappears with an increase in the electrolyte concentration of the external solution (Figure 6). This would indicate that the ion-pair between the amino group and the carboxyl group via a proton was broken by the electrolyte ions in solution. The experimental results of the pH dependence of the apparent surface charge density for the PEG-A and the PEG-C grafted membranes could be explained by the SDM using the pK values, which were pKb ) 8.2 (for aminoethanethiol) and pKa ) 3.4 (for mercaptopropionic acid) (Figure 5c). For the MGC type (PEG-A/PEG-C grafted) amphoteric charged membrane, however, the distinct difference between the experimental result and the calculated one appears in the pH range of 4-9 (Figure 5c). For the experimental data, the rapid decrease in the apparent surface charge density was observed across the isoelectric point (IEP), which is at pH 5.8. This behavior was observed merely in the low-concentration electrolyte solution (1 and 10 mmol/L KCl, see Figures 5c and 7a). In 100 mmol/L KCl solution, the ζ-potential of the MGC type membrane did not show this rapid decrease but showed a two-step decrease near pH 4 and pH 8 with the increase in pH (Figure 7b). The rapid decrease via IEP would be caused by the change in the surface states with pH, that is, the conformational change in the grafted polyelectrolyte chains with the change of the charging

(9) (10)

Therefore, the apparent surface charge density, σs, can be expressed by combining eqs 6 and 7 with eqs 9 and 10. For a multiply charged pore surface, σs is given by

(33) Martell, A. E.; Smith, R. M. Critical Stability Constants; Plenum Press: New York, 1974; Vol. 1; 1975; Vol. 2. For the assignment of cysteine’s pK, see also: Greenstein, J. P.; Winitz, M. Chemistry of the Amino Acids; John Wiley & Sons: New York, 1961; Vol. 1, p 494. Calvin, M. In Glutathione; Colowick, S. P., Ed.; Academic Press: New York, 1954; p 8. (34) Kahaner, D.; Moler, C.; Nash, S. Numerical Methods and Software; Prentice Hall: Englewood Cliffs, NJ, 1989.

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pKa pKb

PEG-A 8.2a PEG-C 3.4a PEG-Cys 1.9a 10.8a PEG-A/PEG-C 3.4a 8.2a HDPE (substrate) a

103Na 103Nb (nm-2) (nm-2) 31.2 12.5 41.5 21.2

41.5 41.8

HDPE (substrate) pKa

103Na (nm-2)

4.3 4.3 4.3 4.3 4.3

16.2 18.7 17.8 15.6 23.5

In ref 33.

Figure 6. Effect of the concentration of the external solution on the pH dependence of the ζ-potential for the ASC type (PEGCys grafted) amphoteric charged membrane.

repulsion between the charge groups along the chain, which depends on the pH and concentration of the electrolyte solution.35-38 A schematic representation of the polymer chain conformation on the pore surface of the MGC type membrane is illustrated in Figure 8. The positively charged grafted chain is more extended on the acidic side, and the negatively charged grafted chain is more extended on the basic side through the IEP. The ζ-potential is generated at the hydrodynamic slipping plane. Therefore, the ζ-potential is more sensitive to the polyelectrolyte chain highly extended into the solution.22 In conclusion, it is impossible to explain the charging property of the MGC type membrane using the classical SDM only based on the dissociation of ionizable groups. 4. Conclusions

Figure 5. pH dependence of the apparent surface charge density for the amphoteric charged membranes: for (a) an unmodified HDPE membrane (substrate), (b) the ASC type (PEG-Cys grafted) amphoteric charged membrane, and (C) the MGC type (PEG-A/PEG-C grafted) amphoteric charged membrane. The solid line corresponds to the theoretical fits using the SDM. The characteristic parameters are listed in Table 2.

property. The conformation of the polyelectrolyte chain in the electrolyte solution is greatly affected by the

Two kinds of amphoteric charged membranes, which are the ASC type (PEG-Cys grafted) and MGC type (PEGA/PEG-C grafted), were prepared by the radiation-induced graft copolymerization of the weakly charged PEG derivatives onto HDPE porous membranes. For the ASC type (PEG-Cys grafted) amphoteric charged membranes, the ζ-potential/pH profile showed a plateau in the range of pH 6-11. This behavior would originate in the dipolar ion structure of the cysteine residue on the grafted chain. This was also supported by the theoretical prediction by (35) Kurihara, K.; Kunitake, T.; Higashi, N.; Niwa, M. Langmuir 1992, 8, 2087. (36) Miklavic, S. J.; Marcˇelja, S. J. Phys. Chem. 1988, 92, 6718. (37) Misra, S.; Varanasi, S.; Varanasi, P. P. Macromolecules 1989, 22, 4173. (38) Zhulina, E. B.; Borisov, O. V.; Birshtein, T. M. J. Phys. II 1992, 2, 63.

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Figure 8. Possible coil conformation on the pore surface of the MGC type (PEG-A/PEG-C grafted) amphoteric charged membrane as a function of pH.

Figure 7. Effect of the concentration of the external solution on the pH dependence of the ζ-potential for the MGC type (PEGA/PEG-C grafted) amphoteric charged membrane, measured in (a) 1 mM and (b) 100 mM KCl solutions.

a SDM using the cysteine’s pK value in the literature.33 This plateau region disappears with the increase in the electrolyte concentration of the external solution. On the other hand, the mixed grafted chain type (PEG-A/PEG-C grafted) membrane showed an intermediate pH dependence of the ζ-potential between the separate positively charged chain- and negatively charged chain-grafted membranes. The apparent surface charge density on the membranes obtained from the ζ-potential was analyzed by a SDM based on the dissociation of charge groups on the pore surface. The experimental results of the pH dependence

of the apparent surface charge density for the PEG-A and PEG-C grafted membranes and the ASC type (PEG-Cys grafted) amphoteric charged membrane could be explained by a SDM using the pK values of the charge groups in the literature.33 For the MGC type (PEG-A/PEG-C grafted) amphoteric charged membrane, however, the difference between the experimental result and the calculated one appeared in the pH range of 4-9. This would be caused by the change in the surface states with pH, that is, the conformational change of the grafted polyelectrolyte chains with the change in the charging state. The SDM approach concerning the pore surface charging phenomena is widely used for the electrokinetic evaluation of charged porous membranes. Using the SDM, however, it is necessary to pay sufficient attention to the factors which influence the ζ-potential except for the dissociation of ionizable groups (e.g., surface morphology, surface wettability, and specific adsorption of the electrolyte ion). We need to extend the SDM to the polyelectrolyte chain-grafted surface where the effect of the conformation change with the change in the charging property is not negligible in the further step. We are concerned about the effect of the unique hydrophilic polyampholyte, PEG-Cys, which forms the dipolar ion structure. In this study, the architecture of the novel amphoteric charged membrane, the ASC type amphoteric charged membrane, by surface modification using PEG-Cys was proposed. There is also interest in the transport and adsorption phenomena in the ASC type membrane. Acknowledgment. We are grateful to Professor Masaaki Kakimoto and Mr. Masaki Takeuchi (Tokyo Institute of Technology) for the NMR measurements and to Mr. Isao Yoda (Research Laboratory for Nuclear Reactors at Tokyo Institute of Technology) for the γ-ray irradiation-induced graft copolymerization. LA001706+