Surface Modification of Zirconium Oxide Particles with Charged

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Langmuir 1996, 12, 3377-3382

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Articles Surface Modification of Zirconium Oxide Particles with Charged Polymers Luc Millesime,† Catherine Amiel,† Franc¸ oise Michel,‡ and Bernard Chaufer*,† Lab. de Physico-Chimie des Biopolyme` res, Universite´ Paris Val de Marne-CNRS (UM 27), 2 rue Henri Dunant, 94 320 Thiais, France, and Lab. de Recherches de Technologie Laitie` re, INRA, 65 rue de Saint-Brieuc, 35 042 Rennes Cedex, France Received June 13, 1995. In Final Form: April 22, 1996X The surface modification of ZrO2 particles with quaternized polyvinylimidazole derivatives has been characterized using diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy and ζ potential measurements. Infrared spectroscopy has been used to determine the adsorbed polymer content. Both the absorbance and the Kubelka-Munk intensities vary linearly with the adsorbed polymer content on the support. ζ potential measurements have been used to study the charge variation of the modified zirconia particles and show that the adsorbed polymer on ZrO2 contributes a net positive charge to ZrO2 in a large range of pH. However the net charge of the particles depends strongly on the physicochemical environment of the support especially with adsorbed electrolytes such as phosphate ions or triethanolamine.

Introduction Surface modifications of mineral oxide materials play an important role in separation processes such as chromatography. The most studied materials were silica particles (SiO2) coated with polymer or copolymer layers which were adsorbed or grafted on the surface.1-8 Only few works have investigated surface modification with cationic polyelectrolytes9 and polymers bearing cationic groups such as quaternary ammonium compounds10 or imines.11 Recently a new chromatographic support based on zirconium oxide (ZrO2) has been used.12 ZrO2 is also the main compound of the active layer of inorganic ultrafiltration membranes due to its ceramic properties (mechanical, thermal, and chemical stability). These membranes are used in several applications such as chemical industries, food and waste water processes, biotechnology, etc. The growing interest in inorganic membranes for the development of membrane technologies have originated several works on the surface properties of ZrO2 * To whom correspondence should be addressed. Present address: Lab. de Proce´de´s de Se´paration, UA Univ. Rennes I-INRA, Baˆt. Nucle´ole, 85 rue de Saint-Brieuc, 35 000 Rennes, France. † Universite ´ Paris Val de Marne-CNRS (UM 27). ‡ INRA. X Abstract published in Advance ACS Abstracts, June 15, 1996. (1) Amiel, C.; Sebille, B. J. Colloid Interface Sci. 1992, 149, 2, 481. (2) Vivarat Perrin, M. P.; Amiel, C.; Sebille, B. Langmuir 1994, 10, 3635. (3) Bohm, J. Th. C.; Lyklema, J. J. Colloid Interface Sci. 1975, 50, 559. (4) Fleer, G. J.; Lyklema, J. Makromol. Chem., Makromol. Symp. 1988, 17, 39. (5) Kawaguchi, M.; Funayama, A.; Yamauchi, S.; Takahashi, A.; Kato, T. J. Colloid Interface Sci. 1989, 121, 130. (6) Yamagiwa, S.; Kawaguchi, M.; Kato, T.; Takahashi, A. Macromolecules 1989, 22, 2199. (7) Kawaguchi, M.; Takahashi, A. Adv. Colloid Interface Sci. 1992, 100, 31. (8) Auroy, P.; Mir, Y.; Auvray, L. Phys. Rev. Lett. 1992, 69, 1, 93. (9) Popping, B.; Deratani, A.; Sebille, B.; Desbois, V.; Lamarche, J. M.; Foissy, A. Colloids Surf. 1992, 64, 125. (10) Denoyel, R.; Durand, G.; Lafuma, F.; Audebert, R. J. Colloid Interface Sci. 1990, 139, 1, 281. (11) Lindquist, G. M.; Straatton, R. A. J. Colloid Interface Sci. 1976, 55, 1, 45. (12) Shafer, W. A.; Carr, P. W. J. Chromatogr. 1991, 587, 149.

S0743-7463(95)00465-3 CCC: $12.00

particles,13-15 which are known as negatively charged substrates at neutral pH. In ultrafiltration, the permeate flux decline depends strongly on the adsorption of solute components of the membrane surface. This phenomenon called fouling is usually caused by large molecules such as proteins.16 Owing to protein-membrane interactions, solute adsorption modifies the performance of all membranes with regard to both retention and permeation.17-18 Contrary to common opinion, the selectivity or retention of an ultrafiltration charged membrane is not based chiefly on its pore size but on the physicochemical environment of the solute and the chemical nature of the surface of the membrane.19 The retention of proteins with inorganic ultrafiltration membranes (ZrO2 active layer) modified with cross-linked and quaternized polyvinylimidazole has been previously studied in order to induce strong interactions between the membranes and the proteins to be concentrated.19-21 It has been shown that the surface modification improves some of the properties of the membranes by giving them better retention and better selectivity. The strength of the interaction forces between the proteins in the mixture solution and the outer surface layer of the fouled membrane plays a leading role in the separation process. The electrostatic component of these forces depends strongly on the net charge of the membrane surface. The aim of this work is to characterize the surface modification of zirconium oxide particles with a positively charged polymer, namely quaternized polyvinylimidazole, (13) Blesa, M. A.; Maroto, A. J. G.; Passagio, S. I.; Figliolia, N. E.; Rigotti, G. J. Mater. Sci. 1985, 26, 4601. (14) Randon, J.; Larbot, A.; Guizart, C.; Julbe, A.; Cot, L. Key Eng. Mater. 1991, 61-62, 155. (15) Dumon, S.; Barnier, H. J. Membr. Sci. 1992, 74, 289. (16) Ingham, K. C.; Busby, T. F.; Sahlestrom, Y.; Castino, F. Polymer Science and Technology, Ultrafiltration membranes and applications; Plenum Press: New York, 1980; Vol. 13, p 141. (17) Zeman, L. J. J. Membr. Sci. 1983, 15, 23. (18) Nystro¨m, M. J. Membr. Sci. 1989, 44, 183. (19) Chaufer, B.; Rollin, M.; Se´bille, B. J. Chromatogr. 1991, 548, 215. (20) Millesime, L.; Dulieu, J.; Chaufer, B. J. Membr. Sci. 1995, 108, 143. (21) Millesime, L.; Dulieu, J.; Chaufer, B. Bioseparation, in press.

© 1996 American Chemical Society

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Figure 1. Structural formulas of the monomers vinylimidazole (VI) and N-vinylpyrrolidone (VP).

and a less positively charged copolymer containing vinylimidazole and a hydrophilic neutral, vinylpyrrolidone. The use of this copolymer for surface modifications of porous silica particles for chromatography purposes has already been the subject of previous studies.22,23 This paper reports a qualitative and quantitative study of the polymer coating on the surface using diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy and a study of charge variation using ζ potential measurements. Materials and Methods 1. Reagents. Polyvinylimidazole (PVI) was synthesized by radical polymerization of vinylimidazole (VI, 276 mM, Polysciences, Eppelheim, Germany) using azobis(isobutyronitrile) (AIBN, 17.5 mM, Prolabo, Paris, France) as initiator in methanol, at 60 °C during 48 h. From the intrinsic viscosity ([η] ) 0.1781 dl g-1 in NaCl 100 mM), the molecular weight was estimated to be about 17 500 g/mol.24 The copolymer of N-vinylpyrrolidone (VP, Fluka, Buchs, Switzerland) and vinylimidazole was synthesized in the same way as for the homopolymer PVI. The acronym of the copolymer (PVI 25) was based on the initial concentration of the two monomers in the reaction batch: 131 mM for VP and 44 mM for VI. The AIBN concentration was 1.2 mM. From elemental analysis of carbon and nitrogen, the chemical composition of the copolymer was about 84% VP units and 16% VI units after 48 h of copolymerization. The molecular weight was unknown due to the lack of a viscosity relationship. The structural formulas of the monomer units are given in Figure 1. Methyl iodide (Prolabo, Paris, France) and the diglycidyl ether of bisphenol A (DGEBA, Epikote 828 LV, kindly provided by Shell, Rueil-Malmaison, France) were used as received for the quaternization and cross-linking of the adsorbed polymers and copolymers on ZrO2, respectively. 2. Preparation of Modified Zirconium Oxide. The solid substrates were porous ZrO2 particles kindly provided by the membrane manufacturer (Tech-Sep, Miribel, France). The surface area was about 33 m2 g-1, and the average particle diameter, measured on a particle analyzer (Coulter Model type N4MD, Coultronics, Hialeah, FL), was 1 µm. In order to remove the physisorbed water and to obtain a reproducible chemical state of the surface, the ZrO2 particles were heated at 120 °C for 24 h before use. The surface modification of the ZrO2 particles was performed as a two-stage process. The first stage was the adsorption of polymer (PVI) or copolymer (PVI 25) onto zirconia particles. The adsorption of the polymer samples was performed on 1 g of ZrO2 particles gently stirred in 10 mL of a 4% polymer solution in methanol for 24 h. The powder was then washed with pure solvent, filtered, and dried at 120 °C for 24 h. The zirconia particles modified with PVI and PVI 25 are further designated as ZrO2-PVI and ZrO2-PVI 25. The second stage of modification consisted in simultaneous quaternization and cross-linking of the amine groups of the adsorbed polymer layer in order to produce the desired amount of surface charges and to prevent desorption of the polymer. Quaternization and crosslinking of the coated particles were then performed with a mixture of methyl iodide and DGEBA. The particles were suspended in 10 mL of the mixture and stirred for 4 h at 70 °C. The zirconia(22) Lemque, R.; Vidal-Madjar, C.; Sebille, B. J. Chromatogr. 1991, 553, 165. (23) Jilge, G.; Sebille, B.; Vidal-Madjar, C.; Lemque, R.; Unger, K. K. Chromatographia 1993, 37, 11, 603. (24) Tan, J. S.; Sochor, A. R. Macromolecules 1981, 14, 1700.

Millesime et al. based particles were washed five times with pure solvent and dried at 120 °C for 24 h. They are further designated as ZrO2KMPVI and ZrO2-KMPVI 25 with the prefixes K and M for epiKote and Methyl iodide, respectively. 3. Quaternized PVI Capacity Measurements. The quaternized PVI capacity ([PVI]quat, µmol/g), which represents the amount of permanent charges afforded by the cross-linking and quaternization step, was determined by a potentiometric backtitration of iodide ions. The modified particles were stirred in a known volume of a 20 mM potassium iodide solution for 16 h. The powder was then allowed to settle down, and the supernatant content in the iodide ions was titrated by a 100 mM silver nitrate solution. A selective electrode (Ag 3) and a reference electrode in mercurous sulfate (S 20) were used with a TT2 processor (Tacussel-Radiometer, Lyon, France). 4. Infrared Measurements. The infrared measurements were performed on a Fourier transform infrared spectrometer (Perkin Elmer type 1760) with a diffuse reflectance FTIR accessory (Spectra-Tech type 018.60791, Eurolabo, Paris, France). The samples were prepared in the following way: after the adsorption, washing, and drying processes, the modified ZrO2 samples were diluted to 5% in dry KBr powder and sampled in a well-defined and reproducible way in a 10 mm diameter cup, without packing the top of the mixed powder. The reflectance spectra were stored against a reference, which was the spectrum of the nonadsorbing diluent medium (KBr). Among many theories that relate reflectance to the analyte concentration, the Kubelka-Munk theory is the most often used.2,25,26 The Kubelka-Munk equation is the diffuse reflectance analogue to Beer’s law for absorbance measurements:

F(R) ) (1 - R)2/2R ) k/s

(1)

where R is the intensity of the reflected light versus the reflected intensity of a standard (KBr), s is a scattering coefficient, and k is the molar absorption coefficient of the medium, for a sample and a standard of infinite thickness. Provided that s remains constant (s is dependent on particle size), a linear relationship is expected between F(R) and the amount of absorbing species in the sample. An alternative method is to use Beer’s law (-log(R) instead of F(R)) because the Kubelka-Munk law tends to accentuate the strong bands and make the weaker bands less intense. Beer’s law is not in principle valid for diffuse reflectance but could be suitable for the analysis of weak bands compared to the absorption of the substrate. 5. ζ Potential Measurements. The ζ potentials of the zirconia particles were obtained from their electrophoretic mobility using a Delsa 440 apparatus (Coultronics, Hialeah, FL). In the calibration procedure, the electroosmotic flow generated by the charges of the cell walls is canceled by using a carboxylated polystyrene latex of 276 nm diameter, of well-known electrophoretic mobility, suspended in a 10 mM KCl solution at pH 7 and 25 °C. The electrophoretic mobility (µ, m2 s-1 V-1) of a charged particle is the ratio of the measured velocity (v, m s-1) to the applied electric field (E, V m-1):

µ ) v/E

(2)

According to the Stoke-Einstein law:

µ ) Q/(6πηr)

(3)

where Q is the effective charge on the particle (C), η is the solution viscosity (Pa s), and r is the radius (m) of the particle, which is assumed to be spherical. The electrophoretic mobility is related to the ζ potential (ζ, V) through the Henry equation:27 (25) Mitchell, M. B.; Chakravarthy, V. R.; White, M. G. Langmuir 1994, 10, 4523. (26) Bummer, P. J.; Guffith, P. R. Anal. Chem. 1986, 58, 2179. (27) Hiemenz, P. C. Principles of colloid and surface chemistry; Marcel Dekker: New York, 1986; Chapter 13. (28) Eng, F. P.; Ishida, H. J. Appl. Polym. Sci. 1986, 32, 5021.

Surface Modification of Zirconium Oxide Particles µ ) 2ζf(κr)/(3η)

Langmuir, Vol. 12, No. 14, 1996 3379 (4)

where  ) r0 ) 78.54 × 8.85 × 10-12 C2 J-1 m-1 (SI units) and f(κr) is a correction parameter dependent on the electrolyte concentration through the reciprocal of the Debye length (κ). For a system with ionic strength about 10 mM and large values of κr (κr > 100), f(κr) approaches 1.5 and eq 4 becomes27

µ ) ζ/η

(5)

At T ) 25 °C the ζ potential is then related to the electrophoretic mobility through the following equation:

ζ (mV) ) 12.89µ (10-8 m2 s-1 V-1)

(6)

Results and Discussion

Table 1. [PVI]total from Elemental Analysis and Quaternary PVI Capacity ([PVI]quat) of ZrO2 Modified with Homopolymer PVI and Copolymer PVI 25 (See Text for Acronyms) coating

[PVI]total (µmol/g)

[PVI]quat (µmol/g)

[PVI]residual (µmol/g)

ZrO2-PVI ZrO2-KMPVI/1 ZrO2-KMPVI/2a ZrO2-PVI 25 ZrO2-KMPVI 25/1 ZrO2-KMPVI 25/2a

161 146 157 30 19 20

0 58 67 0 10 10

161 88 90 30 9 10

a

Duplicate.

Table 2. Selected Bands of PVI and Copolymer PVI/PVP (PVI 25) in Solution (PVI Assignments from Ref 28)

1. Characterization of the Modified ZrO2 Particles. The PVI content ([PVI]total) of the coated particles, calculated from elemental analysis of C, N, H, O and expressed in micromoles per gram of solid, is listed in Table 1. The [PVI]total content of the ZrO2 particles modified with the homopolymer PVI is about sevenfold higher than the PVI content of the ZrO2 particles modified with copolymer PVI 25, which is in good agreement with the chemical composition of the copolymer: 84% VP and 16% VI. As this variation follows the same trend as the molar PVI content of the copolymer, it can be assumed that the copolymer adsorbs on the ZrO2 particles in the same way as the homopolymer. The quaternized PVI capacities ([PVI]quat) correspond to the surface groups exchanging iodide ions. As the quaternization and cross-linking reactions yield is about 40-50%, each sample contains a remaining amount of unmodified PVI ([PVI]residual) which can be deduced from the difference between [PVI]total and [PVI]quat. The ZrO2KMPVI sample (quaternized homopolymer PVI) has a higher quaternized PVI capacity than the ZrO2-KMPVI 25 sample (quaternized copolymer PVI 25), which follows the chemical composition of the copolymer, meaning that cross-linking and quaternization reactions are closed in the homopolymer and copolymer. 2. Adsorbed PVI Content from DRIFT Spectroscopy. The stronger IR bands in the range 800-1800 cm-1, for the homopolymer PVI and the copolymer PVI 25, are summarized in Table 2. In this range, the spectrum of ZrO2 gives two characteristic bands at 1460 and 1560 cm-1 (Figure 2a). These bands become less intense when ZrO2 is coated with polymer due to the thickness of the polymer layer29 (Figure 2b). The spectrum of PVI on the surface layer (Figure 2c) was obtained by subtracting the spectrum of ZrO2 from that of the modified ZrO2-PVI in such a way that the above-mentioned absorption bands of ZrO2 were canceled (interactive difference). This spectrum shows the most characteristic bands of PVI at 917, 1082, 1112, 1230, and 1500 cm-1, respectively, which are listed in Table 2. In the same way, the spectrum obtained from the interactive difference between the spectra of ZrO2 and ZrO2-PVI 25 (Figure 3) shows the bands of the copolymer at 1088, 1110, 1292, 1420, 1495, and 1666 cm-1, respectively. After quaternization and cross-linking of the coated zirconia particles, the spectrum inferred from the interactive difference between the spectra of ZrO2 and ZrO2KMPVI shows bands of PVI at 1112 and 1416 cm-1 and bands of DGEBA at 1184 and 1510 cm-1 (Figure 4). Finally, the 1112 cm-1 band has been chosen to quantify the coating of polymer on the ZrO2 surface because the

zirconia does not absorb in this wavenumber range. This band corresponds to both PVI and PVI 25 and allows the quantitative analysis of both homo- and copolymer. This band, which has been assignated to an in-plane CH bending mode,28 is characteristic of the unmodified PVI only ([PVI]residual) because the CH bending modes are shifted to 1140-1170 cm-1 upon quaternization.30 Thus the 1112 cm-1 band could permit the indirect determination of the amount of quaternized PVI if the total amount of adsorbed PVI ([PVI]total) is known. The residual PVI concentrations and the values of the absorbance at 1112 cm-1 are shown in Table 3 in order to check the validity of the IR analysis. The IR results follow the same trend as the ones obtained from characterization of the samples, since the absorbances of the band at 1112 cm-1 of ZrO2-PVI and ZrO2KMPVI are fourfold higher than those of ZrO2-PVI 25 and ZrO2-KMPVI 25. The absorbance of the band at 1112 cm-1 is roughly linear versus the residual PVI content ([PVI]residual) at the surface of the modified ZrO2, as shown in Figure 5a. Since the beginning of this section, we have implicitly assumed that Beer’s law was valid for the quantitative analysis of the DRIFT results. The spectra have been represented in absorbance values, allowing an easy subtraction of the substrate’s spectrum (ZrO2). The approximation seems to be reasonable, since the band of interest at 1112 cm-1 has a very small height: in a firstorder approximation, the relative variations of heights deduced from Beer’s law or the Kubelka-Munk law should be equivalent. We have checked the validity of this approximation by also doing a Kubelka-Munk analysis. The Kubelka-Munk spectrum of the ZrO2 substrate has been subtracted from the Kubelka-Munk spectrum of the modified ZrO2 particles in such a way that the two

(29) Millesime, L.; Amiel, C.; Chaufer, B. J. Membr. Sci. 1994, 89, 223.

(30) Petrak, K.; Degen, I.; Beynen, P. J. Polym. Sci.: Polym. Chem. Ed. 1982, 20, 783.

sample PVI

PVI 25

band (cm-1)

assignment

1499 1417 1285 1110 1084 916 822 1665 1499 1417 1290 1170 1110 1086 916 841

CdN/CdC stretching CH2 bending ring vibrations CH in-plane bending CH in-plane bending CH out-of-plane bending CH out-of-plane bending CdO stretching (PVP) CdN/CdC stretching (PVI) CH2 bending (PVI) CdN stretching (PVP) CH in-plane bending (PVI) CH in-plane bending (PVI) CH in-plane bending (PVI) CH out-of-plane bending (PVI) CH out-of-plane bending (PVI)

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Figure 4. Diffuse reflectance infrared spectrum of adsorbed KMPVI obtained from interactive difference between the spectra of ZrO2 and ZrO2-KMPVI. Table 3. Absorbance and Kubelka-Munk Intensity (F(R)) of the 1112 cm-1 Band of PVI Coated on ZrO2 coating

[PVI]residualb (µmol/g)

absorbance

F(R)

ZrO2-PVI ZrO2-KMPVI/1 ZrO2-KMPVI/2a ZrO2-PVI 25 ZrO2-KMPVI 25/1 ZrO2-KMPVI 25/2a

161 88 90 30 9 10

0.050 0.026 0.037 0.013 0.009 0.010

2.95 1.85 2.20 1.52 0.99 0.95

a Duplicate. b [PVI] residual refers to un-cross-linked and unquaternized PVI concentration (µmol/g); see text for other abbreviations.

Figure 2. Diffuse reflectance infrared spectra of (a) ZrO2, (b) ZrO2 modified with PVI, and (c) adsorbed PVI obtained from interactive difference of a and b (see text for details).

Figure 5. Correlation between absorbance (a) and the integrated Kubelka-Munk intensity (b) of the unquaternized PVI band at 1112 cm-1 and the residual PVI concentration ([PVI]residual). The straight lines follow the equations (a) y ) 0.0066 + 0.00027x (r2 ) 0.976) and (b) y ) 0.930 + 0.0126x (r2 ) 0.980). Figure 3. Diffuse reflectance infrared spectrum of adsorbed PVI 25 obtained from interactive difference between the spectra of ZrO2 and ZrO2-PVI 25.

characteristic absorption bands of ZrO2 (1460, 1560 cm-1) were canceled.

The plot of the Kubelka-Munk intensity of the 1112 cm-1 band versus the nonquaternized PVI content (Figure 5b) follows also a linear relationship, in good agreement with the plot based on Beer’s law. This proves that the two methods of analysis are equally valid in this particular case or in general when the band of interest is of small height.

Surface Modification of Zirconium Oxide Particles

Langmuir, Vol. 12, No. 14, 1996 3381 Table 4. Influence of Ionic environment (KCl, phosphate, triethanolamine) at Various pH values on ζ Potential (ζ, mV, accuracy (20%) of ZrO2 and ZrO2-KMPVI ζ ZrO2-KMPVI

ZrO2 10 mM KCl 10 mM PO4 5 mM TEA

Figure 6. ζ potentials of ZrO2 (0) and ZrO2-KMPVI (+) at various pH values in 10 mM KCl as indifferent electrolyte.

It appears that the plots do not intercept at 0. These intercepts result from the manner in which the spectra were processed. In fact, the ZrO2 substrate’s contribution was subtracted from the two bands at 1460 and 1560 cm-1, which are far away from the band of interest. Figure 2a shows that the broader and stronger bands of ZrO2 contribute to the absorption in the 1112 cm-1 domain. This absorption might not be well canceled, and the absorption band of ZrO2 at 900 cm-1 could not be used for subtraction of the substrate contribution because it was too broad. Moreover the bands of DGEBA at 1184 and 1240 cm-1 can contribute to the absorption at 1112 cm-1 and were not taken into account for the interactive difference. However this study shows that diffuse reflectance FTIR spectroscopy is an available technique to quickly evaluate the surface modification of inorganic material if well-characterized samples are used for testing the baseline correction during calibration. 3. Surface Charge from ζ Potential Measurements. 3.1. Influence of Coating. Variations of the ζ potentials for unmodified ZrO2 and ZrO2-KMPVI particles in 10 mM KCl electrolyte solution are shown in Figure 6 as a function of pH. These results show that the net charge of the unmodified ZrO2 particles is negative from pH 6 to pH 11. The isoelectric point (pI) of ZrO2 is the pH for which the ζ potential is equal to zero. From Figure 6, the value of the pI of ZrO2 is about 5.5 and is in good agreement with several studies.13,14 The plot of the ζ potential of the ZrO2-KMPVI particles shows how the coating of a charged polymer modifies the ζ potential compared to that of the bare substrate. The pI increases to about 7.2. We can notice that the plots of the ζ potential versus pH are not parallel for the modified and unmodified substrates, as could be expected if the coating contained only permanent charges and neutral components. In fact, the coating is made of two groups: unmodified PVI and quaternized PVI, whose proportions have been determined in sections 1 and 2. The amount of quaternized PVI is about 40% of the total PVI amount for this sample. The positively charged quarternized PVI gives a net positive charge to the substrate which should

pH 3

pH 6

pH 7

pH 11

pH 6

pH 7

pH 11

+28 +4 +31

-1 -24 +6

-5 -33 +7

-24 -19 -13

+7 -13 +28

+1 -26 +15

-23 -35 +1

shift the ζ potential to higher values. Unmodified PVI is a polyelectrolyte whose pKa depends on ionic strength. In a 10 mM KCl solution, its pKa is about 5;9 then the polymer is poorly charged at pH values higher than the pKa. We can then assume that the value of the ζ potential at pH 6 (close to the pI of ZrO2), where ZrO2 is neutral and the unmodified PVI is poorly ionized, corresponds to the charge density of the quaternized PVI content ([PVI]quat). On the other hand, at pH 11, the ζ potentials of the coated and the bare substrate have very close values, the difference between them being much smaller than the one observed at pH 6. This apparent screening of the net positive charge afforded by the charged polymer results from the ionization of zircanol groups, which appears predominant at this pH whatever the chemical treatment was. 3.2. Influence of Ionic Environment. In order to see how inorganic ions interact with the inorganic material at different pH values, some experiments were carried out with solutions containing 10 mM phosphate electrolyte (pK ) 6.80) or 5 mM triethanolamine (TEA) electrolyte (pK ) 7.76). Accordingly, phosphate ions are roughly negative divalent ions at pH > 6.80 and TEA is a positive monovalent ion at pH values lower than 7.76. The ζ potentials are listed in Table 4. The use of 5 mM TEA instead of 10 mM KCl increases the ζ potentials for both modified and unmodified ZrO2. For ZrO2-KMPVI, the ζ potential in TEA is positive in the whole pH range studied. On the other hand, the pI of ZrO2 shifts from 5.5 in KCl to about 9 in TEA. We can assume that some specific adsorption of TEA ions on ZrO2 takes place, which increases the ζ potentials.31 This adsorption seems to occur in the whole pH range studied. However, for ZrO2 at pH 11, the zircanol groups are too strongly negatively charged to be balanced by the TEA adsorption. With phosphate ions, the ζ potentials of unmodified and modified ZrO2 are strongly negative except at pH 3. Phosphate acts a a potential-determining ion, in good accordance with the formation of a negatively charged complex between ZrO2 and phosphate buffer.15,32 This study shows that the ζ potential strongly depends on the ionic environment and on the adsorption properties of the inorganic material. These results give meaningful information for the prediction of adsorption, fouling, flux, and retention of ultrafiltration ZrO2 membranes, especially if the solutions themselves contain charged macromolecules such as proteins. We had previously studied the retention of charged proteins with modified and unmodified inorganic membranes,19-21 and we had particularly investigated the retention of a positively charged protein, lysozyme, with a ZrO2-KMPVI ultrafiltration membrane. We have shown that the lysozyme retention in phosphate buffer at (31) Nystro¨m, M.; Lindstro¨m, M.; Matthiasson, E. Colloids Surf. 1989, 36, 297. (32) Labbe´, J. P.; Quemerais, A.; Michel, F.; Daufin, G. J. Membr. Sci. 1990, 51, 293.

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pH 7 is drastically lower than the lysozyme retention in TEA buffer at pH 7.20 The ζ potential measurements of ZrO2-KMPVI afford a better understanding of these results. This potential is negative in phosphate buffer and positive in TEA buffer. Accordingly, lysozyme acts as a counterion of the phosphated KMPVI support and low retention is observed. On the other hand, lysozyme acts as a co-ion of the KMPVI support in TEA buffer and higher retention than pure molecular sieving is observed due to a superimposed ionic strength-controlled retention.20 Conclusion Surface modification of ZrO2 with cross-linked and quaternized (positively charged) polymers was investigated. The coatings had been characterized by elemental analysis and by ion exchange capacity, allowing the determination of the total coated PVI and of the quaternized PVI, respectively. DRIFT measurements have been proven to be a very sensitive technique to check the surface modification. Moreover, its originality in our case lies in the fact that some quaternized and unquaternized PVI vibrations have different IR responses. This permits

Millesime et al.

a quick and direct determination of the unquaternized PVI amount after canceling the substrate contribution (ZrO2), in good agreement with the other independent measurements whatever the method used (Beer-Lambert or Kubelka-Munk). The apparent net charge of unmodified and modified ZrO2 particles was studied from ζ potential measurements. The charge of solvated ZrO2 was negative above pH 6 in indifferent electrolyte (KCl) whereas the adsorbed quaternized PVI on particles gives a net positive charge. The study in phosphate or triethanolamine electrolyte showed that the ζ potential of modified and unmodified zircone strongly depends on the physicochemical environment. These ions appear to be more or less specifically adsorbed. The influence of charged macromolecules (e.g. protein) on the ζ potential of zirconium oxide and derivatives is under investigation for a better understanding of filtration with similar membranes. Acknowledgment. The authors thank J. Perichon and J. Dulieu for their helpful technical assistance. LA950465G