Intermolecular Forces between a Protein and a Hydrophilic Modified

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Intermolecular Forces between a Protein and a Hydrophilic Modified Polysulfone Film with Relevance to Filtration Jeffrey A. Koehler,† Mathias Ulbricht,‡ and Georges Belfort* Rensselaer Polytechnic Institute, Howard P. Isermann Department of Chemical Engineering, Troy, New York 12180-3590 Received April 3, 2000. In Final Form: September 5, 2000 Correlations between intermolecular forces and ultrafiltration measurements for a thin polysulfone film and membranes modified for increased hydrophilicity by graft polymerization of 2-hydroxyethyl methacrylate and a model protein (hen egg-white lysozyme, Lz) suggest that altering either the chemistry of the polymer surface or the solution conditions should lead to a minimization of protein adhesion and hence fouling for a specific protein/polymer combination. Using the surface forces apparatus, normalized adhesion forces were measured below, at and above the pI of Lz, and compared with corresponding permeation flux ratios from ultrafiltration experiments. Simple exponential correlations were obtained relating the normalized adhesion forces to several different permeation flux ratios. Also, the amount of protein adsorbed onto the membrane from solution during filtration was linearly related to the adhesion force through the choice of solution pH. The correlations imply that protein-polymer adhesive interactions are important during ultrafiltration. The results obtained for both a hydrophilic and a hydrophobic surface were compared. The hydrophilic surface exhibited lower contact angles, reduced adhesion forces, reduced adsorbed amount, and most importantly, reduced protein fouling. Long range attraction between adsorbed protein and hydrophobic polysulfone films was absent with the hydrophilic films. The results provide a fundamental molecular basis to the widely reported and observed phenomenon that hydrophilic membranes are known to foul less than hydrophobic ones during membrane filtration of protein solutions.

Introduction Dissolved proteins or other macromolecules present in a feed solution can severely affect the permeation rate and retention of desired molecules during membrane filtration.1-4 The molecular interaction between such molecules and the external or internal membrane surface is a major cause of concern for improving membrane filtration performance.5-7 Two phenomena are thought to dominate; protein-protein and protein-membrane interactions. The former interactions induce protein aggregation in solution and/or on surfaces preadsorbed with proteins, while the latter interactions affect permeation rates through pore plugging, pore narrowing, and cake deposition. The mechanisms that underlie these attractive interactions at the molecular level are unknown. Thus, with a deeper fundamental knowledge of these interactions, these phenomena, loosely called protein fouling, could be mitigated through design of membrane surface chemistries and choice of environmental conditions that * Corresponding author. Telephone: (518) 276-6948. Fax: (518) 276-4030. E-mail: [email protected]. † Current address: Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA 91125. ‡ Current Address: GKSS-Forschungs Centrum Gaesthacht GMBH, Institut fur Chemie, Kanstrasse 55, Teltow D-14513, Germany. (1) Koehler, J. A.; Ubricht, M.; Belfort, G. Langmuir 1997, 13, 4162. (2) Sirkar, K. K.; Prasad, R. In Membrane Separations in Biotechnology; McGregor, W. C., Ed.; Marcel Dekker: New York, 1986; p 37. (3) Fane, A. G.; Fell, C. J. D.; Suki, A. J. Membr. Sci. 1983, 16, 195. (4) Belfort, G.; Zydney, A. L. Interactions of proteins with polymeric synthetic membranes. In Interfacial Behavior of Biopolymers; Martin Malmsted, Ed.; Marcel Dekker: New York,1998. (5) Nystrom, M.; Laatikainen, M.; Turku, M.; Jarvinen, P. Prog. Colloid Polym. Sci. 1990, 82, 321. (6) McDonogh, R. M.; Bauser, H.; Stroh, N.; Chimel, H. Desalination 1990, 79, 217. (7) Boyd, R. F.; Zydney, A. L. Biotechnol. Bioeng. 1998, 59, 451.

minimize attractive forces between the membrane surface and the protein and between the protein and itself. This, in turn, should yield a longer operational life and exhibit higher performance characteristics (i.e. retention and flux) during membrane filtration of protein solutions. A comprehensive summary of the work that has been performed on membrane filtration with solutions containing various macromolecules including proteins (such as bovine serum albumin, BSA), dextrans, poly(ethylene glycol), and others was previously reviewed.1,3,4 Briefly, it was pointed out that a decline in membrane performance could be attributed to protein adsorption and the influence of pH and ionic strength of the solution, solute buildup at the membrane-solution interface resulting in reduced driving force from an osmotic back-pressure, and buildup and deposition of proteins along with increased fluid viscosity near the membrane surface. These and other phenomenological studies have suggested that, to maximize membrane filtration performance with single pure protein solutions, one should operate (i) away from the pI of the protein,8,9 (ii) at the lowest ionic strength of the solution as possible,10 and (iii) with membrane surface chemistries that are characterized by low protein adsorption.11,12 Since the isoelectric point of a commonly used commercial membrane material such as polyethersulfone is near pH 3 and is negatively charged above this pH,12 operating above the pI of the protein will induce an electrostatic repulsion between the proteins and the membrane surface. Operating well below the pI of the protein may induce electrostatic attraction and denatur(8) Ghosh, R.; Cui, Z. F. J. Membr. Sci. 1998, 139, 17. (9) Kim, K.-J.; Fane, A. G. J. Membr. Sci. 1995, 99, 149. (10) Fane, A. G.; Fell, C. J. D.; Waters, A. G. J. Membr. Sci. 1983, 16, 211. (11) Hannemaaijer, J. H.; Robbertsen, T.; van den Boomgaard, Th.; Gunnink, J. W. Desalination 1988, 68, 93. (12) Nabe, A.; Staude, E.; Belfort, G. J. Membr. Sci. 1997, 133, 57.

10.1021/la000593r CCC: $19.00 © 2000 American Chemical Society Published on Web 11/18/2000

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ation of the protein.13 Also, at low ionic strengths, electrostatic repulsion is maximized. Superimposed upon these electrostatic interactions are the ubiquitous “hydrophobic interactions”. Surfaces that exhibit low protein adsorption are often characterized by a low sessile water drop contact angle. These “hydrophilic” surfaces usually have larger permeation fluxes and a longer operational life than hydrophobic ones. Commercial examples include membrane materials such as regenerated cellulose and hydrophilic additives to the casting solution of polysulfone such as N-vinylpyrrolidinone. Belfort and co-workers14-19 have used modification methods such as low-temperature plasma and ultraviolet with hydrophilic monomer grafting to render the surfaces of membranes more hydrophilic. Protein adsorption decreased, yielding higher fluxes and better separation. The primary goal of this study was to determine the intermolecular adhesion and filtration performance for a hydrophilic membrane in contact with a protein solution and compare the results to those obtained previously for a hydrophobic membrane.1 To do this, we establish correlations between the intermolecular forces and ultrafiltration permeation fluxes (solution and solute) at different pH values of an unmodified thin polysulfone film and membrane and those of a plasma-modified one. 2-Hydroxyethyl methacrylate (HEMA) was grafted onto the membrane surface after low temperature plasma treatment to increase its hydrophilicity. Polysulfone was chosen as the base polymer because it is a commonly used membrane material in industry due to its chemical and structural stability. HEMA modification of the membrane was performed, since it has been studied in the literature as a method for rendering hydrophobic membranes more hydrophilic.16 Because of its well-known properties, hen egg-white lysozyme, Lz, was chosen as the model protein. The work reported here is a natural continuation of our earlier work.1 Also, through molecular measurements, we hope to gain a better understanding of “an extraordinarily complex phenomenon” called protein fouling of synthetic membranes.3 Materials and Methods Chemicals. Potassium hydroxide (99.99%, semiconductor grade), 1,2-dichlorobenzene (99%), methanol (99.9+%, HPLC grade), 2,2-diphenyl-1-picrylhydrazyl (DPPH, 95%), and 2-hydroxyethyl methacrylate (HEMA, 97%, inhibited with 300 ppm methyl ether hydroquinone) were purchased from Aldrich Chemical Co. (Milwaukee, WI). Epoxy resin (EPON 1004) was purchased from Shell Chemical Company (Houston, TX). Nitric acid ampules (0.1 ( 0.001 N analytical concentrate) and RBS pF (surfactant solution) were obtained from J. T. Baker Inc. (Phillipsburg, NJ). Ultrapure, high-resistance (>16 MΩ‚cm) water is prepared by the water system in the Isermann Biochemical Engineering Laboratory in the Ricketts Building, Rensselaer Polytechnic Institute (RPI). The system uses ion exchange, activated carbon adsorption, and reverse osmosis as pretreatment and UV irradiation, ion exchange, and ultrafiltration as the polishing steps. All solutions were made using this water without further treatment. Ruby muscovite mica (Grade 3, Quality V-2) was purchased from S&J Trading Inc. (Glen Oaks, NY). Pure silver wire and tungsten wire evaporation baskets were purchased from Ernest J. Fullam, Inc. (Latham, NY). All of the above chemicals and materials were used without further purification. (13) Anderson, D. E.; Becktel, W. J.; Dahlquist, F. W. Biochemistry 1990, 29, 2403. (14) Ulbricht, M.; Belfort, G. J. Appl. Polym. Sci. 1995, 56, 325. (15) Yamagishi, H.; Crivello, J.; Belfort, G. J. Membr. Sci. 1995, 105, 237. (16) Yamagishi, H.; Crivello, J.; Belfort, G. J. Membr. Sci. 1995, 105, 249.

Koehler et al. The surface forces apparatus (SFA, Mark II) was purchased from Anutech Pty. Ltd. (Canberra, Australia). The glassware for the plasma system, which was designed and built at RPI, was custom-made by Williams Scientific Glassblowing (Montague, MA). The plasma power supply (AM5) and matching network (AMNPS-2A) were purchased from RF Plasma Products, Inc. (Marlton, NJ). The thin channel filtration cell (TCF-2) was donated by Amicon Inc. (now Amicon Div. Millipore Corp., Beverly, MA). Protein. Hen egg-white lysozyme (Lz, E.C. 3.2.1.17) was purchased from Pharmacia Biotech Inc. (Piscataway, NJ). Lysozyme was chosen for these experiments because it is not readily denatured but still can be described as being relatively soft as defined by the adiabatic compressibility, βS. Lz has an intermediate adiabatic compressibility value of 4.67 × 10-12 cm2/ dyn, which is between that for a rigid protein such as cytochrome c (βS ) 0.66 × 10-12 cm2/dyn) and that for a flexible protein such as bovine serum albumin (βS ) 10.5 × 10-12 cm2/dyn). Lysozyme is a prolate ellipsoid with dimensions 30 × 30 × 45 Å3. It has a molecular mass of 14,400 Da with 129 amino acids and 4 disulfide bonds. The isoelectric point of Lz is approximately at pH 11.0. From the space-filled model of the Lz crystal (not shown), it can be seen that Lz has both hydrophobic and hydrophilic residues exposed to the solvent. Film and Membrane. Both the films and membranes were made of polysulfone (PSf), the structure of which is given by

The polysulfone membranes (gr81pp) were obtained from Dow Danske Separation Systems (now called Danish Separation Systems A/S, Nakskov, Denmark) and had a nominal molecular weight cutoff of 6 kD. The polysulfone film was cast from a 3% (w/v) solution of polysulfone (Udel 3500, Union Carbide, Danbury, CT) in 1,2-dichlorobenzene. Since this solvent both wetted mica and dissolved polysulfone, stable films attached to the mica were obtained for the force measurements. A drop of this solution was placed on mica that had been glued on the SFA lens and then spun at a rate of 500 rpm for 5 s on a spin-coating apparatus (Photoresist spinner, Headway Co., Garland, TX) to evenly distribute the solution onto the mica. The speed was immediately increased to 5000 rpm for 40 s to finish smoothing out the film. The film was then dried in a convection oven for 2 h at 80 °C. The films were formed reproducibly with a thickness of 430 ( 15 Å, as measured by ellipsometry (Rudolph Ellipsometer, Fairfield, NJ). For the ellipsometry measurements, the polysulfone was spun onto a silicon wafer for easy thickness determination. Surface Characterization. The films and membranes were studied by attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) and atomic force microscopy (AFM) to determine the chemical nature and morphology of the surfaces, respectively. ATR-FTIR was used to confirm that both surfaces were chemically similar. AFM was used to determine whether the film surface was smooth enough to be used in the SFA. The ATR-FTIR spectra were taken using a 45° germanium crystal on a Nicolet Magna-IR 550 Spectrometer Series II with an Auxiliary Experiment Module (Madison, WI). The AFM scans were performed in noncontact mode using an ultralever with a 0.2 µm tip length on an Autoprobe CP (Park Scientific Instruments, Sunnyvale, CA). The probe tip was made from Si3N4 and had a diameter of approximately 100 Å. The surfaces of the films and the membranes were characterized by captive bubble contact angle measurements using a bubble of air in water underneath an inverted film or membrane.1 Peroxide Determination. The amount of peroxide created on the surface from the plasma modification and air treatment of the surface was measured by a colorimetric assay. DPPH is a free radical which is deep purple in solution and which turns yellow when fully reacted. The amount of radicals on the surface of the plasma-modified membranes was determined by the loss of absorbance at 520 nm. At this wavelength, the fully reacted

Protein-Polysulfone Film Intermolecular Forces

Figure 1. Typical flux-time curve showing flux designations used for filtration experiments. Jw1 is the steady-state buffer flux for the clean membrane (b), Jp is the flux of the protein solution at the end of the protein step (9), and Jw2 is the steadystate buffer flux of the fouled membrane (2). The solute flux is given by the open squares (0). DPPH has an extinction coefficient of 0 L/mol‚cm, which means that the product of the assay does not interfere with the absorbance at 520 nm. Membrane samples that had been plasma modified were placed into a 0.1 mM solution of DPPH in methanol that had been deaerated using ultrahigh purity nitrogen. The radicals were allowed to react at 50 °C for 1 h before the loss in absorbance was measured. Plasma Modification of Surfaces. The polysulfone film or membrane was placed in the plasma apparatus in the reaction chamber. The system was purged with ultrahigh purity helium gas and subsequently evacuated to 0.1750 Torr. Once the desired pressure was obtained, the plasma was generated using a frequency of 13.67 MHz at a power of 25 W for 1 min.14 After the plasma activation of the surface, it was removed from the system and placed in the air for 10 min for peroxide and radical formation. The surface was then placed in a 30% (v/v) HEMA solution in water that had been deaerated with ultrahigh purity nitrogen. The inhibitor was not removed from the HEMA prior to its use, since it inhibits only thermal and ultraviolet polymerization, but not radical polymerization.20 The grafting reaction was performed for 1 h at 50 °C, after which the surface was placed in a large volume of 50% (v/v) ethanol in a water solution for 1 h at 50 °C to stop the polymerization and remove any unreacted HEMA monomer. The HEMA-grafted polysulfone material (film or membrane) is designated HEMA/PSf or HEMA-g-PSf. Filtration Experiments. The filtration cell was cleaned by rinsing several times with a sodium hydroxide solution (pH 12), a hydrochloric acid solution (pH 2), a sodium hydroxide solution (pH 12), and then finally ultrapure water. The original membranes were rinsed to remove their storage solution and then wetted with a 50% (v/v) ethanol in water solution. The HEMAmodified membranes were used immediately after grafting. The membrane was then placed in the cell and filled with a solution containing a mixture of 10-2 M KOH and 10-2 M HNO3. The amounts of the two solutions combined varied depending on the pH of the final solution desired. This allowed solutions of varying pH values with the same ionic strength. The solution was pressured through the membrane at 0.3 MPa nitrogen pressure with a 1 m/s cross-flow velocity for 5 h to allow for equilibration of the membrane. The flux was seen to level off in less than 5 h, so this time was chosen to ensure that all membranes were at a pseudoequilibrium state. This “steady-state” flux was denoted as Jw1 (Figure 1). The salt solution was drained from the cell, and 10 mL of buffer solution was reintroduced to the cell while the cross-flow was maintained. This procedure was necessary so that, upon introduction of the protein solution into the system, it would (17) Ulbricht, M.; Belfort, G. J. Membr. Sci. 1996, 111, 193. (18) Chen, C.; Belfort, G. J. Appl. Polym. Sci., in press. (19) Pieracci, J.; Crivello, J. V.; Belfort, G. J. Membr. Sci., in press. (20) Gineste, J.-L.; Garaud, J.-L.; Pourcelly, G. J. Appl. Polym. Sci. 1993, 48, 2113.

Langmuir, Vol. 16, No. 26, 2000 10421 not come into contact with the membrane under static conditions, a condition which promotes adsorption. Two hundred millileters of lysozyme (50 mg/L) solution was added to the cell, and then, depending on the experiment, either the pressure was reapplied until 50 mL of solution had permeated or no pressure was applied and the solution was recirculated across the membrane surface for 2 h, the approximate amount of time required for 50 mL of permeate to be collected. In some cases, 50 mL of permeate could not be collected, since the membrane was extensively fouled. The flux at the end of the 50 mL of permeate collection was denoted as Jp. The protein solution was drained from the system, and the cell was rinsed two times with the buffer solution before being refilled with the buffer. The pressure was once again applied, and the flux was measured. The “steady-state” flux for this time period was denoted as Jw2. The solute flux was obtained from the product of the permeate concentration, Cp, and the solution volume flux, Jp. Surface Forces Measurements. The SFA was used to measure intermolecular forces between the two layers: one an adsorbed layer of lysozyme and the other a thin film of HEMAgrafted polysulfone or another adsorbed layer of lysozyme. The layers were adsorbed onto mica that had been glued to a halfcylindrical silica lens. The distance between the two layers was determined by interferometry. This separation was used with a known spring force constant to give the forces. The method has been described previously.21 It has been used to measure adhesion andforcesbetweeninorganicsurfaces,22,23 proteins,24-33 surfactants,34-47 polymers,27,38-40 glycolipids,41 biological ligands,42,43 and thin hydrophobic surfaces.1,44-50 In our experiments, the surfaces were submerged in a 10-2 M KOH/HNO3 solution in a Teflon bath. The ionic strength of the solution remained the same while the (21) Israelachvili, J. N.; Adams, G. E. J. Chem. Soc., Faraday Trans. 1978, 74, 975. (22) Ke´kicheff, P.; Ninham, B. W. Europhys. Lett. 1990, 12, 471. (23) Ducker, W. A.; Xu, Z.; Clarke, D. R.; Israelachvili, J. N. J. Am. Ceram. Soc. 1994, 77, 437. (24) Afshar-Rad, T.; Bailey, A. I.; Luckham, P. F.; MacNaughtan, W.; Chapman, D. Biochim. Biophys. Acta 1987, 915, 101. (25) Lee, C. S.; Belfort, G. Proc. Natl. Acad. Sci. U.S.A. 1989, 86, 8392. (26) Belfort, G.; Lee, C. S. Proc. Natl. Acad. Sci. U.S.A. 1991, 88, 9146. (27) Luckham, P. F.; Ansarifar, M. A. Br. Polym. J. 1990, 22, 233. (28) Gallinet, J.-P.; Gauthier-Manuel, B. Eur. Biophys. J. 1993, 22, 195. (29) Blomberg, E.; Claesson, P. M.; Tilton, R. D. J. Colloid. Interface. Sci. 1994, 166, 427. (30) Leckband, D. E.; Schmitt, F.-J.; Israelachvili, J. N.; Knoll, W. Biochemistry 1994, 33, 4611. (31) Nylander, T.; Ke´kicheff, P.; Ninham, B. W. J. Colloid Interface Sci. 1994, 164, 136. (32) Pincet, F.; Perez, E.; Belfort, G. Macromolecules 1994, 27, 3424. (33) Pincet, F.; Perez, E.; Belfort, G. Langmuir 1995, 11, 1229. (34) Christenson, H. K.; Claesson, P. M.; Parker, J. L. J. Phys. Chem. 1992, 96, 6725. (35) Tsao, Y.-H.; Yang, S. X.; Evans, D. F. Langmuir 1992, 8, 1188. (36) Waltermo, A.; Sjo¨berg, M.; Anhede, B.; Claesson, P. M. J. Colloid Interface Sci. 1993, 156, 365. (37) Mao, G.; Tsao, Y.-H.; Tirrell, M.; Hessel, V.; van Esch, J.; Ringsdorf, H.; Davis, H. T. Langmuir 1995, 11, 942. (38) Argillier, J.-F.; Ramachandran, R.; Harris, W. C.; Tirrell, M. J. Colloid Interface Sci. 1991, 146, 242. (39) Kuhl, T. L.; Leckband, D. E.; Lasic, D. D.; Israelachvili. Biophys. J. 1994, 66, 1479. (40) Mangipudi, V.; Pocius, A. V.; Tirrell, M. Langmuir 1995, 11, 19. (41) Luckham, P.; Wood, J.; Swart, R. J. Colloid Interface Sci. 1993, 156, 173. (42) Leckband, D. E.; Kuhl, T.; Wang, H. K.; Herron, J.; Mu¨ller, W.; Ringsdorf, H. Biochemistry 1995, 34, 11467. (43) Sivasankar, S.; Subramanian, S.; Leckband, D. PNAS USA 1998, 95, 12961. (44) Wood, J.; Sharma, R. J. Adhesion Sci. Technol. 1995, 9, 1075. (45) Wood, J.; Sharma, R. Langmuir 1995, 11, 4797. (46) Ducker, W. A.; Xu, Z.; Israelachvili, J. N. Langmuir 1994, 10, 3279. (47) Claesson, P. M.; Christenson, H. K. J. Phys. Chem. 1988, 92, 1650. (48) Herder, P. C. J. Colloid Interface Sci. 1990, 134, 336. (49) Yoon, R.-H.; Flinn, D. H.; Rabinovich, Y. I. J. Colloid Interface Sci. 1997, 185, 363. (50) Yoon, R.-H.; Ravishankar, S. A. J. Colloid Interface Sci. 1996, 179, 391.

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Figure 2. ATR-FTIR spectra of the HEMA-modified (a) polysulfone film (Udel 3500) and (b) membrane (gr81pp). Spectra taken with 45° Ge crystal. pH of the solution was changed from experiment to experiment by mixing proper amounts of 10-2 M KOH or 10-2 M HNO3. The room temperature was controlled at 21 ( 1 °C. Details of the SFA used in this study have been given previously.25-26 The hard wall interaction was chosen as the zero distance reference point for all of the SFA measurements so that all of the results could be easily compared. Because of their pores, membranes are generally opaque and cannot be used in the SFA. For this reason, we use a thin transparent spin-coated smooth nonporous film of the same material as the membrane for the force measurements.

Results and Discussion Characterization of Polymer Surfaces. The DPPH peroxide assay was performed to determine if an upper limit on plasma time existed. Since ablation of the product occurred at the same time as radical creation, a balance needed to be created so that the creation of radicals was greater than their loss. Plasma was generated for times of up to 2 min at 25 W. Over this time frame, peroxide formation increased with time (data not shown). Therefore, the rate of peroxide formation was greater than the loss of the radicals at 1 min, which was the plasma reaction time chosen for our experiments. To compare the force-distance measurements using HEMA-grafted polysulfone films with the filtration results using HEMA grafted onto commercial polysulfone membranes, it was necessary to show that the films and membranes were similar in chemical composition. To this end, ATR-FTIR was used to probe the chemical nature of the surfaces at penetration depths of approximately 0.51.0 µm. Gross estimates of the surface polarity were determined by contact angle measurements. The ATR-FTIR scans for a HEMA-modified polysulfone membrane and film are shown in Figure 2. The two scans confirm that the membrane and film were made of essentially the same material. The aliphatic CH3 stretching is seen from 3000 to 2800 cm-1, and the aromatic CH stretching appears at 3100-3000 cm-1. The strong aromatic ether peak can been seen at ∼1240 cm-1. The symmetric and asymmetric sulfone stretching can be seen at ∼1160 and ∼1328 cm-1, respectively. The HEMA modification of the film and membrane yielded a representative carbonyl peak at 1720 cm-1. The fingerprint region (1600-800 cm-1) also shows good correlation between the two samples. Figure 3a shows the AFM scan of the polysulfone film that was spun on a flat piece of mica and subsequently modified with HEMA. The modified film had an average roughness of 1.5 Å and an RMS

Figure 3. AFM scans of HEMA-modified film and membrane taken in noncontact mode. The cantilever was a 0.2 µm ultralever. The probe tip was Si3N4 and had a tip diameter of approximately 100 Å. (a) Polysulfone film: median height 18 Å, RMS roughness 1.9 Å, average roughness 1.5 Å. (b) Polysulfone membrane: median height 16 Å, RMS roughness 1.6 Å, average roughness 1.3 Å.

(root-mean-square) roughness of 1.9 Å with a median height of 18 Å. These values were smooth enough for SFA measurements. The AFM image of the HEMA-modified gr81pp polysulfone membrane is shown in Figure 3b. The modified membrane had an average roughness of 1.3 Å and an RMS (root-mean-square) roughness of 1.6 Å with a median height of 16 Å. The differences between the film and membrane dimensions are not considered significant. The hysteresis values between the advancing and receding angles for both the modified film and the modified membrane were similar and close to 50° (Table 1). This value is relatively large in comparison to that obtained for the unmodified film and membrane of 18-23°. From the AFM measurements, the polymer surfaces were relatively smooth despite the low temperature plasma treatment and graft polymerization; thus, roughness was not likely responsible for the hysteresis. Likewise, surface restructuring was possible although unlikely, since large polymerized HEMA groups were grafted onto the surface and would have difficulty reverting back into the bulk polymer unless the glass transition temperature was

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Table 1. Captive Air Bubble Contact Angle Measurements (deg) in Watera air/water material

static

receding

advancing

PSU (membrane)b PSU (film)b HEMA/PSU (membrane) HEMA/PSU (film)

99 ( 4 95 ( 1 52 ( 3 53 ( 3

105 ( 4 106 ( 2 80 ( 3 84 ( 2

82 ( 4 88 ( 2 27 ( 3 34 ( 2

a See the text and Ulbricht and Belfort14 for a description of the method. b Taken from Koehler et al.1

sufficiently reduced near the surface due to the presence of HEMA. Also, due to the nature of the graft polymerization, patches of different polarity and hydrophilicity may have been formed. Patchiness of a surface is known to increase hysteresis via contact angle measurements.51 The static contact angle was about halfway between the advancing and receding contact angle values for the modified film and membrane, as expected, since the static contact angle is a combination of the advancing and receding effects. Yoon et al.49,50 have reported that hydrophobic interactions became significant when the contact angle between similar surfaces in water increases above 90°. Grafting HEMA onto the PSf surface reduces the static contact angles from 95-99 ( 4° to 52-53 ( 3° (Table 1). This modification of the PSf surface should reduce the attractive interaction previously observed between Lz and the unmodified PSf surface in our earlier work.1 The similarity between the ATR-FTIR and contact angle results for the modified film and membrane indicates that the two surfaces are chemically similar. Therefore, it is reasonable to compare the intermolecular forces obtained from the HEMA-modified polymer film with the ultrafiltration measurements using the HEMA-grafted membrane. In the SFA, the contact area was about 1 µm2. These dimensions were much larger than the height fluctuations of the modified film surface as measured by AFM; therefore, the modification still gave a surface that was smooth enough for intermolecular force measurements using the SFA. Force Measurements of HEMA/PSf-Mica. To ensure that the HEMA-grafted PSf film was stable in the SFA and did not slide off the curved mica surface, the interaction between the HEMA-grafted PSf surface and mica was measured at all four pH values. Since the compression curves followed a straight line on a semilog plot and the decompression curve showed little or no hysteresis compared to the compression curve (data not shown), the film was considered stable and well formed. The slopes were similar for all four of the pH values studied, indicating that the films’ characteristics were similar at all of the pH values studied. The curves showed repulsion at long distances, suggesting electrostatic repulsion between the mica and modified polymer. Force Measurements of HEMA/PSf-Lz. Next, Lz was adsorbed onto the mica surface opposing the polymer surface at the respective pH values and the HEMA/PSfLz interactions were measured (Figure 4). In all cases, attractive interactions were not observed when the two surfaces were brought into close proximity. This is in contrast to our previous result where a small attraction (