pubs.acs.org/Langmuir © 2009 American Chemical Society
Influence of Zeolite Crystal Size on Zeolite-Polyamide Thin Film Nanocomposite Membranes Mary L. Lind,† Asim K. Ghosh,† Anna Jawor,† Xiaofei Huang,† William Hou,‡ Yang Yang,‡ and Eric M. V. Hoek*,† †
Department of Civil and Environmental Engineering and California NanoSystems Institute and ‡Department of Materials Science and Engineering, University of California, Los Angeles (UCLA), Los Angeles, California Received March 17, 2009. Revised Manuscript Received May 24, 2009
Zeolite-polyamide thin film nanocomposite membranes were coated onto polysulfone ultrafiltration membranes by interfacial polymerization of amine and acid chloride monomers in the presence of Linde type A zeolite nanocrystals. A matrix of three different interfacial polymerization chemistries and three different-sized zeolite crystals produced nanocomposite thin films with widely varying structure, morphology, charge, hydrophilicity, and separation performance (evaluated as reverse osmosis membranes). Pure polyamide film properties were tuned by changing polymerization chemistry, but addition of zeolite nanoparticles produced even greater changes in separation performance, surface chemistry, and film morphology. For fixed polymer chemistry, addition of zeolite nanoparticles formed more permeable, negatively charged, and thicker polyamide films. Smaller zeolites produced greater permeability enhancements, but larger zeolites produced more favorable surface properties; hence, nanoparticle size may be considered an additional “degree of freedom” in designing thin film nanocomposite reverse osmosis membranes. The data presented offer additional support for the hypothesis that zeolite crystals alter polyamide thin film structure when they are present during the interfacial polymerization reaction.
1. Introduction Fresh water is perhaps the most important natural resource for human survival. Water itself is a human necessity, but water availability limits food production, industrial productivity, energy production, and the global economy. Demand is rapidly approaching the available fresh water supply, and hence, purification of nontraditional water sources is gaining popularity.1 Processes based on commercially available reverse osmosis (RO) membranes are capable of producing high-quality water from water sources such as seawater, brackish groundwater, and wastewater. However, RO membranes with higher water permeability, improved contaminant selectivity, and better fouling resistance are needed to reduce the operating costs, chemical consumption, and energy demand of producing high-quality water from these alternative sources. Producing better RO membranes has proven challenging following traditional polymer chemistry approaches. The concept of a mixed-matrix membrane, where a small filler material is dispersed throughout a larger polymeric matrix, has brought new degrees of freedom to the development of advanced membrane materials for numerous separation processes.2 These novel materials often have improved mechanical, chemical, and thermal stability, as well as enhanced separation, reaction, and sorption capacity.2-5 Better membranes have been developed for gas separation, pervaporation, ion exchange, and fuel cell *Corresponding author address: Civil & Environmental Engineering Department 5732-G Boelter Hall, P.O. Box 951593, University of California, Los Angeles, Los Angeles, CA, 90095, USA. Tel: (310) 206-3735. Fax: (310) 206-2222. E-mail:
[email protected]. (1) Elimelech, M. J. Water Supply Res. Technol.-Aqua 2006, 55(1), 3–10. (2) Koros, W. J. AIChE J. 2004, 50(10), 2326–2334. (3) Behera, D.; Banthia, A. K. Polym.-Plast. Technol. Eng. 2007, 46(10-12), 1181–1186. (4) Holt, J. K.; Park, H. G.; Wang, Y. M.; Stadermann, M.; Artyukhin, A. B.; Grigoropoulos, C. P.; Noy, A.; Bakajin, O. Science 2006, 312(5776), 1034–1037. (5) Rittigstein, P.; Priestley, R. D.; Broadbelt, L. J.; Torkelson, J. M. Nat. Mater. 2007, 6(4), 278–282.
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applications by exploiting zeolite or carbon molecular-sieve particles dispersed within relatively thick membrane films.6-9 A goal for most mixed-matrix membrane materials is to incorporate enough of the nanophase material to achieve a “percolation threshold”, which describes a preferential flow path through the nanophase from the feed to permeate the side of the membrane.10 Recently, a new class of mixed-matrix membranes was discovered and evaluated for potential application to reverse osmosis separations. Thin film nanocomposite (TFN) membranes were produced by interfacial polymerization of diamine and acid chloride monomer solutions containing Linde type A (LTA) zeolite nanocrystals.11 A key difference between thin film nanocomposites and other mixed-matrix membranes is that the size of the zeolite molecular sieve particles was designed to match the expected polyamide film layer thickness, thereby providing a preferential flow path through each particle incorporated into the membrane. Later, silverexchanged zeolite-polyamide nanocomposites exhibited antibacterial properties in addition to higher flux and similar rejection relative to conventional polyamide composite membranes.11,12 Other researchers have evaluated TFN membranes produced with silver, silica, and titania nanoparticles.11,13-15 In this paper, we explore the (6) Baglio, V.; Arico, A. S.; Di Blasi, A.; Antonucci, P. L.; Nannetti, F.; Tricoli, V.; Antonucci, V. J. Appl. Electrochem. 2005, 35(2), 207–212. (7) Sholl, D. S.; Johnson, J. K. Science 2006, 312(5776), 1003–1004. (8) Tin, P. S.; Chung, T. S.; Jiang, L. Y.; Kulprathipanja, S. Carbon 2005, 43(9), 2025–2027. (9) Won, J. G.; Seo, J. S.; Kim, J. H.; Kim, H. S.; Kang, Y. S.; Kim, S. J.; Kim, Y. M.; Jegal, J. G. Adv. Mater. 2005, 17(1), 80. (10) Koros, W. J.; Mahajan, R. J. Membr. Sci. 2000, 175(2), 181–196. (11) Jeong, B. H.; Hoek, E. M. V.; Yan, Y. S.; Subramani, A.; Huang, X. F.; Hurwitz, G.; Ghosh, A. K.; Jawor, A. J. Membr. Sci. 2007, 294(1-2), 1–7. (12) Lind, M. L.; Jeong, B.-H.; Subramani, A.; Huang, X.; Hoek, E. M. V. J. Mater. Res. 2009, 24(5), 1624–1631. (13) Lee, S. Y.; Kim, H. J.; Patel, R.; Im, S. J.; Kim, J. H.; Min, B. R. Polym. Adv. Technol. 2007, 18(7), 562–568. (14) Lee, H. S.; Im, S. J.; Kim, J. H.; Kim, H. J.; Kim, J. P.; Min, B. R. Desalination 2008, 219(1-3), 48–56. (15) Singh, P. S.; Aswal, V. K. J. Colloid Interface Sci. 2008, 326(1), 176–185.
Published on Web 06/15/2009
DOI: 10.1021/la900938x
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combined effects of zeolite particle size and polymerization chemistry on the apparent structure, morphology, interface, and permeability of zeolite-polyamide thin film nanocomposite membranes.
2. Experimental Section 2.1. Nomenclature. Nomenclature used in this manuscript is as follows. Chemicals used in the synthesis of the thin polyamide film in the aqueous phase included meta-phenylene diamine (MPD), the salt of triethyl amine (TEA) and (+)-10-camphor sulfonic acid (CSA), sodium lauryl sulfate (SLS), and isopropyl alcohol (IPA). In the organic phase trimesoyl chloride (TMC), Isoparrafin-G (isopar), and Linde type A (LTA) zeolites were used. The chemical formula for fully hydrated LTA is Na12[(AlO2)12 (SiO2)12] 3 27H2O. Solute rejections were tested using sodium chloride (NaCl), magnesium sulfate (MgSO4), and poly(ethylene glycol) with a molecular weight of 200 Da (PEG 200). Thin film composite (TFC) membranes are pure polyamide thin films without nanoparticles, whereas thin film nanocomposite (TFN) membranes are polymerized with nanoparticles present in the isopar-TMC monomer solution. Membranes are designated as follows: thin film type-monomer chemistry-zeolite type; hence, TFN-A-1 refers to a thin film nanocomposite membrane formed using monomer chemistry A and zeolite crystal 1. 2.2. Zeolite Synthesis and Characterization. Colloidal crystals of Linde type A zeolites were prepared in the sodium form (NaA) by a hydrothermal synthesis using a microwave heating system.11,16 A detailed description of both synthesis and characterization is presented elsewhere.11 Zeolite crystal size was determined by dynamic light scattering (DLS) in deionized water (BI-90 Plus; Brookhaven Instruments Corporation; Holtsville, NY, USA). Zeolite crystal zeta potentials were calculated from electrophoretic mobility measurements (ZetaPALS, Brookhaven Instruments Corp.) in 10 mM NaCl solution at unadjusted pH (∼5.8). Pure water contact angles were measured on zeolite films grown on a steel substrate. Physical-chemical properties of the three zeolites used here were previously determined as part of a larger, more systematic study on zeolite synthesis; relevant properties are reproduced in Table 1 for quick reference.16 By varying synthesis temperature and time, average particle diameters were controlled to produce crystals of ∼100 nm, ∼200 nm, and ∼300 nm. Zeolite crystal sizes were selected to span the expected thicknesses of pure polyamide thin films. Also, Linde type A zeolites in the sodium form have pore diameters of approximately 4.2 A˚, are superhydrophilic, and are much more negatively charged than a typical polyamide RO membrane.11,17 The diameters of hydrated sodium and chloride ions are approximately 8-9 A˚; hence, the LTA zeolite is a good candidate to separate salt from water by molecular sieving.
2.3. Polyamide and Zeolite-Polyamide Film Formation. Pure polyamide and zeolite-polyamide nanocomposite thin films were hand-cast on commercially fabricated polysulfone (PSf) ultrafiltration membranes (NanoH2O Inc.; Los Angeles, CA, USA) using the three polymerization chemistries presented in Table 2. The specific chemistries were selected to produce flux and rejection combinations similar to commercial seawater, brackish water, and high-flux RO membranes that were previously characterized following the same experimental method described below.11 In another prior study, pure polyamide films produced by chemistry A exhibited brackish water RO membrane separation performance.18 In chemistry B, similar monomer concentrations were used, but IPA was added to the aqueous-MPD solution to enhance miscibility of the aqueous and organic media, which increases reaction zone thickness and produces less dense, more permeable films.19 Alternatively, chemistry C employed higher (16) Jawor, A.; Jeong, B. H.; Hoek, E. M. V., J. Nanopart. Res. 2008, submitted. (17) Li, Y.; Chung, T. S.; Kulprathipanja, S. AIChE J. 2007, 53(3), 610–616. (18) Ghosh, A. K.; Jeong, B. H.; Huang, X. F.; Hoek, E. M. V. J. Membr. Sci. 2008, 311(1-2), 34–45. (19) Ghosh, A. K.; Hoek, E. M. V. J. Membr. Sci. 2009, 336(1-2), 140–148.
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Lind et al. Table 1. Physical-Chemical Properties of Zeolite Crystals zeolite crystal
dha (nm)
ζa (mV)
θwa,b (°)
1 2 3
97 212 286
-45.9 -50.1 -52.2
2 2 2
a Data previously reported by Jawor et al.16 b Determined for LTA zeolite films grown on steel substrate in Jawor et al.16
Table 2. Polymerization Chemistries Used to Cast TFC and TFN Membranes MPD TEACSA SLS IPA TMC monomer chemistry (% w/v) (% w/v) (% w/v) (% w/v) (% w/v) A B C
2.3 2.5 3.2
6.6 6.6 4.5
0.02 0.02 0.02
0 20 0
0.10 0.10 0.13
concentrations of MPD and TMC (without IPA) to produce relatively thick, less permeable films. Polyamide thin films were hand-cast as follows. A PSf support membrane was immersed for 15 s in a mixture of meta-phenylene diamine (MPD, >99%, Sigma-Aldrich, St. Louis, MO, USA), the salt of triethyl amine (TEA, liquid, 99.5%; Sigma-Aldrich) and (+)-10-camphor sulfonic acid (CSA, powder, 99.0%; SigmaAldrich), sodium lauryl sulfate (SLS, Sigma-Aldrich), and (for chemistry C) isopropyl alcohol (IPA, 99%, Fisher Scientific, Pittsburgh, PA, USA). The TEA-CSA mixture was produced as previously described by Ghosh et al.18 Excess aqueous-MPD solution was removed from the support membrane surface with a custom fabricated air knife. The PSf membrane was then immersed in a solution of trimesoyl chloride (TMC, 98%, SigmaAldrich) in Isopar-G (Gallade Chemicals; Santa Ana, CA, USA), referred to hereafter as “isopar”. After 15 s of reaction, the membrane was removed from the TMC solution. Zeolite-polyamide nanocomposite thin films were cast exactly as described above, except that 0.2% (w/v) of zeolite colloidal crystals dispersed in the isopar-TMC solution. Good dispersion was obtained by ultrasonication for 40 min immediately prior to the interfacial polymerization reaction. The ultrasonication bath temperature was maintained constant at 20 °C by addition of ice. The resulting composite membranes were heat-cured at 82 °C for 10 min, washed thoroughly with deionized water, and stored in light-proof containers filled with deionized water at 5 °C. 2.4. Membrane Separation Performance. The permeability of synthesized thin films was evaluated for pure water, as well as sodium chloride (ACS grade NaCl, Fisher Scientific), magnesium sulfate (ACS grade MgSO4, Fisher), and poly(ethylene glycol) of molecular weight 200 Da (PEG200, Sigma-Aldrich) solutions using a stirred, dead-end filtration cell (HP4750 Stirred Cell, Sterlitech Corp., Kent, WA). The membranes were washed thoroughly for at least 45 min under a pressure of 1.55 MPa, until the membranes were fully compacted and the flux reached a steady state. The volume of permeate collected over 30 min divided by the membrane area (13.8 cm2) and time produced the reported value for permeate flux. The feedwater was changed sequentially to 2000 ppm solutions of (a) NaCl, (b) MgSO4, and (c) PEG200; permeate flux was determined as above and permeate solute concentrations were determined from conductivity (NaCl/MgSO4) or total organic carbon (PEG200) analyses. Between each different feed solution, the membranes were rinsed with DI water for 45 min at 1.55 MPa. Intrinsic solvent (water) permeability, A (=Jv/4p=Qp/Am4p) was determined from the permeate flow (Qp) of deionized water through a given area of membrane (Am) measured for the specified applied pressure (4p). Here, Jv (=Qp/Am) is the volumetric flux through the membrane.20 Observed solute rejections, Xs (=1 - cp/cf), (20) Mulder, J. Basic Principles of Membrane Technology, 2nd ed.; Kluwer Academic Publishers: Dordrecht, 1996; pp 487-488.
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Lind et al. were determined from the difference in feed (cf) and permeate (cp) solute concentrations. The solute permeability coefficient, B (=Jv(1 - Xs)/Xs), was then determined from the flux and rejection as previously described.20 A characteristic pore dimension for the membranes was estimated from PEG200 rejection data through a simple steric-exclusion pore transport model. According to this model, rejection is solely a function of the solute radius, rs, and pore radius, rp, according to Xs=[λ(λ - 2)]2, where λ is the ratio rs/rp.18 The hydrodynamic radius of PEG200 was determined to be 3.5 A˚ from the correlation, rs = 0.184 Mw0.557, derived by fitting previously published molecular dynamic simulations with a power law regression.21 Using observed rejections overestimates the pore size by an amount proportional to the extent of concentration polarization. Also, the steric model neglects many of the known mechanisms for transport through RO membranes. Hence, the pore sizes reported herein should not be taken literally. They are used to illustrate relative differences in characteristic pore dimensions for polyamide and zeolite-polyamide thin films. 2.5. Membrane Structure and Morphology. The film elemental content was assessed by X-ray photoelectron spectroscopy (XPS) using Omicrometer Multiprobe Surface Science Systems with Al KR (1486.6 eV). The X-ray gun was operated at 15 kV and 20 mA. An electron gun was employed during the measurement to reduce charging effects in insulating materials such as polymers. The spectra were taken with takeoff angle of 90° to give a sampling depth of ∼60 A˚.22 Typically, the O/N ratio is used to evaluate the extent of cross-linking in polyamide thin films.23 However, this approach must be modified when considering zeolite-polyamide nanocomposite thin films, because there is excess oxygen from the zeolite. Therefore, relative extent of cross-linking was estimated from the C/N ratio, which increases with the extent of cross-linking. Membrane surface morphology was characterized by scanning electron microscopy, SEM, (S-4700 Hitachi High Technologies America, Inc.) at an accelerating voltage of 10 kV, a magnification of 25 000, and a working distance of 12 mm. Surface roughness was quantified by atomic force microscopy, AFM, (Nanoscope IIIa; Digital Instruments, Santa Barbara, CA, USA). All surface roughness statistics were recorded, but only rms roughness and surface area difference were reported. Characteristic film thickness was estimated as two times the AFM-derived rms roughness value, which in past studies correlated strongly with visual quantifications of hand-cast TFC and TFN film thicknesses by investigating cross-sectional TEM images.11,12,18,19 This thickness is not the effective path length for permeation, because the estimated film thickness does not account for differences in basal film thickness or filling of the support membrane pores. 2.6. Membrane Interfacial Properties. Interfacial charge and hydrophilicity of hand-cast films were determined from zeta potential and contact angle analyses. Streaming potential measurements were performed in 10 mM NaCl solutions at unadjusted pH of 5.8 ( 0.2 (ZetaPals, Brookhaven Instruments Corp.). Zeta potentials of hand-cast membranes were determined from measured streaming potential data using the Helmholtz-Smoluchowski equation. Sessile drop contact angles of deionized water were measured on air-dried samples of synthesized membranes in an environmental chamber mounted to the contact angle goniometer (DSA10, KR€ uSS). The equilibrium value was the steadystate average of left and right angles. The data reported are the averages of 12 measurements on at least 3 different membrane
(21) Lee, H.; Venable, R. M.; MacKerell, A. D.; Pastor, R. W. Biophys. J. 2008, 95(4), 1590–1599. (22) Kim, S. H.; Kwak, S. Y.; Suzuki, T. Environ. Sci. Technol. 2005, 39(6), 1764–1770. (23) Tang, C. Y. Y.; Kwon, Y. N.; Leckie, J. O. J. Membr. Sci. 2007, 287(1), 146– 156.
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Article samples with the high and low values discarded before averaging and computing standard deviations.
3. Results and Discussion 3.1. Separation Performance of TFC and TFN Membranes. Figure 1 presents (a) pure water flux and (b) NaCl rejection for all hand-cast membranes. In every scenario, TFN membrane permeability was higher than that of the base TFC membrane of the same chemistry. For a fixed polymer chemistry, the flux enhancement was inversely correlated with the characteristic size of the zeolite crystal. Membranes TFN-A-1, -2, and -3 exhibited 47%, 27%, and 24% increase in flux over TFC-A. Membranes TFN-B-1, -2, and -3 exhibited 28%, 10%, and 1% increase in flux over TFC-B. Membranes TFN-C-1, -2, and -3 exhibited 1908%, 1583%, and 435% increase in flux over TFC-C. We previously hypothesized that the high charge density, superhydrophilicity, and internal porosity of zeolite molecular sieves provides preferential flow paths for water molecules through nanocomposite thin films, while producing excellent salt rejection and more fouling resistant membrane surfaces.11 Herein, all membranes were formed using identical mass concentration of nanoparticles added to the isopar-TMC solutions; however, for a given mass concentration the number concentration of particles scales inversely with particle size cubed. On the basis of average LTA crystal sizes, there were approximately 25.6 and 2.46 times the number of particles in TFN-A/B/C-1 and TFN-A/B/C-2 membranes, respectively, as there were in TFN-A/B/C-3 precursor solutions. It follows that proportionally as many particles, and hence preferential flow channels, were incorporated into each of the nanocomposite films. Analysis of TEM and SEM images (not shown) was inconclusive, because there were no particles found in many of the small cross sections of nanocomposite thin films imaged. The order of flux enhancement (TFN-A/B/C-1>TFNA/B/C-2>TFN-A/B/C-3>TFC-A/B/C) supports the expected trend, but does not mechanistically prove the preferential flow path hypothesis. In polymerization chemistry B, the addition of 20% IPA to the aqueous amine solution acts to increase the miscibility of aqueous and organic media during the interfacial polymerization reaction, which increases the breadth of the reaction zone and produces a less dense, more permeable film with greater roughness. Miscibility enhancements have been previously shown to increase membrane flux and roughness, while having little impact on the rejection.22 From the flux data presented in Figure 1a, it is seen that the membranes of chemistry B do have enhanced permeability compared to those of chemistry A. Additionally, as seen in Figure 1b, the membranes of chemistry B have relatively similar rejection characteristics to those of chemistry A. Thus, IPA appears to enhance the flux, but the zeolite nanoparticles further enhanced the flux. The main difference between the polymerization chemistries A and C is that C employed higher concentrations of MPD and TMC, which produced higher molecular weight polymer or a thicker film. The TFC-C membranes exhibited much lower flux with correspondingly higher rejection compared to TFC-A (and TFC-B). The addition of zeolites to chemistry C produced the highest overall fluxes, but had a very detrimental effect on rejection. This implies that the interaction between the particles and monomers radically altered the pore structure of the polyamide film, or that the nanoparticles did not form a strong bond with the polymer and defects formed. This is discussed in more detail in section 3.2. 3.2. Spectroscopic and Microscopic Properties of TFC and TFN Membranes. Table 3 presents XPS data for all TFC DOI: 10.1021/la900938x
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Lind et al. Table 3. XPS Results for TFC and TFN Membranes film formulation C (%) O (%) N (%) Al (%) Si (%) C/N (-) TFC-A TFN-A-1 TFC-B TFN-B-1 TFC-C TFN-C-1 TFN-C-2 TFN-C-3
Figure 1. Experimentally measured (a) pure water flux and (b) NaCl rejection for polyamide and zeolite-polyamide membranes.
membranes, all TFN membranes made with the smallest zeolite, and all TFN membranes of chemistry C. This XPS data quantifies the amount of elemental carbon, oxygen, nitrogen, aluminum, and silicon present on the surface of the membrane sample. Since the TFN membranes have higher oxygen contents, it is likely that zeolites incorporated in the membranes were captured by the analysis. However, aluminum and silicon were not detected for samples of chemistry A and chemistry B. Minimal aluminum and silicon appear in the XPS measurements for the membranes of chemistry C for a variety of reasons. The XPS measurement is a surface measurement, probably penetrating a few nanometers below the surface; therefore, zeolites might have been embedded within the polyamide layer or not present in the sample areas analyzed. That said, the consistent increase in oxygen content for TFN membranes suggests zeolites were present in the membrane sample in addition to the possibility of greater cross-linking. The extent of polyamide thin film cross-linking is typically estimated by looking at the atomic ratios of O/N.22,23 Here, we considered the C/N ratio of the membranes, because it also increases with the extent of cross-linking but is not affected by the extra oxygen content brought in by zeolites. By this measure, within a fixed chemistry, all TFN membranes were less cross-linked than the corresponding TFC membranes. These results suggest 10142 DOI: 10.1021/la900938x
72.6 70.2 74.1 72.2 62.5 63.6 59.5 60.0
14.3 19.9 12.6 16.6 11.9 12.4 15.5 20.7
13.1 9.9 13.3 11.2 25.6 24.0 22.8 18.1
0.0 0.0 2.2 0.0
0.0 0.0 0.0 1.2
5.5 7.1 5.6 6.4 2.4 2.7 2.6 3.3
that zeolites alter the polyamide film structure, which is consistent with the findings of Jeong et al.11 who found that “pore filled” zeolites (the polymeric templating agent used in synthesis was not removed, and thus, filled zeolite pore cavities) produce TFN membranes with higher flux than base TFC membranes, but lower flux than TFN membranes made with “pore open” particles (templating agent was completely removed and pores opened). Note that the C/N ratios for the membranes of Chemistry C are below the theoretical C/N ratio for a fully cross-linked film. This is probably the result of excess diamine monomer remaining in the film after curing, which artificially increased the apparent nitrogen content of all membranes. Assuming that the amount of unreacted amine was the same in all films formed, the trend of C/N ratios suggests slightly less cross-linking for nanocomposite membranes of Chemistry C. Figure 2 presents SEM images of the surfaces of the TFCs and TFNs with ∼100 nm zeolites for each of the three polymerization chemistries. The base TFC surface morphology is a function of the polymerization chemistry. Characteristic “leaf-like” structures are visible on the surface of TFC-B (Figure 2c), while TFC-A and TFC-C (Figure 2a,e) exhibit nodular surface structures. As mentioned above, IPA increases the width of the reaction zone and produces this “leaf-like” morphology. Images of the TFN membranes made with ∼100 nm particles are found in Figure 2b,d,f, all of these TFN membranes appear to have more “leaf-like” structures than the TFC membrane of corresponding chemistry. The presence of superhydrophilic zeolites in the organic phase may also enhance the miscibility of the aqueous and organic phases during interfacial polymerization. Gosh et al.18 report that the miscibility of water and hexane increase with temperature, while Ostomel et al.25 report the heat of hydration for LTA zeolites can produce local temperatures up to 95 °C. When zeolites in the organic phase encounter hydrated MPD from the aqueous phase, they will hydrate and release heat, which could locally enhance the miscibility of the aqueous and organic phases having much the same effect as IPA. 3.3. Interfacial Properties of TFC and TFN Membranes. Generally, colloidal and bacterial fouling are reduced as RO membrane interfaces become more smooth, negatively charged, and hydrophilic.26,27 Experimentally determined contact angle and zeta potential data are presented in Figure 3. Generally, there is strong correlation between RO membrane zeta potentials and contact angles. For polyamides, both charge and hydrophilicity increase with increasing TMC content (lower cross-linking) because more pendant carboxylic acid functional groups are exposed at the membrane surface. This is also true for TFN membranes, plus the acidic residues associated with zeolite surfaces increase with increasing zeolite content. (24) Wenzel, R. N. J. Phys. Colloid Chem. 1949, 53(9), 1466–1467. (25) Ostomel, T. A.; Stoimenov, P. K.; Holden, P. A.; Alam, H. B.; Stucky, G. D. J. Thromb. Thrombolysis 2006, 22(1), 55–67. (26) Kim, S.; Hoek, E. M. V. Desalination 2007, 202(1-3), 333–342. (27) Subramani, A.; Hoek, E. M. V. J. Membr. Sci. 2008, 319(1-2), 111–125.
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Figure 2. Representative SEM images (25 000) of polyamide and zeolite-polyamide membrane surfaces: (a) TFC-A, (b) TFN-A-1, (c) TFC-B, (d) TFN-B-1, (e) TFC-C, (f) TFN-C-1.
For chemistries A and B, zeta potentials of TFN membranes were more negative than corresponding TFC membranes and decreased with increasing zeolite size. Contact angles were lower for TFN membranes than corresponding TFC membranes, but increased with increasing zeolite size. For chemistry C, the base TFC membrane had a very large, negative zeta potential and relatively low contact angle. Addition of nanoparticles to chemistry C produced TFN membranes with less negative zeta potentials and higher contact angles. However, TFN membrane zeta potential became more negative and contact angle increased as zeolite size increased, which was consistent with the other polymerization chemistries. Langmuir 2009, 25(17), 10139–10145
Atomic force microscopy (AFM) results presented in Figure 4 elucidate a few trends corresponding to zeolite size. Significant differences in rms and SAD values were not observed for TFC membranes of different polymerization chemistries. Generally, both rms roughness and surface area difference correlated moderately with increasing zeolite size. Considering all twelve membrane types, the correlation coefficient between zeolite size and rms was 76%, while that of zeolite size and SAD was 66%. The rather large variability in roughness statistics for TFN membranes was due to the small sample areas analyzed by AFM (∼10 μm10 μm). Some scans may have samples areas with little or no particles and others may have sampled areas with relatively high concentrations of nanoparticles. The flux of TFC and TFN DOI: 10.1021/la900938x
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Figure 3. Experimentally measured (a) contact angles and (b) zeta potentials for polyamide and zeolite-polyamide membranes.
membranes showed virtually no correlation with either rms (-0.09) or SAD (-0.17) parameters, which is evidence that the available membrane surface area has no impact on hydraulic permeability. 3.4. Characteristic Transport Properties of TFC and TFN Membranes. Table 4 presents characteristic transport properties of TFC and TFN membranes calculated from the experimentally measured data presented in Figures 1 to 4. The pure water transport coefficient (A) was calculated by normalizing the flux with the applied pressure. Since the applied pressure was the same in every experiment, the permeability scaled directly with the flux. However, viewing the solute permeability gives additional perspective on the impact of nanoparticles on film structure. Solute transport coefficients (Bi) are provided for NaCl, MgSO4, and PEG200 in Table 4. For chemistries A and C, the increase in solute transport through TFN membranes relative to TFC membranes was equal to or greater than the relative increase in water transport coefficient. For chemistry C, the TFN-C-1, -2, and -3 membranes had 1908%, 1583%, and 435% increases in water permeability over the base TFC-C. However, the NaCl, MgSO4, and PEG200 transport coefficients increased by even greater percentages. Hence, the TFN-A and -C membranes became “looser” for all three zeolites, but the higher flux attained during solute rejection experiments gave rise to relatively high observed rejections. In contrast, TFN-B-2 and -3 membranes became slightly more permeable to water, MgSO4, and PEG200, 10144 DOI: 10.1021/la900938x
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Figure 4. Experimentally measured (a) rms roughness and (b) surface area difference for polyamide and zeolite-polyamide membranes.
but less permeable to NaCl. Overall, the smallest zeolites produced the largest increases in water and solute passage. Characteristic pore dimensions calculated from PEG200 rejection data using a steric exclusion model are presented in Table 3. The characteristic pore sizes of TFC-A, -B, and -C were 3.9, 4.0, and 3.8 A˚, respectively. In reality, there is a distribution of tortuous paths through the polyamide films probably between about 3 and 6 A˚, where the characteristic pore size gives some estimation of the relative proportion of small versus large pores.22 For TFN membranes, in principle, the size of PEG200 (∼0.35 nm) allows it to access the zeolite pores (∼0.4 nm). Therefore, the characteristic pore size of TFN membranes (potentially) represents a combined average of the zeolite pore size, the PEG200 accessible free-volume of pure polyamide regions, and any voids (defects) between the zeolite and polyamide materials. In general, the characteristic pore size decreased with increasing zeolite crystal size. The estimated pore size correlated almost perfectly with transport coefficients for water and all three solutes. Since TFN solute transport coefficients increased by a greater extent than the water transport coefficients, the extreme hydrophilicity of zeolites did not enhance the relative solubility of the membrane for water over the solutes. Any change in tortuosity for diffusion through the interconnected zeolite framework pores (relative to the irregular pore network of the noncrystalline polymer would be Langmuir 2009, 25(17), 10139–10145
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Article Table 4. Characteristic Transport Properties of TFC and TFN Membranesa
film formulation
A (μm/MPa-s)
BNaCl (μm/s)
TFC-A TFN-A-1 TFN-A-2 TFN-A-3 FFC-B TFN-B-1 TFN-B-2 TFN-B-3 TFC-C TFN-C-1 TFN-C-2 TFN-C-3
7.75 11.4 9.88 9.61 14.9 19.1 16.4 15.0 3.65 73.3 61.4 19.5
0.540 1.29 1.013 0.901 3.00 17.4 2.66 2.37 0.163 644 268 5.09
rP qs -ΔGSL δfilm
0.970 0.319 0.053 -0.095
0.979 0.212 -0.001 -0.212
BMgS04 (μm/s) 0.372 0.928 0.722 0.493 1.79 16.7 2.51 2.31 0.115 644 202.4 3.03
rP (A˚)
BPEG200 (μm/s)
δfilm (nm)
0.183 0.889 0.474 0.227 0.739 11.86 1.85 1.54 0.057 405 151.5 1.427
3.9 4.2 4.0 3.9 4.0 5.8 4.3 4.3 3.8 13.0 9.2 4.1
120 111 257 144 113 135 169 311 118 119 174 268
0.973 0.210 -0.014 -0.217
1.000 0.254 0.078 -0.199
-0.199 0.070 -0.374 1.000
Correlation Coefficients 0.962 0.207 -0.038 -0.220
a Table legend: A=water permeability; B=solute permeability; NaCl, MgSO4, PEG200=test solutes; rp=characteristic pore dimension; δfilm=thin film thickness.
shared by both water and solutes. Finally, the TFN films do not appear to be thinner than TFC analogues. Considering the XPS results, these rather large characteristic pore sizes suggest the zeolites altered the bulk polyamide film structure and created defects in the TFN membranes. While the nature of the zeolite-polyamide interaction is not fully elucidated, smaller nanoparticles produced larger characteristic pores. If the extra water and solute transport were occurring entirely through the zeolite via molecular sieving (no change in polyamide cross-linking, no zeolite-polymer defects, no impact of zeolite hydrophilicity on solution-diffusion transport), then characteristic pore size and the transport characteristics should not significantly change. Assuming each particle produced a small void, the larger number of small particles may have produced a larger number of defects. Alternatively, assuming each particle locally elevates the temperature of the reaction zone, the larger number of small particles would transfer heat more effectively and in a more uniform (less disperse) manner. Within a given polymer chemistry, estimated TFC and TFN film thickness increased with increasing nanoparticle size; however, this AFM-derived film thickness did not account for possible differences in the basal film layer thickness. Through cross-sectional TEM images of TFC and TFN membranes the thickness of the thin film polyamide layer in reverse osmosis membranes has been shown to be approximately 50-200 nm.11,12,18,19 The TFN-A/B/C-1 membranes have ∼100 nm zeolites embedded into the thin polyamide film. These smallest nanoparticles best matched the characteristic film thickness for the three polymer chemistries and produced the greatest permeability enhancement. The larger zeolite crystals exceeded the base TFC film thicknesses. The more negative zeta potentials and lower contact angles confirm that the zeolites were exposed at the membrane surface, and thus, the zeolite crystal size dictated the apparent rms and SAD values. Larger particles may also template film growth during the polymerization reaction in such a way that the polymer
Langmuir 2009, 25(17), 10139–10145
film grows thicker than normal in order to encapsulate the molecular-sieve particles.
Conclusions Changes in interfacial polymerization chemistry can be used to tune polyamide thin film composite RO membrane properties; however, zeolite crystal size offers an additional degree of freedom that can be used to tailor the permeability, selectivity, structure, morphology, and interface of thin film nanocomposite RO membranes. Specifically, smaller zeolites produced greater enhancements in membrane permeability, which translate into high water flux and high solute rejection (at high flux). The dramatic increase in solute permeability might limit the application of TFN membranes use in traditional RO systems designed to operate at extremely low water fluxes. Moreover, these results offer additional support for the hypothesis that zeolite nanoparticles change bulk polyamide film structure possibly by creation of microporous defects as well as reduced cross-linking through heat released during the polymerization reaction. Acknowledgment. Financial support for this research was provided by the UCLA California NanoSystems Institute, NanoH2O Inc., the United States Environmental Protection Agency (US EPA), and the UC Toxic Substances Research and Training Program: Lead Campus on Nanotoxicology. The US EPA funds were awarded through the Desalination Research Innovation Partnership, which is managed by the Metropolitan Water District of Southern California. The authors are also grateful to Prof. Bruce Dunn at UCLA for providing access to the FTIR, Prof. Chi-Ming Ho at UCLA for providing access to the AFM, and Prof. Sharon Walker at UC Riverside for providing access to the streaming potential analyzer. Disclosure: Authors EMVH and AKG have financial interest in one of the project sponsors, NanoH2O Inc., through stock ownership and consulting agreements.
DOI: 10.1021/la900938x
10145
