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Atomic force microscopy of dense and asymmetric cellulose-based membranes
Journal of Membrane Science 160 (1999) 235±242
Atomic force microscopy of dense and asymmetric cellulose-based membranes Dimitris F. Stamatialis, Cristina R. Dias, Maria Norberta de Pinho* Department of Chemical Engineering, Instituto Superior TeÂcnico, Av. Rovisco Pais, 1049-001, Lisboa, Portugal Received 4 January 1999; received in revised form 19 February 1999; accepted 3 March 1999
Abstract The surface structures of dense and integrally skinned cellulose acetate (CA) and cellulose acetate butyrate (CAB) membranes, prepared by phase inversion under different casting conditions, are investigated by tapping mode atomic force microscopy (TM AFM). The results obtained show that: (i) The top and bottom surfaces of the dense CA membrane were quite uniform in comparison with the corresponding faces of asymmetric CA and CAB membranes. Despite the casting conditions the active and support layers of the asymmetric membranes display large differences on the roughness parameters. (ii) The asymmetric membranes prepared with an organic system as a non-solvent pore-former (method IV) display smaller nodule aggregates and lower values of the roughness parameters than the ones prepared using an inorganic system as swelling agent (method I). This is more pronounced for the CA membranes than for the CAB membranes. (iii) In the active layer of asymmetric CA membranes casted at longer evaporation times, the measured values of surface roughness parameters tend to decrease. Also, for these CA membranes, as the evaporation time increases the average size of the depression areas observed on the surface decreases. The laboratory-made CA and CAB membranes display a wide range of nano®ltration and reverse osmosis permeation characteristics. These characteristics are correlated to surface roughness parameters of the active layers. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Cellulose acetate; Cellulose acetate butyrate; Asymmetric membrane; Tapping mode atomic force microscopy
1. Introduction The development of integral asymmetric membranes was a breakthrough towards the industrial application of membranes [1]. These membranes consist of a very dense top layer or skin with a thickness of about 0.1±0.5 mm supported by a porous sublayer with the thickness of about 50±150 mm. They simulta*Corresponding author. Tel.: +351-1-84-17488; fax: +351-1-8499242; e-mail: [email protected]
neously combine the high selectivity of a dense membrane with the high permeation capability of a very thin or porous membrane. It is the structure or the surface morphology of the top active layer that determines the extent of the contribution of the sieving and diffusive mechanisms. The porous sublayer acts mainly as a mechanical support. The phase inversion casting method is intrinsically connected to the asymmetry of these membranes and allows the tailoring of very distinct structures upon the variation of the casting solution composition (system
0376-7388/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 6 - 7 3 8 8 ( 9 9 ) 0 0 0 8 9 - 7
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D.F. Stamatialis et al. / Journal of Membrane Science 160 (1999) 235±242
of solvents, polymers, etc.) and of the casting parameters (evaporation time, coagulation media, etc.). The tailoring of structures does not only deal with more or less dense active layers that can perform as ultra®ltration (UF), nano®ltration (NF) or reverse osmosis (RO) membranes. In fact, Rosa et al. [2] through the use of optical polarizing microscopy observed the existence of anisotropic polymer aggregates dispersed in an amorphous matrix at the skin membrane face of wet cellulose acetate (CA) membranes prepared by phase inversion. The size and the density of these aggregates are dependent on the casting solution, namely on the type and concentration of the non-solvent. This complex morphology of the active layer results in different polymer±water interactions and therefore in different permeation characteristics. Some attempts to correlate such parameters have been performed. Murphy and Pinho [3], using attenuated total re¯ection Fourier transform infrared spectroscopy (ATR-FTIR), show evidence of differences in the state of water (amount and type of water species) present in CA skin layers with very different morphologies. Dias et al. [4] carried out a systematic investigation of the water state in the active layer of asymmetric CA and cellulose acetate butyrate (CAB) membranes prepared under different casting conditions. The authors conclude that the very different morphologies of the active layers are associated to different water structures and membrane permeation characteristics. In the present work, tapping mode atomic force microscopy (TM AFM) is used to comply with the main objective of differentiating surface morphologies of membrane active layers in the RO/NF range. AFM is a useful method to characterize the surface structure of membranes due to the possibility of analyzing dry or wet samples without any pretreatment. AFM imaging of the membrane structure has advantages over scanning electron microscopy (SEM) and transmission electron microscopy (TEM) since the resolution is higher, the sample preparation is minimal and no electron beam damage can occur. Therefore, AFM technique has been applied for studying micro®ltration and ultra®ltration membranes [5± 7], nano®ltration [8,9] and gas separation [10,11] membranes. The CA and CAB membranes under study were prepared in our laboratory by the phase-inversion
method and display a wide range of surface morphologies. The aims of the present work are: (i) the characterization of surface morphologies of NF/RO asymmetric membranes by TM AFM and (ii) the correlation of the active layer surface morphologies with the membrane permeation characteristics. 2. Experimental 2.1. Membrane preparation and drying The cellulose esters used were (i) cellulose acetate, CA-398 (39.8% acetyl content), MW30 270, supplied by Eastmann Kodak and (ii) cellulose acetate butyrate, CAB (17% butyryl content, MW65 000, supplied by Aldrich, Steinheim, Germany). The asymmetric membranes were prepared by phase inversion using the wet process as described by Kunst and Sourirajan [12]. Two methods, labeled as I and IV, corresponding to different casting solutions and ®lm casting conditions, were used. They are described in detail in Table 1. The asymmetric membranes are identi®ed in this text by a three-area code: the ®rst area is relative to the polymer used (CA or CAB), the second to the method (I or IV) and the third is relative to the solvent evaporation time expressed in minutes. Deionized water with conductivity below 0.2 mS/cm was used in all cases. Table 1 Casting solutions and casting conditions for membrane preparation Method Casting solution (wt%) Polymer (CA or CAB) Acetone (p.a., Merck) Mg(ClO4)2xH2O (83% dry wt., Merck) Water Glycerol n-Propanol Triethylphosphate Film casting conditions Temperature of casting solution (8C) Solvent evaporation time (min) Gelation medium
I
IV
13.95 79.07 3.49
20 45 ±
3.49 ± ± ±
2 2 6 25
10
25
1 and 10 Ice cold water
1 Water at 258C
D.F. Stamatialis et al. / Journal of Membrane Science 160 (1999) 235±242
The asymmetric membranes were gently dried by solvent exchange using a series of isopropyl alcohol± hexane mixtures, as described by Lui et al. [13]. After membrane preparation by phase inversion, in order to gradually remove the water, the samples were immersed in aqueous isopropyl alcohol solutions of successively higher alcohol content (25, 50, 75 and 100 vol% isopropyl alcohol) for at least 24 h at each successive step. In order to remove the alcohol, the membranes were then immersed in isopropyl alcohol± hexane solutions of successive higher hexane content (25, 50, 75 and 100 vol% hexane) again for at least 24 h at each successive step. The hexane was then removed by gentle evaporation in a desiccator at ambient temperature for 13 days. More vigorous drying procedures, such as thermovacuum treatment or suspension of the membranes over fresh P2O5, were not undertaken since a complete drying of the membrane under these conditions can irreversibly damage or destroy the membrane structure. Even with the naked eye one can witness major changes. In fact, if the samples are dried with P2O5 they become brittle compared to the soft and ¯exible solvent dried membranes. The dense membranes were prepared by casting a 20 wt% solution of polymer in acetone on a clean glass surface. Acetone evaporation was completed in an atmosphere nearly saturated with acetone vapor for 3 days. The membranes were then kept in a desiccator for 15 days.
237
separated by a porous plate (membrane support). Six membrane cells (surface area of 13.210ÿ4 m2) were placed in series. The membranes were ®rst compacted by circulating deionized water (conductivity below 0.2 mS/cm) at 40 bar, 258C and 0.6 l/min, for at least 8 h. The stabilization time for each experimental run is 30 min. The initial feed concentration of NaCl was 3500 mg/l, and the concentrations at the feed and the permeate were determined by conductivity, at 258C, using a Crison conductimeter (model 525). 2.3. AFM images
(1)
The AFM used to image the membrane surfaces is a multi mode scanning probe microscope with a Nanoscope IIIa Controller, supplied by Digital Instruments, USA. The membrane surface is scanned in intermediate contact (tapping mode) with an oscillating tip. This eliminates shear forces that can damage soft samples and reduce the image resolution. At ®rst, the tip is far from the sample surface bouncing up and down with ``free vibration'' amplitude. The tip then approaches the sample surface and due to the tip-surface interaction the vibration amplitude decreases. During the scanning procedure, the vibration amplitude of the tip is kept constant by changing the scanner height. Small pieces were cut from each membrane, glued onto metal disks and attached to a magnetic sample holder, located on top of the scanner tube. All the TM AFM images were undertaken at 258C. Differences in the membrane surface morphology can be expressed in terms of various roughness parameters, such as: 1. the difference between the highest and the lowest points within the given area, z; 2. the standard deviation of the z values within the given area (Rq). This parameter is calculated as sP zi ÿ zavg 2 (2) Rq Np
where (C)feed and (C)permeate are the concentration of NaCl in the bulk of the feed solution and in the permeate solution, respectively. The laboratory set-up used in permeation experiments has been previously described [14]. The membrane cell is a ¯at plate cell with two detachable parts
where zi is the current z value, zavg is the average of the z values within the given area and Np is the number of points within the given area; 3. the mean roughness (Ra). This parameter represents the mean value of the surface relative to the center plane, the plane for which the volumes enclosed by
2.2. Permeation experiments The permeation experiments were carried out with deionized water, to determine the membrane pure water ¯ux, Jw, and with a reference solution of NaCl p.a. (Pronalab) to determine the apparent rejection to this salt, fNaCl, and the permeate ¯ux, Jv. The apparent rejection to NaCl, fNaCl, is de®ned as fNaCl
Cfeed ÿ Cpermeate Cfeed
100
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the image above and below this plane are equal. It is calculated as 1 Ra Lx L y
ZLx ZLy j f x; yjdxdy 0
(3)
0
where f (x,y) is the surface relative to the center plane and Lx and Ly are the dimensions of the surface in the x and y directions, respectively. All the surface roughness parameters are calculated from the AFM images using an AFM software program. 3. Results and discussion The integrally skinned CA and CAB asymmetric membranes that are the main subject of this work cover a wide range of permeation properties. Accordingly, they display a broad spectrum of AFM 3D images corresponding to very different surface topographies. Table 2 shows the permeation ¯uxes Jw, Jv, and the apparent rejection coef®cient fNaCl of the membranes and the corresponding roughness parameters calculated from the AFM images presented in Figs. 1±7 with the use of an appropriate software program. It should be emphasized that all images are ¯attened in the same way and therefore the roughness values are relative and not absolute values. For comparison purposes, Fig. 1 shows the AFM images of the top and of the bottom surfaces of a dense
Fig. 1. AFM images of the dense symmetric CA membrane: (a) top and (b) bottom surface.
Table 2 Permeation properties [4] and roughness parameters of membranes Membrane
Jw (kg/m2 h)
Jv (kg/m2 h)
fNaCl (%)
Ra (nm)
Rq (nm)
z (nm)
CA dense top surface CA dense bottom surface CA-I-1 active layer CA-I-1 support layer CA-I-10 active layer CA-I-10 support layer CA-IV-1 active layer CA-IV-1 support layer
± ± 29 ± 0.3 ± 18 ±
± ± 28 ± 0.3 ± 19 ±
± ± 30 ± 98 ± 53 ±
0.40 0.93 5.14 16.70 1.86 3.40 0.91 4.82
0.3 1.2 6.9 22.1 2.5 4.2 1.2 6.1
1.1 9.8 53.3 204.6 20.1 31.5 20.5 36.9
CAB-I-1 active layer CAB-I-1 support layer CAB-IV-1 active layer CAB-IV-1 support layer
19 ± 9 ±
17 ± 9 ±
94 ± 98.2 ±
0.87 10.77 0.94 4.05
1.1 14.6 1.2 6.0
6.7 147.4 9.9 27.9
D.F. Stamatialis et al. / Journal of Membrane Science 160 (1999) 235±242
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Fig. 2. AFM image of the active layer of the asymmetric CA-I-1 membrane.
Fig. 4. AFM image of the active layer of the asymmetric CA-I-10 membrane.
symmetric CA membrane. Both surfaces show a relatively uniform structure, despite the fact that the statistical parameters Ra and z are higher for the membrane bottom surface than for the top surface (Table 2). These differences are due to the different nature of the air±solution and of the glass plate± solution interfaces. However, they are much smaller than the differences observed in the asymmetric membranes prepared by the phase inversion method. This asymmetry is always observed irrespective of the casting conditions.
For asymmetric CA membranes, despite the preparation method, the surface morphology of the active layer is characterized by a Ra parameter always at least two times higher than the one of the top surface of the dense CA membrane (see Table 2). These results show that the assumption made by various researchers that the structure of a dense ®lm is a representative model of the active layer of an asymmetric ®lm should be taken with precaution. The Ra parameter of the porous support of the asymmetricmembranesisatleastthreetimeshigherthan theoneof the bottom surface of the CA dense membrane.
Fig. 3. AFM image of the support layer of the asymmetric CA-I-1 membrane: (a) in 20 nm and (b) in 50 nm height data scale.
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D.F. Stamatialis et al. / Journal of Membrane Science 160 (1999) 235±242
Fig. 5. AFM image of the active layer of the asymmetric CA-IV-1 membrane.
Fig. 7. AFM image of the active layer of the asymmetric CAB-IV1 membrane.
In the 3D AFM image of the active layer of a CA-I-1 asymmetric membrane (Fig. 2) one can clearly observe the existence of large darker areas. These are associated to depressions on the surface and they are generally surrounded by high elevation regions made of grains. These grains probably correspond to nodule aggregates [15]. The phenomenon is more pronounced in the case of the support surface of the same CA-I-1 membrane, where large channel-like depressions are observed (see Fig. 3). In this case very high values of Ra and z are obtained. These observations are in agreement with the results
obtained from the ATR-FTIR spectra of the surfaces of the active layer and the porous support in CA and CAB membranes [4]. They show that larger water clusters, which may be associated to larger pores, were observed in the surface of the porous layer. The difference in morphology between the top and the support layer is expected when one considers the mechanism of preparation of asymmetric membranes by the phase inversion method [16]. The effect of the evaporation time is clearly illustrated by comparing the images of the active layers of CA-I-1 and CA-I-10 membranes (Figs. 2 and 4, respectively). In the active layer of the asymmetric membranes prepared using longer evaporation times, the average size of the darker depression areas observed in the AFM 3D images is smaller and their concentration is higher than for the membranes casted at shorter evaporation times. The difference in surface morphologies is also illustrated by the values of the statistical parameters Ra and z, which are much higher for CA-I-1. The higher values of Ra and z are associated with the increase of Jw, Jv and the decrease of fNaCl when the evaporation time decreases. The effect of the casting solution composition is illustrated by comparing the AFM images of the active layers of CA-I-1 and CA-IV-1 (Figs. 2 and 5, respectively) and of CAB-I-1 and CAB-IV-1 (Figs. 6 and 7, respectively) asymmetric membranes. Lower values of Ra and z are observed for the active layer of the CAIV-1 membranes than for the active layer of CA-I-1
Fig. 6. AFM image of the active layer of the asymmetric CAB-I-1 membrane.
D.F. Stamatialis et al. / Journal of Membrane Science 160 (1999) 235±242
membranes. Also, comparing the AFM images of the active layers of these membranes, it is possible to observe that the size of the nodule aggregates and of the depressions observed on the surface is much smaller in the active layer of the CA-IV-1 membrane. Accordingly, the apparent rejection of CA-IV-1 is higher when compared to that of CA-I-1. Also, when comparing CAB-IV-1 to CAB-I-1 membranes (Figs. 7 and 6, respectively) the ®rst ones display smaller and sphere-like depressions on the surface in contrast with larger and ``slit-like'' ones observed in CAB-I-1 membranes. The CA and CAB membranes casted by method I display higher ¯uxes and lower apparent rejection coef®cients when compared to the ones casted by method IV. This difference is correlated to the different casting solutions used. In ``method I'' the pore former additive is an electrophilic salt, Mg(ClO4)2. The aggregation of water (gelation medium) around the electrophilic cations causes the swelling of the polymer matrix and the formation of pores. In ``method IV'', mixtures of organic solvents of different polarities (and therefore of different solvent power) are used as pore additives and may play an important role in reducing the acetone solvent power and the water gelation power with subsequent decrease of pore size and increase of membrane porosity [17,18]. The comparison of the AFM images of the active layers of CA-I-1 and CA-IV-1 with the homologous CAB membranes (i.e., CAB-I-1 and CAB-IV) shows that generally larger nodule aggregates and higher values of the parameters characterizing the membrane surface roughness are observed in CA membranes. This difference in morphology and the higher hydrophilicity of CA could be the reason for the higher Jw, Jv and lower fNaCl values obtained for the CA-I-1 and CA-IV-1 membranes when compared to those obtained for the CAB-I-1 and CAB-IV-1 membranes, respectively. 4. Conclusions The membrane characteristics of both surfaces of a dense CA membranes and of a series of asymmetric CA and CAB membranes were investigated using Atomic Force Microscopy. The asymmetric membranes were prepared under different casting condi-
241
tions and showed a wide range of NF/RO permeation characteristics. These characteristics depend on the surface topographies of the active layers. The analysis of the TM AFM images yield the following conclusions: The top and the bottom surfaces of the dense CA membrane are relatively uniform. The statistical parameters Ra and z, pertaining to surface roughness, are always much smaller for CA symmetric membranes than for the phase inversion asymmetric membranes. The asymmetry displayed by these membranes is clearly shown by the very large difference between the Ra and z values of the surfaces of the active layer and the support layer. This is observed for all the asymmetric membranes irrespective of the casting conditions used. In the preparation of asymmetric membranes, the use of an organic system as a non-solvent pore former (method IV) leads to smaller values of roughness parameters than those obtained with an inorganic swelling agent (method I). This is more pronounced for the CA than for the CAB asymmetric membranes. Smaller nodule aggregates and smaller depression areas are also observed on membranes prepared by method IV. Increasing evaporation times leads to lower values of the roughness parameters. Also, as the evaporation time increases, smaller depression areas on the surface and higher concentration of the darker depression areas are observed in the active layer of asymmetric membranes. The surface morphologies observed at the varying casting conditions are intrinsically associated with the permeation properties. For CA membranes, increasing evaporation times and organic-pore formers lead to higher apparent rejection and lower ¯uxes. These are always correlated to lower values of the roughness parameters, Ra and z. The CAB membranes always display much higher apparent rejection than the CA membranes prepared under the same casting conditions. Such higher apparent rejections are correlated to lower z values. 5. Nomenclature (C)feed
concentration of NaCl in the bulk of the feed solution (mg/l)
242
(C)permeate CA CAB fNaCl Jv Jw NF Np RO SEM TEM ATR-FTIR TM AFM Ra Rq z zavg
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concentration of NaCl in the permeate solution (mg/l) cellulose acetate cellulose acetate butyrate apparent rejection to NaCl (%) permeate flux (kg/m2 h) water permeate flux (kg/m2 h) nanofiltration number of points within the given area used in the calculation of Rq reverse osmosis scanning electron microscopy transmission electron microscopy attenuated total reflectance Fourier transform infrared tapping mode atomic force microscopy mean surface roughness (nm) standard deviation of the z values within the given area (nm) difference between the highest and the lowest points within the given area (nm) average of the z values within the given area (nm)
Acknowledgements The authors wish to thank Prof. Pedro Brogueira (Department of Physics, I.S.T., Lisbon, Portugal) for his assistance with the AFM experiments. References [1] S. Loeb, S. Sourirajan, Sea water demineralisation by means of an osmotic membrane, Adv. Chem. Ser. 38 (1963) 117. [2] M.J. Rosa, M.N. de Pinho, M.H. Godinho, A.F. Martins, Optical polarising studies of cellulose acetate membranes prepared by phase inversion, Mol. Cryst. Liq. Cryst. 258 (1995) 163. [3] D. Murphy, M.N. de Pinho, An ATR-FTIR study of water in cellulose acetate membranes prepared by phase inversion, J. Membr. Sci. 106 (1995) 245. [4] C.R. Dias, M.J. Rosa, M.N. de Pinho, Structure of water in asymmetric cellulose ester membranes ± an ATR-FTIR study, J. Membr. Sci. 138 (1998) 259.
[5] A. Bessieres, M. Meireles, R. Cotatger, J. Beauvillain, V. Sanchez, Investigation of surface properties of polymer membranes by near field microscopy, J. Membr. Sci. 109 (1996) 271. [6] W.R. Bowen, N. Hilal, R.W. Lovitt, R.M. Williams, Atomic force microscope studies of membranes. Surface pore structures of cyclopore and anopore membranes, J. Membr. Sci. 110 (1996) 233. [7] W.R. Bowen, A.W. Mohammad, N. Hilal, Characterisation of nanofiltration membrane for predictive purpose ± use of salts, uncharged solutes and atomic force microscopy, J. Membr. Sci. 126 (1997) 91. [8] W.R. Bowen, N. Hilal, R.W. Lovitt, R.M. Williams, A new technique for membrane characterisation: direct measurement of the force of adhesion of a single particle using an atomic force microscope, J. Membr. Sci. 139 (1998) 269. [9] S. Singh, K.C. Khulbe, T. Matsuura, R.P. Ramamurthy, Membrane characterisation by solute transport and atomic force microscopy, J. Membr. Sci. 142 (1998) 111. [10] K.C. Khulbe, B. Kruczek, G. Chowdhury, S. Gagne, T. Matsuura, Surface morphology of homogenous and asymmetric membranes made from poly(phenylene oxide) by tapping mode atomic force microscope, J. Appl. Polym. Sci. 59 (1996) 1151. [11] K.C. Khulbe, B. Kruczek, G. Chowdhury, S. Gagne, T. Matsuura, S.P. Verma, Characterisation of membranes prepared from PPO by Raman scattering and atomic force microscopy, J. Membr. Sci. 111 (1996) 57. [12] B. Kunst, S. Sourirajan, An approach to the development of cellulose acetate ultrafiltration membranes, J. Appl. Polym. Sci. 18 (1974) 3423. [13] A. Lui, F.D.F. Talbot, A. Fouda, T. Matsuura, S. Sourirajan, Studies of the solvent exchange technique for making dry cellulose acetate membranes for the separation of gaseous mixtures, J. Appl. Polym. Sci. 36 (1988) 1809. [14] M.J. Rosa, Ph.D Thesis, Technical University of Lisbon, Portugal (1995). [15] R.E. Kesting, The four tiers of structure in integrally skinned phase inversion membranes and their relevance to the various separation regimes, J. Appl. Polym. Sci. 41 (1990) 2739. [16] I. Pinnau, W.S. Koros, A qualitative skin layer formation mechanism for membrane made by dry/wet phase inversion, J. Polym. Sci., Part B, Polym. Phys. 31 (1993) 2739. [17] R.E. Kesting, Synthetic polymeric membranes ± A structural perspective, 2nd ed., Wiley, New York, 1985. [18] S. Sourirajan, B. Kunst, Cellulose acetate and other cellulose ester membranes, in: S. Sourirajan (Ed.), Reverse Osmosis and Synthetic Membranes, Chapter 7, NRCC, Ottawa, Canada, 1977, p. 148.
Atomic force microscopy of dense and asymmetric cellulose-based membranes Dimitris F. Stamatialis, Cristina R. Dias, Maria Norberta de Pinho* Department of Chemical Engineering, Instituto Superior TeÂcnico, Av. Rovisco Pais, 1049-001, Lisboa, Portugal Received 4 January 1999; received in revised form 19 February 1999; accepted 3 March 1999
Abstract The surface structures of dense and integrally skinned cellulose acetate (CA) and cellulose acetate butyrate (CAB) membranes, prepared by phase inversion under different casting conditions, are investigated by tapping mode atomic force microscopy (TM AFM). The results obtained show that: (i) The top and bottom surfaces of the dense CA membrane were quite uniform in comparison with the corresponding faces of asymmetric CA and CAB membranes. Despite the casting conditions the active and support layers of the asymmetric membranes display large differences on the roughness parameters. (ii) The asymmetric membranes prepared with an organic system as a non-solvent pore-former (method IV) display smaller nodule aggregates and lower values of the roughness parameters than the ones prepared using an inorganic system as swelling agent (method I). This is more pronounced for the CA membranes than for the CAB membranes. (iii) In the active layer of asymmetric CA membranes casted at longer evaporation times, the measured values of surface roughness parameters tend to decrease. Also, for these CA membranes, as the evaporation time increases the average size of the depression areas observed on the surface decreases. The laboratory-made CA and CAB membranes display a wide range of nano®ltration and reverse osmosis permeation characteristics. These characteristics are correlated to surface roughness parameters of the active layers. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Cellulose acetate; Cellulose acetate butyrate; Asymmetric membrane; Tapping mode atomic force microscopy
1. Introduction The development of integral asymmetric membranes was a breakthrough towards the industrial application of membranes [1]. These membranes consist of a very dense top layer or skin with a thickness of about 0.1±0.5 mm supported by a porous sublayer with the thickness of about 50±150 mm. They simulta*Corresponding author. Tel.: +351-1-84-17488; fax: +351-1-8499242; e-mail: [email protected]
neously combine the high selectivity of a dense membrane with the high permeation capability of a very thin or porous membrane. It is the structure or the surface morphology of the top active layer that determines the extent of the contribution of the sieving and diffusive mechanisms. The porous sublayer acts mainly as a mechanical support. The phase inversion casting method is intrinsically connected to the asymmetry of these membranes and allows the tailoring of very distinct structures upon the variation of the casting solution composition (system
0376-7388/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 6 - 7 3 8 8 ( 9 9 ) 0 0 0 8 9 - 7
236
D.F. Stamatialis et al. / Journal of Membrane Science 160 (1999) 235±242
of solvents, polymers, etc.) and of the casting parameters (evaporation time, coagulation media, etc.). The tailoring of structures does not only deal with more or less dense active layers that can perform as ultra®ltration (UF), nano®ltration (NF) or reverse osmosis (RO) membranes. In fact, Rosa et al. [2] through the use of optical polarizing microscopy observed the existence of anisotropic polymer aggregates dispersed in an amorphous matrix at the skin membrane face of wet cellulose acetate (CA) membranes prepared by phase inversion. The size and the density of these aggregates are dependent on the casting solution, namely on the type and concentration of the non-solvent. This complex morphology of the active layer results in different polymer±water interactions and therefore in different permeation characteristics. Some attempts to correlate such parameters have been performed. Murphy and Pinho [3], using attenuated total re¯ection Fourier transform infrared spectroscopy (ATR-FTIR), show evidence of differences in the state of water (amount and type of water species) present in CA skin layers with very different morphologies. Dias et al. [4] carried out a systematic investigation of the water state in the active layer of asymmetric CA and cellulose acetate butyrate (CAB) membranes prepared under different casting conditions. The authors conclude that the very different morphologies of the active layers are associated to different water structures and membrane permeation characteristics. In the present work, tapping mode atomic force microscopy (TM AFM) is used to comply with the main objective of differentiating surface morphologies of membrane active layers in the RO/NF range. AFM is a useful method to characterize the surface structure of membranes due to the possibility of analyzing dry or wet samples without any pretreatment. AFM imaging of the membrane structure has advantages over scanning electron microscopy (SEM) and transmission electron microscopy (TEM) since the resolution is higher, the sample preparation is minimal and no electron beam damage can occur. Therefore, AFM technique has been applied for studying micro®ltration and ultra®ltration membranes [5± 7], nano®ltration [8,9] and gas separation [10,11] membranes. The CA and CAB membranes under study were prepared in our laboratory by the phase-inversion
method and display a wide range of surface morphologies. The aims of the present work are: (i) the characterization of surface morphologies of NF/RO asymmetric membranes by TM AFM and (ii) the correlation of the active layer surface morphologies with the membrane permeation characteristics. 2. Experimental 2.1. Membrane preparation and drying The cellulose esters used were (i) cellulose acetate, CA-398 (39.8% acetyl content), MW30 270, supplied by Eastmann Kodak and (ii) cellulose acetate butyrate, CAB (17% butyryl content, MW65 000, supplied by Aldrich, Steinheim, Germany). The asymmetric membranes were prepared by phase inversion using the wet process as described by Kunst and Sourirajan [12]. Two methods, labeled as I and IV, corresponding to different casting solutions and ®lm casting conditions, were used. They are described in detail in Table 1. The asymmetric membranes are identi®ed in this text by a three-area code: the ®rst area is relative to the polymer used (CA or CAB), the second to the method (I or IV) and the third is relative to the solvent evaporation time expressed in minutes. Deionized water with conductivity below 0.2 mS/cm was used in all cases. Table 1 Casting solutions and casting conditions for membrane preparation Method Casting solution (wt%) Polymer (CA or CAB) Acetone (p.a., Merck) Mg(ClO4)2xH2O (83% dry wt., Merck) Water Glycerol n-Propanol Triethylphosphate Film casting conditions Temperature of casting solution (8C) Solvent evaporation time (min) Gelation medium
I
IV
13.95 79.07 3.49
20 45 ±
3.49 ± ± ±
2 2 6 25
10
25
1 and 10 Ice cold water
1 Water at 258C
D.F. Stamatialis et al. / Journal of Membrane Science 160 (1999) 235±242
The asymmetric membranes were gently dried by solvent exchange using a series of isopropyl alcohol± hexane mixtures, as described by Lui et al. [13]. After membrane preparation by phase inversion, in order to gradually remove the water, the samples were immersed in aqueous isopropyl alcohol solutions of successively higher alcohol content (25, 50, 75 and 100 vol% isopropyl alcohol) for at least 24 h at each successive step. In order to remove the alcohol, the membranes were then immersed in isopropyl alcohol± hexane solutions of successive higher hexane content (25, 50, 75 and 100 vol% hexane) again for at least 24 h at each successive step. The hexane was then removed by gentle evaporation in a desiccator at ambient temperature for 13 days. More vigorous drying procedures, such as thermovacuum treatment or suspension of the membranes over fresh P2O5, were not undertaken since a complete drying of the membrane under these conditions can irreversibly damage or destroy the membrane structure. Even with the naked eye one can witness major changes. In fact, if the samples are dried with P2O5 they become brittle compared to the soft and ¯exible solvent dried membranes. The dense membranes were prepared by casting a 20 wt% solution of polymer in acetone on a clean glass surface. Acetone evaporation was completed in an atmosphere nearly saturated with acetone vapor for 3 days. The membranes were then kept in a desiccator for 15 days.
237
separated by a porous plate (membrane support). Six membrane cells (surface area of 13.210ÿ4 m2) were placed in series. The membranes were ®rst compacted by circulating deionized water (conductivity below 0.2 mS/cm) at 40 bar, 258C and 0.6 l/min, for at least 8 h. The stabilization time for each experimental run is 30 min. The initial feed concentration of NaCl was 3500 mg/l, and the concentrations at the feed and the permeate were determined by conductivity, at 258C, using a Crison conductimeter (model 525). 2.3. AFM images
(1)
The AFM used to image the membrane surfaces is a multi mode scanning probe microscope with a Nanoscope IIIa Controller, supplied by Digital Instruments, USA. The membrane surface is scanned in intermediate contact (tapping mode) with an oscillating tip. This eliminates shear forces that can damage soft samples and reduce the image resolution. At ®rst, the tip is far from the sample surface bouncing up and down with ``free vibration'' amplitude. The tip then approaches the sample surface and due to the tip-surface interaction the vibration amplitude decreases. During the scanning procedure, the vibration amplitude of the tip is kept constant by changing the scanner height. Small pieces were cut from each membrane, glued onto metal disks and attached to a magnetic sample holder, located on top of the scanner tube. All the TM AFM images were undertaken at 258C. Differences in the membrane surface morphology can be expressed in terms of various roughness parameters, such as: 1. the difference between the highest and the lowest points within the given area, z; 2. the standard deviation of the z values within the given area (Rq). This parameter is calculated as sP zi ÿ zavg 2 (2) Rq Np
where (C)feed and (C)permeate are the concentration of NaCl in the bulk of the feed solution and in the permeate solution, respectively. The laboratory set-up used in permeation experiments has been previously described [14]. The membrane cell is a ¯at plate cell with two detachable parts
where zi is the current z value, zavg is the average of the z values within the given area and Np is the number of points within the given area; 3. the mean roughness (Ra). This parameter represents the mean value of the surface relative to the center plane, the plane for which the volumes enclosed by
2.2. Permeation experiments The permeation experiments were carried out with deionized water, to determine the membrane pure water ¯ux, Jw, and with a reference solution of NaCl p.a. (Pronalab) to determine the apparent rejection to this salt, fNaCl, and the permeate ¯ux, Jv. The apparent rejection to NaCl, fNaCl, is de®ned as fNaCl
Cfeed ÿ Cpermeate Cfeed
100
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the image above and below this plane are equal. It is calculated as 1 Ra Lx L y
ZLx ZLy j f x; yjdxdy 0
(3)
0
where f (x,y) is the surface relative to the center plane and Lx and Ly are the dimensions of the surface in the x and y directions, respectively. All the surface roughness parameters are calculated from the AFM images using an AFM software program. 3. Results and discussion The integrally skinned CA and CAB asymmetric membranes that are the main subject of this work cover a wide range of permeation properties. Accordingly, they display a broad spectrum of AFM 3D images corresponding to very different surface topographies. Table 2 shows the permeation ¯uxes Jw, Jv, and the apparent rejection coef®cient fNaCl of the membranes and the corresponding roughness parameters calculated from the AFM images presented in Figs. 1±7 with the use of an appropriate software program. It should be emphasized that all images are ¯attened in the same way and therefore the roughness values are relative and not absolute values. For comparison purposes, Fig. 1 shows the AFM images of the top and of the bottom surfaces of a dense
Fig. 1. AFM images of the dense symmetric CA membrane: (a) top and (b) bottom surface.
Table 2 Permeation properties [4] and roughness parameters of membranes Membrane
Jw (kg/m2 h)
Jv (kg/m2 h)
fNaCl (%)
Ra (nm)
Rq (nm)
z (nm)
CA dense top surface CA dense bottom surface CA-I-1 active layer CA-I-1 support layer CA-I-10 active layer CA-I-10 support layer CA-IV-1 active layer CA-IV-1 support layer
± ± 29 ± 0.3 ± 18 ±
± ± 28 ± 0.3 ± 19 ±
± ± 30 ± 98 ± 53 ±
0.40 0.93 5.14 16.70 1.86 3.40 0.91 4.82
0.3 1.2 6.9 22.1 2.5 4.2 1.2 6.1
1.1 9.8 53.3 204.6 20.1 31.5 20.5 36.9
CAB-I-1 active layer CAB-I-1 support layer CAB-IV-1 active layer CAB-IV-1 support layer
19 ± 9 ±
17 ± 9 ±
94 ± 98.2 ±
0.87 10.77 0.94 4.05
1.1 14.6 1.2 6.0
6.7 147.4 9.9 27.9
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Fig. 2. AFM image of the active layer of the asymmetric CA-I-1 membrane.
Fig. 4. AFM image of the active layer of the asymmetric CA-I-10 membrane.
symmetric CA membrane. Both surfaces show a relatively uniform structure, despite the fact that the statistical parameters Ra and z are higher for the membrane bottom surface than for the top surface (Table 2). These differences are due to the different nature of the air±solution and of the glass plate± solution interfaces. However, they are much smaller than the differences observed in the asymmetric membranes prepared by the phase inversion method. This asymmetry is always observed irrespective of the casting conditions.
For asymmetric CA membranes, despite the preparation method, the surface morphology of the active layer is characterized by a Ra parameter always at least two times higher than the one of the top surface of the dense CA membrane (see Table 2). These results show that the assumption made by various researchers that the structure of a dense ®lm is a representative model of the active layer of an asymmetric ®lm should be taken with precaution. The Ra parameter of the porous support of the asymmetricmembranesisatleastthreetimeshigherthan theoneof the bottom surface of the CA dense membrane.
Fig. 3. AFM image of the support layer of the asymmetric CA-I-1 membrane: (a) in 20 nm and (b) in 50 nm height data scale.
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Fig. 5. AFM image of the active layer of the asymmetric CA-IV-1 membrane.
Fig. 7. AFM image of the active layer of the asymmetric CAB-IV1 membrane.
In the 3D AFM image of the active layer of a CA-I-1 asymmetric membrane (Fig. 2) one can clearly observe the existence of large darker areas. These are associated to depressions on the surface and they are generally surrounded by high elevation regions made of grains. These grains probably correspond to nodule aggregates [15]. The phenomenon is more pronounced in the case of the support surface of the same CA-I-1 membrane, where large channel-like depressions are observed (see Fig. 3). In this case very high values of Ra and z are obtained. These observations are in agreement with the results
obtained from the ATR-FTIR spectra of the surfaces of the active layer and the porous support in CA and CAB membranes [4]. They show that larger water clusters, which may be associated to larger pores, were observed in the surface of the porous layer. The difference in morphology between the top and the support layer is expected when one considers the mechanism of preparation of asymmetric membranes by the phase inversion method [16]. The effect of the evaporation time is clearly illustrated by comparing the images of the active layers of CA-I-1 and CA-I-10 membranes (Figs. 2 and 4, respectively). In the active layer of the asymmetric membranes prepared using longer evaporation times, the average size of the darker depression areas observed in the AFM 3D images is smaller and their concentration is higher than for the membranes casted at shorter evaporation times. The difference in surface morphologies is also illustrated by the values of the statistical parameters Ra and z, which are much higher for CA-I-1. The higher values of Ra and z are associated with the increase of Jw, Jv and the decrease of fNaCl when the evaporation time decreases. The effect of the casting solution composition is illustrated by comparing the AFM images of the active layers of CA-I-1 and CA-IV-1 (Figs. 2 and 5, respectively) and of CAB-I-1 and CAB-IV-1 (Figs. 6 and 7, respectively) asymmetric membranes. Lower values of Ra and z are observed for the active layer of the CAIV-1 membranes than for the active layer of CA-I-1
Fig. 6. AFM image of the active layer of the asymmetric CAB-I-1 membrane.
D.F. Stamatialis et al. / Journal of Membrane Science 160 (1999) 235±242
membranes. Also, comparing the AFM images of the active layers of these membranes, it is possible to observe that the size of the nodule aggregates and of the depressions observed on the surface is much smaller in the active layer of the CA-IV-1 membrane. Accordingly, the apparent rejection of CA-IV-1 is higher when compared to that of CA-I-1. Also, when comparing CAB-IV-1 to CAB-I-1 membranes (Figs. 7 and 6, respectively) the ®rst ones display smaller and sphere-like depressions on the surface in contrast with larger and ``slit-like'' ones observed in CAB-I-1 membranes. The CA and CAB membranes casted by method I display higher ¯uxes and lower apparent rejection coef®cients when compared to the ones casted by method IV. This difference is correlated to the different casting solutions used. In ``method I'' the pore former additive is an electrophilic salt, Mg(ClO4)2. The aggregation of water (gelation medium) around the electrophilic cations causes the swelling of the polymer matrix and the formation of pores. In ``method IV'', mixtures of organic solvents of different polarities (and therefore of different solvent power) are used as pore additives and may play an important role in reducing the acetone solvent power and the water gelation power with subsequent decrease of pore size and increase of membrane porosity [17,18]. The comparison of the AFM images of the active layers of CA-I-1 and CA-IV-1 with the homologous CAB membranes (i.e., CAB-I-1 and CAB-IV) shows that generally larger nodule aggregates and higher values of the parameters characterizing the membrane surface roughness are observed in CA membranes. This difference in morphology and the higher hydrophilicity of CA could be the reason for the higher Jw, Jv and lower fNaCl values obtained for the CA-I-1 and CA-IV-1 membranes when compared to those obtained for the CAB-I-1 and CAB-IV-1 membranes, respectively. 4. Conclusions The membrane characteristics of both surfaces of a dense CA membranes and of a series of asymmetric CA and CAB membranes were investigated using Atomic Force Microscopy. The asymmetric membranes were prepared under different casting condi-
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tions and showed a wide range of NF/RO permeation characteristics. These characteristics depend on the surface topographies of the active layers. The analysis of the TM AFM images yield the following conclusions: The top and the bottom surfaces of the dense CA membrane are relatively uniform. The statistical parameters Ra and z, pertaining to surface roughness, are always much smaller for CA symmetric membranes than for the phase inversion asymmetric membranes. The asymmetry displayed by these membranes is clearly shown by the very large difference between the Ra and z values of the surfaces of the active layer and the support layer. This is observed for all the asymmetric membranes irrespective of the casting conditions used. In the preparation of asymmetric membranes, the use of an organic system as a non-solvent pore former (method IV) leads to smaller values of roughness parameters than those obtained with an inorganic swelling agent (method I). This is more pronounced for the CA than for the CAB asymmetric membranes. Smaller nodule aggregates and smaller depression areas are also observed on membranes prepared by method IV. Increasing evaporation times leads to lower values of the roughness parameters. Also, as the evaporation time increases, smaller depression areas on the surface and higher concentration of the darker depression areas are observed in the active layer of asymmetric membranes. The surface morphologies observed at the varying casting conditions are intrinsically associated with the permeation properties. For CA membranes, increasing evaporation times and organic-pore formers lead to higher apparent rejection and lower ¯uxes. These are always correlated to lower values of the roughness parameters, Ra and z. The CAB membranes always display much higher apparent rejection than the CA membranes prepared under the same casting conditions. Such higher apparent rejections are correlated to lower z values. 5. Nomenclature (C)feed
concentration of NaCl in the bulk of the feed solution (mg/l)
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(C)permeate CA CAB fNaCl Jv Jw NF Np RO SEM TEM ATR-FTIR TM AFM Ra Rq z zavg
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concentration of NaCl in the permeate solution (mg/l) cellulose acetate cellulose acetate butyrate apparent rejection to NaCl (%) permeate flux (kg/m2 h) water permeate flux (kg/m2 h) nanofiltration number of points within the given area used in the calculation of Rq reverse osmosis scanning electron microscopy transmission electron microscopy attenuated total reflectance Fourier transform infrared tapping mode atomic force microscopy mean surface roughness (nm) standard deviation of the z values within the given area (nm) difference between the highest and the lowest points within the given area (nm) average of the z values within the given area (nm)
Acknowledgements The authors wish to thank Prof. Pedro Brogueira (Department of Physics, I.S.T., Lisbon, Portugal) for his assistance with the AFM experiments. References [1] S. Loeb, S. Sourirajan, Sea water demineralisation by means of an osmotic membrane, Adv. Chem. Ser. 38 (1963) 117. [2] M.J. Rosa, M.N. de Pinho, M.H. Godinho, A.F. Martins, Optical polarising studies of cellulose acetate membranes prepared by phase inversion, Mol. Cryst. Liq. Cryst. 258 (1995) 163. [3] D. Murphy, M.N. de Pinho, An ATR-FTIR study of water in cellulose acetate membranes prepared by phase inversion, J. Membr. Sci. 106 (1995) 245. [4] C.R. Dias, M.J. Rosa, M.N. de Pinho, Structure of water in asymmetric cellulose ester membranes ± an ATR-FTIR study, J. Membr. Sci. 138 (1998) 259.
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