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Modification of cellulose based membranes by γ-radiation: Effect of cellulose content
Journal of Membrane Science 273 (2006) 25–30
Modification of cellulose based membranes by ␥-radiation: Effect of cellulose content R. de Lara, M.I. V´azquez, P. Gal´an, J. Benavente ∗ Grupo de Caracterizaci´on Electrocin´etica en Membranas e Interfases, Departamento de F´ısica Aplicada I, Facultad de Ciencias, Universidad de M´alaga, Campus de Teatinos, E-29071 M´alaga, Spain Received 14 June 2005; received in revised form 29 September 2005; accepted 29 September 2005 Available online 11 November 2005
Abstract Electrochemical characterization of three cellophane membranes with different content of regenerated cellulose was carried out by determining salt diffusion and ion transport numbers, which were obtained from diffusion and membrane potential measurements carried out with the membranes in contact with NaCl solutions at different concentrations. Results show a decrease for salt permeability with the increase of cellulose content in the membranes, which can be related with the decrease in the swelling degree of the samples containing higher amount of cellulose, while a slightly increase in the cation transport number was obtained. The effect of ␥-irradiation in structural and electrochemical parameters of these membranes was also studied. Two different radiation doses (10 J/kg and 80 J/kg) were used, which were delivered by a 60 Co Cobalt Unit. Changes in membrane permeability, cation and water transport numbers were obtained in order to determine structural and electrical modifications in the membrane matrix as a result of irradiation. According to the experimental results, ␥-radiation produces: (i) a reduction in salt permeability, which strongly depends on the membrane cellulose content (between 20% and 40% for 10 J/kg dose), but much lower reduction exist when the two different radiation doses for a given sample are compared; (ii) a slight increase in the negative character of cellophane membranes independently of the cellulose content. © 2005 Elsevier B.V. All rights reserved. Keywords: Regenerated cellulose; ␥-Irradiation; Membrane potential; Diffusional permeability; Water and ions transport numbers
1. Introduction Transport of electrolyte solutions across cellulose based membranes have been extensively studied due to the application of regenerated cellulose and its derivatives in the manufacture of membranes for different separation processes such as dialysis/hemodialysis, ultrafiltration and reverse osmosis [1,2]. Moreover, cellulose is also the basic material in the cell-wall of most plant cells [3] and nowadays cellophane (or regenerated cellulose) sheets are commonly used for food-packing applications [4], because of which transport parameters determination and cellophane membrane modifications are matter of interest in different fields. Cellulosic membranes behave as weak cationexchangers as a consequence of their negative charge associated to the oxidation in air of CH2 OH groups to COOH, which can affect the transport of ions and charged macromolecules (pro-
∗
Corresponding author.
0376-7388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2005.09.057
teins) [5–10]. It is also important to point out that membranes used for medical and biological applications are commonly sterilized by using temperature or radiation [11,12], which can modify the membrane parameters [13–16]. In fact, the effect of these treatments on hydrodynamic and electrical parameters as well as the chemical modification caused in a given cellophane membrane (containing 0.05 kg/m2 regenerated cellulose) has already been considered [17,18]. Moreover, differences in transport parameters across cellulosic membranes depending on the cellulose concentration and chemical conditions in the manufacturing casting solution have recently been reported, and the results show a sharp water flux reduction when the cellulose concentration increases, which is associated to changes in the membrane structure [19]; then, the effect of membrane treatment on transport parameters might be related to the manufacturing conditions. In this work, three cellophane membranes with different content of regenerated cellulose were studied in order to correlate structural and electrochemical parameters with the cellulose
26
R. de Lara et al. / Journal of Membrane Science 273 (2006) 25–30
content. The parameters determined were salt permeability, ion and water transport numbers, which were obtained from salt diffusion and membrane potential measurements carried out with the membranes in contact with NaCl solutions at different concentrations. The effect of ␥-irradiation on the cellophane membranes was established by determining modifications in the electrochemical parameters as a result of irradiation dose, which are associated to electrical and/or morphological changes in the membrane matrix, but they also depend on the content of regenerated cellulose in the membranes. The membrane samples were irradiated with two different doses, and the results show variation in their electrochemical parameters depending on the radiation dose considered, which seems to indicate interactions between electrical and morphological modifications. 2. Experimental 2.1. Membranes Cellophane sheets with different amount of regenerated cellulose (RC) from Cellophane Espa˜nola, S.A. (Burgos, Spain) were used; these membranes are named as C6 (0.06 kg/m2 RC; RC/dry membrane thickness ds = 1419 g/cm3 ), C5 (0.05 kg/m2 RC; ds = 1408 g/cm3 ) and C3 (0.03 kg/m2 RC, ds = 1384 g/cm3 ). The average thickness of dry and water-swollen membranes (xd ) and xw , respectively) was determined in six different points of the membrane samples by means of a digital micrometer, which allows the estimation of the swelling degree of the different membranes: Sw = (xw − xd )/xw . Fig. 1 shows the correlation between these two parameters, xw and Sw , and the amount of regenerated cellulose in the samples (error bars for experimental values are also indicated). As can be observed, the increase in cellulose content increases the membrane thickness but reduces its swelling, which indicates a closer membrane structure; according to that, a decrease in the water (or solution) up taken by the membrane matrix should be expected.
Fig. 1. Wet membrane thickness, xw (×) and swelling degree, Sw (), as a function of regenerate cellulose content of the cellophane membranes.
Samples of these membranes were treated with ionising radiation delivered by a Clinic 60 Co Cobalt Unit, the energy of photons ranged between 1.17 MeV and 1.33 MeV. The membranes were irradiated at the Hospital Regional Universitario “Carlos de Haya”, M´alaga (Spain). In order to ensure the energy delivered to the membrane (irradiation dose) corresponds to the chosen value and the uniformity of radiation, the membranes were placed inside a water-equivalent phantom and in the cross plane to beam axis, the source-phantom surface distance was 80 cm and the membrane placed to a depth of 1 cm in phantom and in the centre of the beam. Membrane irradiation position was chosen to verify the electronic-equilibrium condition (density energy compensation between ionisation loss and gain). To establish possible differences in membrane modification depending on the radiation delivered, two doses of radiation, 10 J/kg and 80 J/kg, were used and these membranes will hereafter be named as C6-Ir10, C6-Ir80, C5-Ir10, C5-Ir80, C3-Ir10 and C3-Ir80, respectively. Membrane thickness and swelling values of irradiated samples were also measured but their values are included in the corresponding error interval of non-treated membranes. Electrochemical measurements were carried out with the membranes in contact with aqueous NaCl solutions at different concentrations (2 × 10−3 ≤ c (M) ≤ 10−1 ), at room temperature t = (25.0 ± 0.3) ◦ C and standard pH (5.8 ± 0.3). Before use, the membranes were immersed for at least 10 h in a solution of the appropriate concentration. 2.2. Salt diffusion and membrane potential measurements The test cell used for the electrochemical measurements is similar to that described elsewhere [20], and it basically consists of two glass half-cells and the membranes were placed between them by using silicone rubber rings. In order to minimize concentration–polarization at the membrane surfaces, a magnetic stirrer was placed at the bottom of each half-cell and measurements were performed at a speed rate of 55 rad/s (or 525 rpm). • In salt diffusion measurements the membrane was initially separating a concentrated solution, C1 = 0.01 M, from a diluted one (initially distilled water, i.e., C2 = 0). Changes in the solution C2 were recorded versus time by means of a conductivity cell connected to a digital conductivity meter (Radiometer CDM 83); a conductivity cell was also placed in reservoir 1 to control the constancy of concentration C1 , replacing it by fresh one if it was necessary. • The electrical potential difference at both sides of the membranes caused by a concentration gradient was measured by two reversible Ag/AgCl electrodes connected to a digital voltmeter (Yokohama 7552, 1 G input resistance). Measurements were carried out by keeping the concentration of the solution at one side of the membrane, C1 , constant (C1 = 0.01 M) and gradually changing the concentration of the solution at the other side, C2 , from 10−3 M to 0.1 M. Membrane potential, Φm , was obtained by subtracting
R. de Lara et al. / Journal of Membrane Science 273 (2006) 25–30
the electrode potential, Φe = −(RT/z F)ln(C1 /C2 ), from the measured values.
27
Table 1 m and water transport numSalt permeability, Ps , true cation transport number, t+ ber, tw , for the different membranes
3. Results and discussion
Membrane
Ps (×10−6 m/s)
m t+
Salt or diffusional permeability across a membrane, Ps , can be obtained from diffusion measurements by means of the Fick’s first law for a quasi-steady state (1). The flux of solute (Js = [(dn/dt)(1/Sm )]) through a membrane can be written as: dC2 Vo Js = Ps (C1 − C2 ) = (1) Sm dt
C6 C6-IrlO C6-Ir80 C5 C5-IrlO C5-Ir80 C3 C3-IrlO C3-Ir80
1.33 1.05 1.02 2.18 1.46 1.36 2.32 1.31 1.10
0.664 0.696 0.692 0.618 0.645 0.709 0.612 0.629 0.600
where Vo and Sm are the volume of the solution at the side of concentration C2 , and the membrane area, respectively. Then, the following expression can be obtained: Sm dC2 Ps dt (2) = [C1 − C2 (t)] Vo if solution conductivity (σ) is considered instead of concentration, Eq. (2) can be written as dσ Sm dσ2 = Ps dt (3) [σ1 − σ2 (t)] Vo dc e where (dσ/dc)e is a parameter characteristic of each electrolyte at a given temperature. Fig. 2a shows the variation in the conductivity of solution 2 (σ 2 ) as a function of time for some of the studied membranes (C3, C5 and C6 to compare the RC content, and C3-Ir10 and C3-Ir80, in order to see the effect of irradiation doses). From the slopes of these straight lines, salt permeability across each membrane sample was obtained by means of Eq. (3), and their values are indicated in Table 1. A significant decrease in salt permeability with the increase in the cellulose content can be seen, which is in agreement with the reduction in the membrane swelling indicated above (compaction of the membrane structure). As can be observed, cellulose content and radiation doses affect the permeability across cellophane membranes. Particularly, a reduction of 43% in the diffusive permeability across membrane C3 was obtained as a result of irradiation with 10 J/kg dose, but this figure decrease to 16% after irradiation dose of 80 J/kg; the same
tw ± ± ± ± ± ± ± ± ±
0.010 0.007 0.012 0.009 0.006 0.016 0.012 0.006 0.010
105 144 178 119 134 198 153 132 104
± ± ± ± ± ± ± ± ±
15 10 19 10 7 18 14 8 14
kind of tendency was obtained for the other membranes, with a reduction in salt permeability of 33% and 7%, respectively, for C5 samples, while the figures for C6 are 21% and 3%. These values are drawn in Fig. 2b to see the linear dependence existing between permeability reduction and RC content for the studied cellophane membranes. The different slope corresponding to each radiation dose seems to indicate different modification (length or cross-linking of the polymer chains) depending on the radiation dose. In order to see the effect of irradiation on the effective fixed charge of the different cellulose samples, membrane potential measurements were performed. Membrane potential, φm , is the electrical potential difference at both sides of a membrane separating two solutions of the same electrolyte but different concentrations (C1 and C2 ). According to the Teorell–Meyer–Sievers theory [21,22], membrane potential can be considered as the sum of two Donnan potentials (one at each membrane–solution interface) plus a diffusion potential in the membrane: φm = φDon(I) + φdif + φDon(II) . Donnan potential can be expressed as [21]: 1/2 2 RT wXf wXf (4) φDon = ln +1 + F 2C 2C where Xf is the membrane fixed charge concentration, w = −1 or +1 for negatively or positively charged membranes, respectively,
Fig. 2. (a) Variation of salt conductivity in compartment 2, σ 2 , vs. time. (♦) Membrane C3, () membrane C5, () membrane C6, () membrane C3-Ir10, () membrane C3-Ir80. (b) Permeability reduction due to ␥-radiation as a function of cellulose content: () 10 J/kg dose, () 80 J/kg dose.
R. de Lara et al. / Journal of Membrane Science 273 (2006) 25–30
28
R and F are the gas and Faraday constants and T is the thermodynamic temperature of the system. Diffusion potential for 1:1 electrolytes (|z+ | = |z− | = 1) is given by [23]: φdif = =
RT F RT F
[t− − t+ ] ln
(1 − 2t+ ) ln
a1 a2 av af
(5)
where ai is the activity of the solution at each side of the membrane (concentration can be used instead of activities when diluted solutions are considered) and ti is the transport number of the ion i in the membrane; ti represents the amount of current transported for ion i with respect to the total current crossing the membrane, ti = Ii /IT , and for single salts, t+ + t− = 1. Fig. 3 shows the membrane potential versus salt concentration, φm versus ln(Cv /Cc ), for the different membrane samples, but theoretical potentials for an ideal cation-exchange membrane (t− = 0 and t+ = 1) for the same range of concentration are also indicated in Fig. 3. Two different φm − ln(Cv /Cc ) relationships can be observed in Fig. 3 depending on the value of the variable salt concentration:
(i) When Cv ranged between 0.002 M and 0.01 M, the membrane potential tendency is similar to that represented by the theoretical values for ideal cation-exchange membranes (Nernst potential), and only slight differences in φm values for this interval of concentration were obtained for the studied membranes, independently of the cellulose content or radiation doses. (ii) For Cv higher than 0.02 M, the Donnan exclusion of anions can be neglected and the membrane potentials are mainly due to the diffusion of counter (t+ ) and co-ions (t− ) through the membrane. Clear differences were obtained when pristine and irradiated samples are compared, which indicates differences in the transport of ions through the different membranes. Assuming the membrane potential for Cv > 0.02 M is practically a diffusion potential, cation transport number in the membranes can be estimated by Eq. (5). However, water transport was not considered in the previous discussion, then the ion transport numbers obtained are refereed to the solution instead to the membrane [24], and they are usually called apparent ion ap ap transport numbers (ti ). Fig. 4 shows the variation of t+ with the average concentration, Cavg = (Cc + Cv )/2, for concentrations higher than 0.02 M, for the untreated and Ir-10 irradiated membranes, but similar dependence was obtained for Ir-80 samples. As can be observed, apparent ion transport number across the
Fig. 3. Membrane potential, Φm , vs. solution activity ratio, av /ac . (a) Membrane C3 (♦), membrane C5 (), membrane C6 (). (b) Membrane C3-Ir10 (), membrane C5-Ir10 (), membrane C6-Ir10 (䊉). (c) Membrane C3-Ir80 (*), membrane C5-Ir80 (), membrane C6-Ir80 ().
R. de Lara et al. / Journal of Membrane Science 273 (2006) 25–30
29
ap
Fig. 4. Apparent cation transport number, t+ , vs. average concentration, Cavg . Membrane C3 (♦), membrane C5 (), membrane C6 (); membrane C3-Ir10 (), membrane C5-Ir10 (), membrane C6-Ir10 (䊉).
membranes depends on the cellulose content; moreover, higher values for the apparent cation transport number across all the irradiated samples were obtained, which indicates the increase of the electronegative character of these membranes as a result of ␥-irradiation. Scatchard obtained the following relationship between apparm ) in a membrane [25]: ent and true cation transport number (t+ ap
m − 0.0018tw (Cavg ) t+ = t+
(6)
where tw represent the water transport number. From the slopes of the straight lines shown in Fig. 4, taking into account Eq. m and t values in the different membrane samples were (6), t+ w obtained and they are also indicated in Table 1. A slightly difference in the cation transport number associated to the increase of cellulose content can be observed, but a more strong effect for water transport number was obtained. In fact, a linear decrease for tw with the decrease in the membrane swelling (or the increase in the cellulose content) was obtained as is shown in Fig. 5 for pristine membranes; this result also indicate a more compact structure for membranes with high cellulose content in agreement with the permeability results determined from diffusion measurements. However, a completely different tw − Sw dependence was obtained for the samples irradiated with a dose of 10 J/kg as can also be seen in Fig. 5, but any kind of correlation for the samples irradiated with the dose of 80 J/kg was m and t for C6 and C5 determined. Then, an increase for both t+ w samples when the irradiation dose increases can be considered, while membrane C3 does not present such effect. Taking into account these results, correlation between structural and electrical modifications in the cellophane matrix associated to both the cellulose content and the irradiation with a dose of 10 J/kg may be established, but another kind of interaction (or more complex effects) might exist as a result of membrane irradiation at the highest dose. In order to correlate variation in transport parameters with membrane matrix modification, infrared analysis with different
Fig. 5. Water transport number, tw , vs. membrane swelling degree, Sw , for different samples: (䊉) non-irradiated membranes, () 10 J/kg irradiation dose.
samples of membrane C5 was carried out in previous studies [16,26]. Those results showed structural modifications due to both membrane irradiation and annealing (sample C5-H) as can be observed in Fig. 6. The small differences obtained in the background for 2000–3000 cm−1 among membrane C5 and the irradiated samples (C5-Ir10 and C5-Ir80) may be due to chemical modification of the cellulose matrix, but the result obtained for the annealed sample (heated at 60 ◦ C for 4 h) indicates that another kind of modification should also exists; particularly, changes in hydrodynamic permeability of the annealed samples were obtained [17]. Similar behaviour is assumed for the other cellulosic membranes [26]. Since heated is a secondary effect of irradiation, both types of modifications must take place in the case of ␥-irradiated samples, which cannot be easily separated. Work to elucidate that point is currently underway. It should be pointed out that modification in transport parameters for cellulose-based membranes must be considered
Fig. 6. Infrared spectra for untreated C5 membrane and differently treated samples: C5 (solid line), C5-H (dash line), C5-Ir10 (dot line), C5-Ir80 (dash dot line).
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R. de Lara et al. / Journal of Membrane Science 273 (2006) 25–30
when ␥-radiation is used (commonly, membrane sterilization for biological and medical applications), which could also be extended to other polymeric membranes.
[9] [10]
4. Conclusions [11]
The increase of regenerated cellulose in the fabrication of dense cellophane membranes causes a decrease in diffusional permeability, but it slightly affects the transport of ions across the membrane as a result of a more tight structure ␥-Irradiation affects the transport parameters of cellulose based membranes by decreasing salt permeability and increasing cation transport number in the membranes when a low radiation dose is delivered (10 J/kg), which seems to indicate structural/morphological changes (polymer chains crosslinking); however, another kind of modification (or interconnected effects) can also appear when high irradiation doses are used for membrane treatment.
[12]
[13] [14]
[15]
Acknowledgement
[16]
We thank the Comisi´on Interministerial de Ciencia y Tecnolog´ıa (CICYT, Spain, Project MAT2003-03328), for financial support.
[17]
[18]
References [1] M. Mulder, Basic Principles of Membrane Technology, Kluwer Acad. Publishers, Dordrecht, The Netherland, 1992. [2] K. Sakai, Determination of pore size and pore size distribution. 2. Dialysis membranes, J. Membr. Sci. 96 (1994) 91. [3] C.J. van Oss, Ultrafiltration membrane performance, Science 139 (1963) 1123. [4] Y. Kimura, H.-J. Lim, T. Ijima, Membrane potential of charged cellulosic membranes, J. Membr. Sci. 18 (1984) 285. [5] Y. Kimura, H.-J. Lim, T. Ijima, Permeability of alkali chlorides through charged cellulosic membranes, Die Angew. Makrom. Chemie 138 (1986) 151. [6] M. Okada, T. Watanabe, K. Imamura, T. Tsurumi, Y. Suma, K. Sakai, Ionic strength affects diffusive permeability to an inorganic phosphate ion of negatively charged dialysis membranes, Trans. Am. Soc. Artif. Organs 36 (1990) 324. [7] J. Benavente, A study on the variation with temperature of fixed charge and membrane structure of cellophane membranes, Sep. Sci. Technol. 26 (1991) 189. [8] A. Ca˜nas, M.J. Ariza, J. Benavente, A comparison of the electrochemical and electrokinetic parameters determined for cellophane membranes in
[19]
[20]
[21] [22] [23] [24] [25] [26]
contact with NaCl and NaNO3 solutions, J. Colloid Interface Sci. 246 (2002) 150. K. Esau, Plant Anatomy, John Wiley & Sons, New York, 1953. M.A. del Nobile, P. Fava, L. Piergiovanni, Water transport properties of cellophane flexible films intended for food packing applications, J. Food Eng. 53 (2002) 295. F. Aucella, M. Vigilante, G. Gatta, E. Grandone, D. Colaizzo, M. Margaglione, S. Modoni, C. Stallone, Effects of ethylene oxide and steam sterilization on dialysis-induced cytokine release by cuprophan membrane, Artif. Organs 26 (2002) 543. M.C. B´elanger, Y. Marois, R. Roy, Y. Mehri, E. Wagner, Z. Zhang, M.W. King, M. Yang, Ch. Hahn, R. Guidoin, Selection of a polyurethane membrane for the manufacture of ventricles for a totally implantable artificial heart: blood compatibility and biocompatibility studies, Artif. Organs 24 (2000) 879. M. Nystr¨om, Fouling of unmodified and modified polysulfone membranes by ovoalbumin, J. Membr. Sci. 44 (1989) 183. J. Benavente, M.I. V´azquez, J. de Abajo, Effect of UV light on different structural and transport parameters of cellophane membranes, Sep. Sci. Technol. 31 (1996) 189. J. Pieracci, J.V. Crivello, G. Belford, Increasing membrane permeability of UV-modified poly(ether sulfone) ultrafiltration membranas, J. Membr. Sci. 202 (2002) 1. L. Shao, T.-S. Chung, S.H. Goh, K.P. Pramoda, Polyimide modification by a linear aliphatic diamine to enhance transport performance and plasticization resistance, J. Membr. Sci. 256 (2005) 46. M.I. V´azquez, J. Benavente, A study of temperature effect on chemical, structural and transport parameters determined for two different regenerated cellulose membranes, J. Membr. Sci. 219 (2003) 59. M.I. V´azquez, P. Gal´an, J. Casado, M.J. Ariza, J. Benavente, Effect of radiation and thermal treatment on structural and transport parameters for cellulose regenerated membranes, Appl. Surf. Sci. 238 (2004) 415. Y.-N. Kuo, J. Hong, A new method for cellulose membrane fabrication and the determination of its characteristics, J. Colloid Interface Sci. 285 (2005) 232. J. Benavente, A. Mu˜noz, A. Heredia, Electrokinetic parameters of ion transport across isolated pepper cuticular membranes, J. Membr. Sci. 139 (1998) 147. K.H. Meyer, J.F. Siever, La permeabilit´e des membranes. I. Theorie de la permeabilit´e ionique, Helv. Chim. Acta 19 (1936) 649. T. Teorell, Transport phenomena in membranes, Discuss. Faraday Soc. 21 (1956) 9. N. Lakshminarayanaiah, Transport Phenomena in Membranes, Academic Press, New York, USA, 1969. A.J. Staverman, Non-equilibrium thermodynamics of membrane processes, Trans. Faraday Soc. 48 (1952) 176. G.J. Scartchard, Ion-exchange electrodes, J. Am. Chem. Soc. 75 (1953) 2883. J. Casado, P. Gal´an, J. Benavente, Modificaci´on estructural y qu´ımica (superficial) de membranas celul´osicas debido a radiaciones ionizantes, F´ısica M´edica, submitted for publication.
Modification of cellulose based membranes by ␥-radiation: Effect of cellulose content R. de Lara, M.I. V´azquez, P. Gal´an, J. Benavente ∗ Grupo de Caracterizaci´on Electrocin´etica en Membranas e Interfases, Departamento de F´ısica Aplicada I, Facultad de Ciencias, Universidad de M´alaga, Campus de Teatinos, E-29071 M´alaga, Spain Received 14 June 2005; received in revised form 29 September 2005; accepted 29 September 2005 Available online 11 November 2005
Abstract Electrochemical characterization of three cellophane membranes with different content of regenerated cellulose was carried out by determining salt diffusion and ion transport numbers, which were obtained from diffusion and membrane potential measurements carried out with the membranes in contact with NaCl solutions at different concentrations. Results show a decrease for salt permeability with the increase of cellulose content in the membranes, which can be related with the decrease in the swelling degree of the samples containing higher amount of cellulose, while a slightly increase in the cation transport number was obtained. The effect of ␥-irradiation in structural and electrochemical parameters of these membranes was also studied. Two different radiation doses (10 J/kg and 80 J/kg) were used, which were delivered by a 60 Co Cobalt Unit. Changes in membrane permeability, cation and water transport numbers were obtained in order to determine structural and electrical modifications in the membrane matrix as a result of irradiation. According to the experimental results, ␥-radiation produces: (i) a reduction in salt permeability, which strongly depends on the membrane cellulose content (between 20% and 40% for 10 J/kg dose), but much lower reduction exist when the two different radiation doses for a given sample are compared; (ii) a slight increase in the negative character of cellophane membranes independently of the cellulose content. © 2005 Elsevier B.V. All rights reserved. Keywords: Regenerated cellulose; ␥-Irradiation; Membrane potential; Diffusional permeability; Water and ions transport numbers
1. Introduction Transport of electrolyte solutions across cellulose based membranes have been extensively studied due to the application of regenerated cellulose and its derivatives in the manufacture of membranes for different separation processes such as dialysis/hemodialysis, ultrafiltration and reverse osmosis [1,2]. Moreover, cellulose is also the basic material in the cell-wall of most plant cells [3] and nowadays cellophane (or regenerated cellulose) sheets are commonly used for food-packing applications [4], because of which transport parameters determination and cellophane membrane modifications are matter of interest in different fields. Cellulosic membranes behave as weak cationexchangers as a consequence of their negative charge associated to the oxidation in air of CH2 OH groups to COOH, which can affect the transport of ions and charged macromolecules (pro-
∗
Corresponding author.
0376-7388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2005.09.057
teins) [5–10]. It is also important to point out that membranes used for medical and biological applications are commonly sterilized by using temperature or radiation [11,12], which can modify the membrane parameters [13–16]. In fact, the effect of these treatments on hydrodynamic and electrical parameters as well as the chemical modification caused in a given cellophane membrane (containing 0.05 kg/m2 regenerated cellulose) has already been considered [17,18]. Moreover, differences in transport parameters across cellulosic membranes depending on the cellulose concentration and chemical conditions in the manufacturing casting solution have recently been reported, and the results show a sharp water flux reduction when the cellulose concentration increases, which is associated to changes in the membrane structure [19]; then, the effect of membrane treatment on transport parameters might be related to the manufacturing conditions. In this work, three cellophane membranes with different content of regenerated cellulose were studied in order to correlate structural and electrochemical parameters with the cellulose
26
R. de Lara et al. / Journal of Membrane Science 273 (2006) 25–30
content. The parameters determined were salt permeability, ion and water transport numbers, which were obtained from salt diffusion and membrane potential measurements carried out with the membranes in contact with NaCl solutions at different concentrations. The effect of ␥-irradiation on the cellophane membranes was established by determining modifications in the electrochemical parameters as a result of irradiation dose, which are associated to electrical and/or morphological changes in the membrane matrix, but they also depend on the content of regenerated cellulose in the membranes. The membrane samples were irradiated with two different doses, and the results show variation in their electrochemical parameters depending on the radiation dose considered, which seems to indicate interactions between electrical and morphological modifications. 2. Experimental 2.1. Membranes Cellophane sheets with different amount of regenerated cellulose (RC) from Cellophane Espa˜nola, S.A. (Burgos, Spain) were used; these membranes are named as C6 (0.06 kg/m2 RC; RC/dry membrane thickness ds = 1419 g/cm3 ), C5 (0.05 kg/m2 RC; ds = 1408 g/cm3 ) and C3 (0.03 kg/m2 RC, ds = 1384 g/cm3 ). The average thickness of dry and water-swollen membranes (xd ) and xw , respectively) was determined in six different points of the membrane samples by means of a digital micrometer, which allows the estimation of the swelling degree of the different membranes: Sw = (xw − xd )/xw . Fig. 1 shows the correlation between these two parameters, xw and Sw , and the amount of regenerated cellulose in the samples (error bars for experimental values are also indicated). As can be observed, the increase in cellulose content increases the membrane thickness but reduces its swelling, which indicates a closer membrane structure; according to that, a decrease in the water (or solution) up taken by the membrane matrix should be expected.
Fig. 1. Wet membrane thickness, xw (×) and swelling degree, Sw (), as a function of regenerate cellulose content of the cellophane membranes.
Samples of these membranes were treated with ionising radiation delivered by a Clinic 60 Co Cobalt Unit, the energy of photons ranged between 1.17 MeV and 1.33 MeV. The membranes were irradiated at the Hospital Regional Universitario “Carlos de Haya”, M´alaga (Spain). In order to ensure the energy delivered to the membrane (irradiation dose) corresponds to the chosen value and the uniformity of radiation, the membranes were placed inside a water-equivalent phantom and in the cross plane to beam axis, the source-phantom surface distance was 80 cm and the membrane placed to a depth of 1 cm in phantom and in the centre of the beam. Membrane irradiation position was chosen to verify the electronic-equilibrium condition (density energy compensation between ionisation loss and gain). To establish possible differences in membrane modification depending on the radiation delivered, two doses of radiation, 10 J/kg and 80 J/kg, were used and these membranes will hereafter be named as C6-Ir10, C6-Ir80, C5-Ir10, C5-Ir80, C3-Ir10 and C3-Ir80, respectively. Membrane thickness and swelling values of irradiated samples were also measured but their values are included in the corresponding error interval of non-treated membranes. Electrochemical measurements were carried out with the membranes in contact with aqueous NaCl solutions at different concentrations (2 × 10−3 ≤ c (M) ≤ 10−1 ), at room temperature t = (25.0 ± 0.3) ◦ C and standard pH (5.8 ± 0.3). Before use, the membranes were immersed for at least 10 h in a solution of the appropriate concentration. 2.2. Salt diffusion and membrane potential measurements The test cell used for the electrochemical measurements is similar to that described elsewhere [20], and it basically consists of two glass half-cells and the membranes were placed between them by using silicone rubber rings. In order to minimize concentration–polarization at the membrane surfaces, a magnetic stirrer was placed at the bottom of each half-cell and measurements were performed at a speed rate of 55 rad/s (or 525 rpm). • In salt diffusion measurements the membrane was initially separating a concentrated solution, C1 = 0.01 M, from a diluted one (initially distilled water, i.e., C2 = 0). Changes in the solution C2 were recorded versus time by means of a conductivity cell connected to a digital conductivity meter (Radiometer CDM 83); a conductivity cell was also placed in reservoir 1 to control the constancy of concentration C1 , replacing it by fresh one if it was necessary. • The electrical potential difference at both sides of the membranes caused by a concentration gradient was measured by two reversible Ag/AgCl electrodes connected to a digital voltmeter (Yokohama 7552, 1 G input resistance). Measurements were carried out by keeping the concentration of the solution at one side of the membrane, C1 , constant (C1 = 0.01 M) and gradually changing the concentration of the solution at the other side, C2 , from 10−3 M to 0.1 M. Membrane potential, Φm , was obtained by subtracting
R. de Lara et al. / Journal of Membrane Science 273 (2006) 25–30
the electrode potential, Φe = −(RT/z F)ln(C1 /C2 ), from the measured values.
27
Table 1 m and water transport numSalt permeability, Ps , true cation transport number, t+ ber, tw , for the different membranes
3. Results and discussion
Membrane
Ps (×10−6 m/s)
m t+
Salt or diffusional permeability across a membrane, Ps , can be obtained from diffusion measurements by means of the Fick’s first law for a quasi-steady state (1). The flux of solute (Js = [(dn/dt)(1/Sm )]) through a membrane can be written as: dC2 Vo Js = Ps (C1 − C2 ) = (1) Sm dt
C6 C6-IrlO C6-Ir80 C5 C5-IrlO C5-Ir80 C3 C3-IrlO C3-Ir80
1.33 1.05 1.02 2.18 1.46 1.36 2.32 1.31 1.10
0.664 0.696 0.692 0.618 0.645 0.709 0.612 0.629 0.600
where Vo and Sm are the volume of the solution at the side of concentration C2 , and the membrane area, respectively. Then, the following expression can be obtained: Sm dC2 Ps dt (2) = [C1 − C2 (t)] Vo if solution conductivity (σ) is considered instead of concentration, Eq. (2) can be written as dσ Sm dσ2 = Ps dt (3) [σ1 − σ2 (t)] Vo dc e where (dσ/dc)e is a parameter characteristic of each electrolyte at a given temperature. Fig. 2a shows the variation in the conductivity of solution 2 (σ 2 ) as a function of time for some of the studied membranes (C3, C5 and C6 to compare the RC content, and C3-Ir10 and C3-Ir80, in order to see the effect of irradiation doses). From the slopes of these straight lines, salt permeability across each membrane sample was obtained by means of Eq. (3), and their values are indicated in Table 1. A significant decrease in salt permeability with the increase in the cellulose content can be seen, which is in agreement with the reduction in the membrane swelling indicated above (compaction of the membrane structure). As can be observed, cellulose content and radiation doses affect the permeability across cellophane membranes. Particularly, a reduction of 43% in the diffusive permeability across membrane C3 was obtained as a result of irradiation with 10 J/kg dose, but this figure decrease to 16% after irradiation dose of 80 J/kg; the same
tw ± ± ± ± ± ± ± ± ±
0.010 0.007 0.012 0.009 0.006 0.016 0.012 0.006 0.010
105 144 178 119 134 198 153 132 104
± ± ± ± ± ± ± ± ±
15 10 19 10 7 18 14 8 14
kind of tendency was obtained for the other membranes, with a reduction in salt permeability of 33% and 7%, respectively, for C5 samples, while the figures for C6 are 21% and 3%. These values are drawn in Fig. 2b to see the linear dependence existing between permeability reduction and RC content for the studied cellophane membranes. The different slope corresponding to each radiation dose seems to indicate different modification (length or cross-linking of the polymer chains) depending on the radiation dose. In order to see the effect of irradiation on the effective fixed charge of the different cellulose samples, membrane potential measurements were performed. Membrane potential, φm , is the electrical potential difference at both sides of a membrane separating two solutions of the same electrolyte but different concentrations (C1 and C2 ). According to the Teorell–Meyer–Sievers theory [21,22], membrane potential can be considered as the sum of two Donnan potentials (one at each membrane–solution interface) plus a diffusion potential in the membrane: φm = φDon(I) + φdif + φDon(II) . Donnan potential can be expressed as [21]: 1/2 2 RT wXf wXf (4) φDon = ln +1 + F 2C 2C where Xf is the membrane fixed charge concentration, w = −1 or +1 for negatively or positively charged membranes, respectively,
Fig. 2. (a) Variation of salt conductivity in compartment 2, σ 2 , vs. time. (♦) Membrane C3, () membrane C5, () membrane C6, () membrane C3-Ir10, () membrane C3-Ir80. (b) Permeability reduction due to ␥-radiation as a function of cellulose content: () 10 J/kg dose, () 80 J/kg dose.
R. de Lara et al. / Journal of Membrane Science 273 (2006) 25–30
28
R and F are the gas and Faraday constants and T is the thermodynamic temperature of the system. Diffusion potential for 1:1 electrolytes (|z+ | = |z− | = 1) is given by [23]: φdif = =
RT F RT F
[t− − t+ ] ln
(1 − 2t+ ) ln
a1 a2 av af
(5)
where ai is the activity of the solution at each side of the membrane (concentration can be used instead of activities when diluted solutions are considered) and ti is the transport number of the ion i in the membrane; ti represents the amount of current transported for ion i with respect to the total current crossing the membrane, ti = Ii /IT , and for single salts, t+ + t− = 1. Fig. 3 shows the membrane potential versus salt concentration, φm versus ln(Cv /Cc ), for the different membrane samples, but theoretical potentials for an ideal cation-exchange membrane (t− = 0 and t+ = 1) for the same range of concentration are also indicated in Fig. 3. Two different φm − ln(Cv /Cc ) relationships can be observed in Fig. 3 depending on the value of the variable salt concentration:
(i) When Cv ranged between 0.002 M and 0.01 M, the membrane potential tendency is similar to that represented by the theoretical values for ideal cation-exchange membranes (Nernst potential), and only slight differences in φm values for this interval of concentration were obtained for the studied membranes, independently of the cellulose content or radiation doses. (ii) For Cv higher than 0.02 M, the Donnan exclusion of anions can be neglected and the membrane potentials are mainly due to the diffusion of counter (t+ ) and co-ions (t− ) through the membrane. Clear differences were obtained when pristine and irradiated samples are compared, which indicates differences in the transport of ions through the different membranes. Assuming the membrane potential for Cv > 0.02 M is practically a diffusion potential, cation transport number in the membranes can be estimated by Eq. (5). However, water transport was not considered in the previous discussion, then the ion transport numbers obtained are refereed to the solution instead to the membrane [24], and they are usually called apparent ion ap ap transport numbers (ti ). Fig. 4 shows the variation of t+ with the average concentration, Cavg = (Cc + Cv )/2, for concentrations higher than 0.02 M, for the untreated and Ir-10 irradiated membranes, but similar dependence was obtained for Ir-80 samples. As can be observed, apparent ion transport number across the
Fig. 3. Membrane potential, Φm , vs. solution activity ratio, av /ac . (a) Membrane C3 (♦), membrane C5 (), membrane C6 (). (b) Membrane C3-Ir10 (), membrane C5-Ir10 (), membrane C6-Ir10 (䊉). (c) Membrane C3-Ir80 (*), membrane C5-Ir80 (), membrane C6-Ir80 ().
R. de Lara et al. / Journal of Membrane Science 273 (2006) 25–30
29
ap
Fig. 4. Apparent cation transport number, t+ , vs. average concentration, Cavg . Membrane C3 (♦), membrane C5 (), membrane C6 (); membrane C3-Ir10 (), membrane C5-Ir10 (), membrane C6-Ir10 (䊉).
membranes depends on the cellulose content; moreover, higher values for the apparent cation transport number across all the irradiated samples were obtained, which indicates the increase of the electronegative character of these membranes as a result of ␥-irradiation. Scatchard obtained the following relationship between apparm ) in a membrane [25]: ent and true cation transport number (t+ ap
m − 0.0018tw (Cavg ) t+ = t+
(6)
where tw represent the water transport number. From the slopes of the straight lines shown in Fig. 4, taking into account Eq. m and t values in the different membrane samples were (6), t+ w obtained and they are also indicated in Table 1. A slightly difference in the cation transport number associated to the increase of cellulose content can be observed, but a more strong effect for water transport number was obtained. In fact, a linear decrease for tw with the decrease in the membrane swelling (or the increase in the cellulose content) was obtained as is shown in Fig. 5 for pristine membranes; this result also indicate a more compact structure for membranes with high cellulose content in agreement with the permeability results determined from diffusion measurements. However, a completely different tw − Sw dependence was obtained for the samples irradiated with a dose of 10 J/kg as can also be seen in Fig. 5, but any kind of correlation for the samples irradiated with the dose of 80 J/kg was m and t for C6 and C5 determined. Then, an increase for both t+ w samples when the irradiation dose increases can be considered, while membrane C3 does not present such effect. Taking into account these results, correlation between structural and electrical modifications in the cellophane matrix associated to both the cellulose content and the irradiation with a dose of 10 J/kg may be established, but another kind of interaction (or more complex effects) might exist as a result of membrane irradiation at the highest dose. In order to correlate variation in transport parameters with membrane matrix modification, infrared analysis with different
Fig. 5. Water transport number, tw , vs. membrane swelling degree, Sw , for different samples: (䊉) non-irradiated membranes, () 10 J/kg irradiation dose.
samples of membrane C5 was carried out in previous studies [16,26]. Those results showed structural modifications due to both membrane irradiation and annealing (sample C5-H) as can be observed in Fig. 6. The small differences obtained in the background for 2000–3000 cm−1 among membrane C5 and the irradiated samples (C5-Ir10 and C5-Ir80) may be due to chemical modification of the cellulose matrix, but the result obtained for the annealed sample (heated at 60 ◦ C for 4 h) indicates that another kind of modification should also exists; particularly, changes in hydrodynamic permeability of the annealed samples were obtained [17]. Similar behaviour is assumed for the other cellulosic membranes [26]. Since heated is a secondary effect of irradiation, both types of modifications must take place in the case of ␥-irradiated samples, which cannot be easily separated. Work to elucidate that point is currently underway. It should be pointed out that modification in transport parameters for cellulose-based membranes must be considered
Fig. 6. Infrared spectra for untreated C5 membrane and differently treated samples: C5 (solid line), C5-H (dash line), C5-Ir10 (dot line), C5-Ir80 (dash dot line).
30
R. de Lara et al. / Journal of Membrane Science 273 (2006) 25–30
when ␥-radiation is used (commonly, membrane sterilization for biological and medical applications), which could also be extended to other polymeric membranes.
[9] [10]
4. Conclusions [11]
The increase of regenerated cellulose in the fabrication of dense cellophane membranes causes a decrease in diffusional permeability, but it slightly affects the transport of ions across the membrane as a result of a more tight structure ␥-Irradiation affects the transport parameters of cellulose based membranes by decreasing salt permeability and increasing cation transport number in the membranes when a low radiation dose is delivered (10 J/kg), which seems to indicate structural/morphological changes (polymer chains crosslinking); however, another kind of modification (or interconnected effects) can also appear when high irradiation doses are used for membrane treatment.
[12]
[13] [14]
[15]
Acknowledgement
[16]
We thank the Comisi´on Interministerial de Ciencia y Tecnolog´ıa (CICYT, Spain, Project MAT2003-03328), for financial support.
[17]
[18]
References [1] M. Mulder, Basic Principles of Membrane Technology, Kluwer Acad. Publishers, Dordrecht, The Netherland, 1992. [2] K. Sakai, Determination of pore size and pore size distribution. 2. Dialysis membranes, J. Membr. Sci. 96 (1994) 91. [3] C.J. van Oss, Ultrafiltration membrane performance, Science 139 (1963) 1123. [4] Y. Kimura, H.-J. Lim, T. Ijima, Membrane potential of charged cellulosic membranes, J. Membr. Sci. 18 (1984) 285. [5] Y. Kimura, H.-J. Lim, T. Ijima, Permeability of alkali chlorides through charged cellulosic membranes, Die Angew. Makrom. Chemie 138 (1986) 151. [6] M. Okada, T. Watanabe, K. Imamura, T. Tsurumi, Y. Suma, K. Sakai, Ionic strength affects diffusive permeability to an inorganic phosphate ion of negatively charged dialysis membranes, Trans. Am. Soc. Artif. Organs 36 (1990) 324. [7] J. Benavente, A study on the variation with temperature of fixed charge and membrane structure of cellophane membranes, Sep. Sci. Technol. 26 (1991) 189. [8] A. Ca˜nas, M.J. Ariza, J. Benavente, A comparison of the electrochemical and electrokinetic parameters determined for cellophane membranes in
[19]
[20]
[21] [22] [23] [24] [25] [26]
contact with NaCl and NaNO3 solutions, J. Colloid Interface Sci. 246 (2002) 150. K. Esau, Plant Anatomy, John Wiley & Sons, New York, 1953. M.A. del Nobile, P. Fava, L. Piergiovanni, Water transport properties of cellophane flexible films intended for food packing applications, J. Food Eng. 53 (2002) 295. F. Aucella, M. Vigilante, G. Gatta, E. Grandone, D. Colaizzo, M. Margaglione, S. Modoni, C. Stallone, Effects of ethylene oxide and steam sterilization on dialysis-induced cytokine release by cuprophan membrane, Artif. Organs 26 (2002) 543. M.C. B´elanger, Y. Marois, R. Roy, Y. Mehri, E. Wagner, Z. Zhang, M.W. King, M. Yang, Ch. Hahn, R. Guidoin, Selection of a polyurethane membrane for the manufacture of ventricles for a totally implantable artificial heart: blood compatibility and biocompatibility studies, Artif. Organs 24 (2000) 879. M. Nystr¨om, Fouling of unmodified and modified polysulfone membranes by ovoalbumin, J. Membr. Sci. 44 (1989) 183. J. Benavente, M.I. V´azquez, J. de Abajo, Effect of UV light on different structural and transport parameters of cellophane membranes, Sep. Sci. Technol. 31 (1996) 189. J. Pieracci, J.V. Crivello, G. Belford, Increasing membrane permeability of UV-modified poly(ether sulfone) ultrafiltration membranas, J. Membr. Sci. 202 (2002) 1. L. Shao, T.-S. Chung, S.H. Goh, K.P. Pramoda, Polyimide modification by a linear aliphatic diamine to enhance transport performance and plasticization resistance, J. Membr. Sci. 256 (2005) 46. M.I. V´azquez, J. Benavente, A study of temperature effect on chemical, structural and transport parameters determined for two different regenerated cellulose membranes, J. Membr. Sci. 219 (2003) 59. M.I. V´azquez, P. Gal´an, J. Casado, M.J. Ariza, J. Benavente, Effect of radiation and thermal treatment on structural and transport parameters for cellulose regenerated membranes, Appl. Surf. Sci. 238 (2004) 415. Y.-N. Kuo, J. Hong, A new method for cellulose membrane fabrication and the determination of its characteristics, J. Colloid Interface Sci. 285 (2005) 232. J. Benavente, A. Mu˜noz, A. Heredia, Electrokinetic parameters of ion transport across isolated pepper cuticular membranes, J. Membr. Sci. 139 (1998) 147. K.H. Meyer, J.F. Siever, La permeabilit´e des membranes. I. Theorie de la permeabilit´e ionique, Helv. Chim. Acta 19 (1936) 649. T. Teorell, Transport phenomena in membranes, Discuss. Faraday Soc. 21 (1956) 9. N. Lakshminarayanaiah, Transport Phenomena in Membranes, Academic Press, New York, USA, 1969. A.J. Staverman, Non-equilibrium thermodynamics of membrane processes, Trans. Faraday Soc. 48 (1952) 176. G.J. Scartchard, Ion-exchange electrodes, J. Am. Chem. Soc. 75 (1953) 2883. J. Casado, P. Gal´an, J. Benavente, Modificaci´on estructural y qu´ımica (superficial) de membranas celul´osicas debido a radiaciones ionizantes, F´ısica M´edica, submitted for publication.