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Modification of cellophane membranes by γ-radiation: Effect of irradiation doses on electrochemical parameters
Journal of Membrane Science 256 (2005) 202–208
Modification of cellophane membranes by ␥-radiation: Effect of irradiation doses on electrochemical parameters M.I. V´azquez, R. de Lara, P. Gal´an, J. Benavente ∗ Grupo de Caracterizaci´on Electrocin´etica y de Transporte en Membranas e Interfases, Departamento de F´ısica Aplicada I, Facultad de Ciencias, Universidad de M´alaga, E-29071 M´alaga, Spain Received 24 November 2004; received in revised form 31 January 2005; accepted 16 February 2005 Available online 25 March 2005
Abstract Modification caused in cellophane matrix (a highly hydrophilic natural polymer) by ␥-irradiation was studied by determining changes in different electrochemical parameters obtained from measurements carried out with the membranes in contact with NaCl and MgCl2 solutions. Three different irradiation doses (10 Gy, 30 Gy and 80 Gy) were used in order to consider its possible effect in membrane modification. Changes in salt permeability, electrical resistance and permselectivity show modifications in the membrane matrix, which seem to produce: (i) a reduction in the inter-chains free space (around 40%), which slightly decreases when the irradiation dose increases (20% at the highest dose); (ii) an increase of the cation-exchange character of cellulose membranes, which is directly dependent on the irradiation doses (between 10% and 20%). Salt permeability and electrical resistance hardly depend on the kind of electrolyte, but higher values were obtained for cationic membrane permselectivity with NaCl solutions for both pristine and irradiated samples. © 2005 Elsevier B.V. All rights reserved. Keywords: Cellophane membranes; ␥-Irradiation; Membrane potential; Salt permeability; Impedance spectroscopy; Irradiation doses
1. Introduction Regenerated cellulose (cellophane), polysulfone and polyamide are among the polymers commonly used for membrane manufacturing due to their rather good chemical and solvent resistance [1,2]. One of the greatest disadvantages of polysulfone membranes comes from the hydrophobic character of this material, and different modifications such as the increase of sulfonic groups [3] or irradiation with ultraviolet (UV) light [4] are proposed in order to increase water transport across these membranes and to reduce the membrane fouling, which is a major problem for ultrafiltration process [5]. It was observed that UV modification of ultrafiltration membranes also affects the zeta potential value [4], which indicates electrical modification in the membrane surface. On the other hand, modification of cellophane membranes due to thermal treatment, UV light and ␥-irradiation, which ∗
Corresponding author. Tel.: +34 95 2131929; fax: +34 95 2132382. E-mail address: J [email protected] (J. Benavente).
0376-7388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2005.02.021
was determined from infrared spectra (FTIR), X-ray photoelectron spectroscopy (XPS) for dry samples and transport parameters (wet membrane samples), was already reported [6,7]. This work studies the effect of ␥-irradiation in cellophane membranes by determining changes in their electrochemical parameters. Cellophane membranes were chosen due to the fact that cellulose is a natural polymer with a high degree of swelling as a result of its hydrophilic character and negative fixed charge (–COOH group), and it is the basic material for many dialysis (hemodialysis) membranes [1,8]. Moreover, cellulose is also the basic material in the cell-wall of most plant cells [9], which may make the study of cellulosebased membranes interesting also for biologists. The electrochemical parameters determined in this work were salt permeability, electrical resistance, ion transport numbers and membrane permselectivity, which were determined from salt diffusion, impedance spectroscopy and membrane potential measurements carried out with the membranes in contact with NaCl and MgCl2 solutions at different concentrations.
M.I. V´azquez et al. / Journal of Membrane Science 256 (2005) 202–208
Comparison of electrochemical parameters for untreated and irradiated samples allows the estimation of structural and electrical modifications, but the use of two different kinds of electrolytes also gives us the possibility to see differences related to the transport of monovalent or bivalent ions. On the other hand, different doses of radiation were used (10 Gy, 30 Gy and 80 Gy) in order to correlate the intensity of radiation and the membrane modification.
2. Experimental 2.1. Membranes A cellophane sheet from Cellophane Espa˜nola, S.A. (Burgos, Spain) with an amount of 25 m2 /kg of regenerated cellulose was used (membrane C25). Some samples were treated with ionising radiation delivered by a Cobalt Unit, photons of 1.21 MeV and 1.33 MeV, which were placed in a waterequivalent phantom to the depth of electronic equilibrium and with enough width of irradiation field to ensure a homogeneous dose in the membrane. The doses delivered in these conditions were 10 Gy, 30 Gy and 80 Gy, and the irradiated membranes will hereafter be named as C25–Ir10, C25–Ir30 and C25–Ir80, respectively. The average thickness of dry (xd ) and water-swollen (xw ) membranes was determined in six different points covering an area of 25 cm2 by means of a digital micrometer, and the following swelling degree, Sw = (xw − xd )/xw , was obtained: Sw = 53 ± 4%. The small differences found among the studied samples do not allow a clear differentiation for each sample since they are included in the margin of error. Electrochemical measurements were carried out with the membranes in contact with aqueous NaCl and MgCl2 solutions at different concentrations (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 8 h in a solution of the appropriate concentration. 2.2. Salt diffusion, membrane potential and impedance spectroscopy measurements The test cell used for the electrochemical measurements is similar to that described elsewhere [10,11]. The membrane was tightly clamped between two glass half-cells by using silicone rubber rings. A magnetic stirrer was placed at the bottom of each half-cell to minimise concentration–polarisation at the membrane surfaces; the measurements were carried out at a stirring rate of 54.9727 rad/s (525 rpm). • In salt diffusion measurements, the membrane was initially separating a concentrated solution, c1 = 0.01 eq./l, 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
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(Radiometer CDM 83); a conductivity cell was also placed in reservoir 1 to control the constancy of concentration c1 . • The electromotive force (E) between 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 eq./l) and gradually changing the concentration of the solution at the other side, c2 , from 10−3 eq./l to 0.1 eq./l. Membrane potential, Φm , was obtained by subtracting the electrode potential, Φe = −(RT/z− F) ln(c1 /c2 ), from the corresponding E measured value. • Impedance spectroscopy (IS) measurements were carried out by using an Impedance Analyzer (Solartron 1260) controlled by a computer, which was connected to the solution in both half-cells via Ag/AgCl electrodes. The experimental data were corrected by software, the influence of connecting cables and other parasite capacitances. The measurements were carried out using 100 different frequencies in the range 10–107 Hz at a maximum voltage of 0.01 V, the solutions in both half-cells having the same concentration.
3. Results and discussion Salt 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 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: dc2 Sm Ps dt (2) = c1 − c2 (t) Vo if solution conductivity (σ) is considered instead of concentration, Eq. (2) can be written as dσ2 Sm dσ = Ps dt (3) σ1 − σ2 (t) Vo dc e where (dσ/dc)e is a parameter characteristic of each electrolyte (at a given temperature). Variation in the conductivity of solution 2, σ 2 , as a function of time is shown in Fig. 1 for the different membranes studied. 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. For all membranes, slightly higher permeability values were obtained for MgCl2 solution than for NaCl, in agreement with its o o higher diffusion coefficient (DMgCl /DNaCl = 1.10); values 2
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Fig. 1. Variation of conductivity in half-cell 2, σ 2 , with time for the different membranes: ( ) C25; (䊉) C25–Ir10; () C25–Ir30 and () C25–Ir80. (a) NaCl solution and (b) MgCl2 solution.
of salt permeability ratio for both electrolytes with each (NaCl) , are also indicated in Table 1. membrane, Ps(MgCl2 ) /Ps Some differences in this ratio depending on the irradiation doses were obtained, but for the pristine sample its value is practically that corresponding to the diffusion coefficient ratio. As can also be observed in Table 1, Ps values for the irradiated samples are lower than for C25 membrane, but these results also show a decrease of this effect when the irradiation dose increases. In fact, the irradiated/pristine sample permeability ratio ranged between 0.65 and 0.80 for NaCl solutions, and 0.75 and 0.80 for those of MgCl2 . These results seem to indicate a modification in the polymer chains as a result of ␥-radiation (e.g., a membrane compaction or reduction in the free space of the membrane matrix), but the increase in the irradiation doses seems to reduce this effect. In order to see other possible changed caused by radiation on characteristic membrane parameters (e.g., electrical parameters), membrane potential and electrical resistance were also measured. Membrane electrical characterisation was carried out by measuring “membrane potential”, øm , which 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 or TMS theory [12,13], the 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, i.e., Φm = øDon(I) + ødif + øDon(II) . The expressions for these potentials, when diluted solutions are considered (concentration are used instead of activities), are [14]: 1/2 2 RT wXf wXf (4) øDon = ln + +1 F 2c 2c
t+ t− c1 = − ln |z− | |z+ | c2 RT c1 = U ln F c2
ødif
RT F
(5)
where Xf is the membrane fixed charge concentration, w = −1 or +1 for negatively or positively charged membranes, respectively, while ti is the transport number of the ion in the membrane and zi is its valence (i = + for cation, − for anion); R and F are the gas and Faraday constants and T is the thermodynamic temperature of the system. ti represents the amount of current transported for one ion with respect to the total current crossing the membrane, ti = Ii /IT , that is, t+ + t− = 1. By combining Eqs. (4) and (5), the following expression for the membrane potential is obtained [14]:
Table 1 Salt permeability, Ps , and fixed charge concentration in the membrane matrix, Xf , determined with NaCl and MgCl2 solutions and permeability ratio, (NaCl) Ps(MgCl2 ) /Ps , for the studied membranes (NaCl)
Ps(MgCl2 ) /Ps
Membrane
NaCl
MgCl2
Ps (m/s)
Xf (eq./l)
Ps (m/s)
Xf (eq./l)
C25 C25–Ir10 C25–Ir30 C25–Ir80
2.19 × 10−6 1.40 × 10−6 1.63 × 10−6 1.76 × 10−6
−1.08 × 10−2 −1.55 × 10−2 −2.42 × 10−2 −2.77 × 10−2
2.38 × 10−6 1.78 × 10−6 1.90 × 10−6 1.92 × 10−6
−1.31 × 10−2 −1.31 × 10−2 −1.58 × 10−2 −1.97 × 10−2
1.09 1.27 1.17 1.09
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Fig. 2. Membrane potential vs. ln(c1 /0.01) for the different membranes: ( ) C25; (䊉) C25–Ir10; () C25–Ir30 and () C25–Ir80. (a) NaCl solutions and (b) MgCl2 solutions. Solid line (—) Nernst potential; dotted line (- - -) diffusion potential.
1/2 c1 [(1 + 4y12 ) + 1] Φm = ln 1/2 c2 [(1 + 4y22 ) + 1] 1/2 (1 + 4y12 ) − wU +wU ln 1/2 (1 + 4y22 ) − wU
RT wzF
(6)
where yj = zj ks cj /wXf and ks is the salt partition coefficient in the membrane. Fig. 2(a and b) shows membrane potential versus salt concentration (øm versus ln(c2 /c1 )) for the different membrane samples with NaCl and MgCl2 solutions, respectively; for comparison, Nernst potential (t− = 1 and t+ = 0) and solution diffusion potentials (øodif ) for the same range of concentration are also drawn in Fig. 2 (øodif was calculated by Eq. (5) using solution transport numbers). Experimental values for pristine and irradiated samples hardly differ when MgCl2 solutions are considered, but some differences exist in the case of NaCl solutions. The points drawn in Fig. 2 show the Donnan exclusion of co-ions at low concentration, which is associated to the small negative charge of cellophane membranes [8,16,17], but at high concentrations the Donnan effect can be neglected and the membrane potentials are mainly due to the diffusion of counter and co-ions through the membrane; thus, ødif ≈ øm . Fixed charge concentration in the studied membranes can be estimated from membrane potential measurements by using the following expression [18]: Xf =
zks Ucext 1/2
(1 − U 2 )
(7)
where cext represents the concentration at the maximum of the curve and it is determined by using the maximum condition (dΦm /dc)ext = 0. Results obtained for the different membrane samples and both electrolytes are indicated in Table 1 (ks value is taken from ref. [15]). As can be observed, these results show an electrical modification of cellophane matrix, particularly the slight increase of Xf values as a result of increasing ␥-irradiation doses. Taking into account these re-
sults, the increase in Donnan exclusion associated to high membrane fixed charge values could be the main reason for the increase in salt permeability values previously determined from diffusion measurements. Cation transport numbers in the membrane at high concentration (0.02–0.1 M) were obtained from membrane potential values by using Eq. (5). Membrane permselectivity, S(i), which is a measure of the selectivity of counter-ions over co-ions in a membrane, can be obtained if the ion transport numbers in the membrane are known [19] S(i) =
ti − tio 1 − tio
(8)
where tio represents the solution transport number of the membrane counter-ion. Membrane cationic permselectivity, S(+), was determined by Eq. (8) and its variation with the average concentration, cavg = (c1 + c2 )/2, is shown in Fig. 3(a and b) for NaCl and MgCl2 solutions, respectively. The higher cationic permselectivity obtained for the irradiated samples is due to their more negative character when compared with C25, and it is in agreement with the higher negative fixed charge previously obtained. Differences in permselectivity depending on the cation charge were also found and lower values for Mg2+ ions were always obtained, which is associated to the higher screening effect on the negative membrane fixed charge of this ion. Impedance spectroscopy measurements permit us to get information about electrical and/or structural membrane parameters [20–22]; particularly, changes in the volume fraction of the inter-chain spaces in ion-exchange membranes might be correlated with modification in the membrane electrical resistance [23]. Fig. 4(a and b) shows the impedance plots for the different samples at a given NaCl solution (c = 0.005 M): (a) Nyquist plot (−Zimg versus Zreal ) and (b) Bode plot (−Zimg versus frequency). The analysis of impedance results was carried out by the complex plane Z* method, which involves plotting the impedance imaginary part (−Zimg ) versus the real part (Zreal ), and using equivalent circuits as models. A
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Fig. 3. Cationic membrane permselectivity, S(+), vs. average concentration, cavg : ( ) C25; (䊉) C25–Ir10; () C25–Ir30 and () C25–Ir80. (a) NaCl solutions and (b) MgCl2 solutions.
Fig. 4. Impedance curves for: ( ) C25; (×) C25–Ir10; () C25–Ir30 and (♦) C25–Ir80 membranes with 0.002 M NaCl solution. (a) Nyquist plot and (b) Bode plot.
single parallel R–C circuit gives rise to a semi-circle in the Z* plane (Fig. 4a), which has intercepts on the axis Zreal at R∞ (ω ⇒ ∞) and Ro (ω ⇒ 0); the resistance of the system is given by (Ro − R∞ ) [24]. The maximum of the semicircle equals 0.5(Ro − R∞ ) and occurs at a frequency in which ωRC = 1, RC being the relaxation time and ω the angular fre-
quency (ω = 2πf). On the other hand, the Bode plot (−Zimg or Zreal versus frequency) allows the determination of the interval of frequency associated to a given relaxation process (Fig. 4b). Impedance curves for the different membranes and concentration studied are similar to those shown in Fig. 4. A
Fig. 5. Variation of Rsm with salt concentration: ( ) C25; (×) C25–Ir10; () C25–Ir30 and (♦) C25–Ir80. (a) NaCl solutions and (b) MgCl2 solutions.
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Fig. 6. Variation of Csm with salt concentration: ( ) C25; (×) C25–Ir10; () C25–Ir30 and (♦) C25–Ir80. (a) NaCl solutions and (b) MgCl2 solutions.
unique relaxation process was obtained for all the electrolyte/membrane/electrolyte systems, as was also reported in the literature for highly porous membranes [20,25]. These curves do not allow a separate estimation of the electrical contribution associated to the membrane and the electrolyte solution, which is attributed to the high degree of swelling of cellulose membranes (high electrolyte up taken) [5,16]. Experimental impedance data were fitted to a parallel resistance–capacitor (Rsm Csm ) circuit by means of a nonlinear program [26] and membrane system electrical resistance (Rsm ) and capacitance (Csm ) were determined. As was previously indicated, Rsm and Csm represent the membrane system resistance and capacitance, which include the contribution of the membrane and the solution layers between the membrane and the electrodes. Variation of Rsm with salt concentration is shown in Fig. 5(a and b); the decrease for the electrical resistance values when salt concentration increases is due to the concentration dependence of the electrolyte, and the values hardly differ depending on the electrolyte considered. Fig. 6(a and b) shows the increases of capacitance values for the different membrane samples with NaCl and MgCl2 solutions; as can be observed, higher capacitance values were obtained for MgCl2 solutions, which indicate higher charge adsorption. As can be observed in Fig. 5, Rsm values for the irradiated membrane systems hardly differ from those determined for C25 at the corresponding concentration, and the following average ratio values were obtained:
permeability results would act in the opposite way [25]. Then, a combination of geometrical and electrical effects acting on the cellulose membrane structure can be assumed, and the possible compaction of the membrane structure as a result of irradiation would be partially compensated by the higher fixed charge and conductivity of these samples.
4. Conclusions This work has studied the modifications caused by ␥irradiation in electrochemical parameters (salt permeability, ionic permselectivity and electrical resistance) for a membrane obtained from a natural polymer (regenerated cellulose), which is a basic material for dialysis membranes and cell plants. The effect of irradiation doses on the chosen electrochemical parameters has also been considered. Changes in the electrochemical parameters are correlated with structural (geometric) and electrical modifications due to membrane irradiation: • The lower salt permeability values obtained for the irradiated samples are attributed to a modification in the polymer chains, which seems to cause a reduction in the free space of the membrane matrix (structural modification). • The increase of cationic permselectivity is due to the increase in the fixed charge for the irradiated samples, which
NaCl MgCl2
(C25 (C25) (C25 – Ir10) –Ir10) = 0.87 ± 0.05 Rsm / Rsm = 0.93 ± 0.03 Rsm / Rsm
Rsm (C25) / Rsm (C25–Ir30) = 0.94 ± 0.05 Rsm (C25) / Rsm (C25–Ir30) = 0.89 ± 0.08
Rsm (C25) / Rsm (C25–Ir80) = 0.93 ± 0.06 Rsm (C25) / Rsm (C25–Ir80) = 0.93 ± 0.05
(C25)
These results show the slight increase of the electrical resistance of the irradiated samples, but this effect is lower that that caused on salt permeability (between 0.65 and 0.80). In fact, taking into account the increase in the fixed charge for the irradiated samples determined from membrane potential results, a decrease in their electrical resistance should be expected, but the membrane compaction assumed from salt
is directly co-related to the increase of the radiation doses. The increase in Donnan exclusion when the irradiation doses increases might be the main reason for the increase of salt permeability in these samples. • The increase in membrane conductivity associated to the increase in the fixed charge of the irradiated samples should
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produce a reduction in their electrical resistance, but membrane compaction associated to irradiation might partially reduce that effect.
Acknowledgement We thank the Comisi´on Interministerial de Ciencia y Tecnolog´ıa (CICYT, Spain), Project MAT2003-03328, for financial support.
References [1] M. Mulder, Basic Principles of Membrane Technology, Kluwer Academic Publishers, Dordrecht, The Netherlands, 1992. [2] V. Gekas, G. Tr¨agard, B. Hallstr¨om, Ultrafiltration Membrane Performance Fundamentals, Swedish Foundation for Membrane Technology, Lund, 1993. [3] J. Benavente, J.M. Garc´ıa, J. de la Campa, J. de Abajo, Sulfonated poly(ether ether sulfones). Characterization and study of dielectrical properties by impedance spectroscopy, J. Membr. Sci. 175 (2000) 43. [4] M. Nystr¨om, Fouling of unmodified and modified polysulfone membranes by ovoalbumin, J. Membr. Sci. 44 (1989) 183. [5] M. Cheryan, Ultrafiltration Handbook, Technomic Publishing Co., Lancaster, USA, 1986. [6] 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. [7] 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. Surface Sci. 238 (2004) 415. [8] K. Sakai, Determination of pore size and pore size distribution. 2. dialysis membranes, J. Membr. Sci. 96 (1994) 91. [9] K. Esau, Plant Anatomy, John Wiley & Sons, New York, 1953. [10] J. Benavente, A. Mu˜noz, A. Heredia, Electrokinetic parameters of ion transport across isolated pepper cuticular membranes, J. Membr. Sci. 139 (1998) 147.
[11] M.I. V´azquez, R. de Lara, J. Benavente, Modification in the transport of NaCl and MgCl2 solutions across a ceramic microporous membrane due to chemical and thermal treatment, Sep. Purif. Technol., in press. [12] K.H. Meyer, J.F. Siever, La permeabilit´e des membranes. I. Theorie de la permeabilit´e ionique, Helv. Chim. Acta 19 (1936) 649. [13] T. Teorell, Transport phenomena in membranes, Discuss. Faraday Soc. 21 (1956) 9. [14] N. Lakshminarayanaiah, Transport Phenomena in Membranes, Academic Press, NY, USA, 1969. [15] Y. Kimura, H.-J. Lim, T. Ijima, Membrane potential of charged cellulosic membranes, J. Membr. Sci. 18 (1984) 285. [16] C.J. van Oss, Ultrafiltration membrane performance, Science 139 (1963) 1123. [17] A. Ca˜nas, M.J. Ariza, J. Benavente, A comparison of the electrochemical and electrokinetic parameters determined for cellophane membranes in contact with NaCl and NaNO3 solutions, J. Colloid Interface Sci. 246 (2002) 150. [18] H.-U. Demisch, W. Pusch, Electric and electrokinetic transport properties of homogeneous weak ion exchange membranes, J. Colloid Interface Sci. 69 (1979) 247. [19] M.L. L´opez, V. Compa˜n, J. Garrido, E. Riande, J.L. Acosta, Proton transport in membranes prepared from sulfonated polystyrene–poly(vinylfluoride) blends, J. Electrochem. Soc. 148 (2002) 372. [20] A. Asaka, Dielectric properties of cellulose acetate reverse osmosis membranes, J. Membr. Sci 50 (1990) 71. [21] A. Ca˜nas, J. Benavente, Electrochemical and structural characterizations of an experimental track-etched membrane in KCl solutions, Sep. Purf. Technol. 20 (2000) 169. [22] A. Ca˜nas, M.J. Ariza, J. Benavente, Characterization of active and porous sublayers of a composite membrane by impedance spectroscopy, streaming and membrane potentials, salt diffusion and Xray photoelectron spectroscopy measurements, J. Membr. Sci. 183 (2001) 135. [23] V.I. Zabolotsky, V.V. Nikonenco, Effect of structural membrane inhomogeneity on transport properties, J. Membr. Sci. 79 (1993) 181. [24] J.R. Macdonald, Impedance Spectroscopy, Wiley, New York, 1987. [25] J. Benavente, J.R. Ramos-Barrado, S. Bruque, M. Matinez, Determination of some electrical parameters for UO2 (O3 PC6 H5 ) films deposited on a porous support, J. Chem. Soc. Faraday Trans. 90 (1994) 3103. [26] B.A. Boukamp, A package for impedance/admittance data analysis., Solid State Ionics 18–19 (1986) 136.
Modification of cellophane membranes by ␥-radiation: Effect of irradiation doses on electrochemical parameters M.I. V´azquez, R. de Lara, P. Gal´an, J. Benavente ∗ Grupo de Caracterizaci´on Electrocin´etica y de Transporte en Membranas e Interfases, Departamento de F´ısica Aplicada I, Facultad de Ciencias, Universidad de M´alaga, E-29071 M´alaga, Spain Received 24 November 2004; received in revised form 31 January 2005; accepted 16 February 2005 Available online 25 March 2005
Abstract Modification caused in cellophane matrix (a highly hydrophilic natural polymer) by ␥-irradiation was studied by determining changes in different electrochemical parameters obtained from measurements carried out with the membranes in contact with NaCl and MgCl2 solutions. Three different irradiation doses (10 Gy, 30 Gy and 80 Gy) were used in order to consider its possible effect in membrane modification. Changes in salt permeability, electrical resistance and permselectivity show modifications in the membrane matrix, which seem to produce: (i) a reduction in the inter-chains free space (around 40%), which slightly decreases when the irradiation dose increases (20% at the highest dose); (ii) an increase of the cation-exchange character of cellulose membranes, which is directly dependent on the irradiation doses (between 10% and 20%). Salt permeability and electrical resistance hardly depend on the kind of electrolyte, but higher values were obtained for cationic membrane permselectivity with NaCl solutions for both pristine and irradiated samples. © 2005 Elsevier B.V. All rights reserved. Keywords: Cellophane membranes; ␥-Irradiation; Membrane potential; Salt permeability; Impedance spectroscopy; Irradiation doses
1. Introduction Regenerated cellulose (cellophane), polysulfone and polyamide are among the polymers commonly used for membrane manufacturing due to their rather good chemical and solvent resistance [1,2]. One of the greatest disadvantages of polysulfone membranes comes from the hydrophobic character of this material, and different modifications such as the increase of sulfonic groups [3] or irradiation with ultraviolet (UV) light [4] are proposed in order to increase water transport across these membranes and to reduce the membrane fouling, which is a major problem for ultrafiltration process [5]. It was observed that UV modification of ultrafiltration membranes also affects the zeta potential value [4], which indicates electrical modification in the membrane surface. On the other hand, modification of cellophane membranes due to thermal treatment, UV light and ␥-irradiation, which ∗
Corresponding author. Tel.: +34 95 2131929; fax: +34 95 2132382. E-mail address: J [email protected] (J. Benavente).
0376-7388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2005.02.021
was determined from infrared spectra (FTIR), X-ray photoelectron spectroscopy (XPS) for dry samples and transport parameters (wet membrane samples), was already reported [6,7]. This work studies the effect of ␥-irradiation in cellophane membranes by determining changes in their electrochemical parameters. Cellophane membranes were chosen due to the fact that cellulose is a natural polymer with a high degree of swelling as a result of its hydrophilic character and negative fixed charge (–COOH group), and it is the basic material for many dialysis (hemodialysis) membranes [1,8]. Moreover, cellulose is also the basic material in the cell-wall of most plant cells [9], which may make the study of cellulosebased membranes interesting also for biologists. The electrochemical parameters determined in this work were salt permeability, electrical resistance, ion transport numbers and membrane permselectivity, which were determined from salt diffusion, impedance spectroscopy and membrane potential measurements carried out with the membranes in contact with NaCl and MgCl2 solutions at different concentrations.
M.I. V´azquez et al. / Journal of Membrane Science 256 (2005) 202–208
Comparison of electrochemical parameters for untreated and irradiated samples allows the estimation of structural and electrical modifications, but the use of two different kinds of electrolytes also gives us the possibility to see differences related to the transport of monovalent or bivalent ions. On the other hand, different doses of radiation were used (10 Gy, 30 Gy and 80 Gy) in order to correlate the intensity of radiation and the membrane modification.
2. Experimental 2.1. Membranes A cellophane sheet from Cellophane Espa˜nola, S.A. (Burgos, Spain) with an amount of 25 m2 /kg of regenerated cellulose was used (membrane C25). Some samples were treated with ionising radiation delivered by a Cobalt Unit, photons of 1.21 MeV and 1.33 MeV, which were placed in a waterequivalent phantom to the depth of electronic equilibrium and with enough width of irradiation field to ensure a homogeneous dose in the membrane. The doses delivered in these conditions were 10 Gy, 30 Gy and 80 Gy, and the irradiated membranes will hereafter be named as C25–Ir10, C25–Ir30 and C25–Ir80, respectively. The average thickness of dry (xd ) and water-swollen (xw ) membranes was determined in six different points covering an area of 25 cm2 by means of a digital micrometer, and the following swelling degree, Sw = (xw − xd )/xw , was obtained: Sw = 53 ± 4%. The small differences found among the studied samples do not allow a clear differentiation for each sample since they are included in the margin of error. Electrochemical measurements were carried out with the membranes in contact with aqueous NaCl and MgCl2 solutions at different concentrations (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 8 h in a solution of the appropriate concentration. 2.2. Salt diffusion, membrane potential and impedance spectroscopy measurements The test cell used for the electrochemical measurements is similar to that described elsewhere [10,11]. The membrane was tightly clamped between two glass half-cells by using silicone rubber rings. A magnetic stirrer was placed at the bottom of each half-cell to minimise concentration–polarisation at the membrane surfaces; the measurements were carried out at a stirring rate of 54.9727 rad/s (525 rpm). • In salt diffusion measurements, the membrane was initially separating a concentrated solution, c1 = 0.01 eq./l, 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
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(Radiometer CDM 83); a conductivity cell was also placed in reservoir 1 to control the constancy of concentration c1 . • The electromotive force (E) between 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 eq./l) and gradually changing the concentration of the solution at the other side, c2 , from 10−3 eq./l to 0.1 eq./l. Membrane potential, Φm , was obtained by subtracting the electrode potential, Φe = −(RT/z− F) ln(c1 /c2 ), from the corresponding E measured value. • Impedance spectroscopy (IS) measurements were carried out by using an Impedance Analyzer (Solartron 1260) controlled by a computer, which was connected to the solution in both half-cells via Ag/AgCl electrodes. The experimental data were corrected by software, the influence of connecting cables and other parasite capacitances. The measurements were carried out using 100 different frequencies in the range 10–107 Hz at a maximum voltage of 0.01 V, the solutions in both half-cells having the same concentration.
3. Results and discussion Salt 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 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: dc2 Sm Ps dt (2) = c1 − c2 (t) Vo if solution conductivity (σ) is considered instead of concentration, Eq. (2) can be written as dσ2 Sm dσ = Ps dt (3) σ1 − σ2 (t) Vo dc e where (dσ/dc)e is a parameter characteristic of each electrolyte (at a given temperature). Variation in the conductivity of solution 2, σ 2 , as a function of time is shown in Fig. 1 for the different membranes studied. 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. For all membranes, slightly higher permeability values were obtained for MgCl2 solution than for NaCl, in agreement with its o o higher diffusion coefficient (DMgCl /DNaCl = 1.10); values 2
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Fig. 1. Variation of conductivity in half-cell 2, σ 2 , with time for the different membranes: ( ) C25; (䊉) C25–Ir10; () C25–Ir30 and () C25–Ir80. (a) NaCl solution and (b) MgCl2 solution.
of salt permeability ratio for both electrolytes with each (NaCl) , are also indicated in Table 1. membrane, Ps(MgCl2 ) /Ps Some differences in this ratio depending on the irradiation doses were obtained, but for the pristine sample its value is practically that corresponding to the diffusion coefficient ratio. As can also be observed in Table 1, Ps values for the irradiated samples are lower than for C25 membrane, but these results also show a decrease of this effect when the irradiation dose increases. In fact, the irradiated/pristine sample permeability ratio ranged between 0.65 and 0.80 for NaCl solutions, and 0.75 and 0.80 for those of MgCl2 . These results seem to indicate a modification in the polymer chains as a result of ␥-radiation (e.g., a membrane compaction or reduction in the free space of the membrane matrix), but the increase in the irradiation doses seems to reduce this effect. In order to see other possible changed caused by radiation on characteristic membrane parameters (e.g., electrical parameters), membrane potential and electrical resistance were also measured. Membrane electrical characterisation was carried out by measuring “membrane potential”, øm , which 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 or TMS theory [12,13], the 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, i.e., Φm = øDon(I) + ødif + øDon(II) . The expressions for these potentials, when diluted solutions are considered (concentration are used instead of activities), are [14]: 1/2 2 RT wXf wXf (4) øDon = ln + +1 F 2c 2c
t+ t− c1 = − ln |z− | |z+ | c2 RT c1 = U ln F c2
ødif
RT F
(5)
where Xf is the membrane fixed charge concentration, w = −1 or +1 for negatively or positively charged membranes, respectively, while ti is the transport number of the ion in the membrane and zi is its valence (i = + for cation, − for anion); R and F are the gas and Faraday constants and T is the thermodynamic temperature of the system. ti represents the amount of current transported for one ion with respect to the total current crossing the membrane, ti = Ii /IT , that is, t+ + t− = 1. By combining Eqs. (4) and (5), the following expression for the membrane potential is obtained [14]:
Table 1 Salt permeability, Ps , and fixed charge concentration in the membrane matrix, Xf , determined with NaCl and MgCl2 solutions and permeability ratio, (NaCl) Ps(MgCl2 ) /Ps , for the studied membranes (NaCl)
Ps(MgCl2 ) /Ps
Membrane
NaCl
MgCl2
Ps (m/s)
Xf (eq./l)
Ps (m/s)
Xf (eq./l)
C25 C25–Ir10 C25–Ir30 C25–Ir80
2.19 × 10−6 1.40 × 10−6 1.63 × 10−6 1.76 × 10−6
−1.08 × 10−2 −1.55 × 10−2 −2.42 × 10−2 −2.77 × 10−2
2.38 × 10−6 1.78 × 10−6 1.90 × 10−6 1.92 × 10−6
−1.31 × 10−2 −1.31 × 10−2 −1.58 × 10−2 −1.97 × 10−2
1.09 1.27 1.17 1.09
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Fig. 2. Membrane potential vs. ln(c1 /0.01) for the different membranes: ( ) C25; (䊉) C25–Ir10; () C25–Ir30 and () C25–Ir80. (a) NaCl solutions and (b) MgCl2 solutions. Solid line (—) Nernst potential; dotted line (- - -) diffusion potential.
1/2 c1 [(1 + 4y12 ) + 1] Φm = ln 1/2 c2 [(1 + 4y22 ) + 1] 1/2 (1 + 4y12 ) − wU +wU ln 1/2 (1 + 4y22 ) − wU
RT wzF
(6)
where yj = zj ks cj /wXf and ks is the salt partition coefficient in the membrane. Fig. 2(a and b) shows membrane potential versus salt concentration (øm versus ln(c2 /c1 )) for the different membrane samples with NaCl and MgCl2 solutions, respectively; for comparison, Nernst potential (t− = 1 and t+ = 0) and solution diffusion potentials (øodif ) for the same range of concentration are also drawn in Fig. 2 (øodif was calculated by Eq. (5) using solution transport numbers). Experimental values for pristine and irradiated samples hardly differ when MgCl2 solutions are considered, but some differences exist in the case of NaCl solutions. The points drawn in Fig. 2 show the Donnan exclusion of co-ions at low concentration, which is associated to the small negative charge of cellophane membranes [8,16,17], but at high concentrations the Donnan effect can be neglected and the membrane potentials are mainly due to the diffusion of counter and co-ions through the membrane; thus, ødif ≈ øm . Fixed charge concentration in the studied membranes can be estimated from membrane potential measurements by using the following expression [18]: Xf =
zks Ucext 1/2
(1 − U 2 )
(7)
where cext represents the concentration at the maximum of the curve and it is determined by using the maximum condition (dΦm /dc)ext = 0. Results obtained for the different membrane samples and both electrolytes are indicated in Table 1 (ks value is taken from ref. [15]). As can be observed, these results show an electrical modification of cellophane matrix, particularly the slight increase of Xf values as a result of increasing ␥-irradiation doses. Taking into account these re-
sults, the increase in Donnan exclusion associated to high membrane fixed charge values could be the main reason for the increase in salt permeability values previously determined from diffusion measurements. Cation transport numbers in the membrane at high concentration (0.02–0.1 M) were obtained from membrane potential values by using Eq. (5). Membrane permselectivity, S(i), which is a measure of the selectivity of counter-ions over co-ions in a membrane, can be obtained if the ion transport numbers in the membrane are known [19] S(i) =
ti − tio 1 − tio
(8)
where tio represents the solution transport number of the membrane counter-ion. Membrane cationic permselectivity, S(+), was determined by Eq. (8) and its variation with the average concentration, cavg = (c1 + c2 )/2, is shown in Fig. 3(a and b) for NaCl and MgCl2 solutions, respectively. The higher cationic permselectivity obtained for the irradiated samples is due to their more negative character when compared with C25, and it is in agreement with the higher negative fixed charge previously obtained. Differences in permselectivity depending on the cation charge were also found and lower values for Mg2+ ions were always obtained, which is associated to the higher screening effect on the negative membrane fixed charge of this ion. Impedance spectroscopy measurements permit us to get information about electrical and/or structural membrane parameters [20–22]; particularly, changes in the volume fraction of the inter-chain spaces in ion-exchange membranes might be correlated with modification in the membrane electrical resistance [23]. Fig. 4(a and b) shows the impedance plots for the different samples at a given NaCl solution (c = 0.005 M): (a) Nyquist plot (−Zimg versus Zreal ) and (b) Bode plot (−Zimg versus frequency). The analysis of impedance results was carried out by the complex plane Z* method, which involves plotting the impedance imaginary part (−Zimg ) versus the real part (Zreal ), and using equivalent circuits as models. A
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Fig. 3. Cationic membrane permselectivity, S(+), vs. average concentration, cavg : ( ) C25; (䊉) C25–Ir10; () C25–Ir30 and () C25–Ir80. (a) NaCl solutions and (b) MgCl2 solutions.
Fig. 4. Impedance curves for: ( ) C25; (×) C25–Ir10; () C25–Ir30 and (♦) C25–Ir80 membranes with 0.002 M NaCl solution. (a) Nyquist plot and (b) Bode plot.
single parallel R–C circuit gives rise to a semi-circle in the Z* plane (Fig. 4a), which has intercepts on the axis Zreal at R∞ (ω ⇒ ∞) and Ro (ω ⇒ 0); the resistance of the system is given by (Ro − R∞ ) [24]. The maximum of the semicircle equals 0.5(Ro − R∞ ) and occurs at a frequency in which ωRC = 1, RC being the relaxation time and ω the angular fre-
quency (ω = 2πf). On the other hand, the Bode plot (−Zimg or Zreal versus frequency) allows the determination of the interval of frequency associated to a given relaxation process (Fig. 4b). Impedance curves for the different membranes and concentration studied are similar to those shown in Fig. 4. A
Fig. 5. Variation of Rsm with salt concentration: ( ) C25; (×) C25–Ir10; () C25–Ir30 and (♦) C25–Ir80. (a) NaCl solutions and (b) MgCl2 solutions.
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Fig. 6. Variation of Csm with salt concentration: ( ) C25; (×) C25–Ir10; () C25–Ir30 and (♦) C25–Ir80. (a) NaCl solutions and (b) MgCl2 solutions.
unique relaxation process was obtained for all the electrolyte/membrane/electrolyte systems, as was also reported in the literature for highly porous membranes [20,25]. These curves do not allow a separate estimation of the electrical contribution associated to the membrane and the electrolyte solution, which is attributed to the high degree of swelling of cellulose membranes (high electrolyte up taken) [5,16]. Experimental impedance data were fitted to a parallel resistance–capacitor (Rsm Csm ) circuit by means of a nonlinear program [26] and membrane system electrical resistance (Rsm ) and capacitance (Csm ) were determined. As was previously indicated, Rsm and Csm represent the membrane system resistance and capacitance, which include the contribution of the membrane and the solution layers between the membrane and the electrodes. Variation of Rsm with salt concentration is shown in Fig. 5(a and b); the decrease for the electrical resistance values when salt concentration increases is due to the concentration dependence of the electrolyte, and the values hardly differ depending on the electrolyte considered. Fig. 6(a and b) shows the increases of capacitance values for the different membrane samples with NaCl and MgCl2 solutions; as can be observed, higher capacitance values were obtained for MgCl2 solutions, which indicate higher charge adsorption. As can be observed in Fig. 5, Rsm values for the irradiated membrane systems hardly differ from those determined for C25 at the corresponding concentration, and the following average ratio values were obtained:
permeability results would act in the opposite way [25]. Then, a combination of geometrical and electrical effects acting on the cellulose membrane structure can be assumed, and the possible compaction of the membrane structure as a result of irradiation would be partially compensated by the higher fixed charge and conductivity of these samples.
4. Conclusions This work has studied the modifications caused by ␥irradiation in electrochemical parameters (salt permeability, ionic permselectivity and electrical resistance) for a membrane obtained from a natural polymer (regenerated cellulose), which is a basic material for dialysis membranes and cell plants. The effect of irradiation doses on the chosen electrochemical parameters has also been considered. Changes in the electrochemical parameters are correlated with structural (geometric) and electrical modifications due to membrane irradiation: • The lower salt permeability values obtained for the irradiated samples are attributed to a modification in the polymer chains, which seems to cause a reduction in the free space of the membrane matrix (structural modification). • The increase of cationic permselectivity is due to the increase in the fixed charge for the irradiated samples, which
NaCl MgCl2
(C25 (C25) (C25 – Ir10) –Ir10) = 0.87 ± 0.05 Rsm / Rsm = 0.93 ± 0.03 Rsm / Rsm
Rsm (C25) / Rsm (C25–Ir30) = 0.94 ± 0.05 Rsm (C25) / Rsm (C25–Ir30) = 0.89 ± 0.08
Rsm (C25) / Rsm (C25–Ir80) = 0.93 ± 0.06 Rsm (C25) / Rsm (C25–Ir80) = 0.93 ± 0.05
(C25)
These results show the slight increase of the electrical resistance of the irradiated samples, but this effect is lower that that caused on salt permeability (between 0.65 and 0.80). In fact, taking into account the increase in the fixed charge for the irradiated samples determined from membrane potential results, a decrease in their electrical resistance should be expected, but the membrane compaction assumed from salt
is directly co-related to the increase of the radiation doses. The increase in Donnan exclusion when the irradiation doses increases might be the main reason for the increase of salt permeability in these samples. • The increase in membrane conductivity associated to the increase in the fixed charge of the irradiated samples should
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produce a reduction in their electrical resistance, but membrane compaction associated to irradiation might partially reduce that effect.
Acknowledgement We thank the Comisi´on Interministerial de Ciencia y Tecnolog´ıa (CICYT, Spain), Project MAT2003-03328, for financial support.
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