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Osmotic behavior of a Nafion membrane in methanol–water electrolyte solutions
Journal of Colloid and Interface Science 263 (2003) 217–222 www.elsevier.com/locate/jcis
Osmotic behavior of a Nafion membrane in methanol–water electrolyte solutions J.P. García-Villaluenga, B. Seoane, V.M. Barragán, and C. Ruiz-Bauzá ∗ Departmento de Física Aplicada I, Facultad de Física, Universidad Complutense de Madrid, 28040 Madrid, Spain Received 18 November 2002; accepted 6 February 2003
Abstract In this paper, an experimental study was carried out in order to investigate the osmotic transport of methanol–water electrolyte solutions through a Nafion membrane. The experimental data indicated that the Nafion membrane showed the typical anomalous osmotic behavior of charged membranes. The influence of some relevant parameters, such as electrolyte concentration difference, weight fraction of methanol on solution, and nature of cation was considered. The results showed that the osmotic volume flow was decreased with the presence of methanol on solvent, but did not alter the anomalous osmotic behavior of the membrane. 2003 Elsevier Science (USA). All rights reserved. Keywords: Anomalous osmosis; Methanol; Nafion membrane
1. Introduction Osmotic transport occurs between solutions of different concentrations separated by a membrane. With nonelectrolyte solutions or with uncharged membranes the flow rate is roughly proportional to the concentration difference between the two solutions, and the flow occurs toward the more concentrated solution. With electrolyte solutions and charged membranes the flow rate is often not proportional to the concentration difference, but appears to be affected by the presence of charges. This phenomenon is called anomalous osmosis and has been discussed by many authors when aqueous solutions are in contact with the membrane [1–5]. If the rise in the height of the level of liquid in the capillary of the chamber containing the more concentrated solution is higher than the theoretical height, the flow is called positive anomalous osmosis. On the other hand, if there is a fall, it is called anomalous negative osmosis. Depending on the membrane and on the electrolyte solutions, either negative or positive anomalous osmosis can be observed. This phenomenon is usually attributed to the permeation velocity of the electrolyte and, consequently, the * Corresponding author.
E-mail addresses: [email protected] (J.P. García-Villaluenga), [email protected] (B. Seoane), [email protected] (V.M. Barragán), [email protected] (C. Ruiz-Bauzá).
water transport accompanying the electrolyte being affected by membrane potentials appearing on both sides of membrane. Thus the total volume flow is the sum of three effects: interdiffusion of solute and solvent components, flow due to pressure gradient, and electro-osmotic flux due to electricpotential gradient [2]. The first term is of a thermodynamic nature, and the second and third terms are mechanical and electrical, respectively. For 1:1 electrolytes the term associated with the interdiffusion of mobile components is positive. The term representing the pressure flow is always positive. On the other hand, the flow due to electric-potential gradient can be either positive or negative, depending on the cation to anion mobility ratio in the membrane. This explains why reverse bulk flows occurred in a certain concentration region. A great variety of anomalies of osmotic flows can be observed in charged membrane electrolyte systems, because the three flow components constituting the total volume flow have greatly different dependencies on both the electrolyte concentrations and the membrane properties. Recently it has become intensively necessary to analyze the behavior of charged membranes in aqueous organic electrolyte solutions due to its importance from the standpoint of medical and industrial applications. However, studies and experimental data available for aqueous organic electrolyte solution systems are, limited compared to aqueous systems. In this paper, the osmotic behavior of a charged membrane in water and methanol–water electrolyte solutions has
0021-9797/03/$ – see front matter 2003 Elsevier Science (USA). All rights reserved. doi:10.1016/S0021-9797(03)00172-3
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Table 1 Membrane features provided by the manufacturer Thickness (mm)
Water uptake (%)
Fixed charge (kmol/m3 )
0.183
35
1.13
been studied. The membrane used was a Nafion membrane. This membrane was chosen because of its good physical and chemical properties and for its wide industrial applications, mainly with respect to its utilization in methanol fuel cells, where the understanding of the transport process in the presence of methanol is fundamental.
2. Experimental 2.1. Materials A commercial membrane Nafion 117 with a nominal equivalent weight of 1100 g/eq was used in this study. The Nafion 117 is a cation-exchange membrane consisting of a polytetrafluoroethylene backbone and long fluorovinyl ether pendent side chains regularly spaced, terminated by a sulfonate ionic group. The thickness, water content, and ion-exchange capacity provided by the manufacturer for this membrane are given in Table 1. The molecular level structure of Nafion focuses on the cluster-network model proposed by Gierke [6] who described the membrane as a series of cluster or inverted micelles, interconnected by narrow pores. In this model, the counterions, the fixed sites, and the swelling water phase are separated from the fluorocarbon matrix into approximately spherical domains connected by short narrow channels. The fixed sites are embedded in the water phase very near to the water fluorocarbon interface. According to many investigations based on different methods, the pore diameter of the cluster has been estimated at 4–5 nm and the diameter of the narrow pores at 1 nm [7,8]. In Nafion polymers, cations migrate along the restricted path through channels of hydrophilic domains, so the membrane-specific conductivity is strongly influenced
by the water content, as well as other transport properties [9]. The materials used in the experiments were solutions of water and methanol at different compositions. Lithium chloride (LiCl), sodium chloride (NaCl), and potassium chloride (KCl) were used as electrolytes. Pure pro-analysis grade chemicals and distilled pure water were used. Before measurements were carried out, the methanol–water solutions were degassed in order to prevent bubble formation during the measurement process. 2.2. Osmosis measurements In order to determine the osmotic flux through the membrane, an experimental setup was designed and constructed. A schematic of the device can be seen in Fig. 1. The main part of the experimental device was a cell, which basically consisted of two equal cylindrical glass chambers. The volume of each container was about 300 cm3 , which is large enough to ensure that the concentration changes in the solution during the measurements may be considered negligible. The membrane was mounted in a Teflon holder, which was positioned between the two chambers. O-rings were employed to ensure there were no liquid leaks in the whole assembly. The effective surface area of the membrane exposed to the flow was 5.7 cm2 . In order to measure the volume flux, two L-shaped capillary tubes were introduced in each chamber, in such a way that the horizontal portions of the tubes were at the same height. All the experiments were carried out under isothermal conditions at 25 ◦ C. Temperature requirements were achieved by immersing the cell in a water-thermostated bath. In order to improve the uniformity of the temperatures and concentrations inside each chamber, the solutions were stirred by a magnetic stirrer assembly. The stirring rate in all the experiments was about 300 rpm. Under these conditions, the temperature was constant within ±0.1 ◦ C. In order to achieve equilibrium, prior to each experiment the membrane was immersed for a minimum of 24 h in the solution of lower concentration. Once the membrane was positioned in the cell, both chambers were filled with
Fig. 1. Sketch of the experimental device used in the osmosis measurements. M, membrane; P, propeller; MS, magnetic stirrer; C, capillary; K, key; T, thermostat; B, bath; E, stirrer engine.
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the solution of lower concentration. When the system was stabilized at the selected temperature and stirring rate, the concentration of the electrolyte was changed in one of the chambers (left) to the desirable value. The volume fractions of methanol in the solutions, which were the same in the two chambers in all measurements, were 0, 25, 50, and 75 vol%. The electrolyte concentration in the left chamber was varied from 0.005 to 0.1 mol/l and the right one was kept free of electrolyte, in such a way that a difference of the electrolyte concentration was established between two sides of the membrane. The measurement consisted of following the displacement of the liquid meniscus in the capillary tube corresponding to the right chamber without electrolyte, as a function of time. The reproducibility of the data was confirmed by repeating each experiment three times, under the same conditions of electrolyte concentration and percentage of methanol.
3. Results and discussion 3.1. Electrolyte–water solutions 3.1.1. Influence of the electrolyte concentration difference Figure 2 shows the volume change in the right chamber as a function of time when KCl is used as electrolyte. Measurements were carried out at four different concentrations, 0.005, 0.01, 0.05, and 0.1 in mol/l, in the left chamber. The results show that the volume in the right chamber increases, indicating that the volume flow takes place from the chamber with the higher electrolyte concentration toward the chamber with the lower electrolyte concentration; consequently, a negative anomalous osmotic behavior is observed. As can be seen in Fig. 2, the time dependence of the volume indicates the existence of a transitory step at the beginning of the measurement in all cases studied. After certain time, the volume variation becomes linear, indicating that steady-state conditions are reached. Linearity was achieved after approximately 1000 s, depending on the experimental conditions. The analysis of the results reveals that there is not any relation between the duration of the transitory step and the electrolyte concentration difference established. The volume flow through the membrane can be estimated from the slope of the linear part of the curves shown in Fig. 2. The volume flows thus obtained are shown afterward in Table 2 as a function of the concentration electrolyte difference. As can be observed, the dependence of the volume flow on the electrolyte concentration difference is not linear, showing the features of anomalous osmosis, i.e., the typical dependence of the volume flow on the logarithmic concentration of electrolyte. These results seem to indicate that the flow due to the electric-potential gradient is negative and opposite to the normal osmosis, therefore the K+ mobility is smaller than the Cl− mobility in the membrane in all concentration ranges studied. The normal osmotic component always
Fig. 2. Volume change in the lower solute concentration chamber as a function of time for different aqueous KCl solution concentrations.
increases when the solute concentration difference across the membrane increases [3,4,10]. For this reason, the decrease of the total flow when the solute concentration difference increases, for concentrations higher than 10−2 mol/l, may be due to the fact that the anomalous component has a higher increase than the normal osmotic component, in this solute concentration range. 3.1.2. Influence of the nature of the electrolyte Figure 3 shows the experimental data obtained with a concentration of 0.01 mol/l of different 1:1 electrolytes in aqueous solutions. As previously noted, in the experiments with KCl solutions, the volume in the right chamber increases with time, indicating that the flow occurs from the chamber containing the aqueous electrolyte solution toward the chamber containing pure water. However, in the case of LiCl and NaCl solutions, the volume flow takes place toward the more concentrated solution, indicating that the membrane shows a positive anomalous osmotic behavior when LiCl and NaCl electrolytes are used. These data are consistent with previous studies in which it was shown that osmotic flow between aqueous electrolyte solutions separated by charged membranes exhibits anomalous osmotic behavior, negative or positive, depending on the membrane nature, the electrolyte, and the experimental conditions [11,12]. The results are also in agreement with the results found in [12] where a positive anomalous osmotic behavior is found for a Nafion membrane with aqueous NaCl solutions. These results indicated that, unlike KCl, in the case of LiCl and NaCl
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Table 2 Volume flow (10−5 cm3 /s) through Nafion 117 membrane as a function of the potassium chloride concentration difference at different percentages of methanol on solvent C (mol/l) 0.005 0.01 0.05 0.1
0% MeOH
25% MeOH
50% MeOH
75% MeOH
7.70 (±0.07) 1.25 (±0.04) 1.36 (±0.04) 2.10 (±0.04)
1.25 (±0.21) 1.07 (±0.04) 0.76 (±0.05) 0.64 (±0.10)
0.34 (±0.05) 0.99 (±0.04) 0.13 (±0.04) 0.10 (±0.07)
– 1.15 (±0.04) 0.83 (±0.12) 1.05 (±0.07)
Fig. 3. Volume change in the lower solute concentration chamber as a function of time for different aqueous electrolyte solutions. The solute concentration was 0.01 M in all cases.
Fig. 4. Osmotic volume flow vs atomic number of the cation corresponding to the aqueous electrolyte solution used.
3.2. Electrolyte methanol–water solutions the mobility of the cation is higher than the anion mobility in the membrane. The relation between the volume of ion and the accompanying water molecules, and that of the channel structure, will determine the mobility of the ion in the membrane. Usually, the amount of water absorbed by the membrane decreases as the naked ionic size increases, resulting in increased cationsulfonate interactions [8,13]. This could be the reason why the interaction between the ion potassium with the –SO− 3 groups is higher than that of the other cations, resulting in a lower mobility in comparison with the ion Cl− . Figure 4 shows the volume flow as a function of the atomic number of the corresponding cation. The volume flow has been considered positive when it takes place in the direction of the normal osmosis. As can be observed, despite very different features observed for the osmosis by using KCl, LiCl, and NaCl electrolytes, it is worth noting that a linear dependence can be found between the volume flow and the atomic number of cation for all water–electrolyte solutions studied.
The influence of the presence of methanol in the negative anomalous behavior of the KCl–Nafion membrane system has been studied. Figures 5a–5d show the volume change with time in the right chamber at different KCl electrolyte concentration differences and percentages of methanol on solvent. These results show that negative anomalous osmotic behavior is also observed for KCl methanol–water solutions, because a volume increase in the chamber with the solution initially free of electrolyte is observed. It can also be seen that the curves representing the time variation of the volume for methanol–water solutions are similar to those obtained for aqueous solutions, with a transitory behavior at the beginning of the process followed by a linear region. No definite relation is observed between the transitory step and the methanol percentage at a given concentration difference. The flow can be estimated from this linear region in a similar way as in aqueous solutions. The volume flow obtained as a function of the concentration difference at different methanol percentages is presented in Table 2.
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(a)
(b)
(c)
(d)
221
Fig. 5. Volume change in the lower solute concentration chamber as a function of time for different volume percentages of methanol on solvent. The data correspond to a (a) 0.005 M, (b) 0.01 M, (c) 0.05 M, and (d) 0.1 M KCl electrolyte.
As can be observed, the presence of methanol affects the osmotic behavior because the value of the volume flow obtained in the absence of methanol is higher (more negative) than the corresponding one observed in presence of methanol, for all the electrolyte concentration differences. It
seems to indicate that the presence of methanol on solvent decreases the negative anomalous behavior of the membrane. In addition, the volume flow decreases (less negative) when the volume fraction of methanol increases, at a given electrolyte concentration, and for methanol fractions lower
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than 50%. At higher percentages of methanol, the volume flow seems to increase, exhibiting therefore a minimum value. The dependence of the volume flow on the electrolyte concentration is also nonlinear when methanol–water mixtures are used as solvent, showing the typical shape of the anomalous osmotic behavior. Moreover, the influence of the presence of methanol in solvent is higher at lower electrolyte concentration differences. The decrease of the volume flow when the percentage of methanol increases may be due to the fact that the presence of methanol on solvent affects the electric potential gradient established between both sides of the membrane. Methanol causes a decrease of the flow due to electric-potential gradient and so, the negative anomalous contribution to the total flow. This behavior is in agreement with studies on the membrane potential in the presence of methanolic solutions reported by other authors, in which the membrane potential decreases with an increase in the weight fraction of methanol in the solutions [14]. This effect is due to the fact that the cation-to-anion mobility ratio in membrane increases when the percentage of methanol increases, while the effective fixed charge decreases, due to the formation of the ion pairs in the membrane. For a 1:1 electrolyte, the other two contributions are always positive, but they will probably also be affected by the presence of methanol in the solution, since the studies carried out with Nafion perfluorinated ionomeric membranes have shown that its ionic conductivity is strongly dependent on the properties of the swelling solvent [15] and that the permeability and the membrane swelling depends on the solvent nature [16]. 4. Summary 1. The osmotic behavior of a Nafion membrane has been studied under different experimental conditions. The membrane shows anomalous osmosis, negative in the case of KCl electrolyte solutions, and positive for LiCl and NaCl solutions when aqueous solutions are used.
2. The influence of the presence of methanol on solvent on the negative anomalous behavior has been analyzed. The results show that the volume flow is higher when aqueous solutions are used and that the negative anomalous osmotic behavior is also observed with methanol–water solutions. 3. The value of the volume flow decreases when the volume fraction of methanol increases, at a given electrolyte concentration, for methanol fractions lower than 50%. At higher percentages of methanol, the volume flow seems to increase, exhibiting a minimum value.
Acknowledgment Financial support from Ministerio de Ciencia y Tecnología of Spain under Project BFM2000-0625 is gratefully acknowledged.
References [1] [2] [3] [4] [5] [6] [7] [8]
[9] [10] [11] [12] [13] [14] [15] [16]
F. Helfferich, Ion-Exchange, Dover, New York, 1995. H. Fujita, Y. Kobatake, J. Colloid Interface Sci. 27 (1968) 609. W.W. Goldstein, F.H. Verhoff, AICHE J. 21 (1975) 229. M. Tasaka, Y. Kondo, M. Nagasawa, J. Phys. Chem. 73 (1969) 3181. Y. Kobatake, J. Chem. Phys. 28 (1958) 442. T.D. Gierke, J. Electrochem. Soc. 124 (1977) 319. J. Ostrowska, A. Narebska, Colloid Polym. Sci. 261 (1983) 93. S. Kjelstrup, T. Okada, M. Ottøy, in: T.S. Sørensen (Ed.), Surface Chemistry and Electrochemistry of Membranes, Dekker, New York, 1999, Chap. 13. T. Okada, G. Xie, O. Gorseth, S. Kjelstrup, N. Nakamura, T. Akimura, Electrochim. Acta 43 (1998) 3741. A. Narebska, W. Kujawski, S. Koter, J. Membr. Sci. 30 (1987) 125. J.S. Vrentas, C.H. Vrentas, Chem. Eng. Sci. 57 (2002) 4199. S. Koter, J. Membr. Sci. 206 (2002) 201. T. Xue, R.B. Longwell, K. Osseo-Asare, J. Membr. Sci. 58 (1991) 175. T. Chou, A. Tanioka, J. Membr. Sci. 144 (1998) 275. M. Doyle, M.E. Lewittes, M.G. Roelofs, S.A. Perusich, R.E. Lowrey, J. Membr. Sci. 184 (2001) 257. D. Rivin, C.E. Kendrick, P.W. Gibson, N.S. Schenider, Polymer 42 (2001) 623.
Osmotic behavior of a Nafion membrane in methanol–water electrolyte solutions J.P. García-Villaluenga, B. Seoane, V.M. Barragán, and C. Ruiz-Bauzá ∗ Departmento de Física Aplicada I, Facultad de Física, Universidad Complutense de Madrid, 28040 Madrid, Spain Received 18 November 2002; accepted 6 February 2003
Abstract In this paper, an experimental study was carried out in order to investigate the osmotic transport of methanol–water electrolyte solutions through a Nafion membrane. The experimental data indicated that the Nafion membrane showed the typical anomalous osmotic behavior of charged membranes. The influence of some relevant parameters, such as electrolyte concentration difference, weight fraction of methanol on solution, and nature of cation was considered. The results showed that the osmotic volume flow was decreased with the presence of methanol on solvent, but did not alter the anomalous osmotic behavior of the membrane. 2003 Elsevier Science (USA). All rights reserved. Keywords: Anomalous osmosis; Methanol; Nafion membrane
1. Introduction Osmotic transport occurs between solutions of different concentrations separated by a membrane. With nonelectrolyte solutions or with uncharged membranes the flow rate is roughly proportional to the concentration difference between the two solutions, and the flow occurs toward the more concentrated solution. With electrolyte solutions and charged membranes the flow rate is often not proportional to the concentration difference, but appears to be affected by the presence of charges. This phenomenon is called anomalous osmosis and has been discussed by many authors when aqueous solutions are in contact with the membrane [1–5]. If the rise in the height of the level of liquid in the capillary of the chamber containing the more concentrated solution is higher than the theoretical height, the flow is called positive anomalous osmosis. On the other hand, if there is a fall, it is called anomalous negative osmosis. Depending on the membrane and on the electrolyte solutions, either negative or positive anomalous osmosis can be observed. This phenomenon is usually attributed to the permeation velocity of the electrolyte and, consequently, the * Corresponding author.
E-mail addresses: [email protected] (J.P. García-Villaluenga), [email protected] (B. Seoane), [email protected] (V.M. Barragán), [email protected] (C. Ruiz-Bauzá).
water transport accompanying the electrolyte being affected by membrane potentials appearing on both sides of membrane. Thus the total volume flow is the sum of three effects: interdiffusion of solute and solvent components, flow due to pressure gradient, and electro-osmotic flux due to electricpotential gradient [2]. The first term is of a thermodynamic nature, and the second and third terms are mechanical and electrical, respectively. For 1:1 electrolytes the term associated with the interdiffusion of mobile components is positive. The term representing the pressure flow is always positive. On the other hand, the flow due to electric-potential gradient can be either positive or negative, depending on the cation to anion mobility ratio in the membrane. This explains why reverse bulk flows occurred in a certain concentration region. A great variety of anomalies of osmotic flows can be observed in charged membrane electrolyte systems, because the three flow components constituting the total volume flow have greatly different dependencies on both the electrolyte concentrations and the membrane properties. Recently it has become intensively necessary to analyze the behavior of charged membranes in aqueous organic electrolyte solutions due to its importance from the standpoint of medical and industrial applications. However, studies and experimental data available for aqueous organic electrolyte solution systems are, limited compared to aqueous systems. In this paper, the osmotic behavior of a charged membrane in water and methanol–water electrolyte solutions has
0021-9797/03/$ – see front matter 2003 Elsevier Science (USA). All rights reserved. doi:10.1016/S0021-9797(03)00172-3
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Table 1 Membrane features provided by the manufacturer Thickness (mm)
Water uptake (%)
Fixed charge (kmol/m3 )
0.183
35
1.13
been studied. The membrane used was a Nafion membrane. This membrane was chosen because of its good physical and chemical properties and for its wide industrial applications, mainly with respect to its utilization in methanol fuel cells, where the understanding of the transport process in the presence of methanol is fundamental.
2. Experimental 2.1. Materials A commercial membrane Nafion 117 with a nominal equivalent weight of 1100 g/eq was used in this study. The Nafion 117 is a cation-exchange membrane consisting of a polytetrafluoroethylene backbone and long fluorovinyl ether pendent side chains regularly spaced, terminated by a sulfonate ionic group. The thickness, water content, and ion-exchange capacity provided by the manufacturer for this membrane are given in Table 1. The molecular level structure of Nafion focuses on the cluster-network model proposed by Gierke [6] who described the membrane as a series of cluster or inverted micelles, interconnected by narrow pores. In this model, the counterions, the fixed sites, and the swelling water phase are separated from the fluorocarbon matrix into approximately spherical domains connected by short narrow channels. The fixed sites are embedded in the water phase very near to the water fluorocarbon interface. According to many investigations based on different methods, the pore diameter of the cluster has been estimated at 4–5 nm and the diameter of the narrow pores at 1 nm [7,8]. In Nafion polymers, cations migrate along the restricted path through channels of hydrophilic domains, so the membrane-specific conductivity is strongly influenced
by the water content, as well as other transport properties [9]. The materials used in the experiments were solutions of water and methanol at different compositions. Lithium chloride (LiCl), sodium chloride (NaCl), and potassium chloride (KCl) were used as electrolytes. Pure pro-analysis grade chemicals and distilled pure water were used. Before measurements were carried out, the methanol–water solutions were degassed in order to prevent bubble formation during the measurement process. 2.2. Osmosis measurements In order to determine the osmotic flux through the membrane, an experimental setup was designed and constructed. A schematic of the device can be seen in Fig. 1. The main part of the experimental device was a cell, which basically consisted of two equal cylindrical glass chambers. The volume of each container was about 300 cm3 , which is large enough to ensure that the concentration changes in the solution during the measurements may be considered negligible. The membrane was mounted in a Teflon holder, which was positioned between the two chambers. O-rings were employed to ensure there were no liquid leaks in the whole assembly. The effective surface area of the membrane exposed to the flow was 5.7 cm2 . In order to measure the volume flux, two L-shaped capillary tubes were introduced in each chamber, in such a way that the horizontal portions of the tubes were at the same height. All the experiments were carried out under isothermal conditions at 25 ◦ C. Temperature requirements were achieved by immersing the cell in a water-thermostated bath. In order to improve the uniformity of the temperatures and concentrations inside each chamber, the solutions were stirred by a magnetic stirrer assembly. The stirring rate in all the experiments was about 300 rpm. Under these conditions, the temperature was constant within ±0.1 ◦ C. In order to achieve equilibrium, prior to each experiment the membrane was immersed for a minimum of 24 h in the solution of lower concentration. Once the membrane was positioned in the cell, both chambers were filled with
Fig. 1. Sketch of the experimental device used in the osmosis measurements. M, membrane; P, propeller; MS, magnetic stirrer; C, capillary; K, key; T, thermostat; B, bath; E, stirrer engine.
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the solution of lower concentration. When the system was stabilized at the selected temperature and stirring rate, the concentration of the electrolyte was changed in one of the chambers (left) to the desirable value. The volume fractions of methanol in the solutions, which were the same in the two chambers in all measurements, were 0, 25, 50, and 75 vol%. The electrolyte concentration in the left chamber was varied from 0.005 to 0.1 mol/l and the right one was kept free of electrolyte, in such a way that a difference of the electrolyte concentration was established between two sides of the membrane. The measurement consisted of following the displacement of the liquid meniscus in the capillary tube corresponding to the right chamber without electrolyte, as a function of time. The reproducibility of the data was confirmed by repeating each experiment three times, under the same conditions of electrolyte concentration and percentage of methanol.
3. Results and discussion 3.1. Electrolyte–water solutions 3.1.1. Influence of the electrolyte concentration difference Figure 2 shows the volume change in the right chamber as a function of time when KCl is used as electrolyte. Measurements were carried out at four different concentrations, 0.005, 0.01, 0.05, and 0.1 in mol/l, in the left chamber. The results show that the volume in the right chamber increases, indicating that the volume flow takes place from the chamber with the higher electrolyte concentration toward the chamber with the lower electrolyte concentration; consequently, a negative anomalous osmotic behavior is observed. As can be seen in Fig. 2, the time dependence of the volume indicates the existence of a transitory step at the beginning of the measurement in all cases studied. After certain time, the volume variation becomes linear, indicating that steady-state conditions are reached. Linearity was achieved after approximately 1000 s, depending on the experimental conditions. The analysis of the results reveals that there is not any relation between the duration of the transitory step and the electrolyte concentration difference established. The volume flow through the membrane can be estimated from the slope of the linear part of the curves shown in Fig. 2. The volume flows thus obtained are shown afterward in Table 2 as a function of the concentration electrolyte difference. As can be observed, the dependence of the volume flow on the electrolyte concentration difference is not linear, showing the features of anomalous osmosis, i.e., the typical dependence of the volume flow on the logarithmic concentration of electrolyte. These results seem to indicate that the flow due to the electric-potential gradient is negative and opposite to the normal osmosis, therefore the K+ mobility is smaller than the Cl− mobility in the membrane in all concentration ranges studied. The normal osmotic component always
Fig. 2. Volume change in the lower solute concentration chamber as a function of time for different aqueous KCl solution concentrations.
increases when the solute concentration difference across the membrane increases [3,4,10]. For this reason, the decrease of the total flow when the solute concentration difference increases, for concentrations higher than 10−2 mol/l, may be due to the fact that the anomalous component has a higher increase than the normal osmotic component, in this solute concentration range. 3.1.2. Influence of the nature of the electrolyte Figure 3 shows the experimental data obtained with a concentration of 0.01 mol/l of different 1:1 electrolytes in aqueous solutions. As previously noted, in the experiments with KCl solutions, the volume in the right chamber increases with time, indicating that the flow occurs from the chamber containing the aqueous electrolyte solution toward the chamber containing pure water. However, in the case of LiCl and NaCl solutions, the volume flow takes place toward the more concentrated solution, indicating that the membrane shows a positive anomalous osmotic behavior when LiCl and NaCl electrolytes are used. These data are consistent with previous studies in which it was shown that osmotic flow between aqueous electrolyte solutions separated by charged membranes exhibits anomalous osmotic behavior, negative or positive, depending on the membrane nature, the electrolyte, and the experimental conditions [11,12]. The results are also in agreement with the results found in [12] where a positive anomalous osmotic behavior is found for a Nafion membrane with aqueous NaCl solutions. These results indicated that, unlike KCl, in the case of LiCl and NaCl
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Table 2 Volume flow (10−5 cm3 /s) through Nafion 117 membrane as a function of the potassium chloride concentration difference at different percentages of methanol on solvent C (mol/l) 0.005 0.01 0.05 0.1
0% MeOH
25% MeOH
50% MeOH
75% MeOH
7.70 (±0.07) 1.25 (±0.04) 1.36 (±0.04) 2.10 (±0.04)
1.25 (±0.21) 1.07 (±0.04) 0.76 (±0.05) 0.64 (±0.10)
0.34 (±0.05) 0.99 (±0.04) 0.13 (±0.04) 0.10 (±0.07)
– 1.15 (±0.04) 0.83 (±0.12) 1.05 (±0.07)
Fig. 3. Volume change in the lower solute concentration chamber as a function of time for different aqueous electrolyte solutions. The solute concentration was 0.01 M in all cases.
Fig. 4. Osmotic volume flow vs atomic number of the cation corresponding to the aqueous electrolyte solution used.
3.2. Electrolyte methanol–water solutions the mobility of the cation is higher than the anion mobility in the membrane. The relation between the volume of ion and the accompanying water molecules, and that of the channel structure, will determine the mobility of the ion in the membrane. Usually, the amount of water absorbed by the membrane decreases as the naked ionic size increases, resulting in increased cationsulfonate interactions [8,13]. This could be the reason why the interaction between the ion potassium with the –SO− 3 groups is higher than that of the other cations, resulting in a lower mobility in comparison with the ion Cl− . Figure 4 shows the volume flow as a function of the atomic number of the corresponding cation. The volume flow has been considered positive when it takes place in the direction of the normal osmosis. As can be observed, despite very different features observed for the osmosis by using KCl, LiCl, and NaCl electrolytes, it is worth noting that a linear dependence can be found between the volume flow and the atomic number of cation for all water–electrolyte solutions studied.
The influence of the presence of methanol in the negative anomalous behavior of the KCl–Nafion membrane system has been studied. Figures 5a–5d show the volume change with time in the right chamber at different KCl electrolyte concentration differences and percentages of methanol on solvent. These results show that negative anomalous osmotic behavior is also observed for KCl methanol–water solutions, because a volume increase in the chamber with the solution initially free of electrolyte is observed. It can also be seen that the curves representing the time variation of the volume for methanol–water solutions are similar to those obtained for aqueous solutions, with a transitory behavior at the beginning of the process followed by a linear region. No definite relation is observed between the transitory step and the methanol percentage at a given concentration difference. The flow can be estimated from this linear region in a similar way as in aqueous solutions. The volume flow obtained as a function of the concentration difference at different methanol percentages is presented in Table 2.
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(a)
(b)
(c)
(d)
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Fig. 5. Volume change in the lower solute concentration chamber as a function of time for different volume percentages of methanol on solvent. The data correspond to a (a) 0.005 M, (b) 0.01 M, (c) 0.05 M, and (d) 0.1 M KCl electrolyte.
As can be observed, the presence of methanol affects the osmotic behavior because the value of the volume flow obtained in the absence of methanol is higher (more negative) than the corresponding one observed in presence of methanol, for all the electrolyte concentration differences. It
seems to indicate that the presence of methanol on solvent decreases the negative anomalous behavior of the membrane. In addition, the volume flow decreases (less negative) when the volume fraction of methanol increases, at a given electrolyte concentration, and for methanol fractions lower
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than 50%. At higher percentages of methanol, the volume flow seems to increase, exhibiting therefore a minimum value. The dependence of the volume flow on the electrolyte concentration is also nonlinear when methanol–water mixtures are used as solvent, showing the typical shape of the anomalous osmotic behavior. Moreover, the influence of the presence of methanol in solvent is higher at lower electrolyte concentration differences. The decrease of the volume flow when the percentage of methanol increases may be due to the fact that the presence of methanol on solvent affects the electric potential gradient established between both sides of the membrane. Methanol causes a decrease of the flow due to electric-potential gradient and so, the negative anomalous contribution to the total flow. This behavior is in agreement with studies on the membrane potential in the presence of methanolic solutions reported by other authors, in which the membrane potential decreases with an increase in the weight fraction of methanol in the solutions [14]. This effect is due to the fact that the cation-to-anion mobility ratio in membrane increases when the percentage of methanol increases, while the effective fixed charge decreases, due to the formation of the ion pairs in the membrane. For a 1:1 electrolyte, the other two contributions are always positive, but they will probably also be affected by the presence of methanol in the solution, since the studies carried out with Nafion perfluorinated ionomeric membranes have shown that its ionic conductivity is strongly dependent on the properties of the swelling solvent [15] and that the permeability and the membrane swelling depends on the solvent nature [16]. 4. Summary 1. The osmotic behavior of a Nafion membrane has been studied under different experimental conditions. The membrane shows anomalous osmosis, negative in the case of KCl electrolyte solutions, and positive for LiCl and NaCl solutions when aqueous solutions are used.
2. The influence of the presence of methanol on solvent on the negative anomalous behavior has been analyzed. The results show that the volume flow is higher when aqueous solutions are used and that the negative anomalous osmotic behavior is also observed with methanol–water solutions. 3. The value of the volume flow decreases when the volume fraction of methanol increases, at a given electrolyte concentration, for methanol fractions lower than 50%. At higher percentages of methanol, the volume flow seems to increase, exhibiting a minimum value.
Acknowledgment Financial support from Ministerio de Ciencia y Tecnología of Spain under Project BFM2000-0625 is gratefully acknowledged.
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