Properties of an anion exchange membrane in contact with aqueous solutions of sodium chloride and sodium benzenecarboxylate or benzenesulfonate

journal of MEMBRANE SCIENCE ELSEVIER

Journal of Membrane Science 103 (1995) 83-94

Properties of an anion exchange membrane in contact with aqueous solutions of sodium chloride and sodium benzenecarboxylate or benzenesulfonate Rachid Delimi, Jacqueline Sandeaux, Roger Sandeaux *, Claude Gavach Laboratoire des MaMriaux et Procddds Membranaires, UMR 9987, CNRS, 1919 route de Mende, B.P. 5051, 34033 Monrpellier Cddex 1, France

Received 30 June 1994;accepted l December 1994

Abstract Some properties of an anion-exchange membrane, ADS (Morgane-Solvay), were studied in aqueous solutions containing both sodium chloride and sodium benzenecarboxylate or benzenesulfonate. Measurements of electric resistance, water content, ion-exchange and self-diffusion flux were performed as a function of the organic salt concentration (10 - 3 to 10-1 M) in the external aqueous solution. The membrane affinity is greater for benzenesulfonate than benzenecarboxylate ion. The electric resistance decreases when the C1- counter-ions are substituted by these organic ions more hydrophobic and less mobile. The self-diffusion flux of the C1- ion through the membrane are strongly dependent on the water content and the concentration of this inorganic anion inside the membrane. Keywords: Ion-exchangemembrane; Benzenecarboxylate;Benzenesulfonate;Sorption;Electric resistance; Self-diffusionflux

1. Introduction Electrodialysis (ED) is increasingly used for the treatment of industrial effluents containing organic compounds. In this application of ED, two limiting effects must be taken into consideration: on the one hand, the competition in the membrane transport between the organic and inorganic ions and on the other hand, the poisoning of the membrane material by charged organic compounds. Several authors have studied the influence of hydrophobic or amphiphilic ions on the membrane properties. Generally, it was observed an increase in electric resistance and a loss in permselectivity of ion-exchange membranes, depend* Correspondingauthor. 0376-7388/95/$9.50 © 1995 Elsevier Science B.V. All rights reserved SSD10376-7388(94)00310-6

ing on the nature and concentration of organic ions present in solution [ 1-8]. The purpose of this paper is to investigate under zero current conditions the properties of an anion-exchange membrane used for ED (ADS, Morgane-Solvay) in contact with aqueous mixtures of sodium chloride and sodium benzenecarboxylate ( q ~ O 2 N a ) or benzenesulfonate (qbSO3Na). The data obtained here will be usefull for analysing the results of a forthcoming study under non zero current conditions. By using radiotracers, it is possible to determine the amount of ionic species (counter-ions and eventually co-ions) inside the membrane and to measure the selfdiffusion flux of ions crossing the membrane without applied overvoltage. From these results, the self-diffusion coefficient of ions into the membrane was cal-

84

R. Delimi et al. / Journal of Membrane Science 103 (1995) 83-94

Table 1 Main characteristics of the ADS XL 10 membrane Nominal properties

ADS XL 10

Type Manufacturer Resistance (ohm.cm 2) in 0.1 M NaCI Ion-exchange capacity (mequiv.g- 1) Transport number in 0.1 M NaC1 Water content (%) Thickness ( ~ m ) Density (g.cm -3)

MEA Morgane-Solvay 1.3 1.27 tcl- = 0.94 32 150 1.42

culated. Measurements of water content and electric resistance of the membrane were also performed under identical equilibrium conditions.

2. Experimental 2.1. Membrane The anion-exchange membrane studied was the ADS membrane produced by Morgane-Solvay. This membrane is a strong ion exchanger and its main characteristics are given in Table 1.

2.2. Ion-exchange equilibrium and amount of sorbed ions Radiotracers were used for the determination of amount of fixed or sorbed ions. 36C1 o r 22Na labelled NaC1 and 14C labelled t/:3CO2H were purchased from the Radiochemical Centre, Amersham, UK. The radioactivity of the samples was measured using a Packard spectrometer.

The area of the membrane samples was ~ 4 cm 2. The membrane samples were immersed for 24 h in 100 ml of radioactive sodium chloride solution containing an organic salt (sodium benzenecarboxylate or sodium benzenesulfonate) at various concentrations, in order to reach a stable physicochemical equilibrium. The samples removed from the labelled solution were washed in deionized water, quickly blotted and immersed in 100 ml of unlabelled 1 M NaCI solution. All the radioactive ions were removed from the membrane and transfered to the solution after a few hours. These chloride ions remaining inside the membrane were only equilibrating the electrical charges of the exchange sites. Their number was deduced from radioactivity measurements of the solution. All results were expressed in mmol/g of dry membrane. For the determination of amount of sorbed ions, the experimental process was identical, except the membrane samples were not washed in water but directly immersed in 100 ml of unlabelled 1 M NaC1 solution. In this case, chloride ions inside the membrane were fixed and sorbed ions.

2.3. Water content of the membrane The area of the membrane samples was also ~ 4 c m 2. After the membrane samples were immersed for 24 h in N a C I and t/~O2Na or t/~SO3Na mixtures, they were wept with two sheets of blotting paper, weighed (p) and dried at 50°C until their weight (Ps) reached a constant value. The following equation relates the wet sample weight (p) to its dry weight (ps): P =Ps +Pw + P i

Table 2 Electric resistance, water content, self-diffusion fluxes of chloride ion and amount of chloride and sodium ions inside the ADS membrane, as a function of the concentration of the NaCl + qbCO2Na mixture INaCI] (M)

10 -~

0

[ t/~CO2Na] (M)

0

10 -3

10 -z

R ( .O.cm2) Pw ( % ) -Po-" 10s (mol.cm- 2.s- l) n o - (fixed) ( m m o l . g - : ) n o - (sorbed) (mmol.g- 1) n~a÷ (sorbed) ( m m o l . g - t )

1.18 31.6 4.80 1.275

1.20 31.6 4.80 1.267

1.32 30.9 4.43 1.197

3.10 -2 1.64 29.2 3.98 1.050 0.043 0.064

5.10 -2 1.92 28.3 3.50 0.949 0.052 0.084

10 -1

10 -1

2.70 26.1 2.79 0.740 0.092 0.164

10.0 20.0 0 0 0 0.061

R. Delimi et al. / Journal of Membrane Science 103 (1995) 83-94

85

Table 3 Electric resistance, water content, self-diffusion fluxes of chloride ions and amount of chloride and sodium ions inside the ADS membrane, as a function of the concentration of the NaCI + q:'SO3Na mixture [NaCI] (M)

10 -1

0

[ t/~SO3Na] (M)

0

10 -3

10 -2

3.10 -2

5.10 -2

10 - l

10 -1

R (g2.cm 2) Pw (%) ~Cl-.108 (mol.cm- 2.s - l ) n o - (fixed) (mmol.g - J ) no - (sorbed) (mmoi.g- 1) nNa+ (sorbed) (mmol.g -1)

1.18 31.6 4.80 1.275

1.20 31.3 4.70 1.248

1.65 28.6 4.00 1.071

2.36 26.8 3.20 0.835 0.024 0.083

3.18 24.5 2.60 0.666 0.082 0.123

4.78 21.5 1.60 0.400 0.108 0.197

13.6 16.3 0 0 0 0.083

where Pw is the water weight and Pi is the sorbed ion weight, calculated from the values of the number of each ionic species present in the membrane given in Tables 2 and 3 and using the equation: no-

+ n R- = N + n N a +

where N is the membrane exchange capacity. The water content (Pw) is expressed by the percentage of weight of water over the weight of the dry membrane: Pw =Pwlo0 P~ 2.4. Electric resistance measurements

Electric resistance measurements were performed at 2500 Hz using a clip cell and a conductimeter (CD60 Tacussel). The area of the platinized platinum electrodes as well of the membrane sample was 1 cm 2. The electric resistance is the difference between the measurements performed with and without the membrane. 2.5. Flux measurements

Flux measurements were performed using radiotracers and a Teflon cell described previously [8]. Solutions at the same concentration circulated in two compartments on both sides of the membrane. The rate of circulation was ~ 10 cm.s- ~. The working area of the membrane was 1 cm 2. In one compartment, a very small volume of labelled chloride was added. The changes of radioactivity in the solution circulating in the other compartment were measured by sampling ali-

quots of solution at regular intervals which allowed the calculation of the number of ions crossing the membrane. At equilibrium, the number of ions appearing in the initially non-radioactive compartment varies linearly with time. The value of the flux was deduced from the slope of this straight line.

3. Results All the experiments were performed with the ADS membrane at 10- ~ M NaC1 and a sodium benzenecarboxylate or benzenesulfonate concentration of 0 to 10-1 M. The results are given in Tables 2 and 3. It should be noted that, on the one hand, the electric resistance increases with the organic salt concentration in the bulk solution, and on the other hand, the water content, amount of fixed chloride ions and self-diffusion fluxes of these ions decrease. Above 3.10 -2 M of organic salt, the presence of sorbed chloride and sodium ions inside the membrane was observed. The amount of these ions are not the same. This means that organic ions, equilibrating sodium co-ions, are also present in the intersticial phase of the membrane. The values ofnc~- and nNa+ (Tables 2 and 3) are too low to be measured at concentrations lower than 3.10 -2 M. For the two organic counter-ions, only self-diffusion fluxes of the benzenecarboxylate ion can be measured because this ion, contrary to the benzenesulfonate, is a commercial labelled compound. Measurements were performed when the membrane was equilibrated with

R. Delimi et al. / Journal of Membrane Science 103 (1995) 83-94

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1

0,9

0,8

/ /

0,7

/

/ / /

0,6

/

/

0,5

/ / 0,4

/ / /

0,3

/ 0,2

0,1

0

0,1

0,2

0,a

0,4

0,5

0,6

03

0,8

0,9

1

XRFig. I. Ion-exchange isotherms between chloride and benzenecarboxylate or benzenesulfonate ions: @, NaCi + tI:JCO2Na; l-I, NaCI + qDSO3Na. Table 4 Values of the selectivity coefficient for the (/~CO~-/CI- and qbSO~-/ CI- ion-exchanges as a function of organic salt concentration in the external aqueous solution [NaCI] (M)

10 -1

[RNa] (M)

10 -~

10 -2

3.10 -2

5.10 -2

10 - I

k (d)CO2/C1-) k (qbSO~-/C1- )

0.658 2.166

0.652 1.9

0.713 1.76

0.686 1.82

0.723 2.18

10 - 1M qbCO2Na solution or 10 - 1M qbCO2Na + 10-1 M NaCI mixture. The results are: 10-1 M qr~CO2Na J~co2- = 0.78.10- s m o l . c m - 2.s - 1 10 -1 M ql~CO2Na+ 10 - 1 M NaCI J°¢co2-= 0.56.10- s m o l . c m - 2.s - 1

These fluxes are much lower than those of the inorganic counter-ion which decreases from 4.8.10 -8 to 2.79.10- 8 m o l . c m - 2.s - i, when the q ~ O 2 N a concentration increases from 0 to 10-1 M. These values have been used to calculate the self-diffusion coefficient of the benzenecarboxylate ion (Table 9). Table 5 Experimental and calculated molalities of Na + co-ion inside the ADS membrane as a function of the concentration of the NaC! + t/~CO2Na mixture [NaCI] (M)

10 -1

[t/~CO2Na]

0

0 10 -3

10 -2

3.10 -2 5.10 -2 10 - l

10 - t

(M) r~P÷. 102 (mmol.g- 1) -caRd 2 mN=÷.10 ( mmol.g - i )

22 0.15 0.15

0.17

0.22

30 0.27

63 0.40

31 0.09

R. Delimi et al. / Journal of Membrane Science 103 (1995) 83-94 Table 6 Experimental and calculated molalities ofNa ÷ co-ion inside the ADS membrane as a function of the concentration of the NaC1 + t/~SO3Na mixture [NaCI] (M)

10 - t

[~SO3Na]

0

0 10 -3

10 -2

3.10 -2

5.10-210 -1

10 -1

(M) ~'~x~÷.102 (mmol.g- l ) m'~.102 (mmoi.g- i )

31 0.15 0.15

0.16

50

0.20

51

92

0.23

0.33

0.08

In all cases, we have checked that the self-diffusion flux of the sodium co-ion is zero. When the membrane was equilibrated with only organic solutions ( [NaCI] = 0, [ q~CO2Na] or [ qbSO3Na] = 10-1 M), an increase in electric resis-

87

tance and a decrease in water content were noted which were slightly higher for benzenesulfonate than benzenecarboxylate. The results of water content are in accordance with those previously obtained by Tasaka et al. [9]. The decrease in water content of Neosepta AFN membranes is higher for the qbSO3 than the qbCO2 counter-ion. The variations are 16% in Neosepta (P) membrane and 59% in ADS membrane for • SO3, and 8% in Neosepta (P) membrane and 37.5% in ADS membrane for q~CO2. All results show that competition between organic and inorganic ions occurs within the membrane phase, changing the membrane properties at equilibrium.

4. Discussion From the results obtained, it is possible to examine the partition of the species, counter-ions, co-ions and

14 rl w

12

10

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Xc~ Fig. 2. Variation of number of water moles per fixed sites versus the chloride ionic fraction inside the membrane:O, NaCI+ q~CO2Na;if], NaC1+ q~SO3Na.

R. Delimi et al. / Journal of Membrane Science 103 (1995) 83-94

88

E

o O

4.5

E

3.5

2.5

1.5

0.5

0 0

I

I

I

I

I

I

I

I

I

t

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Xcr Fig. 3. Variationof self-diffusionfluxesof the chlorideion versusthe chlorideexternal ionic fraction:O, NaCl + qr~CO2Na; I-q, NaC1 + tPSO3Na.

solvent, between the membrane and the aqueous solution, and to calculate the self-diffusion coefficient of the counter-ions inside the membrane. All the experimental or calculated parameters can be analysed as a function of the internal composition of the membrane.

Ion-exchange equilibrium can be characterized by the ion-exchange isotherm which represents the ionic composition of the ion exchanger as a function of the ionic composition of the aqueous solution. The ionic fraction (XR-) of organic ions fixed on the sites of the membrane can be deduced from the number of C I - inside the membrane ( n a - ) :

equilibrated with a 10 - 1 M NaCI solution and no sorbed ions in the membrane, ncl- being fixed ions. The ( C I - - q b C 0 2 ) and (CI--qOS03-) ionexchange isotherms are plotted in Fig. 1. It can be seen that the anion distribution in the membrane is not the same as those in the solution. The ionic fraction of the benzenesulfonate ion is higher inside the membrane than in the bulk solution, and inversely, for the benzenecarboxylate ion. These results show that the membrane has a greater affinity to qbS03 than to C1and a lower affinity to qr~C02 than to C1-. The preference of the membrane for one of the two counter-ions is also expressed by the selectivity coefficient. Since, in this case, they are not sorbed ions inside the membrane, we can consider the following exchange reaction:

X-R- =n'R - / N = ( N - - n o - ) / N

R - + C-I-':- ~ R-- + C I -

4.1. Exchange equilibrium between the chloride and benzenecarboxylate or benzenesulfonate ions

(1)

where N is the membrane exchange capacity. It is the value of ncl- measured when the membrane is only

where R - represents the organic ion and the bars refer to the species inside the membrane.

R. Delimi et al. / Journal of Membrane Science 103 (1995) 83-94

Table 7 Self-diffusion coefficient of the chloride ion inside the ADS membrane as a fiinction of organic salt concentration in the external aqueous solution

The selectivity coefficient is defined by Eq. (2): k=

XR- " XciXR_

•

89

(2)

XCI-

The values of k are given in Table 4 and show that the membrane affinity is twice as great for the benzenesulfonate ion than the chloride ion ( k ~ 2 ) , in the entire concentration range studied. On the contrary, k is lower than unity for the qbCO~-/C1- exchange (k~0.7).

[NaC1] (M)

10 -1

[RNa] (M)

0

D o - 107.cm2.s-! _qbCO~-/CID o - 107.cm2.s -1

10 -3

10 -2 3.10 -2 5.10 -2

10 - l

8.01 8.01

7.54

7.10

6.54

5.76

8.01 7.93

7.31

6.73

5.89

4.83

qbSO~-/C1-

4.2. Co-ion rejection From the amount of sodium and water content inside the membrane (Tables 2 and 3), the experimental molality of the sodium co-ions can be calculated and compared with those obtained from the classical Donnan equation applied to a 1:1 electrolyte [ 10] : --

i---m2

2 1/2

~+d =- ~"P" L-~':':'-'iT± F-fZ±mNa+/ ] / _1 ~

where -mN=N/-mH20 is the fixed ion-exchange site molality, tuNa+ is the sodium ion molality and y+ is the mean activity coefficient of the electrolyte in the bulk solution. The approximation of y ± = l was made. From the values shown in Tables 5 and 6, it can be seen that there is a great difference between the experimental and calculated molalities. It is not surprising

(3)

/

4.5

o~ 3.5

2.5

1.5

0.5

0

I

I

I

I

I

I

I

I

I

I

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Xo _ Fig. 4. Variation of self-diffusion fluxes of the chloride ion versus the chloride internal ionic fraction: O, NaC1 + t/:~CO2Na; 17, NaC1 + qbSO3Na.

R. Delimi et al. / Journal of Membrane Science 103 (1995) 83-94

90

T

9

i"

I

I

I

I

I

I

I

I

q

I

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Xcl Fig. 5. Variation of the self-diffusion coefficient of the chloride ion versus the chloride internal ionic fraction: 0, NaCI+ qbCOzNa; [~, NaCI + qr)SO3Na.

that the Na + concentration increases within the membrane as the Na + external concentration increased. But, for the same co-ion concentration in the bulk solution ( 10-1 M), the co-ion amount inside the membrane was not detected when the counter-ion present in the membrane was only C I - , and the ratio of coion over the total number of counter-ions is about 5 6% when qK202 or @SO~- organic ions penetrate into the membrane. Since, this ratio is about 11-13% for the equimolar mixture, self-diffusion fluxes of Na ÷ ions cannot be detected.

4.3. Swelling In polymeric ion-exchange materials, the swelling results from the balance between the internal osmotic pressure and the counteracting elasticity forces of the polymeric matrix [10]. When an ion-exchange

membrane is immersed in an aqueous electrolyte solution, its water content is related to the hydration state of the charged species: ionized fixed sites and also exchangeable counter-ions. The decrease in water content with the amount of benzenecarboxylate and benzenesulfonate ion inside the membrane can be explained by the substitution of hydrophilic chloride counter-ion by organic ions more hydrophobic. From the values of the water content (Tables 2 and 3), the number of water moles per exchanging sites, nw, can be calculated using the following equation: Pw nw = 18N

(4)

In Fig. 2, the values of nw plotted versus Xc~- give a straight line. Therefore, in the studied concentration range, nw follows a simple variation law. The water

R. Delimi et al. /Journal of Membrane Science 103 (1995) 83-94 0,9

/

E "7

•

0,8

C '~

91

0,7

0,6

[3

0,5

0,4

0,3

0,2

0,1

I

I

I

I

I

I

I

I

t

I

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

I

XCIFig. 6. Variation of the inverse of the membrane electric resistance versus the chloride internal ionic fraction: O, NaCI + qK~O2Na; IS], NaCI + qbSO3Na. Table 8 Comparison between the self-diffusion coefficients of chloride, benzenecarboxylate and benzenesnlfonate ions in aqueous solution and inside the ADS membrane at 10-1 M concentration

Solution Membrane

Dcl-. 105 cm2.s -1

Da:o~ 105 (cmZ.s - ' )

D~so~:105 (cm2.s -1)

1.92 0.277

0.85 0.029

0.985 0.0195

content is d e p e n d e n t on the a m o u n t and the nature o f each ionic species present in the m e m b r a n e . F r o m these results, one notes that the variation Ofnw is b e t w e e n 5 7 m o l . e q u i v - ~ w h e n a C 1 - ion is substituted by an organic ion. This result m a y be related to the values of the hydratation n u m b e r o f the chloride ion in the literature varying f r o m 6 to 9 in a q u e o u s solutions [ 11 ] and 1 to 5 in m e m b r a n e s [ 12]. F r o m transport number measurements, values o f 5 and 6 h a v e be found in A R A m e m b r a n e s [ 13 ].

4.4. Self-diffusion fluxes o f the chloride ion in the membrane Fig. 3 shows the variation versus the C1- external ionic fraction, o f the self-diffusion fluxes o f the C 1 ion through the m e m b r a n e equilibrated with a q u e o u s solutions containing, either the ( C 1 - - q C O f ) or the (CI--q~SO3) mixture. T w o distinct curves are Table 9 Comparison between the self-diffusion coefficients of chloride and benzenecarboxylate ions inside the ADS membrane, calculated from two experimental techniques (self-diffusion flux and electric resistance) D o - . 107 (cm2.s - 1)

Concentration in the bulk solution D- calculated from J D calculated from R

[NaCI] = 10-1 M 8.01 27.7

D,ar,co~ 107 (cm2.s- 1) [ q~O2Na] = 10-1 M 0.94 2.90

92

R. Delimi et al. / Journal of Membrane Science 103 (1995) 83-94

obtained. When the self-diffusion fluxes of the C I - ion are plotted as a function of the C1- internal ionic fraction, a simple straight line starting from zero is obtained (Fig. 4). This result is very interesting. It means that the kinetics of the chloride self-diffusion through this membrane is not dependent on the presence of organic ions inside the membrane. Using the Helfferich equation [ 10], the self-diffusion coefficient of the chloride ion into the membrane can be calculated from experimental results of selfdiffusion fluxes:

-Dcl-

Jcl-d (1- 2Jcl-8 / ~c,k

(5)

m

where D a - is the self-diffusion coefficient of the chloride ion in the membrane, d the membrane thickness and 8 the thickness of the boundary layer. The values of Dcl- were calculated from Eq. (5) using the following experimental values: JcJ- given in Tables 2 and 3 Cc~- deduced from the amount of chloride ions inside the membrane and the water content (Tables 2 and 3) Dc~- = 1.92.10 - 5 cm2.s - l is the self-diffusion coefficient in dilute solution [ 14] d = 15.10 -3 cm (Table 1) 8=5.3.10 -3 cm [7] The Dc~- values given in Table 7, decrease when the organic concentration increase in the mixtures studied. In Fig. 5, it can be seen that D o - varies linearly with the chloride concentration inside the membrane. A similar linear variation of D o - was obtained with the water content in the membrane.

4.5. Electrical properties of the membrane Assuming the membrane phase is a gel model containing ionic species moving independently, the equivalent electrical conductivity of the membrane, K, follows a linear law: K = O/(U cI-XcI- "4-U R - X

R-

"~- U'Na+XNa+

)

(6)

where u are the apparent ionic mobilities of each species and a is a constant. Neglecting the sodium concentration inside the membrane and recalling that X-R-=(1--Xc1-), Eq. (6) gives:

K= a(UR- + (Ucl- --Us-)XC~-

(7)

Fig. 6 shows, in the ionic fraction range 0.5 < xct< 1, that the variation law of r versus xcl- is not a linear relationship. Results of conductance and self-diffusion measurements give different variation laws as a function of the internal composition of the membrane. The apparent diffusion coefficient of counter-ions can be also deduced from conductance by using the Nernst-Einstein relation:

_

RTdl

1

Di=f2 s~iR

(8)

where s is the area, d is the thickness and R is the electric resistance of the membrane. The value of Dcl-, D~co~-and D~so3-can be calculated from Eq. (8) and the values of R given in Tables 2 and 3 at 10- i M NaC1, 4~CO2Na or thSO3Na concentration. Table 8 allows the comparison of the apparent diffusion coefficients of the chloride ion and two organic ions in the membrane and in aqueous solution. The value of the organic self-diffusion coefficient in water is calculated from the following equation:

DR- = Dcl-

)t~l-

(9)

where A° is the limiting equivalent conductance of the individual anion. The mobilities of these organic anions are twice as low as the mobility of the chloride ion in solution and ten times smaller than the mobility of the chloride ion in the membrane. The conductance decrease with decreasing x o - can be explained by this great variation in the mobility of organic counter-ions when they are present in the membrane.

4.6. Comparison between the values of the apparent ionic mobility calculated from electric resistance and self-diffusion measurements The values of D ' o - and D~co~-calculated from Eqs. (5) and (8) are given in Table 9. For two ions, the values deduced from electric resistance measurements are about three times higher than those obtained from self-diffusion flux measurements. This large discrepency shows that the Nernst-Einstein equation which is valid to aqueous solutions can-

R. Delimi et al. / Journal of Membrane Science 103 (1995) 83-94

not be applied to this membrane material. It was found that the electrical values for the counter-ions always exceeded those determined in tracer diffusion [15]. The difference between the two measurements of diffusion coefficients have been generally regarded as a consequence of the electrical mobility taking place against a background flow of electro osmotically driven water travelling in the same direction as the counterions. No such flow is present in a tracer flow experiment. But, to explain such strong effects, the influence of the electro osmotic flux does not seem to be the only reason. A deep analysis at the molecular level of the mechanism of ion movement inside the membrane material is necessary.

p P R R s T u x z 3'

5. Conclusions

References

This study demonstrated the higher affinity of the ADS membrane for the benzenesulfonate than the benzenecarboxylate ion, but, when the same amount of these ions penetrated into the membrane, effects of the same magnitude were noticed. The self-diffusion fluxes of the C1- ion are related to the water content and the amount of this inorganic ion inside the membrane and are not dependent on the nature of the two organic ions studied. The interactions of organic ions with the membrane material can be due, on the one hand, to electrostatic interactions with the functional groups, and on the other hand, to hydrophobic interactions with the polymeric matrix of the membrane. So, the low mobility of these ions inside the membrane leads to a decrease in the membrane conductivity.

6. List of symbols C d D F J k m n N

concentration membrane thickness diffusion coefficient Faraday constant flux selectivity coefficient molality number of moles per unit weight of dry membrane exchange capacity

93

weight water content per unit weight of dry membrane electric resistance gas constant membrane area absolute temperature absolute mobility ionic fraction electrochemical valence activity coefficient boundary layer thickness conductivity

Quantities and symbols with overbars refer to the membrane phase.

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[ 14 ] J. Sandeaux, R. Sandeaux and C. Gavach, Competition between the electrotransports of acetate and chloride ions through a polymeric anion exchange membranes, J. Membrane Sci., 59 ( 1991 ) 265. [15] P. Meares, Trends in ion-exchange membrane science and technology, Ion Exchange Membrane, D.S. Flett (Ed.), 1983.