Synthetic Membranes: Volume I - American Chemical Society

23 Transport of Ions and Water in Sulfonated Polysulfone Membranes N. VINNIKOVA and G. B. TANNY Downloaded by UNIV OF CALIFORNIA SAN DIEGO on June 2, 2015 | http://pubs.acs.org Publication Date: May 21, 1981 | doi: 10.1021/bk-1981-0153.ch023

The Weizmann Institute of Science, Department of Plastics Research, Rehovot, Israel

Ion-exchange type, charged membranes were amongst the first materials recognized to be suitable for desalting water by reverse osmosis (R.O.) (1). However, subsequent to the development of neutral, asymmetric cellulose acetate membranes (2), they were paid relatively little attention. The first step toward the attainment of a charged equivalent of a conventional asymmetric R.O. membrane was the development of thin-film composites of sulphonated poly(phenylene-oxide) (3,4) . These membranes showed some promise, but did not give sufficiently high rejection of concentrated salt solutions to be considered practical for seawater desalting. Sulphonated polysulphone (SPS) membranes, on the other hand, have been shown to have excellent salt rejections and water fluxes (5-7) which are maintained even for feeds of very high salt concentration. In fact, both the magnitude of the salt rejections cited and their independence of feed concentration are remarkably non-characteristic of the behaviour of any charged membrane heretofore examined. Utilizing a previously derived relation between salt rejection and transport number (8) in porous charged membranes it was found that the high salt selectivity of a dense SPS film appeared to be incompatible with a Donnan exclusion mechanism (9). In the present contribution, a more comprehensive study of the transport coefficients has been carried out on dense sulphonated polysulphone membranes. To achieve some physical understanding of these measurements, morphological investigations relating to the distribution of ions in the matrix have also been carried out utilizing the techniques of X-ray scattering and electron microscopy.

0097-6156/81/0153-0351$05.00/0 © 1981 American Chemical Society

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

352

SYNTHETIC

MEMBRANES:

DESALINATION

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on June 2, 2015 | http://pubs.acs.org Publication Date: May 21, 1981 | doi: 10.1021/bk-1981-0153.ch023

Experimental A. M a t e r i a l s . Polysulphone r e s i n P-1700 and P-3500 were obtained from Union Carbide. T h e i r weight average molecular weight, Mw, was determined by u l t r a c e n t r i f u g e : Mw = 35400 f o r P-1700 and Mw = 38600 f o r P-3500. The sodium s a l t o f sulphonated polysulphone Na-SPS was o b t a i ned by r e a c t i o n o f polysulphone r e s i n (P-1700 or P-3500) with chlorosulphonic a c i d i n 1,2-dichloroethane (5). T h i s was followed by d i s s o l u t i o n i n dimethyl formamide, p r e c i p i t a t i o n i n 10% NaCl, and r i n s i n g with large volumes o f water u n t i l the washings were f r e e o f NaCl. The ion-exchange c a p a c i t y (IEC) o f the samples was determined by p o t e n t i o m e t r i c t i t r a t i o n o f the SPS-H form s o l u t i o n (90% d i me thy I formamide, 10% water) with sodium hydroxide. The polymer f i l m was cast on glass p l a t e s from a 20% (by weight) s o l u t i o n o f polymer i n dimethy1formamide. A f t e r the f i l m was d r i e d at 50°C f o r 40 min i t was removed from the glass p l a t e by immersion i n water. No d i f f e r e n c e was found i n the p r o p e r t i e s o f sulphonated products based on the P-1700 r e s i n or P-3500 r e s i n . +

B. Methods. 1. WAXS: Wide angle X-ray s c a t t e r i n g was done on a PW 1830 h o r i zontal goniometer with Cu Ka (1.54A) r a d i a t i o n (32 kV, 20 mA). S l i t s of 1° were employed. 2. SAXS: Small angle d i f f r a c t i o n p a t t e r n s were obtained a£ room temperature u s i n g a " S e a r l e " X-ray camera with Cu Ka (1.54A) r a d i a t i o n (35 kV, 25 mA). The exposure times were about 65 hours. The photographs were microdensitometered u s i n g a Joyce-Loebl double beam micro'densitometer. 3. Transmission e l e c t r o n microscopy: P r i o r to embedding, the 20 urn t h i c k polymer f i l m was converted to i t s C s o r F e form. A mixture o f vinylcyclohexene d i o x i d e (10 g), d i g l y c i d y l ether o f p o l y propyleneglycol (6 g) nonenyl s u c c i n i c anhydride (26 g) and dimethylaminoethanol (0.4 g) was used as an embedding m a t e r i a l . U l t r a - t h i n s e c t i o n s (^800A) were cut with an Ultra-microtome MT-2, using a diamond k n i f e . Micrographs were obtained u s i n g a P h i l l i p s EM 300 e l e c t r o n microscope at an a c c e l e r a t i n g voltage o f 80 kV. 4. F i l t r a t i o n c o e f f i c i e n t : The f i l t r a t i o n c o e f f i c i e n t , Lp, was measured under osmotic pressure u t i l i z i n g thermostated glass c e l l s (±0.05°C) equipped The c e l l s had an e f f e c t i v e membrane area o f 1.77 cm and each compartment contained ^25 cc o f s o l u t i o n . One compartment was f i l l e d with d e i o n i z e d water and the second with a 1 or 2 molal s o l u t i o n o f sucrose (depending upon IEC). To o b t a i n the water flow J y , we followed the r a t e s o f the meniscus movements i n the c a p i l l a r y . The mean value o f s e v e r a l measurements was used to c a l c u l a t e Lp according to the equation(1): +

+ 3

o

2


23.

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Polysulfone

353

Membranes


J

= - a Lp ATT (1) v where a i s r e f l e c t i o n c o e f f i c i e n t , and ATT i s the osmotic pressure d i f f e r e n c e across the membrane. To c a l c u l a t e the osmotic pressure we used values o f osmotic c o e f f i c i e n t s from r e f . (10). T o t a l organic carbon a n a l y s i s (Beckman 914A) o f samples from the water f i l l e d compartment v e r i f i e d that the membranes are impermeable t o sucrose, so that the r e f l e c t i o n c o e f f i c i e n t a i s equal t o u n i t y . The obtained values o f the membrane constant LpAx are c o n s i dered t o c o n t a i n an u n c e r t a i n t y o f ±20%, mostly due t o the uncert a i n t y o f ±15% i n the measurement o f Ax by a d i a l micrometer. 5. The s e l f - d i f f u s i o n measurement: The membrane was clamped between two i d e n t i c a l p l e x i g l a s s h a l f c e l l s (volume o f each, -75 ml). The compartments were f i l l e d with d e i o n i z e d water. After temperature e q u i l i b r a t i o n , 0.1 ml o f t r i t i a t e d water HTO a c t i v i t y 5 mCi/ml was introduced, i n one o f the compartments ("hot" side (')). Simultaneously, s t i r r i n g was s t a r t e d and samples were taken from the "hot" s i d e . At the chosen time i n t e r v a l s , samples from the " c o l d " (") s i d e were taken. The t o t a l number o f samples taken during the experiment was about 14 - two d u p l i c a t e s at every time i n t e r v a l . Each sample volume was 50 u l so the volume changes during experiment can be e f f e c t i v e l y neglected. Each sample was put i n t o a v i a l c o n t a i n i n g 4 ml s c i n t i l l a t i o n mixture (Triton-X s o l u t i o n ) and 0.5 ml o f water. For counting we used a Packard TRI-CARB L i q u i d S c i n t i l l a t i o n spectrometer. P l o t t i n g then cj^ o (number o f counts a t time t , i n the " c o l d " s o l u t i o n ) as a f u n c t i o n o f time t , r e s u l t e d i n a s t r a i g h t l i n e , the slope o f which y i e l d e d the corresponding constant t r i t i a t e d water f l u x Ja dc" T

J

d

=

(

V

/

A

)

~1F^

W 2

where A i s a membrane area, v a r i e d from 9.6 cm f o r lower values of IEC t o 2.5 cm f o r the h i g h e r . Applying F i c k ' s Law, 2

J

d

=

P

™

~2jF^

T

where Ax i s a membrane t h i c k n e s s , P i s a d i f f u s i o n p e r m e a b i l i t y and Ac^rjiQ i s a d i f f e r e n c e i n t r i t i a t e d water concentration across the membrane. The experiment was ended when an amount o f t r a c e r , not l a r g e r than 5% o f the o r i g i n a l , had crossed the membrane, therefore ACJJTO taken equal t o c ^ o " number o f counts i n the "hot" side at the beginning o f the experiment. Thus, the expr e s s i o n shown i n equation (4) was used t o c a l c u l a t e P^: T

c

P

T

a

"

n D

e

a

C ^ M ) ^

A l l the d i f f u s i o n out at 25°±0.05°C.

(4)

measurements were c a r r i e d Membrane t h i c k n e s s was measured by a


354

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MEMBRANES:

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micrometer. The values used f o r c a l c u l a t i o n s were the average o f s e v e r a l measurements and the accuracy o f the membrane thickness measurements was ±15%. 6. E l e c t r i c a l conductance: The conductance measurements were made i n a glass c e l l c o n s i s t i n g ot two equivalent e l e c t r o d e s e c t i o n s . C i r c u l a r p l a t i n i z e d platinum e l e c t r o d e s p a r a l l e l to the plane o f the membrane were embedded i n each s e c t i o n . The membrane area was 0.98 cm . Resistance measurements were made with and without the membrane i n 0.1N NaCl s o l u t i o n . The conductance measurements were made with an Automat i c Capacitance Bridge Assembly (General Radio Company 1680) at a frequency of 1 kHZ. 7. S a l t R e j e c t i o n : The s a l t r e j e c t i o n o f the membrane was c a l c u l a t e d from the c h l o r i d e contents o f the feed, and product as measured by c h l o r i d e t i t r a t i o n ( A m i n c o Cotlove, S i l v e r S p r i n g s ) . The apparent s a l t r e j e c t i o n R ^ i s defined as:


2

Q

R

obs =

1

s

( 5 )

- c;

where C£ i s the s a l t concentration o f the feed, and C i s the product c o n c e n t r a t i o n . Measuring R fo and volume flow ( 5 ) as a func t i o n o f the pressure d i f f e r e n c e , AP,the l i m i t i n g s a l t rejection,Roo, was obtained from 1/R bs - !/ v P l i n e a r r e g r e s s i o n analys i s according to formula (6) (11). 0

v s

s

J

V

l o t

b

v

0

(LTi/Lp - R^LpIT

x +

5 T(*) R , R R J obs oo where Lp i s the hydrodynamic p e r m e a b i l i t y (cm/sec), LTT i s the osmotic p e r m e a b i l i t y (cm/sec) and II i s the osmotic pressure of the feed s o l u t i o n (atm). Two t y p i c a l examples o f data f o r Roo obtained v i a equation (6) f o r various e x t e r n a l s a l t concentrations and membrane IEC are presented i n Table I. 00

TABLE I

IEC (meq/g)

Molality o f NaCl

Slope x

10

5

Intercept

R

oo

Correlation Coefficient to Equation(6)

0.78

0.855

0.1047

1.0204

0.976

0.9837

1.30

0.1

1.077

1.013

0.987

0.9698

8. Annealing procedure: A p i e c e o f SPS i n the Na + form, d r i e d at 60°C i n vacuum over P 2 O 5 overnight, was p l a c e d i n an aluminum f o i l envelope and heated at 280°C during 30 min, a f t e r which i t was immersed i n water.


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355

Results and D i s c u s s i o n A.

Morphological Study

1. Small Angle X-ray S c a t t e r i n g (SAXS). A small angle peak 2° < 26 < 3° was observed f o r the Cs , C a and F e forms o f SPS f o r high and moderate I E C s . This peak i s absent i n the H form o f the polymer ( F i g . 1). The presence o f a s c a t t e r i n g peak shows that h e t e r o g e n e i t i e s on the c o l l o i d a l l e v e l e x i s t i n the e l e c t r o n densit y d i s t r i b u t i o n , and the appearance o f s c a t t e r i n g f o r C s , C a , and F e forms o f SPS, but not f o r the H form, confirms the assump t i o n that these s c a t t e r i n g centers are i o n c o n t a i n i n g regions. The average distance between s c a t t e r i n g centers, which i s connected t o the p e r i o d d e r i v e d from the angular p o s i t i o n o f the peak, cannot be c a l c u l a t e d d i r e c t l y from the c l a s s i c a l Bragg equat i o n which i s meant f o r p o i n t s c a t t e r i n g centers o f n e g l i g i b l e volume. However, i t was proposed (12) that the Bragg equation can be used i n the f o l l o w i n g form: +

z +

3 +

f

+

+


3 +

2 +

+

d

=

K d

Bragg

=

rke

where K i s a dimensionless constant, u s u a l l y between 0.8-1.22 (12). F i g . 2 shows that the angular p o s i t i o n o f the " i o n i c " peak i s a f u n c t i o n o f the IEC. The average distance between s c a t t e r i n g centers, c a l c u l a t e d according t o equation (7) with K=l, decreases as the IEC i n c r e a s e s , u n t i l the IEC approaches a v a l u e ca.lmeq/g. At IEC's above the l a t t e r value, the p o s i t i o n o f the s c a t t e r i n g angle remains constant. We a l s o observed that the SAXS peak i s not i n f l u e n c e d by the water content o f the sample. T h i s i s i n contrast to other ionomer systems i n which the " i o n i c " peak disappears upon water s a t u r a t i o n (12). 2. Transmission E l e c t r o n Microscopy (TEM). P i c t u r e s obtained by TEM show that the e l e c t r o n d e n s i t y o f the membrane i n i t s C s form i s higher than that of the embedding material. However, the membrane i s dark i n a very homogeneous f a s h i o n and no regions o f elevatgd e l e c t r o n d e n s i t y could be observed down t o a r e s o l u t i o n o f ^50A. These observations are i n f a c t i n agreement with the recent work by C. Heitner e t a l . (13). A low temperature study o f hyperfine s p l i t t i n g i n a Mossbauer study o f SPS (IEC=1.21 meq/g) i n the F e 3 form revealed the presence o f three types o f i r o n spec i e s : (a) small u n i t s o r monomers whose diameter i s below 30A (28%); (b) f e r r i c dimers (36%) and (c) c l u s t e r s whose average diameter i s about 34X (35%). Keeping t h i s d i s t r i b u t i o n o f the charge groups i n mind, we can hypothesize that the dimers and monomers decrease the e l e c t r o n d e n s i t y d i f f e r e n c e between the c l u s t e r s and the r e s t o f the matrix and consequently, the TEM s e n s i t i v i t y decreases. In a d d i t i o n , one has t o take i n t o a c c o u n t t h a t the sample i s much t h i c k e r than the c l u s t e r diameter (800A vs. 34A) which a l s o decreases the s e n s i t i v i t y . +

+

o



356

SYNTHETIC

MEMBRANES:

DESALINATION

Figure 1. Comparison of the intensities of the SAXS peak for the acid and its salts of SPS IEC = 1.2 mequiv/g: ( ) W form; (- • - •) Cs* form; ( ; Fe form. +3

0.5

i.o

IEC (meq/g)

Figure 2. Position of the SAXS peak (%) SPS obtained by sulfonation of P-3500 resin; (X) SPS obtained by sulfonation of P-1700 resin) and distance between scattering centers (•) calculated according to the Bragg's equation 7 vs. IEC.


23.


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Polysulfone

Membranes

357

3. Wide Angle X-ray D i f f r a c t i o n Study. The broad peak shown i n Figure 3 was observed f o r both SPS and PS i t s e l f . This type o f s c a t t e r i n g a t wide angle has been seen i n s e v e r a l n o n - c r y s t a l l i n e polymers and i t s appearance i s connected with some type o f short range order, which may be r e l a t e d t o an i n t e r c h a i n distance (14). Rigorous determination o f the a c t u a l i n t e r c h a i n distance must be c a r r i e d out by F o u r i e r transformation o f X-ray i n t e n s i t i e s , and i s u s u a l l y found t o be about 20% greater than that c a l c u l a t e d from Bragg s equation (14). Thus, the a p p l i c a t i o n o f the Bragg s equation t o the s c a t t e r i n g peak at20 = 18° a n d a p p l y i n g a c o r r e c t i o n o f 20% y i e l d s an i n t e r c h a i n distance ca 6A. As a f u n c t i o n o f the degree o f sulphon a t i o n and r e l a t i v e humidity, t h i s distance appears t o be constant. 1

f


!

Q

B.

Transport

Study

1. Water Transport. The dependence o f the d i f f u s i v e permeab i l i t y o f t r i t i a t e d water P , (HTO) and h y d r a u l i c p e r m e a b i l i t y LpAX on the IEC are presented i n Figures 4 and 5. Both P j and LpAX can be seen t o increase e x p o n e n t i a l l y as a f u n c t i o n o f IEC. Since the volume f r a c t i o n o f water, i s also a linear function o f the IEC, a s i m i l a r exponential r e l a t i o n i s obtained f o r these parameters vs. ((). In terms o f the pore model, the increase i n e i t h e r d i f f u s i o n p e r m e a b i l i t y o r the h y d r a u l i c p e r m e a b i l i t y may be caused by one o r both o f the f o l l o w i n g p o s s i b i l i t i e s : (a) an i n c rease i n the number o f passageways, o r (b) by increase i n the radius o f the pores. T h i s question may be r e s o l v e d by examining the g f a c t o r , d e f i n e d as a r a t i o o f two p e r m e a b i l i t i e s (15): LpAxRT g = ~ — (8) TV where V i s the molar volume o f water and RT has i t s usual meaning. F i g . 6 shows that near an IEC =0.9 meq/g the g f a c t o r , which was n e a r l y constant at lower I E C s , begins t o i n c r e a s e . T h i s f i n ding i n d i c a t e s that below an IEC o f ca. 0.9 meq/g, the mechanism o f water t r a n s p o r t through the membrane was p r i m a r i l y d i f f u s i v e i n nature and the increase i n the o v e r a l l water p e r m e a b i l i t y i s due to an increase i n the number o f passageways r a t h e r than an increase i n t h e i r dimension. We may a l s o use g t o c a l c u l a t e an equivalent pore r a d i u s , r , according t o equation (9) d e r i v e d by Kedem and Katchalsky (15): 8 n D V ! r = { % (g-1)}* (9) T

P

W

T

W

W

T

where n d D are the v i s c o s i t y and s e l f d i f f u s i o n c o e f f i c i e n t o f bulk water. While the assumptions u n d e r l y i n g the pore model (bulk water p r o p e r t i e s , P o i s e u i l l e flow, etc.) are c l e a r l y suspect* such an approach does provide a u s e f u l experimental t e s t . F i g . 7 shows that equivalent pore radius remains q u i t e constant below an IEC o f ^0.9 meq/g, while above t h i s IEC an increase i n i o n concentration i s a s s o c i a t e d with an increase i n pore s i z e . a

w

n

w



358

SYNTHETIC

J 2

,

I

i

I

6

10

.

I

•

I

14

18

MEMBRANES:

.

DESALINATION

L 22

28 (degrees)

Figure 3.

+

Wide-angle X-ray scattering, SPS Na form, IEC = 1 mequiv/g

\&

7

id olio

Figure 4. Diffusive permeability, P vs. IEC: (%) SPS obtained by sulfonation of P-3500 resin; (X) SPS obtained by sulfonation of P-l 700 resin T

^

U D

„.

0

iEC(meq/B)



23.


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Polysulfone

Membranes

359

Figure 5. Intrinsic membrane hydraulic permeability vs. IEC: (%) SPS obtained by sulfonation of P-3500 resin; (X) SPS obtained by sulfonation at P-1700 resin); (O) SPS, Ref. 5;(M) > fSPS

Re

6

0.6 0.8 1.0 IEC(f*»q/g)

Figure 6. The g factor vs. IEC: (%) SPS obtained by sulfonation of P-3500 resin; (X) SPS obtained by sulfonation of P-1700 resin.

I E C (meq/g)

Figure 7. An equivalent pore radius, r, vs. IEC: (O) SPS obtained by sulfonation of P-3500 resin; (%) SPS obtained by sulfonation of P-1700 resin.



360

SYNTHETIC

MEMBRANES:

DESALINATION

Returning to the SAXS r e s u l t s , i t may be r e c a l l e d that at t h i s IEC the d i s t a n c e between s c a t t e r i n g centers begins t o approach a constant value d e s p i t e an i n c r e a s e i n the c o n c e n t r a t i o n o f charged groups, i . e . a d d i t i o n a l s c a t t e r i n g centers are not created. One can t h e r e f o r e conclude that at t h i s p o i n t a change occurs i n the nature o f the d i s t r i b u t i o n o f ion-exchange s i t e s between c l u s t e r s , dimers, and monomers. Above an IEC ^0.9 meq/g a d d i t i o n a l charged groups form fewer i o n i c aggregates and are present predominantly as dimers and monomers. One can p i c t u r e the i o n i c aggregates as i s l a n d s o f high concentration o f charge while monomers and dimers are d i s t r i b u t e d i n the i n t e r - c o n n e c t i n g regions between them. Apparently, an increase i n the c o n c e n t r a t i o n o f monomers and d i mers i n the pore causes an i n c r e a s e i n the equivalent pore s i z e . 2. I o n i c Transport, a. C o n d u c t i v i t y : The s p e c i f i c conductance o f the SPS (Na form) membranes i s shown i n F i g . 8, whose data are summarized i n Table I I , i n c l u d i n g values o f an apparent energy o f a c t i v a t i o n . An exponential i n c r e a s e i n i o n i c conductance together with a decrease i n an apparent energy o f a c t i v a t i o n may be r e l a t e d t o a decrease i n a "jump" d i s t a n c e between ion-exchange s i t e s as a f u n c t i o n o f IEC. TABLE I I S p e c i f i c Conductance and an Apparent Energy o f A c t i v a t i o n f o r Various SPS Membranes (Na+ Form) +

IEC

(meq/g)

K-10 ohm cm" 4

0.3 0.75 0,78 0.82 0.98 1.03 1.30

_1

0.006 0.78 1.4 1.85 5.2 7.81 33.3

Ea (Kcal/mol) 17.9 10.1 8.85

In Table I I I the s p e c i f i c conductance and e l e c t r o - o s m o t i c coe f f i c i e n t (8) f o r the SPS membrane are shown together with the data f o r a conventional ion-exchange membrane, AMF C103 (16,17) (polyethylene -styrene g r a f t copolymer c o n t a i n i n g sulphonic a c i d groups). I t appears that there i s a c l o s e s i m i l a r i t y i n p r o p e r t i e s o f both membranes. The 6 value f o r Na - SPS i s s l i g h t l y low (nearly equal t o the water o f h y d r a t i o n o f sodium i o n ) , however, i t i s i n a range common t o other ion-exchange membranes (0.1-0.2 1/F) (1,18). +


VINNIKOVA AND TANNY

23.

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361

TABLE I I I Comparison o f the Transport P r o p e r t i e s o f SPS Membrane with a Conventional Ion-Exchange Membrane Membrane (Na form)


+

IEC (meq/g)

SPS

1.30

AMF-103

1.33

Water cont. (%) 23 18.2 (Ref.16)

K-10 (ohm^cm" ) 5

1

3.3 4.0 (Ref.16)

$ (mol/F) 5.0 7.0 (Ref.17)

b. S a l t r e j e c t i o n : Through measurements o f s a l t r e j e c t i o n at d i f f e r e n t operating pressures, the l i m i t i n g s a l t r e j e c t i o n , R^, was obtained by e x t r a p o l a t i o n o f equation (6) f o r a number o f membranes o f d i f f e r e n t IEC and at two e x t e r n a l s a l t concentrations ( c f . Experimental) . The concentration o f 0.8M NaCl was chosen t o allow a comparison o f the data on Na-SPS t o that o f Sarbolouki and M i l l e r (19), f o r c e l l u l o s e acetate membranes. In Figure 9, R^ f o r various membranes ( o f d i f f e r i n g IEC) has been p l o t t e d versus the equivalent pore r a d i u s , r . I t i s i n t e r e s t i n g that the data f o r Na-SPS and that o f c e l l u l o s e acetate f a l l on the same curve f o r 0.8M concentration. I t a l s o q u i t e s u r p r i s i n g , f o r the f o l l o w i n g reasons: ( i ) the mechanism o f s a l t e x c l u s i o n f o r c e l l u l o s e acetate membranes has been shown t o be p r i m a r i l y v i a the low d i e l e c t r i c constant o f the pore w a l l (20) whereas that o f Na-SPS has been claimed t o be v i a Donnan e x c l u s i o n (7) [Heyde e t a l . (21) d i d see some c o n t r i b u t i o n o f the few charges present i n CA by the a d d i t i o n a l e x c l u s i o n observed at low e x t e r n a l s a l t c o n c e n t r a t i o n s ] . ( i i ) s i n c e the IEC o f the Na-SPS membranes i s not the same f o r each pore r a d i u s , the charge d e n s i t y i n the pore i s not n e c e s s a r i l y i d e n t i c a l and thus the c o n t r i b u t i o n o f the charge d e n s i t y t o the i o n e x c l u s i o n should be d i f f e r e n t at each pore r a d i u s . For both the above reasons one would not expect the i n t r i n s i c r e j e c t i o n data i n F i g . 9 t o c o i n c i d e as w e l l as they do. However, there i s one f e a t u r e which i s very s i m i l a r f o r both membranes and that i s the s t a t e o f water, cont a i n e d w i t h i n the pores. Indeed, n e i t h e r the high s a l t r e j e c t i n g homogeneous CA membranes, nor the SPS-Na membranes show a f r e e z i n g t r a n s i t i o n i n the DSC down t o temperatures o f -60°C (22,25). Furthermore, simultaneous t r a n s p o r t measurements o f HTO and H2O show s i m i l a r r a t i o s o f T / 0 (24,25). Schultz and Asunmaa (26) proposed that the s t r u c t u r e d water found w i t h i n the pores o f CA membranes was r e s p o n s i b l e f o r a s i g n i f i c a n t p o r t i o n o f the membrane's s a l t r e j e c t i n g p r o p e r t i e s . In l i g h t o f the data i n f i g u r e 9, t h i s hypothesis appears t o be very p l a u s i b l e , s i n c e i t i s one feature which i s common t o both membranes and which provides an explanation f o r the remarkable c o i n c i dence o f the data i n the f i g u r e . However, t h i s statement should not be taken t o imply that the presence o f the i o n i c charges i s o f no consequence. Also shown i n f i g u r e 9 i s data at a feed l

1 8


g


362 SYNTHETIC MEMBRANES: DESALINATION



23.

VINNIKOVA AND TANNY

Sulfonated

Polysulfone

Membranes

363

concentration o f 0.1M NaCl. For a given pore r a d i u s , is clearly higher at 0.1M than that at 0.8M, probably due to i o n i c c o n t r i b u t i o n s which are diminished at high e x t e r n a l i o n i c s t r e n g t h . c. E f f e c t s o f Chain Reorganization: Previous d i s c u s s i o n s i n v o l v e d the i n f l u e n c e o f membrane s t r u c t u r e on d i f f e r e n t t r a n s p o r t processes, i n which the changes i n t h i s s t r u c t u r e were considered only as a f u n c t i o n o f IEC. I t t h e r e f o r e seemed to be a c r i t i c a l p o i n t to e f f e c t a change i n the d i s t r i b u t i o n o f ion-exchange s i t e s between monomers, dimers, and c l u s t e r s , while keeping t h e i r t o t a l amount constant. T h i s goal was attempted by annealing the sample at a temperature above i t s Tg. Data f o r the e l e c t r i c a l conductance o f annealed samples shows that i o n i c t r a n s p o r t i s more r e s t r i c t e d i n comparison with those which are nonannealed (Table IV). Since the t o t a l number o f i o n exchange s i t e s i s u n a f f e c t e d by the annealing process ( c f . e x p e r i mental) we may assume that the s i t e to s i t e distance has i n c r e a s e d . TABLE IV Influence of Annealing SPS

on the P r o p e r t i e s o f Na

Before 4

Ax

Membranes

IEC =0.82 meq/g.

Water up-take (%) p-10" * (Ohm cm) p

SPS

T ^w RT

.10

LpAx-10

10

10

2

(cra /sec atm) 2

(cm /sec atm)

g

annealing 14 0.542

After

annealing 11 1.265

o 82

0 50

1.41

0.62

1.7

1.24

6.0

3.5

o

Pore diameter d, A *

S p e c i f i c r e s i s t a n c e o f annealed sample reprepared from DMF s o l u t i o n ^ .529xl0 ohm cm. E f f e c t s o f annealing are a l s o observed on the water t r a n s p o r t properties. Both the d i f f u s i o n a l p e r m e a b i l i t y and the p e r m e a b i l i t y under osmotic pressure decrease i n comparison with the sample bef o r e annealing (Table IV). Moreover, the g r a t i o a l s o decreases, which, i n terms o f the equivalent pore r a d i u s , means that the membrane becomes t i g h t e r upon annealing. To examine the p o s s i b i l i t y that the changes i n membrane prop e r t i e s were caused by permanent chemical changes i n the Na-SPS, a p o r t i o n o f the annealed sample was r e d i s s o l v e d i n DMF, r e c a s t as a f i l m and i t s conductance measured. As Table IV i n d i c a t e s , the r e c a s t f i l m returned to i t s o r i g i n a l conductance, which c l e a r l y proves that the annealing process only e f f e c t e d the p h y s i c a l prop e r t i e s o f the Na-SPS ionomer f i l m . T h i s p o i n t i s r e i n f o r c e d by our d i s c o v e r y that an annealed sample which was s t o r e d i n water over a p e r i o d o f a week slowly changed back to i t s o r i g i n a l conductance value. 4


364

SYNTHETIC

MEMBRANES:

DESALINATION

At present we do not y e t have s u f f i c i e n t information t o determine whether the annealing process exerts an e f f e c t p r i m a r i l y through changes i n chain conformation or the d i s t r i b u t i o n o f i o n i c aggregates, or both. Measurements o f Tg showed no change f o r annealed vs. unannealed samples, while SAXS measurements were i n c o n c l u s i v e and r e q u i r e r e p e t i t i o n . Further Mossbauer measurements would a l s o be r e q u i r e d before d e f i n i t i v e conclusions o f t h i s nature could be drawn.


Literature Cited

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25.

McKelvey, J . G . ; Spiegler, K.S.; Wyllie, M.R.J. Chem. Eng. Progr. Symp. Ser., 1959, 55, 199. Loeb, S.; Sourirajan, S. ACS Advan. Chem. Ser., 1962, 318, 117. Chludzinski, P . J . ; Fickett, A.P.; LaConti, A.G. Amer. Chem. Soc., Div. Polym. Prepr., 1971, 12, 276. Kimura, S.G. Ind. Eng. Chem., Prod. Res. Develop., 1971, 10, 335. Brousse, C.L.; Chapurlat, R.; Quentin, J.P. Desalination, 1976, 18, 137. Noshay, A.; Robenson, L.M. J . Appl. Polym. Sci., 1976, 20, 1885. Quentin, J . P . ; Milas, M.; Rinando, M. 5th Int. Symposium on Fresh Water from the Sea, 1976, 4, 157. Kedem, O. Israel J . of Chem., 1973, 11, 313. Kedem, O.; Tanny, G. Pure & Appl. Chem., 1976, 46, 187. Robinson, R.A.; Stokes, R.M. "Electrolyte Solutions", Butterworths, London, 1959. Push, W. Ber. Bunsenges, Phys. Chem., 1977, 81, 854. Marx, C.L.; Caulfield, D.F.; Cooper, S.L. Macromolecules, 1973, 6, 344. Heitner-Wirguin, C.; Bauminger, E.R.; Levy, A.; Labensky de Kanter, F.; Ofer, S. Polymer, in press. Kavesh, S.; Shultz, J.M. J . Polymer Sci., A-2, 1971, 9, 85. Kedem, O.; Katchalsky, A. J . Gen. Physiol., 1961, 45, 143. Kawabe, H.; Jacobson, H.; Miller, J . F . ; Gregor, H.P. J . Colloid. Interface Sci., 1966, 21, 791. Lakshminarayanaiah, N. "Transport Phenomena in Membranes", Academic Press, New York, 1969, p. 256. Lakshminarayanaiah, N.; Subrahmanyan, V. J. Phys. Chem., 1968, 72. 1253. Sarbolouki, M.N.; Miller, I.F. Desalination, 1973, 12, 343. Sourirajan, S. Ind. Eng. Chem. Fundamen., 1963, 2, 51. Heyde, M.E.; Peters, C.R.; Anderson, J . E . J . Colloid. Interface Sci., 1975, 50, 467. Sivashinsky, N.; Tanny, G. J . Appl. Polym. Sci., in press. Frommer, M.A.; Shporer, M. J . Appl. Polym. Sci., 1973, 17, 2263. This laboratory, unpublished results. Thau, G.; Bloch, R.; Kedem, O. Desalination, 1966, 1, 129.


23.

26.

VINNIKOVA AND TANNY

Sulfonated

Polysulfone

Membranes

365

Shultz, R.D.; Asunmaa, S.K. "Recent Progress in Surface Science", Dannielli, J . F . ; Riddiford, C.A.; Rosenberg, M.D. eds., Academic Press, New York, 1970, 3, p. 291. December 4 , 1980.


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Synthetic Membranes: Volume I - American Chemical Society

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