Negatively charged ultrafiltration membranes of polyacrylonitrile having amphiphilic quaternary ammonium counter ions

journal of

MsEEFE ELSEVIER

Journal of Membrane Science 90 ( 1994 ) 14 1- 150

Negatively charged ultrafiltration membranes of polyacrylonitrile having amphiphilic quaternary ammonium counter ions Takaomi Kobayashi*, Takuro Miyamoto, Toshihiko Nagai, Nobuyuki Fujii Department of Chemistry, Nagaoka Universityof Technology, Kamitomioka, Nagaoka 940-21. Japan (Received June 30, 1993; accepted in revised form December 30, 1993 )

Abstract Charged ultrafiltration membranes of poly (acrylonitrile-mstyrenesulfonate ) having an amphiphilic counter ion were prepared by ion exchange with quatemary ammonium salts such as stearyltrimethylammonium chloride. The ultrafiltration behavior of the charged membranes having amphiphilic groups was examined and compared with those of the corresponding sodium salt. From the uptake of a fluorescence probe, I-anilino-l-naphthalenesulfonic acid sodium salt, by the membranes, it was proved that the hydrophobicity of the charged membrane is large in the amphiphilic quatemary ammonium salt compared with that for the sodium salt. The ultrafiltration rate of the charged membrane and the permeability of dextrans increased in the amphiphilic quatemary ammonium salt. Electrostatic sieve separation of dextran/dextran sulfate mixtures having similar molecular size was also investigated as a function of the content of styrenesulfonate group in the membrane. The separation of the mixtures was enhanced as the charge content of the membrane increased. Key work Charged ultrafiltration membranes; Polyacrylonitrile; Electrostatic barrier; Asymmetric membrane; Amphiphilic quaternary ammonium ion

1. Introduction Much attention has been attracted to charged ultrafiltration membranes because of the permselective barrier caused by the electrostatic potential of the media to the transport of an ionic solute having the same sign of charge as the membrane [ 1,2 1. For preparing charged ultratiltration membranes various synthetic polymers such as polysulfone [ 3,4 1, poly ( vinylidene [ 5 1, poly (phenylene oxide [ 6 1, fluoride) poly (etheretherketone) [ 7 ] and polyacrylonitrile [ 8,9] have been applied. We found by using *Corresponding

author.

03767388/94/%07.00

of polyacrylonitrile (PAN) with various ionic monomers that the electrostatic barrier of a negatively charged membrane made of poly (acrylonitrile-co-sodium styrenesulfonate) markedly restricts the permeation of dextran sulfate [ 1O-l 21. It has become feasible to separate dextran/dextran sulfate mixtures having similar molecular size with high efficiency [ 111. The selectivity of the charged membrane increased with an increase in the charge content in the membrane. Although the charge barrier of the membrane is effective in the restriction of permeation for dextran sulfate [ 10,111 at a high charge content, there are some problems in preparation of the copolymers

0 1994 Elsevier Science B.V. All rights reserved

SSDI0376-7388(93)E0237-E

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membranes. It is difficult to control the ultrafiltration properties by the phase inversion process of the polymer-dimethyl sulfoxide/water system [ 9, lo]. The increase of the charge content lowers the aggregation property in waters because the hydrophilicity of the copolymer increases. Further, the resulting ultrafiltration membrane does not exhibit efficient permeation of fluids. The present work is directed to the ultrafiltration behavior of negatively charged PAN membranes having amphiphilic quaternary ammonium counter ions. We report on a new approach whereby the hydrophobic environment in the charged membrane facilitates the control of ultrafiltration properties with a high charge content of the membrane. By using the electrostatic barrier of the membranes, we are able to separate mixtures of dextran/dextran sulfate having similar molecular size in ultrafiltration. 2. Experimental 2.1. Materials AU reagents used were of reagent grade unless otherwise indicated. Water was distilled and purified by using ion exchange resin. &Anilino1-naphthalenesulfonic acid sodium salt, ANS (Tokyo Kasei ) , was used without further purification. Dextrans (Pharmacia) and dextran sulfate (Tokyo Kasei) having 1.6 mol% content of sodium sulfonate group were employed without further purification.

ing it with water to remove the water soluble additive and solvent. The charged membranes so obtained were N 70-80 pm thick and N 43 mm in diameter. Negatively charged membranes having quaternary ammonium counter ions are shown schematically in Scheme 1. Ion exchange of the sodium ion on the SSS group with quaternary ammonium ions was carried out as follows. Quaternary ammonium chloride ( 10 mmol) having tetramethyl (TMA), octyltrimethyl (OMA), benzyltrimethyl (BMA) or stearyltrimethyl (SMA) groups was dissolved in water ( 100 ml). The negatively charged membrane (0.1 g ) was soaked in the aqueous solution of quaternary ammonium chloride overnight. The membranes obtained were washed with a large quantity of water and kept in water. The exchange of the counter ions of the membrane was checked by ‘H NMR spectra in DMSO-d, at 60°C using a JNM-GX270 F’T-NMR spectrometer. The content of quaternary ammonium ions in the membrane was calculated from integral curves of the NMR spectra for the total aliphatic protons of stearyl and octyl groups and the total aromatic protons of benzyl group. No estimate for the four methyl groups of TMA was carried out because the methyl protons of the TMA group overlap with the signals for the aliphatic main chain of the copolymer. As shown in Table 1, the mole ratio of the proton signal for the quaternary am-

io3-

2.2. Membrane preparation Negatively charged PAN membranes containing sodium styrenesulfonate (SSS) groups were prepared as reported previously [ 10,111. In this work, three types of membrane with SSS group contents of 0.058, 0.11 and 0.51 mmol/g were used. The ultrafiltration membrane was made by spreading a dimethyl sulfoxide solution containing 9 wt% of the base copolymer and 1 wt% of poly (ethylene glycol) on a glass plate, coagulat__ ing the mixture in water at 25 “C and then wash-

CHa

CH3

CHrt&3 bH3

CH3

CHr&HzTMA

&is

(CHz);CH3 OUA

CHS CH3-&Hz-

(CHz);,CHa

bH3 .%A

Scheme 1.

T. Kobayashi et al. /Journal ofMembrane Science 90 (1994) 141-150

monium ion to that of the styrenesulfonate group was near to unity, indicating that the ion exchange takes place stoichiometrically. 2.3. Measurements Fluorescence spectra were measured by using a Hitachi YR-G (FT-MH) spectrofluorometer having a slit width of lo- 15 nm. The excitation of ANS bound on the membrane was carried out under 45” reflection of 350 nm radiation on the surface. The absorbance of ANS was measured UV- 190 double-beam on a Shimadzu spectrophotometer. The morphology of the cross section of the membrane was observed with a scanning electron micrograph (SEM) JXA-733 (Jeol). A wet sample of the membrane was lyophilized and the cross section was obtained by fracturing the membrane at liquid nitrogen temperature and coated with gold by use of an SPM- 112 ( Anelva ) sputter gun. 2.4. Ultrafiltration experiments The apparatus for ultrafiltration was similar to that used previously [ lo- 12 1. The experiment was carried out with an apparatus (Amicon Co. Ltd., type 8050 cell) operated at an applied pressure of 7.5 kPa and with a stirring rate of 300 rpm. The experimentally obtained rejection R is defined as R= ( C’,- C,) /C,, where C,, and C, denote the solute concentrations in the feed and the permeate solution, respectively. The feed and the permeate solution of dextran and dextran sulfate were analyzed on a gel permeation chromatograph (type CCPD W8000, Toyo Soda Co.) with a 30 cm column (TSKgel GSOOO PWxL) equipped with a refractometer (RISOOO). 3. Results and discussion 3.1. Uptake of AN.9 probe by membranes

It is well known that in water ANS is taken up hydrophobically by an organized assembly [ 13 ] having a hydrophobic aliphatic chain, such as

143

micelles, vesicles, and amphiphilic polymers [ 141 and proteins [ 15- 171. In order to examine the association of ANS to the negatively charged membrane in this work, we measured the amount of binding from the absorption change of ANS at 3 15 nm in aqueous solution at 50 @I concentration, in which the weighed membrane was soaked at 30°C overnight. Table I lists the amount of ANS taken up by the negatively charged membranes with a charge content of 0.5 1 mmol/g for various amphiphilic quatemary ammonium salts and sodium salt. Despite the membranes having a negative charge, uptake of ANS by the membrane occurred in each case. The uptake of ANS is responsible for the hydrophobicity of the copolymer such as acrylonitrile segment and the aromatic segment of the SSS group. It is noted that, as shown in Table 1, a marked uptake occurs in the membrane with SMA groups. As well as surfactant micelles and vesicle [ 13 1, the aliphatic long chain of the stearyl group of the quaternary ammonium salt forms a hydrophobic environment. Thus the probe is taken up into the microenvironment by an effective hydrophobic interaction; the hydrophobic association of ANS is strong enough for ANS to be taken up by the membrane by prevailing over the electrostatic repulsion of the anionic ANS and the negatively charged membrane. The emission spectra of ANS reflect the nature of microenvironments such as micelles and vesicles [ 17, I8 1, where ANS binds to the microheterogeneous systems. It is well known that the emission intensity of ANS is strongly enhanced in the nonpolar medium of the system, and the emission gives information about the environment around the ANS. Fig. 1 shows the emission spectra of ANS bound to the membrane surface. A noticeable enhancement of the fluorescence intensity of the probe was observed for the membrane systems, compared with that of aqueous solutions of ANS. The increase of emission intensity for the membrane systems suggests the hydrophobic association of ANS to the hydrophobic parts of the membrane. In addition, it is known that the emission maximum (&,) of the spectrum shifts significantly to the blue region, according to the polarity of the environment

i? Kobayashi et al. /Journal ofMembrane Science 90 (1994) 141-150

144

Table 1 Properties of charged ultrafiltration Counter ion

Na TMA OMA SMA BMA PAN

membranes containing various quarternary ammonium salts”

Mole ratiob

0.99 1.1 0.94

Amount of ANS bound to membrane (flmol/g)

1 max (nm)

Water content (%)

Volume flux X 10m6 (m3/m2 s)

0.22 0.21 0.67 9.0 0.53 0.12

455 454 457 464 455 460

92.6 94.1 92.8 91.1 91.0 93.0

0.81 1.1 2.1 3.5 2.0 5.6

“The charge content of the membrane was 0.5 1 mmol/g. bMole ratio of SSS group to quatemary ammonium salt was measured by integral curves of NMR spectra.

the chain length of the aliphatic group of the quaternary ammonium salt. With increased chain length, the obtained values of A,,, shit? slightly to the red region. This result indicates that the environment of ANS bound to the membrane surface has somewhat polar properties. Presumably ANS is associated to the interface between the hydrophobic parts of the SMA group and the aqueous medium without involving the hydrophobic environment of the stearyl chains. Wavelength

(nm)

Fig. 1. Emission spectra of ANS bound to the membrane surface. Excitation at 350 nm. ANS solution (p) and membrane containing TMA (- -) , BMA ( - - - - ) and SMA groups (---_).

around the probe [ 13,161. An increase of the emission intensity and a blue shift of 2,, take place in nonpolar solvent [ 17 1. For example, Stryer reported that a blue shift of the emission maximum for ANS was observed in octanol in comparison with that in methanol [ 17 1. Similar phenomena were reported [ 13 ] for micro-heterogeneous systems of amphiphilic nature. In the present membrane comprising a PAN copolymer with SSS group content of 0.5 1 mmol/ g, the emission maximum of bound ANS exhibited a marked blue shift compared with the 1,, (4 15 nm ) of aqueous ANS solution. That is, the microenvironment of the membrane in which ANS is bound is a low dielectric medium. We note that the extent of the blue shift is related to

3.2. Ultrafiltrationcharacteristics Table 1 also lists the water content of the membrane and the ultrafiltration rate of water. The water content of membranes containing the various quatemary ammonium salts, i.e., having SMA, OMA and BMA groups is in the range 9 l94%. However, a PAN membrane having essentialIy no fixed charge has a water content the same as that of the charged membranes. The high water content of the PAN membrane results from the porous structure, as reported previously [ 10 1. Fig. 2 shows typical SEM photographs of the cross section of the membranes having as the quatemary ammonium salt (a) SMA and (b) TMA groups. There are many macropores on the plate side surface of both membranes. On the aqueous side, the surface looks dense in both membranes. Also, the photographs of the cross section suggest that the membranes obtained have an asymmetric structure consisting of a dense top layer supported by a porous sublayer,

T. Kobayashi et al. /Journal ofMembrane Science 90 (1994) 141-150

145

W

Aqueous

Plate

of the cross-section

side

20pm

20pm Fig. 2. SEM photographs ammonium salt.

siide

of membranes

by analogy with typical asymmetric ultratiltration membranes [ 2,19 1. In the macroscopic view of the cross-section, the contribution of methyl, octyl, benzyl and stearyl groups in the counter ions to the morphology is small, the structure of the cross section is similar in each case. As shown in Table 1, the ultrafiltration rate of water for these membranes increases according to the increased number of methylene groups in the aliphatic chain of the quaternary ammonium salt. The volume flux data of water for membranes having methyl, octyl and stearyl groups increase in that order. The order appears to be the same as for the hydrophobicity of the aliphatic chain. It appears that the hydrophobicity of the aliphatic chains affects the permeation rate. Presumably, a change of chain conformation in the SSS segment of the membrane takes place owing to ionic association of the amphiphilic

having (a) SMA groups and (b) TMA groups in the quaternary

quaternary ammonium salt with the charged SSS segments of the membrane. In order to estimate the cutoff characteristics of these membranes, dextrans of various molecular weights were used as permeation solutes at a concentration of 0.1 wt%. Fig. 3 shows molecular weight cutoff (MWCO) curves of the negatively charged membranes containing sodium salt and various quatemary ammonium salts. The MWCO curves shift markedly toward high molecular weight of dextran as the aliphatic chain length of the counter ions increases. This implies that the pore size of the membrane increases for the longer aliphatic chains. Further, the sharpness of the cutoff seems to be improved in the membrane having hydrophobic groups. In general, the permeability of solute through an asymmetric ultrafiltration membrane is determined by the microscopic pore size in the top dense sur-

T. Kobayashi et al. /Journal ofMembrane Science 90 (I 994) 141-150

146

0.8

0.2 ,

0 1 o4 Molecular

1 o5 weight

1 oe of

dextrans

Fig. 3. Molecular weight cutoff (MWCO) curves of the negatively charged membranes with sodium salt ( 0 ) and various quaternary ammonium salts: TMA ( n ), OMA (+) and SMA (A ) groups.

face, because of the sieve effect [ 19 1. Therefore the cutoff data in Fig. 3 strongly suggest that the hydrophobic nature of the counter ions extends the pore size in the top surface of the membrane. As reported previously [ lo] for the copolymer consisting of a PAN segment and a hydrophilic charged SSS segment, the aggregation of the PAN part in water is responsible for the formation of a porous membrane of asymmetric structure. The segmental aggregation of the copolymer, however, is lowered by an increase in the content of styrenesulfonate group owing to the lower aggregating property of the charged segment in water. In typical polyelectrolytes, coulombic repulsion occurs between the neighboring charged segments [ 20,2 1 ] and the resulting chain conformation of the charged segment is expanded. Also, amphiphilic polyelectrolytes consisting of hydrophobic groups and electrolyte groups forms an organized structure [ 14 ] such as micelles by self-aggregation of the hydrophobic group in water. As well as micelles and amphiphilic polyelectrolytes, in the present membranes containing amphiphilic quaternary ammonium salt, the charged segment of the SSS group may envelop the PAN segment in water. Therefore we interpret the phenomena of change in the cutoff property in Fig. 3 as follows. In the charged membranes containing sodium salt and TMA quatemary ammonium salt, the

expanded coil conformation of the charged SSS segment reduces the pore size of the membrane as a result of the electrostatic repulsion of the neighboring charged groups around the PAN segment [ 121. Accordingly, the change of pore size in the top surface of the membrane diminishes the permeability of dextran molecules. On the other hand, in the cases of quatemary ammonium salts consisting of hydrophobic groups and positively charged group, the coulombic repulsion of the neighboring charged SSS group on the polymer chains is lowered by ionic association of the hydrophobic counter ion with the SSS segment. The binding of the amphiphilic counter ion to the SSS segment may be larger than that of the sodium ion and the TMA quatemary ammonium ion because of cooperative association by hydrophobic and ionic forces to the base polymer. This results in a shrinking conformation of the charged segment in analogy with an amphiphilic copolymer [ 14 1. Consequently, the volume flux of water increases and the cutoff curves shift towards the high molecular weight region for the dextran permeate as the hydrophobic nature of the counter ion increases. 3.3. Effects of charge content on ultrafiltration properties In charged ultrafiltration membranes containing sodium salts, amounts of charge content in the membrane reflect the ultrafiltration characteristics [ lo,22 ] such as permeation rate, cutoff properties and membrane structure. As shown in Fig. 2, the thickness of the membrane cross section comprising a spongy dense structure at a charge content of 0.5 1 mmol/g is N 10 pm. The thickness of the top layer decreases with a decrease of the charge content in the membrane. At a charge content of 0.058 mmol/g, the thickness of the membrane is in the range 0.1-0.5 p and equals that of a PAN membrane having no charge. In addition, as the charge content in the membrane decreases, the cutoff curves shift to the high molecular weight side as reported previously [ 10 1. Table 2 shows, data of ANS uptake and ultrafiltration rate for the membranes containing so-

147

T. Kobayashi et al. /Journal ofMembrane Science 90 (1994) 141-150

dium salt and amphiphilic SMA quaternary ammonium salt at various charge contents in the membrane. Here, membranes with three different charge contents were used. In contrast to the ANS uptake for the sodium salt, the probe molecules are effectively taken up by hydrophobic association to the membrane with SMA groups. The amount of ANS uptake in the membrane is remarkable with a high charge content of the membrane, in spite of the increased potential coulombic repulsion between the anionic probe molecule and the negatively charged membrane. This may be due to the increased content of SMA groups at a high charge content ofthe membrane. As shown in Table 2, the ultrafiltration rate of the membrane in sodium salt form depends on the charge content in the membrane, whereas the ultrafiltration rate for the amphiphilic quaternary ammonium salt is independent of the charge content. In contrast to the data for the sodium salt, the volume flux of water increases slightly in the case of the quaternary ammonium salt. In particular, the difference between the volume fluxes for the two counter ions becomes large at a the charge content of 0.5 1 mmol/g. Because the amounts of quatemary ammonium ion possessing stearyl groups increase at high charge content, the spread of SSS chains in the PAN membrane decreases with an increase in the hydrophobic quatemary ammonium salt. 3.4. Electrostatic sieve separation We previously reported charged ultrafiltration Table 2 ANS uptake and ultrafiltration Charge content (mmol/g)

0.51 0.11 0.058 0

that a negatively membrane of

properties for membranes

Mole ratio

0.98 0.80 0.81 -

poly ( acrylonitrile-cd-sodium styrenesulfonate ) can separate mixtures of dextran/dextran sulfate having similar molecular size with high efficiency [ IO,111. This effect arises from the electrostatic barrier of the negatively charged membranes to the anionic dextran [ 12,231. In the present work, separation of the solute mixture was carried out by use of membranes with various charge contents of quaternary ammonium salt with stearyl groups. As shown in Fig. 4, the MWCO curves of these membranes have similar characteristics. Here, in order to obtain a similar cutoff property, the cutoff characteristics of the membranes with charge of 0.058 and 0.11 mmol/ g and of a PAN membrane were controlled by addition of 1 wt% poly (ethylene glycol) (molecular weight (Mw ) = 20 000) to a 9 wt% polymer solution. For the membrane of charge 0.51 mmol/g, 3 wt% of poly (ethylene glycol) was added to the 9 wt% polymer solution. Fig. 5 shows plots of volume flux for water, aqueous dextran (D) and dextran/dextran sulfate (D/DS) mixture versus charge content of the membrane. The feed solution of the D/DS mixture was prepared at a weight ratio of D/ DS = I : 9 in 0.1 wt% concentration. The volume flux of water and of an aqueous solution of D of Mw 40 000 is substantially independent of the charge content in the charged membrane. Also, a noticeably low volume flux for the D/DS mixture solution containing DS of Mw= 10 000 is observed. The value of volume flux for the D/ DS solution decreases with increasing the charge content of the membrane. We previously reported that the permeation rate of DS solution

with various charge contents in sodium and SMA salta

ANS uptake @mol/g)

Volumefluxofwaterx10-6 (m’/m* s)

SMA

Na

SMA

9.0 6.4 2.6

0.22 0.11 0.14 (0.12)

3.5 3.4 3.4 -

‘The values in parentheses are for a PAN membrane having no fixed charge.

Na 0.87 2.1

T. Kobayashi et al. /Journal ofMembrane Science 90 (1994) X41-150

148

The same explanation may apply to the D/DS mixture used in the present work. Fig. 6 shows the separation factors of the D/ DS mixture at 0.1, 0.15 and 0.25 wt% total concentration as function of the charge content of a membrane having stearyl groups. For the D/DS solution, the selectivity factor CYD/DS is given by

0.6 0.4

~D/DS=(YD/YD~)/(XDIXD~),

0.2 0 1 o4 Molecular

1 o3

1 o5 weight

10” of dextrans

10’

Fig. 4. MWCO curves of charged membranes having various amounts of charge content in SMA quatemary ammonium salt. Charge content (mmol/g) 0.51 (O), 0.11 (*), 0.058 ( n ) and 0 ( A ). The membranes were prepared from 9 wt% of the casting solution containing poly(ethylene glycol) (Mw=20 000) in concentration of 3 wtl for 0.5 1 mmol/g and 1 wt% concentration for 0.11 and 0.058 mmol/g and for PAN, respectively.

where

YD

and

YDs are the D and DS concentrations in the permeate and & and &s are the D and DS concentrations in the feed [ 19 1. In an uncharged PAN membrane, however, no . . separation 1s achieved [ 111; (YD/DS= 1. On the other hand, tht: separation factor increases with a high charge content in the membrane. Namely, the D component permeates through the membrane, but DS is preferentially rejected by the charged membrane owing to the effective electrostatic barrier to the restricted permeation of DS. In addition, the plots of CYD/DS versus SSS content show that the selectivity of the charged membrane increases with increasing charge content. This suggests that the electrostatic barrier of the charged membrane effectively acts on the restricted permeation of DS. We also measured the separation factor of the D/DS mixture for an SSS group content of 0.11 36 31 26

0

0.1 Charge

0.2

0.3

content

0.4

0.5

0.6

(mmol/g)

Fig. 5. Volume flux of water, 0.1 wt% of D solution and 0.1 wt% of D/DS mixture solution for various charge contents of the membrane in SMA quatemary ammonium salt. ( 0 ) water, (A) 0.1 wt% of D (Mw=40000), (m) 0.1 wt% of D/DS mixture ( 1: 9, w/w).

through a negatively charged membrane of poly (acrylonitrile-co-sodium styrenesulfonate) is slower than that of D of similar molecular size to the anionic solute [ 10,121. This difference in volume flux arises mainly from the electrostatic barrier of the membrane to the anionic solute.

16

6

0

0.1 Charge

0.2

0.3

content

0.4

0.5

0.6

(mmollg)

Fig. 6. Plots of separation factor of D/DS mixture versus charge content of the membrane in SMA quatemary ammonium salt. D/DSmixture (1:9, w/w) for 0.1 (O), 0.15 (W) and 0.25 wt% (e) feed concentration.

T. Kobayashi et al. /Journal ofMembrane Science 90 (1994) 141-150

mmol/g in membranes containing various quaternary ammonium salts. The values of aDIDSfor TMA, OMA, SMA and BMA were 24.0, 10.5, 11.4 and 11.O, respectively. As reported previously [ 111, the value of CYD/D~ for sodium ion is 25. These data show that the coulombic repulsion between DS and the charged membrane decreases for the amphiphilic quatemary ammonium salts. Considering the sodium salt and the quatemary ammonium salt having TMA groups, strong association of the amphiphilic counter ions to the SSS segment may occur. Therefore, by adsorption, neutralization of the electrostatic barrier of the membrane results in a decrease of separation selectivity if the quatemary ammonium salt is amphiphilic in nature. Fig. 7 shows the volume flux of the D/DS mixture solution at various concentrations. The values of volume flux of the D/DS solution are smaller than that of water at three different charge contents. Also, an increase in the charge

1 o-4

I

149

content lowers the permeation rate of the mixture containing DS. As shown in Fig. 5, the volume flux of the D/DS solution at 0.1 wt% concentration is smaller than that of D with Mw = 40 000, even though D and DS have similar molecular sizes. Therefore the decrease in the volume flux in the D/DS mixture may be due to the electrostatic repulsion between DS and the charged membrane. As shown in Fig. 6, the (YD/DS values for membranes containing SMA groups decreased with a high concentration of the feed. The rejection of the DS component by the membrane decreased with an increase of solute concentration. We previously reported the effect of NaCl as electrolyte on the electrostatic interaction between DS and the charged membrane [ 12 1. The rejection of DS decreased and the permeation rate of DS solution through the membrane increased with the salt addition: increase in the salt concentration lowered the charge effect of the membrane. In the present work, the selectivity of the charged membrane decreased with increasing concentration of D/DS solution and then the permeation rate of the D/DS mixture slightly increased as shown in Fig. 7. DS having sodium counter ions is a polyelectrolyte. Accordingly, as well as NaCl, the increase of DS concentration appears to lower the coulombic repulsion of the charged membrane. 4. Conclusions

0

0.05

Concentration

0.1

0.15

of DlDS

0.2

0.25

mixture

0.3 (wt%)

Fig. 7. Volume flux of D/DS mixture for charged membranes with SMA quaternary ammonium salt at various feed concentrations of the mixture. SSS content: 0.058 (M), 0.11 (A ) and0.51 mmol/g (0).

The ultrafiltration behavior of negatively charged membranes of poly (acrylonitrile-costyrenesulfonate) containing quatemary ammonium salt having SMA, OMA, TMA and BMA groups was examined. The ultrafiltration rate and the cutoff characteristics of the membrane depended on the hydrophobicity of the counter ion; with counter ions having a hydrophobic nature, the ultrafiltration rate and the permeability of dextran increased. In addition, the electrostatic barrier of the charged membrane restricted the permeability of DS in mixed feeds of D and DS having similar molecular size. With high selectivity, the separation took place in 0.1 wt% feed concentration.

T. Kobayashi et al. /Journal ofMembrane Science 90 (1994) 141-150

150

5. Acknowledgments This work was partially supported by a Grant in Aid for Encouragement of Young Scientists by the Ministry of Culture, Japan, and for Scientific Research from the Japan Education Center of Environmental Sanitation.

6. References [ 1 ]H. Miyama, H. Yoshida, Y. Nosaka and H. Tanzawa, Negatively charged polyacrylonitrile graft copolymer membrane for permeation and separation of plasma protein, Makromol. Chem., Rapid Commun., 9 ( 1988) [2]6’Nakao, H. Osada, H. Kurata, T. Tsuru and S. Kimura, Separation of proteins by charged ultrafiltration membranes, Desalination, 70 (1988) 191. [ 3]C. Friedrich, A. Driancoart, C. Noel and L. Mohnerie, Asymmetric reverse and ultrafiltration membranes prepared from sulfonated polysulfone, Desalination, 36 (1981) 39. [4]A.Y. Tremblay, C.M. Tam and M.D. Guiver, Variations in the pore size of charged and noncharged hydrophilic polysulfone membrane, Ind. Eng. Chem. Res., 3 1 (1992) 834. [ S]A. Bottino, G. Capannel and S. Munari, Effect of coagulation medium on properties of sulfonated polyvinylidene fluoride membranes, J. Appl. Polym. Sci., 30 (1985) 3009. [ 6 ] J. Schauer, P. Lopour and J. Vacik, The preparation of ultrafiltration membranes from a moderately sulfonated poly [ oxy (2,6-dimethyl- 1,Cphenylene) 1,J. Membrane Sci., 29 (1986) 169. [ 7]K. Nakura, C. Kamizawa, M. Matsuda and H. Masada, Preparation of charged ultrafiltration membrane using poly (etheretherketone) as a membrane material, Maku (Membrane), 17 (1992) 85. [ 8 ] M. Rinaudo and J. Desbrieres, Electrostatic exclusion on anionic membranes for ultrafiltration, Eur. Polym. J., 18 (1982) 175. [ 91 H. Miyama, T. Tanaka, Y. Nosaka, N. Fujii, H. Tanzawa and S. Nagaoka, Charged ultrafiltration membrane for permeation of proteins, J. Appl. Polym. Sci., 36 (1988) 925. [ lO]T. Kobayashi, T. Miyamoto, T. Nagai and N. Fujii, Negatively charged ultrafiltration membranes of poly(acrylonitrile-cu-sodium styrenesulfonate) for per-

meation and separation of dextran and dextran sulfate. J. Appl. Polym. Sci., in press. [ 11 IT. Kobayashi, T. Miyamoto, T. Nagai and N. Fuiii. Electrostatic sieve separation of dextran sulfate and dextran by negatively charged ultrafiltration membranes, Chem. Lett., (1993) 663. 12]T. Kobayashi, T. Nagai, T. Suzuki, Y. Nosaka and N. Fujii, Restricted permeation of dextran sulfate by electrostatic barrier of negatively charged ultrafiltration membranes: salt effect on the permeation, J. Membrane Sci., 86 (1994) 47. 13 ] K. Kalyanasundaram, Photochemistry in Microheterogeneous Systems, Academic Press, New York, 1987, pp. 36-46. [ 14]Y. Morishima, Y. Itoh and S. Nozakura, Copolymers of 2-acrylamido-2-methylpropanesulfonic acid and aromatic vinyl compounds as potential media for photosensitized electron transfer reactions, Makromol. Chem., 182 (1981) 3135. [ 15 ] G. Weber and D.J.R. Laurence, Fluorescent indicators of adsorption in aqueous solution and on the solid phase, B&hem. J., 56 (1954) 31. [ 16]H. Dodiak, H. Kanety and E.M. Koswer, The apomyoglobin-arylaminonaphthalenesulfonate system, insight into fluorescent probe responses by substitutive modulation, J. Phys. Chem., 83 (1979) 515. [ 17]L. Stryer, The interaction of a naphthalene dye with apomyoglobin and apohemoglobin, a fluorescent probe of non-polar binding sites, J. Mol. Biol., 13 ( 1965) 482. [ 18 ]C.J. Selikar and L. Brand, Solvent dependence of the luminescence of N-arylaminonaphthalenesulfonates, Science, 171 (1971) 799. [ 19 ] M. Mulder, Basic Principles of Membrane Technology, Kluwer Academic Publishers, The Hague, 199 1, pp. 1O13. [ 20]M. Nagasawa and A. Holtzen, The helix-coil transition in solution of polyglutamic acid, J. Am. Chem. Sot., 86 (1964) 538. [ 2 1 ] A. Takahashi and M. Nagasawa, Excluded volume of polyelectrolyte in salt solutions, J. Am. Chem. Sot., 86 ( 1964) 543. [ 22]T. Kobayashi, K. Kumagai, Y. Nosaka, H. Miyama and N. Fujii, Permeation behavior of dextrans by charged ultrafiltration membranes of polyacrylonitrile photografted with ionic monomers, J. Appl. Polym. Sci., 43 (1991) 1037. [ 23 ] T. Kobayashi and N. Fujii, Preparation and properties of negatively charged ultrafiltration membrane: photografted sodium styrenesulfonate onto brominated polyacrylonitrile membrane, J. Appl. Polym. Sci., 45 ( 1992) 1897.