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ELSEVIER Journal of Me mb rane Science 131 (1997) 16 7-180

~ ESCIENCE

Membrane surface characterisation by contact angle

measurements us ing the immersed method

M.J. Rosa*, M .N. de Pinho

Chemical Eng ineering Department, lns t i tuto Superior Tdcnico, 1096 Lisboa Codex, Portugal

Received 8 Nov emb er 1996; received in revised form 30 Janua ry 1 997; accepted 4 February 1997

Abstract

A n e w c o n c e p t o f c o n t a c t a n g l e m e a su re m e n t s b y t h e im m e rs e d m e th o d i s a p p l i e d t o m e m b ra n e su r fa c e c h a ra c t e ri sa t i o n .

T h e g e n e r a l s y s t e m o f t w o i m m i s c i b l e l iq u i d s is u s e d w h e r e i n a d r o p l e t o f C C I 4 i s d e p o s i t e d o n t h e m e m b ra n e su r fa c e

im m e rse d i n a n a q u e o u s so lu t i o n . W h e n th e so lu t i o n i s p u re wa ter , m e m b ra n e h y d ro p h i l i c i t y i s e v a lu a te d . Wi th a q u e o u s

so lu t i o n s o f a c a t i o n i c su r fa c t a n t (b e lo w i t s c r i t i c a l m ic e l l e c o n c e n t ra t i o n ) a t d i f f e re n t p H v a lu e s , m e m b ra n e t i t r a t i o n c u rv e s

a re o b t a in e d . F ro m th e se c u rv e s we o b t a in t h e a c id -b a s e b e h a v io u r o f e a ch m e m b ra n e a n d t h e p H v a lu e a t wh ic h h a l f o f th e

m e m b ra n e su r fa c e g ro u p s a re i o n i se d , pK1 /2 . Th i s m e th o d i s t e s t e d wi th f i v e n a n o f il t r a ti o n m e m b ra n e s , a se r i e s o f t h re e

c e l l u lo se a c e t a te m e m b ra n e s , CA -3 1 6 , wi th i n c re a s in g h y d ra u l i c p e rm e a b i l i t i e s a n d two c o m m e rc i a l th in f i lm c o m p o s i t e

m e m b r a n e s , C D - N F - 5 0 o f poly(trans-2,5-dimethyl)piperazin h i o f u ra z a n a m i d e /p o l y e th e r s u l fo n e a n d H R - 9 8 - P P o f p o l y a m i d e /p o ly su l fo n e . Th e r e su l t s sh o w th a t t h e m e th o d ( i ) i s e a sy to p e r fo rm a n d a v o id s d y n a m ic m e a su re m e n t s ( r e q u i r e s 5 m in o f d ro p

d e p o s i t i o n ) ; ( i i) i s r e p ro d u c ib l e (m a x im a l d e v i a t i o n s o f 7 °) ; ( i i i ) s im u la t e s m u l t i p l e m e m b ra n e t e c h n ic a l e n v i ro n m e n t s ( e .g .

p u re w a te r , a q u e o u s so lu t i o n s ); ( i v ) is n o t a f f e c t e d b y t h e p re se n c e o f p o re s i n t h e n a n o f i l t ra t i o n r a n g e o f o p e ra t i o n ; a n d (v ) i s

se n s i t i v e t o m e m b ra n e h y d ro p h i l i c i t y a n d m e m b ra n e a c id i t y /b a s i c i t y ( t i tr a t i o n c u rv e s a n d p K u 2 v a lu e s ) . Fu r th e rm o re , t h e

re su l t s sh o w th e d i r e c t e f f e c t o f t h e a n n e a l in g t r e a tm e n t o n t h e a c id i t y o f t h e CA -3 1 6 m e m b ra n e s a n d t h e y e v id e n c e t h e

im p o r t a n c e o f t h e m e m b ra n e c h e m ic a l p ro p e r t i e s (h y d ro p h i l i c it y , a c id i t y /b a s i c i t y ) o n t h e m e m b ra n e p e rm e a t io n p e r fo rm a n c e .

Keywords: M e m b ra n e su r fa c e c h a ra c t e r isa t i o n ; Co n ta c t a n g l e s ; Im m e rse d m e th o d ; Hy d ro p h i l i c i t y ; Su r fa c t a n t; Ac id -b a se

in t e ra c ti o n s ; Na n o f i l t r a t i o n

1 . S t a t e o f th e a r t

C h a r a c t e r i s a t i o n o f t h e s u r f a c e c h e m i s t r y i s a c ru -

c i a l i s s u e in m e m b r a n e s c i e n c e a n d t e c h n o lo g y ,

b e c a u s e i t i s w e l l k n o w n t h a t m e m b r a n e p e r f o r m a n c e

d e p e n d s n o t o n l y o n f e e d h y d r o d y n a m i c s a n d s t e r i c

h i n d r a n c e s , b u t a ls o o n m e m b r a n e s u r f a c e c h e m i s t r y

*Corresponding author.

0376-7388/97/$17.00 © 1997 Elsevier Science B.V. All r ights reserved.

P I I S 0 3 7 6 - 7 3 8 8 ( 9 7 ) 0 0 0 4 3 - 4

( h y d r o p h i l i c it y , m e m b r a n e s u r f a c e c h a rg e ) a n d o n

m e m b r a n e - s o l u t e ( s ) - s o l v e n t c h e m i c a l i n t er a ct io n s .

C o n t a c t a n g l e i s a m e a s u r e o f t h e w e t t a b i l i t y o f a n

i d e a l s u rf a c e. I t is o n e o f t h e m o s t e a s y , c o m m o n a n d

r e l i a b l e m e t h o d s o f d e t e r m i n i n g s o l i d s u r f a c e p r o p e r -

t i e s w h e n i t i s p o s s i b l e t o s e p a r a t e i n t r i n s i c v a r i a t i o n s

f r o m a p p a r e n t v a r i a t i o n s . T h e s e a p p a r e n t v a r i a t i o n s

a r e i n d u c e d b y s u r f a c e p o r o s i t y a n d r o u g h n e s s , s u r f a c e

m o r p h o l o g i c a l t e r a t io n s d u r in g t h e m e a s u r e m e n t ( e . g .

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168 M.J. Rosa, M.N. de Pinho/Journal of Membrane Science 131 (1997) 167-180

r e s u l ti n g f ro m s u r f ace d ry i n g ) , s u r f ace h e t e ro g en e i t y

and con taminat ions o f the so lu t ion(s ) and /or so l id

surface .

U n t i l n ow , mo s t o f th e co n t ac t an g l e meas u rem en t s

fo r memb ran e ch a rac t e r i s a t i o n h av e b een mad e o nh y d r o p h o b i c m e m b r a n e s u s e d f o r p e r v a p o r a t io n o r g a s

p e rme a t i o n [1 -4 ] an d o n p o ro u s h y d ro p h i l i c o r h y d ro -

p h o b i c memb ran es u s ed i n mi c ro f i l t r a t i o n (MF) o r

u l t ra f i lt ra t ion (UF) [5 ,6 ]. Fan e e t a l . [5 ] cor re la ted f lux

d ec l i n e w i t h t h e v a r ia t i o n o f me mb ran e h y d ro p h i l i c i t y

w h i l e O l d an i an d Sch o ck [6 ] s h o w ed t h a t h y d ro p h i l ic

memb ran es d o h av e b e t t e r f l u x r eco v e r i e s . H o w ev e r ,

t h e s e s t u d i e s h av e s o me d raw b ack s b ecau s e t h ey a r e

p e r f o r m e d w i t h d r y M F o r U F m e m b r a n e s a m p l e s ,

meas u r i n g t h e co n t ac t an g l e o f a d ro p l e t o f w a t e r o n

memb ran e s u r f ace . U s i n g d ry s amp l e s , t h e i n fo rma-

t ion i s l es s re levan t than tha t co l l ec ted in a t echn ica l

en v i ro n men t o f a MF / U F o p e ra t i o n , i. e . me mb ran e i n

co n t ac t w i t h an aq u eo u s s t r eam. Bes i d es , d ry i n g may

i r r ev e r s i b l y d amag e t h e mo rp h o l o g i ca l s t r u c t u r e o f

s o me memb ran es , e . g . c e l l u l o s e ace t a t e memb ran es

p rep a red b y p h as e - i n v e r s io n u s i n g t h e w e t p ro ces s . I n

ad d i ti o n , f o r M F/ U F mem b ran es , t h e s u r f ace p o ro s i t y

an d s u r f ace ro u g h n es s may a f f ec t t h e co n t ac t an g l e

an d u n d e r co v e r i n t r i n s i c v a r i a t i o n s o f memb ran e

hydrophi l i c i ty .

In t h e 7 0 ' s , H ami l t o n [7 ] p ro p o s ed t h e i mmers ed

co n t ac t an g l e me t h o d w h e reb y h e o b t a i n ed t h e mag -

n i t u d e o f t h e p o l a r in t e r ac t i o n b y m eas u rem en t o f t h e

co n t ac t an g l e o f n -o c t an e ( co mp o u n d w i t h a d i s p e r s i v e

surface t ens ion equal to water ) on so l id su r faces

i mmers ed i n w a t e r . D av i d an d Mi s r a [8 ] u s ed t h e

r ev e r s ed s y s t em, t h ey meas u red t h e co n t ac t an g l e o f

w a t e r o n t h e s u r f ace s i mmers ed i n n -o c t an e . Ma t s u -

naga and Ikada [9 ] and Toussa in t and Luner [10]

ev a l u a t ed t he d i s p e r s iv e co m p o n en t o f t h e s u r f ace f r ee

en e rg y o f ce l l u l o s e ma t e r i a ls an d t h e i r n o n -d i s p e r s i v e

in terac t ion wi th d i f feren t po lar l iqu ids (water , g ly -

ce ro l , amo n g o t h e r s ) b a s ed o n a s e r ie s o f co n t ac t an g l ev a l u es o f t h e p o l a r l i q u id o n s o l id s u r f ace s i mm ers ed

i n d i ff e r en t h y d ro ca rb o n s . T h i s s o l i d -w a t e r -h y d ro ca r -

b o n s y s t em i s v e ry ad v an t ag eo u s . I t a l lo w s t h e d e t e r -

mi n a t i o n o f b o t h d i s p e r s i v e an d n o n -d i s p e r s i v e

i n t e r ac ti o n s ( s i n ce h y d ro ca rb o n s d o n o t i n t e r ac t sp e -

c i f i ca l ly wi th the so l id su r face) and i t does no t requ i re

d y n ami c meas u remen t s a s t h e eq u i l i b r i u m b e t w een

the two immisc ib le l iqu ids and the so l id su r face i s

p ro mp t l y r each ed [9 , 1 0 ] . Mo reo v e r , i t t h o ro u g h l y

av o i d s t h e p o l y mer co n t ac t w i t h a i r mo i s t u r e ( s i n ce

s a t u r a t ed i mmers i o n l i q u i d s a r e u s ed ) an d t h e r e fo re ,

t h e s u r f ace s p r ead i n g p r e s s u re may b e n eg l ec t ed

[9,11].

In s t ead o f u s i n g t h e t w o - l iq u i d s i mm ers i o n s y s t em,Z h an g an d H a l l s t r rm [4 ] an d G ek as e t a l . [ 1 2 ] mea -

s u red co n t ac t an g l e s o f a i r o n memb ran e s u r f ace

i mm ers ed i n w a t e r ( cap ti v e b u b b l e me t h o d ) . H o w e v e r ,

t h is s y s t em, a s w e l l a s th e H a mi l t o n ' s m e t h o d r eq u i r e a

mo re s o p h i s t i c a ted apparatus w h e r e t h e m e m b r a n e i s

s u s p en d ed o n t h e t o p o f th e i mm ers i o n ch am b er [4 ] ( in

t h e ca s e o f a s y mmet r i c memb ran e s u s p en d ed u p - s i d e

d o w n w i t h t h e ac t i v e l ay e r f ac i n g t h e b u l k s o l u t io n ) ,

b ecau s e t h e a i r b u b b l e o r t h e n -o c t an e d ro p l e t a r e b o t h

l e ss d en s e r t h an t h e i m mers i o n s o l u t io n o f w a t er .

S t i l l w i t h t h e a i r b u b b l e - l i q u i d -memb ran e s y s t em,

K eu ren t j e s e t a l. [ 1 3 ] d ev e l o p ed a s i mp l e m e t h o d fo r

i n d i r ec t d e te rmi n a t i o n o f t h e co n t ac t an g le . T h ey u s ed

the s t i ck ing bubble t echn ique , i . e . they measured the

p e rcen t ag e o f a i r b u b b l e s t h a t s t ick t o t h e mem b ran e

s u r f ace p l aced o n t h e b o t t o m o f a ce l l , u si n g d i f f e r en t

i mm ers i o n l i q u id s . P l o t t i n g t h e p e r cen t ag e o f s t i ck i n g

bubbles vs . the su r face t en s ion o f the l iqu id they

obta ined the su r face t ens ion o f the l iqu id a t the po in t

o f d e t ach men t , f r o m w h i ch , u s i n g t h e fo r ce b a l an ce ,

t h ey co mp u t e t h e co n t ac t an g l e . T h i s me t h o d i s n o t

i n f l u en ced b y t h e p r e s en ce o f t h e p o re s an d s o i t i s

e s p ec i a l l y ad eq u a t e t o ev a l u a t e t h e h y d ro p h o b i c i t y o f

p o ro u s ma t e r i a l s , s u ch a s MF an d U F memb ran es .

H o w ev e r , i t i s mu ch t o o l ab o r i o u s an d t i me co n s u m i n g

an d i t is n o t w o r t h fo r d en s e m emb ran es , s u ch a s t h o s e

of nanof i l t ra t ion and reverse osmo s i s . In add i t ion , the

a i r b u b b le s w o u l d n o t s t i ck t o h y d ro p h i li c mem b ran es

i mm ers ed i n aq u eo u s s o l u t i o n s b ecau s e t h e w a t e r w e t s

t h e s u r f ace ( a l mo s t ) co mp l e t e l y [1 3 ] , r en d e r i n g t h i s

me t h o d i n ad eq u a t e fo r m o s t p r ac t i c a l s i tu a t io n s .

2 . D e v e l o p m e n t o f a n e w i m m e r s i o n s y s te m f o r

c o n t a c t a n g l e m e a s u r e m e n t

T h e ma i n o b j ec t i v e o f th e p r e s en t w o rk i s t o d ev e l o p

a s i mp l e me t h o d o f d e t e rmi n i n g t h e s u r f ace ch em i ca l

p ro p e r t ie s , n am e l y r e l a t i v e h y d ro p h i l i c i t y an d ac i d i t y /

b a s i c i t y , o f n an o f i l t r a t i o n memb ran es . T h e me t h o d

s h o u l d p r e f e r ab l y a l l o w t h e ev a l u a t i o n o f mem-

b ran e - s o l v en t - s o l u t e ( s ) ch emi ca l i n te r ac t io n s . T h e re -

fo r e , b a s ed o n t h e p r ev i o u s w o rk r ep o r t ed i n t h e

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M.J. Rosa, M.N. de Pinho/Journal o f Membrane Science 131 (1997) 167-180 16 9

a q u e o u s s o lu t io n ( w ) ~ T ° w

_- . Tmw i ~ x .~ u~

m o- -" V / / / / / / / / / ' / / / /7 / / / / / / / / ~ ' / / / / / / / / / /# e m b r a n e ( m )/

F i g . 1 . F i n a l f o r m o f a d r o p l e t o f a n o r g a n i c ( C C1 4 ) o n a m e m b r a n e i m m e r s e d i n a n a q u e o u s s o l u t i o n : c o n t a c t a n g l e a n d m e m b r a n e - - C C l 4 -

so l u t i o n i n t e r fa c i a l fo rc e s .

l i te r a t u re , w e d es i g n ed a t w o - l iq u i d s i m m ers i o n s y s -

t em s en s i ti v e t o p H v a r i a t i o n s an d t o t h e p re s en ce o f a

d i sso lved so lu te .

T h e g en e ra l s y s t em co n s i s t s o f a d ro p o f a n eu t r a l

o rg an i c (o ) , w h i ch i s w a t e r i m m i s c i b l e an d h eav i e r

than water , e .g . carbon te t rach lor ide , p laced on the

m em b ran e s u r f ace (m ) i m m ers ed i n an aq u eo u s s o l u -

t ion (w) . Fig . 1 i l lus t ra tes the conta ct ang le , /9 , o f C C 1 4

o n t h e m em b ran e s u r f ace i m m ers ed i n t h e aq u eo u s

so lu t ion and the in ter rac ia l fo rces : membrane--CCl4 ,

7 m o ; m e m b ra n e - s o l u t i o n , 7 m w ; an d C C 1 4 - s ol u ti o n ,

7 ow . T h e s u r f ace f r ee en e rg y w h i ch i s l o s t w h e n s o l u te

i s ad s o rb ed o n m em b ran e s u r f ace i s r ep re s en t ed b y A d

(w h en t h e i m m ers i o n s o l u t i o n is p u re w a t e r t h e re i s n o

co n t am i n an t an d s o l u t e ad s o rp t i o n (A d ) i s ze ro ) .

C o n t ac t an g l e s s m a l l e r t h an 9 0 ° co r r e s p o n d t o h y d ro -

p h o b i c m em b ran es w h i l e co n t ac t an g l e s l a rg e r t h an

9 0 ° co r r e s p o n d t o h y d ro p h i l i c m em b ran es .

T h e re a r e t w o m a i n r eas o n s fo r ch o o s i n g t h i s s y s -

t em :

( i ) t h e n eed t o s i m u l a t e t h e m em b ran e t ech n i ca l

en v i ro n m en t (w a t e r i m m ers ed ) an d , s i m u l t an eo u s l y ,

t o k eep t h e m em b ran es w e t , av o i d i n g r i s k y an d t i m e

co n s u m i n g d ry i n g p ro ces s es ;

( ii ) th e o p p o r t u n i t y t o t ak e ad v an t ag e f ro m s u r f aceco n t am i n a t i o n (u s u a l l y an u n d es i r ed e f f ec t ) w i t h

so lu te adsorp t ion .

T h e fo rce b a l an ce t o t h e i n t e r face y ie l d s th e Y o u n g -

D u p r6 eq u a t i o n , m o d i f i ed t o i n c l u d e s o l u t e ad s o rp t io n

(A d )

7 o w C O S 0 = 7 m w - - '7 m o - - A d (1 )

Fo r h y d ro p h i l i c s u r f aces ( s u ch a s m o s t o f t h e m em -

branes ) a nd po lar l iqu ids (such as water ) , the adhes ive

fo rces ac t in g o n t h e i n t e r f ace a r e n o w -a -d ay s co n s i d -

e r ed t o h av e a n o n -p o l a r L i f s h it z -V an -d e r -W aa l s an d

a n a c i d - b a s e ( s u c h a s h y d ro g e n b o n d in g ) c o m p o n e n t

[ 1 4 -1 6 ] . T h e ac i d -b as e i n t e r ac t io n r eq u i r e s t h a t ac i d i c

s i tes o f one ph ase in tera ct wi th bas ic s i tes o f the o ther ,

so that i f e i ther phase i s neu t ra l o r i f bo th phases have

o n l y b as i c o r o n l y ac i d i c s it e s, t h e re can b e n o ac i d -

base in teract ion [16] .

M o s t m em b ran e p o l y m ers h av e ac i d i c (u s u a l l y ,

carboxyl ic , su lphonic) and bas ic (usual ly , amine)

g ro u p s an d t h e g l o b a l ch a rac t e r i s d ep en d en t o n t h e

p H o f t h e i m m ers i o n s o l u t io n . Wi t h i n c reas i n g p H , t h e

m em b ran e can g o f ro m p o s i t i v e l y ch a rg ed (b as i c

ch a rac t e r ) t o n eg a t i v e l y ch a rg ed ( ac i d i c ch a rac t e r ) .

W h e n t h e m e m b r a n e i s i m m e r s e d i n a n a q u e o u s

s o l u t i o n , t h i s g l o b a l ch a rac t e r d e t e rm i n es m em -

b ran e - s o l u t e ad s o rp t i o n , t h e re fo re co n d i t i o n i n g t h e

f inal con tact ang le o f C C 1 4 . F i g . 2 i l lustrates the

re l a t i o n s h i p b e t w een ca t i o n i c s u r f ac t an t ad s o rp t i o n

o n an i o n i s ab l e m em b ran e s u r f ace an d t h e co n t ac t

an g l e o f C C 1 4 w h en ( a ) t h e m em b ran e an d t h e s u r f ac -

tan t are bo th pos i t ive ; (b ) the mem bran e i s neu t ra l , low

ad s o rp t i o n ; ( c ) t h e m em b ran e h as a n eg a t i v e ch a rg e ,

c o m p l e t e m o n o l a y e r f o r m a t io n . D e p e n d i n g o n m e m -

b ran e an d s u r f ac t an t ch a rac t e r i s t i c s , w h en t h e m em -b ran e s u r f ace i s h i g h l y n eg a t i v e l y ch a rg ed t h e fo r -

m a t i o n o f a d o u b l e l ay e r (F i g. 2 (d ) ) m ay o ccu r [1 7 ] .

Plo t t ing contact ang les agains t so lu t ion pH resu l t s

i n m em b ran e t i tr a t io n cu rv e g en e ra l l y rep re s en t ed i n

Fig . 3 fo r a bas ic membrane (Fig . 3 (a) ) , fo r an ac id ic

m em b ran e (F ig . 3 (b ) ) an d fo r an am p h o t e r i c (b o t h

acid ic and bas ic) mem bra ne (Fig . 3 (c) ) . In th i s f igure ,

the no tes 2 (a) -2(d) correspond to the s i tua t ions i l lu -

s t ra ted in Fig . 2 and pKt/2 i s t h e p H v a l u e a t w h i ch

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17 0 M . J . R o s a , M . N . d e P i n h o / J o u r n a l o f M e m b r a n e S c i e n c e 1 3 1 ( 1 9 9 7 ) 1 6 7 - 1 8 0

f ° . ° b o n . O \4444( a )

~ / ~ . u t r a , m e m b r a n e ~

I o o o o o I

( b )

( c )

/ / ; e m b r a n e ~ ( ~ ,

TTTTTTTW(d )

Fig . 2 . I l lus t r a t ion of the r e la t ionship be twe e n c a t ionic sur fa c ta n t a dsorp t ion on a ion i sa ble m e m bra ne sur fa c e a nd the c onta c t a ngle of CC14:

( a ) m e m b r a n e a n d s u r f a ct a n t w i th t h e s a m e c h a r g e ; ( b ) n e u tr a l m e m b r a n e ; ( c ) m e m b r a n e w i t h o p p o s i t e c h ar g e , c o m p l e t e m o n o l a y e r f o r m a t i o n ;( d ) m e m b r a n e w i t h m a x i m u m o p p o s i t e c h a rg e , d o u b l e l a y e r f o r m a ti o n .

0

2 ( a )i

b a s e 2 ( b ) a c i d

f~ 2 (c ) , , • c )

,, , ~ _ 2 ( c )a c i d - b a s e ,

I

pK 1/2 pK 1/2 pK 1/2p H " -

( a ) ( b ) ( c )F i g . 3 . M e m b r a n e t i t r a ti o n c u r v e s o b t a i n e d w i t h c o n t a c t a n g l e o f C C 1 4 o n m e m b r a n e s u r f ac e i m m e r s e d i n a c a t i o n i c s u rf a c t an t s o l u t i o n a t

s e ve ra l pH v a lue s for ( a) ba s ic m e m b ra ne ; (b) a c id ic m e m bra ne ; ( c ) a m pho te r ic m e m b ra ne (2(a ) -2(d ) a re the s i tua t ions i l lus t r a te d in F ig . 2) .

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M.J. Rosa, M.N. de Pinho/Journal of Membrane Science 131 (1997) 167-180 1 7 l

5 0 % o f t h e g ro u p s a r e i o n i s ed [1 8 ]. Fo r ex am p l e , f o r

an A H ac i d, t h e eq u i l i b r i a i n v o l v ed a r e :

p H < pKu2 [A H ] > > [A - ]

p H = p K / 2 [ A n ] = [ A - ]

p H > pK,/2 [A H ] < < [A - ]

O t h e r t y p es o f t i t r a t i o n cu rv es w e re o b t a i n ed

by Whi tes ides e t a l . [18] based on con tac t ang le

meas u remen t s u s i n g b u f f e r ed w a t e r a s i mmers i o n

so lu t ion .

In t h e p r e s en t w o rk , t w o t y p es o f imm ers i o n s o l u -

t i o n s a r e u s ed ( i ) p u re w a t e r , t o ev a l u a t e memb ran e

re la t ive hydroph i l i c i ty , and ( i i ) an aqueous so lu t ion o f

a ca t i o n i c s u r f ac t an t a t d i f f e r en t p H v a l u es t o ev a l u a t e

membrane ac id i tyPoas ic i ty .T h e me t h o d i s t e s t ed w i t h f i v e n an o f i lt r a t io n (N F)

mem b ran es (p o re d i ame t e r i n th e r an g e o f 1 0 -9 m ) o f

d i f feren t mater i a l s and poros i t i es . A CA-316 ser i es o f

t h r ee ce l lu l o s e ace t a t e (CA ) m emb ran es w i t h in c r ea s -

ing hydrau l i c permeabi l i t i es ( i . e . wi th increas ing por-

o s i t i e s ) a r e p r ep a red i n t h e l ab o ra t o ry b y t h e p h as e

i n v e r s i o n me t h o d . T w o co mmerc i a l t h i n f i l m co mp o -

s i t e memb ran es a r e u s ed , CD -N F-5 0 o f poly(trans-

2 , 5 -d i me t h y l )p i p e raz i n t h i o fu razan ami d e / p o l y e t h e r -

s u lf o n e ( T F Z / P E S ) a n d H R - 9 8 - P P o f p o l y a m i d e / p o ly -

s u l fo n e (PA / PS) . W i t h t h e s e t o f mem b ran es ch o s en ,

w e t e s t t h e me t h o d fo r w eak l y ac i d i c (CA ) , p r ed o m i -

n an t l y ac i d i c (T FZ ) an d p r ed o mi n an t l y b a s i c (PA )

p o l y mer s an d , f o r t h e s ame p o l y mer , w e s t u d y t h e

e f f ec t o f p o ro s i ty i n t h e r an g e o f n an o m e t e r s (CA -3 1 6

series) .

3. Experimental

3.1. Membranes

L a b o r a t o r y - m a d e a n d c o m m e r c i a l m e m b r a n e s w e r e

inves t iga ted .

A s e r ie s o f f i a t - sh ee t c e l lu l o s e ace t a t e a s y m met r i c

mem b ran es , l ab e l l ed CA -3 1 6 s e ri e s, w e re p r ep a red i n

t h e l ab o ra t o ry b y t h e p h as e i n v e r s i o n me t h o d a s

d es c r i b ed b y K u n s t an d So u r i r a j an [1 9 ] . T h e i n d i v i -

d u a l CA -3 1 6 memb ran es a r e i d en t i f i ed b y t h e n u m-

bers 50 , 68 , and 86 tha t cor respond to the anneal ingt emp era t u r e i n °C . T h i s an n ea l in g w as p e r fo rm ed to

o b t a i n mem b ran es o f d i f f e r en t p e rmeab i l it i e s . D e t a i ls

o n t h e p r ep a ra t i o n an d an n ea l i n g co n d i t i o n s a r e l i s t ed

in Tab le 1 .

T h e ce l l u l o s e ace t a t e ( 3 9 . 8 % ace t y l co n t en t ) w as

s u p p l i ed b y E as t man n K o d ak , MW 3 0 2 7 0 . T h i s i s a

l i n ea r g la s s y p o l y m er w i t h a d eg ree o f s u b s t it u t io n o f

2 .5 , i . e . 2 .5 o f the th ree hydroxyl g roups on the

an h y d ro g l u co s e r i n g b ea r an ace t y l (CH 3 CO -) s u b -

s t i tuen t ins tead o f the hydrogen tha t i s found in the

n a t i v e ce ll u l o se . H y d ro x y l g ro u p s a r e r e s p o n s i b l e fo r

C A - 3 1 6 m e m b r a n e h y d r o p h i l i c it y an d w e a k l y a c i d ic

ch a rac t e r ( -O H d i s s o c i a ti o n co n s t an t i s a ro u n d 1 0 -1 °

to 10 12) . Ac ety l subs t i tuen t s a re res pons ib le fo r

T a b l e 1

F i l m c a s t i n g c o n d i t io n s o f t h e C A - 3 1 6 s e r i es m e m b r a n e s

M e m b r a n e C A - 3 1 6 - 50 C A - 3 1 6 - 6 8 C A - 3 1 6 - 86

C a s t i n g s o l u t i o n ( w t % )

P o l y m e r : C A 3 9 8

S o l v e n t : A c e t o n e

A d d i t i v e : M g ( C 1 O a ) 2 / H 2 0

C a s t i n g c o n d i t i o n s

T e m p e r a t u r e o f s o l u t io n ( ° C )

T e m p e r a t u r e o f a t m o s p h e r e ( ° C )

H u m i d i t y o f a t m o s p h e r e ( % )

S o l v e n t e v a p o r a t io n t i m e ( m i n )

G e l a t i o n m e d i u m

A n n e a l i n g c o n d i t io n s

A n n e a l i n g m e d i u m

A n n e a l i n g t i m e ( m i n )

T e m p e r a t u r e o f m e d i u m ( ° C )

1 7 ( i n a l l c a se s )

6 9 . 2 ( i n a l l c a s e s )

1 . 4 5 /1 2 . 3 5 ( i n a l l c a se s )

9 - 1 2 ( i n a l l c a s e s )

2 2 - 2 3 ( i n a l l c a s e s )

5 0 - 6 5 ( i n a l l c a s e s )

1 ( i n a l l c a s e s )

I c e c o l d w a t e r ( 1 - 2 h ) ( i n a l l c a s e s )

H o t w a t e r ( i n a l l c a s e s )

1 1 ( i n a l l c a se s )

5 0 6 7 . 5 8 6

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17 2 M.J. Rosa, M.N. de Pinho/Journal of Membrane Science 131 (1997) 167-180

m e m b r a n e h y d r o p h o b i c i t y . A t e x t r e m e p H v a l u e s

( ac i d o r a l k a l i n e ) t h e ace t y l g ro u p s may h y d ro l y s e

t o h y d ro x y l g ro u p s an d , t h e r e fo re , CA -3 1 6 m em -

b ran es can n o t b e s u b j ec t ed t o s ev e re p H fo r l o n g

p e r i o d s o f t ime t o av o i d i r rev e r s i b l e ch an g es i n m em -b ran e ch a rac t e r is t ic s . M em b ran e h y d ro p h i l i c i t y w o u l d

i n c rea s e an d t h e p o re r ad iu s a s w e l l , b ecau s e t h e -O H

ster i c h indrance i s lower than tha t o f CH3CO-- .

U s u a l l y , t h e r eco mmen d ed p H l i mi t s o f o p e ra t i o n

a re 2 -8 , t o en s u re a r ea s o n ab l e memb ran e l i f e t i me ,

w h i ch i s 2 -3 y ea r s i n mo s t c a s e s. Fo r v e ry s h o r t ti me

per iods (e .g . 1 -2 days ) , i t i s safe to use a pH in ter va l o f

3 - 1 1 .

T h e t w o co m me rc i a l mem b ran es a r e b o t h t h in f i lm

c o m p o s i t e m e m b r a n e s.

T h e C D - N F - 5 0 m e m b r a n e ( s u p p l i e d b y S e p a r e m ,

I t a l y ) h a s an ac t i v e l ay e r o f T FZ (po ly( t rans-2 ,5 -

d i me t h y l )p i p e raz i n t h i o fu razan ami d e ) ca s t ed o n an

u l t r a f i l t r a t i o n PE S (p o l y e t h e r s u l fo n e ) memb ran e .

T h e C D - N F - 5 0 i s a n a m p h o t e r i c m e m b r a n e w i t h a

p red o mi n an t a c i d i c ch a rac t e r f r o m t h e ca rb o n y l an d

s u l p h u r g ro u p s o f T FZ .

T h e H R - 9 8 - P P m e m b r a n e ( s up p li e d b y D o w / D D S ,

D en mark ) h a s an ac t i v e l ay e r o f PA (p o l y ami d e )

ca s t ed o n a PS (p o l y s u l fo n e ) s up p o r t . I t i s an am p h o -

t e r i c memb ran e w i t h a p r ed o mi n an t b a s i c ch a rac t e r

f ro m t h e ami d e g ro u p s .

T h e f i v e mem b ran es a r e a l l n an o f i lt r a t io n / r ev e r s e

o s mo s i s memb ran es a s ex p re s s ed b y t h e i r h y d rau l i c

p e rmeab i l i t i e s an d av e rag e p o re r ad i i s h o w n i n

Table 2 .

H y d rau l i c p e rmeab i l i t y w as d e t e rmi n ed b y t h e

s l o p e o f t h e p u re w a t e r f l ux v s . t ran s m emb ran e p r e s -

Table 2

Membrane pe rmea t ion cha rac te r i s t i c s ( f rom [20,21] (pre s sure=

3 MPa , t empera ture =2 98 K, f eed t angent i a l ve loc i ty=0.11 m/s ,

f e ed c o n c e n t r a t i o n = 0 . 3 k g / m 3 , m e m b r a n e s u r fa c e a r e a =

13 . 2× 10 4m 2)

M e m br a ne Hydraulic Average

perm eabi l i ty ore radius

(m) ( ,~)

CA-316 -50 1 .55 4 .4

CA-316-68 0.90 4.0

CA-316 -86 0 .16 3 .3

CD-NF-50 2.34 4.2

HR-98-PP 1.63 2.8

s u re [2 0 ] . Memb ran e p o re r ad i u s w as d e t e rmi n ed b y

mo d e l l i n g t h e ex p e r i m en t a l d a t a o f d i l u t e s o l u ti o n s o f

s u g a r s ( e t h an o l , g l y ce ro l , g l u co s e , s acch a ro s e an d

ra f f i n o s e ) b y an i n t eg ra t ed t r an s p o r t mo d e l f o r U F/

N E In t h is mo d e l , t h e av e rag e p o re r ad iu s i s a g l o b a lp a r a m e t e r r e l a t e d t o t h e m e m b r a n e m o r p h o l o g i c a l

s t ru c t u r e ( i . e . p o l y mer mo l ecu l a r a r r an g emen t )

r e s p o n s i b l e fo r t h e d i f fu s i v e an d co n v ec t i v e s t e r i c

h i n d r a n c e s a n d f o r t h e m e m b r a n e - s o l u t e - w a t e r c h e -

mi ca l i n t e rac t i o n s. T h e i n t eg ra ted t r an s p o r t mo d e l , a s

w e l l a s t h e N F ex p e r i men t a l s e t -u p an d p ro ced u re h av e

b een a l r ead y d es c r i b ed b y R o s a an d d e P i n h o [2 0 , 2 1 ].

3 .2 . Sur fa ctan t so lu t ion

A ca t i o n i c s u r f ac t an t w as s e l ec t ed b ecau s e mo s t

c o m m e r c i a l N F m e m b r a n e s h a v e a n a c i di c c h a ra c t e r

a n d w e w a n t e d t o p r o m o t e m e m b r a n e - s u r f a c t a n t

a t t r ac t i o n . T h e s u r f ac t an t u s ed w as N - c e t y l - N ~ , N -

t r i m e t h y l a m m o n i u m b r o m i d e ( C 1 6 T A B ), p r o a n a l y-

s is , f r o m M erck . I n t h e p H r an g e s t u d i ed o f 3 -1 1 t h is

sur fac tan t p resen t s l i t tl e o r no s ign i f i can t pH sens i t iv -

i ty [17] . The sur fac tan t so lu t ion has to be be low the

v a l u e o f c r i ti c a l mi ce l l e co n cen t r a t i o n (CMC ) . O t h e r -

w i s e , t h e s u r f ac t an t mo l ecu l e s w i l l i n t e r ac t w i th eac h

o t h e r f o rmi n g mi ce l l e s i n s t ead o f i n t e r ac t in g w i t h t h e

memb ran e s u r f ace fo rmi n g ad s o rp t i o n l ay e r s . U s i n g

t h e K l ev en s eq u a t i o n [1 7 ] t h e v a l u e o f CMC fo r

C1 6 T A B i s 0 . 8 3 mM . Fo r s a f e t y r ea s o n s , an d b ecau s e

fo r mo s t i o n i c s u r f ac t an t s CMC d ec rea s e s w i t h t h e

i o n i c s t r en g t h o f t h e s o l u t i o n (p ro mo t ed b y t h e p H

ad j u s tmen t ) , a co n cen t r a t i o n o f 0 .3 m M w as u s ed ,

w e l l be l o w t h e C M C o f 0 .8 3 m M .

So l u t i o n p H w as ad j u s t ed w i t h N aO H an d H N O 3

co v e r i n g t h e r an g e o f 3 t o 1 1. D e i o n i s ed w a t e r w i t h a

co n d u c t i v i t y l o w er t h an 0 . 2 ~ tS /cm w as u s ed .

Co n ce rn i n g t h e ab i l i t y t o fo rm a d o u b l e l ay e r ,

a l though th i s i s no t a typ ica l behav iour o f th i s c l ass

o f s u r f ac t an ts d u e t o t h e h i g h v o l u m e o f t h e p o l a r h ead ,C1 6 T A B c an fo rm a d o u b l e l ay e r b ecau s e o f it s lo n g

al iphat i c chain . In fac t , i t i s 32 t imes a C-C leng th

agains t the sphere rad ius o f the po lar head . In add i t ion ,

there i s a lways an ex cess o f su r fac tan t in so lu t ion . Fo r

a mi n i ma l v o l u me o f s o lu t i o n o f 1 ml an d a m i n i ma l

p o l a r h ead r ad i u s o f 1 ~, an d fo r a max i m a l me mb ran e

area o f 2 cm 2 , there are ca . 30 t ime s mo re a d s o r b a b l e

m a t er i a l a r ea (po lar heads ) than a d s o r b en t a r ea (area

avai l ab le fo r adsorp t ion) ( i .e . 0 .3 x 10 -3 ×

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M.J. Rosa, M.N. de Pinho /Journal o f Membrane Science 131 (1997) 167-180 17 3

1 0 - 3 x 6 . 0 2 2 x 1 0 2 3 × 3 .1 4 x 1 0 - 2 ° / 2 × 1 0 - 4 = 2 9 . 3 ) .

T h e fo rma t i o n o f a t h i rd l ay e r o f ad s o rp t i o n i s n o t

p o s s i b l e b ecau s e t h e p o l a r h ead s o f t h e d o u b l e l ay e r

a r e f ac i n g t h e b u l k s o l u t i o n , t h e r e fo re r ep e l l i n g t h e

surfac tan t in so lu t ion .

3.3. Contact angles measurements

M e m b r a n e s a m p l e s h a v e 0 . 5 - 1 c m 2 o f s u r f ac e a r e a.

P r i o r t o t h e meas u remen t s an d b e t w een d i f f e r en t

s o l u t i o n s , t h e memb ran es w e re w as h ed i n an u l t r a -

s o u n d b a t h w i t h d e i o n i s ed w a t e r (< 0 .2 g S / cm ) fo r t w o

t i m e s , 1 5 m i n e a c h . T h e m e m b r a n e s w e r e t h e n

i mmers ed i n t h e aq u eo u s s o l u t i o n , a t r o o m t emp era -

t u r e (2 0 °C) , f o r a t i me p e r i o d o p t i mi s ed t o en s u re t h e

s t ab i l i s a t i o n o f memb ran e - s o l u t i o n i n t e r ac t i o n s .

A f t e rw ard s , t h ey w ere p u t o n a s i n t e r ed g l a s s p l a t e ,

wi th the ac t ive l ayer up , and t rans fer red to a spec t ro -

p h o t o me t r i c q u a r t z ce l l f i l l ed w i t h t h e i mmers i o n

s o l u ti o n . A d ro p l e t ( 2 -5 mm h e i g h t ) o f C C1 4 (p ro

an a l y s is , Merck ) w as d e l i v e r ed b y a mi c ro s y r i n g e .

A f t e r a f i x ed d ep o s i ti o n t ime , t h e i m ag e w as r eco rd ed .

Bo t h s t ab i l i s a t i o n an d d ep o s i t i o n t i mes w e re o p t i -

mi s ed i n p r i o r ex p e r i men t s d e s c r i b ed b e l o w (p o i n t

3 .4 .) . Specia l care was t aken whe n manipu la t ing CC14,

b e i n g a l w ay s av o i d ed t h e d i r ec t co n t ac t ( u s i n g ey e ,

sk in and c lo thes p ro tec t ion) and vapours inhala t ion .

T h e ex p e r i men t a l s e t -u p i n t eg ra t e s a co l o u r v i d eo

c a m e r a J V C O m o u n t e d o n a m i c r o s c o p e W i l d M 3 Z

w h ere t h e q u a r t z ce l l w as ad ap t ed . T h e v i d eo s i g n a l i s

t r an s mi t t ed t o a V i d eo P i x F rameg rab l e r f r o m Su n

Mi c ro s y s t em s , u s in g a co m p u t e r Su n S p a rc s t a ti o n IPC

t o i mp o r t t h e d ro p i mag e . T h e co n t ac t an g l e s w e re

o b t a i n ed b y t h e s e s s i le d ro p m e t h o d u s i n g t h e s o f t w a re

A D S A - P ( A x i s y m m e t r i c D r o p S h a p A n a l y s is - P ro f il e )

d e v e l o p e d b y N e u m a n n a n d c o - w o r k e rs .

Each va lue p resen ted i s the resu l t o f a s t a t i s t i ca l t -

t es t an a lys i s (9 5% conf id ence) , 0 4 - (~ n-1 t95%/v/ -n ) .

T h e n u m b er o f s amp l e s , n , is r e l a ti v e t o t h e n u mb er o fd ro p l e t s d ep o s i t ed in t w o d i f f e r en t mem b ran e s amp l e s

an d n i s a l w ay s eq u a l o r h i g h e r t h an 8 (n > 8 ) .

3.4. Optimisation of the drop deposition time and

stabilisation time

Co n t ac t an g l e s o f C C I 4 o n m e m b r a n e a c t i v e l a y e r

i mm ers ed i n p u re w a t e r (< 0 . 2 ~ tS /cm) w ere m eas u red

af ter 1 , 5 , 60 and 90 ra in o f d rop depos i t ion . Th e

T a b l e 3

C o n t a c t a n g l e s o f C C 14 o n m e m b r a n e a c t i v e l a y e r im m e r s e d i n p u r e

w a t e r ( < 0 . 2 ~ tS /c m , p H = 6 . 5 , i m m e r s i o n t i m e > l w e e k , w a t e r d a i l y

c h a n g e d ) m e a s u r e d a f te r 1 , 5 , 6 0 a n d 9 0 m i n o f d r o p d e p o s i t io n

M e m b r a n e T i m e ( m i n ) 0 ( °)

C A - 3 1 6 - 5 0 1 9 9 4 - 2

5 1004-3

6 0 9 9 4 - 3

9 0 1 0 0 ± 3

C D - N F - 5 0 1 1264-3

5 1 2 7 + 2

60 1254-1

90 1274-3

resu l t s a re show n in Tab le 3 . We tes ted on ly two

m e m b r a n e s , C A - 3 1 6 - 5 0 a n d C D - N F - 5 0 , b e c a u s e

amo n g t h e f i v e memb ran es s t u d i ed t h ey h av e t h e

l a rg e r p o re s ( s ee T ab l e 2 ), t h u s b ecau s e t h e o n es fo r

w h o m t h e re i s a p o s s ib l e v a r i a t io n o f t h e co n t ac t an g l e

w i t h d ep o s i t i o n t i me . T h e memb ran es w e re fu l l y

h y d ra t ed s i n ce t h ey h ad b een s t o r ed i n w a t e r f o r , a t

l e a st , o n e w eek ( t h e w a t e r b e i n g ch an g ed e v e ry d ay ) .

F ro m T ab l e 3 , f iv e mi n u t e s a r e m o re t h an s u f f i c ien t

t o r each t h e eq u i l i b r i u m b e t w een t h e t w o i mmi s c i b l e

s o l u t i o n s an d t h e s u r f ace . T h e re fo re , d y n ami c mea -

s u remen t s a r e n o t n eces s a ry , t hi s b e i n g o n e o f t h e g r ea t

ad v an t ag es o f t h e t w o - l i q u i d s i mmers i o n me t h o d , a s

d i scussed p rev ious ly .

The s t ab i l i sa t ion t ime was op t imised wi th a d i lu te

(0 .3 k g / m 3 ) aq u eo u s s o l u t io n o f 2 - ch l o ro b en z o i c ac i d

(pro analys i s , Merck) , one o f the so lu tes tha t wi l l be

i m p o r t a n t t o s t u d y in t e r m s o f m e m b r a n e - s o l u t e c h e -

mi ca l i n t e r ac t i o n s . Co n t ac t an g l e s o f C C 1 4 o n m e m -

b ran e ac t i v e l ay e r i mmers ed i n ch l o ro b en zo i c ac i d

s o l u t io n fo r d i f f e r en t p e r i o d s o f t i me w ere m eas u red

a f t e r 5 mi n o f d ro p d e p o s i t i o n t ime . T h e r e s u lt s a r e

p re s en t ed i n T ab l e 4 .

F ro m T ab l e 4 w e c o n c l u d e t h a t th e o p t i m u m s ta -b i l is a t io n t i me i s 2 4 4 8 h . L o n g e r t ime p e r i o d s m ay

cause neg at ive s ide ef fec t s , such as so lu te degra dat ion

an d / o r memb ran e d eg rad a t i o n .

Co m p ar i n g T ab l e s 3 an d 4 w e co n c l u d e t h a t th e

v a l u es o b t a i n ed w i t h p u re w a t e r a r e d i f f e r en t f r o m

t h o s e o b t a i n ed w i t h 2 - ch l o ro b en zo i c ac i d s o l u ti o n , i .e .

t h e t w o - l i q u i d s i mmers i o n me t h o d d ev e l o p ed i s s en -

s i t ive to the var ia t ion o f the immers ion so lu t ion (p re-

s en ce o f s o l u te an d / o r s o l u t i o n p H ) .

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174 M.J. Rosa, M.N. de Pinho/Journal of Membrane Science 131 (1997) 167-180

Table 4

Contact angles of CC14 on mem brane active layer imm ersed in 2-chlorobenzoic acid solution (pH= 2.9) for different periods of time

(values measured at room temperature, after 5 min of drop

deposition)

Membrane Time (h) 0 (°)

CA-316-50 1 94±5

24 95±3

48 94±3

CA-316-68 1 98±3

24 97±4

48 95±3

Table 5Contact angles of CC14on mem brane active ayer immersed in pure

water (<0.2 gS/cm, pH =6.5, im mersion time>l week, water daily

changed, values measured at room temperature, after 5 min o f drop

deposition)

Membrane 0 (°)

CA-316-50 100+3

CA-316-68 1045:2

CA-316-86 107± 1

CD-NF-50 127±2

HR-98-PP 146±3

CA-316-86

CD-NF-50

HR-98-PP

1 99±3

24 102±4

48 101 ± 9

14 1394-5

38 141±5

86 141±3

14 140-4-10

38 145±5

86 147±6

4 . R e s u l t s a n d d i s c u s s i o n

4 . 1. I m m e r s i o n s y s t e m I." C C 1 4 - m e m b r a n e - w a t e r

T a b l e 5 s h o w s t h e r e s u l t s r e l a t i v e t o t h e C C 1 4 -

m e m b r a n e - w a t e r s y s te m . F o u r m a i n c o n c l u s io n s

c a n b e d r a w n .

F i r s t , t h e m e t h o d i s r e p r o d u c i b l e a n d h y s t e r e s i s i s

c o n t r o l l e d a s t h e v a l u e s o f c rn - 1 t 9 5 % / x / n r a n g e f r o m 1

t o 3 ° T h e s e v a l u e s a r e s i m i l a r t o ( o r e v e n l o w e r t h a n )

t h o s e r e p o r t e d i n t h e l i t e r a t u r e ( e . g . 0 . 5 - 2 ° [ 9 ] , 4 - 1 6 °

[ 1 2 ]) a n d c o n s i d e r e d t o b e p o s s i b l e t o o b t a i n b y a u t h o r

( i. e . 2 ° [ 2 2 ] ). B e c a u s e t h e y a r e r e l a t i v e t o , a t l e a s t , t w o

m e m b r a n e s a m p l e s t h e y i n d i c a t e t h e m a c r o s c o p i c

h o m o g e n e i t y o f t h e m e m b r a n e a c t iv e l a ye r.S e c o n d , t h e c o n t a c t a n g l e s a r e l a r g e r t h a n 9 0 ° f o r a l l

t h e m e m b r a n e s , t h e r e b y t h e m e m b r a n e s d o p re f e r th e

a q u e o u s s o l u t i o n t o C C 1 4 , i . e . t h e f i v e m e m b r a n e s

s t u d i e d a r e h y d r o p h i l i c . I n a d d i t i o n , t h e s e r e s u l t s

i n d i c a t e t h e r e l a t iv e h y d r o p h i l i c i t y o r d e r o f t he

m e m b r a n e s :

C A - 3 1 6 - 5 0 < C A - 3 1 6 - 6 8 _< C A - 3 1 6 - 8 6

< C D - N F - 5 0 < H R - 9 8 - P P

T h i r d , f o r th e C A - 3 1 6 s e r ie s t h e c o n t a c t a n g l e

i n c r e a s e s w i t h t h e i n c r e a s e o f th e a n n e a l i n g t e m p e r a -

t u re . A c c o r d i n g t o T a b le 2 , t h e m e m b r a n e p o r e r a d i u s

d e c r e a s e s w i t h t he a n n e a l i n g t e m p e r a t u r e . A s a r e su l t ,

i n th is s e r ie s o f h o m o l o g o u s m e m b r a n e s o f th e s a m e

p o l y m e r t h e c o n t a c t a n g l e d e c r e a s e s w i t h t he i n c r e a s e

o f m e m b r a n e p o r e r a d i u s ( s e e T a b l e s 2 a n d 5 ). I n t u m ,

t h e c o n t a c t a n g l e d e v i a t i o n , ~ r n - t t 9 5 % / v ~ , d e c r e a s e s

t h r o u g h C A - 3 1 6 s e r i e s , w h i c h m u s t b e d u e t o t h e

h i g h e r m e m b r a n e s u r f a c e h o m o g e n e i t y o b s e r v e d f o r

m e m b r a n e s w i t h l o w e r a v e r a g e p o r e r a d i u s [ 2 2 ] .

T h e v a r i a t i o n o f th e c o n t a c t a n g l e t h r o u g h C A - 3 1 6

s e r i e s , a l t h o u g h i t i s n o t m u c h p r o n o u n c e d ( 7 ° ) ,

r e v e a l s a n in t r in s i c d e c r e a s e o f t h e m e m b r a n e h y d r o -

p h i l± c i t y w i t h t h e p o r e r a d i u s . I f a n a p p a r e n t e f f e c t

s u c h a s m e m b r a n e r o u g h n e s s w a s i n q u e s t i o n , t h e

o p p o s i t e t r e n d w o u l d b e o b s e r v e d , i . e . t h e c o n t a c t

a n g l e w o u l d i n c r e a s e w i t h p o r e r a d i u s , b e c a u s e w h e n

t h e t r u e a n g l e i s > 9 0 ° th e a p p a r e n t a n g l e o n a r o u g h

s u r f a c e i s g r e a t e r t h a n t h e t r u e v a l u e [ 1 7 ] .

O n t h e o t h e r h a n d , i t is n o t r e a s o n a b l e t o j u s t i f y

t h e o b s e r v e d t r e n d b a s e d o n d r o p p e n e t r a t i o n

e f f e c t s . T h e m e m b r a n e p o r e r a d i i a p p r o a c h m o l e c u l a r

d i m e n s i o n s a n d i t i s n o t p o s s i b l e f o r a C C 1 4 d r o p t o

p e n e t r a t e t h e p o r e s o n l y b y c a p i l l a r y f o r c e s . W h e n

i m m e r s e d i n w a t e r , a C C I 4 d r o p l e t 5 m m h e i g h tw i l l e x e r t a g r a v i t a t i o n a l f o r c e o f a b o u t 3 0 N / m 2

( A p = A p g h , w i th A p ~ 6 0 0 k g /m 3 ). A c c o r d i n g

t o t h e L a p l a c e e q u a t i o n f o r c y l i n d r i c a l p o r e s ( w h i c h ,

i n f a c t , i s m o r e a d e q u a t e f o r m i c r o p o r o u s s u r f a c e s

w i t h r m a x "~ 1 0 - 6 m ) , t h is g r a v i t a t i o n a l f o r c e

w o u l d c o r r e s p o n d t o a " ~ ow c o s / 9 v a r i a t i o n o f

7 5 x 1 0 - 1 ° N / m ( i. e. 1 / 2 A p r m a x w i t h r m a x = 5 ,~ ), a

r a t h e r s m a l l v a l u e c o m p a r e d t o t h e u s u a l 3 'o w r a n g e o f

1 0 - 3 N / m .

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M.J. Rosa, M.N. de Pinh o/Jo urna l o f Mem brane Science 131 (1997) 167 -180 175

Thus , the in t r ins ic var i a t ion o f the hyd roph i l i c i ty in

CA -3 1 6 s e r i e s mu s t b e r e l a t ed t o t h e an n ea l i n g t em-

p e ra t u r e u s ed t o o b t a i n mem b ran es w i t h d i f f e r en t p o re

rad ius .

Actual ly , i t i s repor ted in the l i t e ra tu re tha t mem-b ran es o f t h e s ame p o l y mer can p r e s en t s i g n i f i c an t

var ia t ions o f hydrophi l i c i ty [13] because d i f feren t

co n d i t i o n s o f p r ep a ra t i o n ( e . g . an n ea l in g t emp era -

t u r e s ) c an i n d u ce co n fo rm a t i o n a l ch an g es i n t h e mem -

b ran e p o l y mer i c ma t r i x [2 3 -2 6 ] . I n ad d i t i o n , t h e

au t h o r s o b s e rv e d r ecen t l y t h e i n f l u en ce o f t h e an n ea l -

i n g t emp e ra t u r e o n t h e o p t i ca l an i so t ro p y , an d t h e r e -

fo r e o n t h e ma c ro m o l ecu l a r o rg an i s a ti o n o f t h e ac t iv e

l ay e r o f t h e s e CA -3 1 6 m em b ran es [2 7] .

In fac t , the an neal ing cons i s t s on the in t rodu ct ion o f

t h e rma l en e rg y w h i ch cau s e s t r an s l a t i o n a l mo t i o n o ft h e mac ro m o l ecu l e s an d a l l o w s t h e ap p ro ach o f th e

p o l a r g ro u p s t o fo rm v i r t u a l c ro s s - l i n k s b y d i p o l e -

d ipo le in terac t ions [25] . These cross - l inks t end to

decrease the chain mobi l i ty and , as the anneal ing i s

p e r f o r m e d i n a w a t e r b a th ( a n o n -s o l v e n t m e d i u m f o r

ce l lu lose ace ta te) , they resu l t in an i r revers ib le mem-

brane shr inkage . Th i s sh r inkage i s respons ib le fo r a

d ec rea s e i n t h e av e rag e p o re r ad i u s ( s ee T ab l e 3 )

w h i ch h i n d e r s t h e w a t e r t r an s p o r t t h ro u g h t h e mem-

b ran e ma t r i x an d t h u s r ed u ces t h e memb ran e w a t e r

p e rmeab i l i t y . A T R/ FT IR s t u d i e s w i t h t h e s e CA -3 1 6

mem b ran es s h o w ed t h a t , in f ac t , b o t h t h e w a t e r co n -

t en t an d t h e s t a t e o f w a t e r s o rb ed i n t h e memb ran e

act ive l ayer vary wi th the an neal ing ( i . e . wi th increas -

ing t empera tu re there i s a los t in water con ten t

t o g e t h e r w i t h a d ec rea s e i n h y d ro g en b o n d i n g an d

c l u s t e r s iz e o f t h e w a t e r s o rb ed b y t h e ac t i v e l ay e r o f

t h e memb ran e ) [2 8 ] . N ev e r t h e l e s s , t h e memb ran e

s h r i n k ag e i s a l s o r e s p o n s i b l e fo r a mac ro s co p i c

i n c r ea s e o f th e -O H s u r f ace d ens i ty , w h i ch i n c r ea s e s

t h e memb ran e s u r f ace h y d ro p h i l i c i t y ( i . e . i n c r ea s e s

t h e co n t ac t an g l e ).

T h i s i n c r ea s e i n mem b ran e h y d ro p h i l i c i t y w i t h t h ean n ea l i n g t emp era t u r e i s a v e ry i mp o r t an t r e s u l t

b ecau s e i t ch emi ca l l y f av o u r s t h e w a t e r t r an s p o r t

(memb ran e -w a t e r ch emi ca l i n t e r ac t i o n s a r e f av o u red ,

an d s o a r e t h e w a t e r t r an s p o r t an d t h e memb ran e

c l ean i n g w i t h aq u eo u s s o l u t i o n ) , t h u s en h an c i n g t h e

r e l a t i v e memb ran e p e rmea t i o n p e r fo rman ce . T h i s i s ,

C A - 3 1 6 - 8 6 m e m b r a n e i s le s s p e r m e a b l e th a n C A - 3 1 6 -

5 0 m e m b r a n e ( C A - 3 1 6 -8 6 h a s a d e n s e r p o ly m e r i c

s t ruc tu re , wi th h igher s t e r i c h indrances ) bu t i t would

p ro b ab l y p r e s en t h i g h e r r e l a t i v e f l u x es (p e rm ea t e f l u x

w i t h t h e aq u eo u s s o l u t io n o v e r t h e p u re w a t e r f l ux ) an d

f lux recover ies ( ra t io o f f ina l to in i t i a l pure water

f l u xes d e t e rmi n ed , r e s p ec t iv e l y , a f t e r an d b e fo re p e r -

mea t i n g t h e s o l u t i o n ) t h an CA -3 1 6 -5 0 memb ran e .T h e fo u r t h co n c l u s i o n t o b e t ak en co n ce rn s t h e

c o r r e l a t i o n o f m e m b r a n e p e r m e a t i o n p e r f o r m a n c e

( i n t e rms o f h y d rau l i c p e rmeab i l i t y , T ab l e 2 ) w i t h

me mb ran e h y d ro p h i l i c i t y (T ab l e 5 ). T h e r e s u l ts r e l a -

t i v e t o CD -N F-5 0 an d H R -9 8 -PP c l ea r l y i l lu s t r at e t h a t

t h e s t e r ic h i n d ran ces a r e n o t t h e o n l y f ac t o r p l ay i n g an

i mp o r t an t r o l e o n w a t e r t r an s p o r t. O t h e r e f f ec t s s u ch

as mem b ran e -w a t e r ch em i ca l in t e r ac ti o n s , i .e . h y d ro -

p h i l i c i t y ( an d / o r t h e n u mb er o f p o re s i n t h e ac t i v e

l ay e r ) a r e mo s t i mp o r t an t an d can b e ev a l u a t ed b y

co n t ac t an g l e meas u rem en t s ( h y d rau l i c r e s is t an ce d u et o memb ran e t h i ck n es s a l s o i n t e rv en es an d i s n o t

acco u n t ed i n s u p e r f ic i a l mea s u reme n t s s u ch a s co n t ac t

an g l e s ) . CD -N F-5 0 h as an av e rag e p o re r ad i u s o f

4 . 2 , ~, an i n t e rmed i a t e v a l u e b e t w een t h o s e o f C A -

3 1 6 -5 0 an d CA -3 1 6 -6 8 memb ran es ( r e s p ec t i v e l y ,

b e t w een 4 . 4 an d 4 . 0 ,~ ) b u t i t i s mo re h y d ro p h i l i c

t h an t h e s e memb ran es , i . e . i t h a s a co n t ac t an g l e o f

1 27 ° , w e l l ab o v e t h o s e o f CA -3 1 6 mem b ran es (1 0 0 °

an d 1 0 4 ° f o r CA -3 1 6 -5 0 an d C A -3 1 6 -6 8 , r e s p ec -

t ive ly ) . Therefore , a l though wi th in termedia te s t e r i c

h i nd r a nc e s , C D - N F - 5 0 h a s s t r on g e r m e m b r a n e - w a t e r

ch em i ca l i n t e r ac ti o n s t h an CA -3 1 6 m emb ran es w h i ch

re s u l t s i n t h e h i g h memb ran e h y d rau l i c p e rmeab i l i t y

o f C D - N F - 5 0 m e m b r a n e o v e r t h e C A - 3 1 6 m e m b r a n e s

(2 .34 m agains t 1 .55 m and 0 .90 m, respec t ive ly fo r

C A - 3 1 6 - 5 0 a n d C A - 3 1 6 - 6 8 m e m b r a n e s ) . T h e s a m e

e f f ec t , b u t ev en mo re p ro n o u n ced , i s f o u n d w i t h H R-

9 8 - P P m e m b r a n e o v e r C A - 3 1 6 -8 6 m e m b r a n e . H R - 9 8 -

PP h as a s ma l l e r p o re r ad i u s b u t i t i s mu ch mo re

h y d ro p h i l i c an d 1 0 t imes m o re w a t e r p e rmea b l e t h an

CA -3 1 6 -8 6 (p o re r ad iu s o f 2 . 8 A , co n t ac t an g l e o f

1 46 ° an d h y d rau l i c p e rm eab i l i ty o f 1 .6 3 m fo r H R -9 8 -

PP agains t 3 .3 ,~ , 107 ° and 0 .16 m for C A-31 6-86) .

T h es e r e s u l t s co n t r ib u t e b o t h t o th e co m p reh en s i o n o f

t r an s p o rt p h e n o m e n a t h r o u g h N F / R O m e m b r a n e s a n d

t o mem b ran e s e l ec t io n fo r a g i v en ap p l i ca ti o n .

S i m i la r c o m p a r i s o n o f th e m e m b r a n e p e r f o r m a n c e

w i t h aq u eo u s s o l u t i o n s (o t h e r t h an p u re w a t e r a t

na tu ra l pH) and the resu l t s in Tab le 5 i s no t poss ib le .

A s d i s cu s s ed p r ev i o u sl y , i n mo s t c a s e s , mem b ran e an d

so lu te a re bo th ion i sab le subs tances and , therefo re , the

m e m b r a n e - s o l u t e - s o l v e n t c h e m i c a l i n te r a c ti o n s h a v e

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17 6 M.Z Rosa, M.N. de Pinho/Journal of Membrane Science 131 (1997) 167-180

14 0

1 2 0

C

100

i"-

o 8 0o

6 0

1 4 0 1 4 0

1 2 0

1 0 0

8 0

1 2 0

C A - 3 1 6 - 6 8

4 6 8 10 12

pH

1 0 0

8 0

C A - 3 1 6 - 8 6

6 0 _ 6 0 ~ . . . . . . .

2 4 6 8 10 12 2 2 4 6 8 10 12

pH pH

( a ) ( b ) ( c )

F i g . 4. T i t r a ti o n c u r v e s o f t h e t h r e e C A - 3 1 6 m e m b r a n e s : ( a ) C A - 3 1 6 - 5 0 , ( b ) C A - 3 1 6 - 6 8 , ( c ) C A - 3 1 6 - 8 6 ( c o n t a c t a n g l e s m e a s u r e d a t r o o m

t e m p e r a t u r e , a ft e r 5 m i n o f d r o p d e p o s i t io n , i m m e r s i o n t i m e = 2 4 - 4 8 h ) .

L i f s h i tz - V a n - d e r - W a a l s a n d a c i d - b a s e c o n t ri b u ti o n s.

T h u s , t o e s t i m a t e m e m b r a n e - s o l u t e i n t e r a c t i o n s t h e

c o n t a c t a n g l e s o b t a i n e d w i t h w a t e r a t o n l y o n e p H

v a l u e d o n o t s u f f i c e . T h ey d o n o t i n d i ca t e m em b r an e

ch a r g e s i g n a l b u t o n l y t h e r e l a t i v e s u r f ace ch a r g e

d en s i t y . T o ev a l u a t e m em b r an e ac i d i t y / b a s i c i t y o n e

h as t o h av e v a l u e s a t d i f f e r en t p H v a l u es e i t h e r w i t h

p u r e w a t e r o r, a s i n t h e p r e s en t w o r k , w i t h a s u r f ac t an t

so lu t ion .

4 .2 . I m m e r s i o n s ys t e m H : C C I 4 - m e m b r a n e -

s u r f a c t a n t s o l u t i o n

F i g s . 4 - 6 s h o w t h e r e s u l t s r e l a t i v e t o C C l 4 - m e m -

b r a n e - C 1 6 T A B s o l u t i o n a t d i f f e ren t p H v a l u es , i . e . th e

t it ra t io n c u r v e s o f C A - 3 1 6 , C D - N F - 5 0 a n d H R - 9 8 - P P

m e m b r a n e s , r e s p e c ti v e ly . T w o i m m e d i a t e c o n c l u s i o n s

c a n b e d r a w n .

F i r s t , t h e co n t ac t an g l e h y s t e r e s i s i s n o t m u chi n c r e a s e d w h e n p u r e w a t e r i s r e p l a c e d b y C 1 6 T A B

di lu te so lu t ions . The va lues o f ~r ,_ l t 95% /x /~ are 1 - 3 °

in the f i r s t case and 2 - 7 ° in the second .

S eco n d , a l l th e m em b r an es p r e s en t a v a r i a t io n o f t h e

co n t ac t an g l e w i t h t h e s o l u t i o n p H a s ex p ec t ed f r o m

t h e c h e m i c a l n a tu r e o f th e m e m b r a n e p o l y m e r s

i n v o l v ed .

F i g . 4 p r e s en t s t h e t i t ra t i o n cu r v es o f th e C A - 3 1 6

m e m b r a n e s , a s e ri e s o f w e a k l y a c i d ic m e m b r a n e s d u e

1 4 0

v

t~¢-

13

8

1 3 0

1 2 0

1 1 0

1 0 0

2 4 6 8 10 12

pH

F i g . 5 . T i t r a t i o n c u r v e o f C D - N F - 5 0 m e m b r a n e ( c o n t a c t a n g l e s

m e a s u r e d a t r o o m t e m p e r a t u r e , a f t e r 5 m i n o f d r o p d e p o s i t i o n ,

i m m e r s i o n t i m e = 2 4 - - 4 8 h ).

t o t h e - - O H g r o u p s o f c e l l u l o s e ace t a t e ( s ee S ec t i o n3 .1 ) . As i l lus t r a ted in th i s f igure , the th ree CA-316

m em b r an es p r e s en t s i m i l a r t i t r a t i o n cu r v es w i t h a

m i n i m u m a t p H 5 - 6 , t h e p H t h a t c o r r e s p o n d s t o t h e

i o n i sa t i o n o f 5 0 % o f th e - O H g r o u p s ( t he a c i d i ty

co n s t an t o f a w ea k ac i d s u ch a s an a l co h o l i s a r o u n d

1 0 - t O t O 1 0 - 1 2 , i .e . p K 1 / 2 i s 5 - 6 ) . T h e cu r v es i n F i g . 4

h av e t h e ex p ec t ed s h ap e o f t h e t h eo r e t i c a l t it r a t io n

cu r v e s h o w n i n F i g . 3 ( b ) f o r a c i d i c m em b r an es w i t h

d o u b l e l ay e r f o r m a t i o n . B es i d e s t h e s am e s h ap e ,

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M.J. Rosa, M.N. de Pinho/Journal of Membrane Science 131 (1997) 167-180 17 7

170

150

e.-

~ 1 3 0

i

1 1 0 . . . . J

2 4 6 8 10 12

pH

F i g . 6 . T it r a ti o n c u r v e o f H R - 9 8 - P P m e m b r a n e ( c o n t a c t a n g le s

m e a s u r e d a t r o o m t e m p e r a t u r e , a f t e r 5 m i n o f d r o p d e p o s i t i o n ,

i m m e r s i o n t i m e = 2 4 - - 4 8 h ) .

curves 4 (a) , 4 (b ) and 4 (c) d i sp lay s ign i f i can t d i f fer -

ences . The f i r s t p la teau i s iden t i ca l fo r the th ree

mem b ran es , 1 03 ° f o r CA -3 1 6 -5 0 an d 1 1 0 ° f o r CA -

3 1 6 -6 8 an d C A -3 1 6 -8 6 , b u t t h e mi n i ma l v a l u e o f t h e

con ta c t ang le increase s in the ser i es, be ing 80 ° 95 °

an d 1 05 ° f o r CA -3 1 6 -5 0 , C A -3 1 6 -6 8 an d CA -3 1 6 -8 6 ,

r e s p ec t iv e l y . Fu r t h e rm o re , t h e p H a t w h i ch t h e s eco n d

p l a t eau ( co m p l e t e d o u b l e l ay e r f o rma t i o n ) i s r each ed

al so increases , 7 .5 , 9 and 10 fo r CA-316 -50 , CA -316-

6 8 an d CA -3 1 6 -8 6 , r e s p ec t i v e l y . T h es e o b s e rv a t i o n s

i n d i ca t e th a t t h e m em b ran e s u r f ace ac i d i ty i n c r ea s e s i n

th i s ser i es , in ag reem ent w i th the resu l t s re l a t ive to the

me mb ran e -C C1 4 -w a t e r s y s t em. A t l o w p H v a l u es , t h e

-O H g ro u p s a r e ma i n l y n o t i o n i s ed , t h e r eb y t h e co n -

t ac t an g l e s a r e s i mi l a r f o r t h e t h r ee memb ran es . Fo r

p H v a l u es co r r e s p o n d i n g t o t h e i o n i s a ti o n o f t h e -O H

g ro u p s , i. e. f o r p H > 3 , t h e mem b ran e w i t h h i g h e r -O H

surface dens i ty (h igher ac id i ty ) p resen t s a h igher

n eg a t i v e ch a rg e , t h e r e fo re p r e s en t i n g h i g h e r C C 1 4

con tac t ang les .F i g . 5 p r e s en t s th e t i t r a ti o n cu rv e o f t h e CD -N F-5 0

m e m b r a n e , a n a m p h o t e r i c m e m b r a n e w i t h a p r e d o -

mi n an t l y ac i d i c b eh av i o u r d u e t o t h e ca rb o n y l an d

s u l p h u r g ro u p s o f T F Z ( s ee Sec t i o n 3 . 1 ) . T h e t i t ra t i o n

cu rv e o f t h i s memb ran e s h o w s a p l a t eau fo r p H

ran g i n g f ro m 3 t o 5 , th en an i n c r ea s e u p t o p H 9 a f t e r

w h i ch a s eco n d p l a t eau i s p ro b ab l y r each ed . T h i s

cu rv e ag ree s w i t h t h e t h eo re t i c a l cu rv e s h o w n i n

F i g . 3 ( c ) f o r a c i d -b as e memb ran e w i t h d o u b l e l ay e r

fo rma t i o n . Bo t h f ac t s , t h e ab s en ce o f t h e mi n i mu m

an d t h e h i g h s l o p e o f t h e cu rv e i n d i ca t e t h e s t r o n g

ac i d i c ch a rac t e r o f t h e memb ran e ( ca rb o n y l an d s u l -

p h u r g ro u p s ) r e s p o n s i b l e fo r t h e f a s t ch an g e f ro m a

n eu t r a l t o a h ig h l y n eg a t i v e mem b ran e ch a rg e (d o u b l el ay e r f o rma t i o n ) . T h e v a l u e o f pK x / 2 i s a ro u n d 5 -6

(ca rb o n y l an d s u l p h u r g ro u p s ) an d t h e max i mu m

negat ive charge i s a t t a ined a t pH above 9 . Actual ly ,

F i g . 5 ex t en d s t h e t en d en cy o b s e rv ed t h ro u g h CA -3 1 6

ser ies (Fig . 4 (a -c) ) , i .e . CD -NF -50 (carbo nyl and

su lphur g roups) has an ac id ic charac ter s t ronger than

t h a t o f CA -3 1 6 -8 6 memb ran e (h y d ro x y l g ro u p s ) .

F i g . 6 p r e s en t s t h e t i tr a t io n cu rv e o f th e H R-9 8 -P P

m e m b r a n e , a n a m p h o t e r i c m e m b r a n e w i t h a p r e d o -

mi n an t l y b a s i c b eh av i o u r f ro m t h e ami d e g ro u p s o f PA

(s ee Sec t i o n 3 . 1 ). T h e t i t ra t i o n cu rv e o f t h i s me mb ran e

s h o w s a s l i g h t d ec r ea s e o f t h e co n t ac t an g l e fo r p H

ran g i n g f ro m 5 . 6 to 9 an d t h en a s t eep d ec rea s e f ro m

pH 9 to 11 . Th i s curve agrees wi th the theore t i ca l

cu rv e fo r b a s ic m emb ran es s h o w n i n F i g . 3 ( a ) ( t h e

v a r i a t i o n o f t h e co n t ac t an g l e o ccu r s f o r p H v a l u es

above neu t ra l i ty ) . Th i s ind ica tes tha t , in fac t , the bas ic

ch a rac t e r o f t h e memb ran e s u r f ace f a r ex ceed s i t s

ac id ic charac ter , be ing the ion i sa t ion o f the ac id ic

g ro u p s o n l y s i g n i f ic an t f o r p H ab o v e 1 0 ( fo r co m p ar -

i son , carboxyl ic g roups are 100% ion i sed a t pH 11) .

Su mm ar i s in g , f r o m F i g s. 4 -6 , o n e o b t a in s t h e ac i d -

b as e b eh av i o u r o f t h e f i v e memb ran es :

• CA -3 1 6 mem b ran e s e ri e s p r e s en t an ac i d ic ch a r -

ac t e r ( f ro m t h e -O H g ro u p s o f c e l l u l o s e ) w i t h a

p K 1/2 ~ 5 -6 an d w i t h max i m u m n eg a t i v e ch a rg e a t

pH above 7 .5 , 9 and 10 fo r the anneal ing t em-

pera tu res o f 50 , 68 and 86°C , respect ive ly . Th i s

t rend represen t s the increase in the su r face con-

c e n t ra t io n o f - O H g r o u ps p r o m o t e d b y t h e a n n ea l -

ing t rea tment ;

• CD -N F-5 0 me mb ran e p r e s en t s an ac i d i c ch a rac t e r

( f ro m t h e ca rb o n y l an d s u l p h u r g ro u p s o f T FZ )

s t ro n g e r t h an t h e CA -3 1 6 m emb ran es : p K t / 2 " ~ 5 - 6a n d m a x i m u m n e g a ti v e c h a rg e a t p H > 9 ;

• H R -9 8 -PP me mb ran e h as a b a s ic ch a rac t e r ( f ro m

t h e ami d e g ro u p s o f PA ) p r e s en t in g n eu t r a l o r

w eak l y n eg a t i v e ch a rg e a t p H 9 .

T h es e d a t a i n d i ca t e t h a t , w h en p e rmea t i n g p u re

w a t e r ( i . e . a t p H 6 . 5 ) , H R-9 8 -PP memb ran e i s p o s i -

t i v e l y ch a rg ed w h i l e t h e o t h e r f o u r memb ran es a r e

n eg a t i v e l y ch a rg ed . Bas ed o n t h e mag n i t u d e o f t h e

co n t ac t an g l e v a lu es , a t p H 6 .5 t h e m em b ran e s u r f ace

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17 8 M.J. Rosa, M.N. de Pinho/Journal o f Membrane Science 131 (1997) 167-180

ch a r g e d en s i t y f o l l o w s t h e s eq u en ce :

C A - 3 1 6 - 5 0 ( - ) < C A - 3 1 6 - 6 8 ( - )

< C A - 3 1 6 - 8 6 ( - ) < C D - N F - 5 0 ( - )

< H R - 9 8 - P P ( + )

T h es e r e s u l t s a r e i n ag r eem en t w i t h t h o s e r e l a t i v e t o

p u r e w a t e r a n d t h e y e x p l a i n t h e h i g h w a t e r p e r m e -

a b i li ti e s o f t he s t r o n g l y a ci d ic C D - N F - 5 0 m e m b r a n e

a n d t h e s t ro n g l y b a si c H R - 9 8 - P P m e m b r a n e o v e r th e

l e ss e n e r g e ti c C A - 3 1 6 m e m b r a n e s o f e q u i v a le n t p o r e

rad ius .

D u e t o t h e i r a c i d i c c h a r a c t e r , C D - N F - 5 0 a n d C A -

3 1 6 m em b r an es a r e ad eq u a t e t o p r o ces s a l k a l i n e

aq u eo u s s o l u t io n s o f an i o n ic s o l u t e s ( e .g . a l co h o l s ,

o r g an i c ac i d s ). T h e m em b r an e an d t h e so l u t e ( s ) w o u l d

b e b o t h h i g h l y n e g a t i v e l y c h a r g e d w h i c h m a x i m i s e s

m e m b r a n e - s o l u t e ( s ) r e p u l s i o n a n d i n c r e a s e s m e m -

b r an e p e r m eab i l i t y an d s o l u t e re j ec t i o n . F o r an a l o g o u s

r ea s o n s , H R - 9 8 - P P i s ad eq u a t e t o p r o ces s ac i d aq u -

eo u s s t r eam s w i t h ca t i o n i c s o l u t e s .

• i t i s s en s i t i v e t o m e m b r an e h y d r o p h i l i c i t y an d

m e m b r a n e a c i d it y / b as i c it y ;

• i t g i v e s t h e m em b r an e ti t ra t i o n cu r v es an d t h e

p K 1/ 2 v a l u e s f o r w e ak l y ac i d i c , p r ed o m i n an t l y

a c i d i c a n d p r e d o m i n a n t l y b a s i c m e m b r a n e s .F u r t h e r m o r e , t h e ex p e r i m en t a l d a t a s h o w t h e e f f ec t

o f t h e a n n e a li n g t r e a tm e n t o n C A - 3 1 6 m e m b r a n e

ac i d it y , w h i ch s u g g es t s an i n c r ea s e o f th e s u r f ace

d e n s it y o f - O H g r o u p s w i t h a n n e a l i n g t e m p e r a t u re .

T h e s e d a t a a l s o s h o w t h e i m p o r t a n c e o f t he c h e m i c a l

p r o p e r ti e s o f m e m b r a n e s u r f a c e on m e m b r a n e p e r m e -

ab i l i ty - i . e . m em b r an es w i t h eq u i v a l en t p o r e r ad ii b u t

m o r e h y d r o p h i l i c d o h a v e m u c h ( 2 - 1 0 t i m e s ) h i g h e r

h y d r au l i c p e r m eab i l i t i e s .

T h i s m e t h o d t h e r e f o r e r ep r e s en t s a co n t r i b u t i o n t o

t h e c h a r a c te r i s a ti o n o f m e m b r a n e s u r fa c e c h e m i s t r y

a n d i t s r e l a t i o n w i t h m e m b r a n e p e r m e a t i o n p e r f o r -

m a n c e .

6 . L i s t o f s y m b o l s

5 . C o n c l u s i o n s

A n e w m e t h o d f o r m e m b r a n e s u r f a c e c h a ra c t e r is a -

t i o n i s d ev e l o p ed b as ed o n t h e t w o - l i q u i d s i m m er s i o n

s y s t e m . T h e g e n e r a l s y s te m C C 1 4 - m e m b r a n e - a q u -

eo u s s o l u t i o n i s ad ap t ed t o g i v e m em b r an e r e l a t i v e

h y d r o p h i l i c i t y an d m em b r an e ac i d i t y / b a s i c i t y . I n t h e

f i r s t c a s e t h e i m m er s i o n s o l u t i o n i s p u r e w a t e r an d i n

t h e s eco n d ca s e i s an aq u eo u s s o l u t i o n o f a c a t i o n i c

s u r f ac t an t ( b e l o w i t s c r it i c a l m i ce l l e co n c en t r a t i o n ) a t

d i f f e r en t p H v a l u es .

T h e ex p e r i m en t a l r e s u l t s w i t h f i v e n an o f i l t r a t i o n

m e m b r a n e s o f d if f e re n t p o l y m e r s d e m o n s t r a te d t h at :

• t h e m e t h o d i s r ep r o d u c i b l e , p r e s en t i n g m a x i m a l

dev ia t ion va lues , an -1 t95%/x /~ , o f 7 ° ( co r r e-

s p o n d i n g t o 5 %) ;• i t i s e a s y t o p e r f o r m a n d a v o i d s d y n a m i c m e a s u r e -

m e n t s a s 5 m i n o f d r o p d ep o s i t i o n a r e s u f f i c ien t t o

en s u r e eq u i l i b r i u m ;

• m e a s u r e m e n t s c a n b e p e r f o r m e d w i t h m e m b r a n e s

i m m er s ed i n t h e i r n a t u r a l o p e r a t i n g en v i r o n m en t ,

e .g . water , so lu t ions o f ca t ion ic su r f ac tan ts , so lu -

t io n s o f o r g a n o c h l o r i n a te d c o m p o u n d s ;

• t h e m e t h o d i s n o t a f f ec t ed b y th e p r e s en c e o f p o r e s

i n t h e n an o f i l t r a t io n r an g e o f o p e r a t i o n ;

A d

C A

C 1 6 T A B

C M C

M F

n

N F

P A

P E S

pK1 / 2

P S

rmax

R O

T F Z

U F

m e m b r a n e s u r f a c e e n e r g y s p e n t o n

s o l u t e ad s o r p t i o n , m J / m 2

ce l l u l o s e ace t a t e

N - C e t y l - N , N , N - t r i m e t h y l - a m m o n i u m

b r o m i d e

c r i t i c a l m i ce l l e co n cen t r a t i o n

m i c r o f i l t r a t i o n

n u m b e r o f s a m p l e s

n an o f i l t r a t i o n

p o l y a m i d e

p o l y e t h e r s u l f o n e

p H a t w h i c h 5 0 % o f t h e p o l y m e r

g r o u p s a r e i o n i s ed

p o l y s u l f o n e

m a x i m a l m e m b r a n e p o r e r ad i us

r e v e r se o s m o s i s

poly(trans-2,5-dimethyl)piperazinthio-f u r a z a n a m i d e

u l t r af i l t r a t ion

6.1. Greek letters

A P

7 q

g r av i t a t i o n a l f o r ce ex e r t ed b y t h e C C I 4

d r o p l e t o n m e m b r a n e s u r fa c e , N / m 2

i n t e r r ac i a l t en s i o n b e t w een s u b s t an ces i

a n d j , m J / m 2

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M.J. Rosa, M.N. de Pinho/Journal of Membrane Science 131 (1997) 167-180 17 9

m e m b r a n e - C C 1 4 - s o l u t i o n c o n t a c t

ang le

6.2. Subscripts

m o in t e rf ace m em b r an e - o r g an ic

m w i n te r fa c e m e m b r a n e - w a t e r

o w in t e rf ace o r g an ic - wa te r

Acknowledgements

M aria Joao Ro sa wishes to thank J .N.I .C .T. (Por tu-

gal ) fo r f inancia l suppor t th rough a Ph .D. sch o larsh ip

(BD/1730/91-RM and BD/3788/94) , and I .C .E.M.S.

(Por tugal ) fo r f inancia l con tr ibu t ion . S pecia l thanks to

Dr . Shah (U nivers i ty o f Flor ida , Gainesv i l le , USA ) for

the p rec ious suggest ions o f work on con tac t ang les ,

Prof . B . Saramago and Prof . A. Fernandes ( IST,

L i sb o n ) f o r h av in g b o r r o wed th e wo r k s t a t io n f o r

co n tac t an g le m easu r em en t s , an d Sep a r em f o r h av in g

su p p l ied th e C D- NF- 5 0 m em b r an e .

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U l t r a f il t ra t i o n M e m b r a n e s , O r e n a s , S w e d e n , 1 9 87 .

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f i lt r a t io n m e m b r a n e s b y i n f r a r e d s p e c t ro s c o p y , E S C A , a n d

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