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COLLOIDS

AND A

C ol l o i ds and Surf aces SU R F A C E S

A: Physicochem ical and Engineer ing Aspects 125 (1997) 221 224

L S E V I E R

S p e c tr o p h o to m e tr ic a n d c o n d u c t ime tr i c d e ter min a t io n o f th e

cr it ica l m ice llar conce ntrat ion o f so d ium dod ecy l su l fa te and

c e ty l tr ime th y la m m o n iu m b r o m id e mic e llar sy stem s m o d i f ie d b y

a lcoho ls and sa lt s

I. B e n i to M .A . G a r ci a C . M o n g e J .M . S a z *, M . L . M a r i n a

D e par tam ento de Q uim ica An al l t i ca , Facul tad de Ciene ias , U nivers idad de A lca ld de H ena res , 28871 A lca ld de H enares ,

Madrid), Spain

Received 5 Augu st 1996; accepted 15 Jan ua ry 1997

Abstract

Determination of the critical micellar concentration of some modified micellar systems used as mobile phases in

micellar l iquid chromatography and micellar electrokinetic chromatography has been achieved. Sodium dodecyl

sulfate (SDS) and cetyl t r imethylam mon ium bromide (CTAB) were used as surfactants , n-propanol and n -butanol as

modifiers, a nd pho spha te buffer was included in some o f the micellar systems. 1997 Elsevier Science B.V.

Keywords: Surfactants; M icelles; M icellar systems; Sod ium dodecyl sulfate (SDS); Ce tyltrim ethy lam mo nium brom ide

(STAB); Critical micellar concentration

1 . I n t r od u c t i on

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

i t y o f o r g a n i c c o m p o u n d s i n a q u e o u s m e d i a [1 ] ,

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

s p e c i e s b y c h r o m a t o g r a p h i c t e c h n i q u e s [ 2 , 3 ] a n d

t o i m p r o v e s e n s it i v it y i n s p e c t r o p h o t o m e t r i c

d e t e r m i n a t i o n s [ 4 - 6 ] , a m o n g o t h e r f e a t u r e s o f

a n a l y t i c a l c o n c e r n . D u e t o t h e l a rg e n u m b e r o f

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

s y s t e m s , t h e i r c h a r a c t e r i z a t i o n h a s b e c o m e a n

u s u a l t a s k f o r m a n y l a b o r a t o r i e s .

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

o m e r s g r o u p i n t o a g g r e g a te s , w h o s e s h a p e a n d s i z e

d e p e n d o n t h e n a t u re a n d c o n c e n t r a t io n o f o th e r

s p e c i e s i n s o l u t i o n [ 7 - 9 ] . T h e c o n c e n t r a t i o n o f

* Corresp ondin g author . Fax: +3 4 1 885 4971.

0927-7757/97/ 17.00 1997 Elsevier Science B.V. All r ights reserved

P I I S 0 9 2 7 - 7 75 7 ( 9 7 ) 0 0 0 1 4 - 9

s u r f a c t a n t n e e d e d t o i n i t i a t e m i c e l l e f o r m a t i o n i s

c a l le d t h e C M C ( c ri ti c a l m i c e l l a r c o n c e n t r a t i o n ) .

T h e C M C c a n b e d e t e r m i n e d b y c o n d u c t i m e t r i c

[ 1 0 - 1 3 ] , s p e c t r o p h o t o m e t r i c [ 1 4 - 2 0 ] , s u r f a c e t e n -

s i o n m e a s u r e m e n t [ 2 1 ] , f l u o r i m e t r i c [ 2 2 ] a n d l i g h t

s c a t te r i n g [2 3 ] m e t h o d s , a m o n g o t h e r s.

T h e u s e o f m i c e l la r s y s te m s i n h i g h p e r f o r m a n c e

l i q ui d c h r o m a t o g r a p h y a n d c a p i l la r y e l e c t r o p h o r e -

s is h a s g i v e n r is e t o m i c e l l a r li q u i d c h r o m a t o g r a p h y

( M L C ) [2 4] a n d m i c e ll a r e l e c t ro k i n e t i c c h r o m a -

t o g r a p h y ( M E K C ) [ 25 ], r e sp e c ti v e ly . I n t h e se

t e c h n iq u e s , C M C i s n e c e s s a ry t o p r e d i c t s o l u t e

r e t e n t i o n ( a n d h e n c e t h e s e p a r a t i o n s e l e c t i v i t y )

[ 2 6 ] , a s a n e x a m p l e .

I n t h i s w o r k , t h e C M C o f s o m e m i c e ll a r s y s te m s

l a rg e ly u s e d as m o b i le p h as e s in M L C a n d M E K C

a r e p r e s en t e d , T h e e f f ec t o n t h i s p a r a m e t e r o f s o m e

a d d i t i v e s ( e . g . n - p r o p a n o l , n - b u t a n o l a n d b u f f e r s )

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222

I . Benito et al . / Colloids Surfaces A: Physicochem. En g. Aspe cts 125 1997) 221 -224

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

M L C a n d M E K C i s a ls o s tu d ie d .

2 . Ex per i menta l pro cedure

2 . 1. M i c e l l a r s y s t e m s

S o d i u m d o d e c y l s u l f a t e ( S D S ) a n d c e t y l 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 T A B ) w e r e u se d a s

s u r f a c t a n t s , a n d n - p r o p a n o l ( n - P r O H ) o r n - b u t a -

n o l ( n - B u O H ) ( a t a 3 - 1 0 % v /v ) a s c o s u r f a c t a n t s .

S o d i u m p h o s p h a t e b u f f e r ( P B ) ( 0 . 01 M , p H = 6 .9 )

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

1600

1400

1200

09

1000

~

800

w

o

-~ 600

- o

c -

O 4 0 0

o

200

0 0 , 0 0 5 0 , 0 1 0 , 0 1 5 0 , 0 2 0 ,0 2 5

[SDS] (M)

0,03

2 . 2. C o n d u c t i m e t r i c m e a s u r e m e n t s

C o n d u c t i m e t r i c d e t e r m i n a t i o n o f t h e C M C w a s

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

t h e s p e c i f i c c o n d u c t i v i t y v e r s u s s u r f a c t a n t

c o n c e n t r a t i o n f o r s u r f a c t a n t s o l u t i o n s i s p l o t t e d

( F i g . 1 ( a )) . T h e s t r a i g h t l i n es s h o w e d i n F i g . 1 ( a )

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

t h e f i rs t a n d t h e l a s t f o u r p o i n t s , r e s p e c ti v e l y . T h e

C M C v a l u e s w e r e d e t e r m i n e d b y t h e i n te r s e c ti o n

o f t h e s e l in e s . D a t a s h o w e d i n T a b l e 1 a r e t h e

a v e r a g e o f a t l e a s t t h r e e d i f f e r e n t e x p e r i m e n t s i n

e a c h c a s e . T h e C T A B a n d S D S c o n c e n t r a t i o n s

r a n g e d f r o m l 0 - 4 t o 2 .5 x 10 - 3 M , a n d

1 0 . 3

to

2 .5 x 10 -2 M , r espec t ive ly .

2 . 3. S p e c t r o p h o t o m e t r i c m e a s u r e m e n t s

0,5

O

o

t

t~

o

.~