Ordered Media in Chemical Separations - American Chemical Society

Chapter 1

Organized Surfactant Assemblies in Separation Science Willie L. Hinze

Downloaded by 80.82.78.170 on December 27, 2016 | http://pubs.acs.org Publication Date: June 30, 1987 | doi: 10.1021/bk-1987-0342.ch001

Department of Chemistry, Wake Forest University, P.O. Box 7486, Winston-Salem, NC 27109

A brief description of the structural features and relevant properties of different organized assemblies formed from surfactant molecules is presented. Next, the use and application of these organized surfactant systems in separation science is surveyed. Several possible new areas for future developments employing these ordered media are mentioned. Many separation processes mediated by the presence of surfactant organized assemblies (also referred to as organized or ordered media) have been developed during the past ten years. The growing importance and popularity of such separation techniques i s demonstrated by the fact that numerous recent review a r t i c l e s have been devoted to t h i s subject (1-10). The purpose of t h i s overview i s two-fold. F i r s t , i t i s intended to provide the novice entering the f i e l d with b a s i c , s i m p l i f i e d background information on organized surfactant systems which should f a c i l i t a t e a better understanding of the more s p e c i f i c technical a r t i c l e s that appear i n subsequent chapters of t h i s monograph (or the chemical l i t e r a t u r e ) . Secondly, t h i s overview w i l l attempt to update and summarize the previous reported work i n t h i s area of separation science. Topics not extensively covered i n the previous reviews (or succeeding chapters of t h i s monograph) w i l l be discussed i n greater d e t a i l . Throughout, emphasis w i l l be placed on the p r a c t i c a l applications and p o t e n t i a l future developments. I t i s hoped that t h i s overview w i l l paint a general picture of the s t r u c t u r e , properties, and r o l e of different surfactant organized assemblies i n separation science. Structure and Properties of Different Organized Surfactant Assemblies Structure Formation i n Surfactant S o l u t i o n s . Surfactants, also referred to as soaps, detergents, tensides, or surface a c t i v e agents, are amphiphilic molecules possessing both h y d r o p h i l i c and hydrophobic regions. They can be c l a s s i f i e d as a n i o n i c , c a t i o n i c , z w i t t e r i o n i c , or nonionic (neutral) depending upon the nature of the polar 0097-6156/87/0342-0002$ 18.30/0 © 1987 American Chemical Society

Hinze and Armstrong; Ordered Media in Chemical Separations ACS Symposium Series; American Chemical Society: Washington, DC, 1987.


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Surfactant Assemblies in Separation Science

3

head-group t h a t i s bound t o t h e n o n p o l a r hydrocarbon t a i l . Various c o l l o i d a l - s i z e d o r g a n i z e d s t r u c t u r e s can form when s u r f a c t a n t m o l e c u l e s a r e d i s s o l v e d i n a p a r t i c u l a r s o l v e n t depending upon t h e n a t u r e and c o n c e n t r a t i o n o f t h e s u r f a c t a n t m o l e c u l e , n a t u r e o f t h e s o l v e n t system, and e x a c t e x p e r i m e n t a l c o n d i t i o n s U.e. t e m p e r a t u r e , p r e s s u r e , and/or presence o r absence o f a d d i t i v e s ) (11-16). F i g u r e 1 shows an o v e r s i m p l i f i e d r e p r e s e n t a t i o n o f some o f t h e d i f f e r e n t s u r f a c t a n t s p e c i e s p o s s i b l e as t h e s u r f a c t a n t c o n c e n t r a t i o n i s i n c r e a s e d i n a s u r f a c t a n t - water two-component system. At low c o n c e n t r a t i o n s above t h e K r a f f t t e m p e r a t u r e , t h e s u r f a c t a n t i s p r e s e n t i n i s o l a t e d monomeric m o l e c u l a r form. W i t h f u r t h e r i n c r e a s e s i n c o n c e n t r a t i o n , t h e s u r f a c t a n t m o l e c u l e s can d y n a m i c a l l y a s s o c i a t e t o form m i c e l l a r a s s e m b l i e s (termed aqueous o r normal m i c e l l e s ) . The s u r f a c t a n t c o n c e n t r a t i o n a t which such a g g r e g a t i o n o c c u r s i s r e f e r r e d to a s t h e c r i t i c a l m i c e l l e c o n c e n t r a t i o n (CMC) and t h e number o f s u r f a c t a n t molecules comprising the m i c e l l a r e n t i t y i s c a l l e d i t s a g g r e g a t i o n number ( N ) . A l t h o u g h aqueous normal m i c e l l e s a r e g e n e r a l l y viewed as b e i n g r o u g h l y s p h e r i c a l ( F i g u r e 1 ) , c o n s i d e r a b l e c o n t r o v e r s y s t i l l e x i s t s c o n c e r n i n g t h e e x a c t shape and s t r u c t u r e o f such e n t i t i e s ( 1 5 , 1 7 ) . T y p i c a l l y , such m i c e l l a r aggregates a r e composed o f 40 - 140 monomeric s u r f a c t a n t m o l e c u l e s such t h a t t h e i r h y d r o p h o b i c t a i l s a r e o r i e n t e d inward f o r m i n g a n o n p o l a r c o r e r e g i o n and t h e i r h y d r o p h i l i c headgroups a r e d i r e c t e d toward and i n c o n t a c t w i t h t h e b u l k aqueous s o l v e n t . F u r t h e r i n c r e a s e s i n s u r f a c t a n t c o n c e n t r a t i o n can r e s u l t i n t h e f o r m a t i o n o f o t h e r d i f f e r e n t t y p e s o f o r g a n i z e d a s s e m b l i e s . I n i t i a l l y , t h e r e can be a t r a n s i t i o n from s p h e r i c a l t o r o d l i k e or c y l i n d r i c a l m i c e l l e s (Figure 1). S t i l l higher concentrations lead to formation of various l i q u i d c r y s t a l l i n e aggregates ( F i g u r e 1: m i d d l e , v i s c o u s , and neat l i q u i d c r y s t a l l i n e phases) (jj_, 1_5,J_8). The presence o f a t h i r d component ( o r g a n i c s o l v e n t ) can g i v e r i s e t o an even l a r g e r v a r i e t y o f aggregated s u r f a c t a n t s p e c i e s (j_5). Table I summarizes t h e s t r u c t u r e , name, and m i c e l l a r parameters (CMC and N) o f some t y p i c a l l o n g - c h a i n a l k y l s u r f a c t a n t s employed t o form aqueous normal m i c e l l a r systems (11,19). In a d d i t i o n t o t h e s e t y p e s o f m i c e l l a r - f o r m i n g s u r f a c t a n t s , t h e r e i s another c l a s s o f m o l e c u l e s t h a t can a s s o c i a t e i n water t o form m i c e l l a r a g g r e g a t e s ; namely, t h e b i l e s a l t s ( 2 0 ) . B i l e s a l t s a r e v e r y i m p o r t a n t b i o l o g i c a l d e t e r g e n t - l i k e m o l e c u l e s . However, t h e y d i f f e r from t h e l o n g - c h a i n a l k y l s u r f a c t a n t s p r e v i o u s l y mentioned i n t h a t t h e y possess a hydrophobic and a h y d r o p h i l i c f a c e ( F i g u r e 2 ) . Consequently, b i l e s a l t s e x h i b i t a d i f f e r e n t type o f aggregation b e h a v i o r . That i s , t h e a g g r e g a t i o n p r o c e s s i s viewed as c o n s i s t i n g o f t h e s t e p w i s e f o r m a t i o n o f i n i t i a l p r i m a r y m i c e l l e s which a r e composed of 2 - 8 monomers h e l d t o g e t h e r by h y d r o p h o b i c i n t e r a c t i o n s between the b i l e s a l t n o n p o l a r f a c e s . A t h i g h e r b i l e s a l t c o n c e n t r a t i o n ( o r h i g h i o n i c s t r e n g t h ) , t h e p r i m a r y m i c e l l e s can f u r t h e r aggregate t o form l a r g e r , r o d - l i k e c y l i n d r i c a l l y shaped secondary b i l e s a l t m i c e l l e s due t o i n t e r m o l e c u l a r hydrogen bonding between t h e i r h y d r o x y l groups ( 2 , 2 1 ) . T a b l e I I p r e s e n t s t h e name, s t r u c t u r e , and m i c e l l a r parameters o f some common b i l e s a l t s . A l l s u r f a c t a n t s and b i l e s a l t s mentioned i n T a b l e s I and I I a r e c o m m e r c i a l l y a v a i l a b l e ( 2 2 ) . A r e c e n t m u l t i - v o l u m e s e r i e s l i s t s t h e t r a d e name, c h e m i c a l name, m a n u f a c t u r e r , f o r m , p r o p e r t i e s , t o x i c i t y , c o m p o s i t i o n , p r i n c i p a l and secondary u s e s , e t c . f o r many o f t h e s e s u r f a c t a n t s ( 2 6 ) .


4

O R D E R E D M E D I A IN C H E M I C A L SEPARATIONS

TABLE I .

S t r u c t u r e , Name, A b b r e v i a t i o n , and M i c e l l a r Parameters o f Some Aqueous M i c e l l a r - F o r m i n g S u r f a c t a n t s Employed i n Separation Science

Surfactant

a

S t r u c t u r e ; Name; and [ A b b r e v i a t i o n ]

Anionic Micelle-Forming

CMC, '^ N ^ ' ° mM +

Surfactants of General

Formula R-X M :

R = C , X=0S0 , M=Na; Sodium h e x a d e c y l s u l f a t e 16

[NaHDS]

3

R = C , X=0S0 , M=Na; Sodium d o d e c y l s u l f a t e 12

0.52

[NaLS]

3

R = C , X=0S0 , M=Na; Sodium d e c y l s u l f a t e [NaDS] 1Q

3

8.1

62

33.0

50

136.0

20

[NaL]

24.0

56

R = C , X=C0 , M=K; P o t a s s i u m l a u r a t e [KL]

12.5

48

R=C , X=0S0 , M=Na; Sodium o c t y l s u l f a t e g


100

3

R = C , X=C0 , M=Na; Sodium l a u r a t e 1Q

2

1Q

[NaOS]

2

R = C , X=C0 , M=K; P o t a s s i u m p a l m i t a t e 15

2

[KP]

R = C , X=S0 , M=Na; Sodium d o d e c y l s u l f o n a t e 12

3

Cationic Micelle-Forming

4.0 [NaDDS]


54

9.8 +

Formula R - N ( C H ) X ~ : 3

3

R = C , X=C1; Hexadecyltrimethylammonium c h l o r i d e [CTAC]

1.3

78

R = C , X=Br; Hexadecyltrimethylammonium bromide

0.9

61

15.0

50

65.0

47

16

16

R = C , X=Br; Dodecyltrimethylammonium bromide 12

R = C , X=Br; Decyltrimethylammonium bromide g

o f R = C , C.,, and C. ,

Mixture

predominately CH (CH ) 3

2

CH (CH ) 3

c

10

2

X=Br; C e t r i m i d e

N C H 5

[C]

C l " ; C e t y l p y r i d i n i u m c h l o r i d e [CP]

5

+

1 5

180

[OTAB]

R=C^

+

1 5

[LTAB]

[DTAB]

1Q

R=C , X=Br; O c t y l t r i m e t h y l a m m o n i u m bromide

[CTAB]

N (CH ) (CH C H )Cl"; 3

2

2

6

Micelle-Forming

R = C , n=23; P o l y o x y e t h y l e n e ( 2 3 ) d o d e c a n o l 12

3

3

2

3

octylphenol

2

6

4

3

2

Polysorbate oleate

Formula R ( 0 C H C H ) O H : 2

[Brij-35]

[ T r i t o n X-100 o r TX-100]

7

2

of C

9 >

C

1 Q

g

[Brij-96]

, and C ^ ; n=6; N e o d o l 91-6

80 ( o r P o l y o x y e t h y l e n e

40

0.2

143

0.04 0.37(wt%)-

0.012 Surfactants of General

Formula

R-(CH ) N CH X : 2

n

0.1

+

3

2

s o r b i t a n mono-

[Tween-80]

Zwitterionic Micelle-Forming

62 95

Polyoxyethylene-t-

R = C H ( C H ) C H = C H ( C H ) , n=10; 10 O l e y l e t h e r R=mixture

8.5 0.9

0.27


R = ( C H ) C C H C ( C H ) C H , n=9.5;

50-

Hexadecyldimethyl-

5

benzylammonium c h l o r i d e [CBzAC] Nonionic

2.0-

2

3.9


60

1.


HINZE

Surfactant

S t r u c t u r e ; Name; and [ A b b r e v i a t i o n ]

R = C , X=CH CH S0 ; N - D o d e c y l s u l t a i n e 19

2

2

3

[SB-12]

R = C , X=CH CH S0 ; N - H e x a d e c y l s u l t a i n e 16

2

2

3

R = C , X=C0 ; N - D e c y l b e t a i n e 1Q

2

[DDAA]

R = C , X=C0 ; N - D o d e c y l b e t a i n e 12

2

[DoDAA]

R = C , X= C 0 ; N - H e x a d e c y l b e t a i n e 16

2

[HDAA]

[SB-16]

CMC, mM

a , b

1.2

N

55

0.1 10 21

34

1.5

73

0.02

a


b , C

C r i t i c a l micelle concentration. ^ M i c e l l a r parameters g i v e n a r e f o r aqueous s o l u t i o n s a t 25°C, 1 atm, i n the absence o f any a d d i t i v e s . V a l u e s t a k e n from r e f e r e n c e s (1,5,12,13,15,16). A g g r e g a t i o n number (N).

F i g u r e 1. S i m p l i f i e d r e p r e s e n t a t i o n o f i d e a l i z e d s u r f a c t a n t s p e c i e s t h a t may f o r m i n w a t e r as t h e s u r f a c t a n t c o n c e n t r a t i o n i s progressively increased. "Reproduced w i t h p e r m i s s i o n from R e f . 18. C o p y r i g h t 1979, The C h e m i c a l S o c i e t y


6


). The m i c e l l a r CMC and Ν can a l s o depend upon p r e s s u r e (J_6). However, a t the p r e s s u r e s under w h i c h most s e p a r a t i o n t e c h n i q u e s are conducted (£ 3.5 MPa), the changes i n m i c e l l a r parameters a r e such t h a t t h i s e f f e c t can be n e g l e c t e d i n a l l but the most e x a c t i n g work (64). More d r a s t i c changes i n the CMC and Ν are o b s e r v e d when a d d i t i v e s are p r e s e n t i n the m i c e l l e - f o r m i n g s u r f a c t a n t - water systems. The a d d i t i o n o f i o n i c s p e c i e s Q . e . e l e c t r o l y t e s ) u s u a l l y r e s u l t s i n an i n c r e a s e i n t h e a g g r e g a t i o n number and a r e d u c t i o n i n t h e CMC. T a b l e I I I (and T a b l e I I ) p r e s e n t some d a t a which i l l u s t r a t e t h i s e f f e c t . Depending upon the c o n c e n t r a t i o n , the presence o f water m i s c i b l e o r g a n i c m o l e c u l e s can e i t h e r enhance o r i n h i b i t m i c e l l e f o r m a t i o n . For example, s h o r t - c h a i n a l c o h o l s c a n enhance m i c e l l e f o r m a t i o n U.e. lower t h e CMC) i f p r e s e n t a t v e r y low mole f r a c t i o n and prevent m i c e l l i z a t i o n a t h i g h e r c o n c e n t r a t i o n ( i f X £ 0.05 o r 10—15% by volume) (27,28). Other o r g a n i c s o l v e n t s , l i k e a c e t o n e , d i o x a n e , a c e t o n i t r i l e , t e t r a h y d r o f u r a n , e t c . t h a t form r e l a t i v e l y s t r o n g hydrogen bonds w i t h w a t e r , w i l l g e n e r a l l y have a s l i g h t i n h i b i t o r y e f f e c t on the m i c e l l i z a t i o n p r o c e s s U.e. g r e a t e r CMC v a l u e ) when p r e s e n t a t v e r y low c o n c e n t r a t i o n (28,29)« A t g r e a t e r c o n c e n t r a t i o n s (X £ 0.10 o r 15-20Î by v o l u m e ) , t h e i r presence p r e v e n t s m i c e l l e f o r m a t i o n . L a s t l y , some o r g a n i c s o l v e n t s ( h y d r a z i n e , 1 , 3 ~ p r o p a n e d i o l , formamide, g l y c e r o l ) which can have t h r e e - d i m e n s i o n a l s t r u c t u r e i n t h e i r neat l i q u i d s t a t e , can promote m i c e l l e f o r m a t i o n i f p r e s e n t a t r e l a t i v e l y low c o n c e n t r a t i o n as w e l l a s a l l o w f o r m i c e l l e f o r m a t i o n i n m i x t u r e s o f t h e s e s o l v e n t s w i t h water i n a l l p r o p o r t i o n s (28,29). I f the o r g a n i c a d d i t i v e i s a n o r m a l l y water i m m i s c i b l e s u b s t a n c e , t h e n i t s e f f e c t on the m i c e l l i z a t i o n p r o c e s s can be more c o m p l i c a t e d (65). For i n s t a n c e , t h e a d d i t i o n o f l o n g c h a i n a l c o h o l s ( c o n t a i n i n g 5 o r more c a r b o n atoms) o r a l k a n e s can e i t h e r enhance o r i n h i b i t m i c e l l e f o r m a t i o n depending upon the c o n c e n t r a t i o n o f t h e s u r f a c t a n t p r e s e n t



1.


HINZE

TABLE I I .

S t r u c t u r e and M i c e l l a r

Structure,

Name

R =R =R =H; C h o l a n o i c a c i d

if

R =R =R =0H; C h o l i c

if

x

2

2

3

acid

(CA)/sodium

3

2

if

R =R =0H, R =H; C h e n o d e o x y c h o l i c

if

the a c i d

2

3

c h o l a t e (NaC)

acid

(position

2

taurine; Taurodeoxycholic acid c h o l a t e (NaTDC)

Derivatives

of cholanoic acid

(20, 24, 2 5 ) . At

b

2.8

(CDCA)/sodium

5.7

(NaDCA)

then c a n have t h e c o r r e s p o n d i n g t a u r o d e r i v a t i e s : 3

6.4

24 a c i d c o n j u g a t e d

( 2 0 ) . Data

C

10

C

e

i.e.

d

8.5

References

I n water a l o n e .

pH 7.4.


e

3.5 * c 10

with

t a k e n from

°In 0.15 M N a C l .

14 C

b

4.0 *, c 1.6 ,

(TDC)/sodium t a u r o d e o x y -

I n 0.001 M NaOH.

3°

2.7^

24) i s c o n j u g a t e d w i t h t a u r i n e ,

R^=R =0H, R =H, w i t h p o s i t i o n

b

b

(NaDC)

chenodeoxycholate

e

Ν

12.5

(DCA)/sodium

deoxycholate 1

Salts'

3

R =R =0H; R =H; D e o x y c h o l i c a c i d x

o f Some B i l e

CMC, mM

if

1

Parameters

8

ORDERED MEDIA IN CHEMICAL SEPARATIONS

TABLE I I I .

Comparison o f M i c e l l a r Parameters Under D i f f e r e n t E x p e r i m e n t a l C o n d i t i o n s i n Aqueous Media

Surfactant

Hexadecylpyridinium Bromide ( C P B )

Experimental Factor Varied

CMC, mM

25°C 35 45 55

0.58 0.77 0.89 1.0

Temperature:

a


Added a l c o h o l , Methanol:

Bulk

c) Sodium D o d e c y l Sulfate (NaLS)

0.58 0.75 1.18 1.69 2.81 0.58

0 (w/w) % 6.4 14.7 19.9 26.0 S o l v e n t : Water (25°C) Ethylammonium n i t r a t e (fused s a l t system, 30°

20.0

Added

(25°C, 1 atm) M NaOH M NaOH M NaCl M NaCl M NaCl -5 2+ 1.0 χ: 10 M Mg -5 2+ 1.0 χ: 10 M Fe S o l v e n t : Water (35°C) H y d r a z i n e (35°C) Formamide (60°C) J

8.1 2.7 1.5 1.3

0.7 0.8 8.57 22.0 220.0

Data t a k e n from R e f . (19) u n l e s s o t h e r w i s e i n d i c a t e d . Ref.

C

— — — — — 26

60 42 35 38 50 78

0.1 MPa 40.0 80.0 95.0 120.0 140.0 Electrolytes: none 0.05 0.10 0.15 0.30 0.55

Bulk

— — —

B

Pressure : C

Ν

( 3 0 ) . D a t a t a k e n from R e f . (5,11,13,16,19).

d

Taken

Taken

62

— — 95 117 580

— — — — — from

from R e f .

(63).


1.

HINZE


9


and t h e amount o f o r g a n i c added. I n many i n s t a n c e s , t h e f o r m a t i o n o f microemulsions can r e s u l t , p a r t i c u l a r l y a t higher a l c o h o l o r alkane c o n c e n t r a t i o n s (209)* C o n s e q u e n t l y , t h e v a r i a t i o n i n m i c e l l a r parameters (CMC and N) o r s t r u c t u r e w i t h changes i n t h e e x p e r i m e n t a l c o n d i t i o n s s h o u l d be kept i n mind when one uses s u r f a c t a n t o r g a n i z e d assemblies i n separation science a p p l i c a t i o n s . A p a r t from m i c e l l e f o r m a t i o n when s u r f a c t a n t s a r e added t o w a t e r , v e s i c l e f o r m a t i o n can a l s o o c c u r Π 5 ) . Namely, i f c e r t a i n t y p e s o f s u r f a c t a n t s , i . e . t y p i c a l l y long chain d i a l k y l - c o n t a i n i n g s u r f a c t a n t s , are added t o water and s o n i c a t e d above t h e i r phase t r a n s i t i o n temperature, closed b i - or m u l t i - l a y e r e d s t r u c t u r e s c a l l e d v e s i c l e s can form (15,31-36,211). T a b l e IV l i s t s t h e s t r u c t u r e and some common p r o p e r t i e s o f t h e most s t u d i e d v e s i c l e - f o r m i n g s u r f a c t a n t systems. Compared t o t h e normal m i c e l l a r systems j u s t d e s c r i b e d , such s u r f a c t a n t v e s i c l e s a r e much l a r g e r , more s t a t i c ( i . e . l e s s f l u i d "more r i g i d " ) a g g r e g a t e s . V e s i c l e a g g r e g a t e s , once formed, cannot be d e s t r o y e d by d i l u t i o n whereas m i c e l l e s c a n . Most s y n t h e t i c s u r f a c t a n t v e s i c l e systems a l s o e x h i b i t temperature dependent phase t r a n s i t i o n b e h a v i o r i n c o n t r a s t t o t h e m i c e l l e systems Oj5). A l t h o u g h t h e s u b j e c t o f much r e c e n t s t u d y , s u r f a c t a n t v e s i c l e s have n o t y e t been u t i l i z e d t o any a p p r e c i a b l e e x t e n t i n s e p a r a t i o n s c i e n c e . To d a t e , they have been employed as models f o r t h e s t u d y o f b i o l o g i c a l t r a n s p o r t and membrane - s o l u t e i n t e r a c t i o n s (9). Of c o u r s e , such i n f o r m a t i o n i s u s e f u l and c o u l d l e a d t o development o f s e p a r a t i o n schemes i n v o l v i n g s u r f a c t a n t v e s i c l e s - e s p e c i a l l y i n t h e a r e a o f membrane - based s e p a r a t i o n s . More i n f o r m a t i o n and d e t a i l s o f such v e s i c u l a r and r e l a t e d o r g a n i z e d a s s e m b l i e s i n t h i s c o n t e x t i s p r o v i d e d by t h e f o l l o w i n g Chapter by F e n d l e r i n t h i s Volume (36). In a d d i t i o n t o s t r u c t u r e formation i n water, ordered surfactant a s s e m b l i e s can form i n n o n p o l a r s o l v e n t s as w e l l . F o r i n s t a n c e , when s u r f a c t a n t molecules a r e d i s s o l v e d i n o r g a n i c hydrocarbons i n the presence o f s m a l l amounts o f w a t e r , t h e f o r m a t i o n o f i o n p a i r s as w e l l as s m a l l and l a r g e a g g r e g a t e s i s p o s s i b l e (5,8,11-16,36-41). The term r e v e r s e d o r i n v e r t e d m i c e l l e s i s g i v e n t o such a g g r e g a t e s s i n c e t h e i r p o l a r groups a r e c o n c e n t r a t e d i n t h e i n t e r i o r ( c o r e ) r e g i o n o f t h e s u r f a c t a n t assembly w h i l e t h e i r h y d r o p h o b i c p o r t i o n s e x t e n d i n t o , and are surrounded b y , t h e b u l k n o n p o l a r s o l v e n t m o l e c u l e s . The r e v e r s e d m i c e l l a r i n t e r n a l c o r e r e g i o n c o n t a i n s t h e h y d r o p h i l i c headgroup o f the s u r f a c t a n t i n a d d i t i o n t o an i n n e r p o o l o f c o - s o l u b i l i z e d water (or o t h e r p o l a r s o l v e n t s ) . I t must be s t r e s s e d t h a t t h e u s u a l c o n c e p t s and s t r u c t u r a l models t y p i c a l l y employed t o d e s c r i b e normal aqueous m i c e l l a r f o r m a t i o n i n water a r e n o t always a p p l i c a b l e t o r e v e r s e d m i c e l l a r systems i n o r g a n i c s o l v e n t s (37-41). I n f a c t , s e v e r a l modes o f a g g r e g a t i o n a r e p o s s i b l e depending upon t h e c h a r g e - t y p e s u r f a c t a n t employed. A c c o r d i n g t o M u l l e r ' s c l a s s i f i c a t i o n scheme (£7,_39), t h e m a j o r i t y of s u r f a c t a n t s ( i . e . c a t i o n i c , z w i t t e r i o n i c , and most n o n i o n i c ) undergo s o - c a l l e d Type I a g g r e g a t i o n b e h a v i o r . This i s , aggregation of t h e s e s u r f a c t a n t s proceeds v i a a smooth t r a n s i t i o n o f monomer dimer trimer^ "^n-mer i n d e f i n i t e t y p e o f a s s o c i a t i o n a s opposed t o t h e monomer^η-mer m i c e l l a r e q u i l i b r i u m u s u a l l y observed f o r normal aqueous m i c e l l e s y s t e m s . As a r e s u l t , such r e v e r s e d


10


TABLE

IV.

Structure Systems

Surfactant

and

in

Characteristics

Aqueous

of

Some

6

b

10 M

Structure

R^ ,

w


Surfactant

Vesicle

Solution

A

N

d

e

Τ

H

c

CATIONIC TYPE: (CH)RRNV 3

2

1

2

^ ^ ^ Ι β '

if

X

=

C

1

, D

0

D

A

13

4

' 7.0

C

4 0 0

48,500 36°C

i f = 2 » ' DDDAB ANIONIC TYPE: [CH(CH) C0CH][CH(CH) C0]CHS0"Na (CH )(CH )C(H)(CHS0"Na) 23.0 260 NONIONIC TYPE: [CH(CH)0]P(0)0H, DHP 30.0 600 [CH(CH)0CH]CH0(CHCH0)H 12.0 ZWITTERIONIC TYPE: [(CH)(CH)]N(CH)(CH)S0" 25.0 [CH(CH) ]N(CH)(CH)0P(0)" 17.0 R

R

=C

1

X=BR

12

+

3

2

11

2

2

3

2

11

3

2

+

8

19

7

3

15

2 15

3

6

5,760

3

4

2

2 11

2 2

2

2

none

15

+

3

3

2

U

2

3

2

3

3

+

3

Data

2

17

taken

molecular surfactant

2

3

2

2

from References

weight. molecules

Refers per

38°C

3

(15,31-35). to

the

vesicle

Refers

hydrodynamic aggregate.

to

weight-average

radius. Phase

Number

transition

erature.


of temp

1.

HINZE


11

m i c e l l a r systems do not e x h i b i t a c l e a r - c u t CMC v a l u e as do normal m i c e l l e s . I n s t e a d , a t each s u r f a c t a n t c o n c e n t r a t i o n l e v e l , t h e r e i s a d i s t r i b u t i o n o f aggregates and i n c r e a s e s i n the c o n c e n t r a t i o n l e a d t o f o r m a t i o n o f l a r g e r aggregates i n g r e a t e r p r o p o r t i o n s (J_5,27). These Type I r e v e r s e d aggregates are thus p o l y d i s p e r s e , t h e i r average a g g r e g a t i o n number i s t y p i c a l l y s m a l l (3^N£10), and t h e y a r e p o s t u l a t e d t o have l a m e l l a r type s t r u c t u r e s i n some i n s t a n c e s . The a g g r e g a t i o n number and s i z e o f such r e v e r s e d m i c e l l e s can be s i g n i f i c a n t l y a l t e r e d by the amount o f c o - s o l u b i l i z e d water p r e s e n t


(in). The Type I I r e v e r s e d m i c e l l a r systems ( i . e . those formed from a n i o n i c s u r f a c t a n t s such as a r y l s u l f o n a t e s o r a r y l p h e n o l a t e s ) e x h i b i t a g g r e g a t i o n b e h a v i o r q u i t e s i m i l a r t o t h a t o f normal aqueous s u r f a c t a n t s . That i s , t h e y have f a i r l y w e l l - d e f i n e d CMC v a l u e s and much l a r g e r a g g r e g a t i o n numbers compared t o the Type I systems j u s t d e s c r i b e d . T h e i r a g g r e g a t i o n number and s i z e a r e , however, a l s o dependent upon the water c o n t e n t and r e a c h c o n s t a n t l i m i t i n g v a l u e s under s p e c i f i e d e x p e r i m e n t a l c o n d i t i o n s (37-41)· Due t o the f a c t t h a t t h e s e Type I I a g g r e g a t e systems a r e l e s s complex ( i n terms of the number o f a c t u a l s p e c i e s p r e s e n t ) compared t o the Type I systems, they have been t o u t e d as b e i n g the p r e f e r r e d system of c h o i c e i n any s e p a r a t i o n s c i e n c e a p p l i c a t i o n (42). However, as w i l l be shown from the a p p l i c a t i o n s i n t h e l i t e r a t u r e , both Type I and I I r e v e r s e d m i c e l l e s may be e q u a l l y s u c c e s s f u l l y employed (5). The s t r u c t u r e o f some r e v e r s e d m i c e l l e - f o r m i n g s u r f a c t a n t s as w e l l as d a t a on t h e i r a g g r e g a t i o n b e h a v i o r i n d i f f e r e n t n o n p o l a r s o l v e n t s i s p r e s e n t e d i n T a b l e V. As can be s e e n , a g g r e g a t e s can form at v e r y low s u r f a c t a n t c o n c e n t r a t i o n s i n some cases and the s i z e o f t h e o r g a n i z e d a s s e m b l i e s depends s t r o n g l y o n the amount of water p r e s e n t i n most i n s t a n c e s . C o n s e q u e n t l y , i t i s v e r y i m p o r t a n t t o s t i p u l a t e both the s u r f a c t a n t and water c o n c e n t r a t i o n s when employing r e v e r s e d m i c e l l a r systems i n s e p a r a t i o n s c i e n c e s o t h a t r e p r o d u c i b l e r e s u l t s are o b t a i n a b l e . L a s t l y , mention s h o u l d be made o f s u r f a c t a n t m i c r o e m u l s i o n s . Depending upon the r e l a t i v e c o n c e n t r a t i o n s , t h r e e component systems c o n t a i n i n g a s u r f a c t a n t , w a t e r , and a n o n p o l a r s o l v e n t can form m i c r o e m u l s i o n s (J_5,36,43). The a d d i t i o n o f i n c r e a s i n g amounts o f an o r g a n i c s o l v e n t ( o i l ) t o aqueous normal m i c e l l a r s o l u t i o n s o r i n c r e a s i n g amounts o f s u r f a c t a n t - e n t r a p p e d water t o r e v e r s e d m i c e l l a r s o l u t i o n s can l e a d t o t h e f o r m a t i o n of o i l - i n - w a t e r (o/w) o r w a t e r - i n - o i l (w/o) m i c r o e m u l s i o n s , r e s p e c t i v e l y . A l t h o u g h p o t e n t i a l l y u s e f u l , t h e r e have been very few r e p o r t s o f t h e i r u t i l i z a t i o n i n s e p a r a t i o n s c i e n c e (J_»8). Such systems have, however, been s u c c e s s f u l l y employed i n a v a r i e t y o f i n d u s t r i a l and r e l a t e d p r o c e s s e s i n c l u d i n g enhanced o i l r e c o v e r y which i s a k i n t o a s e p a r a t i o n p r o c e s s (J_5,37,44,45). Due t o space r e s t r i c t i o n s , the u t i l i z a t i o n o f s u r f a c t a n t m i c r o e m u l s i o n s i n c h e m i c a l s e p a r a t i o n s w i l l not be e x t e n s i v e l y d i s c u s s e d i n t h i s r e v i e w a r t i c l e . The i n t e r e s t e d r e a d e r i s r e f e r r r e d t o many f i n e r e f e r e n c e s o n the p r o p e r t i e s and u t i l i z a t i o n o f t h i s type o f o r g a n i z e d s u r f a c t a n t system (15,36,43>46,66,67,215,

216).


12


TABLE V. Solvents

Summary o f Some S u r f a c t a n t s which Aggregate i n A p o l a r

Surfactant

Structure

Bulk Solvent

(Abbreviation)

Cationics of General Structure

2

3

4

U

3


tetradecylammonium

2

0

0

Benzene,

2

butyrate,

w i t h water added: g/5 mL: none 0.5 1.3

TDAB

2

3

4

4

4

tetrabutylammonium

perchlorate, :

R^—R ~R ~C^ , R^—H, X 2

3

2

N0„

-—

4

3-6

J

-2 10 m

t-BAP

2-6

Benzene

o r HSO,

3

4.3 23.2 34.5 10

Benzene

R -R -R -R -C , X = C10 ; 1

Ν

R.R R R,N X 12 3 4

R = R = R = R = C , X= C H ( C H ) C 0 0 " ; 1

Concentration Range

,

tridodecylammonium s a l t s , TLAB o r TLAN R^—R ~R "-Cg, R^—Η, X HSO, \j c SO. , Benzene 4 4

0.4 - 8.0

1-

trioctylammonium s a l t s ,

wt%

3.8

2

3

TOAB o r TOAS

R 1 =R =C , R =R =CH , X=Cf; 2

12

3

4

Benzene

3

didodecyldimethylammonium

1.1 mmole/ 6.5 kg

chloride,

(at 50°)

DDAC R^=C^£, R =R =R =CH , X=C1 ; 2

3

4

CHC1.

3

hexadecyltrimethylammonium

X

=

3

7

CTAC · " 0.003-0.04 7.0

chloride,

CTAC 3

2.0 χ 10" M

Benzene

R = R =R,=?H, X=CH„CH COO 1* 12> 2 3 4 ' 3 2 dodecylammonium p r o p i o n a t e , DAP R

4-5

C

Dichloromethane 0.02-0.04 M

o

0.023 M

4.0

0.05

M

4.0

Dichloromethane 0.11 M

5.0

CCI, 4 Benzene

R=C., R =R =R =CH CH C00 ; 1 4 2 3 4 3 2 ' butylammonium p r o p i o n a t e , BAP o

6.0

3.0

0.025 M

CCI, Anionics: o f G e n e r a l Formula R-C-C=0(CH )CH(S0 ")C=0-0-R M 2

R= 2 - e t h y l h e x y l ,

+

3

M = Na; sodium b i s - Benzene

2.0 χ 10

3

M

G

1323

2-ethylhexylsulfosuccinate CCI,

6.0 χ 10


M 17

1. HINZE

13


Surfactant Structure (Abbreviation)

Bulk Solvent

Concentration Range

Cyclohexane

1 - 3 wt %

Decane

6.5 wt %

Ν

4565 2531

0.4 - 2.8 wt%

Benzene

R= d e c y l , M = Na; sodium d i d e c y l -

916

sulfosuccinate


o f G e n e r a l Formula :

R=C[CH ][CH(CH ) ][CH CH(CH ) ], 3

3

2

2

3

2

M = Na, sodium d i n o n y l n a p h t h a l e n e s u l f o n a t e , NaDNNS

Benzene

0.05

- 0.2 wt%

10

R=C[CH ][CH(CH ) ][CH CH(CH ) ], 3

3

M = Na,

2

2

3

5

sodium d i d o d e c y l n a p h t h a l e n e - Benzene

s u l f o n a t e , NaDDNNS

1,5-Dinonylnaphthalene-4-sulfonic acid

0.5

2.8 wt %

Decane

0.5

2.8 wt%

Benzene

2.0 χ 10 ""M

9.7 15.2

3-12 7.0

5

Hexane

2.0 χ

Toluene

2.0 χ 10

Magnesium d i l a u r a t e , MgDL

Benzene

3.5

7.5 wt%

L i t h i u m decanoate, LiD

Benzene

0.1

0.6 wt% 52 -

DNNSA

10" M M

6.0

16.6

63 Nonionics; S o r b i t a n monooleate, Span-80, SP-80

Benzene

Polyoxyethylene(9.5)-t-octylphenol,

26

CCl^

0.32

M

Cyclo-

0.04

M

1-4

T r i t o n X-100, TX-100 Polyoxyethylene(6)nonylphenol, I g e p a l CO-530, I-C0530 Polyoxylene(20)

hexane

s o r b i t a n monolaurate,

Tween 20,T-20 10 O l e y l E t h e r , B r i j - 9 6 , B-96

Benzene

3-15

Chloroform

1-7

Octane Continued

on next


page

14


T a b l e V.

Continued

Surfactant

Structure

(Abbreviation)

Bulk Solvent

Concentration Range

Ν

Zwitterionics: of General f

R~R

Formula RNH^

+

c o n t a i n i n g more than

"u^CR

1

(where

8 c a r b o n atoms):

!

R=C^2> R = C j ^ ; dodecylammonium


d e c a n o a t e , DAD of General

Benzene

3.5

χ

10 ^m

Benzene

0.001 - 0.01 wt % 0.7 - 1.0 wt%

Formula:

CH 0-CO-R 2

R'-COO-CH

Ο

CH,

C H 0 - Ρ - Ο · CH · CH · N - C H Ο" CH 2

2

2

3

3

Lecithins

(phosphatidylcholines)

Benzene Chloroform a

Refers

to the o p e r a t i o n a l CMC

i n most c a s e s ;

range where r e v e r s e m i c e l l e s a r e p r e s e n t ^Refers

surfactant 5, c

c o n c e n t r a t i o n range.

12-16, 27,

Values

73 68

i . e . concentration

i n the i n d i c a t e d

to the number average a g g r e g a t i o n

80

solvent.

number i n s p e c i f i e d

were taken

from

references

37-40.

T y p i c a l l y e x h i b i t Type I a g g r e g a t i o n

behavior.

Type I I a g g r e g a t i o n b e h a v i o r . e f Taken from r e f e r e n c e 41_. Aggregation

d

Typically

exhibit

d a t a i s f o r the sodium

under b a s i c c o n d i t i o n s .


salt


1.

HINZE


15

R e l e v a n t P r o p e r t i e s o f O r g a n i z e d S u r f a c t a n t Media. The a d d i t i o n o f s u r f a c t a n t t o a s o l v e n t a t s u r f a c t a n t c o n c e n t r a t i o n s / c o n d i t i o n s under which no aggregated s p e c i e s a r e p r e s e n t w i l l u s u a l l y n o t l e a d t o any appreciable a l t e r a t i o n i n the p r o p e r t i e s or processes o c c u r r i n g i n the s o l v e n t a s i d e f o r p o s s i b l e s a l t e f f e c t s upon t h e p r o c e s s and/or i o n p a i r f o r m a t i o n between t h e s u r f a c t a n t and s o l u t e m o l e c u l e s . However, the presence o f o r g a n i z e d s u r f a c t a n t a s s e m b l i e s can a l t e r t h e s o l u b i l i t y o f s o l u t e s , a l t e r c h e m i c a l and p h o t o p h y s i c a l pathways and r a t e s , a l t e r t h e e f f e c t i v e microenvironment about s o l u b i l i z e d s o l u t e s , a l t e r encounter p r o b a b i l i t i e s i n f a s t r e a c t i o n s , m o d i f y t h e p o s i t i o n o f e q u i l i b r i u m p r o c e s s e s , and a l t e r t h e s o l u t i o n p r o p e r t i e s ( v i s c o s i t y , s u r f a c e t e n s i o n , e t c . ) among o t h e r e f f e c t s compared t o t h a t o f t h e b u l k s o l v e n t i n t h e absence o f aggregates (1-1 6). Since such o r g a n i z e d s u r f a c t a n t systems mimic c e r t a i n a s p e c t s o f biomembranes, t h e y have a l s o been r e f e r r e d t o as membrane m i m e t i c agents (.36,47). A l t h o u g h a l l o f t h e mentioned p r o p e r t i e s o f o r g a n i z e d s u r f a c t a n t media can p o t e n t i a l l y a i d t h e s e p a r a t i o n s c i e n t i s t , i n most c a s e s , t h e c r u c i a l factor i n t h e i r successful a p p l i c a t i o n i n separations i s t h e i r a b i l i t y t o s e l e c t i v e l y s o l u b i l i z e and i n t e r a c t w i t h s o l u t e m o l e c u l e s . The presence o f s u r f a c t a n t m i c e l l e s o r v e s i c l e s can d r a m a t i c a l l y enhance t h e s o l u b i l i t y o f a g i v e n s o l u t e compared t o t h a t i n t h e b u l k solvent alone Ο V6,4O,4l_). F o r example, t h e presence o f s u r f a c t a n t i n v e r t e d m i c e l l e s a l l o w s one t o s o l u b i l i z e p o l a r s p e c i e s ( s a l t s , b a s e s , a c i d s , w a t e r ) i n an o r g a n i c s o l v e n t . Whereas t h e s o l u b i l i t y o f water i n a l k a n e s o l v e n t s l i k e h e p t a n e , o c t a n e , o r nonane i s i n t h e range o f 0.01 wt %, homogeneous m i x t u r e s o f a p p r o x i m a t e l y 10% water i n t h e s e s o l v e n t s can be p r e p a r e d i n t h e presence o f r e v e r s e d m i c e l l e s (such as i n 0.015 Μ Α0Τ) ( 4 8 ) . L i k e w i s e , normal aqueous m i c e l l a r media can be employed t o enhance t h e water s o l u b i l i t y o f o r g a n i c m a t e r i a l s . For i n s t a n c e , 1,2-benzphenanthrene and 2,3-benzphenanthrene a r e v i r t u a l l y i n s o l u b l e i n water (water s o l u b i l i t y ύ 9.0 χ 10 M). However, i n t h e presence of.0.50 M p o t a s s i u m dodecanoate, t h e i r s o l u b i l i t y i s r o u g h l y 6.4 χ 10 M (V6). This represents a s o l u b i l i t y enhancement o f 66,000! Many o t h e r examples o f such enhancements i n s o l u b i l i t y a r e r e p o r t e d i n t h e l i t e r a t u r e (1,4,16,40,41). Depending upon t h e n a t u r e o f t h e s o l u t e and o r g a n i z e d s u r f a c t a n t system, a s o l u t e c a n " b i n d " d i f f e r e n t r e g i o n s o f t h e aggregate system. F i g u r e 3 shows some o f t h e s o l u b i l i z a t i o n s i t e s a v a i l a b l e f o r a s o l u t e i n an aqueous normal m i c e l l a r system (4_9). I n i n v e r t e d m i c e l l a r media, p o l a r s o l u t e s can be s o l u b i l i z e d i n t h e i n t e r i o r water p o o l , o r a s s o c i a t e w i t h t h e headgroup o f t h e s u r f a c t a n t m o l e c u l e ( i f o f opposite charge). A d d i t i o n a l l y , l e s s p o l a r s p e c i e s can a l i g n themselves w i t h t h e s u r f a c t a n t m o l e c u l e s v i a b o t h h y d r o p h o b i c and e l e c t r o s t a t i c i n t e r a c t i o n s . The p a r t i t i o n i n g o f a s o l u b i l i z a t e (S) between t h e b u l k s o l v e n t ( s o l ) and o r g a n i z e d s u r f a c t a n t ( s u r ) phase i s a dynamic e q u i l i b r i u m p r o c e s s w i t h t h e degree o f p a r t i t i o n i n g d e f i n e d by a p a r t i t i o n ( o r d i s t r i b u t i o n ) c o e f f i c i e n t P. The p a r t i t i o n c o e f f i c i e n t i s d e f i n e d as t h e r a t i o o f t h e s o l u t e c o n c e n t r a t i o n i n



16


F i g u r e 3. S i m p l i f i e d c r o s s s e c t i o n o f an aqueous normal m i c e l l e showing p o s s i b l e s o l u b i l i z a t i o n s i t e s . A charged s o l u t e (A) would be e l e c t r o s t a t i c a l l y r e p e l l e d from the m i c e l l e s u r f a c e i f i t were o f t h e same c h a r g e - t y p e as the i o n i c m i c e l l e w h i l e an o p p o s i t e l y charged s o l u t e (B) would be e l e c t r o s t a t i c a l l y a t t r a c t e d t o the m i c e l l a r s u r f a c e . Nonpolar s o l u t e s (C) would p a r t i t i o n t o t h e o u t e r p a r t o f the more h y d r o p h o b i c c o r e r e g i o n . A m p h i p h i l i c s o l u t e s (D) would attempt t o a l i g n t h e m s e l v e s so as t o maximize the e l e c t r o s t a t i c and h y d r o p h o b i c i n t e r a c t i o n s poss i b l e between i t s e l f and t h e s u r f a c t a n t m o l e c u l e s . "Reproduced w i t h p e r m i s s i o n f r o m R e f . 49. C o p y r i g h t 1984, E l s e v i e r


1.

HINZE

17

Surfactant Assemblies in Separation Science [S] i n a g g r e g a t e d s u r f a c t a n t [ S l i n bulk solvent

phase

phase

the o r g a n i z e d s u r f a c t a n t assembly phase t o t h a t i n the b u l k s o l v e n t phase ( e q u a t i o n 1) ( 1_). I n d i l u t e s o l u t i o n s , the p a r t i t i o n c o e f f i c i e n t , P, can be r e l a t e d t o a s o l u t e - s u r f a c t a n t a g g r e g a t e b i n d i n g c o n s t a n t , K^, by use o f t h e B e r e z i n e q u a t i o n (50) ( e q u a t i o n 2) i n which ν i s the molar volume of t h e s u r f a c t a n t i n the o r g a n i z e d s u r f a c t a n t medium. The b i n d i n g c o n s t a n t , K , f o r the i n t e r a c t i o n fe

K -(P-1)v b

of s o l u b i l i z a t e w i t h aggregate ( e q u a t i o n

( 2 )

3)

i s merely t h e r a t i o o f

Κ


S + sur

_

N

S«sur

(3)

the c o n c e n t r a t i o n o f the s o l u t e a s s o c i a t e d w i t h the o r g a n i z e d assembly [S«sur] d i v i d e d by the f r e e e q u i l i b r i u m c o n c e n t r a t i o n s of the uncomplexed s o l u t e [ S ] and s u r f a c t a n t a g g r e g a t e [ s u r ] , r e s p e c t i v e l y ( e q u a t i o n 4 ) . The [ s u r ] , sometimes d e s i g n a t e d C , i s g i v e n by m

Κ _ *>

[S-surJ [S] [sur]

(4)

the d i f f e r e n c e between the t o t a l s u r f a c t a n t c o n c e n t r a t i o n (CL) and the c r i t i c a l c o n c e n t r a t i o n d i v i d e d by t h e a g g r e g a t i o n number of the s u r f a c t a n t assembly (1_ 5). S i n c e the a s s o c i a t i o n r a t e o f most s o l u t e s w i t h s u r f a c t a n t a g g r e g a t e s i s c o n s t a n t (* 10 - 10 M~ s ) ( 2 1 6 ) , the l a r g e r the s o l u t e - aggregate b i n d i n g c o n s t a n t , K., the more s t a b l e i s the a s s o c i a t e d s o l u t e - s u r f a c t a n t aggregate complex and t h e l o n g e r i s the s o l u t e ' s r e s i d e n c e t i m e i n the o r g a n i z e d s u r f a c t a n t assembly environment (termed pseudophase). f

T a b l e s VI and V I I p r e s e n t some r e p r e s e n t a t i v e d a t a on the b i n d i n g c o n s t a n t s and p a r t i t i o n c o e f f i c i e n t s r e p o r t e d f o r t h e i n t e r a c t i o n o f s e l e c t e d s o l u t e s w i t h d i f f e r e n t s u r f a c t a n t m i c e l l a r systems. The s t r e n g t h of the a s s o c i a t i o n o f s o l u t e s w i t h s u r f a c t a n t m i c e l l e a s s e m b l i e s i s d i c t a t e d by the net e l e c t r o s t a t i c , hydrogen-bonding, and/or h y d r o p h o b i c i n t e r a c t i o n s p o s s i b l e f o r a g i v e n s o l u t e - m i c e l l e c o m b i n a t i o n under the p r e v a i l i n g e x p e r i m e n t a l c o n d i t i o n s . C o n s e q u e n t l y , as can be seen from the d a t a i n t h e T a b l e s , the c h a r g e - t y p e and c h a i n l e n g t h o f b o t h the s o l u t e and the m i c e l l e f o r m i n g s u r f a c t a n t as w e l l as presence o r absence o f a d d i t i v e s a r e i m p o r t a n t f a c t o r s which can i n f l u e n c e the magnitude o f the b i n d i n g c o n s t a n t s (or p a r t i t i o n c o e f f i c i e n t s ) . For i n s t a n c e , w i t h i n a g i v e n f a m i l y o f s o l u t e s (such as the p o l y c y c l i c a r o m a t i c h y d r o c a r b o n s or quinones i n T a b l e VI o r a l c o h o l s i n T a b l e V I I ) , the degree of p a r t i t i o n i n g / b i n d i n g t o the m i c e l l a r e n t i t y i n c r e a s e s w i t h i n c r e a s e s i n the s o l u t e h y d r o p h o b i c i t y . M e t a l i o n s can e l e c t r o s t a t i c a l l y i n t e r a c t w i t h and b i n d t o a n i o n i c c h a r g e - t y p e s u r f a c t a n t a s s e m b l i e s but not c a t i o n i c s ( r e f e r t o e n t r y f o r c o p p e r ( I I ) i n T a b l e V I ) . For i o n i z a b l e s o l u t e s , b o t h h y d r o p h o b i c and e l e c t r o s t a t i c i n t e r a c t i o n s a r e


18



TABLE V I .

Comparison o f some B i n d i n g C o n s t a n t s f o r t h e I n t e r a c t i o n of S o l u t e s w i t h S e l e c t e d O r g a n i z e d S u r f a c t a n t Systems

Solute

Organized Surfactant Assembly

2-Methyl-l,4-naphthoquinone (menadione) 2,3-Dimethy1-1,4-naphthoquinone Duroquinone

aq. aq.

Κ (M~ )

Ref.

1.2 X 1 0

NaLS m i c e l l e s

2.6 X


ίο

51

4

A

51

4

A

1.3 X 10^ aq. NaLS m i c e l l e s BHAC r e v e r s e d m i c e l l e *3 u 3.5 - 4.4 i n benzene b

Naphthalene Anthracene Pyrene

aq.

1 0

It

52 1,53

2.0 X 5 4.0 X 6 1.7 X 1 0


51

1 0

II

1-Me thy1qu i n o 1 i n i u m ion 10-Methylacridinium ion S i l v e r (I) i o n Nickel (II) ion Copper ( I I ) i o n Hydrogen i o n (H ) Copper-benzoyla c e t o n e complex

f

p,p -DDT

6

A

aq.

53

4.8 X 5 1.4 X 1 0


1 0

II

5

Ο

aq. aq. aq.

NaLS m i c e l l e s DTAC m i c e l l e s NaLS m i c e l l e s

1.3 X 3 2.4 X 1 0 245' 0.002 13.1 8

aq. aq.

NaLS m i c e l l e s DTAC m i c e l l e s

4.9 X 11. i8°

io

aq. + + + +

CTAOH added added added added HexOH

1.5 1.8 3.0 2.7 5.0

10 10^ 10^ 10 10

aq.


53

1 0

II

3

micelles BuOH HexOH KBr KBr & g

g

£

6

χ χ χ χ χ

54 55

3c J C

54

56

BHAC = hexadecylbenzyld^methylammonium c h l o r i d e . R e f e r s t o the e q u i l i b r i u m c o n s t a n t (dm /mol) f o r d i s t r i b u t i o n o f s o l u t e between the r e v e r s e d m i c e l l a r wat^r p o o l and the b u l k o r g a n i c phase. Equilibrium

constant

(dm /mol) a r e g i v e n on a p e r monomer

basis

4

αϊ )· DDT = l , l , l - t r i c h l o r o - 2 , 2 - b i s ( p - c h l o r o p h e n y l ) e t h a n e . ^CTAOH = Hexadecyltrimethylammonium h y d r o x i d e . Amount o f a l c o h o l added i s < 0.07 M. Amount o f KBr added i s < 0.10 M. g


1.

TABLE V I I .


19


HINZE

Summary o f P a r t i t i o n C o e f f i c i e n t s f o r the D i s t r i b u t i o n of S o l u t e s between Normal M i c e l l a r and Aqueous Pseudophases

Solute

Aqueous Normal M i c e l l a r System

Partition Coefficient

Ref.

1- H e p t a n o l

NaDC NaLS

2000 2650 1500 1010 930

207 57

1.8- 0 c t a n e d i o l 1.9- N o n a n e d i o l 1.10- D e c a n e d i o l

NaLS

311 743 3800

57

1-Pentanol

NaDC NaLS NaDeS* SFONa NaLS/SFONa" NaLS/SFONa

100 820 650 535 755 1200

207 58

Chloropentaammine cobalt(III)

NaLS

1.5 χ 10*

59

Propranol Penthianatemethobromide

CTAB CTAB

0.43 0.24

60

2- H e p t a n o l 3- H e p t a n o l 4- H e p t a n o l

a

NaDeS = sodium d e c y l s u l f a t e .

b

SF0Na

sodium

perfluorooctanoate.

Mixed m i c e l l e i n which t h e mol f r a c t i o n o f SFONa i s 0.50. Mixed m i c e l l e i n w h i c h the mol f r a c t i o n o f SFONa i s 0.147.


20


p o s s i b l e . For example, t h e b i n d i n g c o n s t a n t s f o r t h e i n t e r a c t i o n o f p r o t o n a t e d and u n p r o t o n a t e d R - m e t h y l t h i o p h e n o l w i t h c a t i o n i c CTAB normal m i c e l l e s a r e 1.0 χ 1 0 and 8.3 x 10 Μ , r e s p e c t i v e l y ( 6 1 ) . The l a r g e r b i n d i n g c o n s t a n t f o r t h e t h i o p h e n o l a t e i o n merely r e f l e c t s the a d d i t i o n a l e l e c t r o s t a t i c c o n t r i b u t i o n t o t h e b i n d i n g i n t e r a c t i o n compared t o t h a t p o s s i b l e f o r t h e n e u t r a l t h i o p h e n o l . The a d d i t i o n o f a d d i t i v e s (such as s a l t o r a l c o h o l s ) can a l s o i n f l u e n c e t h e magnitude of t h e b i n d i n g i n t e r a c t i o n ( r e f e r t o d a t a on DDT i n T a b l e V I ) .


3

To summarize, t h e b i n d i n g i n t e r a c t i o n o b s e r v e d ( o r d e s i r e d i n particular separation application) for a s p e c i f i c solute with a s u r f a c t a n t assembly can be c o n t r o l l e d by (1) v a r i a t i o n o f t h e s u r f a c t a n t c o n c e n t r a t i o n ( e q u a t i o n s 3 and 4 ) , (2) v a r i a t i o n o f t h e c h a r g e - t y p e and/or carbon c h a i n l e n g t h o f t h e s u r f a c t a n t ( r e f e r t o d a t a on d u r o q u i n o n e , T a b l e VI and 1 - p e n t a n o l , T a b l e V I I ) , and (3) a d d i t i o n o f a p p r o p r i a t e a d d i t i v e s ( r e f e r t o DDT data i n T a b l e V I ) . By m a n i p u l a t i o n o f t h e e x p e r i m e n t a l c o n d i t i o n s j u s t mentioned, i t i s p o s s i b l e t o observe d i f f e r e n c e s i n t h e b i n d i n g / p a r t i t i o n i n g f o r d i f f e r e n t f a m i l i e s o f s o l u t e s (Table V I ) as w e l l as f o r p o s i t i o n a l i s o m e r s (see d a t a i n T a b l e V I I on h e p t a n o l i s o m e r s ) w i t h o r g a n i z e d s u r f a c t a n t media. I n a d d i t i o n , d i f f e r e n c e s i n t h e b i n d i n g o f e n a n t i o m e r s have been o b s e r v e d i n a few cases (J_5,_62). The f a c t t h a t one can u t i l i z e d i f f e r e n t s u f a c t a n t o r g a n i z e d a s s e m b l i e s t o d i f f e r e n t i a l l y s o l u b i l i z e and b i n d a v a r i e t y o f s o l u t e m o l e c u l e s s e r v e s as t h e main b a s i s f o r t h e i r s u c c e s s f u l use i n s e p a r a t i o n s c i e n c e ( j _ , 5 ) . A d d i t i o n a l l y , some o f t h e o t h e r p r e v i s o u l y mentioned unique p r o p e r t i e s o f s u r f a c t a n t s o l u t i o n s and o r g a n i z e d s u r f a c t a n t systems can be j u d i c i o u s l y e x p l o i t e d i n o r d e r t o a i d t h e s e p a r a t i o n s c i e n t i s t i n some s p e c i f i c a p p l i c a t i o n s as w i l l be d e t a i l e d i n l a t e r sections o f t h i s overview. D i f f e r e n t Uses and E x p l o i t a t i o n o f S u r f a c t a n t Systems i n S e p a r a t i o n Science O r g a n i z e d s u r f a c t a n t a s s e m b l i e s have found a m a z i n g l y d i v e r s e and numerous p r a c t i c a l a p p l i c a t i o n s i n many a r e a s o f s e p a r a t i o n s c i e n c e . Space l i m i t a t i o n s p r e c l u d e an e x h a u s t i v e r e v i e w o f a l l such systems and a p p l i c a t i o n s . C o n s e q u e n t l y , o n l y c e r t a i n r e p r e s e n t a t i v e examples w i l l be g i v e n i n many i n s t a n c e s t o i l l u s t r a t e t h e c u r r e n t s t a t e - o f - t h e - a r t , w i t h emphasis g i v e n t o t h e more r e c e n t l y developed techniques. P o t e n t i a l a r e a s f o r f u r t h e r r e s e a r c h and f u t u r e developments w i l l be i d e n t i f i e d . The main t o p i c s t o be covered i n c l u d e : use o f s u r f a c t a n t s as m o b i l e phase a d d i t i v e s and/or s t a t i o n a r y phase m o d i f i c a t i o n r e a g e n t s i n chromatographic s e p a r a t i o n s , w i t h emphasis on m i c e l l a r l i q u i d chromatography; m i c e l l a r e l e c t r o k i n e t i c c a p i l l a r y chromatography; s u r f a c t a n t mediated s o l u b i l i z a t i o n and e x t r a c t i o n schemes; s u r f a c t a n t enhanced d e t e c t i o n schemes i n s e p a r a t i o n s c i e n c e ; a b r i e f d e s c r i p t i o n o f some m i s c e l l a n e o u s a p p l i c a t i o n s o f s u r f a c t a n t s ; and l a s t l y , a s e c t i o n on some e x p e r i m e n t a l c o n s i d e r a t i o n s i n c l u d i n g s u r f a c t a n t and/or s o l u t e recovery i n surfactant-mediated separations. S u r f a c t a n t - M e d i a t e d Chromatographic S e p a r a t i o n s . The s e l e c t i v e i n t e r a c t i o n o f s u r f a c t a n t s w i t h a v a r i e t y o f s o l u t e s (as i o n p a i r s w i t h monomeric s u r f a c t a n t m o l e c u l e s o r as bound ( " a s s o c i a t e d " ) s p e c i e s


1.

HINZE


21


with m i c e l l a r , vesicular, or l i q u i d / c r y s t a l l i n e organized surfactant media) e n a b l e s them t o be a p p l i e d i n chromatography. From an o p e r a t i o n a l v i e w p o i n t , t h e r e can be two t y p e s o f approaches t o t h e i r use i n such chromatographic s e p a r a t i o n s . F i r s t , they can be employed i n c h r o m a t o g r a p h i c m o b i l e phases. An e l u t i n g s o l v e n t ( m o b i l e phase) c o n t a i n s t h e s u r f a c t a n t ( s ) and s o l u t e s d i s t r i b u t e between t h e s t a t i o n a r y phase ( u s u a l l y s u r f a c t a n t m o d i f i e d ) and t h e s u r f a c t a n t (or s u r f a c t a n t a g g r e g a t e ) i n t h e m o b i l e phase ( F i g u r e 4 ) . Alternatively, s u r f a c t a n t s o r s u r f a c t a n t o r g a n i z e d a s s e m b l i e s can be i m m o b i l i z e d i n (or o n t o ) a s t a t i o n a r y phase. S o l u t e s a r e thus d i s t r i b u t e d between a c o n v e n t i o n a l m o b i l e phase and t h e s u r f a c t a n t - m o d i f i e d s t a t i o n a r y phase. Use o f S u r f a c t a n t s i n Chromatographic M o b i l e Phases. (1) P l a n a r and High-Performance L i q u i d Chromatography. Perhaps t h e most r e c e n t development c o n c e r n i n g t h e u t i l i z a t i o n o f s u r f a c t a n t s i n chromatography c o n c e r n s t h e i r use as LC m i c e l l a r m o b i l e phases 0 - 8 ) . S u r f a c t a n t s had p r e v i o u s l y been s u c c e s s f u l l y employed as mobile phase a d d i t i v e s i n s o - c a l l e d i o n - p a i r (68-73»Π 8), soap (74), h y d r o p h o b i c (75,173)» dynamic soap (76), i o n i n t e r a c t i o n (77), h e t a e r i c (78), d e t e r g e n t - b a s e d c a t ion-exchange (7_9) o r s u r f a c t a n t chromatography (_69). There i s s t i l l c o n s i d e r a b l e debate c o n c e r n i n g t h e r e t e n t i o n mechanism i n t h i s p a r t i c u l a r s e p a r a t i o n mode employing s u r f a c t a n t s as a d d i t i v e s i n t h e m o b i l e phase (68,_69,71_,72). F o r t h e s e a p p l i c a t i o n s , the s u r f a c t a n t c o n c e n t r a t i o n s and/or c o n d i t i o n s a r e such t h a t no m i c e l l a r aggregates form. That i s , s u r f a c t a n t c o n c e n t r a t i o n s a r e below t h e CMC v a l u e o r c o n d i t i o n s ( h i g h c o n c e n t r a t i o n s o f added a l c o h o l s ) a r e such t h a t m i c e l l e s do n o t f o r m . I n f a c t , d e v i a t i o n s from t h e expected i o n - p a i r r e t e n t i o n b e h a v i o r observed a t h i g h e r s u r f a c t a n t c o n c e n t r a t i o n s i s u s u a l l y a t t r i b u t e d t o m i c e l l e o r mixed m i c e l l e f o r m a t i o n (71,79-82,118,121 J 7 3 ) . F u r t h e r i n f o r m a t i o n on t h e use o f s u r f a c t a n t s as i o n - p a i r i n g r e a g e n t s i n chromatography a r e g i v e n i n s e v e r a l f i n e r e v i e w s (_69,8l_,83) as w e l l as i n a Chapter by M u l l i n s i n t h i s Symposium Volume (84). The f i r s t i n t e n t i o n a l u s e o f s u r f a c t a n t s i n c h r o m a t o g r a p h i c m o b i l e phases a t c o n c e n t r a t i o n s above t h e CMC was proposed i n 1977 by Armstrong and co-workers (1,86-100). S i n c e t h e i n i t i a l r e p o r t s , t h e g e n e r a l method, dubbed pseudophase l i q u i d chromatography (PLC) o r m i c e l l a r l i q u i d chromatography (MLC), has moved from t h e r e a l m o f an academic n o v e l t y t o a demonstrated p r a c t i c a l s e p a r a t i o n t e c h n i q u e . The b a s i s f o r s e p a r a t i o n employing m i c e l l a r m o b i l e phases stems from t h e i r a b i l i t y t o d i f f e r e n t i a l l y s o l u b i l i z e and b i n d s t r u c t u r a l l y s i m i l a r s o l u t e s . S k e p t i c s view MLC as a f a s c i n a t i n g example o f t h e i n c o r p o r a t i o n o f secondary e q u i l i b r i a f o r c o n t r o l o r adjustment o f r e t e n t i o n (101). However, i t i s t h e u l t i m a t e o f secondary e q u i l i b r i a s i n c e t h e types o f i n t e r a c t i o n s p o s s i b l e w i t h m i c e l l a r aggregates cannot be d u p l i c a t e d by any s i n g l e o t h e r e q u i l i b r i u m system, o r f o r t h a t m a t t e r , any one o r m i x t u r e o f t r a d i t i o n a l normal o r r e v e r s e d phase m o b i l e phase systems. T h i s i s due t o t h e f a c t t h a t s o l u t e s can i n t e r a c t w i t h t h e s u r f a c t a n t aggregates v i a hydrophobic, e l e c t r o s t a t i c , hydrogen b o n d i n g , and/or a c o m b i n a t i o n o f t h e s e factors. A m i c e l l a r m o b i l e phase can be viewed as b e i n g composed o f both t h e s u r f a c t a n t m i c e l l a r a g g r e g a t e s (pseudophase) and t h e r e s t o f t h e



22


F i g u r e 4. A r t i s t i c r e p r e s e n t a t i o n o f t h e s p e c i e s and e q u i l i b r i a p r e s e n t when employing s u r f a c t a n t m i c e l l a r m o b i l e phases i n LC.


1.

HINZE

23



b u l k s o l v e n t ( F i g u r e 4 ) . A s o l u t e t h u s d i s t r i b u t e s between the b u l k s o l v e n t - s u r f a c t a n t m o d i f i e d s t a t i o n a r y phase (P ) and between the b u l k s o l v e n t - m i c e l l a r pseudophase (Ρ ). C o n s e q u e n t l y , t h e r e a r e two p a r t i t i o n c o e f f i c i e n t s which the s e p a r a t i o n s c i e n t i s t can t r y t o m a n i p u l a t e i n o r d e r t o a c h i e v e a d e s i r e d s e p a r a t i o n . The b a s i c f o r m u l a s r e l a t i n g t h e s e two p a r t i t i o n c o e f f i c i e n t s and r e t e n t i o n ( i n terms o f the r e c i p r o c a l o f the c a p a c i t y f a c t o r ) t o the m i c e l l e c o n c e n t r a t i o n are g i v e n i n e q u a t i o n s 5 and 6 f o r TLC and HPLC, respectively:

1

where R^, and k are the r e t a r d a t i o n and c a p a c i t y f a c t o r s , r e s p e c t i v e l y ; φ i s the phase r a t i o ( e q u a l t o V /V where V" and Y are the s t a t i o n a r y - p h a s e and v o i d volumes, r e s p e c t i v e l y ) ; C i s the m i c e l l e c o n c e n t r a t i o n [ e q u a l t o (C -CMC)/N where C i s the t o t a l s u r f a c t a n t c o n c e n t r a t i o n ] ; Κ i s the m i c e l l e - s o l u t e b i n d i n g c o n s t a n t (equal to ( P - 1)v, where Ρ i s the p a r t i t i o n c o e f f i c i e n t f o r d i s t r i b u t i o n o f t h e s o l u t e between the m i c e l l a r and b u l k s o l v e n t p h a s e s ) ; and Ρ i s the p a r t i t i o n c o e f f i c i e n t f o r d i s t r i b u t i o n o f the s o l u t e between the b u l k s o l v e n t and s t a t i o n a r y phases (1,96,98,102). These e q u a t i o n s can be employed t o d e s c r i b e o r p r e d i c t the r e t e n t i o n b e h a v i o r e x h i b i t e d by n e u t r a l s o l u t e s o r i o n i z a b l e s o l u t e s , p r o v i d e d t h a t t h e r e i s o n l y one form of t h e s o l u t e p r e s e n t o v e r the s u r f a c t a n t c o n c e n t r a t i o n range examined a t a p a r t i c u l a r pH ( i n the l a t t e r c a s e , i t would be n e c e s s a r y t o bear i n mind t h a t the K term must be f o r the a c t u a l form o f t h e s p e c i e s p r e s e n t ) . s

s

Q

T

s m

S

fe

I n the case o f an i o n i z a b l e s o l u t e where b o t h the a c i d and c o n j u g a t e base (or base and c o n j u g a t e a c i d ) forms are p r e s e n t , the f o l l o w i n g e q u a t i o n p r e d i c t s the dependence o f k' upon pH (at c o n s t a n t s u r f a c t a n t c o n c e n t r a t i o n ) or s u r f a c t a n t c o n c e n t r a t i o n (at c o n s t a n t pH):

K

^

1+

W

+

C

Vm

)]

+

+

[1

k

+ K

,

c b

bc

[

1

( C

+

m

K

bc

) ]

V

(

C

m

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K

i

/

[

H

+

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(7)

[ H + ]

where and K, a r e the b i n d i n g c o n s t a n t s f o r the i n t e r a c t i o n o f the weak a c i d and i ? s c o n j u g a t e b a s e , r e s p e c t i v e l y ; k^ and k£ a r e the l i m i t i n g c a p a c i t y f a c t o r s o f the weak a c i d and i t s c o n j u g a t e b a s e , r e s p e c t i v e l y ; C i s the m i c e l l e c o n c e n t r a t i o n as p r e v i o u s l y d e f i n e d ; and K.. i s the a p p a r e n t i o n i z a t i o n c o n s t a n t f o r the weak a c i d ( 1 0 3 ) . A s i m i l a r e q u a t i o n can be d e r i v e d f o r weak bases and t h e i r c o n j u g a t e a c i d s (103)· I t s h o u l d be n o t e d t h a t e q u a t i o n s 5, 6, and 7 can be r e - e x p r e s s e d i n terms o f s e v e r a l o t h e r c h r o m a t o g r a p h i c parameters o r by use o f p a r t i t i o n c o e f f i c i e n t s r a t h e r than b i n d i n g c o n s t a n t s U). fe


24



The d e r i v a t i o n o f t h e s e d i f f e r e n t r e t e n t i o n e q u a t i o n s i s i m p o r t a n t i n s e v e r a l r e s p e c t s . F i r s t , they a l l o w f o r c a l c u l a t i o n o f m i c e l l e - s o l u t e b i n d i n g c o n s t a n t s , parameters which a r e i m p o r t a n t i n many a r e a s of m i c e l l a r k i n e t i c s or c h e m i s t r y . There have been s e v e r a l r e p o r t s i n the l i t e r a t u r e d e m o n s t r a t i n g t h i s c h r o m a t o g r a p h i c approach f o r d e t e r m i n a t i o n o f m i c e l l e - s o l u t e b i n d i n g c o n s t a n t s (1,8,104,105). More i m p o r t a n t l y , t h e y a l l o w f o r p r e d i c t i o n of r e t e n t i o n b e h a v i o r as a f u n c t i o n o f s u r f a c t a n t c o n c e n t r a t i o n (or o f pH at c o n s t a n t m i c e l l e c o n c e n t r a t i o n ) , p r o v i d e d t h a t the m i c e l l e - s o l u t e b i n d i n g c o n s t a n t (or s o l u t e i o n i z a t i o n c o n s t a n t ) i s known (which can be d e t e r m i n e d s p e c t r o s c o p i c a l l y or from k i n e t i c s t u d i e s ) (1,96,102). C o n s e q u e n t l y , t h e t h e o r y a l l o w s t h e chromatographer t o determine t h e optimum conditions required for a desired separation. E x a m i n a t i o n of e q u a t i o n s 5, 6, and 7 r e v e a l s t h a t r e t e n t i o n can be c o n t r o l l e d by v a r i a t i o n o f the s u r f a c t a n t m i c e l l e c o n c e n t r a t i o n , v a r i a t i o n o f pH ( f o r i o n i z a b l e s p e c i e s ) , and by m a n i p u l a t i o n o f the s o l u t e - m i c e l l e b i n d i n g c o n s t a n t (K.) w h i c h , i n t u r n can be i n f l u e n c e d by a d d i t i v e s ( s a l t , a l c o h o l ; r e f e r t o d a t a on DDT, T a b l e VI) o r the t y p e (charge and h y d r o p h o b i c i t y ) o f m i c e l l e - f o r m i n g s u r f a c t a n t employed ( r e f e r t o d a t a i n T a b l e V I I f o r 1 - p e n t a n o l ) . T a b l e V I I I summarizes some o f the f a c t o r s t h a t i n f l u e n c e r e t e n t i o n f o r s u r f a c t a n t - c o n t a i n i n g m o b i l e phases and compares the e f f e c t of changes i n t h e s e f a c t o r s upon the r e t e n t i o n b e h a v i o r o b s e r v e d i n b o t h m i c e l l a r l i q u i d and i o n - p a i r chromatography ( 8 1 ) . I n a d d i t i o n t o the f a c t o r s l i s t e d i n T a b l e V I I I , t h e n a t u r e o f the s u r f a c t a n t - m o d i f i e d s t a t i o n a r y phase a f f e c t s Ρ (partition c o e f f i c i e n t f o r d i s t r i b u t i o n o f s o l u t e between bulîPsolvent and m o d i f i e d s t a t i o n a r y phases) and t h u s w i l l i n f l u e n c e the r e t e n t i o n o b s e r v e d . I t s h o u l d be r e a l i z e d t h a t most of the normal and reversed-phase packing m a t e r i a l s w i l l adsorb/absorb s u r f a c t a n t m o l e c u l e s from t h e m o b i l e phase s o l u t i o n and become c o a t e d t o d i f f e r e n t degrees when s u r f a c t a n t m o b i l e phases a r e p a s s e d t h r o u g h them. Numerous a d s o r p t i o n i s o t h e r m s have been r e p o r t e d f o r v a r i o u s s u r f a c t a n t - s t a t i o n a r y phase c o m b i n a t i o n s i l l u s t r a t i n g t h i s p o i n t (82,85,106,115-128,206). The p r e s e n c e o f a d d i t i v e s can mediate the amount o f s u r f a c t a n t s u r f a c e coverage o b t a i n e d (110-129,175,206). I t has been p o s t u l a t e d t h a t the a r c h i t e c t u r e w h i c h adsorbed s u r f a c t a n t m o l e c u l e s can assume on c o n v e n t i o n a l s t a t i o n a r y phases can range from m i c e l l a r , h e m i - m i c e l l a r , o r a d m i c e l l a r t o mono-,bi-, o r m u l t i l a y e r e d , and/or o t h e r l i q u i d c r y s t a l l i n e - t y p e s t r u c t u r e s (93,106,124,128,129, 132,208,212,217,221). I n a few c a s e s , i t has been r e p o r t e d t h a t t h e r e can be a r e l a t i v e l y s l o w r e o r g a n i z a t i o n of t h e s t a t i o n a r y phase s u r f a c t a n t s t r u c t u r e (137) and s i m i l a r a g e i n g ( s t o r a g e ) e f f e c t s on the m i c e l l a r aggregate s t r u c t u r e i n s o l u t i o n have been noted as w e l l ( 1 3 8 ) . A l s o , the s t r u c t u r a l parameters ( i . e . pore diameter and p a r t i c a l s i z e d i s t r i b u t i o n ) of the s t a t i o n a r y phase p a c k i n g m a t e r i a l can be a l t e r e d due t o s u r f a c t a n t a d s o r p t i o n (133-135)· The f a c t t h a t many s t a t i o n a r y phase p r o p e r t i e s a r e s u b s t a n t i a l l y a l t e r e d by the p r o c e s s o f s u r f a c t a n t a d s o r p t i o n has i m p o r t a n t i m p l i c a t i o n s w i t h r e g a r d t o c h r o m a t o g r a p h i c r e t e n t i o n and e f f i c i e n c y (93,106,110,126-128). A r e v i e w on the r o l e o f the s t a t i o n a r y phase i n MLC i s g i v e n i n a Chapter by B e r t h o d , e t a l i n t h i s Volume ( 1 3 6 ) .


TABLE V I I I .

Comparison o f the G e n e r a l E f f e c t o f V a r i a b l e s on R e t e n t i o n i n Reversed-Phase I o n - P a i r (RP-IPC) and M i c e l l a r L i q u i d Chromatography (MLC)

Factor Varied


25


1. HINZE

E f f e c t upon R e t e n t i o n a i n RP-MLC i n RP-IPC

Concentration of surfactant of m o b i l e phase

Increasing concentration increases retenti o n (up t o a l i m i t )

Increasing concentration decreases r e t e n t i o n (down jto a l i m i t i n g value)

P r e s e n c e o f an organic modifier (added a l c o h o l o r acetonitrile)

Retention decreases w i t h i n c r e a s i n g concent r a t i o n or hydrophobic i t y o f the o r g a n i c additive

Same as i n RP-IPC

PH

R e t e n t i o n i n c r e a s e s as pH m a n i p u l a t i o n maxim i z e s the c o n c e n t r a t i o n of t h e i o n i c form o f the s o l u t e

Depends upon t h e nature ( i . e . chargetype and c o n c e n t r a t ion) o f the s u r f a c t ant m i c e l l e and i o n i z a b l e s o l u t e ; eq. 7 predicts a sigmoidaltype dependence between r e t e n t i o n and pH ( a t c o n s t a n t s u r factant c o n c e n t r a t ion)

Temperature

R e t e n t i o n i n c r e a s e s as temperature d e c r e a s e s

Retention decreases s l i g h t l y as temperature increases

Ionic

R e n t i o n d e c r e a s e s as ionic strength i n linear creases dej)çndence__1jet;ween

Strength

^ I n f o r m a t i o n taken 114,121,131.

from Réf. 81.

u

See references

1,96,102,105,106,

°See r e f e r e n c e s 106-112,121,130,131,154,201; some e x c e p t i o n s w i t h s h o r t - c h a i n a l c o h o l s (MeOH) (110). T a b l e IX shows t h e e f f e c t o f a l t e r a t i o n o f a l c o h o l h y d r o p h o b i c i t y i n MLC r e t e n t i o n (112). d e See r e f e r e n c e 103. R e f e r t o r e f e r e n c e 113, F i g u r e 2. f

very

R e f e r t o r e f e r e n c e s 98,99,107,108,110,130,131). There a r e some a p p a r e n t n o t a b l e e x c e p t i o n s t o the g e n e r a l t r e n d , s e e , f o r i n s t a n c e , Ref. 98,99,103,110.


26


TABLE IX.

E f f e c t o f Added A l c o h o l s upon the C h r o m a t o g r a p h i c R e t e n t i o n and E f f i c i e n c y o f 2 - E t h y l a n t h r a q u i n o n e u s i n g a M i c e l l a r Sodium D o d e c y l s u l f a t e M o b i l e Phase and a C-18 Reversed Phase Column


Additive

Capacity Factor

b

Ν

none

37.1

50

5% added methanol

27.3

56

5% added

20.6

100

13.2

320

9.5

725

2% added n - p e n t a n o l 5% "

12.3 7.3

810

5% added DMSO

22.1

ethanol

5% added n - p r o p a n o l 5% added n - b u t a n o l

temperature

23.5° C, d a t a t a k e n from Ref.

112.

^The m i c e l l a r m o b i l e phase i n a l l e x p e r i m e n t s c o n s i s t e d o f aqueous 0.285M NaLS, f l o w r a t e l.OOmL/min, 10-cm column.


1. HINZE

27



From an e x p e r i m e n t a l s t a n d p o i n t , i t i s i m p o r t a n t t o p r o p e r l y e q u i l i b r a t e t h e column w i t h t h e s u r f a c t a n t m o b i l e phase p r i o r t o use so t h a t r e p r o d u c i b l e c h r o m a t o g r a p h i c r e s u l t s can be o b t a i n e d . In terms o f chromatographic a p p l i c a t i o n s , t h e advantages o f employing s u r f a c t a n t m i c e l l a r m o b i l e phases t h a t have been c i t e d i n c l u d e : enhanced s e l e c t i v i t y , low c o s t , low t o x i c i t y , ease o f m o b i l e phase d i s p o s a l , ease o f p u r i f i c a t i o n o f t h e m o b i l e phase ( i . e . water and s u r f a c t a n t ) , and t h e a b i l i t y t o s i m u l t a n e o u s l y chromatograph both h y d r o p h i l i c and h y d r o p h o b i c s o l u t e s among o t h e r s (1-3>88,95,97,111)· More r e c e n t l y , s e v e r a l o t h e r unique c h r o m a t o g r a p h i c advantages o b t a i n a b l e have been r e p o r t e d ( 1 3 9 ) . F i r s t , use o f some m i c e l l a r m o b i l e phases a l l o w s f o r more c o n v e n i e n t and r a p i d g r a d i e n t e l u t i o n ( i . e . g r a d i e n t i n terms o f m i c e l l a r c o n c e n t r a t i o n ) compared t o t h a t p o s s i b l e w i t h c o n v e n t i o n a l h y d r o - o r g a n i c m o b i l e phases (140,141). S e c o n d l y , i t has been r e p o r t e d t h a t u t i l i z a t i o n o f r e v e r s e m i c e l l a r m o b i l e phases (AOT i n hexane) i n normal phase chromatography can g r e a t l y reduce o r e l i m i n a t e t h e s o l u t e r e t e n t i o n dependence upon water c o n t e n t t h a t i s u s u a l l y o b s e r v e d i n normal phase LC ( 1 4 2 ) . T h i r d , t h e use o f some m i c e l l a r m o b i l e phases a l l o w s f o r new o r enhanced modes o f d e t e c t i o n i n TLC or HPLC. More d e t a i l s on t h i s p o i n t w i l l be p r e s e n t e d i n a l a t t e r s e c t i o n o f t h i s r e v i e w a r t i c l e . Some o f t h e s e unique c h r o m a t o g r a p h i c c a p a b i l i t i e s o f m i c e l l a r m o b i l e phases a r e d i s c u s s e d i n more d e t a i l i n a Chapter by Dorsey i n t h i s Symposium Volume ( 1 4 3 ) . A f o u r t h major r e a s o n f o r employing such m i c e l l a r phases i n HPLC i s t h a t they allow f o r the d i r e c t i n j e c t i o n of untreated b i o l o g i c a l f l u i d s ( u r i n e , plasma, s a l i v a ) (144-149,218) as w e l l as waste water samples ( 1 1 2 ) . Thus, t h i s t e c h n i q u e i s v e r y u s e f u l i n t h e r a p e u t i c drug m o n i t o r i n g s i n c e t h e m i c e l l a r s o l u t i o n can s o l u b i l i z e t h e serum/urine p r e v e n t i n g p r o t e i n p r e c i p i t a t i o n and d i s p l a c e t h e d r u g / a n a l y t e from t h e serum/urine components t h u s a l l o w i n g t h e a n a l y t e t o p a r t i t i o n t o t h e s u r f a c t a n t m o d i f i e d s t a t i o n a r y phase (144-149). C o n s e q u e n t l y , m i n i m a l sample p r e p a r a t i o n i s r e q u i r e d and t h e a n a l y s i s t i m e i s r e d u c e d . There w i l l no doubt be f u r t h e r b r e a k - t h r o u g h s i n t h i s f a s t - m o v i n g f i e l d i n t h e near f u t u r e w i t h r e s p e c t t o n o v e l c h r o m a t o g r a p h i c advantages o f s u r f a c t a n t - c o n t a i n i n g m o b i l e phases. L a s t l y , t h e use o f m i c e l l a r m o b i l e phases a l l o w s a c o n v e n i e n t means o f s t u d y i n g m i c e l l e - s o l u t e i n t e r a c t i o n s ( i . e . d e t e r m i n a t i o n o f b i n d i n g c o n s t a n t s ) (1,104,105) as w e l l as d e t e r m i n a t i o n o f s u r f a c t a n t CMC v a l u e s (from b r e a k s i n t h e l o g k* . vs. log C plots) (64,109,148,172). I n t h i s a r e a , t h e more i m p o r t a n t a p p l i c a t i o n i s i t s use i n t h e d e t e r m i n a t i o n o f b i n d i n g c o n s t a n t s (J_). T

The main d i s a d v a n t a g e s o f m i c e l l a r chromatography a r e t h e o b s e r v e d d i m i n i s h e d c h r o m a t o g r a p h i c e f f i c i e n c y , h i g h e r column back p r e s s u r e , and i n p r e p a r a t i v e work, t h e need t o s e p a r a t e t h e f i n a l r e s o l v e d a n a l y t e from t h e s u r f a c t a n t (95) (a l a t e r s e c t i o n o f t h i s r e v i e w w i l l d i s c u s s t h i s l a t t e r problem and i t s r e s o l u t i o n i n f u r t h e r d e t a i l ). The h i g h e r column back p r e s s u r e and p a r t o f t h e d e c r e a s e d e f f i c i e n c y stem from t h e f a c t t h a t s u r f a c t a n t - c o n t a i n i n g m o b i l e phases a r e more v i s c o u s compared t o t h e u s u a l h y d r o - o r g a n i c m o b i l e phases employed i n c o n v e n t i o n a l RP-HPLC ( r e f e r t o v i s c o s i t y d a t a i n T a b l e X)


28


TABLE X.


V i s c o s i t y o f Commonly Employed S o l u t i o n s i n M i c e l l a r L i q u i d Chromatography

Surfactant

System

Methanol alone D i s t i l l e d Water 0.10 0.10 0.10 0.10

M M M M

V i s c o s i t y , cP

0.55 1.01

alone

NaLS CTAC SB-12 NaDC

1.21 1.31 1.16 1.32

0.27 M CTAB + 50% n-Bu0H 0.40 0.40 0.40 0.43 0.04

a

M M M M M

NaLS CTAC SB-12 CPC DODAB ( V e s i c l e

b

System)

4.48 2.27 2.46 1.76 3.40 5.2

D a t a t a k e n a t 25.6° C; R e f e r e n c e 112.

^ P r o b a b l y a m i c r o e m u l s i o n system.



1. HINZE

29


(106,112,118). Due t o t h e r e l a t i v e l y h i g h v i s c o s i t y o f s u r f a c t a n t v e s i c l e and m i c r o e m u l s i o n systems ( r e f e r t o d a t a on DODAB and CTAB/50Î BuOH i n T a b l e X ) , t h e i r use i n HPLC w i l l be l i m i t e d s i n c e lower f l o w r a t e s would be r e q u i r e d which would l e n g t h e n t h e r e q u i r e d t i m e f o r a s e p a r a t i o n . A d d i t i o n a l l y , most s u r f a c t a n t v e s i c u l a r (112) as w e l l as some m i c e l l a r s o l u t i o n s a r e o p t i c a l l y opaque which l i m i t s t h e wavelength range a v a i l a b l e f o r s p e c t r o s c o p i c d e t e c t i o n u n l e s s a postcolumn d i l u t i o n s t e p i s employed (219). The major c o n t r i b u t i o n s which r e s u l t i n t h e r e d u c e d c h r o m a t o g r a p h i c e f f i c i e n c y have been a s c r i b e d t o slow mass t r a n s f e r p r i n c i p a l l y due t o poor w e t t i n g o f t h e s u r f a c t a n t m o d i f i e d s t a t i o n a r y phase ( 1 0 9 ) , poor mass t r a n s f e r between t h e m i c e l l e and s t a t i o n a r y phase ( 1 1 3 ) , and poor mass t r a n s f e r i n t h e s t a t i o n a r y phase (100,106). In some c a s e s , t h e use o f s m a l l amounts o f a l c o h o l a d d i t i v e s (MeOH, n-Pr0H) and o p e r a t i o n a t e l e v a t e d temperature (40 C) r e s u l t i n c h r o m a t o g r a p h i c e f f i c i e n c i e s comparable t o t h a t seen i n t r a d i t i o n a l LC u s i n g h y d r o - o r g a n i c m o b i l e phases (109,113,154,206). I n o u r own work, we have found n-pentanol t o be s u p e r i o r t o n-propanol i n t h i s r e g a r d ( r e f e r t o T a b l e I X ) ( 1 1 2 ) . F u r t h e r work i s c l e a r l y needed i n t h i s e f f i c i e n c y area i n order t o c l a r i f y the exact reason(s) f o r the r e d u c t i o n i n e f f i c i e n c y . I t appears t h a t a c o m b i n a t i o n o f f a c t o r s can c o n t r i b u t e t o t h i s e f f e c t w i t h t h e dominant e f f i c i e n c y r e d u c t i o n mode dependent upon t h e n a t u r e o f t h e s o l u t e , m i c e l l a r m o b i l e phase, and s t a t i o n a r y phase p a c k i n g m a t e r i a l employed (100,112,135). C o n s e q u e n t l y , a l l e x p l a n a t i o n s g i v e n t o date a r e p r o b a b l y c o r r e c t f o r the p a r t i c u l a r l i m i t e d c a s e s examined i n t h e work c i t e d . M i c e l l a r m o b i l e phases have been u t i l i z e d i n numerous r e c e n t p a p e r , t h i n - l a y e r , and h i g h - p e r f o r m a n c e l i q u i d c h r o m a t o g r a p h i c s e p a r a t i o n s . T a b l e XI summarizes t h e s e p a r a t i o n s performed t o d a t e . As can be s e e n , t h e g e n e r a l approach i s amenable t o s e p a r a t i o n o f a wide v a r i e t y o f o r g a n i c , b i o l o g i c a l and i n o r g a n i c s p e c i e s . I t appears t o h o l d p a r t i c u l a r promise i n t h e a r e a s o f m e t a l / a n i o n s p e c i a t i o n (156,162,163) and i n b i o l o g i c a l / p r o t e i n s e p a r a t i o n s (137,165,170, 223). More d e t a i l s c o n c e r n i n g a p p l i c a t i o n o f m i c e l l a r m o b i l e phases i n t h e s e p a r a t i o n o f o r g a n i c and i n o r g a n i c i o n s i s p r e s e n t e d i n a Chapter by M u l l i n s i n t h i s Volume ( 8 4 ) . A r e c e n t r e v i e w by Matson and Goheen (165) o u t l i n e some o f t h e c o n s i d e r a t i o n s and a p p l i c a t i o n s o f u t i l i z i n g d e t e r g e n t - m i c e l l e m o b i l e phases i n t h e HPLC s e p a r a t i o n o f membrane p r o t e i n s . I n many i n s t a n c e s , t h e c o m b i n a t i o n o f s e v e r a l c h r o m a t o g r a p h i c s t e p s , one o r more of w h i c h employed s u r f a c t a n t / m i c e l l e m o b i l e phases, has proven t o be u s e f u l i n t h e s e p a r a t i o n o f b i o l o g i c a l m a t e r i a l s (166,167,171,223,224,233). G

e

l

F i l t r a t i o n . M i c e l l a r s o l u t i o n s have a l s o been u t i l i z e d i n g e l permeation ( f i l t r a t i o n ) chromatography (J_). In f a c t , the f i r s t example o f a s e p a r a t i o n which used a m i c e l l a r m o b i l e phase was i n t h i s a r e a o f e x c l u s i o n l i q u i d chromatography (ELC) (86). The l a s t s i x e n t r i e s i n T a b l e XI summarize some o f t h e s e p a r a t i o n s / w o r k r e p o r t e d c o n c e r n i n g m i c e l l a r m o b i l e phases i n ELC. I n most o f t h e s e a p p l i c a t i o n s , t h e work was conducted w i t h s t a t i o n a r y phases o f r e l a t i v e l y s m a l l pore s i z e . With t h e s e t y p e phases, t h e r e l a t i v e l y l a r g e m i c e l l a r a g g r e g a t e s a r e c o n f i n e d t o t h e e x c l u d e d volume o f t h e column and e l u t e r a p i d l y whereas s m a l l e r s o l u t e m o l e c u l e s i n a m i x t u r e



30 TABLE X I .

Summary o f Some S e l e c t e d S e p a r a t i o n s R e p o r t e d which have U t i l i z e d Surfactant-Containing Mobile Phases


3

Component(s) Separated

Stationary Phase

M o b i l e Phase Composition

Mode

P h e n o l s (28)

Whatman No. 3 paper s t r i p s

Aq. NaLS o r CTAB/ 8% PrOH

PC

Dyestuffs (13), anions ( 2 ) , c a t i o n s (3)

Whatman No. 1 paper

CTAB o r NaLS i n c,d 50%HOH/50% BuOH'

PC

P o l y c y c l i c aromatic hydrocarbons, p e s t i c i d e s (4)

polyamide o r alumina sheets

Aq. NaLS o r CTAB

TLC

87, 91

N u c l e o s i d e s (4)

silanized s i l i c a gel-60

Reversed m i c e l l e s of sodium d i o c t y l s u l f o succinate i n cyclohexane

TLC

87

N u c l e o s i d e s (4)

polyamide

Aq. NaLS

TLC

87

Dyes, P e s t i c i d e s

polyamide o r alumina

Aq.

N L S o r CTAB

TLC

151

Dyes, Food C o l o r s

alumina or polyamide

Aq.

NaLS

TLC

94

Mycotoxins

polyamide, alumina, o r RP s h e e t s

Aq. NaLS

TLC

1,168

Amino a c i d s (3)

column

B r i j - 3 5 / 3 0 % EtOH

P r o t e i n s (6)

supelcosil LC-8

Aq. N e o d e l

Hydroxybenzenes (phenols, q u i n o l s , c a t e c h o l s ) (18)

C-18 RP

Aq.

Dithiocarbamates (5)

Bondapak CN

A n i o n s (5)

T e s t Mix

a

U

Ref.

150

164

column

213

HPLC

152

NaLS

HPLC

105

Aq.

CTAB/30% MeOH

HPLC

108, 206

Spherisorb ODS

Aq.

CTAC

HPLC

107

Polygosil ODS

Aq.

NaLS

HPLC

153

H y p e r s i l ODS

Aq. B r i j - 3 5

HPLC

147

Aq. 0.01M NaLS, pH 3.4

HPLC

218

Aq.

HPLC

220

91-6

Anthracyclines PolyvinylNucleosides, alcohol bases Alkyl-benzenes, C-8 s i l i c a PAH's, p h t h a l a t e s , c h l o r i n a t e d benzenes

0.2M NaLS




1. HINZE


Stationary Phase

3-Alkylbenzenesulfonates

QAE-2SW Aq. DTAB (anion-exchanger)

Substituted benzenes ( 9 ) , e t h y l e s t e r s (5)

RP-18

Aq.

Berberine-type a l k a l o i d s (4) Therapeutic drugs (9)

Bondapak-Ph

Tyrosinyl peptides (5), a r o m a t i c ketones (7) Phenols

(8)

T r i g l y c e r i d e s (10)

Catecholamines (5), 1-phenyla l k a y l a m i n e s (4)

31


Mode

Ref.

HPLC

222

HPLC

128

Aq. NaLS/30% MeOH

HPLC

154

Supelcosil LC-18 o r LC-CN

Aq.

HPLC

144

Hypersil

60:40 Water:MeOH c o n t a i n i n g Tween-20 and NaLS; pH 3.08°

HPLC

76

Ultrasphere octyl

Aq. NaLS ( g r a d i e n t ) pH 2.5

HPLC

155

HPLC

160

HPLC

lby, 222

HPLC

156

SB-12

NaLS

V a r i o u s RP Aq. NaLS, Aq. CTAB C-18 MCH10, b o t h end-capped and non Aq. NaLS, pH 3.5 SP-2SW cation(or 4.6) exchanger

c i s / t r a n s Co(III) complexes

methyl or p h e n y l bonded phases

Aq.

Cu(II)/Ni(II)

methyl or phenyl

Aq. NaLS/5% Me OH

HPLC

156

Zn(II), Pd(II), Cu(II) A r o m a t i c aminosulfonic acids, n u c l e o t i d e s (9)

R a d i a l paksilica octadecylSpherisorb

Aq. NaLS

HPLC

163

Aq. SB-10/20% a c e t o n i t r i l e , pH 4.7 o r 3.0

HPLC

130, 131

Micro-Pak 10

Aq. NaLS

HPLC

95

Microparticu l a t e bonded phases

n-Decylbetaine, pH 5.5 - 6.5

HPLC

130

C-18 endcapped

Brij-35 or T r i t o n X-100/15-30% added acetonitrile

HPLC

120

Bromhexine

Bondapak C-18

NaLS/25% Me0H

HPLC

210

Drugs (5)

Supelcocil CN

Aq.

HPLC

148

g

Phenols,

1

PAN s

t-RNA's (6)

Polypeptides (9), protonated phenylalanine o l i g o m e r s (5)

MCH-

CTAB

Brij-35

c

C o n t i n u e d on next page


32


Table XI.

Continued



Stationary Phase


Mode

Ref.

T e s t mix ( 6 ) , Vanillin/ethyl Vanillin

Radial-PAK C-18

Aq. B r i j - 3 5

HPLC

106

Determination of folylpolyglutamate hydrolase a c t i v i t y

PXS 10/25 ODS

Aq. 0.20M NaLS

HPLC

224

Thiols (8), n i t r o soamines ( 9 ) , and quinones (20)

C-18 o r C-8 RP

HPLC Aq. NaLS, CTAB, CTAC normal m i c e l l e s or AOT/cyclohexane reversed micelles

112

Proline/hydroxyproline

Ultrasphere ODS

Aq. NaLS, pH 2.8

HPLC

162

GF

h

86

t-RNA s (5)

Sephadex G-100- Aq. CTAB/NaCl, pH 8 120

Nucleosides, n u c l e o t i d e s (8)

Sephadex

G-25

Aq. Sodium dodecanoate pH 8

GF

157

Amino a c i d s (14)

Sephadex

G-25

Aq. Sodium dodecanoate

GF

158

N u c l e o t i d e s (5)

Sephadex G-25300 o r G-100120

Aq. CTAB, pH 8.0

GF

159

A l k y l b e n z e n e s (4)

H y p e r s i l s i l i c a Aq. NaLS/2% BuOH

GF

193

GF

194

f

Amino a c i d s

Sephadex

G-25

Aq. NaLS

a

M i c e l l a r m o b i l e ghases u n l e s s o t h e r w i s e s p e c i f i e d . ^ PC = paper chromatography. Presence of m i c e l l e s i s u n c l e a r . Mçst l i k e l y a m i c r o e m s u l i o n system. As i m i n o d i a c e t a t e complexes. As N,N E t h y l e n e - b i s ( a c e t y l a c e t o n e i m i n e ) c h e l a t e s . S e p a g a t e d as t e t r a k i s ( l m e t h y l p y r i d i n i u m - 4 - y l ) p o r p h i n e m e t a l complexes. GF = g e l f i l t r a t i o n . f

g


1.

HINZE

33


can r e s i d e i n the pore volume, t h u s r e q u i r i n g l o n g e r e l u t i o n t i m e s . However, i f t h e s e s m a l l e r s o l u t e m o l e c u l e s can p a r t i t i o n t o the m i c e l l a r pseudophase and b i n d the m i c e l l e e n t i t y , then t h e y w i l l e l u t e more r a p i d l y (J_). C o n s e q u e n t l y , s o l u t e s can be s e p a r a t e d based on t h e i r d i f f e r e n t i a l b i n d i n g a b i l i t y t o a p a r t i c u l a r m i c e l l a r assembly. E q u a t i o n 8 shows the dependence o f t h e e l u t i o n volume, V , c o r r e s p o n d i n g t o the maximum c o n c e n t r a t i o n i n an emerging band, upon the s u r f a c t a n t m i c e l l e c o n c e n t r a t i o n i n the m o b i l e phase e q u i l i b r a t i n g the column (183): Q


- — î - ^ — = ακ C + α V - V b m e ο

(8)

where α i s an e x p e r i m e n t a l c o n s t a n t (see r e f e r e n c e s 1,159,182,183), V i s the e x c l u d e d volume, and Κ i s the m i c e l l e - s o l u t e b i n d i n g c o n s t a n t as p r e v i o u s l y d e f i n e d (1,86,157,158,159,183). V i a use o f e q u a t i o n 8 o r a l t e r n a t i v e r e - e x p r e s s e d v e r s i o n s (V), the b i n d i n g c o n s t a n t (or p a r t i t i o n c o e f f i c i e n t ) o f d i f f e r e n t s o l u t e s t o m i c e l l a r systems have been d e t e r m i n e d (159,182,183,226).

Q

A c u r s o r y r e v i e w o f the l i t e r a t u r e r e v e a l s t h a t the ELC t e c h n i q u e w i t h m i c e l l a r m o b i l e phases has proven t o be very b e n e f i c i a l i n the c h a r a c t e r i z a t i o n o f m i c e l l a r systems (184-186,190-192,227,228). For example, microcolumn e x c l u s i o n LC has been a p p l i e d t o the d e t e r m i n a t i o n o f the CMC v a l u e o f s u r f a c t a n t s (or m i c e l i a r - f o r m i n g p r o t e i n s ) , d e t e r m i n a t i o n of the k i n e t i c r a t e and e q u i l i b r i u m a s s o c i a t i o n c o n s t a n t s f o r s u r f a c t a n t (or p r o t e i n ) m i c e l l i z a t i o n (184,192), d e t e r m i n a t i o n o f the s i z e or s i z e d i s t r i b u t i o n o f m i c e l l e s ( e s p e c i a l l y t h o s e formed from b l o c k copolymers or m i l k c a s e i n ) (185,186,191,192,225) as w e l l as f o r e s t i m a t i o n o f the t i m e r e q u i r e d f o r f o r m a t i o n o f m i c e l l e s (or m i c e l l e - f o r m i n g m a c r o m o l e c u l e s ) (186) among o t h e r s . The s i z e and s t a b i l i t y of r e v e r s e d m i c e l l e s has a l s o been e v a l u a t e d u s i n g ELC (195). The use o f ELC t o c h a r a c t e r i z e m i c e l l a r and r e l a t e d a g g r e g a t e s thus appears t o be p o p u l a r and u s e f u l . I n f a c t , i t s use i n t h i s manner overshadows the a n a l y t i c a l a p p l i c a t i o n s o f m i c e l l a r m o b i l e phases t o a i d ELC s e p a r a t i o n s . However, s e v e r a l r e c e n t r e p o r t s do p o i n t out the advantages o f m i c e l l a r m o b i l e phases i n ELC (187~189) f o r t h e i s o l a t i o n and p u r i f i c a t i o n o f b a c t e r i a l and v i r a l p r o t e i n s . For i n s t a n c e , b a c t e r i o r h o d o p s i n s o l u b i l i z e d i n o c t y l g l u c o s i d e s (0G) was i s o l a t e d a t a n a l y t i c a l and p r e p a r a t i v e l e v e l s from the denatured p r o t e i n and f r e e r e t i n a l (187) and an i n f l u e n z a v i r a l p r o t e i n was i s o l a t e d u s i n g NaLS o r B r i j - 3 5 e l u e n t s w i t h TSK G3000SW o r TSK G5000PW columns (188). Other such a p p l i c a t i o n s w i l l no doubt be f o r t h c o m i n g i n the near f u t u r e . I t has been r e p o r t e d t h a t f o r m i c e l l a r - m e d i a t e d ELC t o d e v e l o p i n t o a v i a b l e t e c h n i q u e r e q u i r e s " t h e development o f a h i g h - p e r f o r m a n c e GPC p a c k i n g m a t e r i a l t h a t has an e x c l u s i o n l i m i t of r o u g h l y 1,000 - 2,000 and i s c o m p a t i b l e w i t h the aqueous m i c e l l a r m o b i l e phase" (J_). F u t u r e work s h o u l d be d i r e c t e d i n t h i s a r e a . S u r f a c t a n t s as S t a t i o n a r y P h a s e s . (1) A p p l i c a t i o n s o f S u r f a c t a n t s " I m m o b i l i z e d " as a S t a t i o n a r y Phase. A p a r t from t h e i r use as m o b i l e phase a d d i t i v e s , t h e r e are i n s t a n c e s where s u r f a c t a n t s have been i m m o b i l i z e d or c o a t e d on s t a t i o n a r y p h a s e s , e s p e c i a l l y f o r


34



p a c k i n g m a t e r i a l s i n GC, GLC, and column, paper o r TLC (176-180,229231). S u r f a c t a n t s such a s A l i q u a t 336, c e t r i m i d e , CTAB, NaLS, TX-100, S u r f y n o l 485, t r i o c t y l a m i n e , e t c . have been coated o r d e p o s i t e d on c a p i l l a r y , m a c r o r e t i c u l a r r e s i n s , Whatman No. 1 p a p e r , chromosorb P, alumina, or s i l i c a supports. B o r o s i l i c a t e g l a s s c a p i l l a r y columns i n which CTAB i s e l e c t r o s t a t i c a l l y i n c o r p o r a t e d t o t h e i n n e r s u r f a c e t h u s f o r m i n g a t h i n f i l m o f h y d r o p h o b i c s t a t i o n a r y phase f o r use i n c a p i l l a r y l i q u i d chromatography have a l s o been d e s c r i b e d (232). When used i n c o n j u n c t i o n w i t h s u r f a c t a n t CTAB m o b i l e phases, e f f i c i e n t s e p a r a t i o n s o f drugs from t h e i r m e t a b o l i t e s a r e p o s s i b l e u s i n g such open t u b u l a r columns (232,233)· The problem w i t h u s i n g s u r f a c t a n t - m o d i f i e d s t a t i o n a r y phases i n LC i s t h a t t h e s u r f a c t a n t w i l l u s u a l l y s l o w l y e l u t e ( b l e e d ) from t h e support t h u s r e s u l t i n g i n d i f f e r e n t r e t e n t i o n b e h a v i o r o f s o l u t e s w i t h t i m e . T h i s i s why most a p p l i c a t i o n s a r e i n t h e a r e a o f GC o r GLC. An e x c i t i n g r e c e n t advance h a s been r e p o r t e d by Okahata, e t a l ( 1 8 1 ) . Namely, a procedure has been d e v e l o p e d f o r i m m o b i l i z i n g a s t a b l e s u r f a c t a n t v e s i c l e b i l a y e r as t h e s t a t i o n a r y phase i n GC. A b i l a y e r p o l y i o n complex composed o f DODAB v e s i c l e s and sodium p o l y ( s t y r e n e s u l f o n a t e ) was d e p o s i t e d on U n i p o r t HP and i t s p r o p e r t i e s a s a GC s t a t i o n a r y phase e v a l u a t e d . U n l i k e p r e v i o u s l i p i d b i l a y e r s which e x h i b i t e d poor p h y s i c a l s t a b i l i t y , t h e DODAB p o l y i o n phase was s t a b l e . A d d i t i o n a l l y , the temperature-retention behavior of t e s t s o l u t e s e x h i b i t e d a phase t r a n s i t i o n i n f l e c t i o n p o i n t . The work demonstrates t h a t i m m o b i l i z e d s u r f a c t a n t v e s i c l e b i l a y e r s t a t i o n a r y phases c a n be employed i n GC s e p a r a t i o n s ( 1 8 1 ) . F u r t h e r work i n t h i s d i r e c t i o n w i l l l i k e l y l e a d t o many such unique gas chromatographic s u p p o r t s and n o v e l separations. 11

11

(2) M i c e l l e s as a L i q u i d Ρseudo-Stationary Phase — M i c e l l a r E l e c t r o k i n e t i c C a p i l l a r y Chromatography (MECC). MECC ( a l s o c a l l e d m i c e l l a r c a p i l l a r y e l e c t r o o s m o t i c chromatography ( 2 0 5 ) ) i s a s e p a r a t i o n t e c h n i q u e f i r s t d e s c r i b e d by Terabe e t a l (196) which combines many o f t h e o p e r a t i o n a l p r i n c i p a l s and advantages o f m i c e l l a r l i q u i d chromatography and c a p i l l a r y zone e l e c t r o p h o r e s i s (196-205). S o l u t e s i n a m i x t u r e (both i o n i c and n e u t r a l ) a r e s e p a r a t e d based on t h e i r d i f f e r e n t i a l p a r t i t i o n i n g between an e l e c t r o o s m o t i c a l l y - p u m p e d aqueous m o b i l e phase and t h e i o n i c s u r f a c t a n t m i c e l l a r a g g r e g a t e which p o s s e s s e s an o v e r a l l f r a c t i o n a l charge and moves a t a v e l o c i t y d i f f e r e n t t h a n t h a t o f t h e aqueous m o b i l e phase due t o e l e c t r o p h o r e t i c e f f e c t s . Thus, t h e s e p a r a t i o n mechanism i s a k i n t o t h a t o f c o n v e n t i o n a l l i q u i d - l i q u i d p a r t i t i o n chromatography, w i t h t h e m i c e l l a r e n t i t y f u n c t i o n i n g a s a " p s e u d o - s t a t i o n a r y " phase (197»201). Some view MECC as an example o f a l a m i n a r m i c r o s c o p i c c o u n t e r - c u r r e n t s e p a r a t i o n t e c h n i q u e O ) . The fundamental c h a r a c t e r i s t i c s and f a c t o r s e f f e c t i n g r e t e n t i o n and e f f i c i e n c y i n MECC have been d e s c r i b e d (196,197,199,202,204,214). The approach r e s u l t s i n e x c e l l e n t r e s o l u t i o n due t o t h e very h i g h e f f i c i e n c y o b t a i n a b l e (200,000 600,000 t h e o r e t i c a l p l a t e s , HETP c a . 1.9 - 3.7 ym) (196,203). MECC has been employed t o s e p a r a t e a v a r i e t y o f e n v i r o n m e n t a l and b i o l o g i c a l - t y p e m i x t u r e s (see T a b l e X I I ) ; i n c l u d i n g t h e a n a l y s i s o f v i t a m i n s i n s p i k e d human u r i n e ( 2 0 1 ) . E l e c t r o k i n e t i c measurements c a n a l s o be employed t o e v a l u a t e s u r f a c t a n t c r i t i c a l m i c e l l e concentrations (236).


TABLE X I I .


35


1. HINZE

Summary o f S u c c e s s f u l A p p l i c a t i o n s e m p l o y i n g M i c e l l a r E l e c t r o k i n e t i c C a p i l l a r y Chromatography (MECC) i n Separations

C l a s s o f Compounds Separated

Micellar Solutions Employed/Conditions

Phenols

Aq.

Reference

0.05 M NaLS, pH 7.0

196

Amino a c i d s (22) [as t h e i r phenylthiohydantoin d e r i v a t i v e s ]

Aq. 0.05 M NaLS o r 0.05 M DTAB, pH 7.0

198

C h l o r i n a t e d phenols

Aq.

0.10 M NaLS, pH 7.0

199

Aq.

0.07 M NaLS, pH 7.0

199

( 8 ) , x y l e n o l s (6)

(7)

Isomeric

chloro-phenols

Aromatic

s u l f i d e s (11)

(222)

Aq. 0.02 o r 0.05 M NaLS; 80:20(%) 0.03 M NaLS: Me OH pH 7.0

M e t a b o l i t e s o f V i t a m i n B. (6)

b

200

Aq. 0.05 M NaLS, 0.01 M 201 phosphate, 0.001 M b o r a t e

S u b s t i t u t e d p u r i n e s (6)

Aq. 0.05 M NaLS, 0.001 M b o r a t e , 0.01 M phosphate

202

N i t r o a r o m a t i c compounds (4)

Aq. 0.01 M NaLS, 0.01 M phosphate

203

Metal ions [Mn(II), C o ( I I ) , Z n ( I I ) , C u ( I I ) ] as t h e i r tetrakis(4-carboxyphenyl)porphinato chelates

Aq. 0.02 M NaLS, 0.05 M 205 phosphate, 0.0125 M b o r a t e

O l i g o n u c l e o t i d e s (7)

Aq.

P o l y t h y m i d i n e s (7)

Aq. 0.05 M NaLS/0.3mM Cu(II)

0.05 M NaLS/3mM Mg(II) 235


235

36



MECC i s t h e most r e c e n t and f a s t e s t d e v e l o p i n g s u r f a c t a n t mediated t e c h n i q u e . F u t u r e work i s r e q u i r e d t o extend t h e range o f r e t e n t i o n p o s s i b l e ( 1 9 7 ) . P r e s e n t l y , t h e t o t a l e l u t i o n range i s r e l a t i v e l y n a r r o w . I t s h o u l d be p o s s i b l e t o m a n i p u l a t e r e t e n t i o n and improve s e p a r a t i o n a t t h e two extremes o f t h e e l u t i o n range by j u d i c i o u s a d d i t i o n of a d d i t i v e s ( o r g a n i c s / s a l t s ) t o the m i c e l l a r s u r f a c t a n t s o l u t i o n (200,234,235). As p r e v i o u s l y mentioned, t h e presence o f such a d d i t i v e s can a l t e r t h e p a r t i t i o n i n g (P ) o f a s o l u t e between t h e aqueous and m i c e l l a r phases. A l t e r n a t i v e l y , t h e use o f o t h e r t y p e s o f c a p i l l a r y m a t e r i a l s (as a l t e r n a t i v e s t o f u s e d s i l i c a ) o r c o a t i n g s o f p o l y m e r i c m a t e r i a l s on t h e i n n e r w a l l o f t h e f u s e d s i l i c a (214) may prove b e n e f i c i a l i n t h i s r e g a r d . More d e t a i l s on t h e c u r r e n t s t a t u s o f t h i s s e p a r a t i o n t e c h n i q u e a r e g i v e n i n a r e v i e w by Armstong Ο ) and a r e p o r t by Sepaniak e t a l (203) i n t h i s Symposium Volume. S u r f a c t a n t - M e d i a t e d S o l v e n t E x t r a c t i o n s . P a r t i t i o n i n g and e x t r a c t i o n s e p a r a t i o n t e c h n i q u e s s e r v e t o p r o v i d e sample p u r i f i c a t i o n as w e l l as a s i m p l e and e f f e c t i v e means f o r improvement o f a n a l y t i c a l methods by enhancement o f b o t h s e n s i t i v i t y (by sample c o n c e n t r a t i o n ) and s e l e c t i v i t y (by removal o f p o t e n t i a l i n t e r f e r e n c e s ) . A l t h o u g h n o t w e l l a p p r e c i a t e d , many s u r f a c t a n t and m i c e l i a r - m e d i a t e d e x t r a c t i o n systems have been d e s c r i b e d i n t h e l i t e r a t u r e , e s p e c i a l l y i n t h e a r e a o f m e t a l s a n a l y s i s and i n b i o l o g i c a l p u r i f i c a t i o n s (1,5.237.238). There a r e s e v e r a l d i f f e r e n t t y p e s o f s u r f a c t a n t - m e d i a t e d e x t r a c t i o n schemes p o s s i b l e depending upon t h e n a t u r e o f t h e a n a l y t e m i x t u r e and t h e e x t r a c t i n g s u r f a c t a n t system employed. These c a n be b r o a d l y d i v i d e d i n t o two t y p e s : (1) those i n v o l v i n g nonpolar s o l v e n t - s u r f a c t a n t (or r e v e r s e d m i c e l l a r ) systems and (2) those i n v o l v i n g aqueous s u r f a c t a n t /mice l i a r media. I n many i n s t a n c e s , t h e p o s s i b i l i t y o f w/o o r o/w m i c r o e m u l s i o n f o r m a t i o n i n t h e s e systems a l s o e x i s t s ( 2 3 9 ) . S u r f a c t a n t s i n o r g a n i c s o l v e n t s have been u t i l i z e d t o e x t r a c t i o n s , complexes, and enzymes from aqueous o r s o l i d m a t r i c e s . L i k e w i s e , some aqueous s u r f a c t a n t / n o r m a l m i c e l l a r systems have been employed to e x t r a c t b i o l o g i c a l , o r g a n i c , o r a g r i c u l t u r a l m a t e r i a l s from o t h e r aqueous, o r g a n i c , o r s o l i d m a t r i c e s . A d d i t i o n a l l y , use o f c e r t a i n aqueous m i c e l l a r media a l l o w s f o r c o n c e n t r a t i o n and p u r i f i c a t i o n o f m e t a l i o n s , o r g a n i c compounds, o r b i o l o g i c a l substances due t o t h e i r phase s e p a r a t i o n b e h a v i o r ( i . e . c l o u d p o i n t phenomena o r c o a c e r v a t i o n b e h a v i o r ) . W h i l e t h e r e have been many p r a c t i c a l a p p l i c a t i o n s u s i n g these s u r f a c t a n t systems i n e x t r a c t i o n s , m e c h a n i s t i c s t u d i e s have lagged b e h i n d due t o t h e c o m p l i c a t e d n a t u r e o f t h e p h y s i c o c h e m i c a l p r o c e s s e s i n v o l v e d and l a c k o f knowledge o f t h e s u r f a c t a n t s t r u c t u r e s p r e s e n t under t h e e x t r a c t i o n c o n d i t i o n s . S i n c e t h e r a t i o n a l d e s i g n o f f u t u r e s e p a r a t i o n systems r e q u i r e s an u n d e r s t a n d i n g o f t h e p r o cesses i n v o l v e d i n these s u r f a c t a n t e x t r a c t i o n procedures, f u t u r e work s h o u l d c o n c e n t r a t e on t h e mechanism o f such s e p a r a t i o n s . N e x t , a b r i e f d e s c r i p t i o n and i l l u s t r a t i v e examples o f each o f t h e s e t y p e s o f s u r f a c t a n t - m e d i a t e d e x t r a c t i o n t e c h n i q u e s w i l l be g i v e n . E x t r a c t i o n s U t i l i z i n g S u r f a c t a n t s i n O r g a n i c S o l v e n t s . The u s e of organic solvents c o n t a i n i n g s u r f a c t a n t s i n e x t r a c t i v e m e t a l l u r g y has p r o b a b l y been t h e most p r e v a l e n t a p p l i c a t i o n o f s u r f a c t a n t s i n c h e m i c a l s e p a r a t i o n s (1.5.240-262). T a b l e X I I I summarizes some o f t h e


1.

37


HINZE

TABLE X I I I .

Name and S t r u c t u r e o f S e v e r a l D i f f e r e n t Types o f Extractants U t i l i z e d f o r Metal Ion Separations Structure

Name A n i o n Exchanger Type:

(CH ) C(CH C(CH ) ) NH

Primene

3

2

R N(CH ) 0

A l i q u a t 336

+

3

2

4

2

C l " where R = C

0

g

- C

10

R^N where R = i s o o c t y l

Adogen 381


3

A l a m i n e 336

R^N where R = C

TOA

R^N where R

Adogen 283

R NH where R = C

g

"10

octyl

2

1 3

Acidic Extractants: Di-2-ethylhexylphosphoric acid

(C H CH(C H )CH 0) P0 H

V e r s a t i c 10

R CCO H where R = C J Ζ ο

Fatty

RCO Η where R = C 2 14

Acids

4

9

2

5

2

2

2

1 7

J

18

SYNEX 1051

Ο TO L

where R C

H

9 19

0 H 3

Solvating Extractants: Tri-n-butylphosphate Trioctylphosphine

R P 0 where R - C,H 0 3 4 9 o

oxide

Dihexylsulfide

R P 0 where R 3

RSR where R •

C

H

" 8 17 C

H

6 13

C h e l a t i n g Type E x t r a c t a n t s : K e l e x 100

©si L I X 63

where R = d o d e c e n y l

C H C H (OH C H )CH(0H)C(N0H)CH(C H )C H 4

9

2

5

2

5

4

9

L I X 34

Ο JΟ Polyols

j . ρ

. ..

where R = p - d o d e c y l NHSO^R benzene 2

C o n t i n u e d on next page


38


Table X I I I .

Continued

Name

Structure

L I X 65N where R^

L I X 54

phenyl, R

= H, and

2

R where ^


s

^^^COCH COR 2

1

= C H and R 3

2

= p- o r m-

dodecyl

^ a t a t a k e n from r e f e r e n c e ( 2 6 4 ) .



1.

HINZE


39

d i f f e r e n t t y p e s o f e x t r a c t a n t s w h i c h have been u t i l i z e d t o e x t r a c t m e t a l i o n s f r o m aqueous s o l u t i o n i n t o an o r g a n i c l a y e r c o n t a i n i n g the e x t r a c t a n t . As c a n be s e e n , many o f t h e s e e x t r a c t a n t s a r e s u r f a c t a n t s . Depending upon t h e s p e c i f i c c o n d i t i o n s and type o f a n a l y t e s p r e s e n t , such s u r f a c t a n t s c a n f u n c t i o n as e i t h e r i o n - p a i r or phase t r a n s f e r agents (264-266). o r e x i s t i n aggregated f o r m as r e v e r s e d m i c e l l e s (240.241.264) o r i n some c a s e s , m i c r o e m u l s i o n s (239.251). A l t h o u g h s t i l l s u b j e c t t o d e b a t e , r e c e n t accumulated e v i d e n c e s t r o n g l y s u p p o r t s t h e argument t h a t r e v e r s e d m i c e l l e s a r e p r e s e n t i n t h e o r g a n i c phase and p l a y a v i t a l r o l e i n many m e t a l i o n e x t r a c t i o n s i n v o l v i n g s u r f a c t a n t e x t r a c t a n t s such as t h o s e d e p i c t e d i n T a b l e X I I I (240-266). F o r example, i t has been r e c e n t l y shown that reversed m i c e l l e s o f di-n-butylphosphate, quaternary a l k y l ammonium s a l t s , m e t a l a I k y l a r y l s u l f o n a t e s , a l k y l s u l f a t e s , d i a l k y l dithiophosphates, di(2-ethylhexyl)phosphoric a c i d , phenols, d i n o n y l n a p h t h a l e n e s u l f o n i c a c i d , and SP-3 c a r b o z o l i n e c a n f o r m i n t h e o r g a n i c l a y e r d u r i n g m e t a l i o n e x t r a c t i v e c o n d i t i o n s (267-283). Under some c o n d i t i o n s , m i c r o e m u l s i o n s form (284.285). I n a d d i t i o n t o t h e s e s u r f a c e a c t i v e e x t r a c t a n t s , many e x t r a c t i o n schemes a l s o have some o t h e r s u r f a c t a n t s p r e s e n t (such as t h o s e g i v e n i n T a b l e V) t h a t c a n f o r m r e v e r s e d m i c e l l e s i n t h e o r g a n i c phase (1.4.5.283.330). The d i a l k y l n a p h t h a l e n e s u l f o n a t e s (see T a b l e V, a n i o n i c s u r f a c t a n t s e c t i o n ) have been e s p e c i a l l y u s e f u l i n t h i s r e g a r d (263). Some o f t h e o r g a n i c - c o n t a i n i n g s u r f a c t a n t systems t h a t have been u t i l i z e d i n t h e e x t r a c t i o n o f a v a r i e t y o f m e t a l i o n s (as c a t i o n s o r m e t a l complexes) f r o m aqueous s o l u t i o n a r e summarized i n T a b l e X I V (286-321). I n s p e c t i o n o f t h e T a b l e i n d i c a t e s t h a t most o f the s u c c e s s f u l e x t r a c t i o n schemes i n v o l v e u s e o f e i t h e r c a t i o n i c o r anionic surfactants/extractants. In contrast, only a r e l a t i v e l y few r e c e n t a p p l i c a t i o n s employed z w i t t e r i o n i c o r n o n i o n i c s u r f a c e a c t i v e a g e n t s . The i n t e r e s t e d r e a d e r i s r e f e r r e d t o s e v e r a l r e c e n t monographs/review a r t i c l e s f o r more e x t e n s i v e c o m p i l a t i o n s o f d i f f e r e n t e x t r a c t i o n systems i n v o l v i n g s u r f a c e a c t i v e agents (330-332. 346.356.358). I t s h o u l d be emphasized t h a t o r g a n i c s p e c i e s t h a t a r e c a p a b l e o f b e i n g i o n i z e d c a n a l s o be e x t r a c t e d from aqueous media o r s o l i d m a t r i c e s ( r e f e r t o t h e l a s t 4 e n t r i e s o f T a b l e XIV) (322-325). L a s t l y , m e n t i o n should be made o f t h e f a c t t h a t many o f t h e systems g i v e n i n T a b l e X I V c a n n o t o n l y be conducted a t a n a l y t i c a l o r p r e p a r a t i v e s c a l e s but a l s o a t the process l e v e l (356). Some o f t h e p r a c t i c a l a p p l i c a t i o n s i n c l u d e r e c o v e r y o f m e t a l s from spent e l e c t r o l y t i c o r s c r a p l e a c h i n g l i q u o r s ( 3 5 3 ) . e x t r a c t i o n s from s y n t h e t i c mixed f i s s i o n p r o d u c t s o l u t i o n s ( 3 5 4 ) . and s e p a r a t i o n o f r a r e e a r t h m e t a l s f r o m o r e s (355). The g e n e r a l approach s h o u l d a l s o be p o t e n t i a l l y u s e f u l i n p y r o m e t a l l u r g i c a l operations i n v o l v i n g melts o r m o l t e n s l a g s ( i f h i g h e r b o i l i n g o r g a n i c s o l v e n t s a r e employed). I n many o f t h e examples p r e s e n t e d i n T a b l e XIV, t h e e x i s t e n c e o f r e v e r s e d m i c e l l e s (275.278.279.282.293.315.326.347) o r m i c r o e m u l s i o n s (3^9-352) i s i m p l i c a t e d and t h e i r p r e s e n c e i s an i m p o r t a n t f a c t o r which influences the c h a r a c t e r i s t i c s of a p a r t i c u l a r extract i o n process. O f t e n , q u a n t i t a t i v e d e s c r i p t i o n s o f such e x t r a c t i o n s i s d i f f i c u l t due t o t h e f a c t t h a t many o f t h e r e v e r s e d m i c e l l a r systems formed undergo an i n d e f i n i t e t y p e o f s e I f - a s s o c i a t i o n i n


40


TABLE XIV.

C o m p i l a t i o n o f S e l e c t e d E x t r a c t i o n Systems t h a t I n v o l v e Use o f O r g a n i c S o l v e n t s i n Presence o f S u r f a c t a n t s *


Component/Aqueous Conditions (Ref.)

Surfactant/ Organic Solvent

Nd, Tb, Tm (286. 287)

Di(2-ethylhexyl) phosphoric acid [HDEHP] i n c y c l o hexane

none glycine 3-mercaptopropionic acid

Cm,Cf ( i n p r e s e n c e o f many o t h e r s a l t s ) (288)

HDEHP

none

Ta,Nb i n aqueous oxalic acid or HC1 (289)

HDEHP i n heptane

none

Co, N i (290)

HDEHP i n x y l e n e , n-dodecanol, o r dodecane

none

Lanthanides/ A c t i n i d e s (291)

HDEHP i n a r o m a t i c solvents

Dinonylnaphthalenesulfonic acid

Zn i n aqueous z i n c s u l f a t e (292)

HDEHP i n k e r o s i n e

none

A l , G , I n (293)

Decanoic a c i d i n benzene o r o c t a n o l

NaCIO, 4

Cu i n aqueous N C 1 0 (294)

Decanoic a c i d i n benzene o r o c t a n o l

Th from o t h e r metals i n a c e t i c a c i d (295)

Versatic-10 i n butanol

none

Cu,Cd,Co,Ni from wastewater (296)

Palmitic, stearic, or l i n o l e i c a c i d i n kerosine

none

Zn,Co from aqueous s o l u t i o n s (297)

(t-dodecylthio) acetic acid i n kerosine

none

Fe from aqueous HNO (298)

Tributylphosphate i n kerosine

none

Mo from aqueous HC1 (299)

1,5-Bis(dioctylphosphiny1)pentane i n chloroform

none

Pd i n n i t r i c (300)

Diheptylsulfide i n benzene o r c h l o r o form

none

a

a

4

acid

Comments

Additives/ Coextractant

Ordered Media in Chemical Separations - American Chemical Society

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