Ordered Media in Chemical Separations

Chapter 7

Amphiphilic Ligands in Chemical Separations E. Pramauro, C. Minero, and E. Pelizzetti

Downloaded by YORK UNIV on November 23, 2012 | http://pubs.acs.org Publication Date: June 30, 1987 | doi: 10.1021/bk-1987-0342.ch007

Dipartimento di Chimica Analitica, Universitàdi Torino, Torino 10125, Italy

A series of 4-alkylamido-2-hydroxybenzoic acids containing a d i f f e r e n t number of carbon atoms i n the a l k y l amido group has been studied as model ligands for metal ion extraction i n aqueous micellar solutions of nonionic surfactants. Their acid-base properties and r e a c t i v i t y towards metal ions i n the presence of micelles were i n vestigated. By operating at a proper temperature, the separation of the iron(III) chelate complexes into a micellar r i c h phase was achieved and the extraction efficiency was correlated with the ligand hydrophobicity. The use of organized molecular assemblies i n analytical chemistry has lead to the improvement of existing methods and to the development of new procedures (1, 2). In particular, its applications i n chemical separations, including chromatography and extraction, seems to be very promising (3, 4). The phase separation of nonionic micellar solutions above the cloud point has been succesfully applied to the l i q u i d - l i q u i d extraction of some metal chelate complexes (5, 6). In these systems the concentration of the analyte takes place i n the micellar r i c h layer, which can be readily analyzed. Although this approach can be interesting i n analytical chemistry because it allows one to conduct extractions without using organic non miscible solvents, no systematic investigations were performed concerning the parameters which can regulate the efficiency of the process, such as the effect of the ligand hydrophobicity, the var i a t i o n of the chemical properties of reagents i n the presence of mic e l l e s , the kinetics of complexation and extraction and so on. In this work, some of the above mentioned features were i n v e s t i gated for a simple extraction model, using suitable complexing amphiphiles having different hydrophobicity.

0097-6156/87/0342-0152$06.00/0 © 1987 American Chemical Society

In Ordered Media in Chemical Separations; Hinze, W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

7.

153

Amphiphilic Ligands in Chemical Separations

PRAMAURO ET AL.

A s e r i e s o f compounds c o n t a i n i n g the same c h e l a t i n g m o i e t y , mely 4 - a m i n o s a l i c y l i c a c i d , w i t h d i f f e r e n t a l k y l c h a i n s , was sized.

The

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

(PAS-C ) i s the

na

synthe

following:

R-CONH.

The m i c e l l a r p a r a m e t e r s were p r e v i o u s l y d e t e r m i n e d

(7) .

S i n c e a g g r e g a t i o n o c c u r s f o r t h e s e compounds o n l y a t h i g h pH

va

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

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

wever, the PAS-C


ce o f n o n i o n i c s u r f a c t a n t s nol) in

A t lower pH,

m o l e c u l e s can be r e a d i l y s o l u b i l i z e d (e.g. B r i j

ho

i n the p r e s e n

35: p o l y o x y e t h y l e n e ( 2 3 ) d o d e c a -

and the o b t a i n e d mixed m i c e l l e s e x h i b i t complexing

capability

a c i d i c media.

tem

In o r d e r t o i n v e s t i g a t e t h e s e p a r a t i o n mechanism, the model s y s i r o n ( I I I ) - P A S - C was chosen and i t s p r o p e r t i e s were s t u d i e d i n η

the

presence of n o n i o n i c m i c e l l e s .

Experimental

Section

Potentiometry.

The d i s s o c i a t i o n c o n s t a n t s o f

benzoic a c i d s i n the presence of B r i j 25°C and 0.10

M ionic

strength

4-alkylamido-2-hydroxy-

35 m i c e l l e s were measured a t

(NaNO^).

The

ligand

(0.002 M)

t r a t e d w i t h 1 M NaOH u s i n g a 6 5 5 - M u l t i - D o s i m a t automated (Metrohm), The

t i

e q u i p p e d w i t h a 605-pH-meter and a 614-Impulsomat u n i t .

t i t r a t i o n s were p e r f o r m e d under

a l l o w the e l e c t r o d e Chromatography.

flow, very slowly, i n order to

equilibration.

The r e t e n t i o n volumes o f PAS-C

a P e r k i n Elmer S-2 tector.

was

titrator

chromatograph,

A μ-Bondapak

C

were measured w i t h

e q u i p p e d w i t h a UV-VIS-LC-55 Β de

r e v e r s e phase

column

(Waters) was

used.

18 M o b i l e phases c o n t a i n i n g B r i j red

t h r o u g h a 0.45

s o l u t e was of

ym

35

( i o n i c s t r e n g t h : 0.10

c e l l u l o s e membrane f i l t e r

dissolved i n Brij

M)

were

(Millipore).

filte Each

35 s o l u t i o n s b e f o r e t h e r u n s ; 5-10

μΐ

the sample s o l u t i o n a t a c o n c e n t r a t i o n i n t h e range 0.001-0.003 M

were i n j e c t e d and the e l u t i o n was (1-2 ml/min),

a t a f i x e d pH

performed a t constant flow r a t e

(2 o r 6 ) , a t room t e m p e r a t u r e

The a b s o r b a n c e s were m o n i t o r e d a t 280 Spectrophotometry. the

presence of B r i j

the

maximum

0.05

Μ HNO^,

The a b s o r b a n c e s o f i r o n ( I I I ) - P A S - C complexes

(520 nm). a t 0.10

(25+ 1 ° C ) .

nm. in

35 m i c e l l e s were measured a t t h e w a v e l e n g t h o f E x p e r i m e n t s were p e r f o r m e d i n the p r e s e n c e o f M i o n i c strength

(NaNO^ was

added), a t 25°C.

The

i n v e s t i g a t e d s u r f a c t a n t c o n c e n t r a t i o n was

i n t h e range 0.001-0.01

M.

The i r o n ( I I I ) p r e s e n t i n the m i c e l l a r r i c h phase

i n extraction


154

ORDERED MEDIA IN CHEMICAL SEPARATIONS

experiments

was

a l s o measured s p e c t r o p h o t o m e t r i e s l y , a t 520

nm,

af-

t e r d i l u t i o n o f an a l i q u o t o f t h i s l a y e r w i t h a b u f f e r e d s o l u t i o n o f T r i t o n X 100 2-5

% w/v,

(polyoxyethylene(9.5)-p-1,1,3,3-tetramethylbutylphenol)

t o ensure

a c l o u d temperature

enough h i g h i n o r d e r t o

a v o i d t u r b i d i t y e f f e c t s d u r i n g t h e measurements. A C a r y 219

spectrophotometer

(Varian) was

used

throughout

the

work. Extraction.

E x t r a c t i o n experiments

were performed

using

suitable

nonionic s u r f a c t a n t s or t h e i r mixtures having cloud p o i n t t r a n s i t i o n temperatures

n o t f a r from t h e room t e m p e r a t u r e .

c e n t r a t i o n was


The

i n t h e range

1-5

a n a l y t e c o n t e n t was

i n excess

%

5-10

The

ppm,

ion).

(acetic acid / acetate or

a c i d / c h l o r o a c e t a t e ) and i n e r t s a l t

(NaNO^) was

The pH

added i n o r d e r t o facilita-

centrifugation.

M i x t u r e s o f T r i t o n X 100 and BL 4.2 n o l ) were used The

(polyoxyethylene(4.2)dodeca-

i n t h i s p a r t o f t h e work.

complex f o r m a t i o n was

a f t e r few m i n u t e s ,

f a s t i n t h e r e p o r t e d c o n d i t i o n s and,

t h e absorbance

o f t h e s o l u t i o n showed no

A f t e r h e a t i n g a t a c o n s t a n t temperature

phase was

then o b t a i n e d .

The

changes.

( c a . 3 5 ° C ) , above t h e

c l o u d p o i n t o f t h e m i x t u r e , t h e heterogeneous f u g e d a t 3400 r.p.m. f o r 15 min.

d i s p e r s i o n was

c e n t r i f u g e v e s s e l s were c a l i b r a t e d i n

o f t h i s l a y e r were t a k i n g w i t h a s y r i n g e f o r t h e DC-Plasma S p e c t r o m e t r y .

aliquots

analysis.

Some c o n t r o l measurements o f t h e a n a l y t e

c o n t e n t i n the aqueous e x t r a c t e d phase were p e r f o r m e d t r a s p a n IV a p p a r a t u s

centri-

A deep v i o l e t m i c e l l a r r i c h upper

o r d e r t o a l l o w t h e measurement o f the m i c e l l a r phase volume;

was

was

chloroacetic

i n c r e a s e t h e d e n s i t y o f the aqueous r i c h lower phase, w h i c h tes f a s t

con-

with a ligand concentration

(ca. t e n times w i t h r e s p e c t t o the metal

adjusted with a proper b u f f e r

surfactant

w/v.

( S p e c t r a m e t r i c s ) . The

u s i n g a Spec-

e m i s s i o n l i n e a t 259.5

nm

used.

R e s u l t s and D i s c u s s i o n Binding Constants of Ligands with Nonionic M i c e l l e s The

acid-base p r o p e r t i e s o f the amphophilic

p r e s e n c e o f m i c e l l a r a g g r e g a t e s due

l i b r i u m o f b o t h t h e a c i d and a n i o n i c form. the a p p a r e n t pK

was

c e l l i z e d surfactant

l i g a n d s change i n t h e

t o t h e w e l l known p a r t i t i o n e q u i A continuous increase i n

observed with i n c r e a s i n g c o n c e n t r a t i o n of

mi-

(see T a b l e I ) .

A c c o r d i n g t o t h e s i m p l e pseudophase model o f B e r e z i n (8), b i n d i n g c o n s t a n t s between the l i g a n d s and c u l a t e d u s i n g the f o l l o w i n g

the

the m i c e l l e s have been

equation:


cal-

7.


PRAMAURO ET AL.

-1

-1

a(app)

a(w)

K

where Κ

+ K HA

155

«1 K C a(w) D

(1)

i s t h e d i s s o c i a t i o n c o n s t a n t o f c a r b o x y l a t e group i n a(app) the p r e s e n c e o f B r i j 35, Κ , i s t h e same c o n s t a n t i n water, Κ is , a(w) HA the b i n d i n g c o n s t a n t o f t h e u n d i s s o c i a t e d PAS-C t o t h e m i c e l l e s and %

ν

C i s t h e c o n c e n t r a t i o n o f m i c e l l i z e d s u r f a c t a n t (C = C - CMC). D D tot The c r i t i c a l m i c e l l a r c o n c e n t r a t i o n f o r B r i j 35, measured w i t h t h e s u r f a c e t e n s i o n method i s 6x10 Table I.

V a l u e s o f pK


Brij

f o r PAS-C , PAS-C 2

oAoffi,

25°C, I =

M, 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 .

i n the Presence

3 35, C x l O (M) D

PAS-C

4

and PAS-C , a t

of B r i j

7

35 M i c e l l e s

Measured pK , a(app) PAS-C PAS-C 2 4 7

0.94

3.12

3.14

1.44

3.14

3.20

1.94

3.16

3.26

2.44

3.20

3.32

2.94

3.26

3.40

3.94

3.29

3.47

4.94

3.33

3.59

6.94

3.55

3.68

9.94

3.65

3.84

14.94 19.94

P l o t s of experimental and PAS-C^, a t t h e lower

data according t o Equation

1, f o r PAS-C

2

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 shown i n F i

gure 1. S i n c e t h e e v a l u a t i o n o f t h i s parameter i s v e r y i m p o r t a n t , a l s o measured u s i n g t h e m i c e l l a r HPLC t e c h n i q u e

i t was

( 9 ) , which a l l o w s a

b e t t e r e s t i m a t i o n o f the p a r t i t i o n c o e f f i c i e n t s i n the presence o f quite high concentrations of surfactant. ter

The c h r o m a t o g r a p h i c

parame

Ρ was measured f o r each l i g a n d 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

centration, according to Equation -1 Ρ = Ρ

+ sw

2:

-1 Κ Ρ C HA sw D

(2)

where P = V / ( V - V ) , V and V a r e t h e volume o f s t a t i o n a r y and , ., s e m s m m o b i l e phase, r e s p e c t i v e l y , and V i s t h e e l u t i o n volume. Ρ repree sw s e n t s t h e p a r t i t i o n c o e f f i c i e n t o f s o l u t e s between t h e s t a t i o n a r y and the aqueous p h a s e .



O R D E R E D M E D I A IN C H E M I C A L S E P A R A T I O N S


7.

157

Amphiphilk Ligands in Chemical Separations

PRAMAURO ET AL.

F o r the i n v e s t i g a t e d PAS-C

l i g a n d s , the b i n d i n g c o n s t a n t

for

the u n d i s s ^ c i a t e d form c l e a r l y i n c r e a s e s w i t h η, from 170 M

for

and

anionic

500

M

f o r C^ up

t o c a . 1500

M

f o r C^,

form i t becomes s i g n i f i c a n t o n l y f o r C i s i n good agreement w i t h the v a l u e s a t low

surfactant concentration,

c h a i n l e n g t h f o r the

whereas f o r the

(110 M

).

estimated

and

The

from pK

C^

obtained

data

, shift, a(app) %

a l l o w us t o d e f i n e a minimum

l i g a n d i n order

t o have a s t r o n g b i n d i n g t o

the

m i c e l l e s , b o t h i n a c i d i c o r a n i o n i c form. Complex F o r m a t i o n C o n s t a n t i n the P r e s e n c e o f M i c e l l e s The

kinetics

(10)

and

the complex f o r m a t i o n

e q u i l i b r i a i n the

presen


ce o f n o n i o n i c m i c e l l e s have been a l s o i n v e s t i g a t e d , a t c o n s t a n t dity.

The

s t o i c h i o m e t r y was

apparent s t a b i l i t y constants Ostwald p r o c e d u r e

For

(12_,

13)

were e v a l u a t e d

according

t o Frank

the and iron/

i r o n / s a l i c y l a t e i n homogeneous aqueous a c i d i c

.

the e q u i l i b r i u m r e a c t i o n : 3+

+

Fe where H S a l

+

HSal

^ΖΞ

+

FeSal

+

Η

i n d i c a t e s the d i s s o c i a t e d c h e l a t i n g m o i e t y o f the l i g a n d ,

the o b s e r v e d changes i n the a p p a r e n t f o r m a t i o n d i r e c t l y r e l a t e d with discussed.

constants

the v a r i a t i o n o f t h e a p p a r e n t pK

(K^)

can

c r e a s i n g the B r i j change i n

values

35 c o n c e n t r a t i o n from 0.001 was

from 2.5x10

M t o 0.01

t o 4.0x10

the

(e.g., by i n M,

the

f o r PAS-C^ and

obser

from

2.3x10

t o 3.0x10

f o r PAS-C^, r e s p e c t i v e l y .

bility,

PAS-C

i n v e s t i g a t e d i n a narrow s u r f a c t a n t c o n c e n t r a t i o n

7

was

range

(0.01-0.02 M);

sured

for this The

a nearly constant

K

c

Due

be

, previously

F o r the l e s s h y d r o p h o b i c l i g a n d s , the i n c r e a s e 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 gave r i s e t o h i g h e r ved

aci

by u s i n g Job's method and

(11) , as p r e v i o u s l y r e p o r t e d f o r the systems

s u l f o s a l i c y l a t e and solution

assessed

value

t o i t s lower s o l u ( c a . 3x10

experiments performed c l e a r l y

showed t h a t , whereas the

p l e x a t i o n o f i r o n ( I I I ) i s n o t v e r y much dependent on d r o p h o b i c i t y , the a s s o c i a t i o n o f the c h a r g e d 1:1 c e l l e s and

) was

mea

compound.

the l i g a n d

c h e l a t e s t o the

t h e n the e f f i c i e n c y o f the c o n c e n t r a t i o n p r o c e s s ,

comhy mi

markedly

increases. E x t r a c t i o n o f I r o n ( I I I ) from M i c e l l a r S o l u t i o n s The

s u r f a c t a n t system T r i t o n X 100

/ BL 4.2

s u i t a b l e c l o u d t e m p e r a t u r e range and towards the l i g a n d s . vestigated The

Table

was

chosen because i t s

the good s o l u b i l i z i n g

capability

I I summarizes the p r o p e r t i e s o f some i n

mixtures.

analyte content,

a f t e r e x t r a c t i o n , was

determined both i n


158


the m i c e l l a r and

i n the

aqueous r i c h phase by

VIS-spectrophotometry.

C a l i b r a t i o n c u r v e s were made w i t h m i c e l l a r phases c o n t a i n i n g solved

ligands,

p a r a t e d by the

i n the

absence o f i r o n ( I I I ) .

To

the

dis

these s o l u t i o n s ,

c e n t r i f u g a t i o n , were added known amounts o f a n a l y t e

se

and

a b s o r b a n c e s were r e c o r d e d . The

s t a n d a r d a d d i t i o n method was

micellar layers containing

a l s o a p p l i e d t o the

iron(III).

The

extracted

r e s u l t s obtained with both

p r o c e d u r e s were found i n good agreement, as w e l l as which o b t a i n e d from DC-plasma s p e c t r o m e t r y a f t e r a n a l y s i s o f the

aqueous d i l u t e pha

ses. The

e x t r a c t i o n e f f i c i e n c y was

i n d e p e n d e n t measurements; the


t e r s was

also

t h e n c a l c u l a t e d from a t l e a s t f o u r

i n f l u e n c e o f the e x p e r i m e n t a l parame

investigated.

Table I I .

Properties

o f Some T r i t o n X 100/BL

Triton X

100 : BL (%,

and

Phase

(°C) 0.50

26.3

-

26.5

:

0.75

26.6

-

26.9

8.5

1.00

:

1.00

26.8

-

27.0

11.0

1.25

:

1.25

26.8

-

27.1

14.3

1.50

:

1.50

27.0

-

27.3

21.1

2.00

:

2.00

27.5

-

27.7

30.6

2.50

:

2.50

27.8

-

28.0

31.0

2 shows the

e f f e c t o f the

BL

4.2

i t can be

(1% w/v);

(PAS-C

1Q

ligand hydrophobicity

added NaNO^: 5% w/v;

the 100

i r o n ( I I I ) : 1x10

seen, q u a n t i t a t i v e r e c o v e r y o f i r o n ( I I I ) has conditions

using

or higher analogues).

c h a i n m o l e c u l e s can

a v a i l a b l e i n the

on

i n the p r e s e n c e o f T r i t o n X

M

M.

o b t a i n e d i n the r e p o r t e d of these long

3.5,

(%)

6.5

:

PAS-C : 2χ1θ"

pounds

w/v)

0.75

and

As

5 %

Volume o f the M i c e l l a r

C l o u d Temperature

r e c o v e r y , measured a t pH

(1% w/v)

3

0.50

Figure analyte

4.2

w/v)

4. 2 M i x t u r e s

C o n d i t i o n s (NaN0

i n the E x p e r i m e n t a l

the more h y d r o f o b i c

However, the

l i m i t the

ligand

lower

been com

solubility

concentration

system, k e e p i n g the volume o f m i c e l l a r

extraction

phase c o n s t a n t . The tions the

e x t r a c t i o n p e r f o r m a n c e s under d i f f e r e n t e x p e r i m e n t a l

( i . e . varying

the pH,

the

composition of surfactant

condi

mixtures,

amount o f c h e l a t i n g compound) were a l s o i n v e s t i g a t e d f o r our

system. All

The

r e s u l t s are

the

e x t r a c t i o n s were p e r f o r m e d i n the p r e s e n c e o f added

(NaNO , 5% w/v).

The

formed a t v a r i o u s

pH

ligand concentration was

test

shown i n T a b l e I I I .

2x10

M.

For

f o r the

salt

experiments p e r

the r u n s i n w h i c h

surfactant



F i g u r e 2. function

P l o t of of

the p e r c e n t r e c o v e r y o f i r o n ( I I I )

ligand a l k y l chain

as

length.


a

160


composition

o r l i g a n d c o n c e n t r a t i o n were changed, t h e pH was c o n s t a n t

(3.5) .

Table I I I .

E x t r a c t i o n E f f i c i e n c y as a F u n c t i o n o f Experimental Parameters f o r I r o n ( I I I ) - P A S - C

pH

% Ε

PAS-C , M 7

% Ε

T r i t o n X 100/

% Ε


BL 4.2, *h w/v -4

2.00

39.0

5.0x10

2.65

74.5

1.0x10

80.0

0.50:0 .50

77.6

87.5

0.75:0 .75

3.10

88.4

93.6

1.5x10

92.0

1.00:1 .00

3.50

93.7

93.7

2.0x10

93.7

1.50:1 .50

3.75

94.3

94.0

-3

Conclusions The

r e s u l t s o b t a i n e d w i t h t h e r e p o r t e d e x t r a c t i o n model showed t h a t

the s e p a r a t i o n o f c h a r g e d s p e c i e s i s p o s s i b l e , p r o v i d e d a s u i t a b l e ligand hydrophobicity.

F u r t h e r a n a l y t i c a l developments o f t h e s e mul

t i p h a s e e x t r a c t i o n systems w i l l

r e q u i r e an a c c u r a t e i n v e s t i g a t i o n o f

the e q u i l i b r i a and k i n e t i c p r o c e s s e s o c c u r r i n g a t t h e i n t e r f a c e s , a s w e l l as t h e study o f the m i c e l l a r host

s t r u c t u r e and p r o p e r t i e s o f t h e

aggregates. Other f u n c t i o n a l i z e d s u r f a c t a n t s having d i f f e r e n t

groups and modular l i p o p h i l i c solubilizing

complexing

chains, together with various nonionic

s u r f a c t a n t s , a r e p r e s e n t l y under i n v e s t i g a t i o n i n o u r

laboratories. Acknowledgments S u p p o r t o f t h i s work by C.N.R (Rome) and European S t a n d a r d i z a t i o n O f f i c e , under C o n t r a c t DAJA 45-85-C-0023, i s g r a t e f u l l y

appreciated.

Literature Cited 1. Hinze, W. L. In "Solution Chemistry of Surfactants"; Mittal, K. L., Ed.; Plenum Press: New York, 1979; p. 79. 2. Pelizzetti, E.; Pramauro, E. Anal. Chim. Acta 1985, 169, 1-29. 3. Cline Love, L. J . ; Habarta, J. G.; Dorsey, J. G. Anal. Chem. 1984, 56, 1133-48 A. 4. Armstrong, D. W. Separation and Purification Methods 1985, 14, 213-304. 5. Watanabe, H.; Tanaka, H. Talanta 1978, 25, 585-9.


7. PRAMAURO ET AL.


161


6. Watanabe, H. In "Solution Behavior of Surfactants: Theoretical and Applied Aspects"; Mittal, K. L.; Fendler, Ε. J., Eds.; Plenum Press: New York, 1982; Vol. II, p. 1305. 7. Pelizzetti, E.; Pramauro, E.; Barni, E.; Savarino, P.; Corti,M.; Degiorgio, V. Ber. Bunsenges. Phys. Chem. 1982, 86, 529-32. 8. Yatsimirskii, A. K.; Martinek, K.; Berezin, I. V. Tetrahedron 1971, 27, 2855-68. 9. Armstrong, D. W.; Nome, F. Anal. Chem. 1981, 53, 1662-6. 10. Pramauro, E.; Pelizzetti, E.; Cavasino, F. P.; Sbriziolo, C. in preparation. 11. Frank, H. S.; Oswalt, R. L. J. Am. Chem. Soc. 1947, 69, 1321-5. 12. Saini, G.; Mentasti, E. Inorg. Chim. Acta 1970, 4, 210-4. 13. Saini, G.; Mentasti, E. Inorg. Chim. Acta 1970, 4, 585-8. RECEIVED October

24, 1986


Ordered Media in Chemical Separations

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