Adsorption From Aqueous Solution

3 Two-dimensional Monomolecular Ion Exchangers

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Kinetics and Equilibrium of Ion Exchange M . DE HEAULME, Y . HENDRIKX,, L . TER M I N A S S I A N - S A R A G A

A . L U Z Z A T I , and

Centre National de la Recherche Scientifique, Physico-Chimie des Surfaces et des Membranes, 45, rue des Saints-Peres, Paris VI°, France

The adsorption

of several cationic

ions at the surface

From the amount of adsorption selective

exchange

the chemical

nature

these sites.

hydrophobic

cations which

Under

This diffusion

conditions

takes place through

of on

particuthe

between

the rate of

ion exchanger

of P o C l 6 2 - towards ion

in

the sites of

and of the distance

our experimental

below the two-dimensional

measured.

depends

character

constitute

by the two-dimensional

by the diffusion

is

the coefficient

This coefficient

ion exchanger,

the ion exchange controlled

(the

solutions

we deduce

(selectivity).

lar) of the organic two-dimensional

soaps and of the P o C l 6 2 -

of their aqueous

a diffusion

is

the

surface.

layer

located

exchanger.

' " p h e first measurements of the a d s o r p t i o n of d i s s o l v e d substances A

the

a i r - s o l u t i o n interface, or

by

soap

films,

by

using

l a b e l l e d w i t h r a d i o a c t i v e elements, w e r e p e r f o r m e d b y D i x o n et al. and by Hutchinson

at

molecules (6)

(11).

D i x o n et al. m e a s u r e d the emission i n t o a i r , a b o v e a n aqueous s o l u t i o n of the surface a c t i v e c o m p o u n d , of soft /?-rays of a

3 5

S l a b e l l e d soap.

T h e y s t u d i e d the kinetics of the a d s o r p t i o n of the soap anions as a f u n c t i o n of t i m e a n d o b t a i n e d the e q u i l i b r i u m values for the a d s o r b e d layer density. It w a s f o u n d that the a d s o r p t i o n m i g h t b e c o n t r o l l e d b y the d i f f u s i o n of the a d s o r b i n g species. T h e a d s o r p t i o n of s m a l l ions (sulfate i o n s ) at the surface of solutions of surface active agents ( c a t i o n i c , a n i o n i c , a n d n o n - a n i o n i c ) w a s m e a s u r e d 23

Weber and Matijevi; Adsorption From Aqueous Solution Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

24

ADSORPTION

b y t h e same authors (13, 14, 15)

F R O M

AQUEOUS

SOLUTION

u s i n g the r a d i o t r a c e r m e t h o d .

Lastly,

t h e y d e m o n s t r a t e d the d i s p l a c e m e n t of one a d s o r b e d soap b y a second, m o r e a c t i v e surface agent, present i n the s o l u t i o n T h e m e t h o d u s e d b y H u t c h i n s o n (11)

(12).

a l l o w s the m e a s u r e m e n t of

the c o m p o s i t i o n of soap films, the constituents of w h i c h ate l a b e l l e d w i t h r a d i o a c t i v e elements.

S u c h a film m a y be f o r m e d b y r a i s i n g a p l a t i n u m

r i n g t h r o u g h the surface of the s o l u t i o n of the soap. T h e r a d i a t i o n e m i t t e d b y the t a g g e d m o l e c u l e s of the film is m e a s u r e d a n d the c o n c e n t r a t i o n of the last ones i n the film are c a l c u l a t e d . T h e m e t h o d s u s e d b y A n i a n s s o n et al. (1, 2, 3), Downloaded by PURDUE UNIV on November 25, 2016 | http://pubs.acs.org Publication Date: June 1, 1968 | doi: 10.1021/ba-1968-0079.ch003

ourselves (7, 20, 21)

N i l s o n (4, 5)

and

are r e l a t e d to the m e t h o d of D i x o n et al. ( 6 ) , w h i l e

the m e t h o d of S h i n o d a a n d a l l (19)

is b a s e d o n that of H u t c h i n s o n ( I I ) .

M o s t of the authors (2, 3, 7, 19, 21)

w h o use l a b e l l e d m o l e c u l e s are

c o n c e r n e d w i t h the process of i o n exchange b y a d s o r b e d i o n i z e d soap m o l e c u l e s a n d c a l c u l a t e the s e p a r a t i n g factors o r the coefficient of selec t i v i t y for the e x c h a n g i n g cations (2, 3,19)

or anions

T h e authors of the present p a p e r (7, 10, 21)

(7,21). u s e d the p l a n a r soap

m o n o l a y e r s as a s i m p l e m o d e l for the c o m p l e x surface of a porous i o n e x c h a n g i n g resin. T h u s , a s t u d y of the effect of the s e p a r a t i o n b e t w e e n the i o n i z e d sites of a " m o d e l " exchanger o n its s e l e c t i v i t y b e t w e e n

two

c o m p e t i n g counterions c a n b e a t t e m p t e d . T h e suggested m o d e l is c o n s i t u t e d b y the aqueous s o l u t i o n of c a t i o n i c soaps, c o n t a i n i n g H C 1 ( 2 M )

a n d traces

(10" -10- M) 7

w h i c h , u n d e r o u r c o n d i t i o n s , is the c o m p o u n d

2 1 0

of a

8

PoCl H 6

2

(4,

2 1 0

P o salt

5).

A t the surface of the s o l u t i o n the c a t i o n i c soap constitutes a p o s i t i v e l a y e r of o r g a n i c cations n e u t r a l i z e d b y C I " a n d P o C l " ions; 6

2

2 1 0

P o emits

a - r a y s . T h e surface d e n s i t y as w e l l as the k i n e t i c s of the a d s o r p t i o n of P o C l " are d e t e r m i n e d b y m e a s u r i n g the r a d i o a c t i v i t y a b o v e the surface 6

2

of the solution. A n analogous t e c h n i q u e is u s e d to d e t e r m i n e the d e n s i t y of the soap a d s o r b e d at the a i r - s o l u t i o n interface, w h e n the soap is labelled w i t h

1 4

C . T h e r e f o r e , these measurements a l l o w a d i r e c t analysis

of the c o m p o s i t i o n of the a d s o r b e d soap films at e q u i h b r i u m a n d d u r i n g their formation. F r o m the e q u i l i b r i u m values w e c a l c u l a t e d the coefficient of selec t i v i t y ( 9 ) a n d the c o r r e s p o n d i n g free energy a n d d e t e r m i n d t h e i r v a r i a t i o n as a f u n c t i o n of the s e p a r a t i o n b e t w e e n the i o n i z e d sites. A p r e l i m i n a r y report (10)

a n d a m o r e d e t a i l e d one (7)

n i q u e s e m p l o y e d are p u b l i s h e d elsewhere.

of the t e c h

F o u r c a t i o n i c soaps are u s e d

as f o l l o w s : d o d e c y l t r i m e t h y l a m m o n i u m b r o m i d e ( d T A B r , I ) ; h e x a d e c y l 2

trimethylammonium

bromide

H ( C i T A B r , I I ) *, 6

hexadecylpyridinium

chloride ( C i P y C l , I I I ) * , and hexadecyldimethylethylolammonium bro G

m i d e ( C i o C h o l B r , I V ) . T h e last c o m p o u n d is r e l a t e d to c h o l i n e b r o m i d e ,


3.

D E H E A U L M E

Monomolecular

E T A L .

Ion

25

Exchangers

one m e t h y l of w h i c h is r e p l a c e d b y a h e x a d e c y l r a d i c a l . m a r k e d w i t h a n asterisk are l a b e l l e d Results and

1 4

The

soaps

C.

Discussion

Kinetics of Adsorption (8)

(Results),

(A)

POC1 ".

T H E IONS

6

2

curves o b t a i n e d b y p l o t t i n g the i n t e n s i t y of the a-rays e m i t t e d b y

The 2 1 0

Po

a d s o r b e d b y the soap film vs. t i m e c o n f o r m to the l a w r e p r e s e n t e d b y t h e F i g u r e 1. T h e slope k of t h e l i n e s h o w n o n F i g u r e 1 is the constant of the rate of exchange of these ions b y t h e p r e - e x i s t i n g film of t h e n e u t r a l a d s o r b e d soap.

N o s u c h a d s o r p t i o n c a n b e f o u n d for a n a n i o n i c soap or


oleic a c i d monolayers.

11 0 Figure

O

i

i

i

i

i

1

10

20

30

40

50

60

1.

Kinetics

of the adsorption ions

\

i

^

70 of

PoCl ' 2

6

A = a-ray emission at equilibrium. A(t): instant a emission, t: time, k = slope of the line. cc TABr = 2 X 10-°M; HCl 2U x

16

(B)

T H E S O A P

F I L M S .

T h e f o r m a t i o n of the soaps films I I a n d I I I

is f o l l o w e d b y m e a s u r i n g the v a r i a t i o n of the i n t e n s i t y of the /?-rays e m i t t e d b y the

1 4

C atoms l a b e l l i n g the o r g a n i c cations of the

soaps.

T h e i r e q u i l i b r i u m surface d e n s i t y is a t t a i n e d l o n g b e f o r e t h a t of

the

P o C l e " ions. 2

Equilibrium of Adsorption,

(A)

T H E SOAP

F I L M S .

T h e isotherms

of a d s o r p t i o n of the o r g a n i c cations of the soaps are r e p r o d u c e d o n the F i g u r e 2. C l a s s i c a l m e t h o d s ( 2 0 ) are u s e d to o b t a i n the surface densities of the soaps I a n d I V .


26

ADSORPTION

(B)

POC1

T H E

6

2

F R O M

AQUEOUS

SOLUTION

C o n s i d e r i n g that the m a x i m u m n u m b e r o f

" IONS.

p o s i t i v e sites is g i v e n b y the surface d e n s i t y 8 o f the o r g a n i c cations, the a m o u n t o f a d s o r p t i o n o f the ions P o C l " , 8 ci6 -> does n o t exceed 1 % 6

of this m a x i m u m v a l u e .

2

2

Po

U n d e r these c o n d i t i o n s 8 p i - is p r o p o r t i o n a l oC

to the c o n c e n t r a t i o n i n b u l k o f the same ions c

P o C

6

2

i 2 - therefore the d i s 6

t r i b u t i o n coefficient of this i o n ^

SpoCle "

^

2

CpoCie " 2

is i n d e p e n d e n t of c c i - - H o w e v e r , i t varies w i t h 8 (7, J O ) . T h e coefficient o f s e l e c t i v i t y ( 9 ) a c o r r e s p o n d i n g to the f o l l o w i n g r e a c t i o n o f exchange Downloaded by PURDUE UNIV on November 25, 2016 | http://pubs.acs.org Publication Date: June 1, 1968 | doi: 10.1021/ba-1968-0079.ch003

P o

6

2

2 C F + P o C l - ^ P o C l - + 2 CI" 6

2

6

(2)

2

can b e d e t e r m i n e d f r o m o u r d a t a . If one assumes that 8 r == (8 - 2 8 c i - ) and, as Spocie < < S r a n d c , 2 - < < c , i t c a n b e s h o w n t h a t a is equal to: C

2-

C

P (

6

C

From AG

2 C

i

P o (

x

£cr

__ ^

cr &

e n t h a l p y of

exchange

ln«.

5x10r

14

[molec/cm*]

Figure 2.

4

5

6

7

i

i

i

i

8

10

9

i

i

i

8

9

10

x lO^C x 10"

[M] J

6

Surface density of 8 of the adsorbed soap films vs. its concentration c in the solution

HCl 2M; 25°C.

2

^

c

8

PoCl6 " 2

E q u a t i o n 3 w e c a l c u l a t e t h e free

V - = -RT

6

c r

C ] 6

8PQCI 2-

a

Po

A (C„TABr); © (CnCholBr); O (C TABr); CuTABr (O-IO-'M); other soaps (0-10~ M) J6

+

(CcPyCl);

s


3.

D E

H E A U L M E

E T

MoYiomolecular

A L .

Ion

27

Exchangers

F i g u r e 3 shows a p l o t of this vs. the q u a n t i t y

w h i c h is p r o

p o r t i o n a l to the separation b e t w e e n the p o s i t i v e charges of the a d s o r b e d soap m o n o l a y e r .


10x10

1x10

6

2,3 RT

6

105.

-jLx10 cm 8

10

4

1

2

3

4

5

6

7

8

9

10

Figure 3. The free enthalpy of ion exchange A G / R T vs. the separation (l/W 8) (A) between the positively charged sites 25°C. From bottom to top, the soaps CuTABr, CmCholBr, CnTABr, CiePyCl F o r a g i v e n s e p a r a t i o n b e t w e e n the c h a r g e d sites a n d for the ions P o C l o " / C l " the c h e m i c a l n a t u r e of the a d s o r b i n g sites m a y affect 2

the

i o n i c s e l e c t i v i t y strongly. T h e h y d r o p h o b i c character of the s u b s t i t u t e d q u a t e r n a r y a m m o n i u m seems to f a v o r the selection of the P o C l ' i o n 6

( c o m p a r e the soaps I a n d I I I ) . If w e consider the exchange

2

PoCl(f~/Cl"

as a n " i n situ i n d i c a t o r " of the h y d r o p h o b i c character of the a d s o r b i n g site o u r results ( F i g u r e 3 ) a l l o w us to class the p o l a r groups of I I , I I I , a n d I V a c c o r d i n g to the i n t e n s i t y of this character, as f o l l o w s : p y r i d i n i u m >

trimethylammonium > dimethylethylolammonium. Kinetics of Adsorption.

DISCUSSION.

T h e i n t e r p r e t a t i o n of the results

r e p r o d u c e d o n the F i g u r e 1 is a t t e m p t e d o n the same lines as the desorpt i o n of s l i g h t l y s o l u b l e m o n o l a y e r s

(20).


28

ADSORPTION

P R O M

AQUEOUS

SOLUTION

It is a s s u m e d that d u r i n g the exchange, the " a d s o r b e d " ions P o C l " 6

2

d i s t r i b u t e l o c a l l y b e t w e e n the soap m o n o l a y e r a n d a v e r y t h i n r e g i o n of n e i g h b o r i n g l i q u i d substrate. T h e l o c a l d i s t r i b u t i o n r a t i o is e q u a l to K o f the E q u a t i o n 1 a n d varies w i t h t i m e f o l l o w i n g the v a r i a t i o n of Spocie "- I n this t h i n r e g i o n the c o n c e n t r a t i o n c c i 2

P o

6

2

i s

l o w e r t h a n i n the

b u l k of the substrate a n d P o C l ' ions m i g r a t e across a t h i c k d i f f u s i o n 6

2

l a y e r c u n t i l the c o n c e n t r a t i o n of the ions P o C l " becomes u n i f o r m 6

2

t h r o u g h o u t a l l the l i q u i d substrate.


U n d e r these c o n d i t i o n s the constant k o f the rate of a d s o r p t i o n is equal to: (4) where D =

0.78 c m . / d a y ( 1 7 ) . 2

I I , I I I , a n d I V (see

Figure 4).

T h e E q u a t i o n 4 is v e r i f i e d b y the soaps T h e h y d r o d y n a m i c constant {D/e)

i n d e p e n d e n t of the n a t u e r of the " a d s o r b e n t " ( s o a p

film).

-^-xlO ^ x cm /molec] 20

2

0,5

Figure K =

4.

The constant of the adsorption (l/X)

rate vs.

distribution coefficient of PoCU*~ (Equation 1) Q CeCholBr; O CuTABr; © C PyCl 16


is

3.

D E HEAULME E T AL.

Motiomolecular

F r o m i t a v a l u e of e ^

29

Ion Exchangers

0.4 m m . is c a l c u l a t e d u s i n g the v a l u e of D

(17). Conclusion T h e kinetics a n d the e q u i l i b r i u m of a d s o r p t i o n of anions P o C l " b y i o n i z e d m o n o l a y e r s of c a t i o n i c soaps h a v e b e e n s t u d i e d a n d b o t h p h e n o m e n a are analogous to those o b s e r v e d w i t h the p o l y m e r i o n exchangers (9). 6

2

It is suggested that the soap films m a y b e c o n s i d e r e d as m o n o m o l e c u -


lar m o d e l ion-exchangers ( t w o - d i m e n s i o n a l ) a n d m a d e use of for the s t u d y of p a r t i c u l a r effects c o n t r i b u t i n g to the u s u a l c o m p l e x process of i o n exchange. A s a first a p p l i c a t i o n , the authors s t u d y the effects of the separation b e t w e e n the e x c h a n g i n g sites a n d of the degree

of

their

h y d r o p h o b i c c h a r a c t e r o n the s e l e c t i v i t y of i o n exchange. Literature

Cited

(1) Aniansson, G . , J. Phys. Coll. Chem. 55,1286 (1951). (2) Aniansson, G . , Steiger, N. H., J. Chem. Phys. 2 1 , 1299 (1953). (3) Aniansson, G . , Steiger, N. H., J. Phys. Chem. 58, 228 (1954). (4) Bagnall, K . W . , d'Eye, R. W . , Freeman, T. W . , J. Chem. Soc. 1955, 2320. (5) Ibid., 1956, 2770. (6) D i x o n , J . R., W e i t h , A . J., Argyle, A . Jr., Salley, D . J . , Nature 163, 845 (1949). (7) de Heaulme, M., Hendrikx, Y . , L u z z a t i , Α., Ter Minassian-Saraga, L., J. Chim. Phys. 6 4 , 1 3 6 3 (1967). (8) de Heaulme, M. (to be published). (9) Helferich, E., "Ion Exchange," M c G r a w - H i l l , N e w York, 1962. (10) Hendrikx, Y . , L u z z a t i , Α., Ter Minassian-Saraga, L., J. Chim. Phys. 59, 481 (1962). (11) Hutchinson, E., J. Coll. Sci. 4, 600 (1949). (12) Judson, G . M., Argyle, Α. Α., Salley, D . J . , D i x o n , J . K . , J. Chem. Phys. 18, 1302 (1950). (13) Ibid., 19, 378 (1951). (14) Judson, G . M., L e r e w , Α. Α., D i x o n , J . K . , Salley, D . J . , J. Chem. Phys. 2 0 , 5 1 9 (1952). (15) Judson, G . W . , L e r e w , Α. Α., D i x o n , J . K . , Salley, D . J . , J. Phys. Chem. 57, 916 (1953). (16) Nilson, G . , J . Phys. Chem. 6 1 , 1 1 3 5 (1957). (17) Servigne, M., J . Chim. Phys. p. 31 (1934). (18) Shinoda, K . , Nakanishi, J . , J. Phys. Chem. 67, 2547 (1943). (19) Shinoda, K . , Ito, K . , J. Phys. Chem. 65, 1499 (1961). (20) Ter Minassian-Saraga, L., J. Chim. Phys. 53, 355 (1956). (21) Ter Minassian-Saraga, L., L u z z a t i , Α., J. Chim. Phys. 59, 481 (1962). RECEIVED

November 24,

1967.


Adsorption From Aqueous Solution

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