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