15 The Relative Adsorbability of Counterions at the Charged Interface
Adsorption From Aqueous Solution Downloaded from pubs.acs.org by YORK UNIV on 12/13/18. For personal use only.
KŌZŌ
SHINODA
and M A S A M I C H I
FUJIHIRA
Yokohama National University, Ookamachi, M i n a m i k u , Yokohama, Japan
The distribution
of counterions
in the bulk solution technique
with
adsorbability of mole
at the charged
was studied
foam fractionation
of counterions adsorbability
technique.
was Cl-:
CH3COO-
Cl-
of sodium
mole/liter),
relative
: Br- :
ratio
phases. ClO -; 3
dodecylammo-
: Br- : C l O 3 - : NO - : SO42- = 3
1 : 1.2 : 2.8 : 1.6 : 2.3 in solution nium(0.015
The
from the
of
and
radiotracer
and in the solution
NO3- = 1 : 0.7 : 1.5 : 3.0 : 4.1 in solution nium(0.011 mole/liter),
surface
the
was determined
ratios in an adsorbed
The relative
combining
+
and Na
dodecylsulfate(0.006
of
dodecyltrimethylammo2+
: Ca
= 1 : 210 in
solution
mole/liter).
he a i r - s o l u t i o n interface of a n aqueous s o l u t i o n of i o n i c surface a c t i v e A
agent forms a u n i f o r m l y c h a r g e d surface o w i n g to t h e a d s o r p t i o n of
surface active ions, o n w h i c h counterions are a d s o r b e d f r o m t h e s o l u t i o n . W h e n t h e s o l u t i o n contains t w o k i n d s of counterions, t h e p r e f e r e n t i a l a d s o r p t i o n is o b s e r v e d at t h e interface. T h e d e t e r m i n a t i o n of t h e r e l a t i v e a d s o r b a b i l i t y of these t w o k i n d s of ions—i.e., t h e r a t i o of t h e ratios of the t w o k i n d s of ions at t h e surface a n d i n t h e s o l u t i o n — i s v e r y i m p o r t a n t i n o r d e r ( 1 ) to k n o w the d i s t r i b u t i o n of ions at the c h a r g e d surface, ( 2 ) t o get i n s i g h t i n t o t h e selective p e r m e a b i l i t y of counterions against i o n exchange r e s i n m e m b r a n e , a n d ( 3 ) to separate or concentrate t h e p a r t i c u l a r ions b y f o a m f r a c t i o n a t i o n . H o w e v e r , t h e d e t e r m i n a t i o n of t h e c o n c e n t r a t i o n o r c o m p o s i t i o n of a d s o r b e d counterions is difficult w i t h o u t the a i d of t h e r a d i o t r a c e r t e c h n i q u e . J u d s o n et al. ( 3 ) h a v e s t u d i e d t h e r e l a t i v e a d s o r p t i o n b e t w e e n c h l o r i d e a n d sulfate anions at t h e a i r - s o l u t i o n interface b y r a d i o a c t i v i t y . W a l l i n g et al. ( 8 ) h a v e f o u n d b y f o a m f r a c t i o n a t i o n that m u l t i v a l e n t ions a r e p r e f e r e n t i a l l y a d s o r b e d f r o m a s o l u t i o n .198
15.
SHINODA A N D F U J I H I R A
Adsorbability
of
Counterions
199
of s o d i u m p a l m i t o y l m e t h y l t a u r i n e . A s the concentrations of the solutions s t u d i e d i n these p a p e r w e r e a b o v e the c . m . c , a q u a n t i t a t i v e i n t e r p r e t a t i o n of the results w a s less accurate b y the c o m p e t i t i v e a d s o r p t i o n of m u l t i v a l e n t ions b y m i c e l l e s . I n the present i n v e s t i g a t i o n a large area of a d s o r b e d surface
was
o b t a i n e d b y c o l l e c t i n g w e l l d r a i n e d foams, a n d the c o n c e n t r a t i o n
was
d e t e r m i n e d b y w e i g h i n g . O n the other h a n d , the a m o u n t of the a d s o r b e d l a b e l e d ions w a s d e t e r m i n e d f r o m the measurements of the r a t i o of counts of the d r i e d samples of c o l l a p s e d foariis a n d of s o l u t i o n . T h e r e l a t i v e a d s o r b a b i l i t y w a s d e t e r m i n e d f r o m these measurements.
Experimental Materials. R a d i o a c t i v e H C 1 , K C 1 0 , a n d H S 0 were obtained f r o m the D a i i c h i P u r e C h e m i c a l s C o . A s the specific a c t i v i t y of these species w a s so h i g h , e a c h r a d i o a c t i v e i o n w a s d i l u t e d w i t h e a c h a c i d ( o r salt) s o l u t i o n . 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 w a s s y n t h e s i z e d from dodecylbromide (extra pure grade) a n d trimethylamine according to the procedures b y T a r t a r ( 4 ) . It was p u r i f i e d b y r e c r y s t a l l i z a t i o n f r o m acetone. T h e other salt—e.g., d o d e c y l t r i m e t h y l a m m o n i u m n i t r a t e — w a s o b t a i n e d a d d i n g s i l v e r n i t r a t e to s o l u t i o n of 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 o d e c y l a m m o n i u m b r o m i d e , n i t r a t e , c h l o r i d e , a n d acetate w e r e o b t a i n e d b y n e u t r a l i z i n g d o d e c y l a m i n e w i t h respective acids. S o d i u m sulfate a n d p o t a s s i u m b r o m i d e u s e d w e r e extra p u r e grade m a t e r i a l s . 3 6
3 6
3
2
3 5
4
Procedures. A q u e o u s solutions of surfactant c o n t a i n i n g t w o k i n d s of counterions, one of w h i c h w a s l a b e l e d , w e r e p r e p a r e d . A b o u t 200 cc. of the s o l u t i o n of k n o w n c o n c e n t r a t i o n w e r e i n t r o d u c e d i n t o a n a p p a r a t u s s h o w n i n F i g u r e 1 ( 5 ) . N a r r o w glass t u b i n g w a s c o n n e c t e d u n d e r the h o r i z o n t a l t u b i n g to p r o m o t e the c i r c u l a t i o n of the s o l u t i o n because of the m o v e m e n t of b u b b l e s . T h e apparatus w a s k e p t i n a n a i r thermostat to c a r r y out the e x p e r i m e n t at constant temperature. T h e b u b b l e s w e r e generated b y the a c t i o n of a c i r c u l a t i n g p u m p . B u b b l e s a t t a i n e d a d s o r p t i o n e q u i l i b r i u m w h i l e they m o v e d f r o m one e n d to the other e n d of the n e a r l y h o r i z o n t a l glass tube of about 50 c m . i n l e n g t h . T h e foams w e r e w e l l d r a i n e d w h i l e t h e y g r a d u a l l y m o v e d u p w a r d s t h r o u g h a t u b e of a b o u t 60 c m . l e n g t h . T h e t u b e was i n c l i n e d to f a c i l i t a t e d r a i n a g e of the foams. T h e f o a m collector w a s c o o l e d to a b o u t 0 ° C . to c o l l a p s e t h e foams. A c o r r e c t i o n because of the c o n d e n s a t i o n of w a t e r v a p o r b y c o o l i n g w a s necessary to d e t e r m i n e the c o n c e n t r a t i o n of foams. F i x e d v o l u m e s of o r i g i n a l s o l u t i o n a n d of c o l l a p s e d foams w e r e t a k e n w i t h a m i c r o ( o v e r f l o w t y p e ) p i p e t a n d t r a n s f e r r e d to respective s a m p l e plates w h i c h w e r e c o a t e d p a r t l y w i t h silicone o i l to p r e v e n t w e t t i n g . T h e s o l u t i o n was d r i e d g r a d u a l l y w i t h i n f r a r e d r a y l a m p a n d t h e n r a d i o a c t i v e counts w e r e d e t e r m i n e d . O n the other h a n d , the c o n c e n t r a t i o n of the s o l u t i o n of c o l l a p s e d b u b b l e s w a s d e t e r m i n e d b y w e i g h ing. T h e a c c u r a c y of this m e t h o d w a s c o n f i r m e d b y d r y i n g the s o l u t i o n of k n o w n c o n c e n t r a t i o n at respective experiments. T h e loss of acetic a c i d
200
ADSORPTION F R O M
AQUEOUS SOLUTION
( a n d therefore d o d e c y l a m i n e ) w a s o b s e r v e d d u r i n g the process o f d r y i n g i n the case of a l k y l a m i n e acetate. T h u s , the c o n c e n t r a t i o n of a d s o r b e d m o l e c u l e s w a s d e t e r m i n e d i n the f o r m of d o d e c y l a m m o n i u m c h l o r i d e b y the a d d i t i o n of H C l p r i o r to the d r y i n g . R e l a t i v e a d s o r b a b i l i t y w a s d e t e r m i n e d f r o m these values b y the procedures d e s c r i b e d i n the next section.
50CM Figure
Results and
1.
Apparatus of relative measurements
adsorption
Discussion
S e v e r a l features of this e x p e r i m e n t ( 5 — 7) as a r a d i o t r a c e r m e t h o d are as f o l l o w s : ( 1 ) since the r e l a t i v e a d s o r b a b i l i t y has b e e n d e t e r m i n e d f r o m the r a t i o of r a d i o c o u n t s of d r i e d samples, it is not affected b y errors i n the absolute values for r a d i o a c t i v i t y a n d ( 2 )
specific a c t i v i t y , w h i c h
has to b e large u s u a l l y for the s t u d y of surface p h e n o m e n a , m a y b e v e r y s m a l l a n d yet the r a d i o a c t i v e measurements y i e l d accurate results.
The
concentrations of solutions e x a m i n e d w e r e a l l close to b u t less t h a n the c.m.c. values of respective surfactant solutions. T h e foams w e r e stable e n o u g h to c a r r y the e x p e r i m e n t i n this c o n c e n t r a t i o n range a n d the m o l e r a t i o of counterions i n the b u l k was e q u a l to the s t o i c h i o m e t r i c m o l e r a t i o —i.e., there w a s no shift i n m o l e r a t i o of d i s p e r s e d counterions o w i n g to the m i c e l l e f o r m a t i o n at the present s t u d y . T h e a d s o r p t i o n of C l " ions at the a i r - s o l u t i o n interface of a n aqueous s o l u t i o n of d o d e c y l a m m o n i u m acetate (0.0140 m o l e / l i t e r ) c o n t a i n i n g a 3 f t
15.
SHINODA A N D F U J I H I R A
Adsorbability
of
201
Counterions
v e r y s m a l l a m o u n t of c h l o r i d e ions (0.000059 e q u i v . / l i t e r of H C 1 ) w a s 3 G
d e t e r m i n e d , a n d the results w e r e s u m m a r i z e d i n T a b l e I. T h e c o n c e n t r a t i o n of the surfactant was m a i n t a i n e d at a n almost constant l e v e l b e l o w the c.m.c. t h r o u g h o u t the experiment. A n a l i q u o t of the s o l u t i o n w a s d r i e d and
its r a d i o a c t i v i t y , R , d e t e r m i n e d is s h o w n i n the 1st l i n e of T a b l e I. s
T h e r a d i o a c t i v i t y of d r i e d s a m p l e of the same v o l u m e of c o l l a p s e d foams, Rf, is s h o w n i n the 2 n d l i n e . T h e c o n c e n t r a t i o n of surfactant ( g r a m / l i t e r ) i n s o l u t i o n is s h o w n i n the 3 r d l i n e a n d that of c o l l a p s e d foams is deter m i n e d b y w e i g h i n g a n d s h o w n i n the 4 t h l i n e . It m a y b e a s s u m e d t h a t the c o m p o s i t i o n of the i n t e r i o r of foams ( f i l m ) is the same as that of the b u l k s o l u t i o n , a n d the v o l u m e of a d s o r b e d molecules c a n b e c o m p a r e d w i t h the v o l u m e of c o l l a p s e d foams. the a m o u n t of a d s o r b e d substances.
Thus, C
f
neglected
— C
8
means
T h e r a t i o of the f r a c t i o n of l a b e l e d
ions i n the a d s o r b e d state against that i n s o l u t i o n is t h e n g i v e n b y , ( R ; ~ Rg) ft*(C; — C«) and
s h o w n i n the 5 t h line.
Where X
X X a d s
a d B
B O l n
and X i s o
n
are the fractions of
l a b e l e d ions a m o n g counterions i n a n a d s o r b e d state a n d i n s o l u t i o n . I n the case w h e n the f r a c t i o n of l a b e l e d ions is v e r y s m a l l c o m p a r e d w i t h u n i t y or the r e l a t i v e a d s o r b a b i l i t y is s m a l l , E q u a t i o n 1 is v e r y close to the r e l a t i v e a d s o r b a b i l i t y of l a b e l e d ions. T h e f r a c t i o n of l a b e l e d ions a m o n g counterions i n s o l u t i o n , X i , is g i v e n i n the 6 t h line. T h e r e l a t i v e a d s o r b s o
n
a b i l i t y ( 5 ) is t h e n g i v e n b y , X a
a d s
1 — Y / ads /
1
A
/
X i _ 1
s o l n
v ^soln
Z
a n d s h o w n i n the 7 t h l i n e of the T a b l e . Table I. Relative Adsorbability of Chloride Against Acetate Ions at the Air-Solution Interface of an Aqueous Solution of Dodecylammonium Acetate (0.0140 mole/liter) at 2 5 ° C . 1 1.
R
8
(cp.m.)
2.
ft; (c.p.m.)
3.
C
4.
C
5. 6. 7.
8
f
(gram/liter) (gram/liter)
3
2
1387
1398
1363
8755
5780
12378
3.43 15.6
3.43 11.1
3.43 22.0
X
