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11 Adsorption and Wetting Phenomena Associated with Graphon in Aqueous Surfactant Solutions
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F . G . G R E E N W O O D , G . D . P A R F I T T , N . H. P I C T O N , and D. G. WHARTON University of Nottingham, Nottingham, E n g l a n d
Adsorption dodecyl
isotherms sulfate
0.1 M
from
from
sodium
chloride,
also
dispersibility
studies the
trolled
by
angle measurements
characteristics powder value
of the
is readily
the solution comes <
bromide
as
electrolyte. solutions
density of
dispersions
Comparison
dispersed
A fairly
with
is not of
the
discrete
ions is necessary by end-over-end
of ionic
strength,
for which
and
trimethyl-
various
properties
sodium
cocon-
system.
were made to assess the
systems.
concentration
dodecyl
in the
shaking.
of surfactant
is independent
for solutions
that dispersibility
electrochemical
coverage
for
the optical
confirms
Contact surface
powder
from end-over-end
agulation
aqueous
using potassium of the
was assessed by measuring resulting
were determined
Graphon
- a m m o n i u m bromide The
at 25°
on
wetting value
before
action.
of the This
and corresponds the contact
angle
to be-
90°.
> X h e p r o b l e m of i n c o r p o r a t i n g a p o w d e r i n t o a l i q u i d to f o r m a d i s p e r i
sion of fine particles is a n i m p o r t a n t aspect of c o l l o i d c h e m i s t r y . T h e o v e r a l l process m a y be c o n s i d e r e d as c o n s i s t i n g of three stages: 1. W e t t i n g of t h e p o w d e r . P o w d e r s consist of aggregates a n d a g glomerates ( t w o w a y s of d e f i n i n g clusters of p r i m a r y p a r t i c l e s ( 9 ) ) so not o n l y the w e t t i n g of the e x t e r n a l surfaces b u t also the d i s p l a c e m e n t of a i r a n d w e t t i n g of the i n t e r n a l surfaces ( b e t w e e n the p a r t i c l e s i n the clusters) m u s t b e c o n s i d e r e d . T h e effectiveness of the w e t t i n g process m a y b e expressed i n terms of t h e s o l i d / l i q u i d / v a p o r contact angle w h i c h m u s t b e zero for spontaneous w e t t i n g of the e x t e r n a l surface (14), a n d less t h a n 90° for spontaneous p e n e t r a t i o n i n t o the agglomerates ( I ) . 135
Weber and Matijevi; Adsorption From Aqueous Solution Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
136
ADSORPTION F R O M
AQUEOUS SOLUTION
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2. B r e a k i n g u p the aggregates a n d agglomerates i n t o c o l l o i d a l p a r ticles. I d e a l l y the w o r k r e q u i r e d to c o m p l e t e this stage s h o u l d b e as s m a l l as possible, a l t h o u g h i n some cases large energies m a y b e i n v o l v e d d e p e n d i n g o n the strength of the b o n d h o l d i n g the p r i m a r y particles together i n the clusters. F o r the systems c o n s i d e r e d i n this p a p e r little effort is a p p a r e n t l y r e q u i r e d for this stage. It has b e e n suggested (16) that the resistance to stress of p a r t i c l e - p a r t i c l e b o n d s c a n be significantly r e d u c e d b y the a d d i t i o n of surface active m a t e r i a l b u t the m e c h a n i s m of the process is not established. 3. C o a g u l a t i o n ( r e d u c t i o n i n p a r t i c l e n u m b e r w i t h t i m e d u e to i r r e v e r s i b l e c o l l i s i o n s ) of the d i s p e r s i o n . T h e resistance to c o a g u l a t i o n , or the s t a b i l i t y of the d i s p e r s i o n , depends o n the r e l a t i v e m a g n i t u d e s of the attractive v a n der W a a l s forces b e t w e e n the p a r t i c l e s , a n d the r e p u l sive force w h i c h i n a system i n v o l v i n g c h a r g e d particles m a y b e asso c i a t e d w i t h the o v e r l a p p i n g of t h e i r e l e c t r i c a l d o u b l e layers. T h e s t a b i l i t y of a c o l l o i d a l d i s p e r s i o n is p r e d i c t e d b y the D e r y a g u i n - L a n d a u - V e r w e y O v e r b e e k ( D L V O ) t h e o r y (6, 7, 21). D i s p e r s i b i l i t y has b e e n defined (12)
as the ease w i t h w h i c h a d r y
p o w d e r m a y b e d i s p e r s e d i n a l i q u i d a n d this t e r m c a n be u s e d to express the effectiveness
of the first t w o stages.
A l t h o u g h i n theory the three
stages m a y b e c o n s i d e r e d q u i t e separately, i n t e r p r e t a t i o n of e x p e r i m e n t a l observations i n terms of these stages m a y b e difficult because t h e y u s u a l l y o v e r l a p i n p r a c t i c e . A great d e a l of a t t e n t i o n has b e e n p a i d to the factors i n v o l v e d i n the s t a b i l i t y of c o l l o i d a l dispersions i n r e l a t i o n to c u r r e n t theories. T h e r e l a t i o n s h i p b e t w e e n d i s p e r s i b i l i t y a n d the v a r i o u s p a r a m e ters o b t a i n i n g i n a n y p a r t i c u l a r system has r e c e i v e d little attention. T h e w e t t i n g characteristics of aqueous surfactant solutions o n o x i d e etc. sur faces is of c o n s i d e r a b l e interest to m i n e r a l processing, a n d o n
carbon
b l a c k s to detergency, b u t s u r p r i s i n g l y f e w attempts h a v e b e e n m a d e to relate the efficiency of the processes to the i n t e r f a c i a l tensions p r e v a i l i n g a n d to the contact angles.
U n f o r t u n a t e l y the measurement of
contact
angle for a l i q u i d w i t h a p o w d e r is beset w i t h difficulties. T h e a d s o r p t i o n of the surface active agent at the s o l i d / l i q u i d i n t e r face is, p r e s u m a b l y , a n i m p o r t a n t p r e r e q u i s i t e to the process associated w i t h d i s p e r s i b i l i t y . Besides the l o w e r i n g of the i n t e r f a c i a l tension, another factor is i n v o l v e d w i t h i o n i c agents n a m e l y the electric p o t e n t i a l associ a t e d w i t h a d s o r p t i o n of ions. mushi
(19)
B o t h factors w e r e c o n s i d e r e d b y T a m a -
to be relevant to the d i s p e r s i o n of p o w d e r s
surfactant solutions.
Some
authors
(15, 20, 22, 23)
in
aqueous
h a v e r e l a t e d the
effects d i r e c t l y to the z e t a p o t e n t i a l , w h i l e others (8, 11, 18) discuss t h e i r observations i n terms of the i n c r e a s i n g degree of h y d r o p h i l i c character of the c a r b o n b l a c k surface as a result of a d s o r p t i o n . F u n d a m e n t a l ener getic considerations s h o w (12)
that the values of the contact angle a n d
the surface tension of the w e t t i n g l i q u i d are i m p o r t a n t parameters c o n t r o l l i n g the d i s p e r s i o n process.
T h e effects of c o a g u l a t i o n , c o n t r o l l e d b y
Weber and Matijevi; Adsorption From Aqueous Solution Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
11.
137
Graphon
GREENWOOD E T A L .
the e l e c t r i c a l properties of the system, m a y be s u p e r i m p o s e d
on
the
d i s p e r s i n g process a n d this m a y l e a d to a n i n c o r r e c t i n t e r p r e t a t i o n of the e x p e r i m e n t a l results. This
paper
describes
(graphitized Spheron 6) fate ( S D S )
a
study
i n aqueous
of
the
d i s p e r s i b i l i t y of
Graphon
solutions of s o d i u m d o d e c y l
an dodecyl trimethylammonium bromide
sul
( D T A B ) , a n d its
r e l a t i o n to the a d s o r p t i o n b e h a v i o r of the surfactants at the s o l i d / l i q u i d interface, w i t h a v i e w to d e t e r m i n e the c o n t r o l l i n g process i n the d i s p e r s i b i l i t y of these systems.
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Experimental M a t e r i a l s . G r a p h o n ( t h e g r a p h i t i z e d f o r m of the m e d i u m - p r o c e s s i n g c h a n n e l b l a c k , S p h e r o n 6 ) was s u p p l i e d b y the C a b o t C o r p o r a t i o n . T h e surface area of G r a p h o n (4) of 78.9 m e t e r / g r a m was d e t e r m i n e d b y the B . E . T . m e t h o d u s i n g n i t r o g e n at - 1 9 6 ° C . a n d o- = 16.2 A . . P u r e samples of D T A B a n d S D S w e r e s u p p l i e d b y G l o v e r s C h e m i c a l s L t d . a n d C y c l o C h e m i c a l s respectively. A n a l y s i s of the surfactants gave t h e f o l l o w i n g results: S D S , C 4 9 . 7 6 % (calc. 5 0 . 0 0 % ), H 8 . 7 3 % (calc. 8 . 6 8 % ), residue 25.12% (calc. 2 4 . 6 6 % ) a n d > 9 9 % C homologue; D T A B , N 4 . 4 0 % ( c a l c . 4 . 5 4 % ) , B r 2 5 . 4 4 % (calc. 2 5 . 9 2 % ) residue 7 0 . 1 6 % ( c a l c . 6 9 . 5 4 % ) a n d > 9 6 % C i h o m o l o g u e . V a l u e s of the 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 (c.m.c.) w e r e d e t e r m i n e d for the t w o surfactants a n d the results for S D S , c.m.c. = 8.0 m M . ( d r o p v o l u m e m e t h o d for surface t e n s i o n ; no m i n i m u m o b s e r v e d ) , a n d D T A B , c.m.c. = 16.0 m M . ( c o n d u c t a n c e ) , w e r e i n g o o d agreement w i t h l i t e r a t u r e values (17, 24), i n d i c a t i n g a satisfactory l e v e l of p u r i t y . B . D . H . L t d . c e t y l p y r i d i n i u m b r o m i d e (standard cationic agent), and A . R . sodium chloride and potassium b r o m i d e w e r e used. 2
2
12
2
Procedure. F o r the a d s o r p t i o n measurements samples of a b o u t 0.3 g r a m G r a p h o n w e r e a c c u r a t e l y w e i g h e d into a d s o r p t i o n tubes, about 10 m l . of surfactant s o l u t i o n a d d e d , the tubes sealed a n d r o t a t e d e n d o v e r - e n d i n a w a t e r thermostat at 25 ± 0.1° for at least t w e l v e h o u r s ; it h a d b e e n established that a m u c h shorter t i m e w a s r e q u i r e d for r e a c h i n g a d s o r p t i o n e q u i l i b r i u m . U s u a l l y it w a s necessary to separate the s o l i d f r o m the s o l u t i o n b y filtering t h r o u g h a n O x o i d m e m b r a n e filter b u t w h e r e possible c e n t r i f u g i n g at 3500 r . p . m . w a s used. T h e clear super natant l i q u i d w a s a n a l y z e d for surfactant b y t i t r a t i o n ( 2 ) against c e t y l p y r i d i n i u m b r o m i d e for D T A B . B r o m o p h e n o l b l u e was u s e d as i n d i c a t o r . I n t h e cases w h e r e b o t h s e p a r a t i o n t e c h n i q u e s w e r e a v a i l a b l e i d e n t i c a l results w e r e o b t a i n e d . T h e effect o n the a d s o r p t i o n of b r e a k i n g u p the G r a p h o n b y i r r a d i a t i o n w i t h ultrasonics w a s assessed i n s i m i l a r e x p e r i ments i n w h i c h the m i x t u r e s w e r e subjected to 40 k c . / s e c . r a d i a t i o n for t w o m i n u t e s u s i n g a 500 w a t t D a w e Instruments L t d . S o n i c l e a n G e n e r a t o r . F o r the assessment of d i s p e r s i b i l i t y , samples of a b o u t 0.1 g r a m of G r a p h o n w e r e a c c u r a t e l y w e i g h e d i n t o s t a n d a r d tubes a p p r o x i m a t e l y 1.5 c m . w i d e a n d 13 c m . l o n g fitted w i t h B 1 4 Q u i c k f i t joints. A k n o w n a m o u n t of s o l u t i o n ( — 1 0 m l . ) w a s a d d e d a n d the tubes w e r e r o t a t e d e n d - o v e r - e n d i n the thermostat at 25° at a p p r o x i m a t e l y 20 r . p . m . for
Weber and Matijevi; Adsorption From Aqueous Solution Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
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ADSORPTION F R O M
AQUEOUS SOLUTION
v a r i o u s times, after w h i c h t h e y w e r e a l l o w e d to s t a n d for 18 h o u r s so that the larger G r a p h o n particles w o u l d settle. T h e o p t i c a l d e n s i t y of the r e m a i n i n g d i s p e r s i o n was m e a s u r e d at 400 m/x i n a 2 m m . c e l l u s i n g a U n i c a m S P 600 spectrophotometer, i n a constant t e m p e r a t u r e r o o m m a i n t a i n e d at 25 ± 1 ° . O p t i c a l densities w e r e c o r r e c t e d to a c o n c e n t r a t i o n of 1 m g . G r a p h o n / m l . s o l u t i o n . T h e w e t t i n g characteristics of the systems w e r e assessed b y m e a s u r i n g the contact angles (0) of the solutions o n the p o w d e r u s i n g the B i k e r m a n m e t h o d ( 3 ) . T o o b t a i n a non-porous flat surface o n w h i c h to p l a c e drops of l i q u i d for measurement, a t h i n l a y e r of G r a p h o n w a s pressed o n a flat paraffin w a x surface. D r o p s of different v o l u m e s (0.001 to 0.008 cc.) w e r e p l a c e d o n the G r a p h o n surface u s i n g a n A g l a syringe, a n d 6 c a l c u l a t e d f r o m measurements, u s i n g a t r a v e l l i n g m i c r o s c o p e , of the diameters of the areas of contact of the drops ( e x t r a p o l a t e d to zero v o l u m e ) , a s s u m i n g e a c h d r o p to have the same shape as a segment of a sphere. I n cases w h e r e this c o n d i t i o n w a s not f u l f i l l e d — i . e . , at l o w 0— anomalous results w e r e o b t a i n e d . C o n t a c t angles for w a t e r w e r e also m e a s u r e d for G r a p h o n pressed o n a v i n y l p l a s t i c t i l e a n d also o n a sheet of P o l y t h e n e , a n d w i t h i n e x p e r i m e n t a l error ( ± 2 % ) the results w e r e the same as those for G r a p h o n o n the w a x surface. C o n t a c t angles of v a r i o u s D T A B solutions o n the w a x surface w e r e f o u n d to b e about 3 0 ° l o w e r t h a n the c o r r e s p o n d i n g values o b t a i n e d for G r a p h o n pressed o n the w a x surface. Results
and
Discussion
T h e a d s o r p t i o n results for S D S o n G r a p h o n f r o m aqueous a n d 0 . 1 M s o d i u m c h l o r i d e solutions are s h o w n i n F i g u r e 1. I n b o t h cases saturation a d s o r p t i o n is r e a c h e d at the c . m . c , the effect of a d d e d salt b e i n g
to
decrease the c.m.c. a n d to increase the m a x i m u m a d s o r p t i o n l e v e l s u c h that the average area p e r a d s o r b e d D S " i o n decreases f r o m 4 2 A . to 3 3 A . . 2
2
F o r aqueous solutions a m a r k e d p o i n t of i n f l e c t i o n is o b s e r v e d at a b o u t h a l f the c . m . c , w h i c h m a y i n d i c a t e a change i n o r i e n t a t i o n , f r o m p a r a l l e l to p e r p e n d i c u l a r , of the a d s o r b e d i o n . A t the p o i n t of inflection the area p e r a d s o r b e d i o n is a p p r o x i m a t e l y 7 0 A . w h i c h w o u l d satisfy the p a r a l l e l 2
orientation model.
S i m i l a r experiments (4)
o n heat-treated samples of
the o r i g i n a l c a r b o n b l a c k S p h e r o n 6 i n d i c a t e that the p o i n t of inflection is associated w i t h the g r a p h i t i z e d , h o m o g e n e o u s surface c o n t a i n i n g v i r t u a l l y no h y d r o p h i l i c sites. T h e p o i n t of i n f l e c t i o n is not a p p a r e n t i n the i s o t h e r m for 0 . 1 M s o d i u m c h l o r i d e p o s s i b l y because of the steep rise i n a d s o r p t i o n at l o w c o n c e n t r a t i o n .
T h e effect of s u b j e c t i n g the G r a p h o n
to u l t r a s o n i c r a d i a t i o n is to increase s l i g h t l y the a d s o r p t i o n at c o n c e n t r a tions a b o v e the p o i n t of inflection. W h e t h e r this increase m a y b e corre l a t e d w i t h a change
i n the w e t t i n g characteristics of
the system
is
uncertain. F i g u r e 2 shows the a d s o r p t i o n d a t a for D T A B , w h i c h h a v e some similarities to those of S D S i n that s a t u r a t i o n is r e a c h e d at the c.m.c. a n d
Weber and Matijevi; Adsorption From Aqueous Solution Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
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.
GREENWOOD E T A L .
139
Graphon
Equilibrium concentration (mM)
Figure 1. Adsorption of SDS on Graphon at 25° from aqueous solution after end-over-end action O and after ultrasonic irradiation X , and from solutions in 0.1 M sodium chloride • (end-over-end) T
i
I
i
i
i
Equilibrium concentration
i
I
i
i
r
(mM)
Figure 2. Adsorption of DTAB on Graphon at 25° from aqueous solution after end-over-end action O and after ultrasonic irradiation X , and from solutions in 0.1 M potassium bromide • (end-over-end)
Weber and Matijevi; Adsorption From Aqueous Solution Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
140
ADSORPTION F R O M
AQUEOUS SOLUTION
t h e a d s o r p t i o n increases o n a d d i t i o n of electrolyte. H o w e v e r , the increase is not as large for D T A B , the average area per D T A
+
ion decreasing
f r o m 4 2 A . to 3 8 A . . T h i s b e h a v i o r is p a r a l l e l e d b y the smaller a p p a r e n t 2
2
increase i n m i c e l l a r w e i g h t o n a d d i t i o n of p o t a s s i u m b r o m i d e to D T A B ( 5 ) c o m p a r e d w i t h that for s o d i u m c h l o r i d e o n S D S ( 1 0 ) , a n d m a y w e l l reflect the screening of the n i t r o g e n b y m e t h y l groups i n D T A B .
Subjecting
the G r a p h o n to u l t r a s o n i c r a d i a t i o n has n o effect ( w i t h i n e x p e r i m e n t a l e r r o r ) o n the a d s o r p t i o n . T h e r e are differences i n i s o t h e r m shape, a n d f o r D T A B the b e h a v i o r is not a m e n a b l e to a s i m p l e e x p l a n a t i o n . O f p a r t i c u l a r interest are plots Downloaded by CORNELL UNIV on October 10, 2016 | http://pubs.acs.org Publication Date: June 1, 1968 | doi: 10.1021/ba-1968-0079.ch011
of the a m o u n t a d s o r b e d against the m e a n i o n i c a c t i v i t y of the surface a c t i v e agent ( i n c l u d i n g the c o u n t e r i o n of the a d d e d e l e c t r o l y t e ) .
I n the
case of D T A B a l l the d a t a , i n c l u d i n g others at v a r i o u s salt concentrations u p to 0 . 5 M , l i e o n one l i n e w h i c h , after a n i n i t i a l steep rise, is l i n e a r to the c.m.c. T h i s indicates that for other t h a n the i n i t i a l strong a d s o r p t i o n at l o w concentrations ( p o s s i b l y because of specific interactions w i t h the surface) the a d s o r p t i o n f o l l o w s the l a w of mass a c t i o n . F o r S D S a s i m i l a r result is o b t a i n e d except that p o s i t i v e deviations f r o m the straight l i n e occur below a
±
— 4 X 1 0 " M for the cases (salt c o n c e n t r a t i o n < 3
0.1M)
w h e n there is a p o i n t of i n f l e c t i o n i n the i s o t h e r m . T h e s e deviations m a y reflect specific interactions of the D S " w i t h the surface w h e n the ions are adsorbed i n parallel orientation. D u r i n g the a d s o r p t i o n experiments
greater difficulty w a s
experi
e n c e d i n s e p a r a t i n g the G r a p h o n d i s p e r s e d i n surfactant solutions at concentrations a b o v e the c . m . c , a n d this difficulty increases i n m a g n i t u d e w i t h the l e n g t h of the p e r i o d subjected to e n d - o v e r - e n d a c t i o n . the s e p a r a t i o n is c o m p l e t e , observed
Unless
a m a x i m u m i n the a d s o r p t i o n i s o t h e r m is
since the t o t a l a m o u n t of surfactant a n a l y z e d is larger t h a n
that c o r r e s p o n d i n g to the t r u e a d s o r p t i o n . W e f o u n d the a m o u n t of s o l i d remaining suspended
that w o u l d l e a d to a n a d s o r p t i o n m a x i m u m , to
be deceivingly small.
A n i l l u s t r a t i o n of the r e l a t i o n b e t w e e n the e q u i
l i b r i u m c o n c e n t r a t i o n of D T A B a n d the a m o u n t of s o l i d m a t e r i a l r e m a i n ing
suspended
after
s t a n d i n g for
some days
a c t i o n for 30 hours, is g i v e n i n F i g u r e 3.
following
end-over-end
T h e i n i t i a l change, w h i c h is
f a i r l y a b r u p t , occurs at a c o n c e n t r a t i o n b e l o w the c . m . c , a n d c o m p a r i s o n w i t h the a d s o r p t i o n i s o t h e r m i n F i g u r e 2 shows there to be n o a p p a r e n t c o r r e l a t i o n b e t w e e n the effect a n d the n a t u r e of the a d s o r b e d layer. S u c h is the case for a l l the systems discussed i n this p a p e r .
F u r t h e r m o r e , the
effect bears no r e l a t i o n to the s t a b i l i t y to c o a g u l a t i o n of the Measurements (13)
systems.
of the rate of c o a g u l a t i o n of dispersions p r e p a r e d
u s i n g u l t r a s o n i c i r r a d i a t i o n s h o w that the G r a p h o n , once d i s p e r s e d , is i n d e f i n i t e l y stable i n aqueous surfactant solutions at a l l concentrations. It is the same for solutions c o n t a i n i n g salt a l t h o u g h i n these cases at l o w
Weber and Matijevi; Adsorption From Aqueous Solution Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
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11.
141
Graphon
GREENWOOD E T A L .
Figure 3. The dispersibility of Graphon in aqueous solutions of DTAB at concentrations from left to right, in mM: 2.0, 5.1, 9.1,12.3, 15.0, 19.3, 22.6, 26.0, 30.2, 33.7 surfactant concentrations
the dispersions are r e l a t i v e l y unstable.
Also,
measurements of e l e c t r o p h o r e t i c m o b i l i t y i n d i c a t e that for a l l the systems the zeta p o t e n t i a l is constant over the range of c o n c e n t r a t i o n at w h i c h there is a m a r k e d change i n d i s p e r s i b i l i t y . It seems clear that the d i s p e r s i b i l i t y of the G r a p h o n is not c o n t r o l l e d b y the e l e c t r o c h e m i c a l p r o p erties of the system. T h e d i s p e r s i b i l i t y of G r a p h o n i n S D S solutions, b o t h w i t h a n d w i t h out s o d i u m c h l o r i d e , is i l l u s t r a t e d i n F i g u r e 4 i n terms of the o p t i c a l d e n s i t y ( o n a s t a n d a r d w e i g h t basis) of the dispersions w h i c h r e m a i n after v a r i o u s periods of e n d - o v e r - e n d a c t i o n . S i m i l a r plots w e r e o b t a i n e d for D T A B .
F o r a l l the plots e x t r a p o l a t i o n to zero o p t i c a l density of the
a p p r o x i m a t e l y l i n e a r r e g i o n of r a p i d l y i n c r e a s i n g o p t i c a l density leads to a f a i r l y discrete v a l u e of the surface coverage of a d s o r b e d ions (46 i t 1A.
2
D S " a n d 52 ±
p e r s i b i l i t y occurs.
1 A . for D T A ) at w h i c h the a b r u p t change i n d i s 2
+
T h e s e d a t a i n d i c a t e that the d i s p e r s i b i l i t y of G r a p h o n
is r e l a t e d to the h y d r o p h i l i c character of the surface associated w i t h the a d s o r b e d surfactant ions. Spontaneous w e t t i n g of the external surface of a s o l i d is associated w i t h zero contact angle, otherwise some w o r k is necessary for
complete
w e t t i n g to be a c h i e v e d . I n the case of a p o w d e r w e m u s t also consider the p e n e t r a t i o n of l i q u i d into the s m a l l channels i n s i d e a n d b e t w e e n
the
aggregates of the d r y p o w d e r , a n d this is t h e o r e t i c a l l y spontaneous
only
when 6 <
may
90°
(assuming a hypothetical cylindrical pore).
It
therefore b e a s s u m e d that for the p o w d e r to b e d i s p e r s e d i n the l i q u i d as fine particles it is necessary for 6 < would we
9 0 ° , a n d that o n l y w h e n 0 =
expect the w h o l e w e t t i n g process to be
0
spontaneous—i.e.,
Weber and Matijevi; Adsorption From Aqueous Solution Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
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142
ADSORPTION F R O M
Concentration
SDS
AQUEOUS SOLUTION
(mM)
Figure 4. The dispersibility of Graphon in aqueous solutions of SDS shown as values of optical density put on a standard weight basis: (a) without sodium chloride, (b) with 0.02M sodium chloride, (c) with 0.1M sodium chloride. The numbers indicate the number of hours subjected to end-over-end action. Arrows indicate the c.m.c. r e q u i r e n o e x t e r n a l w o r k . T h a t this is the case w i t h the present system is b o r n e out b y the measurements of 6 for the v a r i o u s solutions o n G r a p h o n ( F i g u r e 5 ) . I n a l l cases i t is o b s e r v e d that t h e G r a p h o n cannot b e d i s p e r s e d e a s i l y unless 0 < 9 0 ° . It seems
reasonable to c o n c l u d e ,
therefore, that d i s p e r s i b i l i t y is
r e l a t e d to the w e t t i n g of the p o w d e r b y t h e l i q u i d r a t h e r t h a n to t h e e l e c t r o c h e m i c a l properties of the system.
Weber and Matijevi; Adsorption From Aqueous Solution Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
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11.
GREENWOOD E T A L .
Graphotl
4
143
6 8 10 Concentration (mM)
Figure 5. Contact angles on Graphon for aqueous solutions of DTAB • and SDS X , for DTAB in 0.1M potassium bromide O , and for SDS in 0.1M sodium chloride O Acknowledgments T h e authors are g r a t e f u l to C . D . M o o r e of G l o v e r s C h e m i c a l s L t d . for p r e p a r i n g the s a m p l e of D T A B , to A c h e s o n I n d u s t r i e s ( E u r o p e ) L t d . for a grant to D . G . W . a n d to the R e s e a r c h A s s o c i a t i o n of B r i t i s h P a i n t , C o l o u r a n d V a r n i s h M a n u f a c t u r e r s f o r a grant to N . H . P . Literature (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16)
Cited
Adamson, A. W . , "Physical Chemistry of Surfaces," p. 475, 2nd ed., John W i l e y and Sons, N e w York, 1967. Barr, T., Oliver, J . , Stubbings, W . V . , J. Soc. Chem. Ind. 67, 45 (1948). Bikerman, J . J . , Ind. Eng. Chem. 13, 443 (1941). D a y , R. E., Greenwood, F . G., Parfitt, G . D . , 4th Int. Cong. Surface Active Substances V o l . 2, 1005 (1964). Debye, P., Ann. N.Y. Acad. Sci. 51, 575 (1949). Deryaguin, B. V., Trans. Faraday Soc. 36, 203 (1940). Deryaguin, B . V., L a n d a u , L. D., Acta Physicochim. U.S.S.R. 14, 633 (1941). Doscher, T. M., J. Coll. Sci. 5, 100 (1950). Gerstner, W., J. Oil and Colour Chem. Assoc. 49, 954 (1966). Huisman, H. F., Proc. Kon. Ned. Akad. van Wet. 67, 367 (1964). Meguro, K . ,J.Chem. Soc. Japan (Ind. Chem. Sect.) 58, 905 (1955). Parfitt, G . D., J. Oil and Colour Chem. Assoc. 50, 822 (1967). Parfitt, G . D . , Picton, N . H., Trans. Faraday Soc. 64, 1955 (1968). Patton, T . C., "Paint F l o w and Pigment Dispersion," Chapt. 8, Interscience N e w York 1964 Ray, L. N., Hutchinson, A." W . , J. Phys. Coll. Chem. 55, 1334 (1951). Rehbinder, P., Colloid J. U.S.S.R. 20, 493 (1958).
Weber and Matijevi; Adsorption From Aqueous Solution Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
144 (17) (18) (19) (20) (21) (22) (23) (24)
ADSORPTION F R O M
AQUEOUS
SOLUTION
Scott, A . B., Tartar, H. V., J. Am. Chem. Soc. 65, 692 (1943). Tamaki, K . , J. Japan Oil Chem. Soc. 9, 426 (1960). Tamamushi, B., " C o l l o i d a l Surfactants," p. 244, Shinoda et al., eds., Academic Press, L o n d o n , 1963. U r b a i n , W . M., Jensen, L. B., J. Phys. Chem. 40, 821 (1936). Verwey, E . J . W . , Overbeek, J . T h . G., "Theory of the Stability of L y o phobic Colloids," Elsevier, Amsterdam, 1948. V o i d , R. D . , Greiner, L., J. Phys. Coll. Chem. 53, 67 (1949). V o l d , R. D . , Konecny, C . C., J. Phys. Coll. Chem. 53, 1262 (1949). W i l l i a m s , R. J., Phillips, J. N., Mysels, K . J., Trans. Faraday Soc. 51, 728 October 26,
1967.
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RECEIVED
Weber and Matijevi; Adsorption From Aqueous Solution Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
