Adsorption From Aqueous Solution

11 Adsorption and Wetting Phenomena Associated with Graphon in Aqueous Surfactant Solutions

Downloaded by CORNELL UNIV on October 10, 2016 | http://pubs.acs.org Publication Date: June 1, 1968 | doi: 10.1021/ba-1968-0079.ch011

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
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


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


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.


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



138

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



.

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)


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



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

Adsorption From Aqueous Solution

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